* Re: [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
@ 2013-09-18 5:00 ` pinskia
2013-09-18 5:00 ` [Bug c/58454] " pinskia at gmail dot com
` (5 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: pinskia @ 2013-09-18 5:00 UTC (permalink / raw)
To: mednafen at gmail dot com; +Cc: gcc-bugs
All of these functions overflow the loop induction variable so only -fwrapv will provide the behavior you want for all of the functions. The inconsistent behavior is due to the overflows happening for induction variables. If it was not an induction variable then -fno-strict-overflow would have worked..-fno-strict-overflow is only defined to work for non loop variables.
Thanks,
Andrew
On Sep 17, 2013, at 6:28 PM, "mednafen at gmail dot com" <gcc-bugzilla@gcc.gnu.org> wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
>
> Bug ID: 58454
> Summary: Potentially wrong(or at least weird/inconsistent) code
> generation with -O2 -fno-strict-overflow
> Product: gcc
> Version: 4.8.1
> Status: UNCONFIRMED
> Severity: normal
> Priority: P3
> Component: c
> Assignee: unassigned at gcc dot gnu.org
> Reporter: mednafen at gmail dot com
>
> Created attachment 30844
> --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30844&action=edit
> Testcase program.
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O0 -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> ????:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****Aborted
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2
> -fno-aggressive-loop-optimizations -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3:
> ****************************************************************************************************Aborted
>
>
> Broken:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1:
> *********************************************************************************************Aborted
>
>
> Broken:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2
> -fno-aggressive-loop-optimizations -fno-strict-overflow -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1:
> *********************************************************************************************Aborted
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -fno-tree-vrp -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -fwrapv -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -v
> Using built-in specs.
> COLLECT_GCC=/usr/local/gcc-4.8.1/bin/gcc
> COLLECT_LTO_WRAPPER=/usr/local/gcc-4.8.1/libexec/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
> Target: x86_64-unknown-linux-gnu
> Configured with: ../gcc-4.8.1/configure --prefix=/usr/local/gcc-4.8.1
> Thread model: posix
> gcc version 4.8.1 (GCC)
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
2013-09-18 5:00 ` pinskia
@ 2013-09-18 5:00 ` pinskia at gmail dot com
2013-09-18 5:56 ` mednafen at gmail dot com
` (4 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: pinskia at gmail dot com @ 2013-09-18 5:00 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
--- Comment #1 from pinskia at gmail dot com <pinskia at gmail dot com> ---
All of these functions overflow the loop induction variable so only -fwrapv
will provide the behavior you want for all of the functions. The inconsistent
behavior is due to the overflows happening for induction variables. If it was
not an induction variable then -fno-strict-overflow would have
worked..-fno-strict-overflow is only defined to work for non loop variables.
Thanks,
Andrew
On Sep 17, 2013, at 6:28 PM, "mednafen at gmail dot com"
<gcc-bugzilla@gcc.gnu.org> wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
>
> Bug ID: 58454
> Summary: Potentially wrong(or at least weird/inconsistent) code
> generation with -O2 -fno-strict-overflow
> Product: gcc
> Version: 4.8.1
> Status: UNCONFIRMED
> Severity: normal
> Priority: P3
> Component: c
> Assignee: unassigned at gcc dot gnu.org
> Reporter: mednafen at gmail dot com
>
> Created attachment 30844
> --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30844&action=edit
> Testcase program.
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O0 -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> ????:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3: ****IMm3:
> ****IMm3: ****Aborted
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2
> -fno-aggressive-loop-optimizations -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3:
> ****************************************************************************************************Aborted
>
>
> Broken:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1:
> *********************************************************************************************Aborted
>
>
> Broken:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2
> -fno-aggressive-loop-optimizations -fno-strict-overflow -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1:
> *********************************************************************************************Aborted
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -fno-tree-vrp -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> Working:
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -Wall -O2 -fno-strict-overflow
> -fwrapv -o halt halt.c
> XXX@willow:~/halt$ ./halt
> IMm3: ****
> IMm2: ***
> IMm1: **
> IM: *
>
>
> XXX@willow:~/halt$ /usr/local/gcc-4.8.1/bin/gcc -v
> Using built-in specs.
