* [patch, fortran, doc] Explicitly mention undefined overflow
@ 2023-03-19 8:32 Thomas Koenig
2023-03-19 13:12 ` Paul Richard Thomas
2023-03-19 13:15 ` Andreas Schwab
0 siblings, 2 replies; 6+ messages in thread
From: Thomas Koenig @ 2023-03-19 8:32 UTC (permalink / raw)
To: fortran, gcc-patches
[-- Attachment #1: Type: text/plain, Size: 238 bytes --]
Here's also an update on the docs to explicitly mention behavior
on overflow.
Maybe this will reach another 0.05% of users...
OK for trunk?
Best regards
Thomas
gcc/fortran/ChangeLog:
* gfortran.texi: Mention behavior on overflow.
[-- Attachment #2: overflow.diff --]
[-- Type: text/x-patch, Size: 1414 bytes --]
diff --git a/gcc/fortran/gfortran.texi b/gcc/fortran/gfortran.texi
index c483e13686d..93c66b18938 100644
--- a/gcc/fortran/gfortran.texi
+++ b/gcc/fortran/gfortran.texi
@@ -820,6 +820,7 @@ might in some way or another become visible to the programmer.
* File operations on symbolic links::
* File format of unformatted sequential files::
* Asynchronous I/O::
+* Behavior on integer overflow::o
@end menu
@@ -1160,6 +1161,23 @@ sytems, such as Linux, it is necessary to specify @option{-pthread},
@c Maybe this chapter should be merged with the 'Standards' section,
@c whenever that is written :-)
+@node Behavior on integer overflow
+@section Behavior on integer overflow
+@cindex integer overflow
+@cindex overflow handling
+
+Integer overflow is prohibited by the Fortran standard. The behavior
+of gfortran on integer overflow is undefined by default. Traditional
+code, like linear congruential pseudo-random number generators in old
+programs that rely on specific, non-standard behavior may generate
+unexpected results. The @option{-fsanitize=undefined} option can be
+used to detect such code at runtime.
+
+It is recommended to use the intrinsic subroutine @code{RANDOM_NUMBER}
+for random number generators or, if the old behavior is desired, to
+use the @option{-fwrapv} option. Note that this option can impact
+performance.
+
@node Extensions
@chapter Extensions
@cindex extensions
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [patch, fortran, doc] Explicitly mention undefined overflow
2023-03-19 8:32 [patch, fortran, doc] Explicitly mention undefined overflow Thomas Koenig
@ 2023-03-19 13:12 ` Paul Richard Thomas
2023-03-19 17:40 ` Thomas Koenig
2023-03-19 13:15 ` Andreas Schwab
1 sibling, 1 reply; 6+ messages in thread
From: Paul Richard Thomas @ 2023-03-19 13:12 UTC (permalink / raw)
To: Thomas Koenig; +Cc: fortran, gcc-patches
[-- Attachment #1: Type: text/plain, Size: 653 bytes --]
Hi Thomas,
Yes, that's fine for trunk. I wonder if it is worth being explicit that
linear congruential pseudo-random number generators can and do fail at -O3?
Thanks for the doc patches!
Paul
On Sun, 19 Mar 2023 at 08:32, Thomas Koenig via Fortran <fortran@gcc.gnu.org>
wrote:
> Here's also an update on the docs to explicitly mention behavior
> on overflow.
>
> Maybe this will reach another 0.05% of users...
>
> OK for trunk?
>
> Best regards
>
> Thomas
>
> gcc/fortran/ChangeLog:
>
> * gfortran.texi: Mention behavior on overflow.
>
--
"If you can't explain it simply, you don't understand it well enough" -
Albert Einstein
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [patch, fortran, doc] Explicitly mention undefined overflow
2023-03-19 8:32 [patch, fortran, doc] Explicitly mention undefined overflow Thomas Koenig
2023-03-19 13:12 ` Paul Richard Thomas
@ 2023-03-19 13:15 ` Andreas Schwab
1 sibling, 0 replies; 6+ messages in thread
From: Andreas Schwab @ 2023-03-19 13:15 UTC (permalink / raw)
To: Thomas Koenig via Gcc-patches; +Cc: fortran, Thomas Koenig
On Mär 19 2023, Thomas Koenig via Gcc-patches wrote:
> diff --git a/gcc/fortran/gfortran.texi b/gcc/fortran/gfortran.texi
> index c483e13686d..93c66b18938 100644
> --- a/gcc/fortran/gfortran.texi
> +++ b/gcc/fortran/gfortran.texi
> @@ -820,6 +820,7 @@ might in some way or another become visible to the programmer.
> * File operations on symbolic links::
> * File format of unformatted sequential files::
> * Asynchronous I/O::
> +* Behavior on integer overflow::o
s/o$//
--
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
"And now for something completely different."
