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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect fabs optimization when fdiv or fmul is involved"
href="https://bugs.llvm.org/show_bug.cgi?id=47943">47943</a>
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<tr>
<th>Summary</th>
<td>Incorrect fabs optimization when fdiv or fmul is involved
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<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>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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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>juneyoung.lee@sf.snu.ac.kr
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>----------------------------------------
// Transforms/InstCombine/fmul.ll
define float @fabs_squared(float %x) {
%0:
%x.fabs = fabs float %x
%mul = fmul float %x.fabs, %x.fabs
ret float %mul
}
=>
define float @fabs_squared(float %x) {
%0:
%mul = fmul float %x, %x
ret float %mul
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
float %x = undef
Source:
float %x.fabs = NaN [based on undef value]
float %mul = NaN [based on undef value]
Target:
float %mul = #x80000000 (-0.0)
Source value: NaN
Target value: #x80000000 (-0.0)
----------------------------------------
// Transforms/InstCombine/fdiv.ll
define float @fabs_same_op(float %x) {
%0:
%a = fabs float %x
%r = fdiv float %a, %a
ret float %r
}
=>
define float @fabs_same_op(float %x) {
%0:
%r = fdiv float %x, %x
ret float %r
}
Transformation doesn't verify!
ERROR: Value mismatch
----------------------------------------
After optimization, `fmul %x, %x` isn't guaranteed to be non-negative because
the result can be negative if %x was undef.
Similarly for the `fdiv` case.</pre>
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