On Mon, 11 Apr 2022 at 23:14, Richard Henderson <richard.hender...@linaro.org> wrote: > > On 4/11/22 09:18, Peter Maydell wrote: > >> + ESB ---- 0011 0010 0000 1111 ---- 0001 0000 > >> + ] > > > > Why don't we decode bits [11:8] here? I see it's the same > > as YIELD/WFE/WFI, but I'm not sure why we're not decoding > > those bits in those insns either... > > See page F4-7074 in H.a, where bits [11:8] of the imm12 field are described > with 'xxxx'.
Hmm. That just means "decodes to the NOP/WFI/ESB/whatever instruction-description whatever the value of those bits", but when the specific instruction-description then marks those bits as "(0)" or "(1)", that has the usual CONSTRAINED UNPREDICTABLE meaning described in section F1.7.2, where we get a free choice of UNDEF, NOP, ignore the bit, or any-dest-regs-are-UNKNOWN. So we're within the spec to not decode [11:8] but I think it would be more consistent with how we try to handle those (0) and (1) bits generally if we insist that [11:8] is all zeroes here. For this series, I guess go along with the current way we handle hint instructions, and maybe fix this as a separate cleanup later. -- PMM
