code: void _emitRexR(int w, int opReg, int regCode) {
          if (is64()) {
  
              boolean r = (opReg & 0x8) != 0;
              boolean b = (regCode & 0x8) != 0;
  
              // w Default operand size(0=Default, 1=64 bits).
              // r Register field (1=high bit extension of the ModR/M REG field).
              // x Index field not used in RexR
              // b Base field (1=high bit extension of the ModR/M or SIB Base field).
              if (w != 0 || r || b || (_properties & (1 << PROPERTY_X86_FORCE_REX)) != 0) {
                  _emitByte(0x40 | (w << 3) | (intValue(r) << 2) | intValue(b));
              }
          }
      }
docs: Emit REX prefix (64 bit mode only).