diff --git a/build/Android.common.mk b/build/Android.common.mk
index a4f13b8..75d360c 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -251,7 +251,11 @@
 ifeq ($(TARGET_CPU_SMP),true)
   ART_TARGET_CFLAGS += -DANDROID_SMP=1
 else
-  ART_TARGET_CFLAGS += -DANDROID_SMP=0
+  ifeq ($(TARGET_CPU_SMP),false)
+    ART_TARGET_CFLAGS += -DANDROID_SMP=0
+  else
+    $(error TARGET_CPU_SMP must be (true|false), found $(TARGET_CPU_SMP))
+  endif
 endif
 ART_TARGET_CFLAGS += $(ART_DEFAULT_GC_TYPE_CFLAGS)
 
diff --git a/compiler/dex/quick/arm/call_arm.cc b/compiler/dex/quick/arm/call_arm.cc
index bb84fb7..5d74b8d 100644
--- a/compiler/dex/quick/arm/call_arm.cc
+++ b/compiler/dex/quick/arm/call_arm.cc
@@ -295,8 +295,11 @@
     LoadConstantNoClobber(rs_r3, 0);
     // Is lock unheld on lock or held by us (==thread_id) on unlock?
     OpRegReg(kOpCmp, rs_r1, rs_r2);
-    LIR* it = OpIT(kCondEq, "TEE");
-    GenMemBarrier(kStoreLoad);
+
+    LIR* it = OpIT(kCondEq, "EE");
+    if (GenMemBarrier(kStoreLoad)) {
+      UpdateIT(it, "TEE");
+    }
     Store32Disp/*eq*/(rs_r0, mirror::Object::MonitorOffset().Int32Value(), rs_r3);
     // Go expensive route - UnlockObjectFromCode(obj);
     LoadWordDisp/*ne*/(rs_rARM_SELF, QUICK_ENTRYPOINT_OFFSET(4, pUnlockObject).Int32Value(),
diff --git a/compiler/dex/quick/arm/codegen_arm.h b/compiler/dex/quick/arm/codegen_arm.h
index 6696cf7..1ee59c6 100644
--- a/compiler/dex/quick/arm/codegen_arm.h
+++ b/compiler/dex/quick/arm/codegen_arm.h
@@ -141,7 +141,7 @@
     void GenFusedFPCmpBranch(BasicBlock* bb, MIR* mir, bool gt_bias, bool is_double);
     void GenFusedLongCmpBranch(BasicBlock* bb, MIR* mir);
     void GenSelect(BasicBlock* bb, MIR* mir);
-    void GenMemBarrier(MemBarrierKind barrier_kind);
+    bool GenMemBarrier(MemBarrierKind barrier_kind);
     void GenMonitorEnter(int opt_flags, RegLocation rl_src);
     void GenMonitorExit(int opt_flags, RegLocation rl_src);
     void GenMoveException(RegLocation rl_dest);
@@ -160,6 +160,7 @@
     LIR* OpDecAndBranch(ConditionCode c_code, RegStorage reg, LIR* target);
     LIR* OpFpRegCopy(RegStorage r_dest, RegStorage r_src);
     LIR* OpIT(ConditionCode cond, const char* guide);
+    void UpdateIT(LIR* it, const char* new_guide);
     void OpEndIT(LIR* it);
     LIR* OpMem(OpKind op, RegStorage r_base, int disp);
     LIR* OpPcRelLoad(RegStorage reg, LIR* target);
diff --git a/compiler/dex/quick/arm/int_arm.cc b/compiler/dex/quick/arm/int_arm.cc
index 2e0e559..2d4834c 100644
--- a/compiler/dex/quick/arm/int_arm.cc
+++ b/compiler/dex/quick/arm/int_arm.cc
@@ -67,6 +67,34 @@
   return NewLIR2(kThumb2It, code, mask);
 }
 
