ports/181838: [patch] x11-servers/xorg-server: fix TLS variables with clang

[Niclas Zeising]

The following reply was made to PR ports/181838; it has been noted by GNATS.

From: Niclas Zeising <zeising at freebsd.org>
To: Tijl Coosemans <tijl at FreeBSD.org>
Cc: bug-followup at FreeBSD.org
Subject: Re: ports/181838: [patch] x11-servers/xorg-server: fix TLS variables
 with clang
Date: Sat, 07 Sep 2013 01:52:44 +0200

 On 09/06/13 23:22, Tijl Coosemans wrote:
 > On Fri, 06 Sep 2013 22:18:27 +0200 Niclas Zeising wrote:
 >> Can you please elaborate a bit on this, exactly what happends when
 >> setting that configure variable?
 > 
 > The configure script tests how to declare a thread-local variable.  For
 > clang and gcc this is done using the __thread keyword like this:
 > 
 > __thread int thread_specific_int_var;
 > 
 > Other compilers use __declspec(thread) instead of __thread.
 > 
 > The code in configure comes from the XORG_TLS call in configure.ac.
 > XORG_TLS is defined in m4/xorg-tls.m4. Here are the relevant bits:
 > 
 >     AC_MSG_CHECKING(for thread local storage (TLS) support)
 >     AC_CACHE_VAL(ac_cv_tls, [
 >         ac_cv_tls=none
 >         keywords="__thread __declspec(thread)"
 >         for kw in $keywords ; do
 >             AC_TRY_COMPILE([int $kw test;], [], ac_cv_tls=$kw)
 >         done
 >     ])
 >     AC_MSG_RESULT($ac_cv_tls)
 > 
 > The for-loop runs over the elements in $keywords and sets ac_cv_tls
 > to the keyword if the compiler accepts it.  The problem is that clang
 > accepts __declspec(thread) but it doesn't actually work (there's a
 > warning about this but no error) so when the for-loop ends ac_cv_tls
 > is set to __declspec(thread).  Because the test is inside AC_CACHE_VAL
 > it can be skipped by defining ac_cv_tls which is what the patch does.
 > 
 > I found a similar test in pixman but there the for-loop contains a
 > break so it ends on the first successful keyword (i.e. __thread).
 > 
 >> Have you tried and ensured that this doesn't break xserver built with
 >> gcc?
 > 
 > No, I currently don't have gcc installed, but the keyword for gcc is
 > also __thread.  You could search for __thread in a config.log generated
 > with gcc to verify this.
 > 
 
 Thank you for your very excellent description!
 I'll look into this more during the weekend or beginning of next week.
 Your patch looks fine, now that I understand it properly ;). I just want
 to make sure that gcc still works, check the old xorg-server version,
 and look through the rest of the xorg ports, in case this is needed
 somewhere else as well.
 Regards!
 -- 
 Niclas Zeising