Reland "[Assembler][x64] Make Operand immutable"

This is a reland of e7f9fb4a.

Original change's description:
> [Assembler][x64] Make Operand immutable
> 
> This CL removes all setters from the Operand and removes the friendship
> relation between Assembler and Operand. All data fields of the Operand
> are set exactly once in the constructor, the Operand is immutable
> afterwards.
> In order to construct the data of an Operand easily, the OperandBuilder
> is introduced. After building an Operand, the data is copied to the
> const field of the Operand.
> 
> R=mstarzinger@chromium.org
> 
> Bug: v8:7310
> Change-Id: I1628052b8a0c47cbfbc3645dfdac5a0e9705977b
> Reviewed-on: https://chromium-review.googlesource.com/936741
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51563}

Bug: v8:7310
Change-Id: I84df5e11b1811585fbba7309e3bb9c6b17e18c0b
Reviewed-on: https://chromium-review.googlesource.com/936628Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51573}

src/compiler/x64/code-generator-x64.cc

@@ -3382,11 +3382,9 @@ void CodeGenerator::AssembleSwap(InstructionOperand* source,
         frame_access_state()->IncreaseSPDelta(1);
         unwinding_info_writer_.MaybeIncreaseBaseOffsetAt(__ pc_offset(),
                                                          kPointerSize);
-        Operand dst = g.ToOperand(destination);
-        __ movq(src, dst);
+        __ movq(src, g.ToOperand(destination));
         frame_access_state()->IncreaseSPDelta(-1);
-        dst = g.ToOperand(destination);
-        __ popq(dst);
+        __ popq(g.ToOperand(destination));
         unwinding_info_writer_.MaybeIncreaseBaseOffsetAt(__ pc_offset(),
                                                          -kPointerSize);
       } else {

src/wasm/baseline/x64/liftoff-assembler-x64.h

@@ -39,6 +39,21 @@ inline Operand GetContextOperand() { return Operand(rbp, -16); }
 // stack for a call to C.
 static constexpr Register kCCallLastArgAddrReg = rax;
 
+inline Operand GetMemOp(LiftoffAssembler* assm, Register addr, Register offset,
+                        uint32_t offset_imm, LiftoffRegList pinned) {
+  if (offset_imm > kMaxInt) {
+    // The immediate can not be encoded in the operand. Load it to a register
+    // first.
+    Register total_offset = assm->GetUnusedRegister(kGpReg, pinned).gp();
+    assm->movl(total_offset, Immediate(offset_imm));
+    if (offset != no_reg) {
+      assm->emit_ptrsize_add(total_offset, addr, offset);
+    }
+    return Operand(addr, total_offset, times_1, 0);
+  }
+  if (offset == no_reg) return Operand(addr, offset_imm);
+  return Operand(addr, offset, times_1, offset_imm);
+}
 }  // namespace liftoff
 
 uint32_t LiftoffAssembler::PrepareStackFrame() {
@@ -110,19 +125,8 @@ void LiftoffAssembler::Load(LiftoffRegister dst, Register src_addr,
                             Register offset_reg, uint32_t offset_imm,
                             LoadType type, LiftoffRegList pinned,
                             uint32_t* protected_load_pc) {
-  Operand src_op = offset_reg == no_reg
-                       ? Operand(src_addr, offset_imm)
-                       : Operand(src_addr, offset_reg, times_1, offset_imm);
-  if (offset_imm > kMaxInt) {
-    // The immediate can not be encoded in the operand. Load it to a register
-    // first.
-    Register src = GetUnusedRegister(kGpReg, pinned).gp();
-    movl(src, Immediate(offset_imm));
-    if (offset_reg != no_reg) {
-      emit_ptrsize_add(src, src, offset_reg);
-    }
-    src_op = Operand(src_addr, src, times_1, 0);
-  }
+  Operand src_op =
+      liftoff::GetMemOp(this, src_addr, offset_reg, offset_imm, pinned);
   if (protected_load_pc) *protected_load_pc = pc_offset();
   switch (type.value()) {
     case LoadType::kI32Load8U:
@@ -170,19 +174,8 @@ void LiftoffAssembler::Store(Register dst_addr, Register offset_reg,
                              uint32_t offset_imm, LiftoffRegister src,
                              StoreType type, LiftoffRegList pinned,
                              uint32_t* protected_store_pc) {
-  Operand dst_op = offset_reg == no_reg
-                       ? Operand(dst_addr, offset_imm)
-                       : Operand(dst_addr, offset_reg, times_1, offset_imm);
-  if (offset_imm > kMaxInt) {
-    // The immediate can not be encoded in the operand. Load it to a register
-    // first.
-    Register dst = GetUnusedRegister(kGpReg, pinned).gp();
-    movl(dst, Immediate(offset_imm));
-    if (offset_reg != no_reg) {
-      emit_ptrsize_add(dst, dst, offset_reg);
-    }
-    dst_op = Operand(dst_addr, dst, times_1, 0);
-  }
+  Operand dst_op =
+      liftoff::GetMemOp(this, dst_addr, offset_reg, offset_imm, pinned);
   if (protected_store_pc) *protected_store_pc = pc_offset();
   switch (type.value()) {
     case StoreType::kI32Store8:

