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gnat_ugn.texi: The "Run-Time Checks" section said "arithmetic overflow checking for integer...

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2008-08-08  Bob Duff  <duff@adacore.com>

	* gnat_ugn.texi: The "Run-Time Checks" section said "arithmetic overflow
	checking for integer operations (including division by zero)", which
	is wrong -- divide by zero is not part of overflow checking.
	Also added misc clarification about what check-suppression means.

	* gnat_rm.texi: Clarify the meaning of pragma Suppress.

From-SVN: r138879
This commit is contained in:
Arnaud Charlet 2008-08-08 15:00:00 +02:00
parent e8c4584bd7
commit a3ea913742
3 changed files with 103 additions and 24 deletions
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48
gcc/ada/ChangeLog
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@ -1,3 +1,51 @@
2008-08-08 Thomas Quinot <quinot@adacore.com>
* gsocket.h:
On Windows, include <errno.h> and redefine only selected errno values
from their <winsock2.h> definitions.
* s-osinte-freebsd.ads: Minor reformatting
* s-osinte-hpux.ads, s-osinte-irix.ads: Minor reformatting
* g-soccon.ads: New file.
* g-stheme.adb, g-socthi-vms.adb, g-socthi-vxworks.adb,
g-socthi-mingw.adb, g-sttsne-vxworks.adb, g-socthi.adb,
g-stsifd-sockets.adb, g-socket.adb, g-socket.ads,
g-sothco.adb, g-sothco.ads: Add back GNAT.Sockets.Constants as a child
unit, to allow building software that depends on this internal unit
with both older and newer compilers.
2008-08-08 Robert Dewar <dewar@adacore.com>
* s-strxdr.adb: Minor reformatting
2008-08-08 Bob Duff <duff@adacore.com>
* gnat_ugn.texi: The "Run-Time Checks" section said "arithmetic overflow
checking for integer operations (including division by zero)", which
is wrong -- divide by zero is not part of overflow checking.
Also added misc clarification about what check-suppression means.
* gnat_rm.texi: Clarify the meaning of pragma Suppress.
2008-08-08 Jerome Lambourg <lambourg@adacore.com>
* g-comlin.adb (Add_Switch): Handle addition of switches at the
begining of the command line.
(Append, Add): Renaming of Append to Add as this now allows addition
at the begining of the list.
* g-comlin.ads (Add_Switch): Handle addition of switches at the
begining of the command line.
2008-08-08 Thomas Quinot <quinot@adacore.com>
* g-sercom.ads:
(Name): Document application scope (only legacy PC serial ports on
Linux and Windows).
2008-08-08 Thomas Quinot <quinot@adacore.com>
* gcc-interface/Makefile.in, gcc-interface/Make-lang.in: Revert

13
gcc/ada/gnat_rm.texi
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@ -3413,7 +3413,7 @@ Syntax:
@smallexample @c ada
pragma Obsolescent
(Entity => NAME [, static_string_EXPRESSION [,Ada_05]]);
[(Entity => NAME [, static_string_EXPRESSION [,Ada_05]])];
@end smallexample
@noindent
@ -4447,6 +4447,17 @@ on addresses used in address clauses. Such checks can also be suppressed
by suppressing range checks, but the specific use of @code{Alignment_Check}
allows suppression of alignment checks without suppressing other range checks.
Note that pragma Suppress gives the compiler permission to omit
checks, but does not require the compiler to omit checks. The compiler
will generate checks if they are essentially free, even when they are
suppressed. In particular, if the compiler can prove that a certain
check will necessarily fail, it will generate code to do an
unconditional ``raise'', even if checks are suppressed. The compiler
warns in this case.
Of course, run-time checks are omitted whenever the compiler can prove
that they will not fail, whether or not checks are suppressed.
@node Pragma Suppress_All
@unnumberedsec Pragma Suppress_All
@findex Suppress_All

