AVX generic mode tuning discussion.
Mon Oct 31 21:21:00 GMT 2011
> > > We would like to propose changing AVX generic mode tuning to
> > 128-bit
> > > AVX instead of 256-bit AVX.
> > You indicate a 3% reduction on bulldozer with avx256.
> > How does avx128 compare to -mno-avx -msse4.2?
> We see these % differences going from SSE42 to AVX128 to AVX256 on
> Bulldozer with "-mtune=generic -Ofast".
> (Positive is improvement, negative is degradation)
> AVX128/SSE42 AVX256/AVX-128
> 410.bwaves -1.4% -1.4%
> 416.gamess -1.1% 0.0%
> 433.milc 0.5% -2.4%
> 434.zeusmp 9.7% -2.1%
> 435.gromacs 5.1% 0.5%
> 436.cactusADM 8.2% -23.8%
> 437.leslie3d 8.1% 0.4%
> 444.namd 3.6% 0.0%
> 447.dealII -1.4% -0.4%
> 450.soplex -0.4% -0.4%
> 453.povray 0.0% -1.5%
> 454.calculix 15.7% -8.3%
> 459.GemsFDTD 4.9% 1.4%
> 465.tonto 1.3% -0.6%
> 470.lbm 0.9% 0.3%
> 481.wrf 7.3% -3.6%
> 482.sphinx3 5.0% -9.8%
> SPECFP 3.8% -3.2%
> > Will the next AMD generation have a useable avx256?
> > I'm not keen on the idea of generic mode being tune
> > for a single processor revision that maybe shouldn't
> > actually be using avx at all.
> We see a substantial gain in several SPECFP benchmarks going from SSE42
> to AVX128 on Bulldozer.
> IMHO, accomplishing even a 5% gain in an individual benchmark takes a
> hardware company several man months.
> The loss with AVX256 for Bulldozer is much more significant than the
> gain for SandyBridge.
> While the general trend in the industry is a move toward AVX256, for
> now we would be disadvantaging Bulldozer with this choice.
> We have several customers who use -mtune=generic and it is default,
> unless a user explicitly overrides it with -mtune=native. They are the
> ones who want to experiment with latest ISA using gcc, but want to keep
> their ISA selection and tuning agnostic on x86/64. IMHO, it is with
> these customers in mind that generic was introduced in the first place.
Since stage 1 closure is around the corner, just wanted to ping to see if the maintainers have made up their mind on this one.
AVX-128 is an improvement over SSE42 for Bulldozer and AVX-256 wipes out pretty much all of that gain in generic mode.
Until there is a convergence on AVX-256 for x86/64, we would like to propose having generic generate avx-128 by default and have a user override to avx-256 manually when known to benefit performance.
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