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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [Optimisation] Systems with load-linked/store-conditional could `merge' some atomic operations"
href="http://llvm.org/bugs/show_bug.cgi?id=16341">16341</a>
</td>
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<tr>
<th>Summary</th>
<td>[Optimisation] Systems with load-linked/store-conditional could `merge' some atomic operations
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: ARM
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ed@80386.nl
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the following piece of code:
---------------------------------------------------------------------
#include <assert.h>
#include <stdatomic.h>
#include <stdbool.h>
_Atomic(int) cur = ATOMIC_VAR_INIT(0);
static const int max = 100;
bool
allocate(void)
{
int old;
old = atomic_load_explicit(&cur, memory_order_relaxed);
do {
if (old >= max)
return false;
} while (!atomic_compare_exchange_weak_explicit(&cur, &old, old + 1,
memory_order_relaxed, memory_order_relaxed));
return true;
}
void
deallocate(void)
{
int old;
old = atomic_fetch_sub_explicit(&cur, 1, memory_order_relaxed);
assert(old > 0);
}
---------------------------------------------------------------------
In other words, it implements a simple counter to constrain the number of
users. Examples: maximum number of file descriptors/sockets/etc in an OS
kernel.
In theory, allocate() could be translated to the following sequence of
instructions:
...
ldrex r0, [...]
cmp r0, #100
bge ...
add r1, r0, #1
strex r0, r1, [...]
...
Instead, I see the following:
00000000 <acquire>:
// Do a load + add
0: e59fc03c ldr ip, [pc, #60] ; 44 <acquire+0x44>
4: e59c2000 ldr r2, [ip]
8: e3a00000 mov r0, #0 ; 0x0
c: e3520063 cmp r2, #99 ; 0x63
10: c12fff1e bxgt lr
14: e2820001 add r0, r2, #1 ; 0x1
// Do a compare + exchange
18: e19c3f9f ldrex r3, [ip]
1c: e1530002 cmp r3, r2
20: 1a000002 bne 30 <acquire+0x30>
24: e18c1f90 strex r1, r0, [ip]
28: e3510000 cmp r1, #0 ; 0x0
2c: 1afffff9 bne 18 <acquire+0x18>
// Handle compare + exchange result
30: e1530002 cmp r3, r2
34: e3a00001 mov r0, #1 ; 0x1
38: e1a02003 mov r2, r3
3c: 1afffff1 bne 8 <acquire+0x8>
40: e12fff1e bx lr
44: 00000000 .word 0x00000000
LLVM/Clang version used:
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
Target: armv6-unknown-freebsd10.0
Thread model: posix</pre>
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