diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 2d60c35afb1..dce4e7abab1 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2723,6 +2723,72 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
   return false;
 }
 
+/// Check if passing a value to an instruction will cause undefined behavior.
+static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
+  Constant *C = dyn_cast<Constant>(V);
+  if (!C)
+    return false;
+
+  if (!I->hasOneUse()) // FIXME: There is no reason to limit this to one use.
+    return false;
+
+  if (C->isNullValue()) {
+    Instruction *Use = I->use_back();
+
+    // Now make sure that there are no instructions in between that can alter
+    // control flow (eg. calls)
+    for (BasicBlock::iterator i = ++BasicBlock::iterator(I); &*i != Use; ++i)
+      if (i == I->getParent()->end() ||
+          !i->isSafeToSpeculativelyExecute())
+        return false;
+
+    // Look through GEPs. A load from a GEP derived from NULL is still undefined
+    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
+      if (GEP->getPointerOperand() == I)
+        return passingValueIsAlwaysUndefined(V, GEP);
+
+    // Look through bitcasts.
+    if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
+      return passingValueIsAlwaysUndefined(V, BC);
+
+    // load from null is undefined
+    if (isa<LoadInst>(Use))
+      return true;
+
+    // store to null is undef
+    if (isa<StoreInst>(Use) && Use->getOperand(1) == I)
+      return true;
+  }
+  return false;
+}
+
+/// If BB has an incoming value that will always trigger undefined behavior
+/// (eg. null pointer derefence), remove the branch leading here.
+static bool removeUndefIntroducingPredecessor(BasicBlock *BB) {
+  for (BasicBlock::iterator i = BB->begin();
+       PHINode *PHI = dyn_cast<PHINode>(i); ++i)
+    for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i)
+      if (passingValueIsAlwaysUndefined(PHI->getIncomingValue(i), PHI)) {
+        TerminatorInst *T = PHI->getIncomingBlock(i)->getTerminator();
+        IRBuilder<> Builder(T);
+        if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
+          BB->removePredecessor(PHI->getIncomingBlock(i));
+          // Turn uncoditional branches into unreachables and remove the dead
+          // destination from conditional branches.
+          if (BI->isUnconditional())
+            Builder.CreateUnreachable();
+          else
+            Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1) :
+                                                         BI->getSuccessor(0));
+          BI->eraseFromParent();
+          return true;
+        }
+        // TODO: SwitchInst.
+      }
+
+  return false;
+}
+
 bool SimplifyCFGOpt::run(BasicBlock *BB) {
   bool Changed = false;
 
@@ -2746,6 +2812,9 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
   // Check for and eliminate duplicate PHI nodes in this block.
   Changed |= EliminateDuplicatePHINodes(BB);
 
+  // Check for and remove branches that will always cause undefined behavior.
+  Changed |= removeUndefIntroducingPredecessor(BB);
+
   // Merge basic blocks into their predecessor if there is only one distinct
   // pred, and if there is only one distinct successor of the predecessor, and
   // if there are no PHI nodes.
diff --git a/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll b/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll
new file mode 100644
index 00000000000..65d888ea01e
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll
@@ -0,0 +1,87 @@
+; RUN: opt -simplifycfg -S < %s | FileCheck %s
+
+declare void @bar() nounwind
+
+define i32 @test1(i32* %a, i32 %b, i32* %c, i32 %d) nounwind {
+entry:
+  %tobool = icmp eq i32 %b, 0
+  br i1 %tobool, label %if.else, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.else:                                          ; preds = %entry
+  %tobool3 = icmp eq i32 %d, 0
+  br i1 %tobool3, label %if.end7, label %if.then4
+
+if.then4:                                         ; preds = %if.else
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.end7:                                          ; preds = %if.else, %if.then4, %if.then
+  %x.0 = phi i32* [ %a, %if.then ], [ %c, %if.then4 ], [ null, %if.else ]
+  %tmp9 = load i32* %x.0
+  ret i32 %tmp9
+
+; CHECK: @test1
+; CHECK: if.else:
+; CHECK: br label %if.end7
+
+; CHECK: phi i32* [ %a, %if.then ], [ %c, %if.else ]
+}
+
+define i32 @test2(i32* %a, i32 %b, i32* %c, i32 %d) nounwind {
+entry:
+  %tobool = icmp eq i32 %b, 0
+  br i1 %tobool, label %if.else, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.else:                                          ; preds = %entry
+  %tobool3 = icmp eq i32 %d, 0
+  br i1 %tobool3, label %if.end7, label %if.then4
+
+if.then4:                                         ; preds = %if.else
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.end7:                                          ; preds = %if.else, %if.then4, %if.then
+  %x.0 = phi i32* [ %a, %if.then ], [ null, %if.then4 ], [ null, %if.else ]
+  %tmp9 = load i32* %x.0
+  ret i32 %tmp9
+; CHECK: @test2
+; CHECK: if.else:
+; CHECK: unreachable
+
+; CHECK-NOT: phi
+}
+
+define i32 @test3(i32* %a, i32 %b, i32* %c, i32 %d) nounwind {
+entry:
+  %tobool = icmp eq i32 %b, 0
+  br i1 %tobool, label %if.else, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.else:                                          ; preds = %entry
+  %tobool3 = icmp eq i32 %d, 0
+  br i1 %tobool3, label %if.end7, label %if.then4
+
+if.then4:                                         ; preds = %if.else
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.end7:                                          ; preds = %if.else, %if.then4, %if.then
+  %x.0 = phi i32* [ %a, %if.then ], [ null, %if.then4 ], [ null, %if.else ]
+  tail call void @bar() nounwind
+  %tmp9 = load i32* %x.0
+  ret i32 %tmp9
+; CHECK: @test3
+; CHECK: if.end7:
+; CHECK: phi i32* [ %a, %if.then ], [ null, %if.then4 ], [ null, %if.else ]
+}