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

David Laight
2024-07-28T14:20:45Z

__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@aculab.com>
---
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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