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<h1 class="post-title">How C++ Lambda Function Is Compiled</h1>
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<a href="https://raptorsun.github.io/author/haoyu-sun.html">Haoyu sun</a>
| <time datetime="Mon 17 June 2019">Mon 17 June 2019</time>
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<p><a href="https://en.cppreference.com/w/cpp/language/lambda">Lambda function</a> is a convenient way of defining an anonymous function in the place where it is invoked or assigned as an argurment or variable. It is largely used to pass a short stateless function to an algorithm (comparison to sort algorithm), and as a callback from asynchronous methods. I wonder how lambda functions are compiled. Are they functions with a random name? Are they inline functions expressing simple logics?</p>
<p>Lets start with a minimalistic case.</p>
<div class="highlight"><pre><span></span> <span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="k">return</span> <span class="p">[]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">6</span><span class="p">;</span>
<span class="p">}</span> <span class="p">();</span>
<span class="p">}</span>
</pre></div>
<p>It calls a lambda function which does nothing else than returning an integer 6. The corresponding assembly shows that these codes generate 2 functions: the main and the lambda function.</p>
<div class="highlight"><pre><span></span> <span class="na">.section</span> <span class="no">__TEXT</span><span class="p">,</span><span class="no">__text</span><span class="p">,</span><span class="no">regular</span><span class="p">,</span><span class="no">pure_instructions</span>
<span class="na">.build_version</span> <span class="no">macos</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">14</span> <span class="no">sdk_version</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">14</span>
<span class="na">.globl</span> <span class="no">_main</span> <span class="c">## -- Begin function main</span>
<span class="na">.p2align</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">0x90</span>
<span class="nl">_main:</span> <span class="c">## @main</span>
<span class="na">.cfi_startproc</span>
<span class="c">## %bb.0:</span>
<span class="nf">pushq</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_offset</span> <span class="mi">16</span>
<span class="na">.cfi_offset</span> <span class="nv">%rbp</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span>
<span class="nf">movq</span> <span class="nv">%rsp</span><span class="p">,</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_register</span> <span class="nv">%rbp</span>
<span class="nf">subq</span> <span class="no">$32</span><span class="p">,</span> <span class="nv">%rsp</span>
<span class="nf">movl</span> <span class="no">$0</span><span class="p">,</span> <span class="p">-</span><span class="mi">4</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="nv">%edi</span><span class="p">,</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movq</span> <span class="nv">%rsi</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">leaq</span> <span class="p">-</span><span class="mi">24</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">callq</span> <span class="no">__ZZ4mainENK3$_0clEv</span>
<span class="nf">addq</span> <span class="no">$32</span><span class="p">,</span> <span class="nv">%rsp</span>
<span class="nf">popq</span> <span class="nv">%rbp</span>
<span class="nf">retq</span>
<span class="na">.cfi_endproc</span>
<span class="c">## -- End function</span>
<span class="na">.p2align</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">0x90</span> <span class="c">## -- Begin function _ZZ4mainENK3$_0clEv (lambda function)</span>
<span class="nl">__ZZ4mainENK3$_0clEv:</span> <span class="c">## @"_ZZ4mainENK3$_0clEv"</span>
<span class="na">.cfi_startproc</span>
<span class="c">## %bb.0:</span>
<span class="nf">pushq</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_offset</span> <span class="mi">16</span>
<span class="na">.cfi_offset</span> <span class="nv">%rbp</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span>
<span class="nf">movq</span> <span class="nv">%rsp</span><span class="p">,</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_register</span> <span class="nv">%rbp</span>
<span class="nf">movq</span> <span class="nv">%rdi</span><span class="p">,</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="no">$6</span><span class="p">,</span> <span class="nv">%eax</span>
<span class="nf">popq</span> <span class="nv">%rbp</span>
<span class="nf">retq</span>
<span class="na">.cfi_endproc</span>
<span class="c">## -- End function</span>
<span class="na">.subsections_via_symbols</span>
</pre></div>
<p>The "anonymous" function does have a name, which is _ZZ4mainENK3$_0clEv. Let's demangle it to see what does it mean.</p>
<div class="highlight"><pre><span></span><span class="err">➜ lambda_cpp nm lambda.out | c++filt</span>
<span class="err">0000000100000fa0 unsigned short main::$_0::operator()() const</span>
<span class="err">0000000100000000 T __mh_execute_header</span>
<span class="err">0000000100000f70 T _main</span>
<span class="err"> U dyld_stub_binder</span>
</pre></div>
<p>It is defined as a const function under the namespace of "main", returing an unsigned short, which is obviously deduced from the constant "6". The name "$_0::operator()()" seems like a wrapper class "$0" with a function "operator()()". The name $_0 may suggest a auto-increasing key for each lambda function in the same scope. Lets try define multiple lambda function and see how they are named.</p>
