[llvm-dev] RFC: [X86] Introducing command line options to prefer narrower vector instructions even when wider instructions are available

[Craig Topper via llvm-dev]

Reviews of the initial plumbing have been posted

https://reviews.llvm.org/D39575
https://reviews.llvm.org/D39576

~Craig

On Thu, Nov 2, 2017 at 4:57 AM, Tobias Grosser <tobias.grosser at inf.ethz.ch>
wrote:

> Hi Craig,
>
> this sounds like a good idea.
>
> Best,
> Tobias
>
> On Thu, Nov 2, 2017, at 00:35, Craig Topper via llvm-dev wrote:
> > Hello all,
> >
> >
> >
> > I would like to propose adding the -mprefer-avx256 and -mprefer-avx128
> > command line flags supported by latest GCC to clang. These flags will be
> > used to limit the vector register size presented by TTI to the
> > vectorizers.
> > The backend will still be able to use wider registers for code written
> > using the instrinsics in x86intrin.h. And the backend will still be able
> > to
> > use AVX512VL instructions and the additional XMM16-31 and YMM16-31
> > registers.
> >
> >
> >
> > Motivation:
> >
> > -Using 512-bit operations on some Intel CPUs may cause a decrease in CPU
> > frequency that may offset the gains from using the wider register size.
> > See
> > section 15.26 of Intel® 64 and IA-32 Architectures Optimization Reference
> > Manual published October 2017.
> >
> > -The vector ALUs on ports 0 and 1 of the Skylake Server microarchitecture
> > are only 256-bits wide. 512-bit instructions using these ALUs must use
> > both
> > ports. See section 2.1 of Intel® 64 and IA-32 Architectures Optimization
> > Reference Manual published October 2017.
> >
> >
> >
> > Implementation Plan:
> >
> > -Add prefer-avx256 and prefer-avx128 as SubtargetFeatures in X86.td not
> > mapped to any CPU.
> >
> > -Add mprefer-avx256 and mprefer-avx128 and the corresponding
> > -mno-prefer-avx128/256 options to clang's driver Options.td file. I
> > believe
> > this will allow clang to pass these straight through to the
> > -target-feature
> > attribute in IR.
> >
> > -Modify X86TTIImpl::getRegisterBitWidth to only return 512 if AVX512 is
> > enabled and prefer-avx256 and prefer-avx128 is not set. Similarly return
> > 256 if AVX is enabled and prefer-avx128 is not set.
> >
> >
> >
> > There may be some other backend changes needed, but I plan to address
> > those
> > as we find them.
> >
> >
> > At a later point, consider making -mprefer-avx256 the default for Skylake
> > Server due to the above mentioned performance considerations.
> >
> >
> >
> > Does this sound reasonable?
> >
> >
> >
> > *Latest Intel Optimization manual available here:
> > https://software.intel.com/en-us/articles/intel-sdm#optimization
> >
> >
> > -Craig Topper
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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