PING for Review/ACK. Come on fellow reviewers, it's only 5 lines of code!
The mempool library cannot build with MSVC without this patch series. Other patches are also being held back, waiting for this MSVC compatible DPDK macro for __builtin_constant_p(). The macro for MSVC can be improved as suggested by Stephen later. > From: Morten Brørup [mailto:m...@smartsharesystems.com] > Sent: Monday, 1 April 2024 10.35 > > > 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.
