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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] rsqrt and div generated for 1/sqrt(x * x * x) with -Ofast"
href="https://bugs.llvm.org/show_bug.cgi?id=46406">46406</a>
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<th>Summary</th>
<td>[X86] rsqrt and div generated for 1/sqrt(x * x * x) with -Ofast
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>craig.topper@gmail.com
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>InstCombine transforms sqrt(x * x * x) into fabs(x) * sqrt(x) under fast math.
This means (1 / sqrt(x*x*x)) becomes (1 / (fabs(x) * sqrt(x))) which the
backend is unable to match to reciprocal sqrt approximation. Instead we do a
square root approximation which uses rsqrt followed by a regular divide.
Ideally we'd just do the x*x*x and then approximate the reciprocal square root
of that.
<a href="https://godbolt.org/z/fR66c4">https://godbolt.org/z/fR66c4</a></pre>
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