[llvm] [RFC][Transforms] Prefer unreachable insn over optimizaiton with undef (PR #131020)

[via llvm-commits]

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@@ -4829,9 +4829,9 @@ static Value *simplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
     if (isa<PoisonValue>(CondC))
       return PoisonValue::get(TrueVal->getType());
 
-    // select undef, X, Y -> X or Y
+    // select undef, X, Y -> undef
     if (Q.isUndefValue(CondC))
-      return isa<Constant>(FalseVal) ? FalseVal : TrueVal;
+      return UndefValue::get(TrueVal->getType());
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yonghong-song wrote:

Thanks for clarification. Do we have any way to expose such a transformation to user?  For example, through some Diagnostic or something else? This will be very useful for users to get early signal about their codes.

https://github.com/llvm/llvm-project/pull/131020