From: "Morten Brørup" <mb@smartsharesystems.com>
To: <dev@dpdk.org>, "Andrew Rybchenko" <andrew.rybchenko@oktetlabs.ru>
Cc: "Stephen Hemminger" <stephen@networkplumber.org>,
"Tyler Retzlaff" <roretzla@linux.microsoft.com>
Subject: RE: [PATCH 1/2] eal: provide macro for GCC builtin constant intrinsic
Date: Mon, 27 May 2024 13:58:34 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F4B2@smartserver.smartshare.dk> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F345@smartserver.smartshare.dk>
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:mb@smartsharesystems.com]
> Sent: Monday, 1 April 2024 10.35
>
> > 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.
next prev parent reply other threads:[~2024-05-27 11:58 UTC|newest]
Thread overview: 15+ 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
2024-04-01 8:34 ` Morten Brørup
2024-05-27 11:58 ` Morten Brørup [this message]
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
2024-06-14 14:32 ` David Marchand
2024-06-27 13:28 ` [PATCH 0/2] provide toolchain abstracted __builtin_constant_p Thomas Monjalon
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