diff --git a/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp b/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
index 20770a17ef2f..8a18dc0fc79d 100644
--- a/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
+++ b/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
@@ -628,10 +628,10 @@ void TwoAddressInstructionPass::ProcessCopy(MachineInstr *MI,
   if (IsDstPhys && !IsSrcPhys)
     DstRegMap.insert(std::make_pair(SrcReg, DstReg));
   else if (!IsDstPhys && IsSrcPhys) {
-    bool isNew =
-      SrcRegMap.insert(std::make_pair(DstReg, SrcReg)).second;
-    isNew = isNew; // Silence compiler warning.
-    assert(isNew && "Can't map to two src physical registers!");
+    bool isNew = SrcRegMap.insert(std::make_pair(DstReg, SrcReg)).second;
+    if (!isNew)
+      assert(SrcRegMap[DstReg] == SrcReg &&
+             "Can't map to two src physical registers!");
 
     SmallVector<unsigned, 4> VirtRegPairs;
     bool isCopy = false;
@@ -653,8 +653,9 @@ void TwoAddressInstructionPass::ProcessCopy(MachineInstr *MI,
         break;
       }
       bool isNew = SrcRegMap.insert(std::make_pair(NewReg, DstReg)).second;
-      isNew = isNew; // Silence compiler warning.
-      assert(isNew && "Can't map to two src physical registers!");
+      if (!isNew)
+      assert(SrcRegMap[NewReg] == DstReg &&
+             "Can't map to two src physical registers!");
       VirtRegPairs.push_back(NewReg);
       DstReg = NewReg;
     }
@@ -666,8 +667,9 @@ void TwoAddressInstructionPass::ProcessCopy(MachineInstr *MI,
         unsigned FromReg = VirtRegPairs.back();
         VirtRegPairs.pop_back();
         bool isNew = DstRegMap.insert(std::make_pair(FromReg, ToReg)).second;
-        isNew = isNew; // Silence compiler warning.
-        assert(isNew && "Can't map to two dst physical registers!");
+        if (!isNew)
+          assert(DstRegMap[FromReg] == ToReg &&
+                 "Can't map to two dst physical registers!");
         ToReg = FromReg;
       }
     }
diff --git a/llvm/test/CodeGen/X86/2009-04-13-2AddrAssert.ll b/llvm/test/CodeGen/X86/2009-04-13-2AddrAssert.ll
new file mode 100644
index 000000000000..7f94c6ca947e
--- /dev/null
+++ b/llvm/test/CodeGen/X86/2009-04-13-2AddrAssert.ll
@@ -0,0 +1,16 @@
+; RUN: llvm-as < %s | llc
+; rdar://6781755
+; PR3934
+
+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-f80:128:128"
+target triple = "x86_64-undermydesk-freebsd8.0"
+
+define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
+entry:
+        %call = tail call i32 (...)* @getpid()          ; <i32> [#uses=1]
+        %conv = trunc i32 %call to i16          ; <i16> [#uses=1]
+        %0 = tail call i16 asm "xchgb ${0:h}, ${0:b}","=Q,0,~{dirflag},~{fpsr},~{flags}"(i16 %conv) nounwind           ; <i16> [#uses=0]
+        ret i32 undef
+}
+
+declare i32 @getpid(...)