> From: Stephen Hemminger [mailto:step...@networkplumber.org] > Sent: Monday, 1 April 2024 00.03 > > On Wed, 20 Mar 2024 14:33:35 -0700 > Tyler Retzlaff <roret...@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.uec...@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.
