Date: Fri, 6 Sep 2013 21:30:00 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: <201309062130.r86LU0oS059285@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: Fri, 6 Sep 2013 23:22:29 +0200
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.
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