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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loop counting top bits set badly vectorized on X86"
href="https://bugs.llvm.org/show_bug.cgi?id=46053">46053</a>
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<th>Summary</th>
<td>Loop counting top bits set badly vectorized on X86
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Loop Optimizer
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nikita.ppv@gmail.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
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<pre>Originally reported at: <a href="https://github.com/rust-lang/rust/issues/72355">https://github.com/rust-lang/rust/issues/72355</a>.
A loop counting the number of bytes with the top bit set:
#include <stddef.h>
#include <stdint.h>
uint64_t count_non_ascii(unsigned *buffer, size_t len) {
uint64_t count = 0;
for (size_t i = 0; i < len; i++) {
count += buffer[i] >= 0x80;
}
return count;
}
Gets vectorized to some complicated code: <a href="https://c.godbolt.org/z/jTU3rX">https://c.godbolt.org/z/jTU3rX</a>
While this could be just movmsk + popcnt.</pre>
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