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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hi Tim (and other reviewers),<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>MachineCopyPropagation has special logic for removing COPY instructions. It will remove plain COPYs using eraseFromParent(), but if the COPY has imp-defs/imp-uses it will convert it to a KILL, to keep the imp-def around.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This actually totally breaks and causes the machine verifier to cry in several cases, one of which being:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>%RAX<def> = COPY %RCX<kill><o:p></o:p></p><p class=MsoNormal>%ECX<def> = COPY %EAX<kill>, %RAX<imp-use,kill><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>These subregister copies are together identified as noops, so are both removed. However, the second one as it has an imp-use gets converted into a kill:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>%ECX<def> = KILL %EAX<kill>, %RAX<imp-use,kill><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>As the original COPY has been removed, the verifier goes into tears at the use of undefined EAX and RAX.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>There are several hacky solutions to this hacky problem (which is all to do with imp-use/def weirdnesses), but the least hacky I’ve come up with is to *<b>always</b>* remove COPYs by converting to KILLs. KILLs are no-ops to the code generator so the generated code doesn’t change (which is why they were partially used in the first place), but using them also keeps the def/use and imp-def/imp-use chains alive:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>%RAX<def> = KILL %RCX<kill><o:p></o:p></p><p class=MsoNormal>%ECX<def> = KILL %EAX<kill>, %RAX<imp-use,kill><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The patch passes all test cases including the ones that check the removal of MOVs in this circumstance, along with an extra test I added to check subregister behaviour (which made the machine verifier fall over before my patch).<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The patch also adds some DEBUG() statements because the file hadn’t got any.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Please review!<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Cheers,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>James<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>