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

[Tobias Grosser via llvm-dev]

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
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