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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine: 'X % -Y -> X % Y' incorrect if Y < 0"
href="http://llvm.org/bugs/show_bug.cgi?id=21256">21256</a>
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<th>Summary</th>
<td>InstCombine: 'X % -Y -> X % Y' incorrect if Y < 0
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nunoplopes@sapo.pt
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In InstCombiner::visitSRem(), the transformation 'X % -Y -> X % Y' should be
applied only to constants, since we don't know what's the value of Y if it's
not a constant and we could be introducing undefined behavior.
if (Value *RHSNeg = dyn_castNegVal(Op1))
if (!isa<Constant>(RHSNeg) ||
(isa<ConstantInt>(RHSNeg) &&
cast<ConstantInt>(RHSNeg)->getValue().isStrictlyPositive())) {
// X % -Y -> X % Y
Worklist.AddValue(I.getOperand(1));
I.setOperand(1, RHSNeg);
return &I;
}
I propose we drop the '!isa<Constant>(RHSNeg) ||' part.
This is what Alive says:
%Op1 = sub 0, %X
%r = srem %Op0, %Op1
=>
%r = srem %Op0, %X
ERROR: Domain of definedness of Target is smaller than Source's for i4 %r
Example:
%X i4 = 0xF (15, -1)
%Op0 i4 = 0x8 (8, -8)
%Op1 i4 = 0x1 (1)
Source value: 0x0 (0)
Target value: undef</pre>
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