src/x64/assembler-x64-inl.h

@@ -93,11 +93,11 @@ void Assembler::emit_rex_64(Register reg, XMMRegister rm_reg) {
 }
 
 void Assembler::emit_rex_64(Register reg, Operand op) {
-  emit(0x48 | reg.high_bit() << 2 | op.rex_);
+  emit(0x48 | reg.high_bit() << 2 | op.data().rex);
 }
 
 void Assembler::emit_rex_64(XMMRegister reg, Operand op) {
-  emit(0x48 | (reg.code() & 0x8) >> 1 | op.rex_);
+  emit(0x48 | (reg.code() & 0x8) >> 1 | op.data().rex);
 }
 
 
@@ -106,14 +106,14 @@ void Assembler::emit_rex_64(Register rm_reg) {
   emit(0x48 | rm_reg.high_bit());
 }
 
-void Assembler::emit_rex_64(Operand op) { emit(0x48 | op.rex_); }
+void Assembler::emit_rex_64(Operand op) { emit(0x48 | op.data().rex); }
 
 void Assembler::emit_rex_32(Register reg, Register rm_reg) {
   emit(0x40 | reg.high_bit() << 2 | rm_reg.high_bit());
 }
 
 void Assembler::emit_rex_32(Register reg, Operand op) {
-  emit(0x40 | reg.high_bit() << 2  | op.rex_);
+  emit(0x40 | reg.high_bit() << 2 | op.data().rex);
 }
 
 
@@ -121,7 +121,7 @@ void Assembler::emit_rex_32(Register rm_reg) {
   emit(0x40 | rm_reg.high_bit());
 }
 
-void Assembler::emit_rex_32(Operand op) { emit(0x40 | op.rex_); }
+void Assembler::emit_rex_32(Operand op) { emit(0x40 | op.data().rex); }
 
 void Assembler::emit_optional_rex_32(Register reg, Register rm_reg) {
   byte rex_bits = reg.high_bit() << 2 | rm_reg.high_bit();
@@ -129,12 +129,12 @@ void Assembler::emit_optional_rex_32(Register reg, Register rm_reg) {
 }
 
 void Assembler::emit_optional_rex_32(Register reg, Operand op) {
-  byte rex_bits =  reg.high_bit() << 2 | op.rex_;
+  byte rex_bits = reg.high_bit() << 2 | op.data().rex;
   if (rex_bits != 0) emit(0x40 | rex_bits);
 }
 
 void Assembler::emit_optional_rex_32(XMMRegister reg, Operand op) {
-  byte rex_bits =  (reg.code() & 0x8) >> 1 | op.rex_;
+  byte rex_bits = (reg.code() & 0x8) >> 1 | op.data().rex;
   if (rex_bits != 0) emit(0x40 | rex_bits);
 }
 
@@ -166,7 +166,7 @@ void Assembler::emit_optional_rex_32(XMMRegister rm_reg) {
 }
 
 void Assembler::emit_optional_rex_32(Operand op) {
-  if (op.rex_ != 0) emit(0x40 | op.rex_);
+  if (op.data().rex != 0) emit(0x40 | op.data().rex);
 }
 
 
@@ -180,7 +180,7 @@ void Assembler::emit_vex3_byte1(XMMRegister reg, XMMRegister rm,
 
