Re: [Xenomai-core] __thread instead of pthread_get/setspecific

[Jan Kiszka <jan.kiszka@domain.hid>]

Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Gilles Chanteperdrix wrote:
>>> Jan Kiszka wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> Jan Kiszka wrote:
>>>>>> Hi,
>>>>>>
>>>>>> looking into the "xeno_in_primary_mode" thing I wondered how to make the
>>>>>> thread state quickly retrievable. Going via pthread_getspecific as we do
>>>>>> for xeno_get_current appears logical - but not optimal. Though
>>>>>> getspecific is optimized for speed, it remains a function call, a few
>>>>>> sanity checks, and only finally a TLS variable access. That could be
>>>>>> achieved in a much lighter way by using a __thread variable.
>>>>>>
>>>>>> But can we assume that all target we support also support the __thread
>>>>>> storage class? TLS is surely mandatory now: I assume pthread_getspecific
>>>>>> would become non-RT safe without it, right? Is there anything we
>>>>>> can/must check for during configure to verify __thread support?
>>>>> I really think that this optimization is not worth the trouble. Anyway,
>>>> As long as we cannot specify the amount of "trouble", it's hard to
>>>> decide. Me current feeling is that it should rather simplify the
>>>> implementation + save us quite a few ops in the fast path (even more
>>>> with upcoming thread-mode check).
>>> The trouble is to make some reliable detections in the configure script,
>>> so that the user will know early that Xenomai can not work with its
>>> current toolchain. And to make this detection work with uclibc as well
>>> as with glibc, gcc 4 versus gcc 3, etc...
>> Will work out a test program for configure.
> 
> glibc does that:
> 
> AC_ARG_WITH([__thread],
>             AC_HELP_STRING([--without-__thread],
>                            [do not use TLS features even when supporting them]),
>             [use__thread=$withval],
>             [use__thread=yes])
> 
> dnl Check whether the compiler supports the __thread keyword.
> if test "x$use__thread" != xno; then
>   AC_CACHE_CHECK([for __thread], libc_cv_gcc___thread,
>   [cat > conftest.c <<\EOF
> __thread int a = 42;
> EOF
>   if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -c conftest.c >&AS_MESSAGE_LOG_F
> D]); then
>     libc_cv_gcc___thread=yes
>   else
>     libc_cv_gcc___thread=no
>   fi
>   rm -f conftest*])
>   if test "$libc_cv_gcc___thread" = yes; then
>     AC_DEFINE(HAVE___THREAD)
>   fi
> else
>   libc_cv_gcc___thread=no
> fi
> 

Cool, thanks. Will play with this for xeno_get_current soon. Then we can
review and test if it works and makes sense based on some real code.

Jan

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