diff --git a/lib/CodeGen/MachineBasicBlock.cpp b/lib/CodeGen/MachineBasicBlock.cpp
index 8b3f01fe58c..58e71bbe2e2 100644
--- a/lib/CodeGen/MachineBasicBlock.cpp
+++ b/lib/CodeGen/MachineBasicBlock.cpp
@@ -1208,8 +1208,15 @@ MachineBasicBlock::computeRegisterLiveness(const TargetRegisterInfo *TRI,
       if (Info.DeadDef)
         return LQR_Dead;
       // Register is (at least partially) live after a def.
-      if (Info.Defined)
-        return LQR_Live;
+      if (Info.Defined) {
+        if (!Info.PartialDeadDef)
+          return LQR_Live;
+        // As soon as we saw a partial definition (dead or not),
+        // we cannot tell if the value is partial live without
+        // tracking the lanemasks. We are not going to do this,
+        // so fall back on the remaining of the analysis.
+        break;
+      }
       // Register is dead after a full kill or clobber and no def.
       if (Info.Killed || Info.Clobbered)
         return LQR_Dead;
diff --git a/test/CodeGen/X86/cmpxchg-clobber-flags.ll b/test/CodeGen/X86/cmpxchg-clobber-flags.ll
index 74599cbdc17..f2b9dee9103 100644
--- a/test/CodeGen/X86/cmpxchg-clobber-flags.ll
+++ b/test/CodeGen/X86/cmpxchg-clobber-flags.ll
@@ -167,11 +167,10 @@ define i32 @test_feed_cmov(i32* %addr, i32 %desired, i32 %new) {
 
 ; x8664-sahf-LABEL: test_feed_cmov:
 ; x8664-sahf: cmpxchgl
-; x8664-sahf: pushq %rax
+; RAX is dead, do not push or pop it.
 ; x8664-sahf-NEXT: seto %al
 ; x8664-sahf-NEXT: lahf
 ; x8664-sahf-NEXT: movq %rax, [[FLAGS:%.*]]
-; x8664-sahf-NEXT: popq %rax
 ; x8664-sahf-NEXT: callq foo
 ; x8664-sahf-NEXT: pushq %rax
 ; x8664-sahf-NEXT: movq [[FLAGS]], %rax