[llvm] [InstCombine] Enable select freeze poison folding when storing value (PR #129776)

[John McIver via llvm-commits]

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@@ -4812,15 +4812,20 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
   //
   // TODO: This could use getBinopAbsorber() / getBinopIdentity() to avoid
   //       duplicating logic for binops at least.
-  auto getUndefReplacement = [&I](Type *Ty) {
-    Constant *BestValue = nullptr;
-    Constant *NullValue = Constant::getNullValue(Ty);
+  auto getUndefReplacement = [&I, &AC = this->AC, &DT = this->DT](Type *Ty) {
+    Value *BestValue = nullptr;
+    Value *NullValue = Constant::getNullValue(Ty);
     for (const auto *U : I.users()) {
-      Constant *C = NullValue;
+      Value *C = NullValue;
       if (match(U, m_Or(m_Value(), m_Value())))
         C = ConstantInt::getAllOnesValue(Ty);
       else if (match(U, m_Select(m_Specific(&I), m_Constant(), m_Value())))
         C = ConstantInt::getTrue(Ty);
+      else if (match(&I, m_Freeze(m_Poison())) &&
+               match(U, m_c_Select(m_Specific(&I), m_Value(C)))) {
+        if (!isGuaranteedNotToBeUndefOrPoison(C, &AC, &I, &DT))
----------------
jmciver wrote:

I believe we need `isGuaranteedNotToBeUndefOrPoison` to check each of the select instructions otherwise we might incorrectly pick the replacement, i.e., is the select argument nonundef.

This is what I am thinking:
```c++
auto getUndefReplacement = [&I, &AC = this->AC, &DT = this->DT](Type *Ty) {
    Value *BestValue = nullptr;
    Value *NullValue = Constant::getNullValue(Ty);
    for (const auto *U : I.users()) {
      Value *C = NullValue;
      Value *SelectArgument = nullptr;
      if (match(U, m_Or(m_Value(), m_Value())))
        C = ConstantInt::getAllOnesValue(Ty);
      else if (match(U, m_Select(m_Specific(&I), m_Constant(), m_Value())))
        C = ConstantInt::getTrue(Ty);
      else if (match(&I, m_Freeze(m_Poison())) &&
               match(U, m_c_Select(m_Specific(&I), m_Value(SelectArgument))) &&
               isGuaranteedNotToBeUndefOrPoison(SelectArgument, &AC, &I, &DT))
        C = SelectArgument;

      if (!BestValue)
        BestValue = C;
      else if (BestValue != C)
        BestValue = NullValue;
    }
    assert(BestValue && "Must have at least one use");
    return BestValue;
  };
```
Let me know if I am misinterpreting your suggestion.


https://github.com/llvm/llvm-project/pull/129776