Re: powerpc __tls_get_addr call optimization

["Carlos O'Donell" <carlos at redhat dot com>]

On 03/20/2015 12:19 PM, H.J. Lu wrote:
> 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.

Agreed. As should the TLS specification instead of a URL reference to
tls.pdf which is going to get out of date.

Cheers,
Carlos.