+void ArmMir2Lir::UpdateIT(LIR* it, const char* new_guide) {
+  int mask;
+  int mask3 = 0;
+  int mask2 = 0;
+  int mask1 = 0;
+  ArmConditionCode code = static_cast<ArmConditionCode>(it->operands[0]);
+  int cond_bit = code & 1;
+  int alt_bit = cond_bit ^ 1;
+
+  // Note: case fallthroughs intentional
+  switch (strlen(new_guide)) {
+    case 3:
+      mask1 = (new_guide[2] == 'T') ? cond_bit : alt_bit;
+    case 2:
+      mask2 = (new_guide[1] == 'T') ? cond_bit : alt_bit;
+    case 1:
+      mask3 = (new_guide[0] == 'T') ? cond_bit : alt_bit;
+      break;
+    case 0:
+      break;
+    default:
+      LOG(FATAL) << "OAT: bad case in UpdateIT";
+  }
+  mask = (mask3 << 3) | (mask2 << 2) | (mask1 << 1) |
+      (1 << (3 - strlen(new_guide)));
+  it->operands[1] = mask;
+}
+
 void ArmMir2Lir::OpEndIT(LIR* it) {
   // TODO: use the 'it' pointer to do some checks with the LIR, for example
   //       we could check that the number of instructions matches the mask
@@ -934,7 +962,7 @@
   return OpCondBranch(c_code, target);
 }
 
-void ArmMir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
+bool ArmMir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
 #if ANDROID_SMP != 0
   // Start off with using the last LIR as the barrier. If it is not enough, then we will generate one.
   LIR* barrier = last_lir_insn_;
@@ -952,15 +980,21 @@
       break;
   }
 
+  bool ret = false;
+
   // If the same barrier already exists, don't generate another.
   if (barrier == nullptr
       || (barrier != nullptr && (barrier->opcode != kThumb2Dmb || barrier->operands[0] != dmb_flavor))) {
     barrier = NewLIR1(kThumb2Dmb, dmb_flavor);
+    ret = true;
   }
 
   // At this point we must have a memory barrier. Mark it as a scheduling barrier as well.
   DCHECK(!barrier->flags.use_def_invalid);
   barrier->u.m.def_mask = ENCODE_ALL;
+  return ret;
+#else
+  return false;
 #endif
 }
 
diff --git a/compiler/dex/quick/arm64/call_arm64.cc b/compiler/dex/quick/arm64/call_arm64.cc
index 51e97cd..1bcf19b 100644
--- a/compiler/dex/quick/arm64/call_arm64.cc
+++ b/compiler/dex/quick/arm64/call_arm64.cc
@@ -278,6 +278,7 @@
     MarkPossibleNullPointerException(opt_flags);
     LoadConstantNoClobber(rs_x3, 0);
     LIR* slow_unlock_branch = OpCmpBranch(kCondNe, rs_x1, rs_x2, NULL);
+    GenMemBarrier(kStoreLoad);
     Store32Disp(rs_x0, mirror::Object::MonitorOffset().Int32Value(), rs_x3);
     LIR* unlock_success_branch = OpUnconditionalBranch(NULL);
 
@@ -295,7 +296,6 @@
 
     LIR* success_target = NewLIR0(kPseudoTargetLabel);
     unlock_success_branch->target = success_target;
-    GenMemBarrier(kStoreLoad);
   } else {
     // Explicit null-check as slow-path is entered using an IT.
     GenNullCheck(rs_x0, opt_flags);
diff --git a/compiler/dex/quick/arm64/codegen_arm64.h b/compiler/dex/quick/arm64/codegen_arm64.h
index af0029c..418a989 100644
--- a/compiler/dex/quick/arm64/codegen_arm64.h
+++ b/compiler/dex/quick/arm64/codegen_arm64.h
@@ -142,7 +142,7 @@
     void GenFusedFPCmpBranch(BasicBlock* bb, MIR* mir, bool gt_bias, bool is_double);
     void GenFusedLongCmpBranch(BasicBlock* bb, MIR* mir);
     void GenSelect(BasicBlock* bb, MIR* mir);
-    void GenMemBarrier(MemBarrierKind barrier_kind);
+    bool GenMemBarrier(MemBarrierKind barrier_kind);
     void GenMonitorEnter(int opt_flags, RegLocation rl_src);
     void GenMonitorExit(int opt_flags, RegLocation rl_src);
     void GenMoveException(RegLocation rl_dest);
diff --git a/compiler/dex/quick/arm64/int_arm64.cc b/compiler/dex/quick/arm64/int_arm64.cc
index 0465249..f2a57e7 100644
--- a/compiler/dex/quick/arm64/int_arm64.cc
+++ b/compiler/dex/quick/arm64/int_arm64.cc
@@ -712,7 +712,7 @@
   return OpCondBranch(c_code, target);
 }
 
