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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clang -O1 is incorrectly removing a comparison"
href="https://llvm.org/bugs/show_bug.cgi?id=25421">25421</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Clang -O1 is incorrectly removing a comparison
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Global Analyses
</td>
</tr>
<tr>
<th>Assignee</th>
<td>sanjoy@playingwithpointers.com
</td>
</tr>
<tr>
<th>Reporter</th>
<td>rtrieu@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Suspected revision is r251050. Inside the loop, the comparison (i <
result_size - 1) is being treated as true for all cases, even on the last loop
iterator where i is equal to result_size - 1
test4.cc:
#include <iostream>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
int main(int argc, char** argv) {
vector<int> vv = {1, 2};
long int result_size = vv.back();
for (long int i = 0; i < result_size; ++i) {
cout << i << " < " << result_size
<< " - 1 = " << ((i < result_size - 1) ? "true" : "false") << endl;
}
return 0;
}
clang++ -std=c++11 test4.cc -O1 ; ./a.out
0 < 2 - 1 = true
1 < 2 - 1 = true
clang++ -std=c++11 test4.cc ; ./a.out
0 < 2 - 1 = true
1 < 2 - 1 = false</pre>
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