6. GNAT and Program Execution

This chapter covers several topics:

6.1. Running and Debugging Ada Programs

This section discusses how to debug Ada programs.

An incorrect Ada program may be handled in three ways by the GNAT compiler:

  • The illegality may be a violation of the static semantics of Ada. In that case GNAT diagnoses the constructs in the program that are illegal. It is then a straightforward matter for the user to modify those parts of the program.
  • The illegality may be a violation of the dynamic semantics of Ada. In that case the program compiles and executes, but may generate incorrect results, or may terminate abnormally with some exception.
  • When presented with a program that contains convoluted errors, GNAT itself may terminate abnormally without providing full diagnostics on the incorrect user program.

6.1.1. The GNAT Debugger GDB

GDB is a general purpose, platform-independent debugger that can be used to debug mixed-language programs compiled with gcc, and in particular is capable of debugging Ada programs compiled with GNAT. The latest versions of GDB are Ada-aware and can handle complex Ada data structures.

See Debugging with GDB, for full details on the usage of GDB, including a section on its usage on programs. This manual should be consulted for full details. The section that follows is a brief introduction to the philosophy and use of GDB.

When GNAT programs are compiled, the compiler optionally writes debugging information into the generated object file, including information on line numbers, and on declared types and variables. This information is separate from the generated code. It makes the object files considerably larger, but it does not add to the size of the actual executable that will be loaded into memory, and has no impact on run-time performance. The generation of debug information is triggered by the use of the -g switch in the gcc or gnatmake command used to carry out the compilations. It is important to emphasize that the use of these options does not change the generated code.

The debugging information is written in standard system formats that are used by many tools, including debuggers and profilers. The format of the information is typically designed to describe C types and semantics, but GNAT implements a translation scheme which allows full details about Ada types and variables to be encoded into these standard C formats. Details of this encoding scheme may be found in the file exp_dbug.ads in the GNAT source distribution. However, the details of this encoding are, in general, of no interest to a user, since GDB automatically performs the necessary decoding.

When a program is bound and linked, the debugging information is collected from the object files, and stored in the executable image of the program. Again, this process significantly increases the size of the generated executable file, but it does not increase the size of the executable program itself. Furthermore, if this program is run in the normal manner, it runs exactly as if the debug information were not present, and takes no more actual memory.

However, if the program is run under control of GDB, the debugger is activated. The image of the program is loaded, at which point it is ready to run. If a run command is given, then the program will run exactly as it would have if GDB were not present. This is a crucial part of the GDB design philosophy. GDB is entirely non-intrusive until a breakpoint is encountered. If no breakpoint is ever hit, the program will run exactly as it would if no debugger were present. When a breakpoint is hit, GDB accesses the debugging information and can respond to user commands to inspect variables, and more generally to report on the state of execution.

6.1.2. Running GDB

This section describes how to initiate the debugger.

The debugger can be launched from a GNAT Studio menu or directly from the command line. The description below covers the latter use. All the commands shown can be used in the GNAT Studio debug console window, but there are usually more GUI-based ways to achieve the same effect.

The command to run GDB is

$ gdb program

where program is the name of the executable file. This activates the debugger and results in a prompt for debugger commands. The simplest command is simply run, which causes the program to run exactly as if the debugger were not present. The following section describes some of the additional commands that can be given to GDB.

6.1.3. Introduction to GDB Commands

GDB contains a large repertoire of commands. See Debugging with GDB for extensive documentation on the use of these commands, together with examples of their use. Furthermore, the command help invoked from within GDB activates a simple help facility which summarizes the available commands and their options. In this section we summarize a few of the most commonly used commands to give an idea of what GDB is about. You should create a simple program with debugging information and experiment with the use of these GDB commands on the program as you read through the following section.

  • set args arguments
    The arguments list above is a list of arguments to be passed to the program on a subsequent run command, just as though the arguments had been entered on a normal invocation of the program. The set args command is not needed if the program does not require arguments.
  • run
    The run command causes execution of the program to start from the beginning. If the program is already running, that is to say if you are currently positioned at a breakpoint, then a prompt will ask for confirmation that you want to abandon the current execution and restart.
  • breakpoint location
    The breakpoint command sets a breakpoint, that is to say a point at which execution will halt and GDB will await further commands. location is either a line number within a file, given in the format file:linenumber, or it is the name of a subprogram. If you request that a breakpoint be set on a subprogram that is overloaded, a prompt will ask you to specify on which of those subprograms you want to breakpoint. You can also specify that all of them should be breakpointed. If the program is run and execution encounters the breakpoint, then the program stops and GDB signals that the breakpoint was encountered by printing the line of code before which the program is halted.
  • catch exception name
    This command causes the program execution to stop whenever exception name is raised. If name is omitted, then the execution is suspended when any exception is raised.
  • print expression
    This will print the value of the given expression. Most simple Ada expression formats are properly handled by GDB, so the expression can contain function calls, variables, operators, and attribute references.
  • continue
    Continues execution following a breakpoint, until the next breakpoint or the termination of the program.
  • step
    Executes a single line after a breakpoint. If the next statement is a subprogram call, execution continues into (the first statement of) the called subprogram.
  • next
    Executes a single line. If this line is a subprogram call, executes and returns from the call.
  • list
    Lists a few lines around the current source location. In practice, it is usually more convenient to have a separate edit window open with the relevant source file displayed. Successive applications of this command print subsequent lines. The command can be given an argument which is a line number, in which case it displays a few lines around the specified one.
  • backtrace
    Displays a backtrace of the call chain. This command is typically used after a breakpoint has occurred, to examine the sequence of calls that leads to the current breakpoint. The display includes one line for each activation record (frame) corresponding to an active subprogram.
  • up
    At a breakpoint, GDB can display the values of variables local to the current frame. The command up can be used to examine the contents of other active frames, by moving the focus up the stack, that is to say from callee to caller, one frame at a time.
  • down
    Moves the focus of GDB down from the frame currently being examined to the frame of its callee (the reverse of the previous command),
  • frame n
    Inspect the frame with the given number. The value 0 denotes the frame of the current breakpoint, that is to say the top of the call stack.
  • kill
    Kills the child process in which the program is running under GDB. This may be useful for several purposes:
    • It allows you to recompile and relink your program, since on many systems you cannot regenerate an executable file while it is running in a process.
    • You can run your program outside the debugger, on systems that do not permit executing a program outside GDB while breakpoints are set within GDB.
    • It allows you to debug a core dump rather than a running process.

The above list is a very short introduction to the commands that GDB provides. Important additional capabilities, including conditional breakpoints, the ability to execute command sequences on a breakpoint, the ability to debug at the machine instruction level and many other features are described in detail in Debugging with GDB. Note that most commands can be abbreviated (for example, c for continue, bt for backtrace).

6.1.4. Using Ada Expressions

GDB supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is

  • That GDB should provide basic literals and access to operations for arithmetic, dereferencing, field selection, indexing, and subprogram calls, leaving more sophisticated computations to subprograms written into the program (which therefore may be called from GDB).
  • That type safety and strict adherence to Ada language restrictions are not particularly relevant in a debugging context.
  • That brevity is important to the GDB user.

Thus, for brevity, the debugger acts as if there were implicit with and use clauses in effect for all user-written packages, thus making it unnecessary to fully qualify most names with their packages, regardless of context. Where this causes ambiguity, GDB asks the user’s intent.

For details on the supported Ada syntax, see Debugging with GDB.

6.1.5. Calling User-Defined Subprograms

An important capability of GDB is the ability to call user-defined subprograms while debugging. This is achieved simply by entering a subprogram call statement in the form:

call subprogram-name (parameters)

The keyword call can be omitted in the normal case where the subprogram-name does not coincide with any of the predefined GDB commands.

The effect is to invoke the given subprogram, passing it the list of parameters that is supplied. The parameters can be expressions and can include variables from the program being debugged. The subprogram must be defined at the library level within your program, and GDB will call the subprogram within the environment of your program execution (which means that the subprogram is free to access or even modify variables within your program).

The most important use of this facility is in allowing the inclusion of debugging routines that are tailored to particular data structures in your program. Such debugging routines can be written to provide a suitably high-level description of an abstract type, rather than a low-level dump of its physical layout. After all, the standard GDB print command only knows the physical layout of your types, not their abstract meaning. Debugging routines can provide information at the desired semantic level and are thus enormously useful.

For example, when debugging GNAT itself, it is crucial to have access to the contents of the tree nodes used to represent the program internally. But tree nodes are represented simply by an integer value (which in turn is an index into a table of nodes). Using the print command on a tree node would simply print this integer value, which is not very useful. But the PN routine (defined in file treepr.adb in the GNAT sources) takes a tree node as input, and displays a useful high level representation of the tree node, which includes the syntactic category of the node, its position in the source, the integers that denote descendant nodes and parent node, as well as varied semantic information. To study this example in more detail, you might want to look at the body of the PN procedure in the stated file.

Another useful application of this capability is to deal with situations of complex data which are not handled suitably by GDB. For example, if you specify Convention Fortran for a multi-dimensional array, GDB does not know that the ordering of array elements has been switched and will not properly address the array elements. In such a case, instead of trying to print the elements directly from GDB, you can write a callable procedure that prints the elements in the desired format.

6.1.6. Using the next Command in a Function

When you use the next command in a function, the current source location will advance to the next statement as usual. A special case arises in the case of a return statement.

Part of the code for a return statement is the ‘epilogue’ of the function. This is the code that returns to the caller. There is only one copy of this epilogue code, and it is typically associated with the last return statement in the function if there is more than one return. In some implementations, this epilogue is associated with the first statement of the function.

The result is that if you use the next command from a return statement that is not the last return statement of the function you may see a strange apparent jump to the last return statement or to the start of the function. You should simply ignore this odd jump. The value returned is always that from the first return statement that was stepped through.

6.1.7. Stopping When Ada Exceptions Are Raised

You can set catchpoints that stop the program execution when your program raises selected exceptions.

  • catch exception
    Set a catchpoint that stops execution whenever (any task in the) program raises any exception.
  • catch exception name
    Set a catchpoint that stops execution whenever (any task in the) program raises the exception name.
  • catch exception unhandled
    Set a catchpoint that stops executing whenever (any task in the) program raises an exception for which there is no handler.
  • info exceptions, info exceptions regexp
    The info exceptions command permits the user to examine all defined exceptions within Ada programs. With a regular expression, regexp, as argument, prints out only those exceptions whose name matches regexp.

6.1.8. Ada Tasks

GDB allows the following task-related commands:

  • info tasks

    This command shows a list of current Ada tasks, as in the following example:

    (gdb) info tasks
      ID       TID P-ID   Thread Pri State                 Name
       1   8088000   0   807e000  15 Child Activation Wait main_task
       2   80a4000   1   80ae000  15 Accept/Select Wait    b
       3   809a800   1   80a4800  15 Child Activation Wait a
    *  4   80ae800   3   80b8000  15 Running               c

    In this listing, the asterisk before the first task indicates it to be the currently running task. The first column lists the task ID that is used to refer to tasks in the following commands.

  • break``*linespec* ``task taskid, break linespec task taskid if ...

    These commands are like the break ... thread .... linespec specifies source lines.

    Use the qualifier task taskid with a breakpoint command to specify that you only want GDB to stop the program when a particular Ada task reaches this breakpoint. taskid is one of the numeric task identifiers assigned by GDB, shown in the first column of the info tasks display.

    If you do not specify task taskid when you set a breakpoint, the breakpoint applies to all tasks of your program.

    You can use the task qualifier on conditional breakpoints as well; in this case, place task taskid before the breakpoint condition (before the if).

  • task taskno

    This command allows switching to the task referred by taskno. In particular, this allows browsing of the backtrace of the specified task. It is advisable to switch back to the original task before continuing execution otherwise the scheduling of the program may be perturbed.

For more detailed information on the tasking support, see Debugging with GDB.

6.1.9. Debugging Generic Units

GNAT always uses code expansion for generic instantiation. This means that each time an instantiation occurs, a complete copy of the original code is made, with appropriate substitutions of formals by actuals.

