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

[Peter Collingbourne]

Hi spatel,

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

http://reviews.llvm.org/D10726

Files:
  lib/Transforms/Scalar/GVN.cpp
  test/Transforms/GVN/br-identical.ll

Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -2808,6 +2808,10 @@
   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;
Index: test/Transforms/GVN/br-identical.ll
===================================================================
--- /dev/null
+++ test/Transforms/GVN/br-identical.ll
@@ -0,0 +1,61 @@
+; RUN: opt -gvn -S -o - %s | FileCheck %s
+
+define void @widget() {
+bb21:
+  br label %bb22
+
+bb22:
+  %tmp = phi i64 [ 0, %bb21 ], [ %tmp75, %bb74 ]
+  %tmp24 = add i64 %tmp, 1
+  br i1 undef, label %bb27, label %bb59
+
+bb27:
+  %tmp28 = sub i64 %tmp24, 0
+  %tmp29 = icmp ult i64 0, %tmp28
+  br i1 %tmp29, label %bb30, label %bb55
+
+bb30:
+  br label %bb32
+
+bb32:
+  br i1 undef, label %bb38, label %bb32
+
+  %tmp35 = call i8* @f()
+  br label %bb36
+
+bb36:
+  br label %bb38
+
+bb38:
+  %tmp39 = phi i8* [ %tmp35, %bb36 ], [ null, %bb32 ]
+  %tmp40 = bitcast i8* %tmp39 to double*
+  br label %bb52
+
+bb52:
+  %tmp53 = getelementptr double, double* %tmp40
+  %tmp54 = ptrtoint double* %tmp53 to i64
+  br label %bb55
+
+bb55:
+  ; CHECK-NOT: phi {{.*}} undef
+  %1 = phi i64 [ %tmp54, %bb52 ], [ 0, %bb27 ]
+  br label %bb59
+
+bb59:
+  br i1 undef, label %bb73, label %bb72
+
+bb72:
+  br i1 true, label %bb74, label %bb74
+
+bb73:
+  br i1 true, label %bb74, label %bb74
+
+bb74:
+  %tmp75 = add i64 %tmp, 1
+  br i1 undef, label %bb22, label %bb77
+
+bb77:
+  ret void
+}
+
+declare i8* @f()

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