 // byte 1 of 3-byte VEX
 void Assembler::emit_vex3_byte1(XMMRegister reg, Operand rm, LeadingOpcode m) {
-  byte rxb = ~((reg.high_bit() << 2) | rm.rex_) << 5;
+  byte rxb = ~((reg.high_bit() << 2) | rm.data().rex) << 5;
   emit(rxb | m);
 }
 
@@ -226,7 +226,7 @@ void Assembler::emit_vex_prefix(Register reg, Register vreg, Register rm,
 void Assembler::emit_vex_prefix(XMMRegister reg, XMMRegister vreg, Operand rm,
                                 VectorLength l, SIMDPrefix pp, LeadingOpcode mm,
                                 VexW w) {
-  if (rm.rex_ || mm != k0F || w != kW0) {
+  if (rm.data().rex || mm != k0F || w != kW0) {
     emit_vex3_byte0();
     emit_vex3_byte1(reg, rm, mm);
     emit_vex3_byte2(w, vreg, l, pp);
@@ -413,49 +413,6 @@ void RelocInfo::Visit(ObjectVisitor* visitor) {
   }
 }
 
-// -----------------------------------------------------------------------------
-// Implementation of Operand
-
-void Operand::set_modrm(int mod, Register rm_reg) {
-  DCHECK(is_uint2(mod));
-  buf_[0] = mod << 6 | rm_reg.low_bits();
-  // Set REX.B to the high bit of rm.code().
-  rex_ |= rm_reg.high_bit();
-}
-
-
-void Operand::set_sib(ScaleFactor scale, Register index, Register base) {
-  DCHECK_EQ(len_, 1);
-  DCHECK(is_uint2(scale));
-  // Use SIB with no index register only for base rsp or r12. Otherwise we
-  // would skip the SIB byte entirely.
-  DCHECK(index != rsp || base == rsp || base == r12);
-  buf_[1] = (scale << 6) | (index.low_bits() << 3) | base.low_bits();
-  rex_ |= index.high_bit() << 1 | base.high_bit();
-  len_ = 2;
-}
-
-void Operand::set_disp8(int disp) {
-  DCHECK(is_int8(disp));
-  DCHECK(len_ == 1 || len_ == 2);
-  int8_t* p = reinterpret_cast<int8_t*>(&buf_[len_]);
-  *p = disp;
-  len_ += sizeof(int8_t);
-}
-
-void Operand::set_disp32(int disp) {
-  DCHECK(len_ == 1 || len_ == 2);
-  int32_t* p = reinterpret_cast<int32_t*>(&buf_[len_]);
-  *p = disp;
-  len_ += sizeof(int32_t);
-}
-
-void Operand::set_disp64(int64_t disp) {
-  DCHECK_EQ(1, len_);
-  int64_t* p = reinterpret_cast<int64_t*>(&buf_[len_]);
-  *p = disp;
-  len_ += sizeof(disp);
-}
 }  // namespace internal
 }  // namespace v8
 

src/x64/assembler-x64.cc

@@ -157,132 +157,189 @@ Address RelocInfo::js_to_wasm_address() const {
 // -----------------------------------------------------------------------------
 // Implementation of Operand
 
-Operand::Operand(Register base, int32_t disp) : rex_(0) {
-  len_ = 1;
-  if (base == rsp || base == r12) {
-    // SIB byte is needed to encode (rsp + offset) or (r12 + offset).
-    set_sib(times_1, rsp, base);
-  }
+namespace {
+class OperandBuilder {
+ public:
+  OperandBuilder(Register base, int32_t disp) {
+    if (base == rsp || base == r12) {
+      // SIB byte is needed to encode (rsp + offset) or (r12 + offset).
+      set_sib(times_1, rsp, base);
+    }
 
-  if (disp == 0 && base != rbp && base != r13) {
-    set_modrm(0, base);
-  } else if (is_int8(disp)) {
-    set_modrm(1, base);
-    set_disp8(disp);
-  } else {
-    set_modrm(2, base);
-    set_disp32(disp);
+    if (disp == 0 && base != rbp && base != r13) {
+      set_modrm(0, base);
+    } else if (is_int8(disp)) {
+      set_modrm(1, base);
+      set_disp8(disp);
+    } else {
+      set_modrm(2, base);
+      set_disp32(disp);
+    }
   }
-}
 