It is not possible to refer to the original generic entities in GDB, but it is always possible to debug a particular instance of a generic, by using the appropriate expanded names. For example, if we have

procedure g is

   generic package k is
      procedure kp (v1 : in out integer);
   end k;

   package body k is
      procedure kp (v1 : in out integer) is
         v1 := v1 + 1;
      end kp;
   end k;

   package k1 is new k;
   package k2 is new k;

   var : integer := 1;

   k1.kp (var);
   k2.kp (var);
   k1.kp (var);
   k2.kp (var);

Then to break on a call to procedure kp in the k2 instance, simply use the command:

(gdb) break g.k2.kp

When the breakpoint occurs, you can step through the code of the instance in the normal manner and examine the values of local variables, as for other units.

6.1.10. Remote Debugging with gdbserver

On platforms where gdbserver is supported, it is possible to use this tool to debug your application remotely. This can be useful in situations where the program needs to be run on a target host that is different from the host used for development, particularly when the target has a limited amount of resources (either CPU and/or memory).

To do so, start your program using gdbserver on the target machine. gdbserver then automatically suspends the execution of your program at its entry point, waiting for a debugger to connect to it. The following commands starts an application and tells gdbserver to wait for a connection with the debugger on localhost port 4444.

$ gdbserver localhost:4444 program
Process program created; pid = 5685
Listening on port 4444

Once gdbserver has started listening, we can tell the debugger to establish a connection with this gdbserver, and then start the same debugging session as if the program was being debugged on the same host, directly under the control of GDB.

$ gdb program
(gdb) target remote targethost:4444
Remote debugging using targethost:4444
0x00007f29936d0af0 in ?? () from /lib64/ld-linux-x86-64.so.
(gdb) b foo.adb:3
Breakpoint 1 at 0x401f0c: file foo.adb, line 3.
(gdb) continue

Breakpoint 1, foo () at foo.adb:4
4       end foo;

It is also possible to use gdbserver to attach to an already running program, in which case the execution of that program is simply suspended until the connection between the debugger and gdbserver is established.

For more information on how to use gdbserver, see the Using the gdbserver Program section in Debugging with GDB. GNAT provides support for gdbserver on x86-linux, x86-windows and x86_64-linux.

6.1.11. GNAT Abnormal Termination or Failure to Terminate

When presented with programs that contain serious errors in syntax or semantics, GNAT may on rare occasions experience problems in operation, such as aborting with a segmentation fault or illegal memory access, raising an internal exception, terminating abnormally, or failing to terminate at all. In such cases, you can activate various features of GNAT that can help you pinpoint the construct in your program that is the likely source of the problem.

The following strategies are presented in increasing order of difficulty, corresponding to your experience in using GNAT and your familiarity with compiler internals.

  • Run gcc with the -gnatf. This first switch causes all errors on a given line to be reported. In its absence, only the first error on a line is displayed.

    The -gnatdO switch causes errors to be displayed as soon as they are encountered, rather than after compilation is terminated. If GNAT terminates prematurely or goes into an infinite loop, the last error message displayed may help to pinpoint the culprit.

  • Run gcc with the -v (verbose) switch. In this mode, gcc produces ongoing information about the progress of the compilation and provides the name of each procedure as code is generated. This switch allows you to find which Ada procedure was being compiled when it encountered a code generation problem.

  • Run gcc with the -gnatdc switch. This is a GNAT specific switch that does for the front-end what -v does for the back end. The system prints the name of each unit, either a compilation unit or nested unit, as it is being analyzed.
  • Finally, you can start gdb directly on the gnat1 executable. gnat1 is the front-end of GNAT, and can be run independently (normally it is just called from gcc). You can use gdb on gnat1 as you would on a C program (but The GNAT Debugger GDB for caveats). The where command is the first line of attack; the variable lineno (seen by print lineno), used by the second phase of gnat1 and by the gcc backend, indicates the source line at which the execution stopped, and input_file name indicates the name of the source file.

6.1.12. Naming Conventions for GNAT Source Files

In order to examine the workings of the GNAT system, the following brief description of its organization may be helpful:

  • Files with prefix sc contain the lexical scanner.

  • All files prefixed with par are components of the parser. The numbers correspond to chapters of the Ada Reference Manual. For example, parsing of select statements can be found in par-ch9.adb.

  • All files prefixed with sem perform semantic analysis. The numbers correspond to chapters of the Ada standard. For example, all issues involving context clauses can be found in sem_ch10.adb. In addition, some features of the language require sufficient special processing to justify their own semantic files: sem_aggr for aggregates, sem_disp for dynamic dispatching, etc.

  • All files prefixed with exp perform normalization and expansion of the intermediate representation (abstract syntax tree, or AST). these files use the same numbering scheme as the parser and semantics files. For example, the construction of record initialization procedures is done in exp_ch3.adb.

  • The files prefixed with bind implement the binder, which verifies the consistency of the compilation, determines an order of elaboration, and generates the bind file.

  • The files atree.ads and atree.adb detail the low-level data structures used by the front-end.

  • The files sinfo.ads and sinfo.adb detail the structure of the abstract syntax tree as produced by the parser.

  • The files einfo.ads and einfo.adb detail the attributes of all entities, computed during semantic analysis.

  • Library management issues are dealt with in files with prefix lib.

  • Ada files with the prefix a- are children of Ada, as defined in Annex A.

  • Files with prefix i- are children of Interfaces, as defined in Annex B.

  • Files with prefix s- are children of System. This includes both language-defined children and GNAT run-time routines.

  • Files with prefix g- are children of GNAT. These are useful general-purpose packages, fully documented in their specs. All the other .c files are modifications of common gcc files.

6.1.13. Getting Internal Debugging Information

Most compilers have internal debugging switches and modes. GNAT does also, except GNAT internal debugging switches and modes are not secret. A summary and full description of all the compiler and binder debug flags are in the file debug.adb. You must obtain the sources of the compiler to see the full detailed effects of these flags.

The switches that print the source of the program (reconstructed from the internal tree) are of general interest for user programs, as are the options to print the full internal tree, and the entity table (the symbol table information). The reconstructed source provides a readable version of the program after the front-end has completed analysis and expansion, and is useful when studying the performance of specific constructs. For example, constraint checks are indicated, complex aggregates are replaced with loops and assignments, and tasking primitives are replaced with run-time calls.

6.1.14. Stack Traceback

Traceback is a mechanism to display the sequence of subprogram calls that leads to a specified execution point in a program. Often (but not always) the execution point is an instruction at which an exception has been raised. This mechanism is also known as stack unwinding because it obtains its information by scanning the run-time stack and recovering the activation records of all active subprograms. Stack unwinding is one of the most important tools for program debugging.

The first entry stored in traceback corresponds to the deepest calling level, that is to say the subprogram currently executing the instruction from which we want to obtain the traceback.

Note that there is no runtime performance penalty when stack traceback is enabled, and no exception is raised during program execution. Non-Symbolic Traceback

Note: this feature is not supported on all platforms. See GNAT.Traceback spec in g-traceb.ads for a complete list of supported platforms.

Tracebacks From an Unhandled Exception

A runtime non-symbolic traceback is a list of addresses of call instructions. To enable this feature you must use the -E gnatbind option. With this option a stack traceback is stored as part of exception information. You can retrieve this information using the addr2line tool.

Here is a simple example:

procedure STB is

   procedure P1 is
      raise Constraint_Error;
   end P1;

   procedure P2 is
   end P2;

end STB;
$ gnatmake stb -bargs -E
$ stb

Execution terminated by unhandled exception
Exception name: CONSTRAINT_ERROR
Message: stb.adb:5
Call stack traceback locations:
0x401373 0x40138b 0x40139c 0x401335 0x4011c4 0x4011f1 0x77e892a4

As we see the traceback lists a sequence of addresses for the unhandled exception CONSTRAINT_ERROR raised in procedure P1. It is easy to guess that this exception come from procedure P1. To translate these addresses into the source lines where the calls appear, the addr2line tool, described below, is invaluable. The use of this tool requires the program to be compiled with debug information.

$ gnatmake -g stb -bargs -E
$ stb

Execution terminated by unhandled exception
Exception name: CONSTRAINT_ERROR
Message: stb.adb:5
Call stack traceback locations:
0x401373 0x40138b 0x40139c 0x401335 0x4011c4 0x4011f1 0x77e892a4

$ addr2line --exe=stb 0x401373 0x40138b 0x40139c 0x401335 0x4011c4
   0x4011f1 0x77e892a4

00401373 at d:/stb/stb.adb:5
0040138B at d:/stb/stb.adb:10
0040139C at d:/stb/stb.adb:14
00401335 at d:/stb/b~stb.adb:104
004011C4 at /build/.../crt1.c:200
004011F1 at /build/.../crt1.c:222
77E892A4 in ?? at ??:0

The addr2line tool has several other useful options:

--functions to get the function name corresponding to any location
--demangle=gnat to use the gnat decoding mode for the function names. Note that for binutils version 2.9.x the option is simply --demangle.
$ addr2line --exe=stb --functions --demangle=gnat 0x401373 0x40138b
   0x40139c 0x401335 0x4011c4 0x4011f1

00401373 in stb.p1 at d:/stb/stb.adb:5
0040138B in stb.p2 at d:/stb/stb.adb:10
0040139C in stb at d:/stb/stb.adb:14
00401335 in main at d:/stb/b~stb.adb:104
004011C4 in <__mingw_CRTStartup> at /build/.../crt1.c:200
004011F1 in <mainCRTStartup> at /build/.../crt1.c:222

From this traceback we can see that the exception was raised in stb.adb at line 5, which was reached from a procedure call in stb.adb at line 10, and so on. The b~std.adb is the binder file, which contains the call to the main program. Running gnatbind. The remaining entries are assorted runtime routines, and the output will vary from platform to platform.

It is also possible to use GDB with these traceback addresses to debug the program. For example, we can break at a given code location, as reported in the stack traceback:

$ gdb -nw stb

Furthermore, this feature is not implemented inside Windows DLL. Only the non-symbolic traceback is reported in this case.

(gdb) break *0x401373
Breakpoint 1 at 0x401373: file stb.adb, line 5.

It is important to note that the stack traceback addresses do not change when debug information is included. This is particularly useful because it makes it possible to release software without debug information (to minimize object size), get a field report that includes a stack traceback whenever an internal bug occurs, and then be able to retrieve the sequence of calls with the same program compiled with debug information.

Tracebacks From Exception Occurrences

Non-symbolic tracebacks are obtained by using the -E binder argument. The stack traceback is attached to the exception information string, and can be retrieved in an exception handler within the Ada program, by means of the Ada facilities defined in Ada.Exceptions. Here is a simple example:

with Ada.Text_IO;
with Ada.Exceptions;

procedure STB is

   use Ada;
   use Ada.Exceptions;

   procedure P1 is
      K : Positive := 1;
      K := K - 1;
      when E : others =>
         Text_IO.Put_Line (Exception_Information (E));
   end P1;

   procedure P2 is
   end P2;

end STB;

This program will output:

$ stb

Exception name: CONSTRAINT_ERROR
Message: stb.adb:12
Call stack traceback locations:
0x4015e4 0x401633 0x401644 0x401461 0x4011c4 0x4011f1 0x77e892a4

Tracebacks From Anywhere in a Program

It is also possible to retrieve a stack traceback from anywhere in a program. For this you need to use the GNAT.Traceback API. This package includes a procedure called Call_Chain that computes a complete stack traceback, as well as useful display procedures described below. It is not necessary to use the -E gnatbind option in this case, because the stack traceback mechanism is invoked explicitly.

In the following example we compute a traceback at a specific location in the program, and we display it using GNAT.Debug_Utilities.Image to convert addresses to strings:

with Ada.Text_IO;
with GNAT.Traceback;
with GNAT.Debug_Utilities;

procedure STB is

   use Ada;
   use GNAT;
   use GNAT.Traceback;

   procedure P1 is
      TB  : Tracebacks_Array (1 .. 10);
      --  We are asking for a maximum of 10 stack frames.
      Len : Natural;
      --  Len will receive the actual number of stack frames returned.
      Call_Chain (TB, Len);

      Text_IO.Put ("In STB.P1 : ");

      for K in 1 .. Len loop
         Text_IO.Put (Debug_Utilities.Image (TB (K)));
         Text_IO.Put (' ');
      end loop;

   end P1;

   procedure P2 is
   end P2;

end STB;
$ gnatmake -g stb
$ stb

In STB.P1 : 16#0040_F1E4# 16#0040_14F2# 16#0040_170B# 16#0040_171C#
16#0040_1461# 16#0040_11C4# 16#0040_11F1# 16#77E8_92A4#

You can then get further information by invoking the addr2line tool as described earlier (note that the hexadecimal addresses need to be specified in C format, with a leading ‘0x’). Symbolic Traceback

A symbolic traceback is a stack traceback in which procedure names are associated with each code location.

