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Re: powerpc __tls_get_addr call optimization


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.

-- 
H.J.


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