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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 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Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.1.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.2.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.3.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.4.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.5.</strong> time</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a 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aria-hidden="true">5.11.</strong> pgo</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">6.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">6.1.</strong> systemd</a></li><li class="chapter-item expanded "><a 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><li class="chapter-item expanded "><a href="../linux/input.html"><strong aria-hidden="true">6.6.</strong> input</a></li><li class="chapter-item expanded "><a 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                    <h1 class="menu-title">Notes</h1>

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                        <h1 id="profile-guided-optimization-pgo"><a class="header" href="#profile-guided-optimization-pgo">Profile guided optimization (pgo)</a></h1>
<p><code>pgo</code> is an optimization technique to optimize a program for its usual
workload.</p>
<p>It is applied in two phases:</p>
<ol>
<li>Collect profiling data (best with representative benchmarks).</li>
<li>Optimize program based on collected profiling data.</li>
</ol>
<p>The following simple program is used as demonstrator.</p>
<pre><code class="language-c">#include &lt;stdio.h&gt;

#define NOINLINE __attribute__((noinline))

NOINLINE void foo() { puts(&quot;foo()&quot;); }
NOINLINE void bar() { puts(&quot;bar()&quot;); }

int main(int argc, char *argv[]) {
  if (argc == 2) {
    foo();
  } else {
    bar();
  }
}
</code></pre>
<h2 id="clang"><a class="header" href="#clang">clang</a></h2>
<p>On the actual machine with <code>clang 15.0.7</code>, the following code is generated for
the <code>main()</code> function.</p>
<pre><code class="language-x86asm"># clang -o test test.c -O3

0000000000001160 &lt;main&gt;:
    1160:  50                   push   rax
    ; Jump if argc != 2.
    1161:  83 ff 02             cmp    edi,0x2
    1164:  75 09                jne    116f &lt;main+0xf&gt;
    ; foor() is on the hot path (fall-through).
    1166:  e8 d5 ff ff ff       call   1140 &lt;_Z3foov&gt;
    116b:  31 c0                xor    eax,eax
    116d:  59                   pop    rcx
    116e:  c3                   ret
    ; bar() is on the cold path (branch).
    116f:  e8 dc ff ff ff       call   1150 &lt;_Z3barv&gt;
    1174:  31 c0                xor    eax,eax
    1176:  59                   pop    rcx
    1177:  c3                   ret
</code></pre>
<p>The following shows how to compile with profiling instrumentation and how to
optimize the final program with the collected profiling data (<a href="https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization">llvm
pgo</a>).</p>
<p>The arguments to <code>./test</code> are chosen such that <code>9/10</code> runs call <code>bar()</code>, which
is currently on the <code>cold path</code>.</p>
<pre><code class="language-bash"># Compile test program with profiling instrumentation.
clang -o test test.cc -O3 -fprofile-instr-generate

# Collect profiling data from multiple runs.
for i in {0..10}; do
    LLVM_PROFILE_FILE=&quot;prof.clang/%p.profraw&quot; ./test $(seq 0 $i)
done

# Merge raw profiling data into single profile data.
llvm-profdata merge -o pgo.profdata prof.clang/*.profraw

# Optimize test program with profiling data.
clang -o test test.cc -O3 -fprofile-use=pgo.profdata
</code></pre>
<blockquote>
<p>NOTE: If <code>LLVM_PROFILE_FILE</code> is not given the profile data is written to
<code>default.profraw</code> which is re-written on each run. If the <code>LLVM_PROFILE_FILE</code>
contains a <code>%m</code> in the filename, a unique integer will be generated and
consecutive runs will update the same generated profraw file,
<code>LLVM_PROFILE_FILE</code> can specify a new file every time, however that requires
more storage in general.</p>
</blockquote>
<p>After optimizing the program with the profiling data, the <code>main()</code> function
looks as follows.</p>
<pre><code class="language-x86asm">0000000000001060 &lt;main&gt;:
    1060:  50                    push   rax
    ; Jump if argc == 2.
    1061:  83 ff 02              cmp    edi,0x2
    1064:  74 09                 je     106f &lt;main+0xf&gt;
    ; bar() is on the hot path (fall-through).
    1066:  e8 e5 ff ff ff        call   1050 &lt;_Z3barv&gt;
    106b:  31 c0                 xor    eax,eax
    106d:  59                    pop    rcx
    106e:  c3                    ret
    ; foo() is on the cold path (branch).
    106f:  e8 cc ff ff ff        call   1040 &lt;_Z3foov&gt;
    1074:  31 c0                 xor    eax,eax
    1076:  59                    pop    rcx
    1077:  c3                    ret
</code></pre>
<h2 id="gcc"><a class="header" href="#gcc">gcc</a></h2>
<p>With <code>gcc 13.2.1</code> on the current machine, the optimizer puts <code>bar()</code> on the
<code>hot path</code> by default.</p>
<pre><code class="language-x86asm">0000000000001040 &lt;main&gt;:
    1040:  48 83 ec 08          sub    rsp,0x8
    ; Jump if argc == 2.
    1044:  83 ff 02             cmp    edi,0x2
    1047:  74 0c                je     1055 &lt;main+0x15&gt;
    ; bar () is on the hot path (fall-through).
    1049:  e8 22 01 00 00       call   1170 &lt;_Z3barv&gt;
    104e:  31 c0                xor    eax,eax
    1050:  48 83 c4 08          add    rsp,0x8
    1054:  c3                   ret
    ; foo() is on the cold path (branch).
    1055:  e8 06 01 00 00       call   1160 &lt;_Z3foov&gt;
    105a:  eb f2                jmp    104e &lt;main+0xe&gt;
    105c:  0f 1f 40 00          nop    DWORD PTR [rax+0x0]

</code></pre>
<p>The following shows how to compile with profiling instrumentation and how to
optimize the final program with the collected profiling data.</p>
<p>The arguments to <code>./test</code> are chosen such that <code>2/3</code> runs call <code>foo()</code>, which
is currently on the <code>cold path</code>.</p>
<pre><code class="language-bash">gcc -o test test.cc -O3 -fprofile-generate
./test 1
./test 1
./test 2 2
gcc -o test test.cc -O3 -fprofile-use
</code></pre>
<blockquote>
<p>NOTE: Consecutive runs update the generated <code>test.gcda</code> profile data file
rather than re-write it.</p>
</blockquote>
<p>After optimizing the program with the profiling data, the <code>main()</code> function</p>
<pre><code class="language-x86asm">0000000000001040 &lt;main.cold&gt;:
    ; bar() is on the cold path (branch).
    1040:  e8 05 00 00 00       call   104a &lt;_Z3barv&gt;
    1045:  e9 25 00 00 00       jmp    106f &lt;main+0xf&gt;

0000000000001060 &lt;main&gt;:
    1060:  51                   push   rcx
    ; Jump if argc != 2.
    1061:  83 ff 02             cmp    edi,0x2
    1064:  0f 85 d6 ff ff ff    jne    1040 &lt;main.cold&gt;
    ; for() is on the hot path (fall-through).
    106a:  e8 11 01 00 00       call   1180 &lt;_Z3foov&gt;
    106f:  31 c0                xor    eax,eax
    1071:  5a                   pop    rdx
    1072:  c3                   ret
</code></pre>

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