GVN: If a branch has two identical successors, we cannot declare either dead.

This previously caused miscompilations as a result of phi nodes receiving
undef incoming values from blocks dominated by such successors.

Differential Revision: http://reviews.llvm.org/D10726

llvm-svn: 240670

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index e93833c..60903c8 100644 (file)
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2804,6 +2804,10 @@ bool GVN::processFoldableCondBr(BranchInst *BI) {
   if (!BI || BI->isUnconditional())
     return false;
 
+  // If a branch has two identical successors, we cannot declare either dead.
+  if (BI->getSuccessor(0) == BI->getSuccessor(1))
+    return false;
+
   ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
   if (!Cond)
     return false;

diff --git a/llvm/test/Transforms/GVN/br-identical.ll b/llvm/test/Transforms/GVN/br-identical.ll
new file mode 100644 (file)
index 0000000..dfb7abe
--- /dev/null
+++ b/llvm/test/Transforms/GVN/br-identical.ll
@@ -0,0 +1,38 @@
+; RUN: opt -gvn -S -o - %s | FileCheck %s
+
+; If a branch has two identical successors, we cannot declare either dead.
+
+define void @widget(i1 %p) {
+entry:
+  br label %bb2
+
+bb2:
+  %t1 = phi i64 [ 0, %entry ], [ %t5, %bb7 ]
+  %t2 = add i64 %t1, 1
+  %t3 = icmp ult i64 0, %t2
+  br i1 %t3, label %bb3, label %bb4
+
+bb3:
+  %t4 = call i64 @f()
+  br label %bb4
+
+bb4:
+  ; CHECK-NOT: phi {{.*}} undef
+  %foo = phi i64 [ %t4, %bb3 ], [ 0, %bb2 ]
+  br i1 %p, label %bb5, label %bb6
+
+bb5:
+  br i1 true, label %bb7, label %bb7
+
+bb6:
+  br i1 true, label %bb7, label %bb7
+
+bb7:
+  %t5 = add i64 %t1, 1
+  br i1 %p, label %bb2, label %bb8
+
+bb8:
+  ret void
+}
+
+declare i64 @f()