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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 --- - Infinite loop in atomic function when built with -O0 on AArch64"
href="https://llvm.org/bugs/show_bug.cgi?id=25611">25611</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Infinite loop in atomic function when built with -O0 on AArch64
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>FreeBSD
</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>Backend: AArch64
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>andrew@fubar.geek.nz
</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>When building the following with -O0 it generates code that may, on some
hardware, enter an infinite loop. This is because it is implementation defined
if any store will leave the local-monitor in exclusive mode or not.
int
test(int *ptr, int old_value, int new_value)
{
return __sync_val_compare_and_swap(ptr, old_value, new_value);
}
When building with -O0 on clang trunk I get the following asm. This is broken
on ThunderX as the local monitor will leave exclusive-mode when it executes the
store at address 40 below.
0000000000000000 <test>:
0: d100c3ff sub sp, sp, #0x30
4: f90017e0 str x0, [sp,#40]
8: b90027e1 str w1, [sp,#36]
c: b90023e2 str w2, [sp,#32]
10: f94017e0 ldr x0, [sp,#40]
14: b94027e1 ldr w1, [sp,#36]
18: b94023e2 ldr w2, [sp,#32]
1c: f9000fe0 str x0, [sp,#24]
20: b90017e1 str w1, [sp,#20]
24: b90013e2 str w2, [sp,#16]
28: f9400fe8 ldr x8, [sp,#24]
2c: 885ffd09 ldaxr w9, [x8] // Load-exclusive
30: 2a0903e0 mov w0, w9
34: 2a0003e9 mov w9, w0
38: b94017ea ldr w10, [sp,#20]
3c: 6b0a013f cmp w9, w10
40: b9000fe9 str w9, [sp,#12] // Unrelated store
44: 54000101 b.ne 64 <test+0x64>
48: b94013e8 ldr w8, [sp,#16]
4c: 2a0803e9 mov w9, w8
50: 2a0903e8 mov w8, w9
54: f9400fe9 ldr x9, [sp,#24]
58: 880afd28 stlxr w10, w8, [x9] // Store-exclusive
5c: 35fffe6a cbnz w10, 28 <test+0x28>
60: 14000003 b 6c <test+0x6c>
64: d5033f5f clrex
68: 14000001 b 6c <test+0x6c>
6c: b9400fe0 ldr w0, [sp,#12]
70: 9100c3ff add sp, sp, #0x30
74: d65f03c0 ret
The following example will print a different value based on if WITH_STORE is
defined or not.
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
uint32_t tmp, a, val, *pa;
uint32_t b, *pb;
int res;
a = 1;
val = 1;
pa = &a;
pb = malloc(sizeof(int));;
__asm __volatile(
"1: ldxr %w0, [%2] \n"
" add %w0, %w0, %w3 \n"
#if WITH_STORE
" str %w0, [%4] \n"
#endif
" stxr %w1, %w0, [%2] \n"
: "=&r"(tmp), "=&r"(res) : "r" (pa), "r" (val), "r" (pb)
: "cc", "memory"
);
printf("res = %x\n", res);
printf("a = %u\n", a);
return (0);
}
# clang atomic_test.c -DWITH_STORE
# ./a.out
res = 1
a = 1
# clang atomic_test.c
# ./a.out
res = 0
a = 2</pre>
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