Use constant length for memcpy on A64 simulator.

src/a64/simulator-a64.h

Index: src/a64/simulator-a64.h
diff --git a/src/a64/simulator-a64.h b/src/a64/simulator-a64.h
index 959644b773e21bec17eea32c840ebd413fc799c2..acca27c1302dbf935daea99dd471af49def42b0a 100644
--- a/src/a64/simulator-a64.h
+++ b/src/a64/simulator-a64.h
@@ -166,10 +166,18 @@ class SimRegisterBase {
   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
@@ -178,8 +186,14 @@ class SimRegisterBase {
   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;
   }