Add a mechanism to override the detected cpu features.

src/ia32/assembler-ia32.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 517 matching lines @@
 //   }
 class CpuFeatures : public AllStatic {
  public:
   // Detect features of the target CPU. Set safe defaults if the serializer
   // is enabled (snapshots must be portable).
   static void Probe();
 
   // Check whether a feature is supported by the target CPU.
   static bool IsSupported(CpuFeature f) {
     ASSERT(initialized_);
+    if ((force_feature_ & (static_cast<uint64_t>(1) << f)) != 0) return true;

[Sven Panne, 2013/09/16 13:48:05]

Lift out

   static_cast<uint64_t>(1) << f

into a private helper now that it's all over the place.

     if (f == SSE2 && !FLAG_enable_sse2) return false;
     if (f == SSE3 && !FLAG_enable_sse3) return false;
     if (f == SSE4_1 && !FLAG_enable_sse4_1) return false;
     if (f == CMOV && !FLAG_enable_cmov) return false;
     return (supported_ & (static_cast<uint64_t>(1) << f)) != 0;
   }
 
   static bool IsFoundByRuntimeProbingOnly(CpuFeature f) {
     ASSERT(initialized_);
     return (found_by_runtime_probing_only_ &
             (static_cast<uint64_t>(1) << f)) != 0;
   }
 
   static bool IsSafeForSnapshot(CpuFeature f) {
     return (IsSupported(f) &&
-            (!Serializer::enabled() || !IsFoundByRuntimeProbingOnly(f)));
+            (!Serializer::enabled() || !IsFoundByRuntimeProbingOnly(f))) ||
+           ((force_feature_ & (static_cast<uint64_t>(1) << f)) != 0);
+  }
+
+  static void ForceFeature(CpuFeature f) {

[Sven Panne, 2013/09/16 13:48:05]

I am not sure about the future use cases, perhaps some scoped form might be more
appropriate.

+    force_feature_ = force_feature_ | (static_cast<uint64_t>(1) << f);
+  }
+
+  static void ClearForcedFeatures() {
+    force_feature_ = 0;
   }
 
  private:
 #ifdef DEBUG
   static bool initialized_;
 #endif
+  static uint64_t force_feature_;
   static uint64_t supported_;
   static uint64_t found_by_runtime_probing_only_;
 
   friend class ExternalReference;
   DISALLOW_COPY_AND_ASSIGN(CpuFeatures);
 };
 
 
 class Assembler : public AssemblerBase {
  private:
@@ skipping 664 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_IA32_ASSEMBLER_IA32_H_