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3114
2020 Nov 19
1
Problems with undef subranges in identity copies
...y
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 special case has been an
issue.
2. Should effectively empty segments be a verifier error? I'm not sure
what the exact rule wou...