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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 - Missed optimization: useless for-loop must be eliminated"
href="https://bugs.llvm.org/show_bug.cgi?id=39515">39515</a>
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<th>Summary</th>
<td>Missed optimization: useless for-loop must be eliminated
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>clang(trunk) with '-O3 -std=c++17' for this code:
#include <vector>
#include <algorithm>
int foo(std::vector<int> v) {
int l = v[0];
for(const auto& x : v) {
l = std::min(l, x);
}
for(const auto& x : v) {
l = std::max(l, x);
}
return l;
}
gcc doesn't eliminate first loop, but gcc can, because first loop has no effect
in this function.
Same result for the version without std::vector and std::max:
int min(int a, int b)
{
return a < b ? a : b;
}
int max(int a, int b)
{
return a > b ? a : b;
}
int foo(int* v, int size) {
int l = v[0];
for(int i=0; i < size; ++i)
{
l = min(l, v[i]);
}
for(int i=0; i < size; ++i)
{
l = max(l, v[i]);
}
return l;
}</pre>
</div>
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