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<h1 class="menu-title">Notes</h1>
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<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>
<h3><a class="header" href="#target-options" id="target-options">Target options</a></h3>
<pre><code class="language-bash"># List all target options with their description.
gcc --help=target
# Configure for current cpu arch and query (-Q) value of options.
gcc -march=native -Q --help=target
</code></pre>
<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 "
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(); }
}
" | 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-x86asm">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>
<h2><a class="header" href="#abi-linux" id="abi-linux">ABI (Linux)</a></h2>
<ul>
<li>C ABI - <a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SystemV ABI</a></li>
<li>C++ ABI - <a href="https://itanium-cxx-abi.github.io/cxx-abi">C++ Itanium ABI</a></li>
</ul>
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