Zen5 tuning part 1: avoid FMA chains

testing matrix multiplication benchmarks shows that FMA on a critical chain
is a perofrmance loss over separate multiply and add. While the latency of 4
is lower than multiply + add (3+2) the problem is that all values needs to
be ready before computation starts.

While on znver4 AVX512 code fared well with FMA, it was because of the split
registers. Znver5 benefits from avoding FMA on all widths.  This may be different
with the mobile version though.

On naive matrix multiplication benchmark the difference is 8% with -O3
only since with -Ofast loop interchange solves the problem differently.
It is 30% win, for example, on S323 from TSVC:

real_t s323(struct args_t * func_args)
{

//    recurrences
//    coupled recurrence

    initialise_arrays(__func__);
    gettimeofday(&func_args->t1, NULL);

    for (int nl = 0; nl < iterations/2; nl++) {
        for (int i = 1; i < LEN_1D; i++) {
            a[i] = b[i-1] + c[i] * d[i];
            b[i] = a[i] + c[i] * e[i];
        }
        dummy(a, b, c, d, e, aa, bb, cc, 0.);
    }

    gettimeofday(&func_args->t2, NULL);
    return calc_checksum(__func__);
}

gcc/ChangeLog:

	* config/i386/x86-tune.def (X86_TUNE_AVOID_128FMA_CHAINS): Enable for
	znver5.
	(X86_TUNE_AVOID_256FMA_CHAINS): Likewise.
	(X86_TUNE_AVOID_512FMA_CHAINS): Likewise.

diff --git a/gcc/config/i386/x86-tune.def b/gcc/config/i386/x86-tune.def
index 3d29bffc49c32724dc8ec7f3546d69dd38082b50..da1a3d6a3c6c00b3823d7c16035948b84103e56f 100644 (file)
--- a/gcc/config/i386/x86-tune.def
+++ b/gcc/config/i386/x86-tune.def
@@ -508,17 +508,18 @@ 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_ZNVER4
+DEF_TUNE (X86_TUNE_AVOID_128FMA_CHAINS, "avoid_fma_chains", m_ZNVER
           | m_YONGFENG | m_SHIJIDADAO | 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_ZNVER4
-         | m_CORE_HYBRID | m_SAPPHIRERAPIDS | m_CORE_ATOM | m_GENERIC)
+DEF_TUNE (X86_TUNE_AVOID_256FMA_CHAINS, "avoid_fma256_chains",
+         m_ZNVER2 | m_ZNVER3 | m_ZNVER4 | m_ZNVER5 | 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.  */
-DEF_TUNE (X86_TUNE_AVOID_512FMA_CHAINS, "avoid_fma512_chains", m_NONE)
+DEF_TUNE (X86_TUNE_AVOID_512FMA_CHAINS, "avoid_fma512_chains", m_ZNVER5)
 
 /* X86_TUNE_V2DF_REDUCTION_PREFER_PHADDPD: Prefer haddpd
    for v2df vector reduction.  */