Enable AVX512 512bit vectors by default on Zen4
Commit Message
Hi,
this patch enables AVX512 by default on Zen4. While internally 512
registers are splits into two 256 halves, 512 bit vectors reduces number
of instructions to retire and has chance to improve paralelism.
There are few tsvc benchmarks that improves significantly:
runtime
benchmark 256bit 512bit
s2275 48.57 20.67 -58%
s311 32.29 16.06 -50%
s312 32.30 16.07 -50%
vsumr 32.30 16.07 -50%
s314 10.77 5.42 -50%
s313 21.52 10.85 -50%
vdotr 43.05 21.69 -50%
s316 10.80 5.64 -48%
s235 61.72 33.91 -45%
s161 15.91 9.95 -38%
s3251 32.13 20.31 -36%
And there are no benchmarks with off-noise regression. The basic matrix
multiplication loop improves by 32% (for 1000x1000 marices). It is also
expected that 512 bit vectors are more power effecient (I can't masure
that).
The down side is that loops with low trip counts slower for an iteration
ranges where the epilogue is hit more often. In SPECfp this problem
happens with x264 (12% regression) and bwaves (6% regression) and this
is tracked in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108410
and will need more work on vectorizer to support masked epilogues.
After some additional testing it seems that using 512 bit vectors by
default is now overall better choice.
Bootstrapped/regtested x86_64-linux. Plan to commit it tomorrow.
* config/i386/x86-tune.def (X86_TUNE_AVX256_OPTIMAL): Turn off
for znver4.
@@ -551,7 +551,7 @@ DEF_TUNE (X86_TUNE_AVX128_OPTIMAL, "avx128_optimal", m_BDVER | m_BTVER2
/* X86_TUNE_AVX256_OPTIMAL: Use 256-bit AVX instructions instead of 512-bit AVX
instructions in the auto-vectorizer. */
-DEF_TUNE (X86_TUNE_AVX256_OPTIMAL, "avx256_optimal", m_CORE_AVX512 | m_ZNVER4)
+DEF_TUNE (X86_TUNE_AVX256_OPTIMAL, "avx256_optimal", m_CORE_AVX512)
/* X86_TUNE_AVX256_SPLIT_REGS: if true, AVX512 ops are split into two AVX256 ops. */
DEF_TUNE (X86_TUNE_AVX512_SPLIT_REGS, "avx512_split_regs", m_ZNVER4)