diff --git a/lib/CodeGen/MachineSink.cpp b/lib/CodeGen/MachineSink.cpp
index e47ba7c2cc3..e6596190157 100644
--- a/lib/CodeGen/MachineSink.cpp
+++ b/lib/CodeGen/MachineSink.cpp
@@ -126,6 +126,11 @@ bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
   // Can't sink anything out of a block that has less than two successors.
   if (MBB.succ_size() <= 1 || MBB.empty()) return false;
 
+  // Don't bother sinking code out of unreachable blocks. In addition to being
+  // unprofitable, it can also lead to infinite looping, because in an unreachable
+  // loop there may be nowhere to stop.
+  if (!DT->isReachableFromEntry(&MBB)) return false;
+
   bool MadeChange = false;
 
   // Walk the basic block bottom-up.  Remember if we saw a store.
diff --git a/test/CodeGen/X86/unreachable-loop-sinking.ll b/test/CodeGen/X86/unreachable-loop-sinking.ll
new file mode 100644
index 00000000000..35f69175a86
--- /dev/null
+++ b/test/CodeGen/X86/unreachable-loop-sinking.ll
@@ -0,0 +1,30 @@
+; RUN: llc < %s
+; PR6777
+
+; MachineSink shouldn't try to sink code in unreachable blocks, as it's
+; not worthwhile, and there are corner cases which it doesn't handle.
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define double @fn1(i8* %arg, i64 %arg1) {
+Entry:
+  br i1 undef, label %Body, label %Exit
+
+Exit:                                             ; preds = %Brancher7, %Entry
+  ret double undef
+
+Body:                                             ; preds = %Entry
+  br i1 false, label %Brancher7, label %Body3
+
+Body3:                                            ; preds = %Body6, %Body3, %Body
+  br label %Body3
+
+Body6:                                            ; preds = %Brancher7
+  %tmp = fcmp oeq double 0xC04FBB2E40000000, undef ; <i1> [#uses=1]
+  br i1 %tmp, label %Body3, label %Brancher7
+
+Brancher7:                                        ; preds = %Body6, %Body
+  %tmp2 = icmp ult i32 undef, 10                  ; <i1> [#uses=1]
+  br i1 %tmp2, label %Body6, label %Exit
+}