MINOR: compiler: add a __has_builtin() macro to detect features more easily

We already have a __has_attribute() macro to detect when the compiler
supports a specific attribute, but we didn't have the equivalent for
builtins. clang-3 and gcc-10 have __has_builtin() for this. Let's just
bring it using the same mechanism as __has_attribute(), which will allow
us to simply define the macro's value for older compilers. It will save
us from keeping that many compiler-specific tests that are incomplete
(e.g. the __builtin_unreachable() test currently doesn't cover clang).

(cherry picked from commit 96cfcb1df36fdf8fb963371ad5e167c733d842b1)
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/include/haproxy/compiler.h b/include/haproxy/compiler.h
index 2699f73..e0d8b01 100644 (file)
--- a/include/haproxy/compiler.h
+++ b/include/haproxy/compiler.h
@@ -66,6 +66,14 @@
 #define __has_attribute(x) __equals_1(__has_attribute_ ## x)
 #endif
 
+/* gcc 10 and clang 3 brought __has_builtin() to test if a builtin exists.
+ * Just like above, if it doesn't exist, we remap it to a macro allowing us
+ * to define these ourselves by defining __has_builtin_<name> to 1.
+ */
+#ifndef __has_builtin
+#define __has_builtin(x) __equals_1(__has_builtin_ ## x)
+#endif
+
 /* The fallthrough attribute arrived with gcc 7, the same version that started
  * to emit the fallthrough warnings and to parse the comments. Comments do not
  * manage to stop the warning when preprocessing is split from compiling (e.g.