https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91652
Bug ID: 91652 Summary: -march=skylake-avx512 -mno-fma -O2 generates FMA instructions Product: gcc Version: 9.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: joseph.weening at gmail dot com Target Milestone: --- Created attachment 46807 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46807&action=edit program to demonstrate generation of FMA instructions On x86_64, compiling C code with -march=skylake-avx512 -mno-fma -O2 generates FMA instructions. (-mno-fma works correctly with -march=skylake or -march=haswell.) The attached program demonstrates the problem. It prints "d is 0.000000" when not using FMA, and "d is -1.000000" when using FMA. % gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/var/tmp/gcc/libexec/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: ../gcc-9.2.0/configure --prefix=/var/tmp/gcc --disable-multilib --enable-languages=c --with-gmp=/usr/local/gmp/6.1.2 --with-mpfr=/usr/local/mpfr/4.0.1 --with-mpc=/usr/local/mpc/1.1.0 Thread model: posix gcc version 9.2.0 (GCC) Command to demonstrate the bug: % gcc -march=skylake-avx512 -mno-fma -O2 fmabug.i && a.out Compiler errors, warnings: (none)