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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - unreachable reduces performance"
href="https://llvm.org/bugs/show_bug.cgi?id=25952">25952</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>unreachable reduces performance
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>3.7
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>g.bergmann.official@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15526" name="attach_15526" title="brainfuck interpreter and example data">attachment 15526</a> <a href="attachment.cgi?id=15526&action=edit" title="brainfuck interpreter and example data">[details]</a></span>
brainfuck interpreter and example data
Delaring cases in a switch statement like below as unreachable may reduce
performance:
switch x {
case 0: // ...
break;
case 1: // ...
break;
// ....
case 7: // ...
break;
default:
__builtin_unreachable();
}
Removing the __builtin_unreachable() improves the performance. On my PC the
attached example needs about 25 with it and 17 without.
The attached code is a simple brainfuck interpreter that can be tested by
calling "brainfuck-optimized mandel.brainfuck".
It was test with options -O3 and -O2 as 64 bit build on a haswell processor.
The problem is reproducible in Rust.
Profiling shows a suspiciously high number of branch mispredictions. From what
I have seen of the assembler code, I think that nested conditional branches
like a binary search are faster than a branch table by causing fewer
mispredictions.</pre>
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