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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Reduction loop repeated due to loop invariant variable"
href="https://bugs.llvm.org/show_bug.cgi?id=47020">47020</a>
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<th>Summary</th>
<td>Reduction loop repeated due to loop invariant variable
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>florian_hahn@apple.com, listmail@philipreames.com, llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
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<div>
<pre>Pulled out of a code example discussed here:
What Everyone Should Know About How Amazing Compilers Are - Matt Godbolt [C++
on Sea 2019]
<a href="https://www.youtube.com/watch?v=w0sz5WbS5AM">https://www.youtube.com/watch?v=w0sz5WbS5AM</a>
#include <vector>
typedef int T;
static T sumA = 0;
static T sumB = 0;
void v3(bool useA, const std::vector<T> &x) {
for(auto v : x) {
if (useA)
sumA += v*v;
else
sumB += v*v;
}
}
-g0 -O3 -march=haswell
<a href="https://godbolt.org/z/x79oPx">https://godbolt.org/z/x79oPx</a>
We repeat the entire reduction loop, with the sumA/sumB result variable
load/store being hoisted out to the start/end - it should be possible to merge
these loops and just select which reference to use before the loop.
If I perform this manually we loose vectorization entirely:
#include <vector>
typedef int T;
static T sumA = 0;
static T sumB = 0;
void v3(bool useA, const std::vector<T> &x) {
T &sum = useA ? sumA : sumB;
for(auto v : x) {
sum += v*v;
}
}
<a href="https://godbolt.org/z/168GMz">https://godbolt.org/z/168GMz</a></pre>
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