Fix awesomeness in X64 assembler.

src/x64/assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/macro-assembler.h"
@@ skipping 1159 matching lines @@
     emit_optional_rex_32(dst, src);
   }
   emit(0x8A);
   emit_operand(dst, src);
 }
 
 
 void Assembler::movb(Register dst, Immediate imm) {
   EnsureSpace ensure_space(this);
   if (!dst.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
     emit_rex_32(dst);
   }
   emit(0xB0 + dst.low_bits());
   emit(imm.value_);
 }
 
 
 void Assembler::movb(const Operand& dst, Register src) {
   EnsureSpace ensure_space(this);
   if (!src.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
     emit_rex_32(src, dst);
   } else {
     emit_optional_rex_32(src, dst);
   }
   emit(0x88);
   emit_operand(src, dst);
 }
 
 
 void Assembler::movb(const Operand& dst, Immediate imm) {
@@ skipping 191 matching lines @@
   emit(0x0F);
   emit(0xB6);
   emit_operand(dst, src);
 }
 
 
 void Assembler::emit_movzxb(Register dst, Register src, int size) {
   EnsureSpace ensure_space(this);
   // 32 bit operations zero the top 32 bits of 64 bit registers.  Therefore
   // there is no need to make this a 64 bit operation.
-  emit_optional_rex_32(dst, src);
+  if (!src.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
+    emit_rex_32(dst, src);
+  } else {
+    emit_optional_rex_32(dst, src);
+  }
   emit(0x0F);
   emit(0xB6);
   emit_modrm(dst, src);
 }
 
 
 void Assembler::emit_movzxw(Register dst, const Operand& src, int size) {
   EnsureSpace ensure_space(this);
   // 32 bit operations zero the top 32 bits of 64 bit registers.  Therefore
   // there is no need to make this a 64 bit operation.
@@ skipping 235 matching lines @@
 }
 
 
 void Assembler::setcc(Condition cc, Register reg) {
   if (cc > last_condition) {
     movb(reg, Immediate(cc == always ? 1 : 0));
     return;
   }
   EnsureSpace ensure_space(this);
   DCHECK(is_uint4(cc));
-  if (!reg.is_byte_register()) {  // Use x64 byte registers, where different.
+  if (!reg.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
     emit_rex_32(reg);
   }
   emit(0x0F);
   emit(0x90 | cc);
   emit_modrm(0x0, reg);
 }
 
 
 void Assembler::shld(Register dst, Register src) {
   EnsureSpace ensure_space(this);
@@ skipping 1319 matching lines @@
 
 
 bool RelocInfo::IsInConstantPool() {
   return false;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64