[PATCH 4/6] typesafe: cast_if_type to allow macros functions which take more than one type.

[Rusty Russell <rusty@rustcorp.com.au>]

To create functions which can take two types, but still warn on any
other types, we need a way of casting one type and no others.

To make things more complex, it should correctly handle function args,
NULL, and be usable in initializers.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r e6626cc7bdc2 include/linux/compiler-gcc.h
--- a/include/linux/compiler-gcc.h	Sun Jan 20 18:51:51 2008 +1100
+++ b/include/linux/compiler-gcc.h	Sun Jan 20 18:57:14 2008 +1100
@@ -53,3 +53,20 @@
 #define  noinline			__attribute__((noinline))
 #define __attribute_const__		__attribute__((__const__))
 #define __maybe_unused			__attribute__((unused))
+
+/**
+ * cast_if_type - allow an alternate type
+ * @expr: the expression to optionally cast
+ * @oktype: the type to allow.
+ * @desttype: the type to cast to.
+ *
+ * This is used to accept a particular alternate type for an expression:
+ * because any other types will not be cast, they will cause a warning as
+ * normal.
+ *
+ * Note that the unnecessary trinary forces functions to devolve into
+ * function pointers as users expect. */
+#define cast_if_type(expr, oktype, desttype)				\
+  __builtin_choose_expr(__builtin_types_compatible_p(typeof(1?(expr):NULL), \
+						     oktype),		\
+			(desttype)(expr), (expr))
diff -r e6626cc7bdc2 include/linux/compiler-intel.h
--- a/include/linux/compiler-intel.h	Sun Jan 20 18:51:51 2008 +1100
+++ b/include/linux/compiler-intel.h	Sun Jan 20 18:57:14 2008 +1100
@@ -29,3 +29,5 @@
 #endif
 
 #define uninitialized_var(x) x
+
+#define cast_if_type(expr, oktype, desttype) ((desttype)(expr))