[InstSimplify] Don't fold phi of poison and trapping const expr (PR49839)

Folding this case would result in the constant expression being
executed unconditionally, which may introduce a new trap.

Fixes llvm#49839.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4817,11 +4817,18 @@ static Value *SimplifyPHINode(PHINode *PN, ArrayRef<Value *> IncomingValues,
   if (!CommonValue)
     return UndefValue::get(PN->getType());
 
-  // If we have a PHI node like phi(X, undef, X), where X is defined by some
-  // instruction, we cannot return X as the result of the PHI node unless it
-  // dominates the PHI block.
-  if (HasUndefInput)
+  if (HasUndefInput) {
+    // We cannot start executing a trapping constant expression on more control
+    // flow paths.
+    auto *CE = dyn_cast<ConstantExpr>(CommonValue);
+    if (CE && CE->canTrap())
+      return nullptr;
+
+    // If we have a PHI node like phi(X, undef, X), where X is defined by some
+    // instruction, we cannot return X as the result of the PHI node unless it
+    // dominates the PHI block.
     return valueDominatesPHI(CommonValue, PN, Q.DT) ? CommonValue : nullptr;
+  }
 
   return CommonValue;
 }

llvm/test/Transforms/InstSimplify/phi.ll

@@ -162,7 +162,8 @@ define i64 @pr49839_with_poison(i1 %c) {
 ; CHECK:       if:
 ; CHECK-NEXT:    br label [[JOIN]]
 ; CHECK:       join:
-; CHECK-NEXT:    ret i64 srem (i64 1, i64 ptrtoint (i32* @g to i64))
+; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ poison, [[IF]] ], [ srem (i64 1, i64 ptrtoint (i32* @g to i64)), [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i64 [[PHI]]
 ;
 entry:
   br i1 %c, label %if, label %join