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

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 4/1/24 11:34, Morten Brørup wrote:
>> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
>> Sent: Monday, 1 April 2024 00.03
>>
>> On Wed, 20 Mar 2024 14:33:35 -0700
>> Tyler Retzlaff <roretzla@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@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.
> 

LGTM too, but meanwhile we can continue without it just to unblock build 
on MSVC