[PATCH 1/2] eal: provide macro for GCC builtin constant intrinsic

[Stephen Hemminger]

On Wed, 20 Mar 2024 14:33:35 -0700
Tyler Retzlaff <roretzla at linux.microsoft.com> wrote:

> +#ifdef RTE_TOOLCHAIN_MSVC
> +#define __rte_constant(e) 0
> +#else
> +#define __rte_constant(e) __extension__(__builtin_constant_p(e))
> +#endif
> +


I did some looking around and some other project have macros
for expressing constant expression vs constant.

Implementing this with some form of sizeof math is possible.
For example in linux/compiler.h

/*
 * 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 at 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)))