> COLLECT_GCC=/usr/local/gcc-4.8.1/bin/gcc
> COLLECT_LTO_WRAPPER=/usr/local/gcc-4.8.1/libexec/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
> Target: x86_64-unknown-linux-gnu
> Configured with: ../gcc-4.8.1/configure --prefix=/usr/local/gcc-4.8.1
> Thread model: posix
> gcc version 4.8.1 (GCC)
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
2013-09-18 5:00 ` pinskia
2013-09-18 5:00 ` [Bug c/58454] " pinskia at gmail dot com
@ 2013-09-18 5:56 ` mednafen at gmail dot com
2013-09-18 8:45 ` rguenth at gcc dot gnu.org
` (3 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: mednafen at gmail dot com @ 2013-09-18 5:56 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
--- Comment #2 from mednafen at gmail dot com ---
Your assertion conflicts with the gcc 4.2 release change-list at
http://gcc.gnu.org/gcc-4.2/changes.html when the strict-overflow options were
added.
Additionally, -fwrapv produces unnecessarily bloated code compared to
-fno-strict-overflow, in my experience.
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
` (2 preceding siblings ...)
2013-09-18 5:56 ` mednafen at gmail dot com
@ 2013-09-18 8:45 ` rguenth at gcc dot gnu.org
2013-09-20 0:08 ` ian at airs dot com
` (2 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: rguenth at gcc dot gnu.org @ 2013-09-18 8:45 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |documentation
Status|UNCONFIRMED |NEW
Last reconfirmed| |2013-09-18
CC| |iant at google dot com
Ever confirmed|0 |1
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
-fno-strict-overflow should have never been added because it certainly does not
disable all use of undefined signed overflow behavior. Which makes it kind-of
useless.
Ian, the documentation of -fno-strict-overflow suggests that the compiler
will not rely on signed overflow not happening. That's not true, so this
is definitely a documentation bug.
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
` (3 preceding siblings ...)
2013-09-18 8:45 ` rguenth at gcc dot gnu.org
@ 2013-09-20 0:08 ` ian at airs dot com
2013-09-20 7:45 ` manu at gcc dot gnu.org
2021-05-14 10:00 ` [Bug tree-optimization/58454] " jakub at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: ian at airs dot com @ 2013-09-20 0:08 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
Ian Lance Taylor <ian at airs dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ian at airs dot com
--- Comment #4 from Ian Lance Taylor <ian at airs dot com> ---
Richi, as you know, -fno-strict-overflow was added to prevent people from
configuring GCC to make -fwrapv the default. I know you hate the option but I
continue to think that adding was the right tactical move at the time. We
can't just ignore the world outside the compiler development community.
It's true that -fno-strict-overflow does not apply to loop optimizations, which
are covered by -fno-aggressive-loop-optimizations. I agree that that is a doc
omission.
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
` (4 preceding siblings ...)
2013-09-20 0:08 ` ian at airs dot com
@ 2013-09-20 7:45 ` manu at gcc dot gnu.org
2021-05-14 10:00 ` [Bug tree-optimization/58454] " jakub at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: manu at gcc dot gnu.org @ 2013-09-20 7:45 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |manu at gcc dot gnu.org
--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Ian Lance Taylor from comment #4)
> It's true that -fno-strict-overflow does not apply to loop optimizations,
> which are covered by -fno-aggressive-loop-optimizations. I agree that that
> is a doc omission.
The examples above suggest that this is not true either. Only -fno-tree-vrp or
-fwrapv do the trick.
>From gcc-bugs-return-430231-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Fri Sep 20 07:46:57 2013
Return-Path: <gcc-bugs-return-430231-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Delivered-To: listarch-gcc-bugs@gcc.gnu.org
Received: (qmail 3963 invoked by alias); 20 Sep 2013 07:46:57 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Delivered-To: mailing list gcc-bugs@gcc.gnu.org
Received: (qmail 3601 invoked by uid 48); 20 Sep 2013 07:46:53 -0000
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58464] [4.9 Regression] Crashes with SIGSEGV (infinite recursion in phi_translate)
Date: Fri, 20 Sep 2013 07:46:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.9.0
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: ASSIGNED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: rguenth at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 4.9.0
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields: bug_status assigned_to
Message-ID: <bug-58464-4-yb2du9alxP@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-58464-4@http.gcc.gnu.org/bugzilla/>
References: <bug-58464-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2013-09/txt/msg01471.txt.bz2
Content-length: 454
http://gcc.gnu.org/bugzilla/show_bug.cgi?idX464
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Mine.
^ permalink raw reply [flat|nested] 8+ messages in thread* [Bug tree-optimization/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow
2013-09-18 1:28 [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow mednafen at gmail dot com
` (5 preceding siblings ...)
2013-09-20 7:45 ` manu at gcc dot gnu.org
@ 2021-05-14 10:00 ` jakub at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-05-14 10:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58454
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED
CC| |jakub at gcc dot gnu.org
Target Milestone|8.5 |8.0
--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed then.
^ permalink raw reply [flat|nested] 8+ messages in thread