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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Most negative integer greater than zero"
href="http://llvm.org/bugs/show_bug.cgi?id=19757">19757</a>
</td>
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<tr>
<th>Summary</th>
<td>Most negative integer greater than zero
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.4
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</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>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>akeep@cisco.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12516" name="attach_12516" title="C program that demonstrates the bug when compiled with -O2">attachment 12516</a> <a href="attachment.cgi?id=12516&action=edit" title="C program that demonstrates the bug when compiled with -O2">[details]</a></span>
C program that demonstrates the bug when compiled with -O2
Clang reports that the most negative 32-bit integer (0x80000000) is a positive
integer when the -O2 flag is specified. (The same problem occurs for the most
negative 64-bit number when compiling to a 64-bit target.)
Here is the example program that demonstrates the problem (also added as an
attachment):
#include <stdio.h>
unsigned int bogus(unsigned int b) {
return b;
}
int main(int argc, char * argv[]) {
unsigned int n = bogus(0x80000000);
signed int final = -(signed int) n;
if (final > 0) {
printf("clang failed final (%d) > 0 where n is (%u)\n", final, n);
} else {
printf("clang worked! final (%d) < 0 where n is (%u)\n", final, n);
}
}
(Note: bogus here is the identity function and is used simply to interfere with
constant folding that would normally make this go away---this is a simplified
demonstration of the bug from the actual case that led us to the problem.)
Compiled on Mac OS X Mavericks with the default Xcode clang:
% clang -m32 -Wextra -Werror -Wno-unused-parameter -O2 -msse2
-mmacosx-version-min=10.4 -o break_clang break_clang.c
% ./break_clang
clang failed final (-2147483648) > 0 where n is (2147483648)
% clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
The same problem exists with a 64-bit build:
% clang -m64 -Wextra -Werror -Wno-unused-parameter -O2 -msse2
-mmacosx-version-min=10.4 -o break_clang break_clang.c
% ./break_clang
clang failed final (-2147483648) > 0 where n is (2147483648)
% clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM
3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
The program works correctly when not running at -O2:
% clang -m32 -Wextra -Werror -Wno-unused-parameter -msse2
-mmacosx-version-min=10.4 -o break_clang break_clang.c
% ./break_clang
clang worked! final (-2147483648) < 0 where n is (2147483648)</pre>
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