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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Slow code generated by Clang for 525.x264_r-like loop"
href="https://bugs.llvm.org/show_bug.cgi?id=43580">43580</a>
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<th>Summary</th>
<td>Slow code generated by Clang for 525.x264_r-like loop
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>Pattern:
typedef signed char schar;
void foo (schar *__restrict a, schar *__restrict b,
schar *__restrict c, int n, int step)
{
for (int j = 0; j < n; ++j)
{
for (int i = 0; i < 16; ++i)
a[i] = (b[i] + c[i]) >> 1;
a += step;
b += step;
c += step;
}
}
Current codegen:
<a href="https://godbolt.org/z/0Lo66v">https://godbolt.org/z/0Lo66v</a>
Clang generates 2x slower code than icc/gcc.
Microbenchmark:
<a href="https://pastebin.com/rECq65sa">https://pastebin.com/rECq65sa</a>
Clang SVN -O3 0m5,619s
ICC 19 -O3 0m2,525s
GCC SVN -O3 0m2,432s
With -march=haswell
Clang SVN 0m3,540s
ICC 19 0m1,892s
GCC SVN 0m2,608s</pre>
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