Index: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
@@ -19717,13 +19717,6 @@
   }
 }
 
-static bool hasMFENCE(const X86Subtarget &Subtarget) {
-  // Use mfence if we have SSE2 or we're on x86-64 (even if we asked for
-  // no-sse2). There isn't any reason to disable it if the target processor
-  // supports it.
-  return Subtarget.hasSSE2() || Subtarget.is64Bit();
-}
-
 LoadInst *
 X86TargetLowering::lowerIdempotentRMWIntoFencedLoad(AtomicRMWInst *AI) const {
   unsigned NativeWidth = Subtarget.is64Bit() ? 64 : 32;
@@ -19763,7 +19756,7 @@
     // the IR level, so we must wrap it in an intrinsic.
     return nullptr;
 
-  if (!hasMFENCE(Subtarget))
+  if (!Subtarget.hasMFence())
     // FIXME: it might make sense to use a locked operation here but on a
     // different cache-line to prevent cache-line bouncing. In practice it
     // is probably a small win, and x86 processors without mfence are rare
@@ -19794,7 +19787,7 @@
   // The only fence that needs an instruction is a sequentially-consistent
   // cross-thread fence.
   if (FenceOrdering == SequentiallyConsistent && FenceScope == CrossThread) {
-    if (hasMFENCE(Subtarget))
+    if (Subtarget.hasMFence())
       return DAG.getNode(X86ISD::MFENCE, dl, MVT::Other, Op.getOperand(0));
 
     SDValue Chain = Op.getOperand(0);
Index: llvm/trunk/lib/Target/X86/X86InstrInfo.td
===================================================================
--- llvm/trunk/lib/Target/X86/X86InstrInfo.td
+++ llvm/trunk/lib/Target/X86/X86InstrInfo.td
@@ -845,6 +845,7 @@
 def FavorMemIndirectCall  : Predicate<"!Subtarget->callRegIndirect()">;
 def NotSlowIncDec : Predicate<"!Subtarget->slowIncDec()">;
 def HasFastMem32 : Predicate<"!Subtarget->isUnalignedMem32Slow()">;
+def HasMFence    : Predicate<"Subtarget->hasMFence()">;
 
 //===----------------------------------------------------------------------===//
 // X86 Instruction Format Definitions.
Index: llvm/trunk/lib/Target/X86/X86InstrSSE.td
===================================================================
--- llvm/trunk/lib/Target/X86/X86InstrSSE.td
+++ llvm/trunk/lib/Target/X86/X86InstrSSE.td
@@ -3762,6 +3762,8 @@
 
 let SchedRW = [WriteFence] in {
 // Load, store, and memory fence
+// TODO: As with mfence, we may want to ease the availablity of sfence/lfence
+// to include any 64-bit target.
 def SFENCE : I<0xAE, MRM_F8, (outs), (ins),
                "sfence", [(int_x86_sse_sfence)], IIC_SSE_SFENCE>,
                PS, Requires<[HasSSE1]>;
@@ -3770,7 +3772,7 @@
                TB, Requires<[HasSSE2]>;
 def MFENCE : I<0xAE, MRM_F0, (outs), (ins),
                "mfence", [(int_x86_sse2_mfence)], IIC_SSE_MFENCE>,
-               TB, Requires<[HasSSE2]>;
+               TB, Requires<[HasMFence]>;
 } // SchedRW
 
 def : Pat<(X86SFence), (SFENCE)>;
Index: llvm/trunk/lib/Target/X86/X86Subtarget.h
===================================================================
--- llvm/trunk/lib/Target/X86/X86Subtarget.h
+++ llvm/trunk/lib/Target/X86/X86Subtarget.h
@@ -446,6 +446,11 @@
   bool isSLM() const { return X86ProcFamily == IntelSLM; }
   bool useSoftFloat() const { return UseSoftFloat; }
 
+  /// Use mfence if we have SSE2 or we're on x86-64 (even if we asked for
+  /// no-sse2). There isn't any reason to disable it if the target processor
+  /// supports it.
+  bool hasMFence() const { return hasSSE2() || is64Bit(); }
+
   const Triple &getTargetTriple() const { return TargetTriple; }
 
   bool isTargetDarwin() const { return TargetTriple.isOSDarwin(); }
Index: llvm/trunk/test/CodeGen/X86/mfence.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/mfence.ll
+++ llvm/trunk/test/CodeGen/X86/mfence.ll
@@ -1,11 +1,37 @@
-; RUN: llc < %s -mtriple=i386-unknown-unknown -mattr=+sse2 | FileCheck %s
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i386-unknown-unknown -mattr=+sse2 | FileCheck %s --check-prefix=X32
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=-sse2 | FileCheck %s --check-prefix=X64
+
+; It doesn't matter if an x86-64 target has specified "no-sse2"; we still can use mfence.
 
 define void @test() {
-; CHECK-LABEL: test:
-; CHECK:       # BB#0:
-; CHECK-NEXT:    mfence
-; CHECK-NEXT:    retl
+; X32-LABEL: test:
+; X32:       # BB#0:
+; X32-NEXT:    mfence
+; X32-NEXT:    retl
+;
+; X64-LABEL: test:
+; X64:       # BB#0:
+; X64-NEXT:    mfence
+; X64-NEXT:    retq
   fence seq_cst
   ret void
 }
 
+define i32 @fence(i32* %ptr) {
+; X32-LABEL: fence:
+; X32:       # BB#0:
+; X32-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X32-NEXT:    mfence
+; X32-NEXT:    movl (%eax), %eax
+; X32-NEXT:    retl
+;
+; X64-LABEL: fence:
+; X64:       # BB#0:
+; X64-NEXT:    mfence
+; X64-NEXT:    movl (%rdi), %eax
+; X64-NEXT:    retq
+  %atomic = atomicrmw add i32* %ptr, i32 0 seq_cst
+  ret i32 %atomic
+}
+