On Tue, Dec 12, 2023 at 10:38 PM Jan Hubicka <hubi...@ucw.cz> wrote: > > Hi, > this patch disables use of FMA in matrix multiplication loop for generic (for > x86-64-v3) and zen4. I tested this on zen4 and Xenon Gold Gold 6212U. > > For Intel this is neutral both on the matrix multiplication microbenchmark > (attached) and spec2k17 where the difference was within noise for Core. > > On core the micro-benchmark runs as follows: > > With FMA: > > 578,500,241 cycles:u # 3.645 GHz > ( +- 0.12% ) > 753,318,477 instructions:u # 1.30 insn per > cycle ( +- 0.00% ) > 125,417,701 branches:u # 790.227 M/sec > ( +- 0.00% ) > 0.159146 +- 0.000363 seconds time elapsed ( +- 0.23% ) > > > No FMA: > > 577,573,960 cycles:u # 3.514 GHz > ( +- 0.15% ) > 878,318,479 instructions:u # 1.52 insn per > cycle ( +- 0.00% ) > 125,417,702 branches:u # 763.035 M/sec > ( +- 0.00% ) > 0.164734 +- 0.000321 seconds time elapsed ( +- 0.19% ) > > So the cycle count is unchanged and discrete multiply+add takes same time as > FMA. > > While on zen: > > > With FMA: > 484875179 cycles:u # 3.599 GHz > ( +- 0.05% ) (82.11%) > 752031517 instructions:u # 1.55 insn per > cycle > 125106525 branches:u # 928.712 M/sec > ( +- 0.03% ) (85.09%) > 128356 branch-misses:u # 0.10% of all > branches ( +- 0.06% ) (83.58%) > > No FMA: > 375875209 cycles:u # 3.592 GHz > ( +- 0.08% ) (80.74%) > 875725341 instructions:u # 2.33 insn per > cycle > 124903825 branches:u # 1.194 G/sec > ( +- 0.04% ) (84.59%) > 0.105203 +- 0.000188 seconds time elapsed ( +- 0.18% ) > > The diffrerence is that Cores understand the fact that fmadd does not need > all three parameters to start computation, while Zen cores doesn't. > > Since this seems noticeable win on zen and not loss on Core it seems like good > default for generic. > > I plan to commit the patch next week if there are no compplains. The generic part LGTM.(It's exactly what we proposed in [1])
[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-November/637721.html > > Honza > > #include <stdio.h> > #include <time.h> > > #define SIZE 1000 > > float a[SIZE][SIZE]; > float b[SIZE][SIZE]; > float c[SIZE][SIZE]; > > void init(void) > { > int i, j, k; > for(i=0; i<SIZE; ++i) > { > for(j=0; j<SIZE; ++j) > { > a[i][j] = (float)i + j; > b[i][j] = (float)i - j; > c[i][j] = 0.0f; > } > } > } > > void mult(void) > { > int i, j, k; > > for(i=0; i<SIZE; ++i) > { > for(j=0; j<SIZE; ++j) > { > for(k=0; k<SIZE; ++k) > { > c[i][j] += a[i][k] * b[k][j]; > } > } > } > } > > int main(void) > { > clock_t s, e; > > init(); > s=clock(); > mult(); > e=clock(); > printf(" mult took %10d clocks\n", (int)(e-s)); > > return 0; > > } > > * confg/i386/x86-tune.def (X86_TUNE_AVOID_128FMA_CHAINS, > X86_TUNE_AVOID_256FMA_CHAINS) > Enable for znver4 and Core. > > diff --git a/gcc/config/i386/x86-tune.def b/gcc/config/i386/x86-tune.def > index 43fa9e8fd6d..74b03cbcc60 100644 > --- a/gcc/config/i386/x86-tune.def > +++ b/gcc/config/i386/x86-tune.def > @@ -515,13 +515,13 @@ DEF_TUNE (X86_TUNE_USE_SCATTER_8PARTS, > "use_scatter_8parts", > > /* X86_TUNE_AVOID_128FMA_CHAINS: Avoid creating loops with tight 128bit or > smaller FMA chain. */ > -DEF_TUNE (X86_TUNE_AVOID_128FMA_CHAINS, "avoid_fma_chains", m_ZNVER1 | > m_ZNVER2 | m_ZNVER3 > - | m_YONGFENG) > +DEF_TUNE (X86_TUNE_AVOID_128FMA_CHAINS, "avoid_fma_chains", m_ZNVER1 | > m_ZNVER2 | m_ZNVER3 | m_ZNVER4 > + | m_YONGFENG | m_GENERIC) > > /* X86_TUNE_AVOID_256FMA_CHAINS: Avoid creating loops with tight 256bit or > smaller FMA chain. */ > -DEF_TUNE (X86_TUNE_AVOID_256FMA_CHAINS, "avoid_fma256_chains", m_ZNVER2 | > m_ZNVER3 > - | m_CORE_HYBRID | m_SAPPHIRERAPIDS | m_CORE_ATOM) > +DEF_TUNE (X86_TUNE_AVOID_256FMA_CHAINS, "avoid_fma256_chains", m_ZNVER2 | > m_ZNVER3 | m_ZNVER4 > + | m_CORE_HYBRID | m_SAPPHIRERAPIDS | m_CORE_ATOM | m_GENERIC) > > /* X86_TUNE_AVOID_512FMA_CHAINS: Avoid creating loops with tight 512bit or > smaller FMA chain. */ -- BR, Hongtao