Note that this feature is not supported on all platforms. See GNAT.Traceback.Symbolic spec in g-trasym.ads for a complete list of currently supported platforms.

Note that the symbolic traceback requires that the program be compiled with debug information. If it is not compiled with debug information only the non-symbolic information will be valid.

Tracebacks From Exception Occurrences

Here is an example:

with Ada.Text_IO;
with GNAT.Traceback.Symbolic;

procedure STB is

   procedure P1 is
      raise Constraint_Error;
   end P1;

   procedure P2 is
   end P2;

   procedure P3 is
   end P3;

   when E : others =>
      Ada.Text_IO.Put_Line (GNAT.Traceback.Symbolic.Symbolic_Traceback (E));
end STB;
$ gnatmake -g .\stb -bargs -E
$ stb

0040149F in stb.p1 at stb.adb:8
004014B7 in stb.p2 at stb.adb:13
004014CF in stb.p3 at stb.adb:18
004015DD in ada.stb at stb.adb:22
00401461 in main at b~stb.adb:168
004011C4 in __mingw_CRTStartup at crt1.c:200
004011F1 in mainCRTStartup at crt1.c:222
77E892A4 in ?? at ??:0

In the above example the .\ syntax in the gnatmake command is currently required by addr2line for files that are in the current working directory. Moreover, the exact sequence of linker options may vary from platform to platform. The above -largs section is for Windows platforms. By contrast, under Unix there is no need for the -largs section. Differences across platforms are due to details of linker implementation.

Tracebacks From Anywhere in a Program

It is possible to get a symbolic stack traceback from anywhere in a program, just as for non-symbolic tracebacks. The first step is to obtain a non-symbolic traceback, and then call Symbolic_Traceback to compute the symbolic information. Here is an example:

with Ada.Text_IO;
with GNAT.Traceback;
with GNAT.Traceback.Symbolic;

procedure STB is

   use Ada;
   use GNAT.Traceback;
   use GNAT.Traceback.Symbolic;

   procedure P1 is
      TB  : Tracebacks_Array (1 .. 10);
      --  We are asking for a maximum of 10 stack frames.
      Len : Natural;
      --  Len will receive the actual number of stack frames returned.
      Call_Chain (TB, Len);
      Text_IO.Put_Line (Symbolic_Traceback (TB (1 .. Len)));
   end P1;

   procedure P2 is
   end P2;

end STB;

Automatic Symbolic Tracebacks

Symbolic tracebacks may also be enabled by using the -Es switch to gnatbind (as in gprbuild -g ... -bargs -Es). This will cause the Exception_Information to contain a symbolic traceback, which will also be printed if an unhandled exception terminates the program.

6.1.15. Pretty-Printers for the GNAT runtime

As discussed in Calling User-Defined Subprograms, GDB’s print command only knows about the physical layout of program data structures and therefore normally displays only low-level dumps, which are often hard to understand.

An example of this is when trying to display the contents of an Ada standard container, such as Ada.Containers.Ordered_Maps.Map:

with Ada.Containers.Ordered_Maps;

procedure PP is
   package Int_To_Nat is
      new Ada.Containers.Ordered_Maps (Integer, Natural);

   Map : Int_To_Nat.Map;
   Map.Insert (1, 10);
   Map.Insert (2, 20);
   Map.Insert (3, 30);

   Map.Clear; --  BREAK HERE
end PP;

When this program is built with debugging information and run under GDB up to the Map.Clear statement, trying to print Map will yield information that is only relevant to the developers of our standard containers:

(gdb) print map
$1 = (
  tree => (
    first => 0x64e010,
    last => 0x64e070,
    root => 0x64e040,
    length => 3,
    tc => (
      busy => 0,
      lock => 0

Fortunately, GDB has a feature called pretty-printers, which allows customizing how GDB displays data structures. The GDB shipped with GNAT embeds such pretty-printers for the most common containers in the standard library. To enable them, either run the following command manually under GDB or add it to your .gdbinit file:

python import gnatdbg; gnatdbg.setup()

Once this is done, GDB’s print command will automatically use these pretty-printers when appropriate. Using the previous example:

(gdb) print map
$1 = pp.int_to_nat.map of length 3 = {
  [1] = 10,
  [2] = 20,
  [3] = 30

Pretty-printers are invoked each time GDB tries to display a value, including when displaying the arguments of a called subprogram (in GDB’s backtrace command) or when printing the value returned by a function (in GDB’s finish command).

To display a value without involving pretty-printers, print can be invoked with its /r option:

(gdb) print/r map
$1 = (
  tree => (...

Finer control of pretty-printers is also possible: see GDB’s online documentation for more information.

6.2. Profiling

This section describes how to use the gprof profiler tool on Ada programs.

6.2.1. Profiling an Ada Program with gprof

This section is not meant to be an exhaustive documentation of gprof. Full documentation for it can be found in the GNU Profiler User’s Guide documentation that is part of this GNAT distribution.

Profiling a program helps determine the parts of a program that are executed most often, and are therefore the most time-consuming.

gprof is the standard GNU profiling tool; it has been enhanced to better handle Ada programs and multitasking. It is currently supported on the following platforms

  • linux x86/x86_64
  • windows x86

In order to profile a program using gprof, several steps are needed:

  1. Instrument the code, which requires a full recompilation of the project with the proper switches.
  2. Execute the program under the analysis conditions, i.e. with the desired input.
  3. Analyze the results using the gprof tool.

The following sections detail the different steps, and indicate how to interpret the results. Compilation for profiling

In order to profile a program the first step is to tell the compiler to generate the necessary profiling information. The compiler switch to be used is -pg, which must be added to other compilation switches. This switch needs to be specified both during compilation and link stages, and can be specified once when using gnatmake:

$ gnatmake -f -pg -P my_project

Note that only the objects that were compiled with the -pg switch will be profiled; if you need to profile your whole project, use the -f gnatmake switch to force full recompilation. Program execution

Once the program has been compiled for profiling, you can run it as usual.

The only constraint imposed by profiling is that the program must terminate normally. An interrupted program (via a Ctrl-C, kill, etc.) will not be properly analyzed.

Once the program completes execution, a data file called gmon.out is generated in the directory where the program was launched from. If this file already exists, it will be overwritten. Running gprof

The gprof tool is called as follow:

$ gprof my_prog gmon.out

or simply:

$  gprof my_prog

The complete form of the gprof command line is the following:

$ gprof [switches] [executable [data-file]]

gprof supports numerous switches. The order of these switch does not matter. The full list of options can be found in the GNU Profiler User’s Guide documentation that comes with this documentation.

The following is the subset of those switches that is most relevant:

--demangle[=style], --no-demangle
These options control whether symbol names should be demangled when printing output. The default is to demangle C++ symbols. The --no-demangle option may be used to turn off demangling. Different compilers have different mangling styles. The optional demangling style argument can be used to choose an appropriate demangling style for your compiler, in particular Ada symbols generated by GNAT can be demangled using --demangle=gnat.
-e function_name
The -e function option tells gprof not to print information about the function function_name (and its children...) in the call graph. The function will still be listed as a child of any functions that call it, but its index number will be shown as [not printed]. More than one -e option may be given; only one function_name may be indicated with each -e option.
-E function_name
The -E function option works like the -e option, but execution time spent in the function (and children who were not called from anywhere else), will not be used to compute the percentages-of-time for the call graph. More than one -E option may be given; only one function_name may be indicated with each -E` option.
-f function_name
The -f function option causes gprof to limit the call graph to the function function_name and its children (and their children...). More than one -f option may be given; only one function_name may be indicated with each -f option.
-F function_name
The -F function option works like the -f option, but only time spent in the function and its children (and their children...) will be used to determine total-time and percentages-of-time for the call graph. More than one -F option may be given; only one function_name may be indicated with each -F option. The -F option overrides the -E option. Interpretation of profiling results

The results of the profiling analysis are represented by two arrays: the ‘flat profile’ and the ‘call graph’. Full documentation of those outputs can be found in the GNU Profiler User’s Guide.

The flat profile shows the time spent in each function of the program, and how many time it has been called. This allows you to locate easily the most time-consuming functions.

The call graph shows, for each subprogram, the subprograms that call it, and the subprograms that it calls. It also provides an estimate of the time spent in each of those callers/called subprograms.

6.3. Improving Performance

This section presents several topics related to program performance. It first describes some of the tradeoffs that need to be considered and some of the techniques for making your program run faster.

It then documents the unused subprogram/data elimination feature, which can reduce the size of program executables.

6.3.1. Performance Considerations

The GNAT system provides a number of options that allow a trade-off between

  • performance of the generated code
  • speed of compilation
  • minimization of dependences and recompilation
  • the degree of run-time checking.

The defaults (if no options are selected) aim at improving the speed of compilation and minimizing dependences, at the expense of performance of the generated code:

  • no optimization
  • no inlining of subprogram calls
  • all run-time checks enabled except overflow and elaboration checks

These options are suitable for most program development purposes. This section describes how you can modify these choices, and also provides some guidelines on debugging optimized code. Controlling Run-Time Checks

By default, GNAT generates all run-time checks, except stack overflow checks, and checks for access before elaboration on subprogram calls. The latter are not required in default mode, because all necessary checking is done at compile time.

The gnat switch, -gnatp allows this default to be modified. See Run-Time Checks.

Our experience is that the default is suitable for most development purposes.

Elaboration checks are off by default, and also not needed by default, since GNAT uses a static elaboration analysis approach that avoids the need for run-time checking. This manual contains a full chapter discussing the issue of elaboration checks, and if the default is not satisfactory for your use, you should read this chapter.

For validity checks, the minimal checks required by the Ada Reference Manual (for case statements and assignments to array elements) are on by default. These can be suppressed by use of the -gnatVn switch. Note that in Ada 83, there were no validity checks, so if the Ada 83 mode is acceptable (or when comparing GNAT performance with an Ada 83 compiler), it may be reasonable to routinely use -gnatVn. Validity checks are also suppressed entirely if -gnatp is used.

Note that the setting of the switches controls the default setting of the checks. They may be modified using either pragma Suppress (to remove checks) or pragma Unsuppress (to add back suppressed checks) in the program source. Use of Restrictions

The use of pragma Restrictions allows you to control which features are permitted in your program. Apart from the obvious point that if you avoid relatively expensive features like finalization (enforceable by the use of pragma Restrictions (No_Finalization), the use of this pragma does not affect the generated code in most cases.

One notable exception to this rule is that the possibility of task abort results in some distributed overhead, particularly if finalization or exception handlers are used. The reason is that certain sections of code have to be marked as non-abortable.

If you use neither the abort statement, nor asynchronous transfer of control (select ... then abort), then this distributed overhead is removed, which may have a general positive effect in improving overall performance. Especially code involving frequent use of tasking constructs and controlled types will show much improved performance. The relevant restrictions pragmas are

pragma Restrictions (No_Abort_Statements);
pragma Restrictions (Max_Asynchronous_Select_Nesting => 0);

It is recommended that these restriction pragmas be used if possible. Note that this also means that you can write code without worrying about the possibility of an immediate abort at any point. Optimization Levels

Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the subprogram and get exactly the results you would expect from the source code.

Turning on optimization makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

The default is optimization off. This results in the fastest compile times, but GNAT makes absolutely no attempt to optimize, and the generated programs are considerably larger and slower than when optimization is enabled. You can use the -O switch (the permitted forms are -O0, -O1 -O2, -O3, and -Os) to gcc to control the optimization level:

  • -O0

    No optimization (the default); generates unoptimized code but has the fastest compilation time.

    Note that many other compilers do substantial optimization even if ‘no optimization’ is specified. With gcc, it is very unusual to use -O0 for production if execution time is of any concern, since -O0 means (almost) no optimization. This difference between gcc and other compilers should be kept in mind when doing performance comparisons.

  • -O1

    Moderate optimization; optimizes reasonably well but does not degrade compilation time significantly.

  • -O2

    Full optimization; generates highly optimized code and has the slowest compilation time.