<div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="k">auto</span> <span class="n">funcA</span> <span class="o">=</span> <span class="p">[]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">1</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">auto</span> <span class="n">funcB</span> <span class="o">=</span> <span class="p">[]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">2</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">auto</span> <span class="n">funcC</span> <span class="o">=</span> <span class="p">[]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">3</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">return</span> <span class="n">funcA</span><span class="p">()</span> <span class="o">+</span> <span class="n">funcB</span><span class="p">()</span> <span class="o">+</span> <span class="n">funcC</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
<p>This time we got main::$_[0-2]::operator()() functions. That wrapper class is in fact a number. The naming convention is under the namespace where it is defined, a wrapper class with an increasing number to disinguish one from another, then the function itself "operator ()" with its arguments' list.</p>
<div class="highlight"><pre><span></span><span class="err">0000000100000f80 unsigned short main::$_0::operator()() const</span>
<span class="err">0000000100000f90 unsigned short main::$_1::operator()() const</span>
<span class="err">0000000100000fa0 unsigned short main::$_2::operator()() const</span>
</pre></div>
<p>Now let's try adding parameters into the lambda function, starting with the capture clause "[]". We put a local variable into the capture clause so that it can be used inside the lambda function. In this example we use the copy capture clause, which is the default behavior.</p>
<div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">var</span> <span class="o">=</span> <span class="mi">7</span><span class="p">;</span>
<span class="k">auto</span> <span class="n">funcA</span> <span class="o">=</span> <span class="p">[</span><span class="o">=</span><span class="p">]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">var</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">return</span> <span class="n">funcA</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
<p>From the output code we can see in this case it is exactly the same sequence as a regular function call: copy local variable into stack, move the stack upward and then call the function.</p>
<div class="highlight"><pre><span></span> <span class="nf">movl</span> <span class="no">$7</span><span class="p">,</span> <span class="p">-</span><span class="mi">20</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="p">-</span><span class="mi">20</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%edi</span>
<span class="nf">movl</span> <span class="nv">%edi</span><span class="p">,</span> <span class="p">-</span><span class="mi">24</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">leaq</span> <span class="p">-</span><span class="mi">24</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">callq</span> <span class="no">__ZZ4mainENK3$_0clEv</span>
</pre></div>
<p>Meanwhile in lambda function has just a normal function body, getting the argument from stack and copy it into the return register.</p>
<div class="highlight"><pre><span></span> <span class="na">.p2align</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">0x90</span> <span class="c">## -- Begin function _ZZ4mainENK3$_0clEv</span>
<span class="nl">__ZZ4mainENK3$_0clEv:</span> <span class="c">## @"_ZZ4mainENK3$_0clEv"</span>
<span class="na">.cfi_startproc</span>
<span class="c">## %bb.0:</span>
<span class="nf">pushq</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_offset</span> <span class="mi">16</span>
<span class="na">.cfi_offset</span> <span class="nv">%rbp</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span>
<span class="nf">movq</span> <span class="nv">%rsp</span><span class="p">,</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_register</span> <span class="nv">%rbp</span>
<span class="nf">movq</span> <span class="nv">%rdi</span><span class="p">,</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movq</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">movl</span> <span class="p">(</span><span class="nv">%rdi</span><span class="p">),</span> <span class="nv">%eax</span>
<span class="nf">popq</span> <span class="nv">%rbp</span>
<span class="nf">retq</span>
<span class="na">.cfi_endproc</span>
<span class="c">## -- End function</span>
</pre></div>
<p>By changing the capture clause to reference mode, we get almost the same code but this time it is the address of variable pushed into the stack.</p>
<div class="highlight"><pre><span></span> <span class="k">auto</span> <span class="n">funcA</span> <span class="o">=</span> <span class="p">[</span><span class="o">=</span><span class="p">]()</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">var</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
<div class="highlight"><pre><span></span> <span class="na">...</span>
<span class="nf">movl</span> <span class="no">$7</span><span class="p">,</span> <span class="p">-</span><span class="mi">20</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">leaq</span> <span class="p">-</span><span class="mi">20</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rsi</span> <span class="c">## address of "var" into %rsi</span>
