Re: using gcc built-ins for bitops?

[Andrew Morton <akpm@osdl.org>]

Arjan van de Ven <arjanv@redhat.com> wrote:
>
> gcc 3.4 gained support for several typical bitops as builtin directives.
>  Using these over inline asm has a few advantages:
>  * gcc can optimize constants into these better
>  * gcc can reorder and schedule the code better
>  * gcc can allocate registers etc better for the code
> 
>  The question is if we consider it desirable to go down this road or not. In
>  order to help that discussion I've attached a patch below that switches the
>  i386 ffz() function to the gcc builtin version, conditional on gcc having
>  support for this. Before I go down the road of converting more functions
>  and/or architectures.... is this worth doing?

I guess it depends on the resulting code size and quality.  Some extra
conversions would be needed for that.

For the implementation it would be nice to have the old-style
implementations in one header and the new-style ones in a separate header. 
That would create a bit of an all-or-nothing situation, but that should be
OK?

>  +static inline unsigned long ffz (unsigned long word) 
>  +{ 
>  +	return __builtin_ctzl (~word); 
>  +}

eww, whitepsace innovations.

static inline unsigned long ffz(unsigned long word) 
{ 
	return __builtin_ctzl(~word); 
}