Sign Extend Word sign-extends a word to the size of the register, and writes the result to the destination register.
符号扩展将双字扩展到寄存器的大小，并将结果写入目标寄存器。
sxtw x6, w7 // Sign-extend word in w7 to 64-bits
x6 = (int_64) w7;
4037d0: 11000480 add w0, w4, #0x1
4037d4: 93407c00 sxtw x0, w0
4037d8: 38606b21 ldrb w1, [x25, x0]