diff --git a/lib/CodeGen/SimpleRegisterCoalescing.cpp b/lib/CodeGen/SimpleRegisterCoalescing.cpp
index 43da9f2daa4..b61735fdf4d 100644
--- a/lib/CodeGen/SimpleRegisterCoalescing.cpp
+++ b/lib/CodeGen/SimpleRegisterCoalescing.cpp
@@ -660,6 +660,22 @@ bool SimpleRegisterCoalescing::ReMaterializeTrivialDef(LiveInterval &SrcInt,
       return false;
   }
 
+  // If destination register has a sub-register index on it, make sure it mtches
+  // the instruction register class.
+  if (DstSubIdx) {
+    const TargetInstrDesc &TID = DefMI->getDesc();
+    if (TID.getNumDefs() != 1)
+      return false;
+    const TargetRegisterClass *DstRC = mri_->getRegClass(DstReg);
+    const TargetRegisterClass *DstSubRC =
+      DstRC->getSubRegisterRegClass(DstSubIdx);
+    const TargetRegisterClass *DefRC = TID.OpInfo[0].getRegClass(tri_);
+    if (DefRC == DstRC)
+      DstSubIdx = 0;
+    else if (DefRC != DstSubRC)
+      return false;
+  }
+
   MachineInstrIndex DefIdx = li_->getDefIndex(CopyIdx);
   const LiveRange *DLR= li_->getInterval(DstReg).getLiveRangeContaining(DefIdx);
   DLR->valno->setCopy(0);
diff --git a/test/CodeGen/X86/2009-09-07-CoalescerBug.ll b/test/CodeGen/X86/2009-09-07-CoalescerBug.ll
new file mode 100644
index 00000000000..d3333f5ee74
--- /dev/null
+++ b/test/CodeGen/X86/2009-09-07-CoalescerBug.ll
@@ -0,0 +1,48 @@
+; RUN: llvm-as < %s | llc -triple=x86_64-unknown-freebsd7.2 -code-model=kernel | FileCheck %s
+; PR4689
+
+%struct.__s = type { [8 x i8] }
+%struct.pcb = type { i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i16, i8* }
+%struct.pcpu = type { i32*, i32*, i32*, i32*, %struct.pcb*, i64, i32, i32, i32, i32 }
+
+define i64 @hammer_time(i64 %modulep, i64 %physfree) nounwind ssp noredzone noimplicitfloat {
+; CHECK: hammer_time:
+; CHECK: movq $Xrsvd, %rax
+; CHECK: movq $Xrsvd, %rdi
+; CHECK: movq $Xrsvd, %r8
+entry:
+  br i1 undef, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.inc, %if.end
+  switch i32 undef, label %if.then76 [
+    i32 9, label %for.inc
+    i32 10, label %for.inc
+    i32 11, label %for.inc
+    i32 12, label %for.inc
+  ]
+
+if.then76:                                        ; preds = %for.body
+  unreachable
+
+for.inc:                                          ; preds = %for.body, %for.body, %for.body, %for.body
+  br i1 undef, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc
+  call void asm sideeffect "mov $1,%gs:$0", "=*m,r,~{dirflag},~{fpsr},~{flags}"(%struct.__s* bitcast (%struct.pcb** getelementptr (%struct.pcpu* null, i32 0, i32 4) to %struct.__s*), i64 undef) nounwind
+  br label %for.body170
+
+for.body170:                                      ; preds = %for.body170, %for.end
+  store i64 or (i64 and (i64 or (i64 ptrtoint (void (i32, i32, i32, i32)* @Xrsvd to i64), i64 2097152), i64 2162687), i64 or (i64 or (i64 and (i64 shl (i64 ptrtoint (void (i32, i32, i32, i32)* @Xrsvd to i64), i64 32), i64 -281474976710656), i64 140737488355328), i64 15393162788864)), i64* undef
+  br i1 undef, label %for.end175, label %for.body170
+
+for.end175:                                       ; preds = %for.body170
+  unreachable
+}
+
+declare void @Xrsvd(i32, i32, i32, i32) ssp noredzone noimplicitfloat