compiler.h: Explain how __is_constexpr() works

The __is_constexpr() macro is dark magic. Shed some light on it with
a comment to explain how and why it works.

Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20240301044428.work.411-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/compiler.h

@@ -231,6 +231,45 @@ static inline void *offset_to_ptr(const int *off)
  * This returns a constant expression while determining if an argument is
  * a constant expression, most importantly without evaluating the argument.
  * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
+ *
+ * Details:
+ * - sizeof() return an integer constant expression, and does not evaluate
+ *   the value of its operand; it only examines the type of its operand.
+ * - The results of comparing two integer constant expressions is also
+ *   an integer constant expression.
+ * - The first literal "8" isn't important. It could be any literal value.
+ * - The second literal "8" is to avoid warnings about unaligned pointers;
+ *   this could otherwise just be "1".
+ * - (long)(x) is used to avoid warnings about 64-bit types on 32-bit
+ *   architectures.
+ * - The C Standard defines "null pointer constant", "(void *)0", as
+ *   distinct from other void pointers.
+ * - If (x) is an integer constant expression, then the "* 0l" resolves
+ *   it into an integer constant expression of value 0. Since it is cast to
+ *   "void *", this makes the second operand a null pointer constant.
+ * - If (x) is not an integer constant expression, then the second operand
+ *   resolves to a void pointer (but not a null pointer constant: the value
+ *   is not an integer constant 0).
+ * - The conditional operator's third operand, "(int *)8", is an object
+ *   pointer (to type "int").
+ * - The behavior (including the return type) of the conditional operator
+ *   ("operand1 ? operand2 : operand3") depends on the kind of expressions
+ *   given for the second and third operands. This is the central mechanism
+ *   of the macro:
+ *   - When one operand is a null pointer constant (i.e. when x is an integer
+ *     constant expression) and the other is an object pointer (i.e. our
+ *     third operand), the conditional operator returns the type of the
+ *     object pointer operand (i.e. "int *). Here, within the sizeof(), we
+ *     would then get:
+ *       sizeof(*((int *)(...))  == sizeof(int)  == 4
+ *   - When one operand is a void pointer (i.e. when x is not an integer
+ *     constant expression) and the other is an object pointer (i.e. our
+ *     third operand), the conditional operator returns a "void *" type.
+ *     Here, within the sizeof(), we would then get:
+ *       sizeof(*((void *)(...)) == sizeof(void) == 1
+ * - The equality comparison to "sizeof(int)" therefore depends on (x):
+ *     sizeof(int) == sizeof(int)     (x) was a constant expression
+ *     sizeof(int) != sizeof(void)    (x) was not a constant expression
  */
 #define __is_constexpr(x) \
 	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))