Use constant length for memcpy on A64 simulator.

src/a64/simulator-a64.h

 // Copyright 2013 the V8 project authors. 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 148 matching lines @@
 
 
 // Represent a register (r0-r31, v0-v31).
 template<int kSizeInBytes>
 class SimRegisterBase {
  public:
   template<typename T>
   void Set(T new_value, unsigned size = sizeof(T)) {
     ASSERT(size <= kSizeInBytes);
     ASSERT(size <= sizeof(new_value));
+    STATIC_ASSERT(kXRegSize == kDRegSize);
+    STATIC_ASSERT(kWRegSize == kSRegSize);
     // All AArch64 registers are zero-extending; Writing a W register clears the
     // top bits of the corresponding X register.
-    memset(value_, 0, kSizeInBytes);
-    memcpy(value_, &new_value, size);
+    if (size == kXRegSize) {
+      memcpy(value_, &new_value, kXRegSize);
+    } else if (size == kWRegSize) {
+      memset(value_, 0, kSizeInBytes);
+      memcpy(value_, &new_value, kWRegSize);
+    } else {
+      UNREACHABLE();
+    }
   }
 
   // Copy 'size' bytes of the register to the result, and zero-extend to fill
   // the result.
   template<typename T>
   T Get(unsigned size = sizeof(T)) const {
     ASSERT(size <= kSizeInBytes);
     T result;
-    memset(&result, 0, sizeof(result));
-    memcpy(&result, value_, size);
+    if (size == kXRegSize) {
+      memcpy(&result, value_, kXRegSize);
+    } else if (size == kWRegSize) {
+      memset(&result, 0, sizeof(result));
+      memcpy(&result, value_, kWRegSize);
+    } else {
+      UNREACHABLE();
+    }
     return result;
   }
 
  protected:
   uint8_t value_[kSizeInBytes];
 };
 typedef SimRegisterBase<kXRegSize> SimRegister;      // r0-r31
 typedef SimRegisterBase<kDRegSize> SimFPRegister;    // v0-v31
 
 
@@ skipping 706 matching lines @@
   static void UnregisterCTryCatch() {
     Simulator::current(Isolate::Current())->PopAddress();
   }
 };
 
 #endif  // !defined(USE_SIMULATOR)
 
 } }  // namespace v8::internal
 
 #endif  // V8_A64_SIMULATOR_A64_H_