+  OperandBuilder(Register base, Register index, ScaleFactor scale,
+                 int32_t disp) {
+    DCHECK(index != rsp);
+    set_sib(scale, index, base);
+    if (disp == 0 && base != rbp && base != r13) {
+      // This call to set_modrm doesn't overwrite the REX.B (or REX.X) bits
+      // possibly set by set_sib.
+      set_modrm(0, rsp);
+    } else if (is_int8(disp)) {
+      set_modrm(1, rsp);
+      set_disp8(disp);
+    } else {
+      set_modrm(2, rsp);
+      set_disp32(disp);
+    }
+  }
 
-Operand::Operand(Register base,
-                 Register index,
-                 ScaleFactor scale,
-                 int32_t disp) : rex_(0) {
-  DCHECK(index != rsp);
-  len_ = 1;
-  set_sib(scale, index, base);
-  if (disp == 0 && base != rbp && base != r13) {
-    // This call to set_modrm doesn't overwrite the REX.B (or REX.X) bits
-    // possibly set by set_sib.
+  OperandBuilder(Register index, ScaleFactor scale, int32_t disp) {
+    DCHECK(index != rsp);
     set_modrm(0, rsp);
-  } else if (is_int8(disp)) {
-    set_modrm(1, rsp);
-    set_disp8(disp);
-  } else {
-    set_modrm(2, rsp);
+    set_sib(scale, index, rbp);
     set_disp32(disp);
   }
-}
 
+  OperandBuilder(Label* label, int addend) {
+    data_.addend = addend;
+    DCHECK_NOT_NULL(label);
+    DCHECK(addend == 0 || (is_int8(addend) && label->is_bound()));
+    set_modrm(0, rbp);
+    set_disp64(reinterpret_cast<intptr_t>(label));
+  }
 
-Operand::Operand(Register index,
-                 ScaleFactor scale,
-                 int32_t disp) : rex_(0) {
-  DCHECK(index != rsp);
-  len_ = 1;
-  set_modrm(0, rsp);
-  set_sib(scale, index, rbp);
-  set_disp32(disp);
-}
-
-Operand::Operand(Label* label, int addend) : rex_(0), len_(1), addend_(addend) {
-  DCHECK_NOT_NULL(label);
-  DCHECK(addend == 0 || (is_int8(addend) && label->is_bound()));
-  set_modrm(0, rbp);
-  set_disp64(reinterpret_cast<intptr_t>(label));
-}
-
-Operand::Operand(Operand operand, int32_t offset) {
-  DCHECK_GE(operand.len_, 1);
-  // Operand encodes REX ModR/M [SIB] [Disp].
-  byte modrm = operand.buf_[0];
-  DCHECK_LT(modrm, 0xC0);  // Disallow mode 3 (register target).
-  bool has_sib = ((modrm & 0x07) == 0x04);
-  byte mode = modrm & 0xC0;
-  int disp_offset = has_sib ? 2 : 1;
-  int base_reg = (has_sib ? operand.buf_[1] : modrm) & 0x07;
-  // Mode 0 with rbp/r13 as ModR/M or SIB base register always has a 32-bit
-  // displacement.
-  bool is_baseless = (mode == 0) && (base_reg == 0x05);  // No base or RIP base.
-  int32_t disp_value = 0;
-  if (mode == 0x80 || is_baseless) {
-    // Mode 2 or mode 0 with rbp/r13 as base: Word displacement.
-    disp_value = *bit_cast<const int32_t*>(&operand.buf_[disp_offset]);
-  } else if (mode == 0x40) {
-    // Mode 1: Byte displacement.
-    disp_value = static_cast<signed char>(operand.buf_[disp_offset]);
+  OperandBuilder(Operand operand, int32_t offset) {
+    DCHECK_GE(operand.data().len, 1);
+    // Operand encodes REX ModR/M [SIB] [Disp].
+    byte modrm = operand.data().buf[0];
+    DCHECK_LT(modrm, 0xC0);  // Disallow mode 3 (register target).
+    bool has_sib = ((modrm & 0x07) == 0x04);
+    byte mode = modrm & 0xC0;
+    int disp_offset = has_sib ? 2 : 1;
+    int base_reg = (has_sib ? operand.data().buf[1] : modrm) & 0x07;
+    // Mode 0 with rbp/r13 as ModR/M or SIB base register always has a 32-bit
+    // displacement.
+    bool is_baseless =
+        (mode == 0) && (base_reg == 0x05);  // No base or RIP base.
+    int32_t disp_value = 0;
+    if (mode == 0x80 || is_baseless) {
+      // Mode 2 or mode 0 with rbp/r13 as base: Word displacement.
+      disp_value = *bit_cast<const int32_t*>(&operand.data().buf[disp_offset]);
+    } else if (mode == 0x40) {
+      // Mode 1: Byte displacement.
+      disp_value = static_cast<signed char>(operand.data().buf[disp_offset]);
+    }
+
+    // Write new operand with same registers, but with modified displacement.
+    DCHECK(offset >= 0 ? disp_value + offset > disp_value
+                       : disp_value + offset < disp_value);  // No overflow.
+    disp_value += offset;
+    data_.rex = operand.data().rex;
+    if (!is_int8(disp_value) || is_baseless) {
+      // Need 32 bits of displacement, mode 2 or mode 1 with register rbp/r13.
+      data_.buf[0] = (modrm & 0x3F) | (is_baseless ? 0x00 : 0x80);
+      data_.len = disp_offset + 4;
+      Memory::int32_at(&data_.buf[disp_offset]) = disp_value;
+    } else if (disp_value != 0 || (base_reg == 0x05)) {
+      // Need 8 bits of displacement.
+      data_.buf[0] = (modrm & 0x3F) | 0x40;  // Mode 1.
+      data_.len = disp_offset + 1;
+      data_.buf[disp_offset] = static_cast<byte>(disp_value);
+    } else {
+      // Need no displacement.
+      data_.buf[0] = (modrm & 0x3F);  // Mode 0.
+      data_.len = disp_offset;
+    }
+    if (has_sib) {
+      data_.buf[1] = operand.data().buf[1];
+    }
   }
 
