[PATCH v2 3/8] compiler.h: Add __if_constexpr(expr, if_const, if_not_const)

[David Laight]

__if_constexpr(expr, if_const, if_not_const) returns 'if_const' if 'expr'
is a 'constant integer expression' otherwise 'if_not_const'.
The two values may have different types.

__is_constexpr(expr) is equivalent to __if_constexpr(expr, 1, 0).

Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
---
v2:
- Don't change __is_constexpr()

 include/linux/compiler.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 2594553bb30b..35d5b2fa4786 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -242,6 +242,23 @@ static inline void *offset_to_ptr(const int *off)
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
+/**
+ * __if_constexpr - Check whether an expression is an 'integer
+ *		constant expression'
+ * @expr: Expression to test, not evaluated, can be a pointer
+ * @if_const: return value if constant
+ * @if_not_const: return value if not constant
+ *
+ * The return values @if_const and @if_not_const can have different types.
+ *
+ * Relies on typeof(x ? NULL : ptr_type) being ptr_type and
+ * typeof(x ? (void *)y : ptr_type) being 'void *'.
+ */
+#define __if_constexpr(expr, if_const, if_not_const)		\
+       _Generic(0 ? ((void *)((long)(expr) * 0l)) : (char *)0,	\
+		char *: (if_const),				\
+		void *: (if_not_const))
+
 /*
  * This returns a constant expression while determining if an argument is
  * a constant expression, most importantly without evaluating the argument.
-- 
2.17.1

-
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