https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77270
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
The problem is in the fact that for -march=native, the driver will pass
-mno-prfchw, since the relevant bit is not present in cpuid flags.
Following code in i386.c:
/* Enable prefetch{,w} instructions for -m3dnow and -mprefetchwt1. */
if (TARGET_3DNOW_P (opts->x_ix86_isa_flags)
|| TARGET_PREFETCHWT1_P (opts->x_ix86_isa_flags))
opts->x_ix86_isa_flags
|= OPTION_MASK_ISA_PRFCHW & ~opts->x_ix86_isa_flags_explicit;
will then try to set the option, but it won't succeed, due to the above
handling of explicit compile flags.
