diff --git a/lib/Transforms/Scalar/LoopUnswitch.cpp b/lib/Transforms/Scalar/LoopUnswitch.cpp
index 3918738cc8b..de124d10aaa 100644
--- a/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -677,15 +677,22 @@ void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
   LoopProcessWorklist.push_back(NewLoop);
   redoLoop = true;
 
+  // Keep a WeakVH holding onto LIC.  If the first call to RewriteLoopBody
+  // deletes the instruction (for example by simplifying a PHI that feeds into
+  // the condition that we're unswitching on), we don't rewrite the second
+  // iteration.
+  WeakVH LICHandle(LIC);
+  
   // Now we rewrite the original code to know that the condition is true and the
   // new code to know that the condition is false.
   RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
-  
-  // It's possible that simplifying one loop could cause the other to be
-  // deleted.  If so, don't simplify it.
-  if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
-    RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
 
+  // It's possible that simplifying one loop could cause the other to be
+  // changed to another value or a constant.  If its a constant, don't simplify
+  // it.
+  if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
+      LICHandle && !isa<Constant>(LICHandle))
+    RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
 }
 
 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
@@ -981,6 +988,8 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
       continue;
     }
     
+    // FIXME: Change this to use instruction simplify interfaces!
+    
     // Special case hacks that appear commonly in unswitched code.
     switch (I->getOpcode()) {
     case Instruction::Select:
diff --git a/test/Transforms/LoopUnswitch/crash.ll b/test/Transforms/LoopUnswitch/crash.ll
index fac55a6bb1a..101fb7a2c2c 100644
--- a/test/Transforms/LoopUnswitch/crash.ll
+++ b/test/Transforms/LoopUnswitch/crash.ll
@@ -45,3 +45,22 @@ for.body:                                         ; preds = %for.body, %bb.nph
 for.end:                                          ; preds = %for.body, %entry
   ret void
 }
+
+; PR6879
+define i32* @test3(i32** %p_45, i16 zeroext %p_46, i64 %p_47, i64 %p_48, i16 signext %p_49) nounwind {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond4, %entry
+  br i1 false, label %for.cond4, label %for.end88
+
+for.cond4:                                        ; preds = %for.cond
+  %conv46 = trunc i32 0 to i8                     ; <i8> [#uses=2]
+  %cmp60 = icmp sgt i8 %conv46, 124               ; <i1> [#uses=1]
+  %or.cond = and i1 undef, %cmp60                 ; <i1> [#uses=1]
+  %cond = select i1 %or.cond, i8 %conv46, i8 undef ; <i8> [#uses=0]
+  br label %for.cond
+
+for.end88:                                        ; preds = %for.cond
+  ret i32* undef
+}