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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Sub-optimal optimization of abs(x) % n"
href="https://bugs.llvm.org/show_bug.cgi?id=49763">49763</a>
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<th>Summary</th>
<td>Sub-optimal optimization of abs(x) % n
</td>
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<th>Product</th>
<td>tools
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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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>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>rifkin@purdue.edu
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>LLVM does not find all optimizations related to taking the remainder of the
absolute value of a signed number.
The following equivilent expressions result in sub-optimal code generation:
x % 2 == 1 || x % 2 == -1
std::abs(x % 2) == 1
std::abs(x) % 2 == 1
LLVM may be ignoring poison values for std::abs / @llvm.abs.* in making
optimizations for the remainder operation. Alternatively, LLVM may not be
taking
the absolute value into account when performing algebraic operations on the
remainder instructions.
If x is guaranteed to be positive (e.g. it is unsigned or an induction
variable),
LLVM does produce expected codegen. When std::abs is present, LLVM should be
able
to take advantage of the remainder operation being odd (i.e. -x % n = -(x %
n)).
The expected codegen is that of x % 2 != 0, x & 1, where LLVM generates a
single
and instruction.
Godbolt comparison of these expressions: <a href="https://godbolt.org/z/1YPW1z4v7">https://godbolt.org/z/1YPW1z4v7</a></pre>
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</p>
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