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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 --- - miscompile of select at O1"
href="https://llvm.org/bugs/show_bug.cgi?id=27575">27575</a>
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<tr>
<th>Summary</th>
<td>miscompile of select at O1
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kcc@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>#include <stdio.h>
int main(int argc, char **argv) {
int i = argc - 1;
int a = 1 << i;
int b = a >> 1;
double c = b > 0 ? (double)b : 0.5;
printf("%d %g\n", b, c);
}
for f in 0 1 2; do clang++ -O$f bug.cc && ./a.out ; done
0 0.5
0 0
0 0
This is what I see in IR:
*** IR Dump After Dead Argument Elimination ***; ModuleID = 'bug.cc'
%cond = select i1 %cmp, double %conv, double 5.000000e-01
*** IR Dump After Combine redundant instructions ***
%1 = select i1 %0, i32 %shr, i32 0
I don't see how this transformation could be legal.</pre>
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</p>
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