  • -O3

    Full optimization as in -O2; also uses more aggressive automatic inlining of subprograms within a unit (Inlining of Subprograms) and attempts to vectorize loops.

  • -Os

    Optimize space usage (code and data) of resulting program.

Higher optimization levels perform more global transformations on the program and apply more expensive analysis algorithms in order to generate faster and more compact code. The price in compilation time, and the resulting improvement in execution time, both depend on the particular application and the hardware environment. You should experiment to find the best level for your application.

Since the precise set of optimizations done at each level will vary from release to release (and sometime from target to target), it is best to think of the optimization settings in general terms. See the Options That Control Optimization section in Using the GNU Compiler Collection (GCC) for details about the -O settings and a number of -f options that individually enable or disable specific optimizations.

Unlike some other compilation systems, gcc has been tested extensively at all optimization levels. There are some bugs which appear only with optimization turned on, but there have also been bugs which show up only in unoptimized code. Selecting a lower level of optimization does not improve the reliability of the code generator, which in practice is highly reliable at all optimization levels.

Note regarding the use of -O3: The use of this optimization level ought not to be automatically preferred over that of level -O2, since it often results in larger executables which may run more slowly. See further discussion of this point in Inlining of Subprograms. Debugging Optimized Code

Although it is possible to do a reasonable amount of debugging at nonzero optimization levels, the higher the level the more likely that source-level constructs will have been eliminated by optimization. For example, if a loop is strength-reduced, the loop control variable may be completely eliminated and thus cannot be displayed in the debugger. This can only happen at -O2 or -O3. Explicit temporary variables that you code might be eliminated at level -O1 or higher.

The use of the -g switch, which is needed for source-level debugging, affects the size of the program executable on disk, and indeed the debugging information can be quite large. However, it has no effect on the generated code (and thus does not degrade performance)

Since the compiler generates debugging tables for a compilation unit before it performs optimizations, the optimizing transformations may invalidate some of the debugging data. You therefore need to anticipate certain anomalous situations that may arise while debugging optimized code. These are the most common cases:

  • The ‘hopping Program Counter’: Repeated step or next commands show the PC bouncing back and forth in the code. This may result from any of the following optimizations:

    • Common subexpression elimination: using a single instance of code for a quantity that the source computes several times. As a result you may not be able to stop on what looks like a statement.
    • Invariant code motion: moving an expression that does not change within a loop, to the beginning of the loop.
    • Instruction scheduling: moving instructions so as to overlap loads and stores (typically) with other code, or in general to move computations of values closer to their uses. Often this causes you to pass an assignment statement without the assignment happening and then later bounce back to the statement when the value is actually needed. Placing a breakpoint on a line of code and then stepping over it may, therefore, not always cause all the expected side-effects.
  • The ‘big leap’: More commonly known as cross-jumping, in which two identical pieces of code are merged and the program counter suddenly jumps to a statement that is not supposed to be executed, simply because it (and the code following) translates to the same thing as the code that was supposed to be executed. This effect is typically seen in sequences that end in a jump, such as a goto, a return, or a break in a C switch statement.

  • The ‘roving variable’: The symptom is an unexpected value in a variable. There are various reasons for this effect:

    • In a subprogram prologue, a parameter may not yet have been moved to its ‘home’.
    • A variable may be dead, and its register re-used. This is probably the most common cause.
    • As mentioned above, the assignment of a value to a variable may have been moved.
    • A variable may be eliminated entirely by value propagation or other means. In this case, GCC may incorrectly generate debugging information for the variable

    In general, when an unexpected value appears for a local variable or parameter you should first ascertain if that value was actually computed by your program, as opposed to being incorrectly reported by the debugger. Record fields or array elements in an object designated by an access value are generally less of a problem, once you have ascertained that the access value is sensible. Typically, this means checking variables in the preceding code and in the calling subprogram to verify that the value observed is explainable from other values (one must apply the procedure recursively to those other values); or re-running the code and stopping a little earlier (perhaps before the call) and stepping to better see how the variable obtained the value in question; or continuing to step from the point of the strange value to see if code motion had simply moved the variable’s assignments later.

In light of such anomalies, a recommended technique is to use -O0 early in the software development cycle, when extensive debugging capabilities are most needed, and then move to -O1 and later -O2 as the debugger becomes less critical. Whether to use the -g switch in the release version is a release management issue. Note that if you use -g you can then use the strip program on the resulting executable, which removes both debugging information and global symbols. Inlining of Subprograms

A call to a subprogram in the current unit is inlined if all the following conditions are met:

  • The optimization level is at least -O1.

  • The called subprogram is suitable for inlining: It must be small enough and not contain something that gcc cannot support in inlined subprograms.

  • Any one of the following applies: pragma Inline is applied to the subprogram; the subprogram is local to the unit and called once from within it; the subprogram is small and optimization level -O2 is specified; optimization level -O3 is specified.

Calls to subprograms in withed units are normally not inlined. To achieve actual inlining (that is, replacement of the call by the code in the body of the subprogram), the following conditions must all be true:

  • The optimization level is at least -O1.
  • The called subprogram is suitable for inlining: It must be small enough and not contain something that gcc cannot support in inlined subprograms.
  • There is a pragma Inline for the subprogram.
  • The -gnatn switch is used on the command line.

Even if all these conditions are met, it may not be possible for the compiler to inline the call, due to the length of the body, or features in the body that make it impossible for the compiler to do the inlining.

Note that specifying the -gnatn switch causes additional compilation dependencies. Consider the following:

package R is
   procedure Q;
   pragma Inline (Q);
end R;
package body R is
end R;

with R;
procedure Main is
end Main;

With the default behavior (no -gnatn switch specified), the compilation of the Main procedure depends only on its own source, main.adb, and the spec of the package in file r.ads. This means that editing the body of R does not require recompiling Main.

On the other hand, the call R.Q is not inlined under these circumstances. If the -gnatn switch is present when Main is compiled, the call will be inlined if the body of Q is small enough, but now Main depends on the body of R in r.adb as well as on the spec. This means that if this body is edited, the main program must be recompiled. Note that this extra dependency occurs whether or not the call is in fact inlined by gcc.

The use of front end inlining with -gnatN generates similar additional dependencies.

Note: The -fno-inline switch overrides all other conditions and ensures that no inlining occurs, unless requested with pragma Inline_Always for gcc back-ends. The extra dependences resulting from -gnatn will still be active, even if this switch is used to suppress the resulting inlining actions.

Note: The -fno-inline-functions switch can be used to prevent automatic inlining of subprograms if -O3 is used.

Note: The -fno-inline-small-functions switch can be used to prevent automatic inlining of small subprograms if -O2 is used.

Note: The -fno-inline-functions-called-once switch can be used to prevent inlining of subprograms local to the unit and called once from within it if -O1 is used.

Note regarding the use of -O3: -gnatn is made up of two sub-switches -gnatn1 and -gnatn2 that can be directly specified in lieu of it, -gnatn being translated into one of them based on the optimization level. With -O2 or below, -gnatn is equivalent to -gnatn1 which activates pragma Inline with moderate inlining across modules. With -O3, -gnatn is equivalent to -gnatn2 which activates pragma Inline with full inlining across modules. If you have used pragma Inline in appropriate cases, then it is usually much better to use -O2 and -gnatn and avoid the use of -O3 which has the additional effect of inlining subprograms you did not think should be inlined. We have found that the use of -O3 may slow down the compilation and increase the code size by performing excessive inlining, leading to increased instruction cache pressure from the increased code size and thus minor performance improvements. So the bottom line here is that you should not automatically assume that -O3 is better than -O2, and indeed you should use -O3 only if tests show that it actually improves performance for your program. Floating_Point_Operations

On almost all targets, GNAT maps Float and Long_Float to the 32-bit and 64-bit standard IEEE floating-point representations, and operations will use standard IEEE arithmetic as provided by the processor. On most, but not all, architectures, the attribute Machine_Overflows is False for these types, meaning that the semantics of overflow is implementation-defined. In the case of GNAT, these semantics correspond to the normal IEEE treatment of infinities and NaN (not a number) values. For example, 1.0 / 0.0 yields plus infinitiy and 0.0 / 0.0 yields a NaN. By avoiding explicit overflow checks, the performance is greatly improved on many targets. However, if required, floating-point overflow can be enabled by the use of the pragma Check_Float_Overflow.

Another consideration that applies specifically to x86 32-bit architectures is which form of floating-point arithmetic is used. By default the operations use the old style x86 floating-point, which implements an 80-bit extended precision form (on these architectures the type Long_Long_Float corresponds to that form). In addition, generation of efficient code in this mode means that the extended precision form will be used for intermediate results. This may be helpful in improving the final precision of a complex expression. However it means that the results obtained on the x86 will be different from those on other architectures, and for some algorithms, the extra intermediate precision can be detrimental.

In addition to this old-style floating-point, all modern x86 chips implement an alternative floating-point operation model referred to as SSE2. In this model there is no extended form, and furthermore execution performance is significantly enhanced. To force GNAT to use this more modern form, use both of the switches:

-msse2 -mfpmath=sse

A unit compiled with these switches will automatically use the more efficient SSE2 instruction set for Float and Long_Float operations. Note that the ABI has the same form for both floating-point models, so it is permissible to mix units compiled with and without these switches. Vectorization of loops

You can take advantage of the auto-vectorizer present in the gcc back end to vectorize loops with GNAT. The corresponding command line switch is -ftree-vectorize but, as it is enabled by default at -O3 and other aggressive optimizations helpful for vectorization also are enabled by default at this level, using -O3 directly is recommended.

You also need to make sure that the target architecture features a supported SIMD instruction set. For example, for the x86 architecture, you should at least specify -msse2 to get significant vectorization (but you don’t need to specify it for x86-64 as it is part of the base 64-bit architecture). Similarly, for the PowerPC architecture, you should specify -maltivec.

The preferred loop form for vectorization is the for iteration scheme. Loops with a while iteration scheme can also be vectorized if they are very simple, but the vectorizer will quickly give up otherwise. With either iteration scheme, the flow of control must be straight, in particular no exit statement may appear in the loop body. The loop may however contain a single nested loop, if it can be vectorized when considered alone:

A : array (1..4, 1..4) of Long_Float;
S : array (1..4) of Long_Float;

procedure Sum is
   for I in A'Range(1) loop
      for J in A'Range(2) loop
         S (I) := S (I) + A (I, J);
      end loop;
   end loop;
end Sum;

The vectorizable operations depend on the targeted SIMD instruction set, but the adding and some of the multiplying operators are generally supported, as well as the logical operators for modular types. Note that compiling with -gnatp might well reveal cases where some checks do thwart vectorization.

Type conversions may also prevent vectorization if they involve semantics that are not directly supported by the code generator or the SIMD instruction set. A typical example is direct conversion from floating-point to integer types. The solution in this case is to use the following idiom:

Integer (S'Truncation (F))

if S is the subtype of floating-point object F.

In most cases, the vectorizable loops are loops that iterate over arrays. All kinds of array types are supported, i.e. constrained array types with static bounds:

type Array_Type is array (1 .. 4) of Long_Float;

constrained array types with dynamic bounds:

type Array_Type is array (1 .. Q.N) of Long_Float;

type Array_Type is array (Q.K .. 4) of Long_Float;

type Array_Type is array (Q.K .. Q.N) of Long_Float;

or unconstrained array types:

type Array_Type is array (Positive range <>) of Long_Float;

The quality of the generated code decreases when the dynamic aspect of the array type increases, the worst code being generated for unconstrained array types. This is so because, the less information the compiler has about the bounds of the array, the more fallback code it needs to generate in order to fix things up at run time.

It is possible to specify that a given loop should be subject to vectorization preferably to other optimizations by means of pragma Loop_Optimize:

pragma Loop_Optimize (Vector);

placed immediately within the loop will convey the appropriate hint to the compiler for this loop.

