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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __finite math calls generated prevents optimizations."
href="https://bugs.llvm.org/show_bug.cgi?id=48317">48317</a>
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<th>Summary</th>
<td>__finite math calls generated prevents optimizations.
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>venkataramanan.kumar.llvm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I am on Ubuntu 18 machine and it has finite math header <bits/math-finite.h>.
This header is included by the glibc 2.27. This header has this following
definition.
extern double log (double) asm ("" "log_finite") attribute__ ((nothrow ));
Consider the following test case
#include <math.h>
double mylog (double d) {
return log(d);
}
Recently clang started supporting asm labels.
Ref: <a href="https://reviews.llvm.org/D88712">https://reviews.llvm.org/D88712</a>
Before this patch clang generates the below LLVM IR for -O2 -ffast-math
--Snip--
; Function Attrs: nounwind readnone uwtable
define dso_local double @mlog(double %d) local_unnamed_addr #0 {
entry:
%0 = tail call fast double @llvm.log.f64(double %d)
ret double %0
}
---Snip--
After this patch for -O2 -ffast-math, clang generates
--Snip--
; Function Attrs: nounwind readnone uwtable
define dso_local double @mlog(double %d) local_unnamed_addr #0 {
entry:
%call = tail call fast double @__log_finite(double %d) #2
ret double %call
}
--Snip--
Note on latest Ubuntu 20.04.1 LTS with Glibc 2.31 <bits/math-finite.h> header
is not present. It is removed by glibc. so there I see the @llvm.log.f64
intrinsic calls.
So on machines with lesser glibc versions where the header file is present this
will be a problem as it generates finite calls now.
Note the LLVM "opt" does optimizations like vectorization only on the
intrinsic, but fails to do so in the presence of calls to @__log_finite.
Example <a href="https://llvm.godbolt.org/z/765We8">https://llvm.godbolt.org/z/765We8</a>
Older glibc versions are still supported OS distros. We are seeing performance
issues due to this finite calls.</pre>
</div>
</p>
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