[Pixman] [PATCH] Fix pixman build with older GCC releases

[Brad Smith]

The following patch fixes building pixman with older GCC releases
such as GCC 3.3 and older (OpenBSD; some older archs use GCC 3.3.6)
by changing the method of detecting the presence of __builtin_clz
to utilizing an autoconf check to determine its presence. Compilers
that pretend to be GCC, implement __builtin_clz and are already
utilizing the intrinsic include LLVM/Clang, Open64, EKOPath and
PCC.
---
 configure.ac           | 16 ++++++++++++++++
 pixman/pixman-matrix.c |  2 +-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 263c63e..a269f21 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1036,6 +1036,22 @@ fi
 
 AC_MSG_RESULT($support_for_float128)
 
+dnl =====================================
+dnl __builtin_clz
+
+support_for_builtin_clz=no
+
+AC_MSG_CHECKING(for __builtin_clz)
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+unsigned int x = 11; int main (void) { __builtin_clz(x); }
+]])], support_for_builtin_clz=yes)
+
+if test x$support_for_builtin_clz = xyes; then
+   AC_DEFINE([HAVE_BUILTIN_CLZ], [], [Whether the compiler supports __builtin_clz])
+fi
+
+AC_MSG_RESULT($support_for_builtin_clz)
+
 dnl ==================
 dnl libpng
 
diff --git a/pixman/pixman-matrix.c b/pixman/pixman-matrix.c
index 89b9682..4032c13 100644
--- a/pixman/pixman-matrix.c
+++ b/pixman/pixman-matrix.c
@@ -37,7 +37,7 @@
 static force_inline int
 count_leading_zeros (uint32_t x)
 {
-#ifdef __GNUC__
+#ifdef HAVE_BUILTIN_CLZ
     return __builtin_clz (x);
 #else
     int n = 0;
-- 
1.8.4


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