[PATCH] D43079: [TTI CostModel] change default cost of FP ops to 1 (PR36280)

[Adam Nemet via Phabricator via llvm-commits]

anemet added a comment.

In https://reviews.llvm.org/D43079#1013321, @spatel wrote:

> In https://reviews.llvm.org/D43079#1013269, @eastig wrote:
>
> > Hi Sanjay,
> >
> > The patch caused regressions in the LLVM benchmarks and in Spec2k/Spec2k6 benchmarks on AArch64 Cortex-A53:
> >
> > SingleSource/Benchmarks/Misc/matmul_f64_4x4:  49%
> >  MultiSource/Benchmarks/TSVC/LoopRerolling-flt/LoopRerolling-flt: 5.32%
> >  CFP2000/188.ammp/188.ammp: 3.58%
> >  CFP2000/177.mesa/177.mesa: 2.48%
> >  CFP2006/444.namd/444.namd: 2.49%
> >
> > The regression of SingleSource/Benchmarks/Misc/matmul_f64_4x4 can also be seen on the public bot: http://lnt.llvm.org/db_default/v4/nts/90636 
> >  It is 128.85%.
> >
> > The main difference in generated code is FMUL(FP, scalar) instead of FMUL(SIMD, scalar):
> >
> >   fmul d20, d16, d2
> >
> >
> > instead of
> >
> >   fmul v17.2d, v1.2d, v5.2d
> >
> >
> > This also caused code size increase: 6.04% in SingleSource/Benchmarks/Misc/matmul_f64_4x4
> >
> > I am working on a reproducer.
>
>
> Thanks. We knew this change was likely to cause perf regressions based on some of the x86 diffs, so having those reductions will help tune the models in general and specifically for AArch64.
>
> Ie, we should be able to solve the AArch64 problems with AArch64-specific cost model changes rather than reverting this. For example as @fhahn mentioned, we might want to make the int-to-FP ratio 3:2 for some cores. Another possibility is overriding the fmul/fsub/fadd AArch64 costs to be more realistic (as we also probably have to do for x86).


Please revert until these things get worked out so that we can properly track performance.   We are seeing many regressions including 17% on 444.namd and 12% on 482.sphinx3 in SPECfp 2006.


Repository:
  rL LLVM

https://reviews.llvm.org/D43079