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Date: Tue, 14 Oct 2008 11:58:22 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-core] __thread instead of pthread_get/setspecific
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To: Jan Kiszka <jan.kiszka@domain.hid>
Cc: xenomai-core <xenomai@xenomai.org>

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



-- 
                                                 Gilles.