<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 - -ffast-math breaks strtod() due to "== HUGE_VAL" considered impossible"
href="https://bugs.llvm.org/show_bug.cgi?id=51775">51775</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>-ffast-math breaks strtod() due to "== HUGE_VAL" considered impossible
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>12.0
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>xavier.roche@algolia.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=25236" name="attach_25236" title="Test aimed to reproduce the -ffast-math comparison bug">attachment 25236</a> <a href="attachment.cgi?id=25236&action=edit" title="Test aimed to reproduce the -ffast-math comparison bug">[details]</a></span>
Test aimed to reproduce the -ffast-math comparison bug
This is a regression from clang-11 to clang-12, but could be considered normal
if we consider -ffast-math as being "by design" unreliable w.r.t comparisons.
However, this breaks the use of functions such as strtod(), which can be
considered a more serious issue, because the "== HUGE_VAL" comparison is
considered by the compiler impossible and statically optimized as false
The attached program demonstrates the issue:
$ clang-11 -O1 -ffast-math
/tmp/ffast-math-strtod-overflow-condition-misoptimization-bug.c -o bug && ./bug
bad value
$ clang-12 -O1 -ffast-math
/tmp/ffast-math-strtod-overflow-condition-misoptimization-bug.c -o bug && ./bug
good value: inf
Clang versions 12 and above now are not seeing HUGE_VAL anymore</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>