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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
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<!-- 2021-01-23 Sat 18:03 -->
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<title>golang汇编基础知识</title>
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<div id="org-div-home-and-up">
<a accesskey="h" href="index.html"> UP </a>
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<h1 class="title">golang汇编基础知识</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#org45e8e1f">1. golang汇编基础知识</a>
<ul>
<li><a href="#org1264c7d">1.1. 前言</a></li>
<li><a href="#org0c10809">1.2. 寄存器</a></li>
<li><a href="#org790433a">1.3. 跟例子学汇编</a></li>
<li><a href="#org75d01ff">1.4. 第一个例子</a></li>
<li><a href="#org7ff5117">1.5. 第二个例子</a></li>
<li><a href="#org0e0adfb">1.6. 参考资料</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-org45e8e1f" class="outline-2">
<h2 id="org45e8e1f"><span class="section-number-2">1</span> golang汇编基础知识</h2>
<div class="outline-text-2" id="text-1">
</div>
<div id="outline-container-org1264c7d" class="outline-3">
<h3 id="org1264c7d"><span class="section-number-3">1.1</span> 前言</h3>
<div class="outline-text-3" id="text-1-1">
<p>
golang的汇编基于plan9汇编,是一个中间汇编方式。这样可以忽略底层不同架构之间的一些差别。汇编主要了解各种寄存器的使用跟寻址方式。根据汇编我们能够一探golang的底层实现。比如内存如何分配，栈如何扩张。接口如何转变。
</p>
</div>
</div>
<div id="outline-container-org0c10809" class="outline-3">
<h3 id="org0c10809"><span class="section-number-3">1.2</span> 寄存器</h3>
<div class="outline-text-3" id="text-1-2">
<p>
各种伪计数器:
</p>
<ul class="org-ul">
<li>FP: Frame pointer: arguments and locals.(指向当前栈帧)</li>
<li>PC: Program counter: jumps and branches.(指向指令地址)</li>
<li>SB: Static base pointer: global symbols.(指向全局符号表)</li>
<li>SP: Stack pointer: top of stack.(指向当前栈顶部)</li>
</ul>
<p>
注意: 栈是向下整长
golang的汇编是调用者维护参数返回值跟返回地址。所以FP的值小于参数跟返回值。
</p>
</div>
</div>
<div id="outline-container-org790433a" class="outline-3">
<h3 id="org790433a"><span class="section-number-3">1.3</span> 跟例子学汇编</h3>
<div class="outline-text-3" id="text-1-3">
</div>
<div id="outline-container-org75d01ff" class="outline-3">
<h3 id="org75d01ff"><span class="section-number-3">1.4</span> 第一个例子</h3>
<div class="outline-text-3" id="text-1-4">
<div class="org-src-container">
<pre class="src src-go"><span style="color: #89DDFF;">package</span> main
<span style="color: #676E95;">//</span><span style="color: #676E95;">go:noinline</span>
<span style="color: #89DDFF;">func</span> <span style="color: #82aaff;">add</span>(<span style="color: #ffcb6b;">a</span>, <span style="color: #ffcb6b;">b</span> <span style="color: #c792ea;">int32</span>) (<span style="color: #c792ea;">int32</span>, <span style="color: #c792ea;">bool</span>) {
<span style="color: #89DDFF;">return</span> a + b, <span style="color: #f78c6c;">true</span>
}
<span style="color: #89DDFF;">func</span> <span style="color: #82aaff;">main</span>() {
<span style="color: #82aaff;">add</span>(10, 32)
}
</pre>
</div>
<p>
如上的这段代码会被编译为以下汇编:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #c3e88d;">""</span>.add STEXT nosplit size=20 args=0x10 locals=0x0
<span style="color: #89DDFF;">0x0000</span> 00000 (call.go:4) TEXT <span style="color: #c3e88d;">""</span>.add(SB), NOSPLIT, $0-16
<span style="color: #89DDFF;">0x0000</span> 00000 (call.go:4) FUNCDATA $0, gclocals·f207267fbf96a0178e8758c6e3e0ce28(SB)
<span style="color: #89DDFF;">0x0000</span> 00000 (call.go:4) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
<span style="color: #89DDFF;">0x0000</span> 00000 (call.go:4) MOVL <span style="color: #c3e88d;">""</span>.b+12(SP), AX
<span style="color: #89DDFF;">0x0004</span> 00004 (call.go:4) MOVL <span style="color: #c3e88d;">""</span>.a+8(SP), CX
<span style="color: #89DDFF;">0x0008</span> 00008 (call.go:5) ADDL CX, AX
<span style="color: #89DDFF;">0x000a</span> 00010 (call.go:5) MOVL AX, <span style="color: #c3e88d;">""</span>.~r2+16(SP)
<span style="color: #89DDFF;">0x000e</span> 00014 (call.go:5) MOVB $1, <span style="color: #c3e88d;">""</span>.~r3+20(SP)
<span style="color: #89DDFF;">0x0013</span> 00019 (call.go:5) RET
<span style="color: #89DDFF;">0x0000</span> 8b 44 24 0c 8b 4c 24 08 01 c8 89 44 24 10 c6 44 .D$..L$....D$..D
<span style="color: #89DDFF;">0x0010</span> 24 14 01 c3 $...
