From: Stephen Hemminger <stephen@networkplumber.org>
To: Tyler Retzlaff <roretzla@linux.microsoft.com>
Cc: dev@dpdk.org, "Morten Brørup" <mb@smartsharesystems.com>,
"Andrew Rybchenko" <andrew.rybchenko@oktetlabs.ru>
Subject: Re: [PATCH 1/2] eal: provide macro for GCC builtin constant intrinsic
Date: Sun, 31 Mar 2024 15:03:27 -0700 [thread overview]
Message-ID: <20240331150327.71312354@hermes.local> (raw)
In-Reply-To: <1710970416-27841-2-git-send-email-roretzla@linux.microsoft.com>
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)))
next prev parent reply other threads:[~2024-03-31 22:03 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-20 21:33 [PATCH 0/2] provide toolchain abstracted __builtin_constant_p Tyler Retzlaff
2024-03-20 21:33 ` [PATCH 1/2] eal: provide macro for GCC builtin constant intrinsic Tyler Retzlaff
2024-03-26 9:57 ` Morten Brørup
2024-03-31 22:03 ` Stephen Hemminger [this message]
2024-04-01 8:34 ` Morten Brørup
2024-05-27 11:58 ` Morten Brørup
2024-05-29 11:42 ` Andrew Rybchenko
2024-05-27 12:00 ` Bruce Richardson
2024-05-29 11:42 ` Andrew Rybchenko
2024-03-20 21:33 ` [PATCH 2/2] mempool: use rte constant macro instead of GCC builtin Tyler Retzlaff
2024-03-26 9:57 ` Morten Brørup
2024-05-29 11:42 ` Andrew Rybchenko
2024-05-29 14:51 ` Thomas Monjalon
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