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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong optimizations for pointers: `p == q ? p : q` -> `q`"
href="https://bugs.llvm.org/show_bug.cgi?id=44374">44374</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Wrong optimizations for pointers: `p == q ? p : q` -> `q`
</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>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>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>Similar to <a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong optimizations for pointers: `if (q == p) use p` -> `if (q == p) use q`"
href="show_bug.cgi?id=44313">bug 44313</a>.
The optimizer sometimes changes `p == q ? p : q` to `q`. This is wrong when the
actual provenance of `p` differs from that of `q`.
There are two forms -- with the actual conditional operator and with the `if`
statement.
The ideal example would be constructed with the help of restricted pointers but
it's run into a theoretical problem -- see the first testcase in <a class="bz_bug_link
bz_status_NEW "
title="NEW - Optimization with `restrict`: is `p == q ? p : q` "based" on `p`?"
href="show_bug.cgi?id=44373">bug 44373</a>.
My other examples require two conditionals to eliminate the possibility of UB.
Comparison of integers should give stable results, hopefully that would be
enough to demonstrate the problem.
gcc bug -- <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93052">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93052</a>.
Example with the conditional operator and with dead malloc (the wrong
optimization seems to be applied in Early CSE):
----------------------------------------------------------------------
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
__attribute__((noipa,optnone)) // imagine it in a separate TU
static void *opaque(void *p) { return p; }
int main()
{
int *q = malloc(sizeof(int));
opaque(q);
uintptr_t iq = (uintptr_t)(void *)q;
free(q);
int *p = malloc(sizeof(int));
opaque(p);
uintptr_t ip = (uintptr_t)(void *)p;
uintptr_t ir = ip == iq ? ip : iq;
if (ip == iq) {
*p = 1;
*(int *)(void *)ir = 2;
printf("result: %d\n", *p);
}
}
----------------------------------------------------------------------
$ clang -std=c11 -Weverything -Wno-unknown-attributes test.c && ./a.out
result: 2
$ clang -std=c11 -Weverything -Wno-unknown-attributes -O3 test.c && ./a.out
result: 1
----------------------------------------------------------------------
clang x86-64 version: clang version 10.0.0
(<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
fccac1ec16951e9a9811abf19e2c18be147854fc)
----------------------------------------------------------------------
The idea of problems arising from `p == q ? p : q` is from Chung-Kil Hur via
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752#c15">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752#c15</a>.</pre>
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