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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Comment describing isFsqrtCheap is probably wrong"
href="https://llvm.org/bugs/show_bug.cgi?id=24572">24572</a>
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<th>Summary</th>
<td>Comment describing isFsqrtCheap is probably wrong
</td>
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<th>Product</th>
<td>new-bugs
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>schnetter@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Line 221 of include/llvm/Target/TargetLowering.h describes isFsqrtCheap, and
says
/// Return true if sqrt(x) is as cheap or cheaper than 1 / rsqrt(x)
This sounds wrong: When one calculates sqrt(x) via rsqrt(x), one does not use
1/rsqrt(x) -- rather, one uses x*rsqrt(x).
I did not check what code LLVM generates in this case, but I assume it does not
use a division.</pre>
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