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* [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified @ 2020-11-03 18:05 vincenzo.innocente at cern dot ch 2020-11-04 8:24 ` [Bug tree-optimization/97707] avx512 " rguenth at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: vincenzo.innocente at cern dot ch @ 2020-11-03 18:05 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97707 Bug ID: 97707 Summary: avx12 math function invoked even if -mprefer-vector-width=256 specified Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: vincenzo.innocente at cern dot ch Target Milestone: --- this code will invoke _ZGVeN8v_sin instead of _ZGVdN4v_sin making use of zmm registers #include<cmath> int main() { double res=0; for (int x=0; x<1024;x++) { double y = x; res += std::sin(y); } return res > 0.5; } NOTE if I specify for (long long x=0; x<1024;x++) { it will correcty invoke _ZGVdN4v_sin (no zmm) compiler options -Ofast -march=skylake-avx512 -mprefer-vector-width=256 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/97707] avx512 math function invoked even if -mprefer-vector-width=256 specified 2020-11-03 18:05 [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified vincenzo.innocente at cern dot ch @ 2020-11-04 8:24 ` rguenth at gcc dot gnu.org 2020-11-04 8:47 ` jakub at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: rguenth at gcc dot gnu.org @ 2020-11-04 8:24 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97707 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW CC| |jakub at gcc dot gnu.org Blocks| |53947 Target| |x86_64-*-* i?86-*-* Ever confirmed|0 |1 Last reconfirmed| |2020-11-04 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- -mprefer-vector-width=256 means it _prefers_ AVX256 but then in case vectorization fails (for whatever bogus reason) with AVX256 it will still try AVX512: static unsigned int ix86_autovectorize_vector_modes (vector_modes *modes, bool all) { if (TARGET_AVX512F && !TARGET_PREFER_AVX256) { modes->safe_push (V64QImode); modes->safe_push (V32QImode); modes->safe_push (V16QImode); } else if (TARGET_AVX512F && all) { modes->safe_push (V32QImode); modes->safe_push (V16QImode); modes->safe_push (V64QImode); } the odd thing is that the dumps suggest we use V4DFmode. We use a VF of 8 because of V8SI and it seems that call vectorization then chooses the larger vector mode because it is supported - it seems to have its own idea of costing there (probably reasonable). Do you think this is a bad idea? Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947 [Bug 53947] [meta-bug] vectorizer missed-optimizations ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/97707] avx512 math function invoked even if -mprefer-vector-width=256 specified 2020-11-03 18:05 [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified vincenzo.innocente at cern dot ch 2020-11-04 8:24 ` [Bug tree-optimization/97707] avx512 " rguenth at gcc dot gnu.org @ 2020-11-04 8:47 ` jakub at gcc dot gnu.org 2020-11-04 8:57 ` vincenzo.innocente at cern dot ch 2020-11-04 8:59 ` jakub at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: jakub at gcc dot gnu.org @ 2020-11-04 8:47 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97707 --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- The prefer option is exactly that, try that first and if it fails, use the others. So I guess the main question is why it failed for the VF=4 case. E.g. for -mavx512f it wouldn't surprise me as much, because then AVX512VL is not in and it could be that AVX2 isn't able to deal with that. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/97707] avx512 math function invoked even if -mprefer-vector-width=256 specified 2020-11-03 18:05 [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified vincenzo.innocente at cern dot ch 2020-11-04 8:24 ` [Bug tree-optimization/97707] avx512 " rguenth at gcc dot gnu.org 2020-11-04 8:47 ` jakub at gcc dot gnu.org @ 2020-11-04 8:57 ` vincenzo.innocente at cern dot ch 2020-11-04 8:59 ` jakub at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: vincenzo.innocente at cern dot ch @ 2020-11-04 8:57 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97707 --- Comment #3 from vincenzo Innocente <vincenzo.innocente at cern dot ch> --- the main point in using -mprefer-vector-width=256 is to avoid clock throttling in "mixed" workloads. In small benchmarks like this one avx512 is faster (even on an old Silver) even if trigger a slower clock. (and the test should be performed with the machine fully loaded). Still if I ask -mprefer-vector-width=256 I would like to see no 512-wide instructions to be used. A disturbing feature is also the difference between using int or long long as loop index. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/97707] avx512 math function invoked even if -mprefer-vector-width=256 specified 2020-11-03 18:05 [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified vincenzo.innocente at cern dot ch ` (2 preceding siblings ...) 2020-11-04 8:57 ` vincenzo.innocente at cern dot ch @ 2020-11-04 8:59 ` jakub at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: jakub at gcc dot gnu.org @ 2020-11-04 8:59 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97707 --- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- If you want no AVX512* instructions, the option to say that is -mno-avx512f ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2020-11-04 8:59 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2020-11-03 18:05 [Bug tree-optimization/97707] New: avx12 math function invoked even if -mprefer-vector-width=256 specified vincenzo.innocente at cern dot ch 2020-11-04 8:24 ` [Bug tree-optimization/97707] avx512 " rguenth at gcc dot gnu.org 2020-11-04 8:47 ` jakub at gcc dot gnu.org 2020-11-04 8:57 ` vincenzo.innocente at cern dot ch 2020-11-04 8:59 ` jakub at gcc dot gnu.org
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