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为什么写这篇文章？
昨天在技术群上，有人问了个问题：
随便一猜：golang 肯定是直接 copy 整个结构体。
为了确认是否真的是这样，最直白的方式就是直接看 golang 生成的汇编代码。
从图中的汇编代码中，我们可以清楚的看到：golang 的确是执行了完整的结构体 copy 。
然后群友给了这样的反馈…
看着自己日益升高的发际线，我陷入了沉思…
好了，进入正题。
本文将以 1 + … + 100 的代码为例，介绍以下几种语言查看“汇编代码”的方式。
（这里的“汇编代码”只是个统称，大家不用太计较）
- GolangLuaJavaScript（ V8 ）Rust Python 等等 …
1. Golang 生成汇编代码
源码
package main
func main(){
var sum = 0
for i:=1 ; i <= 100; i++ {
sum = sum + i
}
}
查看方式
go tool compile -S . test .go
go tool objdump .test.o
分析
行 1 : 表示 将数值 1 放到 AX 中
行 2 : 表示 跳转到 行 4
行 3 : 表示 对 AX 中的数值执行 + 1 操作
行 4 : 比较 AX 是否在 100 以内。（0x64 是 数值 100 的 16 进制）
行 5 : 跳转到 行 3
嗯… 怎么感觉就是在空循环，我们的 sum 变量哪里去了？
事实上：golang 检测到 sum 没有被使用，直接就帮我们优化掉了，只留下一个空循环。
（但它没有彻底的帮我们把这个空循环也删掉…)
如果我们把代码改成这样子：
package main
func main(){
}
func getSum()int{
var sum = 0
for i:=1 ; i <= 100; i++ {
sum = sum + i
}
return sum
}
那么 sum 累加部分的汇编就是可以正常的显示出来，如图所示：
2. Lua 生成汇编代码
源码
function getSum() local sum = 0 for i=1, 100 do sum = sum + i end return sum end
查看方式
luac.exe -l test.lua
分析
Lua 虚拟机是基于寄存器来实现的。这段汇编代码读起来，就不像golang那么好理解了。
（关于寄存器虚拟机的相关内容，我在文章的评论中补充了一些。有兴趣的话，可以看看）
这里我就简单的分析下：
行 1-4：将常量表里的 0，1，100，1分别加载到寄存器中。
LOADK 指令后面的跟着 2 个参数，分别是：参数1 寄存器索引，参数2 常量表索引
分号后面的数值是： 具体的常量值
这四个数字中：
第一个数字 0 ，就是 sum 的初始值 。
后面三个数（1， 100， 1）：表示了循环从 1 开始，到 100 结束，步长为 1
（ 也就是说 i 从 1 开始，每次循环自动+1，直到达到 100）
行 5-7：执行循环
Lua 通过 FORPREP、FORLOOP 两条指令来实现循环。
它们的第一个参数：表示指向 循环所需的三个数字 的起始寄存器索引，也就是 寄存器 1。
（这样虚拟机就知道了，循环所需的三个数字：1，100，1，从而准确的控制循环的逻辑）
它们的第二个参数，表示PC指令跳转的距离。
注意：FORLOOP 会把 i+1 的结果 放在寄存器 4 中，对应 add 的第 3 个参数
行 6：执行 Add 操作
Add 后面跟着 3 个参数。含义如下：
参数1：存放结果的寄存器索引
参数2、参数3： 分别是两个加数的索引位置
既然已经分析了 golang 和 lua 两个语言生成的汇编代码，后面的语言就不再详细的分析了，基本大同小异。
3. JavaScript 生成汇编代码
function getSum(){
let sum = 0;
for(let i = 1 ; i <= 100; i++){
sum += i
}
return sum
}
查看方式
node --print-bytecode .test.js
4. Rust 生成汇编代码
rust 就比较有意思了（和 c++ 差不多），值得稍微提一下。
fn main() {
println!("{}", get_sum());
}
fn get_sum() -> i32{
let mut sum = 0;
for i in 1..=100 {
sum += i
}
return sum
}
查看方式
rustc -O --emit asm=test_rust.s .test_rust.rs
加入了 -O 优化之后，生成的汇编代码是这个样子，它直接用 5050 来赋值，省略了计算的过程。
5. Python 生成汇编代码
import dis def getSum(): sum = 0 for i in range(1,101): sum = sum + i return sum dis.dis(getSum)
查看方式
python .test.py # 也可以通过 python -m py_compile .test.py 来编译出 pyc 文件 # 再通过第三方工具，来查看 pyc 的字节码
其它语言:有时间再补充
…
最后
那么了解汇编层面的东西，有什么用呢？
嗯…对于绝大多数开发者来说，的确没什么用…
但是如果你有深入底层的追求，那么了解汇编的知识，会让你对程序理解的更加透彻。
而且，不管是 Rust，还是 C++ 都支持 内联汇编：可以在代码里嵌入汇编语言，直接控制 CPU，从而获取极致的底层操作、以及性能上的提升。
比如：腾讯开源的C++协程库：Tencent/libco 就用到了该技术方案。
学习C/C++的伙伴可以私信回复小编“学习”领取全套免费C/C++学习资料、视频
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