Using condition flags as GNU C inline asm outputs

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Starting with GCC6 on x86 you can actually use "=@ccCOND" as output (where COND is any valid x86 condition code).

Example originally from here, cleaned up by David’s suggestions:

int variable_test_bit(long n, volatile const unsigned long *addr)
{
    int oldbit;
    asm volatile("bt %[value],%[bit]"
                 : "=@ccc" (oldbit)
                 : [value] "m" (*addr), [bit] "Jr" (n));
    return oldbit;
}

Before using this, you should test if __GCC_ASM_FLAG_OUTPUTS__ is defined.

Documentation at https://gcc.gnu.org.

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