[Bug target/114560] Compilation error when using _mm512_maskz_expandloadu_epi16 with only -mavx512vbmi2

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114560

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Meirav Grimberg from comment #2)
> (In reply to Jakub Jelinek from comment #1)
> > AVX512BW is needed to be able to use __mmask32/__mmask64, those aren't
> > supported in AVX512F, which only supports __mmask16.  __mmask8 needs
> > AVX512DQ (though, guess for that one one can just use KMOV with 16-bit mask).
> > In GCC 13 and later, -mavx512bw has been added as the implicit requirement of
> > -mavx512vbmi2
> > https://gcc.gnu.org/pipermail/gcc-patches/2023-April/615906.html
> > and -mavx512bitalg
> > https://gcc.gnu.org/pipermail/gcc-patches/2023-April/615905.html
> 
> Hi,
> thank you for the quick reply.
> 
> As i mentioned Intel Intrinsics Guide specifically specifies only the
> AVX512_VBMI2 flag without referencing AVX512BW. Could you shed some light on
> this?

That is just a bug in the Intrinsic Guide IMNSHO.

> Moreover, I noticed that both Clang and Intel's compiler allow compilation
> without additional flags, suggesting an implementation that aligns with the
> hardware requirements. Could you provide insights into why GCC necessitates
> an additional flag?

The intrinsic needs to load the 32-bit mask into one of the %k{0,1,2,3,4,5,6,7}
registers.  And without AVX512BW there is just not an instruction for that.
If you'll compile your testcase with clang with -O0 -mavx512vbmi2, you can see
kmovd   %ecx, %k1
instruction, which requires AVX512BW CPUID.  So, supposedly it does what GCC14
and later does, enabling -mavx512bw implicitly when -mavx512vbmi2 is requested.
While the vpexpandw instruction indeed maybe only needs AVX512VBMI2, you can't
implement the intrinsic without AVX512BW.
When I check clang -E -dD -mavx512vbmi2 output on godbolt, I see
#define __AVX512BW__ 1
#define __AVX512VBMI2__ 1
defined there.

> Regarding the term "implicit requirement," could you please clarify its
> meaning? I didn't observe any apparent differences when attempting
> compilation with GCC 13.

Ah, sorry, it is indeed in GCC 14 only.  I was misled by the commit date of
January 2023, but it has been actually pushed into GCC trunk only in April
after GCC 13 branched.
In GCC 11-13 you need to use both -mavx512vbmi2 -mavx512bw to use these
intrinsics.