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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Instcombine turns -(x/INT_MIN) to x/INT_MIN"
href="http://llvm.org/bugs/show_bug.cgi?id=20186">20186</a>
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<th>Summary</th>
<td>Instcombine turns -(x/INT_MIN) to x/INT_MIN
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Transformation Utilities
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>davemm@cs.rutgers.edu
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, regehr@cs.utah.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The transformation of -(X/C) to X/(-C) is invalid if C == INT_MIN.
Specifically, if I have
define i32 @foo(i32 %x) #0 {
entry:
%div = sdiv i32 %x, -2147483648
%sub = sub nsw i32 0, %div
ret i32 %sub
}
then opt -instcombine will produce
define i32 @foo(i32 %x) #0 {
entry:
%sub = sdiv i32 %x, -2147483648
ret i32 %sub
}
You can observe this with the following test case:
#include <stdio.h>
#include <limits.h>
int foo(int x)
{
return -(x/INT_MIN);
}
int main (void)
{
printf ("%d\n", foo(INT_MIN));
}
This will print -1 or 1, depending on the optimization level.
This appears to be the relevant code:
InstCombineAddSub.cpp:1556
// 0 - (X sdiv C) -> (X sdiv -C)
if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) &&
match(Op0, m_Zero()))
return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));</pre>
</div>
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