[Bug target/91652] -march=skylake-avx512 -mno-fma -O2 generates FMA instructions

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91652

Jakub Jelinek  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |INVALID

--- Comment #3 from Jakub Jelinek  ---
I don't believe this is a bug.
If you look up the Intel CPU manuals, the FMA CPUID Feature Flag guards the VEX
encoded VF{,N}M{ADD,SUB,ADDSUB}{132,213,231}{S,P}{S,D} instructions, but not
the EVEX encoded ones, those are guarded just by AVX512F or AVX512F and
AVX512VL CPUID Feature Flags.  Thus, -mno-fma shouldn't affect them,
-mno-avx512f should.
And you can use -ffp-contract= to decide if code that isn't already using
explicit fma should be converted to FMA or not if the instructions are
available.

[Bug target/91652] -march=skylake-avx512 -mno-fma -O2 generates FMA instructions

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91652

Jakub Jelinek  changed:

   What|Removed |Added

 CC||jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek  ---
If you don't want fma matched for code that originally didn't have it, perhaps
better use -ffp-contract=off ?

[Bug target/91652] -march=skylake-avx512 -mno-fma -O2 generates FMA instructions

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91652

H.J. Lu  changed:

   What|Removed |Added

 CC||hjl.tools at gmail dot com

--- Comment #1 from H.J. Lu  ---
VEX FMA is enabled by -mfma and EVEX FMA is enabled by -mavx512f.
-mno-fma disables VEX FMA, but not EVEX FMA.  Adding -mno-avx512f
disables EVEX FMA.