[IR] Disable select ? C : undef -> C fold in ConstantFoldSelectInstruction unless we know C isn't poison.

This matches the recent change to InstSimplify from D83440.

Differential Revision: https://reviews.llvm.org/D83535

1 files changed, 22 insertions, 2 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index f3c3e9a..f02246c 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -779,10 +779,30 @@ Constant *llvm::ConstantFoldSelectInstruction(Constant *Cond,
     if (isa<UndefValue>(V1)) return V1;
     return V2;
   }
-  if (isa<UndefValue>(V1)) return V2;
-  if (isa<UndefValue>(V2)) return V1;
+
   if (V1 == V2) return V1;
 
+  // If the true or false value is undef, we can fold to the other value as
+  // long as the other value isn't poison.
+  auto NotPoison = [](Constant *C) {
+    // TODO: We can analyze ConstExpr by opcode to determine if there is any
+    //       possibility of poison.
+    if (isa<ConstantExpr>(C))
+      return false;
+
+    if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) || isa<ConstantFP>(C) ||
+        isa<ConstantPointerNull>(C) || isa<Function>(C))
+      return true;
+
+    if (C->getType()->isVectorTy())
+      return !C->containsUndefElement() && !C->containsConstantExpression();
+
+    // TODO: Recursively analyze aggregates or other constants.
+    return false;
+  };
+  if (isa<UndefValue>(V1) && NotPoison(V2)) return V2;
+  if (isa<UndefValue>(V2) && NotPoison(V1)) return V1;
+
   if (ConstantExpr *TrueVal = dyn_cast<ConstantExpr>(V1)) {
     if (TrueVal->getOpcode() == Instruction::Select)
       if (TrueVal->getOperand(0) == Cond)