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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded "><a 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href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li 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href="../linux/coredump.html"><strong aria-hidden="true">6.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">6.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">6.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">6.5.</strong> swap</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">7.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">7.1.</strong> tcpdump</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">8.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong 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<h1 id="gcc1"><a class="header" href="#gcc1">gcc(1)</a></h1>
<h2 id="cli"><a class="header" href="#cli">CLI</a></h2>
<h3 id="preprocessing"><a class="header" href="#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>
<li><code>-###</code> dry-run, outputting exact compiler/linker invocations</li>
<li><code>-print-multi-lib</code> print available multilib configurations</li>
</ul>
<h3 id="target-options"><a class="header" href="#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 id="builtins"><a class="header" href="#builtins"><a href="https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html">Builtins</a></a></h2>
<h3 id="__builtin_expectexpr-cond"><a class="header" href="#__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>
<p>The semantics of this hint are as follows, the compiler prioritises <code>expr == cond</code>. So <code>__builtin_expect(expr, 0)</code> means that we expect the <code>expr</code> to be <code>0</code>
most of the time.</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 id="abi-linux"><a class="header" href="#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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