Fix bug in the x64 assembler's movw instruction.

runtime/vm/assembler_x64_test.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
@@ skipping 517 matching lines @@
   __ ret();
 }
 
 
 ASSEMBLER_TEST_RUN(MoveExtendMemory, entry) {
   typedef int (*MoveExtendMemory)();
   EXPECT_EQ(0xff - 1 + 0xffff, reinterpret_cast<MoveExtendMemory>(entry)());
 }
 
 
+ASSEMBLER_TEST_GENERATE(MoveWord, assembler) {
+  __ xorq(RAX, RAX);
+  __ pushq(Immediate(0));
+  __ movq(RAX, RSP);
+  __ movq(RCX, Immediate(-1));
+  __ movw(Address(RAX, 0), RCX);
+  __ movzxw(RAX, Address(RAX, 0));  // RAX = 0xffff
+  __ addq(RSP, Immediate(kWordSize));
+  __ ret();
+}
+
+
+ASSEMBLER_TEST_RUN(MoveWord, entry) {
+  typedef int (*MoveWord)();
+  EXPECT_EQ(0xffff, reinterpret_cast<MoveWord>(entry)());
+}
+
+
+ASSEMBLER_TEST_GENERATE(MoveWordRex, assembler) {
+  __ pushq(Immediate(0));
+  __ movq(R8, RSP);
+  __ movq(R9, Immediate(-1));
+  __ movw(Address(R8, 0), R9);
+  __ movzxw(R8, Address(R8, 0));  // 0xffff
+  __ xorq(RAX, RAX);
+  __ addq(RAX, R8);  // RAX = 0xffff
+  __ addq(RSP, Immediate(kWordSize));
+  __ ret();
+}
+
+
+ASSEMBLER_TEST_RUN(MoveWordRex, entry) {
+  typedef int (*MoveWordRex)();
+  EXPECT_EQ(0xffff, reinterpret_cast<MoveWordRex>(entry)());
+}
+
+
 ASSEMBLER_TEST_GENERATE(Bitwise, assembler) {
   __ movl(RCX, Immediate(42));
   __ xorl(RCX, RCX);
   __ orl(RCX, Immediate(256));
   __ movl(RAX, Immediate(4));
   __ orl(RCX, RAX);
   __ movl(RAX, Immediate(0xfff0));
   __ andl(RCX, RAX);
   __ movl(RAX, Immediate(1));
   __ orl(RCX, RAX);
@@ skipping 1356 matching lines @@
   EXPECT_EQ(0, res);
   res = reinterpret_cast<ExtractSignBits>(entry)(-1.0);
   EXPECT_EQ(1, res);
   res = reinterpret_cast<ExtractSignBits>(entry)(-0.0);
   EXPECT_EQ(1, res);
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64