<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 - Wrong optimization: conditional equivalence vs. values with several representations"
href="https://bugs.llvm.org/show_bug.cgi?id=45101">45101</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Wrong optimization: conditional equivalence vs. values with several representations
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ch3root@openwall.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The problem happens when:
- conditional equivalence propagation replaces an expression with a variable or
a constant that has the same value but a different representation, and
- this happens in a computation where representation is accessed.
Example with a pseudo-denormal in long double:
----------------------------------------------------------------------
#include <stdio.h>
__attribute__((noipa,optnone)) // imagine it in a separate TU
static void *opaque(void *p) { return p; }
int main()
{
long double x, y;
unsigned char *px = (unsigned char *)&x;
unsigned char *py = (unsigned char *)&y;
// make x pseudo-denormal
x = 0;
px[7] = 0x80;
opaque(&x); // hide it from the optimizer
y = x;
if (y == 0x1p-16382l)
printf("py[8] = %d\n", py[8]);
printf("py[8] = %d\n", py[8]);
}
----------------------------------------------------------------------
$ clang -std=c11 -Weverything -Wno-unknown-attributes -O3 test.c && ./a.out
py[8] = 1
py[8] = 0
----------------------------------------------------------------------
clang x86-64 version: clang version 11.0.0
(<a href="https://github.com/llvm/llvm-project">https://github.com/llvm/llvm-project</a> 9284abd0040afecfd619dbcf1b244a8b533291c9)
----------------------------------------------------------------------
The value 0x1p-16382l admits two representations:
00 00 80 00 00 00 00 00 00 00 pseudo-denormal
00 01 80 00 00 00 00 00 00 00 normalized value
So both 0 and 1 for py[8] are fine but the testcase should print the same value
both times, i.e. the representation of y should be stable.
DR 260 Q1 allows for unstable representation but IMO this is wrong.
gcc bug -- <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94035">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94035</a></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>