问题描述
正如标题所说。
我这样做：
As title says.My way to do this:
; eax holds 32bit integer
sal rax, 32
sar rax, 32
;after operation int is converted (rax holds the same value on 64 bits)
有更优雅/更好/更快的方法吗？
Is there more elegant/better/faster way to do this??
推荐答案
TL; DR ： movsxd ，或者对于eax特例， cdqe 。
TL;DR: movsxd, or for the eax special-case, cdqe.
就像我在你的一个，您可以通过查看编译器自己回答这些问题输出。 真的很容易编写一个符号扩展整数的函数，然后查看它在手册中使用的指令：
Like I explained in one of your previous questions, you can answer these yourself by looking at compiler output. It's really easy to write a function that sign-extends an integer, then go look up the instruction it uses in the manual:
#include <stdint.h>
int64_t sign_extend(int32_t a) { return a; }
movsxd rax, edi
ret
来自 tp：//www.felixcloutier.com/x86/\"rel =nofollow noreferrer>指令集参考，你就完成了。 （这是英特尔PDF的转换（。 8086没有 movsx 直到386，只有al - > ax cbw 。
There's a special-case for rax, called cdqe. 8086 didn't have movsx until 386, just the al -> ax cbw.
这是和其他只有ax的东西是为什么英特尔在 8086.AMD可能应该为AMD64收回这些内容，用于未来的指令集扩展，因为AMD64是第一个（并且很可能只是很长一段时间）打破向后兼容8086操作码或386的32位模式机器代码的机会。
This is and other ax-only stuff is why Intel spent 8 opcodes on single-byte encodings for xchg ax, reg in 8086. AMD arguably should have reclaimed these for AMD64, for use in future instruction-set extensions, since AMD64 was the first (and probably only for a very long time) opportunity to break backwards compatibility with 8086 opcodes, or 386's 32bit-mode machine-code.
这是单字节NOP（ 90 ）编码的来源： xchg eax，eax 被认为是一个有效执行的特例（不是免费的，但比 mov eax，eax 便宜）。在64位模式下，它不会将eax零扩展到rax中。 （英特尔文件为一个单独的指令（以及长NOP编码），并没有在）
This is where the single-byte NOP (90) encoding comes from: xchg eax,eax is recognized as a special-case that's executed efficiently (not free, but cheaper than mov eax,eax for example). In 64bit mode, it doesn't zero-extend eax into rax. (Intel documents nop as a separate instruction (along with the long-NOP encoding), and doesn't mention this special-case behaviour in the entry for xchg)
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