[Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[mednafen at gmail dot com]

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)

[Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[pinskia at gmail dot com]

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)

Re: [Bug c/58454] New: Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[pinskia]

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)

[Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[mednafen at gmail dot com]

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.

[Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[rguenth at gcc dot gnu.org]

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.

[Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[ian at airs dot com]

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.

[Bug c/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[manu at gcc dot gnu.org]

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.
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Subject: [Bug tree-optimization/58464] [4.9 Regression] Crashes with SIGSEGV (infinite recursion in phi_translate)
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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.

[Bug tree-optimization/58454] Potentially wrong(or at least weird/inconsistent) code generation with -O2 -fno-strict-overflow

[jakub at gcc dot gnu.org]

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.