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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [AArch64] Wrong code generated (?) for compare_exchange_strong on __int128_t"
href="https://bugs.llvm.org/show_bug.cgi?id=51102">51102</a>
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<th>Summary</th>
<td>[AArch64] Wrong code generated (?) for compare_exchange_strong on __int128_t
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>efriedma@quicinc.com
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</tr>
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<th>CC</th>
<td>arnaud.degrandmaison@arm.com, llvm-bugs@lists.llvm.org, smithp352@googlemail.com, Ties.Stuij@arm.com
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<p>
<div>
<pre>Consider:
#include <stdatomic.h>
__int128_t ff(_Atomic __int128_t *p) {
__int128_t e = 0;
atomic_compare_exchange_strong(p, &e, 0);
return e;
}
ff:
mov x8, x0
.LBB0_1:
ldaxp x0, x1, [x8]
orr x9, x0, x1
cbnz x9, .LBB0_4
stlxp w9, xzr, xzr, [x8]
cbnz w9, .LBB0_1
ret
.LBB0_4:
clrex
ret
This sequence is subtly wrong in the case where the cmpxchg fails.
Specifically, the ldaxp is not single-copy atomic, so the returned value might
tear. At least, that's my understanding of the rules here. So we need to
replace the "clrex" with a sequence like "stlxp w9, x0, x1, [x8]; cbnz w9,
.LBB0_1".
Not sure if this sort of tearing can actually happen in practice on commonly
used AArch64 chips.
I'll throw together a patch.</pre>
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