</pre>
</div>
<p>
<code>TEXT "".add(SB), NOSPLIT, $0-16</code> 这行代码声明了一个add函数，NOSPLIT告诉编译器不要插入分裂栈检查点。这里可以不用检查栈的大小是因为这里add函数的栈大小为0. $0 表示栈大小为0, $16 表示参数大小为16个字节。
<code>FUNCDATA</code> 跟垃圾回收有关，暂时不用理会
SP 指向当前栈顶部 .a+8(SP) 之前的.a表示助记符，没有特别含义。8(SP)表示地址SP+8 这里是第一参数a, 因为当前栈为0且SP+0存储了函数返回地址。同理12(SP) 表示第二个参数b。16(SP)表示第一个返回值，20(SP)表示第二个返回值。
</p>
</div>
</div>
<div id="outline-container-org7ff5117" class="outline-3">
<h3 id="org7ff5117"><span class="section-number-3">1.5</span> 第二个例子</h3>
<div class="outline-text-3" id="text-1-5">
<div class="org-src-container">
<pre class="src src-go"><span style="color: #89DDFF;">package</span> main
<span style="color: #676E95;">//</span><span style="color: #676E95;">go:noinline</span>
<span style="color: #89DDFF;">func</span> <span style="color: #82aaff;">add</span>(<span style="color: #ffcb6b;">a</span>, <span style="color: #ffcb6b;">b</span> <span style="color: #c792ea;">int32</span>) (<span style="color: #c792ea;">int32</span>, <span style="color: #c792ea;">bool</span>) {
<span style="color: #89DDFF;">return</span> a + b, <span style="color: #f78c6c;">true</span>
}
<span style="color: #676E95;">//</span><span style="color: #676E95;">go:noinline</span>
<span style="color: #89DDFF;">func</span> <span style="color: #82aaff;">callAdd</span>() <span style="color: #c792ea;">int32</span> {
<span style="color: #ffcb6b;">a</span>, <span style="color: #ffcb6b;">_</span> := <span style="color: #82aaff;">add</span>(10, 20)
<span style="color: #89DDFF;">return</span> a
}
<span style="color: #89DDFF;">func</span> <span style="color: #82aaff;">main</span>() {
<span style="color: #82aaff;">callAdd</span>()
}
</pre>
</div>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #c3e88d;">""</span>.callAdd STEXT size=73 args=0x8 locals=0x18
<span style="color: #82aaff;">0x0000</span> <span style="color: #89DDFF;">00000</span> (call.go:9) TEXT <span style="color: #c3e88d;">""</span>.callAdd(SB), $24-8
<span style="color: #676E95;">; </span><span style="color: #676E95;">这三行代码检查是否需要扩展栈, 需要的话，跳到0x0042</span>
<span style="color: #82aaff;">0x0000</span> <span style="color: #89DDFF;">00000</span> (call.go:9) MOVQ (TLS), CX
<span style="color: #82aaff;">0x0009</span> <span style="color: #89DDFF;">00009</span> (call.go:9) CMPQ SP, 16(CX)
<span style="color: #82aaff;">0x000d</span> <span style="color: #89DDFF;">00013</span> (call.go:9) JLS 66
<span style="color: #82aaff;">0x000f</span> <span style="color: #89DDFF;">00015</span> (call.go:9) SUBQ $24, SP
<span style="color: #82aaff;">0x0013</span> <span style="color: #89DDFF;">00019</span> (call.go:9) MOVQ BP, 16(SP)
<span style="color: #82aaff;">0x0018</span> <span style="color: #89DDFF;">00024</span> (call.go:9) LEAQ 16(SP), BP
<span style="color: #82aaff;">0x001d</span> <span style="color: #89DDFF;">00029</span> (call.go:9) FUNCDATA $0, gclocals·2a5305abe05176240e61b8620e19a815(SB)
<span style="color: #82aaff;">0x001d</span> <span style="color: #89DDFF;">00029</span> (call.go:9) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
