Pip Cet writes:
> On Tue, Aug 18, 2020 at 6:52 AM Senthil Kumar Selvaraj > <senthil.theco...@gmail.com> wrote: >> > recognize such insns, but as it stands that define_insn would >> > recognize the incorrect insn: >> > >> > [(set (reg:QI 0) (const_int 0)) >> > (clobber (scratch:CC))] >> >> get_cc_reg_clobber_rtx also looks at the insn itself (i.e. whatever >> the erstwhile NOTICE_UPDATE_CC hook did), so if the cc attribute is LDI, >> and operands[1] is const0_rtx, it would return cc_reg_rtx as the clobber reg. >> >> AFAICT, some passes after reload verify if operands match constraints, >> and that would work in this case - I'm not sure if the pattern you >> mentioned could show up, outside of wrong splitters/peepholes in the md file. > > I don't think they can, but it's still another case of lying to GCC. > > At the very least, output_movqi should assert that it isn't asked to > produce code for this situation. I agree. > >> Another approach would be to conservatively use a 'c' constraint and >> clobber REG_CC for all reg-reg moves. > > I'd prefer splitting the constraint alternatives to have one clear-r0 > alternative, an ldi alternative, and a clear -r1_31 alternative. > > As for define_subst, is it really worth it? If I'm reading the > documentation correctly, it's not powerful enough to deal with scratch > operands on its own, so we'd probably need three or four variants of > define_subst just to handle those cases. Is this related to cmpelim not looking at REG_CC clobber if there are other (clobber scratch..) preceding it? > > I'm probably missing something obvious, but what's the reason for > keeping the CC-clobbering post-reload splitter when we already have a > CC-setting one? Are there significant post-reload optimizations that > can deal with a clobber but not an arbitrary assignment to CC? If so, > wouldn't it be better to fix those optimizations? The post-reload splitter introduces the clobber. The wiki suggests that approach if most insns clobber REG_CC, perhaps because of the missed optimizations you describe below? > > (There are at least some pre-reload optimizations that don't work with > the CC-clobbering insn patterns. lower_subreg.c's simple_move, for > example, recognizes only two-operand sets. This resulted in pessimized > code, but is easy to fix.) Regards Senthil
