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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Infinite Empty Loops are incorrectly optimized out (C)"
href="https://llvm.org/bugs/show_bug.cgi?id=31217">31217</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Infinite Empty Loops are incorrectly optimized out (C)
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.8
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</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>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>npmazzuca@gmail.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>```C
void foo() { while (1) { } }
void create_null() {
foo();
int i = 0;
while (i < 100) { i += 1; }
}
__attribute__((noreturn))
void use_null() {
__builtin_unreachable();
}
int main() {
create_null();
use_null();
}
```
This crashes under optimization level 2; this is invalid behavior under the C11
standard:
```
An iteration statement whose controlling expression is not a constant
expression, [note 156] that performs no input/output operations,
does not access volatile objects, and performs no synchronization or
atomic operations in its body, controlling expression, or (in the case of
a for statement) its expression-3, may be assumed by the
implementation to terminate. [note 157]
156: An omitted controlling expression is replaced by a nonzero constant,
which is a constant expression.
157: This is intended to allow compiler transformations such as
removal of empty loops even when termination cannot be proven.
```
Note "whose controlling expression is not a constant expression" - in `while
(1) { }`, `1` is a constant expression, and thus the loop may not be removed.</pre>
</div>
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