Revert "Partial revert of r21901"

src/base/atomicops_internals_x86_gcc.h

Index: src/base/atomicops_internals_x86_gcc.h
diff --git a/src/base/atomicops_internals_x86_gcc.h b/src/base/atomicops_internals_x86_gcc.h
index ec87c42121246f7b75d2cd3939ba2f28195925ad..90e1acf1eeae803828b11a2024b10acb78e52117 100644
--- a/src/base/atomicops_internals_x86_gcc.h
+++ b/src/base/atomicops_internals_x86_gcc.h
@@ -17,9 +17,6 @@ namespace base {
 struct AtomicOps_x86CPUFeatureStruct {
   bool has_amd_lock_mb_bug;  // Processor has AMD memory-barrier bug; do lfence
                              // after acquire compare-and-swap.
-#if !defined(__SSE2__)
-  bool has_sse2;             // Processor has SSE2.
-#endif
 };
 extern struct AtomicOps_x86CPUFeatureStruct AtomicOps_Internalx86CPUFeatures;
 
@@ -94,10 +91,7 @@ inline void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value) {
   *ptr = value;
 }
 
-#if defined(__x86_64__) || defined(__SSE2__)
-
-// 64-bit implementations of memory barrier can be simpler, because it
-// "mfence" is guaranteed to exist.
+// We require SSE2, so mfence is guaranteed to exist.
 inline void MemoryBarrier() {
   __asm__ __volatile__("mfence" : : : "memory");
 }
@@ -107,28 +101,6 @@ inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
   MemoryBarrier();
 }
 
-#else
-
-inline void MemoryBarrier() {
-  if (AtomicOps_Internalx86CPUFeatures.has_sse2) {
-    __asm__ __volatile__("mfence" : : : "memory");
-  } else {  // mfence is faster but not present on PIII
-    Atomic32 x = 0;
-    NoBarrier_AtomicExchange(&x, 0);  // acts as a barrier on PIII
-  }
-}
-
-inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
-  if (AtomicOps_Internalx86CPUFeatures.has_sse2) {
-    *ptr = value;
-    __asm__ __volatile__("mfence" : : : "memory");
-  } else {
-    NoBarrier_AtomicExchange(ptr, value);
-                          // acts as a barrier on PIII
-  }
-}
-#endif
-
 inline void Release_Store(volatile Atomic32* ptr, Atomic32 value) {
   ATOMICOPS_COMPILER_BARRIER();
   *ptr = value;  // An x86 store acts as a release barrier.