Date: Sun, 8 Sep 2013 10:20:00 GMT From: Tijl Coosemans <tijl@FreeBSD.org> To: freebsd-x11@FreeBSD.org Subject: Re: ports/181838: [patch] x11-servers/xorg-server: fix TLS variables with clang Message-ID: <201309081020.r88AK0nW000530@freefall.freebsd.org>
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The following reply was made to PR ports/181838; it has been noted by GNATS. From: Tijl Coosemans <tijl@FreeBSD.org> To: Niclas Zeising <zeising@freebsd.org> Cc: bug-followup@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.
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