66
gcc/ada/gnat_ugn.texi
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@ -4022,12 +4022,12 @@ catches that cannot be dealt with in the front-end.
@item -gnato
@cindex @option{-gnato} (@command{gcc})
Enable numeric overflow checking (which is not normally enabled by
default). Not that division by zero is a separate check that is not
default). Note that division by zero is a separate check that is not
controlled by this switch (division by zero checking is on by default).
@item -gnatp
@cindex @option{-gnatp} (@command{gcc})
Suppress all checks.
Suppress all checks. See @ref{Run-Time Checks} for details.
@item -gnatP
@cindex @option{-gnatP} (@command{gcc})
@ -6222,14 +6222,11 @@ clears any previously set style checks.
@cindex Checks, stack overflow checking
@noindent
If you compile with the default options, GNAT will insert many run-time
checks into the compiled code, including code that performs range
checking against constraints, but not arithmetic overflow checking for
integer operations (including division by zero), checks for access
before elaboration on subprogram calls, or stack overflow checking. All
other run-time checks, as required by the Ada Reference Manual, are
generated by default. The following @command{gcc} switches refine this
default behavior:
By default, the following checks are suppressed: integer overflow
checks, stack overflow checks, and checks for access before
elaboration on subprogram calls. All other checks, including range
checks and array bounds checks, are turned on by default. The
following @command{gcc} switches refine this default behavior.
@table @option
@c !sort!
@ -6238,13 +6235,33 @@ default behavior:
@cindex Suppressing checks
@cindex Checks, suppressing
@findex Suppress
Suppress all run-time checks as though @code{pragma Suppress (all_checks})
Suppress all run-time checks as though @code{pragma Suppress (All_checks)}
had been present in the source. Validity checks are also suppressed (in
other words @option{-gnatp} also implies @option{-gnatVn}.
Use this switch to improve the performance
of the code at the expense of safety in the presence of invalid data or
program bugs.
Note that when checks are suppressed, the compiler is allowed, but not
required, to omit the checking code. If the run-time cost of the
checking code is zero or near-zero, the compiler will generate it even
if checks are suppressed. In particular, if the compiler can prove
that a certain check will necessarily fail, it will generate code to
do an unconditional ``raise'', even if checks are suppressed. The
compiler warns in this case.
Of course, run-time checks are omitted whenever the compiler can prove
that they will not fail, whether or not checks are suppressed.
Note that if you suppress a check that would have failed, program
execution is erroneous, which means the behavior is totally
unpredictable. The program might crash, or print wrong answers, or
do anything else. It might even do exactly what you wanted it to do
(and then it might start failing mysteriously next week or next
year). The compiler will generate code based on the assumption that
the condition being checked is true, which can result in disaster if
that assumption is wrong.
@item -gnato
@cindex @option{-gnato} (@command{gcc})
@cindex Overflow checks
@ -6271,22 +6288,23 @@ X4 := Integer (F);
@end smallexample
@noindent
Note that if explicit values are assigned at compile time, the compiler may
be able to detect overflow at compile time, in which case no run-time check
is required, and the setting of -gnato is irrelevant. That's why the assigned
values in the above fragment are in quotes, the meaning is "assign a value
not known to the compiler that happens to be equal to ...". The remaining
discussion assumes that the compiler cannot detect the values at compile time.
Note that if explicit values are assigned at compile time, the
compiler may be able to detect overflow at compile time, in which case
no actual run-time checking code is required, and Constraint_Error
will be raised unconditionally, with or without
@option{-gnato}. That's why the assigned values in the above fragment
are in quotes, the meaning is "assign a value not known to the
compiler that happens to be equal to ...". The remaining discussion
assumes that the compiler cannot detect the values at compile time.
Here the first addition results in a value that is outside the base range
of Integer, and hence requires an overflow check for detection of the
constraint error. Thus the first assignment to @code{X1} raises a
@code{Constraint_Error} exception only if @option{-gnato} is set.
The second increment operation results in a violation
of the explicit range constraint, and such range checks are always
performed (unless specifically suppressed with a pragma @code{suppress}
or the use of @option{-gnatp}).
The second increment operation results in a violation of the explicit
range constraint; such range checks are performed by default, and are
unaffected by @option{-gnato}.
The two conversions of @code{F} both result in values that are outside
the base range of type @code{Integer} and thus will raise
@ -6342,6 +6360,8 @@ explicitly use the -gnato switch either on the @command{gnatmake} or
@cindex Check, elaboration
Enables dynamic checks for access-before-elaboration
on subprogram calls and generic instantiations.
Note that @option{-gnatE} is not necessary for safety, because in the
default mode, GNAT ensures statically that the checks would not fail.
For full details of the effect and use of this switch,
@xref{Compiling Using gcc}.
@ -9548,8 +9568,8 @@ some guidelines on debugging optimized code.
@subsection Controlling Run-Time Checks
@noindent
By default, GNAT generates all run-time checks, except arithmetic overflow
checking for integer operations and checks for access before elaboration on
By default, GNAT generates all run-time checks, except integer overflow
checks, 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.
@cindex @option{-gnatp} (@command{gcc})