gistfile1.cpp

// Returns number of bytes used, including *data.
int DisassemblerX64::JumpConditionalShort(byte* data) {
  byte cond = *data & 0x0F;
  byte b = *(data + 1);
  byte* dest = data + static_cast<int8_t>(b) + 2;
  const char* mnem = conditional_code_suffix[cond];
  AppendToBuffer("j%s %s", mnem, NameOfAddress(dest));
  return 2;
}


//
//
//  elsewhere....
//
//

Operand::Operand(const Operand& operand, int32_t offset) {
  ASSERT(operand.len_ >= 1);
  // Operand encodes REX ModR/M [SIB] [Disp].
  byte modrm = operand.buf_[0];
  ASSERT(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 = *BitCast<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]);
  }

  // Write new operand with same registers, but with modified displacement.
  ASSERT(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;
  }
  if (has_sib) {
    buf_[1] = operand.buf_[1];
  }
}