[PATCH 1/3] typecheck: introduce assign_if() and assign_if_enabled()

[Josh Cartwright]

The assign_if() and assign_if_enable() macros are intended to be used
in static initializers for function pointers, where the pointer is
expected to be NULL when a compile-time condition does not hold.

These macros allow for implementing this behavior, without requiring the
functions be wrapped in #ifdef conditionals, and while providing
typesafety regardless of the value of the conditional.

For example, the following pattern is common:

	#ifdef CONFIG_FOO
	static void foo_callback(void)
	{
	}
	#else
	#define foo_callback NULL
	#endif

	static struct foo_object foo_obj = {
		.callback = foo_callback,
	};

Usage of assign_if_enabled() allows for achieving the same effect
without the preprocessor conditional, and in addition, allowing the
compiler to typecheck the function regardless of CONFIG_FOO.

	static void foo_callback(void)
	{
	}

	static struct foo_object foo_obj = {
		.callback = assign_if_enabled(CONFIG_FOO, foo_callback),
	};

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Josh Cartwright <joshc@xxxxxxxxxxxxxx>
---
 include/linux/typecheck.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h
index eb5b74a..04134c7 100644
--- a/include/linux/typecheck.h
+++ b/include/linux/typecheck.h
@@ -21,4 +21,22 @@
 	(void)__tmp; \
 })
 
+/*
+ * Intended for use in static object initializers,
+ * assign_if(const_expr, function) evaluates to 'function' if 'const_expr',
+ * otherwise NULL.
+ *
+ * The type of the assign_if() expression is typeof(function), and therefore
+ * can provide typechecking regardless of 'const_expr'.
+ *
+ * gcc considers 'function' to be used and will not generate a 'defined but not
+ * used' warning when not 'const_expr', however, gcc is smart enough to
+ * eliminate 'function' if assign_if() is the only reference.
+ */
+#define assign_if(const_expr,function)	\
+	((const_expr) ? function : NULL)
+
+#define assign_if_enabled(option,function)	\
+	assign_if(IS_ENABLED(option), function)
+
 #endif		/* TYPECHECK_H_INCLUDED */
-- 
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