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

[Tijl Coosemans]

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

From: Tijl Coosemans <tijl at FreeBSD.org>
To: Niclas Zeising <zeising at freebsd.org>
Cc: bug-followup at FreeBSD.org
Subject: Re: ports/181838: [patch] x11-servers/xorg-server: fix TLS
 variables with clang
Date: Sun, 8 Sep 2013 12:18:15 +0200

 On Sat, 07 Sep 2013 23:51:32 +0200 Niclas Zeising wrote:
 > 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.
 > 
 > Looking through the git history of xorg-server, I noticed that this test
 > has changed in later versions.  I changed the TLS test to mimic this
 > way, and it seems to work.  I just want a second set of eyes, can you
 > please test the attached patch?
 
 Yes that works too.  It's the same as in the pixman configure script.