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [patch, fortran, doc] Explicitly mention undefined overflow
2023-03-19 13:12 ` Paul Richard Thomas
@ 2023-03-19 17:40 ` Thomas Koenig
2023-03-20 7:14 ` Thomas Koenig
0 siblings, 1 reply; 6+ messages in thread
From: Thomas Koenig @ 2023-03-19 17:40 UTC (permalink / raw)
To: Paul Richard Thomas; +Cc: fortran, gcc-patches
Hi Paul,
> Yes, that's fine for trunk. I wonder if it is worth being explicit that
> linear congruential pseudo-random number generators can and do fail at -O3?
I don't think we should put this into the docs, because that can change
at any time. Maybe into porting_to.html, though (where I have only
mentioned this as a general issue with linear congruential generators,
without mentioning specific options. Current text can be seen at
https://gcc.gnu.org/gcc-13/porting_to.html ).
Hm....
Best regards
Thomas
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [patch, fortran, doc] Explicitly mention undefined overflow
2023-03-19 17:40 ` Thomas Koenig
@ 2023-03-20 7:14 ` Thomas Koenig
2023-03-20 21:32 ` Harald Anlauf
0 siblings, 1 reply; 6+ messages in thread
From: Thomas Koenig @ 2023-03-20 7:14 UTC (permalink / raw)
To: Paul Richard Thomas; +Cc: fortran, gcc-patches
I wrote:
>
>> Yes, that's fine for trunk. I wonder if it is worth being explicit that
>> linear congruential pseudo-random number generators can and do fail at
>> -O3?
>
> I don't think we should put this into the docs, because that can change
> at any time. Maybe into porting_to.html, though (where I have only
> mentioned this as a general issue with linear congruential generators,
> without mentioning specific options. Current text can be seen at
> https://gcc.gnu.org/gcc-13/porting_to.html ).
>
> Hm....
Breaking things actually goes back further than I thought.
Taking the random number generator from rnseed and running
it 10 times with my system gfortran 9.4.0 gets me, at
different levels of optimization:
$ for a in -O0 -O1 -O2; do echo $a; gfortran $a genuni.f90 && ./a.out; done
-O0
0.269411892
0.891386986
0.444042951
0.779210865
0.500058949
0.666437685
0.666963458
0.462416053
0.376364112
2.90278494E-02
-O1
0.269411892
0.891386986
0.444042951
0.779210865
0.500058949
0.666437685
0.666963458
0.462416053
0.376364112
2.90278494E-02
-O2
-0.730588138
0.891386986
-0.555957019
-0.220789105
-0.499941051
0.666437685
-0.333036542
0.462416053
0.376364112
2.90278494E-02
and for current trunk it is
$ for a in -O0 -O1 -O2; do echo $a; gfortran $a genuni.f90 && ./a.out; done
-O0
0.269411892
0.891386986
0.444042951
0.779210865
0.500058949
0.666437685
0.666963458
0.462416053
0.376364112
2.90278494E-02
-O1
0.269411892
0.891386986
0.444042951
0.779210865
0.500058949
0.666437685
0.666963458
0.462416053
0.376364112
2.90278494E-02
-O2
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
0.211324871
so it the general problem is not restricted to -O3 and not
to current trunk, it depends on the details.
I doubt that the result from 9.4.0 was expected, but rather
nobody noticed. Or, bringing out the pseudo-RNG into a
different setting changed things.
So... any suggestions on how to improve the current wording?
Best regards
Thomas
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [patch, fortran, doc] Explicitly mention undefined overflow
2023-03-20 7:14 ` Thomas Koenig
@ 2023-03-20 21:32 ` Harald Anlauf
0 siblings, 0 replies; 6+ messages in thread
From: Harald Anlauf @ 2023-03-20 21:32 UTC (permalink / raw)
To: Thomas Koenig, Paul Richard Thomas; +Cc: fortran, gcc-patches
Hi Thomas,
Am 20.03.23 um 08:14 schrieb Thomas Koenig via Gcc-patches:
> so it the general problem is not restricted to -O3 and not
> to current trunk, it depends on the details.
>
> I doubt that the result from 9.4.0 was expected, but rather
> nobody noticed. Or, bringing out the pseudo-RNG into a
> different setting changed things.
>
> So... any suggestions on how to improve the current wording?
how about changing:
"... relying on a specific, non-standard behavior may now generate
unexpected results."
to
"... relying on a specific, non-standard behavior may generate
unexpected results depending on optimization level and other compiler
flags."
We cannot know all the codes used in the wild ...
Cheers,
Harald
> Best regards
>
> Thomas
>
>
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-03-20 21:32 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-03-19 8:32 [patch, fortran, doc] Explicitly mention undefined overflow Thomas Koenig
2023-03-19 13:12 ` Paul Richard Thomas
2023-03-19 17:40 ` Thomas Koenig
2023-03-20 7:14 ` Thomas Koenig
2023-03-20 21:32 ` Harald Anlauf
2023-03-19 13:15 ` Andreas Schwab
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).