[SimplifyCFG] Improve FoldTwoEntryPHINode when one of phi values is undef

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3376,7 +3376,8 @@ static bool FoldCondBranchOnValueKnownInPredecessor(BranchInst *BI,
 /// Given a BB that starts with the specified two-entry PHI node,
 /// see if we can eliminate it.
 static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
-                                DomTreeUpdater *DTU, const DataLayout &DL) {
+                                DomTreeUpdater *DTU, const DataLayout &DL,
+                                AssumptionCache *AC) {
   // Ok, this is a two entry PHI node.  Check to see if this is a simple "if
   // statement", which has a very simple dominance structure.  Basically, we
   // are trying to find the condition that is being branched on, which
@@ -3543,9 +3544,20 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
     Value *TrueVal = PN->getIncomingValueForBlock(IfTrue);
     Value *FalseVal = PN->getIncomingValueForBlock(IfFalse);
 
-    Value *Sel = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", DomBI);
-    PN->replaceAllUsesWith(Sel);
-    Sel->takeName(PN);
+    Value *ReplaceVal = nullptr;
+    if (isa<UndefValue>(TrueVal) &&
+        isGuaranteedNotToBePoison(FalseVal, AC, IfFalse->getTerminator(),
+                                  /*DT*/ nullptr, /*Depth*/ 0)) {
+      ReplaceVal = FalseVal;
+    } else if (isa<UndefValue>(FalseVal) &&
+               isGuaranteedNotToBePoison(TrueVal, AC, IfTrue->getTerminator(),
+                                         /*DT*/ nullptr, /*Depth*/ 0)) {
+      ReplaceVal = TrueVal;
+    } else {
+      ReplaceVal = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", DomBI);
+      ReplaceVal->takeName(PN);
+    }
+    PN->replaceAllUsesWith(ReplaceVal);
     PN->eraseFromParent();
   }
 
@@ -7406,7 +7418,7 @@ bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
     // eliminate it, do so now.
     if (auto *PN = dyn_cast<PHINode>(BB->begin()))
       if (PN->getNumIncomingValues() == 2)
-        if (FoldTwoEntryPHINode(PN, TTI, DTU, DL))
+        if (FoldTwoEntryPHINode(PN, TTI, DTU, DL, Options.AC))
           return true;
   }
 

llvm/test/Transforms/SimplifyCFG/pr67342.ll

@@ -0,0 +1,50 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+; Tests from PR67342
+define i16 @test1(i32 %err) {
+; CHECK-LABEL: define i16 @test1(
+; CHECK-SAME: i32 [[ERR:%.*]]) {
+; CHECK-NEXT:  bb3:
+; CHECK-NEXT:    [[_3:%.*]] = icmp slt i32 [[ERR]], 0
+; CHECK-NEXT:    [[OK:%.*]] = trunc i32 [[ERR]] to i16
+; CHECK-NEXT:    ret i16 [[OK]]
+;
+  %_3 = icmp slt i32 %err, 0
+  br i1 %_3, label %bb1, label %bb2
+
+bb2:
+  %ok = trunc i32 %err to i16
+  br label %bb3
+
+bb1:
+  br label %bb3
+
+bb3:
+  %r.sroa.3.0 = phi i16 [ undef, %bb1 ], [ %ok, %bb2 ]
+  ret i16 %r.sroa.3.0
+}
+
+; commuted test
+define i16 @test2(i32 %err) {
+; CHECK-LABEL: define i16 @test2(
+; CHECK-SAME: i32 [[ERR:%.*]]) {
+; CHECK-NEXT:  bb3:
+; CHECK-NEXT:    [[_3:%.*]] = icmp slt i32 [[ERR]], 0
+; CHECK-NEXT:    [[OK:%.*]] = trunc i32 [[ERR]] to i16
+; CHECK-NEXT:    ret i16 [[OK]]
+;
+  %_3 = icmp slt i32 %err, 0
+  br i1 %_3, label %bb1, label %bb2
+
+bb2:
+  br label %bb3
+
+bb1:
+  %ok = trunc i32 %err to i16
+  br label %bb3
+
+bb3:
+  %r.sroa.3.0 = phi i16 [ %ok, %bb1 ], [ undef, %bb2 ]
+  ret i16 %r.sroa.3.0
+}