It is also possible to help the compiler generate better vectorized code for a given loop by asserting that there are no loop-carried dependencies in the loop. Consider for example the procedure:

type Arr is array (1 .. 4) of Long_Float;

procedure Add (X, Y : not null access Arr; R : not null access Arr) is
  for I in Arr'Range loop
    R(I) := X(I) + Y(I);
  end loop;

By default, the compiler cannot unconditionally vectorize the loop because assigning to a component of the array designated by R in one iteration could change the value read from the components of the array designated by X or Y in a later iteration. As a result, the compiler will generate two versions of the loop in the object code, one vectorized and the other not vectorized, as well as a test to select the appropriate version at run time. This can be overcome by another hint:

pragma Loop_Optimize (Ivdep);

placed immediately within the loop will tell the compiler that it can safely omit the non-vectorized version of the loop as well as the run-time test. Other Optimization Switches

Since GNAT uses the gcc back end, all the specialized gcc optimization switches are potentially usable. These switches have not been extensively tested with GNAT but can generally be expected to work. Examples of switches in this category are -funroll-loops and the various target-specific -m options (in particular, it has been observed that -march=xxx can significantly improve performance on appropriate machines). For full details of these switches, see the Submodel Options section in the Hardware Models and Configurations chapter of Using the GNU Compiler Collection (GCC). Optimization and Strict Aliasing

The strong typing capabilities of Ada allow an optimizer to generate efficient code in situations where other languages would be forced to make worst case assumptions preventing such optimizations. Consider the following example:

procedure R is
   type Int1 is new Integer;
   type Int2 is new Integer;
   type Int1A is access Int1;
   type Int2A is access Int2;
   Int1V : Int1A;
   Int2V : Int2A;

   for J in Data'Range loop
      if Data (J) = Int1V.all then
         Int2V.all := Int2V.all + 1;
      end if;
   end loop;
end R;

In this example, since the variable Int1V can only access objects of type Int1, and Int2V can only access objects of type Int2, there is no possibility that the assignment to Int2V.all affects the value of Int1V.all. This means that the compiler optimizer can “know” that the value Int1V.all is constant for all iterations of the loop and avoid the extra memory reference required to dereference it each time through the loop.

This kind of optimization, called strict aliasing analysis, is triggered by specifying an optimization level of -O2 or higher or -Os and allows GNAT to generate more efficient code when access values are involved.

However, although this optimization is always correct in terms of the formal semantics of the Ada Reference Manual, difficulties can arise if features like Unchecked_Conversion are used to break the typing system. Consider the following complete program example:

package p1 is
   type int1 is new integer;
   type int2 is new integer;
   type a1 is access int1;
   type a2 is access int2;
end p1;

with p1; use p1;
package p2 is
   function to_a2 (Input : a1) return a2;
end p2;

with Unchecked_Conversion;
package body p2 is
   function to_a2 (Input : a1) return a2 is
      function to_a2u is
        new Unchecked_Conversion (a1, a2);
      return to_a2u (Input);
   end to_a2;
end p2;

with p2; use p2;
with p1; use p1;
with Text_IO; use Text_IO;
procedure m is
   v1 : a1 := new int1;
   v2 : a2 := to_a2 (v1);
   v1.all := 1;
   v2.all := 0;
   put_line (int1'image (v1.all));

This program prints out 0 in -O0 or -O1 mode, but it prints out 1 in -O2 mode. That’s because in strict aliasing mode, the compiler can and does assume that the assignment to v2.all could not affect the value of v1.all, since different types are involved.

This behavior is not a case of non-conformance with the standard, since the Ada RM specifies that an unchecked conversion where the resulting bit pattern is not a correct value of the target type can result in an abnormal value and attempting to reference an abnormal value makes the execution of a program erroneous. That’s the case here since the result does not point to an object of type int2. This means that the effect is entirely unpredictable.

However, although that explanation may satisfy a language lawyer, in practice an applications programmer expects an unchecked conversion involving pointers to create true aliases and the behavior of printing 1 seems plain wrong. In this case, the strict aliasing optimization is unwelcome.

Indeed the compiler recognizes this possibility, and the unchecked conversion generates a warning:

p2.adb:5:07: warning: possible aliasing problem with type "a2"
p2.adb:5:07: warning: use -fno-strict-aliasing switch for references
p2.adb:5:07: warning:  or use "pragma No_Strict_Aliasing (a2);"

Unfortunately the problem is recognized when compiling the body of package p2, but the actual “bad” code is generated while compiling the body of m and this latter compilation does not see the suspicious Unchecked_Conversion.

As implied by the warning message, there are approaches you can use to avoid the unwanted strict aliasing optimization in a case like this.

One possibility is to simply avoid the use of -O2, but that is a bit drastic, since it throws away a number of useful optimizations that do not involve strict aliasing assumptions.

A less drastic approach is to compile the program using the option -fno-strict-aliasing. Actually it is only the unit containing the dereferencing of the suspicious pointer that needs to be compiled. So in this case, if we compile unit m with this switch, then we get the expected value of zero printed. Analyzing which units might need the switch can be painful, so a more reasonable approach is to compile the entire program with options -O2 and -fno-strict-aliasing. If the performance is satisfactory with this combination of options, then the advantage is that the entire issue of possible “wrong” optimization due to strict aliasing is avoided.

To avoid the use of compiler switches, the configuration pragma No_Strict_Aliasing with no parameters may be used to specify that for all access types, the strict aliasing optimization should be suppressed.

However, these approaches are still overkill, in that they causes all manipulations of all access values to be deoptimized. A more refined approach is to concentrate attention on the specific access type identified as problematic.

First, if a careful analysis of uses of the pointer shows that there are no possible problematic references, then the warning can be suppressed by bracketing the instantiation of Unchecked_Conversion to turn the warning off:

pragma Warnings (Off);
function to_a2u is
  new Unchecked_Conversion (a1, a2);
pragma Warnings (On);

Of course that approach is not appropriate for this particular example, since indeed there is a problematic reference. In this case we can take one of two other approaches.

The first possibility is to move the instantiation of unchecked conversion to the unit in which the type is declared. In this example, we would move the instantiation of Unchecked_Conversion from the body of package p2 to the spec of package p1. Now the warning disappears. That’s because any use of the access type knows there is a suspicious unchecked conversion, and the strict aliasing optimization is automatically suppressed for the type.

If it is not practical to move the unchecked conversion to the same unit in which the destination access type is declared (perhaps because the source type is not visible in that unit), you may use pragma No_Strict_Aliasing for the type. This pragma must occur in the same declarative sequence as the declaration of the access type:

type a2 is access int2;
pragma No_Strict_Aliasing (a2);

Here again, the compiler now knows that the strict aliasing optimization should be suppressed for any reference to type a2 and the expected behavior is obtained.

Finally, note that although the compiler can generate warnings for simple cases of unchecked conversions, there are tricker and more indirect ways of creating type incorrect aliases which the compiler cannot detect. Examples are the use of address overlays and unchecked conversions involving composite types containing access types as components. In such cases, no warnings are generated, but there can still be aliasing problems. One safe coding practice is to forbid the use of address clauses for type overlaying, and to allow unchecked conversion only for primitive types. This is not really a significant restriction since any possible desired effect can be achieved by unchecked conversion of access values.

The aliasing analysis done in strict aliasing mode can certainly have significant benefits. We have seen cases of large scale application code where the time is increased by up to 5% by turning this optimization off. If you have code that includes significant usage of unchecked conversion, you might want to just stick with -O1 and avoid the entire issue. If you get adequate performance at this level of optimization level, that’s probably the safest approach. If tests show that you really need higher levels of optimization, then you can experiment with -O2 and -O2 -fno-strict-aliasing to see how much effect this has on size and speed of the code. If you really need to use -O2 with strict aliasing in effect, then you should review any uses of unchecked conversion of access types, particularly if you are getting the warnings described above. Aliased Variables and Optimization

There are scenarios in which programs may use low level techniques to modify variables that otherwise might be considered to be unassigned. For example, a variable can be passed to a procedure by reference, which takes the address of the parameter and uses the address to modify the variable’s value, even though it is passed as an IN parameter. Consider the following example:

procedure P is
   Max_Length : constant Natural := 16;
   type Char_Ptr is access all Character;

   procedure Get_String(Buffer: Char_Ptr; Size : Integer);
   pragma Import (C, Get_String, "get_string");

   Name : aliased String (1 .. Max_Length) := (others => ' ');
   Temp : Char_Ptr;

   function Addr (S : String) return Char_Ptr is
      function To_Char_Ptr is
        new Ada.Unchecked_Conversion (System.Address, Char_Ptr);
      return To_Char_Ptr (S (S'First)'Address);

   Temp := Addr (Name);
   Get_String (Temp, Max_Length);

where Get_String is a C function that uses the address in Temp to modify the variable Name. This code is dubious, and arguably erroneous, and the compiler would be entitled to assume that Name is never modified, and generate code accordingly.

However, in practice, this would cause some existing code that seems to work with no optimization to start failing at high levels of optimzization.

What the compiler does for such cases is to assume that marking a variable as aliased indicates that some “funny business” may be going on. The optimizer recognizes the aliased keyword and inhibits optimizations that assume the value cannot be assigned. This means that the above example will in fact “work” reliably, that is, it will produce the expected results. Atomic Variables and Optimization

There are two considerations with regard to performance when atomic variables are used.

First, the RM only guarantees that access to atomic variables be atomic, it has nothing to say about how this is achieved, though there is a strong implication that this should not be achieved by explicit locking code. Indeed GNAT will never generate any locking code for atomic variable access (it will simply reject any attempt to make a variable or type atomic if the atomic access cannot be achieved without such locking code).

That being said, it is important to understand that you cannot assume that the entire variable will always be accessed. Consider this example:

type R is record
   A,B,C,D : Character;
end record;
for R'Size use 32;
for R'Alignment use 4;

RV : R;
pragma Atomic (RV);
X : Character;
X := RV.B;

You cannot assume that the reference to RV.B will read the entire 32-bit variable with a single load instruction. It is perfectly legitimate if the hardware allows it to do a byte read of just the B field. This read is still atomic, which is all the RM requires. GNAT can and does take advantage of this, depending on the architecture and optimization level. Any assumption to the contrary is non-portable and risky. Even if you examine the assembly language and see a full 32-bit load, this might change in a future version of the compiler.

If your application requires that all accesses to RV in this example be full 32-bit loads, you need to make a copy for the access as in:

   RV_Copy : constant R := RV;
   X := RV_Copy.B;

Now the reference to RV must read the whole variable. Actually one can imagine some compiler which figures out that the whole copy is not required (because only the B field is actually accessed), but GNAT certainly won’t do that, and we don’t know of any compiler that would not handle this right, and the above code will in practice work portably across all architectures (that permit the Atomic declaration).

The second issue with atomic variables has to do with the possible requirement of generating synchronization code. For more details on this, consult the sections on the pragmas Enable/Disable_Atomic_Synchronization in the GNAT Reference Manual. If performance is critical, and such synchronization code is not required, it may be useful to disable it. Passive Task Optimization

A passive task is one which is sufficiently simple that in theory a compiler could recognize it an implement it efficiently without creating a new thread. The original design of Ada 83 had in mind this kind of passive task optimization, but only a few Ada 83 compilers attempted it. The problem was that it was difficult to determine the exact conditions under which the optimization was possible. The result is a very fragile optimization where a very minor change in the program can suddenly silently make a task non-optimizable.

With the revisiting of this issue in Ada 95, there was general agreement that this approach was fundamentally flawed, and the notion of protected types was introduced. When using protected types, the restrictions are well defined, and you KNOW that the operations will be optimized, and furthermore this optimized performance is fully portable.

Although it would theoretically be possible for GNAT to attempt to do this optimization, but it really doesn’t make sense in the context of Ada 95, and none of the Ada 95 compilers implement this optimization as far as we know. In particular GNAT never attempts to perform this optimization.

In any new Ada 95 code that is written, you should always use protected types in place of tasks that might be able to be optimized in this manner. Of course this does not help if you have legacy Ada 83 code that depends on this optimization, but it is unusual to encounter a case where the performance gains from this optimization are significant.

Your program should work correctly without this optimization. If you have performance problems, then the most practical approach is to figure out exactly where these performance problems arise, and update those particular tasks to be protected types. Note that typically clients of the tasks who call entries, will not have to be modified, only the task definition itself.

6.3.2. Text_IO Suggestions

The Ada.Text_IO package has fairly high overheads due in part to the requirement of maintaining page and line counts. If performance is critical, a recommendation is to use Stream_IO instead of Text_IO for volume output, since this package has less overhead.

If Text_IO must be used, note that by default output to the standard output and standard error files is unbuffered (this provides better behavior when output statements are used for debugging, or if the progress of a program is observed by tracking the output, e.g. by using the Unix tail -f command to watch redirected output.

