[Pixman] [PATCH] Do CPU features detection from 'constructor' function when compiled with gcc

[Siarhei Siamashka]

From: Siarhei Siamashka <siarhei.siamashka at nokia.com>

There is attribute 'constructor' supported since gcc 2.7 which allows
to have a constructor function for library initialization. This eliminates
an extra branch for each composite operation and also helps to avoid
complains from race condition detection tools like helgrind.

The other compilers may or may not support this attribute properly.
Ideally, the compilers should fail to compile the code with unknown
attribute, so the configure check should do the right job. But in
reality the problems are surely possible. Fortunately such problems
should be quite easy to find because NULL pointer dereference should
happen almost immediately if the constructor fails to run.

clang 2.7:
  supports __attribute__((constructor)) properly and pretends to be gcc

tcc 0.9.25:
  ignores __attribute__((constructor)), but does not pretend to be gcc
---
 configure.ac    |   27 +++++++++++++++++++++++++++
 pixman/pixman.c |   15 ++++++++++++---
 2 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index fc08def..08768cc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -740,6 +740,33 @@ AC_SUBST(HAVE_PTHREAD_SETSPECIFIC)
 AC_SUBST(PTHREAD_LDFLAGS)
 AC_SUBST(PTHREAD_LIBS)
 
+dnl =====================================
+dnl __attribute__((constructor))
+
+support_for_attribute_constructor=no
+
+AC_MSG_CHECKING(for __attribute__((constructor)))
+AC_LINK_IFELSE([
+#if defined(__GNUC__) && (__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 7))
+/* attribute 'constructor' is supported since gcc 2.7, but some compilers
+ * may only pretend to be gcc, so let's try to actually use it
+ */
+static int x = 1;
+static void __attribute__((constructor)) constructor_function () { x = 0; }
+int main (void) { return x; }
+#else
+#error not gcc or gcc version is older than 2.7
+#endif
+], support_for_attribute_constructor=yes)
+
+if test x$support_for_attribute_constructor = xyes; then 
+   AC_DEFINE([TOOLCHAIN_SUPPORTS_ATTRIBUTE_CONSTRUCTOR],
+             [],[Whether the tool chain supports __attribute__((constructor))])
+fi
+
+AC_MSG_RESULT($support_for_attribute_constructor)
+AC_SUBST(TOOLCHAIN_SUPPORTS_ATTRIBUTE_CONSTRUCTOR)
+
 AC_OUTPUT([pixman-1.pc
            pixman-1-uninstalled.pc
            Makefile
diff --git a/pixman/pixman.c b/pixman/pixman.c
index bd9305b..b7164d8 100644
--- a/pixman/pixman.c
+++ b/pixman/pixman.c
@@ -30,14 +30,23 @@
 
 #include <stdlib.h>
 
+static pixman_implementation_t *global_implementation;
+
+#ifdef TOOLCHAIN_SUPPORTS_ATTRIBUTE_CONSTRUCTOR
+static void __attribute__((constructor))
+pixman_constructor (void)
+{
+    global_implementation = _pixman_choose_implementation ();
+}
+#endif
+
 static force_inline pixman_implementation_t *
 get_implementation (void)
 {
-    static pixman_implementation_t *global_implementation;
-
+#ifndef TOOLCHAIN_SUPPORTS_ATTRIBUTE_CONSTRUCTOR
     if (!global_implementation)
 	global_implementation = _pixman_choose_implementation ();
-
+#endif
     return global_implementation;
 }
 
-- 
1.7.2.2