[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[Vincent Mailhol via B4 Relay <devnull+mailhol.vincent.wanadoo.fr@kernel.org>]

From: Vincent Mailhol <mailhol.vincent@wanadoo.fr>

__is_constexpr(), while being one of the most glorious one liner hack
ever witnessed by mankind, is overly complex. Following the adoption
of C11 in the kernel, this macro can be simplified through the use of
a _Generic() selection.

First, split the macro in two:

  - __is_const_zero(x): an helper macro; tells whether x is the
    integer constant expression 0 or something else.

  - is_const(x): replacement of __is_constexpr(); tells whether x is a
    integer constant expression.

The split serves two purposes: first make it easier to understand;
second, __is_const_zero() will be reused as a building block for other
is_const_*() macros that will be introduced later on.

The core principle of __is_constexpr() to abuse the return type of the
ternary operator remains, but all the surrounding sizeof() hack
disappear.

On a side note, while not relevant to the kernel, __is_constexpr()
relied on the GNU extension that sizeof(void) is 1. const_expr() does
not use any GNU extensions, making it ISO C compliant.

__is_constexpr() is temporarily kept and will be removed once all its
users get migrated to is_const() (or one of its friend).

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 include/linux/compiler.h | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index a2a56a50dd85227a4fdc62236a2710ca37c5ba52..30ce06df4153cfdc0fad9bc7bffab9097f8b0450 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -316,6 +316,47 @@ static inline void *offset_to_ptr(const int *off)
 #define statically_true(x) (__builtin_constant_p(x) && (x))
 #define statically_false(x) (__builtin_constant_p(x) && (x) == 0)
 
+/*
+ * Whether x is the integer constant expression 0 or something else.
+ *
+ * Details:
+ *   - The C11 standard defines in §6.3.2.3.3
+ *       (void *)<integer constant expression with the value 0>
+ *     as a null pointer constant (c.f. the NULL macro).
+ *   - If x evaluates to the integer constant expression 0,
+ *       (void *)(x)
+ *     is a null pointer constant. Else, it is a void * expression.
+ *   - In a ternary expression:
+ *       condition ? operand1 : operand2
+ *     if one of the two operands is of type void * and the other one
+ *     some other pointer type, the C11 standard defines in §6.5.15.6
+ *     the resulting type as below:
+ *       if one operand is a null pointer constant, the result has the
+ *       type of the other operand; otherwise [...] the result type is
+ *       a pointer to an appropriately qualified version of void.
+ *   - As such, in
+ *       0 ? (void *)(x) : (char *)0
+ *     if x is the integer constant expression 0, operand1 is a null
+ *     pointer constant and the resulting type is that of operand2:
+ *     char *. If x is anything else, the type is void *.
+ *   - The (long) cast silences a compiler warning for when x is not 0.
+ *   - Finally, the _Generic() dispatches the resulting type into a
+ *     Boolean.
+ *
+ * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
+ */
+#define __is_const_zero(x) \
+	_Generic(0 ? (void *)(long)(x) : (char *)0, char *: 1, void *: 0)
+
+/*
+ * Returns a constant expression while determining if its argument is a
+ * constant expression, most importantly without evaluating the argument.
+ *
+ * If getting a constant expression is not relevant to you, use the more
+ * powerful __builtin_constant_p() instead.
+ */
+#define is_const(x) __is_const_zero(0 * (x))
+
 /*
  * This is needed in functions which generate the stack canary, see
  * arch/x86/kernel/smpboot.c::start_secondary() for an example.

-- 
2.45.2