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2020 Nov 19
1
Problems with undef subranges in identity copies
...r0 undef %0.sub1:vreg_64 = COPY killed $vgpr0 bb.1: %1:vreg_64 = COPY killed %0 %0:vreg_64 = COPY %1 // Indirect identity copy S_CBRANCH_EXECNZ %bb.1, implicit $exec bb.2: undef %2.sub1:vreg_64 = COPY %0.sub1 %0:vreg_64 = COPY killed %2 S_BRANCH %bb.1 ... Here, pruneSubRegValues (https://github.com/llvm/llvm-project/blob/3ded927cf80ac519f9f9c4664fef08787f7c537d/llvm/lib/CodeGen/RegisterCoalescer.cpp#L3114) ends up leaving behind a similarly dead segment. 1. Is omitting undef lanes from the live segments a good idea in the first place? This isn't the first time this...