v8 atomicops: Drop SSE2 detection if __SSE2__ is defined.

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 70 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_