<span class="nf">movq</span> <span class="nv">%rsi</span><span class="p">,</span> <span class="p">-</span><span class="mi">32</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">leaq</span> <span class="p">-</span><span class="mi">32</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">callq</span> <span class="no">__ZZ4mainENK3$_0clEv</span>
<span class="na">...</span>
<span class="na">.p2align</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">0x90</span> <span class="c">## -- Begin function _ZZ4mainENK3$_0clEv</span>
<span class="nl">__ZZ4mainENK3$_0clEv:</span> <span class="c">## @"_ZZ4mainENK3$_0clEv"</span>
<span class="na">.cfi_startproc</span>
<span class="c">## %bb.0:</span>
<span class="nf">pushq</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_offset</span> <span class="mi">16</span>
<span class="na">.cfi_offset</span> <span class="nv">%rbp</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span>
<span class="nf">movq</span> <span class="nv">%rsp</span><span class="p">,</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_register</span> <span class="nv">%rbp</span>
<span class="nf">movq</span> <span class="nv">%rdi</span><span class="p">,</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movq</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">movq</span> <span class="p">(</span><span class="nv">%rdi</span><span class="p">),</span> <span class="nv">%rdi</span>
<span class="nf">movl</span> <span class="p">(</span><span class="nv">%rdi</span><span class="p">),</span> <span class="nv">%eax</span>
<span class="nf">popq</span> <span class="nv">%rbp</span>
<span class="nf">retq</span>
<span class="na">.cfi_endproc</span>
<span class="c">## -- End function</span>
</pre></div>
<p>Now let's try constexpr on lambda expression. The constexpr objects will be evaluated during compilation time and reduced to a constant in the final code. Let's see.</p>
<div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="k">auto</span> <span class="n">funcA</span> <span class="o">=</span> <span class="p">[</span><span class="o">&</span><span class="p">](</span><span class="kt">int</span> <span class="n">a</span><span class="p">)</span> <span class="k">constexpr</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="mi">3</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">constexpr</span> <span class="kt">int</span> <span class="n">result</span> <span class="o">=</span> <span class="n">funcA</span><span class="p">(</span><span class="mi">7</span><span class="p">);</span>
<span class="k">return</span> <span class="n">result</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
<p>The lambda function is decalred as constexpr, thus during compilation time, the result of funcA(7) can be calculated as 10. The code is then equivalent to:</p>
<div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="k">constexpr</span> <span class="kt">int</span> <span class="n">result</span> <span class="o">=</span> <span class="mi">10</span><span class="p">;</span>
<span class="k">return</span> <span class="n">result</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
<p>That leads to an integer 10 in stack <code>movl $10, -28(%rbp)</code> and a direct return of 10 <code>movl $10, %eax</code>:</p>
<div class="highlight"><pre><span></span> <span class="na">.section</span> <span class="no">__TEXT</span><span class="p">,</span><span class="no">__text</span><span class="p">,</span><span class="no">regular</span><span class="p">,</span><span class="no">pure_instructions</span>
<span class="na">.build_version</span> <span class="no">macos</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">14</span> <span class="no">sdk_version</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">14</span>
<span class="na">.globl</span> <span class="no">_main</span> <span class="c">## -- Begin function main</span>
<span class="na">.p2align</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">0x90</span>
<span class="nl">_main:</span> <span class="c">## @main</span>
<span class="na">.cfi_startproc</span>
<span class="c">## %bb.0:</span>
<span class="nf">pushq</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_offset</span> <span class="mi">16</span>
<span class="na">.cfi_offset</span> <span class="nv">%rbp</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span>
<span class="nf">movq</span> <span class="nv">%rsp</span><span class="p">,</span> <span class="nv">%rbp</span>
<span class="na">.cfi_def_cfa_register</span> <span class="nv">%rbp</span>
<span class="nf">movl</span> <span class="no">$0</span><span class="p">,</span> <span class="p">-</span><span class="mi">4</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="nv">%edi</span><span class="p">,</span> <span class="p">-</span><span class="mi">8</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movq</span> <span class="nv">%rsi</span><span class="p">,</span> <span class="p">-</span><span class="mi">16</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="no">$10</span><span class="p">,</span> <span class="p">-</span><span class="mi">28</span><span class="p">(</span><span class="nv">%rbp</span><span class="p">)</span>
<span class="nf">movl</span> <span class="no">$10</span><span class="p">,</span> <span class="nv">%eax</span>
<span class="nf">popq</span> <span class="nv">%rbp</span>
<span class="nf">retq</span>
<span class="na">.cfi_endproc</span>
<span class="c">## -- End function</span>
<span class="na">.subsections_via_symbols</span>
</pre></div>
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