-  // Write new operand with same registers, but with modified displacement.
-  DCHECK(offset >= 0 ? disp_value + offset > disp_value
-                     : disp_value + offset < disp_value);  // No overflow.
-  disp_value += offset;
-  rex_ = operand.rex_;
-  if (!is_int8(disp_value) || is_baseless) {
-    // Need 32 bits of displacement, mode 2 or mode 1 with register rbp/r13.
-    buf_[0] = (modrm & 0x3F) | (is_baseless ? 0x00 : 0x80);
-    len_ = disp_offset + 4;
-    Memory::int32_at(&buf_[disp_offset]) = disp_value;
-  } else if (disp_value != 0 || (base_reg == 0x05)) {
-    // Need 8 bits of displacement.
-    buf_[0] = (modrm & 0x3F) | 0x40;  // Mode 1.
-    len_ = disp_offset + 1;
-    buf_[disp_offset] = static_cast<byte>(disp_value);
-  } else {
-    // Need no displacement.
-    buf_[0] = (modrm & 0x3F);  // Mode 0.
-    len_ = disp_offset;
+  void set_modrm(int mod, Register rm_reg) {
+    DCHECK(is_uint2(mod));
+    data_.buf[0] = mod << 6 | rm_reg.low_bits();
+    // Set REX.B to the high bit of rm.code().
+    data_.rex |= rm_reg.high_bit();
   }
-  if (has_sib) {
-    buf_[1] = operand.buf_[1];
+
+  void set_sib(ScaleFactor scale, Register index, Register base) {
+    DCHECK_EQ(data_.len, 1);
+    DCHECK(is_uint2(scale));
+    // Use SIB with no index register only for base rsp or r12. Otherwise we
+    // would skip the SIB byte entirely.
+    DCHECK(index != rsp || base == rsp || base == r12);
+    data_.buf[1] = (scale << 6) | (index.low_bits() << 3) | base.low_bits();
+    data_.rex |= index.high_bit() << 1 | base.high_bit();
+    data_.len = 2;
   }
-}
 
