> On 07/12/2011 02:22 PM, [email protected] wrote:
> > 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.
Thanks,
Harsha
