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authorjohannst <johannst@users.noreply.github.com>2020-03-19 21:21:17 +0000
committerjohannst <johannst@users.noreply.github.com>2020-03-19 21:21:17 +0000
commitf7f091a2e6d3167fed4e1063dea266cdfff3ab55 (patch)
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+ <main>
+ <h1><a class="header" href="#gcc1" id="gcc1">gcc(1)</a></h1>
+<h2><a class="header" href="#cli" id="cli">CLI</a></h2>
+<h3><a class="header" href="#preprocessing" id="preprocessing">Preprocessing</a></h3>
+<p>While debugging can be helpful to just pre-process files.</p>
+<pre><code class="language-bash">gcc -E [-dM] ...
+</code></pre>
+<ul>
+<li><code>-E</code> run only preprocessor</li>
+<li><code>-dM</code> list only <code>#define</code> statements</li>
+</ul>
+<h2><a class="header" href="#a-hrefhttpsgccgnuorgonlinedocsgccother-builtinshtmlbuiltinsa" id="a-hrefhttpsgccgnuorgonlinedocsgccother-builtinshtmlbuiltinsa"><a href="https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html">Builtins</a></a></h2>
+<h3><a class="header" href="#__builtin_expectexpr-cond" id="__builtin_expectexpr-cond"><code>__builtin_expect(expr, cond)</code></a></h3>
+<p>Give the compiler a hint which branch is hot, so it can lay out the code
+accordingly to reduce number of jump instructions.
+See on <a href="https://godbolt.org/z/MbTHAP">compiler explorer</a>.</p>
+<pre><code class="language-bash">echo &quot;
+extern void foo();
+extern void bar();
+void run0(int x) {
+ if (__builtin_expect(x,0)) { foo(); }
+ else { bar(); }
+}
+void run1(int x) {
+ if (__builtin_expect(x,1)) { foo(); }
+ else { bar(); }
+}
+&quot; | gcc -O2 -S -masm=intel -o /dev/stdout -xc -
+</code></pre>
+<p>Will generate something similar to the following.</p>
+<ul>
+<li><code>run0</code>: <code>bar</code> is on the path without branch</li>
+<li><code>run1</code>: <code>foo</code> is on the path without branch</li>
+</ul>
+<pre><code class="language-c">run0:
+ test edi, edi
+ jne .L4
+ xor eax, eax
+ jmp bar
+.L4:
+ xor eax, eax
+ jmp foo
+run1:
+ test edi, edi
+ je .L6
+ xor eax, eax
+ jmp foo
+.L6:
+ xor eax, eax
+ jmp bar
+</code></pre>
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