avx512erintrin.h: uninitialized variable warning (optimized build)

avx512erintrin.h: uninitialized variable warning (optimized build)

[James Addison]

Hi,

During GCC 12.2.0 compilation of a file that includes[1] immintrin.h
with both code-optimization and uninitialized-variable-warnings
enabled, a warning is emitted:

    /usr/lib/gcc/x86_64-linux-gnu/12/include/avx512erintrin.h:55:20:
warning: ‘__W’ is used uninitialized [-Wuninitialized]

The minimal repro compilation command appears to be:

    gcc -O -Wuninitialized -mavx512er -mavx512pf
./numpy/distutils/checks/cpu_avx512_knl.c

My question is: does the warning indicate a possible bug that should
be reported, or is there a reason that the relevant code[2] does not
initialize the variable (for example, for performance reasons)?

Thank you,
James

[1] - https://github.com/numpy/numpy/blob/bb2769e12a8646f3d63097e9464592aa6e20058d/numpy/distutils/checks/cpu_avx512_knl.c#L14

[2] - https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/i386/avx512erintrin.h;h=6b3b679a17675612f45a1090f227012b80b871a6#l54

Re: avx512erintrin.h: uninitialized variable warning (optimized build)

[Sam James]

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> On 12 Jan 2023, at 00:26, James Addison via Gcc <gcc@gcc.gnu.org> wrote:
> 
> Hi,
> 
> During GCC 12.2.0 compilation of a file that includes[1] immintrin.h
> with both code-optimization and uninitialized-variable-warnings
> enabled, a warning is emitted:
> 
>    /usr/lib/gcc/x86_64-linux-gnu/12/include/avx512erintrin.h:55:20:
> warning: ‘__W’ is used uninitialized [-Wuninitialized]
> 
> The minimal repro compilation command appears to be:
> 
>    gcc -O -Wuninitialized -mavx512er -mavx512pf
> ./numpy/distutils/checks/cpu_avx512_knl.c
> 
> My question is: does the warning indicate a possible bug that should
> be reported, or is there a reason that the relevant code[2] does not
> initialize the variable (for example, for performance reasons)?

See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105593, might
be the same thing?

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Re: avx512erintrin.h: uninitialized variable warning (optimized build)

[James Addison]

Thanks, Sam,

Yep, that does seem closely related.  The problems mentioned there
look similar enough that it may be possible to resolve an entire class
of these problems in the same area of the code in a single pass.  I'll
add some more details there soon.

On Thu, 12 Jan 2023 at 06:51, Sam James <sam@gentoo.org> wrote:
>
>
>
> > On 12 Jan 2023, at 00:26, James Addison via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Hi,
> >
> > During GCC 12.2.0 compilation of a file that includes[1] immintrin.h
> > with both code-optimization and uninitialized-variable-warnings
> > enabled, a warning is emitted:
> >
> >    /usr/lib/gcc/x86_64-linux-gnu/12/include/avx512erintrin.h:55:20:
> > warning: ‘__W’ is used uninitialized [-Wuninitialized]
> >
> > The minimal repro compilation command appears to be:
> >
> >    gcc -O -Wuninitialized -mavx512er -mavx512pf
> > ./numpy/distutils/checks/cpu_avx512_knl.c
> >
> > My question is: does the warning indicate a possible bug that should
> > be reported, or is there a reason that the relevant code[2] does not
> > initialize the variable (for example, for performance reasons)?
>
> See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105593, might
> be the same thing?