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Re: powerpc __tls_get_addr call optimization


On Fri, Mar 20, 2015 at 9:14 AM, Rich Felker <dalias@libc.org> wrote:
> On Fri, Mar 20, 2015 at 08:51:39AM -0700, H.J. Lu wrote:
>> On Fri, Mar 20, 2015 at 8:48 AM, Carlos O'Donell <carlos@redhat.com> wrote:
>> > On 03/20/2015 11:27 AM, Rich Felker wrote:
>> >> 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.
>> >
>> > I agree. And this was my initial inclination, but I'm not against what
>> > Alan has implemented. As a machine maintainer he should be allowed some
>> > leeway to argue this implementation is "N instructions less" and therefore
>> > must be faster, but that such speed is harder to show in a microbenchmark,
>> > it would in the mean result in say less CPU usage over billions of cycles.
>> >
>> > IBM has to accept that the downside to all of this is that breakage in
>> > this area may take longer to fix, and get less fixes than those arches
>> > already using TLS DESC.
>>
>> Speaking of TLS DESC, are there any tests for TLS DESC in
>> glibc?  I never implemented TLS DESC for x32 since I didn't
>> find any run-time tests for TLS DESC in GCC nor glibc.
>
> Not that I know of. i386 TLSDESC was broken in binutils for several
> years and only recently fixed... Until a couple months ago nobody
> noticed. :-(
>
> This situation really should be set right (with proper tests and
> timeline for changing the default to TLSDESC) so we can put an end to
> the invalid use of IE-model in shared libraries.

Another thing,  x86 and x86-64 TLS DESC spec should be
in x86 and x86-64 psABIs, not a URL.


-- 
H.J.


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