[InstSimplify] Fold all poison phi to poison instead of undef

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -5248,11 +5248,16 @@ static Value *simplifyPHINode(PHINode *PN, ArrayRef<Value *> IncomingValues,
   // If all of the PHI's incoming values are the same then replace the PHI node
   // with the common value.
   Value *CommonValue = nullptr;
+  bool HasPoisonInput = false;
   bool HasUndefInput = false;
   for (Value *Incoming : IncomingValues) {
     // If the incoming value is the phi node itself, it can safely be skipped.
     if (Incoming == PN)
       continue;
+    if (isa<PoisonValue>(Incoming)) {
+      HasPoisonInput = true;
+      continue;
+    }
     if (Q.isUndefValue(Incoming)) {
       // Remember that we saw an undef value, but otherwise ignore them.
       HasUndefInput = true;
@@ -5263,12 +5268,13 @@ static Value *simplifyPHINode(PHINode *PN, ArrayRef<Value *> IncomingValues,
     CommonValue = Incoming;
   }
 
-  // If CommonValue is null then all of the incoming values were either undef or
-  // equal to the phi node itself.
+  // If CommonValue is null then all of the incoming values were either undef,
+  // poison or equal to the phi node itself.
   if (!CommonValue)
-    return UndefValue::get(PN->getType());
+    return HasUndefInput ? UndefValue::get(PN->getType())
+                         : PoisonValue::get(PN->getType());
 
-  if (HasUndefInput) {
+  if (HasPoisonInput || HasUndefInput) {
     // 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.

llvm/test/Transforms/InstCombine/shift.ll

@@ -1151,7 +1151,7 @@ define void @test61(i128 %arg, i1 %c1, i1 %c2, i1 %c3, i1 %c4) {
 ; CHECK:       bb7:
 ; CHECK-NEXT:    br i1 [[C3:%.*]], label [[BB8]], label [[BB2]]
 ; CHECK:       bb8:
-; CHECK-NEXT:    br i1 undef, label [[BB11:%.*]], label [[BB12]]
+; CHECK-NEXT:    br i1 poison, label [[BB11:%.*]], label [[BB12]]
 ; CHECK:       bb11:
 ; CHECK-NEXT:    br i1 [[C4:%.*]], label [[BB1]], label [[BB12]]
 ; CHECK:       bb12:

llvm/test/Transforms/InstSimplify/phi.ll

@@ -141,7 +141,7 @@ define i8 @only_poison(i1 %cond) {
 ; CHECK:       B:
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       EXIT:
-; CHECK-NEXT:    ret i8 undef
+; CHECK-NEXT:    ret i8 poison
 ;
   br i1 %cond, label %A, label %B
 A:

llvm/test/Transforms/LoopDeletion/update-scev.ll

@@ -66,7 +66,7 @@ define void @test2(ptr %bx, i64 %by) local_unnamed_addr align 2 {
 
 ; SCEV-EXPRS-LABEL: test2
 ; SCEV-EXPRS:     %inc.lcssa.1 = phi i64 [ poison, %for.body7.preheader.1 ]
-; SCEV-EXPRS-NEXT: -->  undef
+; SCEV-EXPRS-NEXT: -->  poison
 entry:
   %cmp = icmp sgt i64 %by, 0
   br label %for.cond.preheader