<span style="color: #82aaff;">0x001d</span> <span style="color: #89DDFF;">00029</span> (call.go:10) MOVQ $85899345930, AX
<span style="color: #82aaff;">0x0027</span> <span style="color: #89DDFF;">00039</span> (call.go:10) MOVQ AX, (SP)
<span style="color: #82aaff;">0x002b</span> <span style="color: #89DDFF;">00043</span> (call.go:10) PCDATA $0, $0
<span style="color: #82aaff;">0x002b</span> <span style="color: #89DDFF;">00043</span> (call.go:10) CALL <span style="color: #c3e88d;">""</span>.add(SB)
<span style="color: #82aaff;">0x0030</span> <span style="color: #89DDFF;">00048</span> (call.go:10) MOVL 8(SP), AX
<span style="color: #82aaff;">0x0034</span> <span style="color: #89DDFF;">00052</span> (call.go:11) MOVL AX, <span style="color: #c3e88d;">""</span>.~r0+32(SP)
<span style="color: #82aaff;">0x0038</span> <span style="color: #89DDFF;">00056</span> (call.go:11) MOVQ 16(SP), BP
<span style="color: #82aaff;">0x003d</span> <span style="color: #89DDFF;">00061</span> (call.go:11) ADDQ $24, SP
<span style="color: #82aaff;">0x0041</span> <span style="color: #89DDFF;">00065</span> (call.go:11) RET
<span style="color: #676E95;">; </span><span style="color: #676E95;">扩展栈, 扩展完了后跳到最开始</span>
<span style="color: #82aaff;">0x0042</span> <span style="color: #89DDFF;">00066</span> (call.go:11) NOP
<span style="color: #82aaff;">0x0042</span> <span style="color: #89DDFF;">00066</span> (call.go:9) PCDATA $0, $-1
<span style="color: #82aaff;">0x0042</span> <span style="color: #89DDFF;">00066</span> (call.go:9) CALL runtime.morestack_noctxt(SB)
<span style="color: #82aaff;">0x0047</span> <span style="color: #89DDFF;">00071</span> (call.go:9) JMP 0
</pre>
</div>
<p>
现在我们添加一层函数调用，看看生成后的样子, 主要关注callAdd函数。在这个例子中我们看到这里的栈大小是24byte(2个参数2个返回值(bool 对齐到4字节, 4 * 4 = 16 byte)，再加上一个8byte的bp)，参数是8byte(对齐到8byte)。
我们从0x000f说起，前面那段代码主要用于分裂栈检查。
0x000f 把栈减了24个字节。增大了栈空间。(向下增长)
0x0013-0x0018保存老的bp设置新的bp。这里的bp是真实的寄存器，这几行代码属于惯用法。在汇编中可以用bp来引用参数跟本地变量。
0x001d 是10跟20合起来的(八字节)二进制表示。本来需要两次move，现在只需要一次move，也算个小优化了吧。
接下来就是调用add方法，移动返回值。虽然我们没有使用add的第二个返回值，但是我们也要为他分配内存。
0x0038恢复BP寄存器，0x003d缩减栈空间.
RET指令加载返回地址，跳到返回地址。
</p>
</div>
</div>
</div>
<div id="outline-container-org0e0adfb" class="outline-3">
<h3 id="org0e0adfb"><span class="section-number-3">1.6</span> 参考资料</h3>
<div class="outline-text-3" id="text-1-6">
<ul class="org-ul">
<li><a href="https://lrita.github.io/2017/12/12/golang-asm/">https://lrita.github.io/2017/12/12/golang-asm/</a></li>
<li><a href="https://lrita.github.io/images/posts/go/GoFunctionsInAssembly.pdf">https://lrita.github.io/images/posts/go/GoFunctionsInAssembly.pdf</a></li>
<li><a href="https://github.com/go-internals-cn/go-internals">https://github.com/go-internals-cn/go-internals</a></li>
<li><a href="https://golang.org/doc/asm">https://golang.org/doc/asm</a></li>
<li><a href="https://9p.io/sys/doc/asm.html">https://9p.io/sys/doc/asm.html</a></li>
</ul>
</div>
</div>
</div>
</div>
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