+  void set_disp8(int disp) {
+    DCHECK(is_int8(disp));
+    DCHECK(data_.len == 1 || data_.len == 2);
+    int8_t* p = reinterpret_cast<int8_t*>(&data_.buf[data_.len]);
+    *p = disp;
+    data_.len += sizeof(int8_t);
+  }
+
+  void set_disp32(int disp) {
+    DCHECK(data_.len == 1 || data_.len == 2);
+    int32_t* p = reinterpret_cast<int32_t*>(&data_.buf[data_.len]);
+    *p = disp;
+    data_.len += sizeof(int32_t);
+  }
+
+  void set_disp64(int64_t disp) {
+    DCHECK_EQ(1, data_.len);
+    int64_t* p = reinterpret_cast<int64_t*>(&data_.buf[data_.len]);
+    *p = disp;
+    data_.len += sizeof(disp);
+  }
+
+  const Operand::Data& data() const { return data_; }
+
+ private:
+  Operand::Data data_;
+};
+}  // namespace
+
+Operand::Operand(Register base, int32_t disp)
+    : data_(OperandBuilder(base, disp).data()) {}
+
+Operand::Operand(Register base, Register index, ScaleFactor scale, int32_t disp)
+    : data_(OperandBuilder(base, index, scale, disp).data()) {}
+
+Operand::Operand(Register index, ScaleFactor scale, int32_t disp)
+    : data_(OperandBuilder(index, scale, disp).data()) {}
+
+Operand::Operand(Label* label, int addend)
+    : data_(OperandBuilder(label, addend).data()) {}
+
+Operand::Operand(Operand operand, int32_t offset)
+    : data_(OperandBuilder(operand, offset).data()) {}
 
 bool Operand::AddressUsesRegister(Register reg) const {
   int code = reg.code();
-  DCHECK_NE(buf_[0] & 0xC0, 0xC0);  // Always a memory operand.
-  // Start with only low three bits of base register. Initial decoding doesn't
-  // distinguish on the REX.B bit.
-  int base_code = buf_[0] & 0x07;
+  DCHECK_NE(data_.buf[0] & 0xC0, 0xC0);  // Always a memory operand.
+  // Start with only low three bits of base register. Initial decoding
+  // doesn't distinguish on the REX.B bit.
+  int base_code = data_.buf[0] & 0x07;
   if (base_code == rsp.code()) {
     // SIB byte present in buf_[1].
     // Check the index register from the SIB byte + REX.X prefix.
-    int index_code = ((buf_[1] >> 3) & 0x07) | ((rex_ & 0x02) << 2);
+    int index_code = ((data_.buf[1] >> 3) & 0x07) | ((data_.rex & 0x02) << 2);
     // Index code (including REX.X) of 0x04 (rsp) means no index register.
     if (index_code != rsp.code() && index_code == code) return true;
     // Add REX.B to get the full base register code.
-    base_code = (buf_[1] & 0x07) | ((rex_ & 0x01) << 3);
+    base_code = (data_.buf[1] & 0x07) | ((data_.rex & 0x01) << 3);
     // A base register of 0x05 (rbp) with mod = 0 means no base register.
-    if (base_code == rbp.code() && ((buf_[0] & 0xC0) == 0)) return false;
+    if (base_code == rbp.code() && ((data_.buf[0] & 0xC0) == 0)) return false;
     return code == base_code;
   } else {
     // A base register with low bits of 0x05 (rbp or r13) and mod = 0 means
     // no base register.
-    if (base_code == rbp.code() && ((buf_[0] & 0xC0) == 0)) return false;
-    base_code |= ((rex_ & 0x01) << 3);
+    if (base_code == rbp.code() && ((data_.buf[0] & 0xC0) == 0)) return false;
+    base_code |= ((data_.rex & 0x01) << 3);
     return code == base_code;
   }
 }
@@ -528,19 +585,20 @@ void Assembler::GrowBuffer() {
 
 void Assembler::emit_operand(int code, Operand adr) {
   DCHECK(is_uint3(code));
-  const unsigned length = adr.len_;
+  const unsigned length = adr.data().len;
   DCHECK_GT(length, 0);
 
   // Emit updated ModR/M byte containing the given register.
-  DCHECK_EQ(adr.buf_[0] & 0x38, 0);
-  *pc_++ = adr.buf_[0] | code << 3;
+  DCHECK_EQ(adr.data().buf[0] & 0x38, 0);
+  *pc_++ = adr.data().buf[0] | code << 3;
 
