[llvm] [InstCombine] Preserve profile after folding select instructions with conditionals (PR #159666)

[Mircea Trofin via llvm-commits]

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@@ -4521,8 +4522,21 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   auto FoldSelectWithAndOrCond = [&](bool IsAnd, Value *A,
                                      Value *B) -> Instruction * {
     if (Value *V = simplifySelectInst(B, TrueVal, FalseVal,
-                                      SQ.getWithInstruction(&SI)))
-      return SelectInst::Create(A, IsAnd ? V : TrueVal, IsAnd ? FalseVal : V);
+                                      SQ.getWithInstruction(&SI))) {
+      Value *NewTrueVal = IsAnd ? V : TrueVal;
+      Value *NewFalseVal = IsAnd ? FalseVal : V;
+
+      // If the True and False values don't change, then preserve the branch
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mtrofin wrote:

Neat! IIUC we can't determine the probability of the new condition because we only have the composite probability, but you're saying, if the values selected don't change, then the probability won't change, right?

Could the values get flipped, in which case you'd know the probability, too?

https://github.com/llvm/llvm-project/pull/159666