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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loop layering based on a monotonic condition between two loop indvars"
href="https://bugs.llvm.org/show_bug.cgi?id=44156">44156</a>
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</tr>
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<th>Summary</th>
<td>Loop layering based on a monotonic condition between two loop indvars
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>
<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>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>void init(int colIn, int colOut);
void sink(int colIn, int colOut);
void bad(int inputWidth, int outWidth) {
for(int colIn = 0, colOut = 0; colOut < outWidth; ++colIn, ++colOut) {
if(colIn == inputWidth) {
init(colIn, colOut);
colIn = 0;
}
sink(colIn, colOut);
}
}
Every iteration of innermost branch we have a conditional branch.
But we can avoid it, by having two loops, with now-innermost
running for precomputed number of iterations with no such branching:
#include <algorithm>
void init(int colIn, int colOut);
void sink(int colIn, int colOut);
void good(int inputWidth, int outWidth) {
for(int colOut = 0, colIn = 0; colOut < outWidth; ) {
if(colIn == inputWidth) {
init(colIn, colOut);
colIn = 0;
}
int outColForNextInRow = outWidth - colIn;
for( ; colOut < std::min(outWidth, outColForNextInRow)
; ++colIn, ++colOut)
sink(colIn, colOut);
}
}
<a href="https://godbolt.org/z/MMMaKb">https://godbolt.org/z/MMMaKb</a></pre>
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