Re: using gcc built-ins for bitops?

[Arjan van de Ven <arjanv@redhat.com>]

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On Thu, Jun 24, 2004 at 02:00:22AM -0700, Andrew Morton wrote:
> 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 ffz() the exact same assembly instructions are generated in the cases I looked at
(kernel/signal.c); eg no extra code at all.



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