[llvm] [InstCombine] Missing optimization: fold mul (select a, b), (select b, a) to mul a, b (PR #74953)

[Nikita Popov via llvm-commits]

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@@ -1132,6 +1132,13 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
   };
 
   if (LHSIsSelect && RHSIsSelect && A == D) {
+    // op(select(%v, %x, %y), select(%v, %y, %x)) --> op(%x, %y)
+    if (I.isCommutative() && B == F && C == E) {
+      Value *BI = Builder.CreateBinOp(I.getOpcode(), B, E);
+      BI->takeName(&I);
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nikic wrote:

It doesn't -- dropping poison generating flags undoes the preceding flag copy. You should only call copyIRFlags(), not dropPoisonGeneratingFlags().

https://github.com/llvm/llvm-project/pull/74953