Partial revert of r21901 (2nd attempt)

src/base/atomicops_internals_x86_gcc.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This module gets enough CPU information to optimize the
 // atomicops module on x86.
 
 #include <string.h>
 
 #include "src/base/atomicops.h"
@@ skipping 24 matching lines @@
 #if defined(cpuid)        // initialize the struct only on x86
 
 namespace v8 {
 namespace base {
 
 // Set the flags so that code will run correctly and conservatively, so even
 // if we haven't been initialized yet, we're probably single threaded, and our
 // default values should hopefully be pretty safe.
 struct AtomicOps_x86CPUFeatureStruct AtomicOps_Internalx86CPUFeatures = {
   false,          // bug can't exist before process spawns multiple threads
+#if !defined(__SSE2__)
+  false,          // no SSE2
+#endif
 };
 
 } }  // namespace v8::base
 
 namespace {
 
 // Initialize the AtomicOps_Internalx86CPUFeatures struct.
 void AtomicOps_Internalx86CPUFeaturesInit() {
   using v8::base::AtomicOps_Internalx86CPUFeatures;
 
@@ skipping 25 matching lines @@
   // non-locked read-modify-write instruction.  Rev F has this bug in
   // pre-release versions, but not in versions released to customers,
   // so we test only for Rev E, which is family 15, model 32..63 inclusive.
   if (strcmp(vendor, "AuthenticAMD") == 0 &&       // AMD
       family == 15 &&
       32 <= model && model <= 63) {
     AtomicOps_Internalx86CPUFeatures.has_amd_lock_mb_bug = true;
   } else {
     AtomicOps_Internalx86CPUFeatures.has_amd_lock_mb_bug = false;
   }
+
+#if !defined(__SSE2__)
+  // edx bit 26 is SSE2 which we use to tell use whether we can use mfence
+  AtomicOps_Internalx86CPUFeatures.has_sse2 = ((edx >> 26) & 1);
+#endif
 }
 
 class AtomicOpsx86Initializer {
  public:
   AtomicOpsx86Initializer() {
     AtomicOps_Internalx86CPUFeaturesInit();
   }
 };
 
 
 // A global to get use initialized on startup via static initialization :/
 AtomicOpsx86Initializer g_initer;
 
 }  // namespace
 
 #endif  // if x86
 
 #endif  // ifdef V8_BASE_ATOMICOPS_INTERNALS_X86_GCC_H_