[PATCH] D94380: [InstCombine] Update transformations to use poison for insertelement/shufflevector's placeholder value (1/2)

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added inline comments.


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp:1728
       // Vec.
       if (IdxN % DstNumElts != 0 || IdxN + DstNumElts > VecNumElts) {
         replaceInstUsesWith(CI, UndefValue::get(CI.getType()));
----------------
Guarded by this branch condition, the for loop at line 1740 never adds a mask that makes `Shuffle` point to the second operand vector (which was previously Undef, but now Poison).


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:2613
-      return InsertElementInst::Create(UndefValue::get(DestTy), Elem,
-                     Constant::getNullValue(Type::getInt32Ty(CI.getContext())));
     }
----------------
This creates `insertelement <1 x X86_mmx> undef, Elem, 0`
Replacing undef with poison has no visible effect.


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Comment at: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp:1693
       Value *NewRHS = ConstOp1 ? NewC : V1;
-      return createBinOpShuffle(NewLHS, NewRHS, Mask);
+      return createBinOpShuffle(NewLHS, NewRHS, Mask, false);
     }
----------------
Be conservative, and create shufflevector with undef if Mask may point to the second vector


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D94380/new/

https://reviews.llvm.org/D94380