[PATCH] D84948: [InstCombine] Fold select/and/or with freeze(undef) that is used in the op only

Fri Jul 31 00:35:08 PDT 2020

nikic added inline comments.

================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:1803
+      match(Op1, m_OneUse(m_Freeze(m_Undef()))))
+    return replaceInstUsesWith(I, Constant::getNullValue(Op0->getType()));
+
----------------
So, these changes duplicate some undef folds from InstSimplify (here: https://github.com/llvm/llvm-project/blob/9b04fec0021112d913058e81bf864e4f33c33dcc/llvm/lib/Analysis/InstructionSimplify.cpp#L2004-L2006) into InstCombine.

I think it would be legal to instead make the fold in InstSimplify also work for one-use freeze of undef. Per the rules, the InstSimplify result needs to be used for a replacement, which will also remove the freeze. Of course, this will only really be safe in conjunction with D84792 for consumers that don't perform replacements.

Though maybe, if we consider the long-term direction (undef replaced by freeze poison), having it in InstCombine and dropping the InstSimplify folds may make more sense -- because we could also fold the multi-use case here, if we replace the whole freeze instruction.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D84948/new/

https://reviews.llvm.org/D84948