   // Recognize RIP relative addressing.
-  if (adr.buf_[0] == 5) {
+  if (adr.data().buf[0] == 5) {
     DCHECK_EQ(9u, length);
-    Label* label = *bit_cast<Label* const*>(&adr.buf_[1]);
+    Label* label = *bit_cast<Label* const*>(&adr.data().buf[1]);
     if (label->is_bound()) {
-      int offset = label->pos() - pc_offset() - sizeof(int32_t) + adr.addend_;
+      int offset =
+          label->pos() - pc_offset() - sizeof(int32_t) + adr.data().addend;
       DCHECK_GE(0, offset);
       emitl(offset);
     } else if (label->is_linked()) {
@@ -554,7 +612,7 @@ void Assembler::emit_operand(int code, Operand adr) {
     }
   } else {
     // Emit the rest of the encoded operand.
-    for (unsigned i = 1; i < length; i++) *pc_++ = adr.buf_[i];
+    for (unsigned i = 1; i < length; i++) *pc_++ = adr.data().buf[i];
   }
 }
 

src/x64/assembler-x64.h

@@ -333,6 +333,13 @@ enum ScaleFactor : int8_t {
 
 class Operand {
  public:
+  struct Data {
+    byte rex = 0;
+    byte buf[9];
+    byte len = 1;   // number of bytes of buf_ in use.
+    int8_t addend;  // for rip + offset + addend.
+  };
+
   // [base + disp/r]
   Operand(Register base, int32_t disp);
 
@@ -355,46 +362,37 @@ class Operand {
   // [rip + disp/r]
   explicit Operand(Label* label, int addend = 0);
 
+  Operand(const Operand&) = default;
+
   // Checks whether either base or index register is the given register.
   // Does not check the "reg" part of the Operand.
   bool AddressUsesRegister(Register reg) const;
 
   // Queries related to the size of the generated instruction.
   // Whether the generated instruction will have a REX prefix.
-  bool requires_rex() const { return rex_ != 0; }
+  bool requires_rex() const { return data_.rex != 0; }
   // Size of the ModR/M, SIB and displacement parts of the generated
   // instruction.
-  int operand_size() const { return len_; }
-
- private:
-  byte rex_;
-  byte buf_[9];
-  // The number of bytes of buf_ in use.
-  byte len_;
-
-  int8_t addend_;  // for rip + offset + addend
-
-  // Set the ModR/M byte without an encoded 'reg' register. The
-  // register is encoded later as part of the emit_operand operation.
-  // set_modrm can be called before or after set_sib and set_disp*.
-  inline void set_modrm(int mod, Register rm);
+  int operand_size() const { return data_.len; }
 
-  // Set the SIB byte if one is needed. Sets the length to 2 rather than 1.
-  inline void set_sib(ScaleFactor scale, Register index, Register base);
+  const Data& data() const { return data_; }
 
-  // Adds operand displacement fields (offsets added to the memory address).
-  // Needs to be called after set_sib, not before it.
-  inline void set_disp8(int disp);
-  inline void set_disp32(int disp);
-  inline void set_disp64(int64_t disp);  // for labels.
-
-  // TODO(clemensh): Get rid of this friendship, or make Operand immutable.
-  friend class Assembler;
+ private:
+  const Data data_;
 };
 static_assert(sizeof(Operand) <= 2 * kPointerSize,
               "Operand must be small enough to pass it by value");
+// Unfortunately, MSVC 2015 is broken in that both is_trivially_destructible and
+// is_trivially_copy_constructible are true, but is_trivially_copyable is false.
+// (status at 2018-02-26, observed on the msvc waterfall bot).
+#if V8_CC_MSVC
+static_assert(std::is_trivially_copy_constructible<Operand>::value &&
+                  std::is_trivially_destructible<Operand>::value,
+              "Operand must be trivially copyable to pass it by value");
+#else
 static_assert(IS_TRIVIALLY_COPYABLE(Operand),
               "Operand must be trivially copyable to pass it by value");
+#endif
 
 #define ASSEMBLER_INSTRUCTION_LIST(V) \
   V(add)                              \