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

[Morten Brørup]

> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Monday, 1 April 2024 00.03
> 
> 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)))

Nice!
If the author is willing to license it under the BSD license, we can copy it as is.

We might want to add a couple of build time checks to verify that it does what is expected; to catch any changes in compiler behavior.