If you are generating large volumes of output with Text_IO and performance is an important factor, use a designated file instead of the standard output file, or change the standard output file to be buffered using Interfaces.C_Streams.setvbuf.

6.3.3. Reducing Size of Executables with Unused Subprogram/Data Elimination

This section describes how you can eliminate unused subprograms and data from your executable just by setting options at compilation time. About unused subprogram/data elimination

By default, an executable contains all code and data of its composing objects (directly linked or coming from statically linked libraries), even data or code never used by this executable.

This feature will allow you to eliminate such unused code from your executable, making it smaller (in disk and in memory).

This functionality is available on all Linux platforms except for the IA-64 architecture and on all cross platforms using the ELF binary file format. In both cases GNU binutils version 2.16 or later are required to enable it. Compilation options

The operation of eliminating the unused code and data from the final executable is directly performed by the linker.

In order to do this, it has to work with objects compiled with the following options: -ffunction-sections -fdata-sections.

These options are usable with C and Ada files. They will place respectively each function or data in a separate section in the resulting object file.

Once the objects and static libraries are created with these options, the linker can perform the dead code elimination. You can do this by setting the -Wl,--gc-sections option to gcc command or in the -largs section of gnatmake. This will perform a garbage collection of code and data never referenced.

If the linker performs a partial link (-r linker option), then you will need to provide the entry point using the -e / --entry linker option.

Note that objects compiled without the -ffunction-sections and -fdata-sections options can still be linked with the executable. However, no dead code elimination will be performed on those objects (they will be linked as is).

The GNAT static library is now compiled with -ffunction-sections and -fdata-sections on some platforms. This allows you to eliminate the unused code and data of the GNAT library from your executable. Example of unused subprogram/data elimination

Here is a simple example:

with Aux;

procedure Test is
   Aux.Used (10);
end Test;

package Aux is
   Used_Data   : Integer;
   Unused_Data : Integer;

   procedure Used   (Data : Integer);
   procedure Unused (Data : Integer);
end Aux;

package body Aux is
   procedure Used (Data : Integer) is
      Used_Data := Data;
   end Used;

   procedure Unused (Data : Integer) is
      Unused_Data := Data;
   end Unused;
end Aux;

Unused and Unused_Data are never referenced in this code excerpt, and hence they may be safely removed from the final executable.

$ gnatmake test

$ nm test | grep used
020015f0 T aux__unused
02005d88 B aux__unused_data
020015cc T aux__used
02005d84 B aux__used_data

$ gnatmake test -cargs -fdata-sections -ffunction-sections \\
     -largs -Wl,--gc-sections

$ nm test | grep used
02005350 T aux__used
0201ffe0 B aux__used_data

It can be observed that the procedure Unused and the object Unused_Data are removed by the linker when using the appropriate options.

6.4. Overflow Check Handling in GNAT

This section explains how to control the handling of overflow checks.

6.4.1. Background

Overflow checks are checks that the compiler may make to ensure that intermediate results are not out of range. For example:

A : Integer;
A := A + 1;

If A has the value Integer'Last, then the addition may cause overflow since the result is out of range of the type Integer. In this case Constraint_Error will be raised if checks are enabled.

A trickier situation arises in examples like the following:

A, C : Integer;
A := (A + 1) + C;

where A is Integer'Last and C is -1. Now the final result of the expression on the right hand side is Integer'Last which is in range, but the question arises whether the intermediate addition of (A + 1) raises an overflow error.

The (perhaps surprising) answer is that the Ada language definition does not answer this question. Instead it leaves it up to the implementation to do one of two things if overflow checks are enabled.

  • raise an exception (Constraint_Error), or
  • yield the correct mathematical result which is then used in subsequent operations.

If the compiler chooses the first approach, then the assignment of this example will indeed raise Constraint_Error if overflow checking is enabled, or result in erroneous execution if overflow checks are suppressed.

But if the compiler chooses the second approach, then it can perform both additions yielding the correct mathematical result, which is in range, so no exception will be raised, and the right result is obtained, regardless of whether overflow checks are suppressed.

Note that in the first example an exception will be raised in either case, since if the compiler gives the correct mathematical result for the addition, it will be out of range of the target type of the assignment, and thus fails the range check.

This lack of specified behavior in the handling of overflow for intermediate results is a source of non-portability, and can thus be problematic when programs are ported. Most typically this arises in a situation where the original compiler did not raise an exception, and then the application is moved to a compiler where the check is performed on the intermediate result and an unexpected exception is raised.

Furthermore, when using Ada 2012’s preconditions and other assertion forms, another issue arises. Consider:

procedure P (A, B : Integer) with
  Pre => A + B <= Integer'Last;

One often wants to regard arithmetic in a context like this from a mathematical point of view. So for example, if the two actual parameters for a call to P are both Integer'Last, then the precondition should be regarded as False. If we are executing in a mode with run-time checks enabled for preconditions, then we would like this precondition to fail, rather than raising an exception because of the intermediate overflow.

However, the language definition leaves the specification of whether the above condition fails (raising Assert_Error) or causes an intermediate overflow (raising Constraint_Error) up to the implementation.

The situation is worse in a case such as the following:

procedure Q (A, B, C : Integer) with
  Pre => A + B + C <= Integer'Last;

Consider the call

Q (A => Integer'Last, B => 1, C => -1);

From a mathematical point of view the precondition is True, but at run time we may (but are not guaranteed to) get an exception raised because of the intermediate overflow (and we really would prefer this precondition to be considered True at run time).

6.4.2. Management of Overflows in GNAT

To deal with the portability issue, and with the problem of mathematical versus run-time interpretation of the expressions in assertions, GNAT provides comprehensive control over the handling of intermediate overflow. GNAT can operate in three modes, and furthemore, permits separate selection of operating modes for the expressions within assertions (here the term ‘assertions’ is used in the technical sense, which includes preconditions and so forth) and for expressions appearing outside assertions.

The three modes are:

  • Use base type for intermediate operations (STRICT)

    In this mode, all intermediate results for predefined arithmetic operators are computed using the base type, and the result must be in range of the base type. If this is not the case then either an exception is raised (if overflow checks are enabled) or the execution is erroneous (if overflow checks are suppressed). This is the normal default mode.

  • Most intermediate overflows avoided (MINIMIZED)

    In this mode, the compiler attempts to avoid intermediate overflows by using a larger integer type, typically Long_Long_Integer, as the type in which arithmetic is performed for predefined arithmetic operators. This may be slightly more expensive at run time (compared to suppressing intermediate overflow checks), though the cost is negligible on modern 64-bit machines. For the examples given earlier, no intermediate overflows would have resulted in exceptions, since the intermediate results are all in the range of Long_Long_Integer (typically 64-bits on nearly all implementations of GNAT). In addition, if checks are enabled, this reduces the number of checks that must be made, so this choice may actually result in an improvement in space and time behavior.

    However, there are cases where Long_Long_Integer is not large enough, consider the following example:

    procedure R (A, B, C, D : Integer) with
      Pre => (A**2 * B**2) / (C**2 * D**2) <= 10;

    where A = B = C = D = Integer'Last. Now the intermediate results are out of the range of Long_Long_Integer even though the final result is in range and the precondition is True (from a mathematical point of view). In such a case, operating in this mode, an overflow occurs for the intermediate computation (which is why this mode says most intermediate overflows are avoided). In this case, an exception is raised if overflow checks are enabled, and the execution is erroneous if overflow checks are suppressed.

  • All intermediate overflows avoided (ELIMINATED)

    In this mode, the compiler avoids all intermediate overflows by using arbitrary precision arithmetic as required. In this mode, the above example with A**2 * B**2 would not cause intermediate overflow, because the intermediate result would be evaluated using sufficient precision, and the result of evaluating the precondition would be True.

    This mode has the advantage of avoiding any intermediate overflows, but at the expense of significant run-time overhead, including the use of a library (included automatically in this mode) for multiple-precision arithmetic.

    This mode provides cleaner semantics for assertions, since now the run-time behavior emulates true arithmetic behavior for the predefined arithmetic operators, meaning that there is never a conflict between the mathematical view of the assertion, and its run-time behavior.

    Note that in this mode, the behavior is unaffected by whether or not overflow checks are suppressed, since overflow does not occur. It is possible for gigantic intermediate expressions to raise Storage_Error as a result of attempting to compute the results of such expressions (e.g. Integer'Last ** Integer'Last) but overflow is impossible.

Note that these modes apply only to the evaluation of predefined arithmetic, membership, and comparison operators for signed integer arithmetic.

For fixed-point arithmetic, checks can be suppressed. But if checks are enabled then fixed-point values are always checked for overflow against the base type for intermediate expressions (that is such checks always operate in the equivalent of STRICT mode).

For floating-point, on nearly all architectures, Machine_Overflows is False, and IEEE infinities are generated, so overflow exceptions are never raised. If you want to avoid infinities, and check that final results of expressions are in range, then you can declare a constrained floating-point type, and range checks will be carried out in the normal manner (with infinite values always failing all range checks).

6.4.3. Specifying the Desired Mode

The desired mode of for handling intermediate overflow can be specified using either the Overflow_Mode pragma or an equivalent compiler switch. The pragma has the form

pragma Overflow_Mode ([General =>] MODE [, [Assertions =>] MODE]);

where MODE is one of

  • STRICT: intermediate overflows checked (using base type)
  • MINIMIZED: minimize intermediate overflows
  • ELIMINATED: eliminate intermediate overflows

The case is ignored, so MINIMIZED, Minimized and minimized all have the same effect.

If only the General parameter is present, then the given MODE applies to expressions both within and outside assertions. If both arguments are present, then General applies to expressions outside assertions, and Assertions applies to expressions within assertions. For example:

pragma Overflow_Mode
  (General => Minimized, Assertions => Eliminated);

specifies that general expressions outside assertions be evaluated in ‘minimize intermediate overflows’ mode, and expressions within assertions be evaluated in ‘eliminate intermediate overflows’ mode. This is often a reasonable choice, avoiding excessive overhead outside assertions, but assuring a high degree of portability when importing code from another compiler, while incurring the extra overhead for assertion expressions to ensure that the behavior at run time matches the expected mathematical behavior.

The Overflow_Mode pragma has the same scoping and placement rules as pragma Suppress, so it can occur either as a configuration pragma, specifying a default for the whole program, or in a declarative scope, where it applies to the remaining declarations and statements in that scope.

Note that pragma Overflow_Mode does not affect whether overflow checks are enabled or suppressed. It only controls the method used to compute intermediate values. To control whether overflow checking is enabled or suppressed, use pragma Suppress or Unsuppress in the usual manner.

Additionally, a compiler switch -gnato? or -gnato?? can be used to control the checking mode default (which can be subsequently overridden using pragmas).

Here ? is one of the digits 1 through 3:

1 use base type for intermediate operations (STRICT)
2 minimize intermediate overflows (MINIMIZED)
3 eliminate intermediate overflows (ELIMINATED)

As with the pragma, if only one digit appears then it applies to all cases; if two digits are given, then the first applies outside assertions, and the second within assertions. Thus the equivalent of the example pragma above would be -gnato23.

If no digits follow the -gnato, then it is equivalent to -gnato11, causing all intermediate operations to be computed using the base type (STRICT mode).

6.4.4. Default Settings

The default mode for overflow checks is

General => Strict

which causes all computations both inside and outside assertions to use the base type.

This retains compatibility with previous versions of GNAT which suppressed overflow checks by default and always used the base type for computation of intermediate results.

The switch -gnato (with no digits following) is equivalent to

General => Strict

which causes overflow checking of all intermediate overflows both inside and outside assertions against the base type.

The pragma Suppress (Overflow_Check) disables overflow checking, but it has no effect on the method used for computing intermediate results.

The pragma Unsuppress (Overflow_Check) enables overflow checking, but it has no effect on the method used for computing intermediate results.

6.4.5. Implementation Notes

In practice on typical 64-bit machines, the MINIMIZED mode is reasonably efficient, and can be generally used. It also helps to ensure compatibility with code imported from some other compiler to GNAT.

Setting all intermediate overflows checking (CHECKED mode) makes sense if you want to make sure that your code is compatible with any other possible Ada implementation. This may be useful in ensuring portability for code that is to be exported to some other compiler than GNAT.