-void Arm64Mir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
+bool Arm64Mir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
 #if ANDROID_SMP != 0
   // Start off with using the last LIR as the barrier. If it is not enough, then we will generate one.
   LIR* barrier = last_lir_insn_;
@@ -730,15 +730,21 @@
       break;
   }
 
+  bool ret = false;
+
   // If the same barrier already exists, don't generate another.
   if (barrier == nullptr
       || (barrier->opcode != kA64Dmb1B || barrier->operands[0] != dmb_flavor)) {
     barrier = NewLIR1(kA64Dmb1B, dmb_flavor);
+    ret = true;
   }
 
   // At this point we must have a memory barrier. Mark it as a scheduling barrier as well.
   DCHECK(!barrier->flags.use_def_invalid);
   barrier->u.m.def_mask = ENCODE_ALL;
+  return ret;
+#else
+  return false;
 #endif
 }
 
diff --git a/compiler/dex/quick/mips/codegen_mips.h b/compiler/dex/quick/mips/codegen_mips.h
index b7ea34f..c5b40da 100644
--- a/compiler/dex/quick/mips/codegen_mips.h
+++ b/compiler/dex/quick/mips/codegen_mips.h
@@ -139,7 +139,7 @@
     void GenFusedFPCmpBranch(BasicBlock* bb, MIR* mir, bool gt_bias, bool is_double);
     void GenFusedLongCmpBranch(BasicBlock* bb, MIR* mir);
     void GenSelect(BasicBlock* bb, MIR* mir);
-    void GenMemBarrier(MemBarrierKind barrier_kind);
+    bool GenMemBarrier(MemBarrierKind barrier_kind);
     void GenMoveException(RegLocation rl_dest);
     void GenMultiplyByTwoBitMultiplier(RegLocation rl_src, RegLocation rl_result, int lit,
                                        int first_bit, int second_bit);
diff --git a/compiler/dex/quick/mips/target_mips.cc b/compiler/dex/quick/mips/target_mips.cc
index 2821209..35345e8 100644
--- a/compiler/dex/quick/mips/target_mips.cc
+++ b/compiler/dex/quick/mips/target_mips.cc
@@ -433,9 +433,12 @@
   FreeTemp(rs_rMIPS_ARG3);
 }
 
-void MipsMir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
+bool MipsMir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
 #if ANDROID_SMP != 0
   NewLIR1(kMipsSync, 0 /* Only stype currently supported */);
+  return true;
+#else
+  return false;
 #endif
 }
 
diff --git a/compiler/dex/quick/mir_to_lir.h b/compiler/dex/quick/mir_to_lir.h
index 6a0f3b2..836d2ac 100644
--- a/compiler/dex/quick/mir_to_lir.h
+++ b/compiler/dex/quick/mir_to_lir.h
@@ -1192,8 +1192,9 @@
      * barrier, then it will be used as such. Otherwise, a new LIR will be generated
      * that can keep the semantics.
      * @param barrier_kind The kind of memory barrier to generate.
+     * @return whether a new instruction was generated.
      */
-    virtual void GenMemBarrier(MemBarrierKind barrier_kind) = 0;
+    virtual bool GenMemBarrier(MemBarrierKind barrier_kind) = 0;
 
