Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Gilles Chanteperdrix wrote:
>>>>>> Jan Kiszka wrote:
>>>>>>> It will always remain orders of magnitude heavier than __thread
>>>>>>> variables which are a) inlined and b) should only need two memory
>>>>>>> accesses at worst. Moreover, it is clearly the future, while the
>>>>>>> importance of pthread_getspecific will decrease over the time. The
>>>>>>> __thread storage class is C99 standard (though its implementation
>>>>>>> remains a separate topic).
>>>>>> You are exagerating a bit: pthread_getspecific is pretty efficient
>>>>>> already (from the few things that I have timed on ARM, it is the only
>>>>>> one which takes under the microsecond). That you will gain something
>>>>>> with __thread is not guaranteed by the C99 standard either: in fact the
>>>>>> implementation could use exactly the same functions.
>>>>> As long as we do not loose anything (performance or portability),
>>>> You loose portability. But I agree that we do not care much.
>>> The fallback remains - must remain in order to obtain true optimization
>>> from the TLS-based version without locking out some corner-case usage.
>>> Find a proposal below (on top of handle-based xeno_get_current).
>>>
>>> We have to set initial-exec as TLS model, otherwise we end up with a
>>> dynamic lookup similar (maybe still faster, dunno) to the pthread
>>> service. This model requires start-time linking, will not work with
>>> dlopen (I strongly assume the linker will bail out). But I consider
>>> runtime loading of Xenomai libs as a uncommon corner case, and the user
>>> can still re-enable it via --without-__thread.
>> glibc has a separate test to know whether the tsl_model attribute is
>> supported:
>>
>> if test "$libc_cv_gcc___thread" = yes; then
>>   dnl Check whether the compiler supports the tls_model attribute.
>>   AC_CACHE_CHECK([for tls_model attribute], libc_cv_gcc_tls_model_attr, [dnl
>>   cat > conftest.c <<\EOF
>> extern __thread int a __attribute__((tls_model ("initial-exec")));
>> EOF
>>   if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -S -Werror conftest.c
>>> &AS_MESSA
>> GE_LOG_FD]); then
>>     libc_cv_gcc_tls_model_attr=yes
>>   else
>>     libc_cv_gcc_tls_model_attr=no
>>   fi
>>   rm -f conftest*])
>>   if test "$libc_cv_gcc_tls_model_attr" = yes; then
>>     AC_DEFINE(HAVE_TLS_MODEL_ATTRIBUTE)
>>   fi
>> fi
>>
> 
> OK, but for us the question is if we want __thread without initial-exec
> at all. If not (I think so), I could add -Werror to the __thread test
> and that should combine both tests into a single one, sufficient for our
> use case.

Yes, my point was that if an implementation supports __thread, it does
not necessarily mean that it supports the tls_model attribute. We should
fallback to pthread_key if either one is not supported. However, other
parts of Xenomai skins use pthread_specific, so, if we implement
something based on __thread, I think we should factor it and use it
everywhere.

-- 
                                                 Gilles.

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