<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - accuracy error by optimization of pow(x, -0.5) when -ffast-math is disabled"
href="https://bugs.llvm.org/show_bug.cgi?id=44330">44330</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>accuracy error by optimization of pow(x, -0.5) when -ffast-math is disabled
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>9.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Transformation Utilities
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>utsumi.yuichiro@fujitsu.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The following code causes accuracy error even when -ffast-math is disabled.
--------------- pow.c --------------------------
#include <math.h>
#include <stdio.h>
double (*libpow)(double, double) = pow;
double x = 1000000001.0;
int main() {
double a = libpow(x, -0.5); // not optimized
double b = pow(x, -0.5); // optimized
printf("lib pow: %.20g, pow: %.20g\n", a, b);
return 0;
}
--------------------------------------------------
This code includes `pow(x, -0.5)`.
The following is the result.
-------------------------------------------------------------------
$ clang -O1 pow.c -lm
$ ./a.out
lib pow: 3.1622776585872407456e-05, pow: 3.162277658587240068e-05
-------------------------------------------------------------------
The specified compile option is only `-O1` and `-ffast-math` is disabled but
the value of `lib pow` and `pow` differ.
This behavior is caused by
<a href="https://github.com/llvm/llvm-project/blob/release/9.x/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp#L1431">https://github.com/llvm/llvm-project/blob/release/9.x/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp#L1431</a>
I think the optimization pow(x, -0.5) -> 1.0/sqrt(x) is admitted only when fast
math flag exist.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>