http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52381
Timo Kreuzer <timo.kreuzer at reactos dot org> changed:
What |Removed |Added
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CC| |timo.kreuzer at reactos dot
| |org
--- Comment #3 from Timo Kreuzer <timo.kreuzer at reactos dot org> 2013-01-10
13:49:11 UTC ---
I support this feature request.
As an exmple I'd like to mention the x86 instruction cmpxchg.
While GCC has 2 CAS builtins (__sync_bool_compare_and_swap and
__sync_val_compare_and_swap) neither of these supports what the x86 cmpxchg
instruction does, namely returning both a boolean value indicating success AND
the previous value of the memory operand in case of failure.
So here's the code that would handle this, if output operand was usable.
try_again:
// do something
asm goto
(
"lock cmpxchg %%ecx, %[Destination] \t\n"
"jnz %l[try_again] \t\n"
: "=c"(Exchange) // <= does not work
: [Destination]"m"(Destination), [Comparand]"a"(Comparand),
[Exchange]"c"(Exchange)
: "memory"
: try_again
);
Note that in this case the output is needed on the jump path.