     virtual void GenMoveException(RegLocation rl_dest) = 0;
     virtual void GenMultiplyByTwoBitMultiplier(RegLocation rl_src, RegLocation rl_result, int lit,
diff --git a/compiler/dex/quick/x86/codegen_x86.h b/compiler/dex/quick/x86/codegen_x86.h
index 11e7ff9..ef8c33c 100644
--- a/compiler/dex/quick/x86/codegen_x86.h
+++ b/compiler/dex/quick/x86/codegen_x86.h
@@ -142,7 +142,7 @@
     void GenFusedFPCmpBranch(BasicBlock* bb, MIR* mir, bool gt_bias, bool is_double);
     void GenFusedLongCmpBranch(BasicBlock* bb, MIR* mir);
     void GenSelect(BasicBlock* bb, MIR* mir);
-    void GenMemBarrier(MemBarrierKind barrier_kind);
+    bool GenMemBarrier(MemBarrierKind barrier_kind);
     void GenMoveException(RegLocation rl_dest);
     void GenMultiplyByTwoBitMultiplier(RegLocation rl_src, RegLocation rl_result, int lit,
                                        int first_bit, int second_bit);
diff --git a/compiler/dex/quick/x86/target_x86.cc b/compiler/dex/quick/x86/target_x86.cc
index 2db9845..e3312a2 100644
--- a/compiler/dex/quick/x86/target_x86.cc
+++ b/compiler/dex/quick/x86/target_x86.cc
@@ -421,11 +421,12 @@
     return false;
 }
 
-void X86Mir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
+bool X86Mir2Lir::GenMemBarrier(MemBarrierKind barrier_kind) {
 #if ANDROID_SMP != 0
   // Start off with using the last LIR as the barrier. If it is not enough, then we will update it.
   LIR* mem_barrier = last_lir_insn_;
 
+  bool ret = false;
   /*
    * According to the JSR-133 Cookbook, for x86 only StoreLoad barriers need memory fence. All other barriers
    * (LoadLoad, LoadStore, StoreStore) are nops due to the x86 memory model. For those cases, all we need
@@ -435,11 +436,13 @@
     // If no LIR exists already that can be used a barrier, then generate an mfence.
     if (mem_barrier == nullptr) {
       mem_barrier = NewLIR0(kX86Mfence);
+      ret = true;
     }
 
     // If last instruction does not provide full barrier, then insert an mfence.
     if (ProvidesFullMemoryBarrier(static_cast<X86OpCode>(mem_barrier->opcode)) == false) {
       mem_barrier = NewLIR0(kX86Mfence);
+      ret = true;
     }
   }
 
@@ -451,6 +454,9 @@
     DCHECK(!mem_barrier->flags.use_def_invalid);
     mem_barrier->u.m.def_mask = ENCODE_ALL;
   }
+  return ret;
+#else
+  return false;
 #endif
 }
 
diff --git a/runtime/Android.mk b/runtime/Android.mk
index 72f1774..052d12e 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -270,8 +270,7 @@
 	runtime_linux.cc \
 	thread_linux.cc
 
-ifeq ($(HOST_ARCH),x86)
-ifneq ($(BUILD_HOST_64bit),)
+ifeq ($(HOST_ARCH),x86_64)
 LIBART_HOST_SRC_FILES += \
 	arch/x86_64/context_x86_64.cc \
 	arch/x86_64/entrypoints_init_x86_64.cc \
@@ -282,6 +281,7 @@
 	arch/x86_64/fault_handler_x86_64.cc \
 	monitor_pool.cc
 else
+  ifeq ($(HOST_ARCH),x86)
 LIBART_HOST_SRC_FILES += \
 	arch/x86/context_x86.cc \
 	arch/x86/entrypoints_init_x86.cc \
@@ -290,10 +290,10 @@
 	arch/x86/quick_entrypoints_x86.S \
 	arch/x86/fault_handler_x86.cc \
 	arch/x86/thread_x86.cc
-endif
-else # HOST_ARCH != x86
+  else # HOST_ARCH != x86 && HOST_ARCH != x86_64
 $(error unsupported HOST_ARCH=$(HOST_ARCH))
-endif # HOST_ARCH != x86
+  endif
+endif
 
 
 LIBART_ENUM_OPERATOR_OUT_HEADER_FILES := \