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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Replace loop-based variable with loop invariant variable"
href="https://bugs.llvm.org/show_bug.cgi?id=42700">42700</a>
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<tr>
<th>Summary</th>
<td>Replace loop-based variable with loop invariant variable
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int f(bool* arr, bool x) {
int ss = 0;
for (int i = 0; i < 16; ++i) {
if (arr[i] == x) {
// [0]
ss += (arr[i]) ? 1 : -1;
}
}
return ss;
}
[0] x is not modified in this branch so try to replace:
ss += (arr[i]) ? 1 : -1;
with:
ss += (x) ? 1 : -1;
int f2(bool* arr, bool x) {
int ss = 0;
for (int i = 0; i < 16; ++i) {
if (arr[i] == x) {
ss += (x) ? 1 : -1;
}
}
return ss;
}
It may help LICM to hoist some code, help Loop
distribution/unswitch/vectorization pass, etc..
For my motivating example, current Clang trunk -O3 just unrolls 'f' (for N=16)
/ vectorizes 'f' (N=64+). Clang vectorizes 'f2' even for N=16+.
Godbolt: <a href="https://godbolt.org/z/c7HwFi">https://godbolt.org/z/c7HwFi</a></pre>
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