Re: powerpc __tls_get_addr call optimization

[Rich Felker <dalias at libc dot org>]

On Fri, Mar 20, 2015 at 06:25:02PM +1030, Alan Modra wrote:
> On Thu, Mar 19, 2015 at 11:33:16PM -0400, Carlos O'Donell wrote:
> > On 03/18/2015 10:56 PM, Alan Modra wrote:
> > > On Wed, Mar 18, 2015 at 01:07:32PM -0400, Carlos O'Donell wrote:
> > >> On 03/18/2015 02:11 AM, Alan Modra wrote:
> > >>> Now that Alex's fixes for static TLS have gone in, I figure it's worth
> > >>> revisiting an old patch of mine.
> > >>> https://sourceware.org/ml/libc-alpha/2009-03/msg00053.html
> > >>
> > >> I'm not against this patch, but it certainly seems like you would be
> > >> better served by just implementing tls descriptors?
> > > 
> > > I think this is one better than tls descriptors, because powerpc
> > > avoids the indirect function call used by tls descriptors.
> > 
> > You mean to say it is "faster" than tls descriptors, but at the same
> 
> To be honest, there isn't much difference in the optimized case where
> static TLS is available.  It boils down to an indirect call to a
> function that loads one value vs. a direct call to a stub that loads
> two values and compares one against zero.  I think what I've
> implemented is slightly better for PowerPC, but whether that would
> carry over to other architectures is debatable.

If the performance difference isn't measurable in real-world
applications, I would think uniformity between targets would be a lot
more valuable.

I also don't see how your approach is a "direct call". The function
being called is in a different DSO so it has to go through a pointer
in the GOT or similar, in which case it's just as "indirect" as the
TLSDESC call would be.

Rich