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

[Hal Finkel via llvm-dev]

On 11/01/2017 06:35 PM, 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.
>

I'd certainly like to see these options (especially for this reason).

  -Hal

> -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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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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