The Ada standard allows the reassociation of expressions at the same precedence level if no parentheses are present. For example, A+B+C parses as though it were (A+B)+C, but the compiler can reintepret this as A+(B+C), possibly introducing or eliminating an overflow exception. The GNAT compiler never takes advantage of this freedom, and the expression A+B+C will be evaluated as (A+B)+C. If you need the other order, you can write the parentheses explicitly A+(B+C) and GNAT will respect this order.

The use of ELIMINATED mode will cause the compiler to automatically include an appropriate arbitrary precision integer arithmetic package. The compiler will make calls to this package, though only in cases where it cannot be sure that Long_Long_Integer is sufficient to guard against intermediate overflows. This package does not use dynamic allocation, but it does use the secondary stack, so an appropriate secondary stack package must be present (this is always true for standard full Ada, but may require specific steps for restricted run times such as ZFP).

Although ELIMINATED mode causes expressions to use arbitrary precision arithmetic, avoiding overflow, the final result must be in an appropriate range. This is true even if the final result is of type [Long_[Long_]]Integer'Base, which still has the same bounds as its associated constrained type at run-time.

Currently, the ELIMINATED mode is only available on target platforms for which Long_Long_Integer is 64-bits (nearly all GNAT platforms).

6.5. Performing Dimensionality Analysis in GNAT

The GNAT compiler supports dimensionality checking. The user can specify physical units for objects, and the compiler will verify that uses of these objects are compatible with their dimensions, in a fashion that is familiar to engineering practice. The dimensions of algebraic expressions (including powers with static exponents) are computed from their constituents.

This feature depends on Ada 2012 aspect specifications, and is available from version 7.0.1 of GNAT onwards. The GNAT-specific aspect Dimension_System allows you to define a system of units; the aspect Dimension then allows the user to declare dimensioned quantities within a given system. (These aspects are described in the Implementation Defined Aspects chapter of the GNAT Reference Manual).

The major advantage of this model is that it does not require the declaration of multiple operators for all possible combinations of types: it is only necessary to use the proper subtypes in object declarations.

The simplest way to impose dimensionality checking on a computation is to make use of one of the instantiations of the package System.Dim.Generic_Mks, which are part of the GNAT library. This generic package defines a floating-point type MKS_Type, for which a sequence of dimension names are specified, together with their conventional abbreviations. The following should be read together with the full specification of the package, in file s-digemk.ads.

type Mks_Type is new Float_Type
   Dimension_System => (
     (Unit_Name => Meter,    Unit_Symbol => 'm',   Dim_Symbol => 'L'),
     (Unit_Name => Kilogram, Unit_Symbol => "kg",  Dim_Symbol => 'M'),
     (Unit_Name => Second,   Unit_Symbol => 's',   Dim_Symbol => 'T'),
     (Unit_Name => Ampere,   Unit_Symbol => 'A',   Dim_Symbol => 'I'),
     (Unit_Name => Kelvin,   Unit_Symbol => 'K',   Dim_Symbol => "Theta"),
     (Unit_Name => Mole,     Unit_Symbol => "mol", Dim_Symbol => 'N'),
     (Unit_Name => Candela,  Unit_Symbol => "cd",  Dim_Symbol => 'J'));

The package then defines a series of subtypes that correspond to these conventional units. For example:

subtype Length is Mks_Type
   Dimension => (Symbol => 'm', Meter  => 1, others => 0);

and similarly for Mass, Time, Electric_Current, Thermodynamic_Temperature, Amount_Of_Substance, and Luminous_Intensity (the standard set of units of the SI system).

The package also defines conventional names for values of each unit, for example:

m   : constant Length           := 1.0;
kg  : constant Mass             := 1.0;
s   : constant Time             := 1.0;
A   : constant Electric_Current := 1.0;

as well as useful multiples of these units:

 cm  : constant Length := 1.0E-02;
 g   : constant Mass   := 1.0E-03;
 min : constant Time   := 60.0;
 day : constant Time   := 60.0 * 24.0 * min;

There are three instantiations of System.Dim.Generic_Mks defined in the GNAT library:

  • System.Dim.Float_Mks based on Float defined in s-diflmk.ads.
  • System.Dim.Long_Mks based on Long_Float defined in s-dilomk.ads.
  • System.Dim.Mks based on Long_Long_Float defined in s-dimmks.ads.

Using one of these packages, you can then define a derived unit by providing the aspect that specifies its dimensions within the MKS system, as well as the string to be used for output of a value of that unit:

subtype Acceleration is Mks_Type
  with Dimension => ("m/sec^2",
                     Meter => 1,
                     Second => -2,
                     others => 0);

Here is a complete example of use:

with System.Dim.MKS; use System.Dim.Mks;
with System.Dim.Mks_IO; use System.Dim.Mks_IO;
with Text_IO; use Text_IO;
procedure Free_Fall is
  subtype Acceleration is Mks_Type
    with Dimension => ("m/sec^2", 1, 0, -2, others => 0);
  G : constant acceleration := 9.81 * m / (s ** 2);
  T : Time := 10.0*s;
  Distance : Length;

  Put ("Gravitational constant: ");
  Put (G, Aft => 2, Exp => 0); Put_Line ("");
  Distance := 0.5 * G * T ** 2;
  Put ("distance travelled in 10 seconds of free fall ");
  Put (Distance, Aft => 2, Exp => 0);
  Put_Line ("");
end Free_Fall;

Execution of this program yields:

Gravitational constant:  9.81 m/sec^2
distance travelled in 10 seconds of free fall 490.50 m

However, incorrect assignments such as:

Distance := 5.0;
Distance := 5.0 * kg;

are rejected with the following diagnoses:

Distance := 5.0;
   >>> dimensions mismatch in assignment
   >>> left-hand side has dimension [L]
   >>> right-hand side is dimensionless

Distance := 5.0 * kg:
   >>> dimensions mismatch in assignment
   >>> left-hand side has dimension [L]
   >>> right-hand side has dimension [M]

The dimensions of an expression are properly displayed, even if there is no explicit subtype for it. If we add to the program:

Put ("Final velocity: ");
Put (G * T, Aft =>2, Exp =>0);
Put_Line ("");

then the output includes:

Final velocity: 98.10 m.s**(-1)

The type Mks_Type is said to be a dimensionable type since it has a Dimension_System aspect, and the subtypes Length, Mass, etc., are said to be dimensioned subtypes since each one has a Dimension aspect.

The Dimension aspect of a dimensioned subtype S defines a mapping from the base type’s Unit_Names to integer (or, more generally, rational) values. This mapping is the dimension vector (also referred to as the dimensionality) for that subtype, denoted by DV(S), and thus for each object of that subtype. Intuitively, the value specified for each Unit_Name is the exponent associated with that unit; a zero value means that the unit is not used. For example:

   Acc : Acceleration;

Here DV(Acc) = DV(Acceleration) = (Meter=>1, Kilogram=>0, Second=>-2, Ampere=>0, Kelvin=>0, Mole=>0, Candela=>0). Symbolically, we can express this as Meter / Second**2.

The dimension vector of an arithmetic expression is synthesized from the dimension vectors of its components, with compile-time dimensionality checks that help prevent mismatches such as using an Acceleration where a Length is required.

The dimension vector of the result of an arithmetic expression expr, or DV(expr), is defined as follows, assuming conventional mathematical definitions for the vector operations that are used:

  • If expr is of the type universal_real, or is not of a dimensioned subtype, then expr is dimensionless; DV(expr) is the empty vector.
  • DV(op expr), where op is a unary operator, is DV(expr)
  • DV(expr1 op expr2) where op is “+” or “-” is DV(expr1) provided that DV(expr1) = DV(expr2). If this condition is not met then the construct is illegal.
  • DV(expr1 * expr2) is DV(expr1) + DV(expr2), and DV(expr1 / expr2) = DV(expr1) - DV(expr2). In this context if one of the exprs is dimensionless then its empty dimension vector is treated as (others => 0).
  • DV(expr ** power) is power * DV(expr), provided that power is a static rational value. If this condition is not met then the construct is illegal.

Note that, by the above rules, it is illegal to use binary “+” or “-” to combine a dimensioned and dimensionless value. Thus an expression such as acc-10.0 is illegal, where acc is an object of subtype Acceleration.

The dimensionality checks for relationals use the same rules as for “+” and “-”, except when comparing to a literal; thus

acc > len

is equivalent to

acc-len > 0.0

and is thus illegal, but

acc > 10.0

is accepted with a warning. Analogously a conditional expression requires the same dimension vector for each branch (with no exception for literals).

The dimension vector of a type conversion T(expr) is defined as follows, based on the nature of T:

  • If T is a dimensioned subtype then DV(T(expr)) is DV(T) provided that either expr is dimensionless or DV(T) = DV(expr). The conversion is illegal if expr is dimensioned and DV(expr) /= DV(T). Note that vector equality does not require that the corresponding Unit_Names be the same.

    As a consequence of the above rule, it is possible to convert between different dimension systems that follow the same international system of units, with the seven physical components given in the standard order (length, mass, time, etc.). Thus a length in meters can be converted to a length in inches (with a suitable conversion factor) but cannot be converted, for example, to a mass in pounds.

  • If T is the base type for expr (and the dimensionless root type of the dimension system), then DV(T(expr)) is DV(expr). Thus, if expr is of a dimensioned subtype of T, the conversion may be regarded as a “view conversion” that preserves dimensionality.

    This rule makes it possible to write generic code that can be instantiated with compatible dimensioned subtypes. The generic unit will contain conversions that will consequently be present in instantiations, but conversions to the base type will preserve dimensionality and make it possible to write generic code that is correct with respect to dimensionality.

  • Otherwise (i.e., T is neither a dimensioned subtype nor a dimensionable base type), DV(T(expr)) is the empty vector. Thus a dimensioned value can be explicitly converted to a non-dimensioned subtype, which of course then escapes dimensionality analysis.

The dimension vector for a type qualification T'(expr) is the same as for the type conversion T(expr).

An assignment statement

Source := Target;

requires DV(Source) = DV(Target), and analogously for parameter passing (the dimension vector for the actual parameter must be equal to the dimension vector for the formal parameter).

6.7. Memory Management Issues

This section describes some useful memory pools provided in the GNAT library and in particular the GNAT Debug Pool facility, which can be used to detect incorrect uses of access values (including ‘dangling references’).

It also describes the gnatmem tool, which can be used to track down “memory leaks”.

6.7.1. Some Useful Memory Pools

The System.Pool_Global package offers the Unbounded_No_Reclaim_Pool storage pool. Allocations use the standard system call malloc while deallocations use the standard system call free. No reclamation is performed when the pool goes out of scope. For performance reasons, the standard default Ada allocators/deallocators do not use any explicit storage pools but if they did, they could use this storage pool without any change in behavior. That is why this storage pool is used when the user manages to make the default implicit allocator explicit as in this example:

type T1 is access Something;
 -- no Storage pool is defined for T2

type T2 is access Something_Else;
for T2'Storage_Pool use T1'Storage_Pool;
-- the above is equivalent to
for T2'Storage_Pool use System.Pool_Global.Global_Pool_Object;

The System.Pool_Local package offers the Unbounded_Reclaim_Pool storage pool. The allocation strategy is similar to Pool_Local except that the all storage allocated with this pool is reclaimed when the pool object goes out of scope. This pool provides a explicit mechanism similar to the implicit one provided by several Ada 83 compilers for allocations performed through a local access type and whose purpose was to reclaim memory when exiting the scope of a given local access. As an example, the following program does not leak memory even though it does not perform explicit deallocation:

with System.Pool_Local;
procedure Pooloc1 is
   procedure Internal is
      type A is access Integer;
      X : System.Pool_Local.Unbounded_Reclaim_Pool;
      for A'Storage_Pool use X;
      v : A;
      for I in  1 .. 50 loop
         v := new Integer;
      end loop;
   end Internal;
   for I in  1 .. 100 loop
   end loop;
end Pooloc1;

The System.Pool_Size package implements the Stack_Bounded_Pool used when Storage_Size is specified for an access type. The whole storage for the pool is allocated at once, usually on the stack at the point where the access type is elaborated. It is automatically reclaimed when exiting the scope where the access type is defined. This package is not intended to be used directly by the user and it is implicitly used for each such declaration:

type T1 is access Something;
for T1'Storage_Size use 10_000;

6.7.2. The GNAT Debug Pool Facility

The use of unchecked deallocation and unchecked conversion can easily lead to incorrect memory references. The problems generated by such references are usually difficult to tackle because the symptoms can be very remote from the origin of the problem. In such cases, it is very helpful to detect the problem as early as possible. This is the purpose of the Storage Pool provided by GNAT.Debug_Pools.

