[x64] Fix testw with immediates.

Assembler::testw(Register, Immediate) and Assembler::testw(const Operand&, Immediate) were emitting only the first 8 bits of a 16-bit immediate, causing unexpected crashes.

This went unnoticed because before http://crrev.com/1948453002 no compiler was using them.

Review-Url: https://codereview.chromium.org/1962563003
Cr-Commit-Position: refs/heads/master@{#36110}

src/x64/assembler-x64.cc

@@ -2095,14 +2095,14 @@ void Assembler::testw(Register reg, Immediate mask) {
   emit(0x66);
   if (reg.is(rax)) {
     emit(0xA9);
-    emit(mask.value_);
+    emitw(mask.value_);
   } else {
     if (reg.low_bits() == 4) {
       emit_rex_32(reg);
     }
     emit(0xF7);
     emit_modrm(0x0, reg);
-    emit(mask.value_);
+    emitw(mask.value_);
   }
 }
 
@@ -2113,7 +2113,7 @@ void Assembler::testw(const Operand& op, Immediate mask) {
   emit_optional_rex_32(rax, op);
   emit(0xF7);
   emit_operand(rax, op);
-  emit(mask.value_);
+  emitw(mask.value_);
 }
 
 void Assembler::testw(const Operand& op, Register reg) {

test/cctest/test-assembler-x64.cc

@@ -328,6 +328,32 @@ TEST(AssemblerX64TestlOperations) {
   CHECK_EQ(1u, result);
 }
 
+TEST(AssemblerX64TestwOperations) {
+  typedef uint16_t (*F)(uint16_t * x);
+  CcTest::InitializeVM();
+  // Allocate an executable page of memory.
+  size_t actual_size;
+  byte* buffer = static_cast<byte*>(v8::base::OS::Allocate(
+      Assembler::kMinimalBufferSize, &actual_size, true));
+  CHECK(buffer);
+  Assembler assm(CcTest::i_isolate(), buffer, static_cast<int>(actual_size));
+
+  // Set rax with the ZF flag of the testl instruction.
+  Label done;
+  __ movq(rax, Immediate(1));
+  __ testw(Operand(arg1, 0), Immediate(0xf0f0));
+  __ j(not_zero, &done, Label::kNear);
+  __ movq(rax, Immediate(0));
+  __ bind(&done);
+  __ ret(0);
+
+  CodeDesc desc;
+  assm.GetCode(&desc);
+  // Call the function from C++.
+  uint16_t operand = 0x8000;
+  uint16_t result = FUNCTION_CAST<F>(buffer)(&operand);
+  CHECK_EQ(1u, result);
+}
 
 TEST(AssemblerX64XorlOperations) {
   CcTest::InitializeVM();