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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [InstCombine] recognize ub-safe negation pattern"
href="https://llvm.org/bugs/show_bug.cgi?id=27817">27817</a>
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<th>Summary</th>
<td>[InstCombine] recognize ub-safe negation pattern
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In D20434 ( <a href="http://reviews.llvm.org/D20434">http://reviews.llvm.org/D20434</a> ), I learned a better way to avoid
undefined behavior due to signed integer overflow when negating INT_MIN:
$ cat ub.c
#include <limits.h>
int goo(int x) {
if (x == INT_MIN)
return x;
return -x;
}
int foo(int x) {
return -(unsigned)x;
}
----------------------------------------------------------------------------
Those should be equivalent in optimized IR, but:
$ ./clang -O1 ub.c -S -o - -emit-llvm
define i32 @goo(i32 %x) #0 {
%cmp = icmp eq i32 %x, -2147483648
%sub = sub nsw i32 0, %x
%sel = select i1 %cmp, i32 -2147483648, i32 %sub
ret i32 %sel
}
define i32 @foo(i32 %x) #0 {
%sub = sub i32 0, %x
ret i32 %sub
}</pre>
</div>
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