2 files changed, 60 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 0dde18522f4..bc9574221b3 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1010,6 +1010,8 @@ static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
   return true;
 }
 
+static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I);
+
 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
 /// BB2, hoist any common code in the two blocks up into the branch block.  The
 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
@@ -1094,6 +1096,12 @@ HoistTerminator:
       if (BB1V == BB2V)
         continue;
 
+      // Check for passingValueIsAlwaysUndefined here because we would rather
+      // eliminate undefined control flow then converting it to a select.
+      if (passingValueIsAlwaysUndefined(BB1V, PN) ||
+          passingValueIsAlwaysUndefined(BB2V, PN))
+       return Changed;
+
       if (isa<ConstantExpr>(BB1V) && !isSafeToSpeculativelyExecute(BB1V, DL))
         return Changed;
       if (isa<ConstantExpr>(BB2V) && !isSafeToSpeculativelyExecute(BB2V, DL))
@@ -1508,6 +1516,11 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
     if (ThenV == OrigV)
       continue;
 
+    // Don't convert to selects if we could remove undefined behavior instead.
+    if (passingValueIsAlwaysUndefined(OrigV, PN) ||
+        passingValueIsAlwaysUndefined(ThenV, PN))
+      return false;
+
     HaveRewritablePHIs = true;
     ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
     ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
diff --git a/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll b/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
index 5fff37bc71f..21428c62f53 100644
--- a/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
+++ b/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
@@ -71,3 +71,50 @@ U:
 T:
         ret i32 2
 }
+
+
+;; We can either convert the following control-flow to a select or remove the
+;; unreachable control flow because of the undef store of null. Make sure we do
+;; the latter.
+
+define void @test5(i1 %cond, i8* %ptr) {
+
+; CHECK-LABEL: test5
+; CHECK: entry:
+; CHECK-NOT: select
+; CHECK:  store i8 2, i8* %ptr
+; CHECK:  ret
+
+entry:
+  br i1 %cond, label %bb1, label %bb3
+
+bb3:
+ br label %bb2
+
+bb1:
+ br label %bb2
+
+bb2:
+  %ptr.2 = phi i8* [ %ptr, %bb3 ], [ null, %bb1 ]
+  store i8 2, i8* %ptr.2, align 8
+  ret void
+}
+
+; CHECK-LABEL: test6
+; CHECK: entry:
+; CHECK-NOT: select
+; CHECK:  store i8 2, i8* %ptr
+; CHECK:  ret
+
+define void @test6(i1 %cond, i8* %ptr) {
+entry:
+  br i1 %cond, label %bb1, label %bb2
+
+bb1:
+  br label %bb2
+
+bb2:
+  %ptr.2 = phi i8* [ %ptr, %entry ], [ null, %bb1 ]
+  store i8 2, i8* %ptr.2, align 8
+  ret void
+}