> > > We would like to propose changing AVX generic mode tuning to > generate > > 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) > > Bulldozer: > 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. Thanks, Harsha
