http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50583
--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> 2011-09-30 18:37:42
UTC ---
The same problem with
`TYPE __sync_add_and_fetch (TYPE *ptr, TYPE value, ...)'
`TYPE __sync_sub_and_fetch (TYPE *ptr, TYPE value, ...)'
`TYPE __sync_or_and_fetch (TYPE *ptr, TYPE value, ...)'
`TYPE __sync_and_and_fetch (TYPE *ptr, TYPE value, ...)'
`TYPE __sync_xor_and_fetch (TYPE *ptr, TYPE value, ...)'
`TYPE __sync_nand_and_fetch (TYPE *ptr, TYPE value, ...)'
These builtins perform the operation suggested by the name, and
return the new value. That is,
{ *ptr OP= value; return *ptr; }
{ *ptr = ~(*ptr & value); return *ptr; } // nand
[hjl@gnu-33 pr50583]$ cat z.i
int
foo (int *p)
{
return __sync_and_and_fetch (p, 7);
}
[hjl@gnu-33 pr50583]$ gcc -O2 -S z.i
[hjl@gnu-33 pr50583]$ cat z.s
.file "z.i"
.text
.p2align 4,,15
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
movl (%rdi), %eax
.L2:
movl %eax, %edx
andl $7, %edx
lock cmpxchgl %edx, (%rdi)
jne .L2
movl %edx, %eax
ret
.cfi_endproc
.LFE0:
.size foo, .-foo
.ident "GCC: (GNU) 4.6.0 20110603 (Red Hat 4.6.0-10)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-33 pr50583]$
It is
{
tmp = *ptr;
do
{
tmp1 = tmp OP value;
}
while (__sync_val_compare_and_swap (ptr, tmp, tmp1) != tmp);
return tmp1;
}
If *ptr is changed between load and cmpxchg, we get an infinite loop.