In order to use the GNAT specific debugging pool, the user must associate a debug pool object with each of the access types that may be related to suspected memory problems. See Ada Reference Manual 13.11.

type Ptr is access Some_Type;
Pool : GNAT.Debug_Pools.Debug_Pool;
for Ptr'Storage_Pool use Pool;

GNAT.Debug_Pools is derived from a GNAT-specific kind of pool: the Checked_Pool. Such pools, like standard Ada storage pools, allow the user to redefine allocation and deallocation strategies. They also provide a checkpoint for each dereference, through the use of the primitive operation Dereference which is implicitly called at each dereference of an access value.

Once an access type has been associated with a debug pool, operations on values of the type may raise four distinct exceptions, which correspond to four potential kinds of memory corruption:

  • GNAT.Debug_Pools.Accessing_Not_Allocated_Storage
  • GNAT.Debug_Pools.Accessing_Deallocated_Storage
  • GNAT.Debug_Pools.Freeing_Not_Allocated_Storage
  • GNAT.Debug_Pools.Freeing_Deallocated_Storage

For types associated with a Debug_Pool, dynamic allocation is performed using the standard GNAT allocation routine. References to all allocated chunks of memory are kept in an internal dictionary. Several deallocation strategies are provided, whereupon the user can choose to release the memory to the system, keep it allocated for further invalid access checks, or fill it with an easily recognizable pattern for debug sessions. The memory pattern is the old IBM hexadecimal convention: 16#DEADBEEF#.

See the documentation in the file g-debpoo.ads for more information on the various strategies.

Upon each dereference, a check is made that the access value denotes a properly allocated memory location. Here is a complete example of use of Debug_Pools, that includes typical instances of memory corruption:

with Gnat.Io; use Gnat.Io;
with Unchecked_Deallocation;
with Unchecked_Conversion;
with GNAT.Debug_Pools;
with System.Storage_Elements;
with Ada.Exceptions; use Ada.Exceptions;
procedure Debug_Pool_Test is

   type T is access Integer;
   type U is access all T;

   P : GNAT.Debug_Pools.Debug_Pool;
   for T'Storage_Pool use P;

   procedure Free is new Unchecked_Deallocation (Integer, T);
   function UC is new Unchecked_Conversion (U, T);
   A, B : aliased T;

   procedure Info is new GNAT.Debug_Pools.Print_Info(Put_Line);

   Info (P);
   A := new Integer;
   B := new Integer;
   B := A;
   Info (P);
   Free (A);
      Put_Line (Integer'Image(B.all));
      when E : others => Put_Line ("raised: " & Exception_Name (E));
      Free (B);
      when E : others => Put_Line ("raised: " & Exception_Name (E));
   B := UC(A'Access);
      Put_Line (Integer'Image(B.all));
      when E : others => Put_Line ("raised: " & Exception_Name (E));
      Free (B);
      when E : others => Put_Line ("raised: " & Exception_Name (E));
   Info (P);
end Debug_Pool_Test;

The debug pool mechanism provides the following precise diagnostics on the execution of this erroneous program:

Debug Pool info:
  Total allocated bytes :  0
  Total deallocated bytes :  0
  Current Water Mark:  0
  High Water Mark:  0

Debug Pool info:
  Total allocated bytes :  8
  Total deallocated bytes :  0
  Current Water Mark:  8
  High Water Mark:  8

Debug Pool info:
  Total allocated bytes :  8
  Total deallocated bytes :  4
  Current Water Mark:  4
  High Water Mark:  8

6.7.3. The gnatmem Tool

The gnatmem utility monitors dynamic allocation and deallocation activity in a program, and displays information about incorrect deallocations and possible sources of memory leaks. It is designed to work in association with a static runtime library only and in this context provides three types of information:

  • General information concerning memory management, such as the total number of allocations and deallocations, the amount of allocated memory and the high water mark, i.e., the largest amount of allocated memory in the course of program execution.
  • Backtraces for all incorrect deallocations, that is to say deallocations which do not correspond to a valid allocation.
  • Information on each allocation that is potentially the origin of a memory leak. Running gnatmem

gnatmem makes use of the output created by the special version of allocation and deallocation routines that record call information. This allows it to obtain accurate dynamic memory usage history at a minimal cost to the execution speed. Note however, that gnatmem is only supported on GNU/Linux and Windows.

The gnatmem command has the form

$ gnatmem [ switches ] [ DEPTH ] user_program

The program must have been linked with the instrumented version of the allocation and deallocation routines. This is done by linking with the libgmem.a library. For correct symbolic backtrace information, the user program should be compiled with debugging options (see Compiler Switches). For example to build my_program:

$ gnatmake -g my_program -largs -lgmem

As library libgmem.a contains an alternate body for package System.Memory, s-memory.adb should not be compiled and linked when an executable is linked with library libgmem.a. It is then not recommended to use gnatmake with switch -a.

When my_program is executed, the file gmem.out is produced. This file contains information about all allocations and deallocations performed by the program. It is produced by the instrumented allocations and deallocations routines and will be used by gnatmem.

In order to produce symbolic backtrace information for allocations and deallocations performed by the GNAT run-time library, you need to use a version of that library that has been compiled with the -g switch (see Rebuilding the GNAT Run-Time Library).

gnatmem must be supplied with the gmem.out file and the executable to examine. If the location of gmem.out file was not explicitly supplied by -i switch, gnatmem will assume that this file can be found in the current directory. For example, after you have executed my_program, gmem.out can be analyzed by gnatmem using the command:

$ gnatmem my_program

This will produce the output with the following format:

$ gnatmem my_program

Global information
   Total number of allocations        :  45
   Total number of deallocations      :   6
   Final Water Mark (non freed mem)   :  11.29 Kilobytes
   High Water Mark                    :  11.40 Kilobytes

Allocation Root # 2
 Number of non freed allocations    :  11
 Final Water Mark (non freed mem)   :   1.16 Kilobytes
 High Water Mark                    :   1.27 Kilobytes
 Backtrace                          :
   my_program.adb:23 my_program.alloc

The first block of output gives general information. In this case, the Ada construct new was executed 45 times, and only 6 calls to an Unchecked_Deallocation routine occurred.

Subsequent paragraphs display information on all allocation roots. An allocation root is a specific point in the execution of the program that generates some dynamic allocation, such as a new construct. This root is represented by an execution backtrace (or subprogram call stack). By default the backtrace depth for allocations roots is 1, so that a root corresponds exactly to a source location. The backtrace can be made deeper, to make the root more specific. Switches for gnatmem

gnatmem recognizes the following switches:

Quiet. Gives the minimum output needed to identify the origin of the memory leaks. Omits statistical information.
DEPTH is an integer literal (usually between 1 and 10) which controls the depth of the backtraces defining allocation root. The default value for DEPTH is 1. The deeper the backtrace, the more precise the localization of the root. Note that the total number of roots can depend on this parameter, in other words there may be more roots when the requested backtrace depth is higher. This parameter must be specified before the name of the executable to be analyzed, to avoid ambiguity.
-b N
This switch has the same effect as just a depth parameter N.
-i file
Do the gnatmem processing starting from file, rather than gmem.out in the current directory.
-m n
This switch causes gnatmem to mask the allocation roots that have less than n leaks. The default value is 1. Specifying the value of 0 will allow examination of even the roots that did not result in leaks.
-s order
This switch causes gnatmem to sort the allocation roots according to the specified order of sort criteria, each identified by a single letter. The currently supported criteria are n, h, and w standing respectively for number of unfreed allocations, high watermark, and final watermark corresponding to a specific root. The default order is nwh.
This switch causes memory allocated size to be always output in bytes. Default gnatmem behavior is to show memory sizes less then 1 kilobyte in bytes, from 1 kilobyte till 1 megabyte in kilobytes and the rest in megabytes. Example of gnatmem Usage

The following example shows the use of gnatmem on a simple memory-leaking program. Suppose that we have the following Ada program:

with Unchecked_Deallocation;
procedure Test_Gm is

   type T is array (1..1000) of Integer;
   type Ptr is access T;
   procedure Free is new Unchecked_Deallocation (T, Ptr);
   A : Ptr;

   procedure My_Alloc is
      A := new T;
   end My_Alloc;

   procedure My_DeAlloc is
      B : Ptr := A;
      Free (B);
   end My_DeAlloc;

   for I in 1 .. 5 loop
      for J in I .. 5 loop
      end loop;
   end loop;

The program needs to be compiled with the debugging option and linked with the gmem library:

$ gnatmake -g test_gm -largs -lgmem

Then we execute the program as usual:

$ test_gm

Then gnatmem is invoked simply with

$ gnatmem test_gm

which produces the following output (result may vary on different platforms):

Global information
   Total number of allocations        :  18
   Total number of deallocations      :   5
   Final Water Mark (non freed mem)   :  53.00 Kilobytes
   High Water Mark                    :  56.90 Kilobytes

Allocation Root # 1
 Number of non freed allocations    :  11
 Final Water Mark (non freed mem)   :  42.97 Kilobytes
 High Water Mark                    :  46.88 Kilobytes
 Backtrace                          :
   test_gm.adb:11 test_gm.my_alloc

Allocation Root # 2
 Number of non freed allocations    :   1
 Final Water Mark (non freed mem)   :  10.02 Kilobytes
 High Water Mark                    :  10.02 Kilobytes
 Backtrace                          :
   s-secsta.adb:81 system.secondary_stack.ss_init

Allocation Root # 3
 Number of non freed allocations    :   1
 Final Water Mark (non freed mem)   :  12 Bytes
 High Water Mark                    :  12 Bytes
 Backtrace                          :
   s-secsta.adb:181 system.secondary_stack.ss_init

Note that the GNAT runtime contains itself a certain number of allocations that have no corresponding deallocation, as shown here for root #2 and root #3. This is a normal behavior when the number of non-freed allocations is one, it allocates dynamic data structures that the run time needs for the complete lifetime of the program. Note also that there is only one allocation root in the user program with a single line back trace: test_gm.adb:11 test_gm.my_alloc, whereas a careful analysis of the program shows that ‘My_Alloc’ is called at 2 different points in the source (line 21 and line 24). If those two allocation roots need to be distinguished, the backtrace depth parameter can be used:

$ gnatmem 3 test_gm

which will give the following output:

Global information
   Total number of allocations        :  18
   Total number of deallocations      :   5
   Final Water Mark (non freed mem)   :  53.00 Kilobytes
   High Water Mark                    :  56.90 Kilobytes

Allocation Root # 1
 Number of non freed allocations    :  10
 Final Water Mark (non freed mem)   :  39.06 Kilobytes
 High Water Mark                    :  42.97 Kilobytes
 Backtrace                          :
   test_gm.adb:11 test_gm.my_alloc
   test_gm.adb:24 test_gm
   b_test_gm.c:52 main

Allocation Root # 2
 Number of non freed allocations    :   1
 Final Water Mark (non freed mem)   :  10.02 Kilobytes
 High Water Mark                    :  10.02 Kilobytes
 Backtrace                          :
   s-secsta.adb:81  system.secondary_stack.ss_init
   s-secsta.adb:283 <system__secondary_stack___elabb>
   b_test_gm.c:33   adainit

Allocation Root # 3
 Number of non freed allocations    :   1
 Final Water Mark (non freed mem)   :   3.91 Kilobytes
 High Water Mark                    :   3.91 Kilobytes
 Backtrace                          :
   test_gm.adb:11 test_gm.my_alloc
   test_gm.adb:21 test_gm
   b_test_gm.c:52 main

Allocation Root # 4
 Number of non freed allocations    :   1
 Final Water Mark (non freed mem)   :  12 Bytes
 High Water Mark                    :  12 Bytes
 Backtrace                          :
   s-secsta.adb:181 system.secondary_stack.ss_init
   s-secsta.adb:283 <system__secondary_stack___elabb>
   b_test_gm.c:33   adainit

The allocation root #1 of the first example has been split in 2 roots #1 and #3, thanks to the more precise associated backtrace.