[llvm] dc42ff6 - [InstCombine] add FIXME comment to shuffle transform; NFC

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2020-02-04T13:02:06-05:00
New Revision: dc42ff6697db1847a6e2f154de07f6e1f3d97ae4

URL: https://github.com/llvm/llvm-project/commit/dc42ff6697db1847a6e2f154de07f6e1f3d97ae4
DIFF: https://github.com/llvm/llvm-project/commit/dc42ff6697db1847a6e2f154de07f6e1f3d97ae4.diff

LOG: [InstCombine] add FIXME comment to shuffle transform; NFC

Existing tests:
rG5d04e008f708
rG2a191cf8500f
...should verify that the underlying analysis doesn't improve
too much without updating this user code.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index f6bc83347e05..6f22190582b3 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1701,6 +1701,9 @@ Instruction *InstCombiner::foldVectorBinop(BinaryOperator &Inst) {
         BO->getOpcode() != Opcode)
       return nullptr;
 
+    // FIXME: This may not be safe if the analysis allows undef elements. By
+    //        moving 'Y' before the splat shuffle, we are implicitly assuming
+    //        that it is not undef/poison at the splat index.
     Value *Y, *OtherOp;
     if (isSplatValue(BO->getOperand(0), SplatIndex->getZExtValue())) {
       Y = BO->getOperand(0);