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From: "timo.kreuzer at reactos dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/52381] asm goto output operands diagnostics
Date: Thu, 10 Jan 2013 13:49:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52381

Timo Kreuzer <timo.kreuzer at reactos dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 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.