> >I've observed SPEC2006 failure on avx512-vlbwdq branch. > >It was caused by hardreg_cprop. In maybe_mode_change it was > >assumed, that all values of the same register class and same mode. > >are ok. This is not the case for i386/avx512. We need to honor > >HARD_REGNO_MODE_OK.
> One could argue that having a class where some members are OK for being used > in a particular mode, but other members are not is the core issue here. > > Can you describe a bit about why you've got a class of that nature? > Background on that would be useful. > AVX512 added new 16 xmm registers (xmm16-xmm31). Those registers require evex encoding. Only 512-bit wide versions of instructions have evex encoding with avx512f, but all versions have it with avx512vl. Most instructions have same macroized pattern for 128/256/512 vector length. They all use constraint 'v', which corresponds to class ALL_SSE_REGS (xmm0 - xmm31). To disallow e. g. xmm20 in 256-bit case (avx512f) and allow it only in avx512vl case we have HARD_REGNO_MODE_OK checking for regno being evex-only and disallowing it if mode is not 512-bit.
