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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - warn when loop condition comparison uses different size operands"
href="https://llvm.org/bugs/show_bug.cgi?id=31705">31705</a>
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<th>Summary</th>
<td>warn when loop condition comparison uses different size operands
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>MacOS X
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Static Analyzer
</td>
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<th>Assignee</th>
<td>kremenek@apple.com
</td>
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<th>Reporter</th>
<td>nivek.research@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>I recently encountered a infinite loop where the variable in a for loop
overflowed before reaching the termination value. For example, the 16-bit
unsigned int below is compared to a 64-bit unsigned long (on macOS) and since
the 16-bit value will overflow after 65535 it never reaches 65536 and the loop
will never terminates. I think it would be a great check for either the
compiler or more probably the static analyzer to warn that loop may not
terminate because of comparison may always be false due to overflow. I believe
the check would be good for for, while and do until loops.
#include <cstddef>
#include <cstdint>
#include <cstdio>
int main() {
size_t limit = 65536;
for (uint16_t index = 0; index < limit; index++) {
}
printf("completedi\n");
}
I believe similar warnings could be reported for
uint16_t index = 0;
while (index < limit) {
index++;
}
and
do {
index++;
} while (index < limit);
The compiler does produce a warning if limit is replaced by a constant value:
main.cpp:11:15: warning: comparison of constant 65536 with expression of type
'uint16_t' (aka 'unsigned short') is always true
[-Wtautological-constant-out-of-range-compare]
while (index < 65536) {
~~~~~ ^ ~~~~~
So perhaps this can just be enhanced.</pre>
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