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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Loop is incorrectly optimized away"
href="https://llvm.org/bugs/show_bug.cgi?id=24402">24402</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Loop is incorrectly optimized away
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.6
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>bugz@madler.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The C code below gives different results with -O0/-O1 vs. -O2/-O3. -O1 gives
the correct result, whereas -O2 gives an incorrect result. See the comments in
the code.
/*
Example of a clang optimization bug.
Mark Adler, August 8, 2015.
Using -O0 or -O1 takes a little while and gives the correct result:
47 bits set (4294967296 loops)
Using -O2 or -O3 optimizes out the loop, returning immediately with:
0 bits set (4294967296 loops)
Of course, there weren't really that many loops. The number of loops was
calculated, correctly, by the compiler when optimizing. But it got the
number of bits set wrong.
This is with:
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
*/
#include <stdio.h>
#include <inttypes.h>
/* bit vector of 1<<32 bits, initialized to all zeros */
static uint64_t vec[1 << 26] = {0};
int main(void)
{
/* set 47 of the bits. */
vec[31415927] = UINT64_C(0xb9fe2f2fedf7ebbd);
/* count the set bits */
uint64_t count = 0;
uint64_t loops = 0;
uint32_t x = 0;
do {
if (vec[x >> 6] & ((uint64_t)1 << (x & 0x3f)))
count++;
x++;
loops++;
} while (x);
printf("%" PRIu64 " bits set (%" PRIu64 " loops)\n", count, loops);
return 0;
}</pre>
</div>
</p>
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