linux-kernel - Re: [PATCH v4 1/2] compiler.h: add const

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Message-ID: <CAHk-=wiq=GUNWJwWh1CRAYchW73UmOaSkaCovLatfDKeveZctA@mail.gmail.com>
Date: Sun, 17 Nov 2024 10:00:12 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: Vincent Mailhol <mailhol.vincent@...adoo.fr>, Yury Norov <yury.norov@...il.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Luc Van Oostenryck <luc.vanoostenryck@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>, 
	Rikard Falkeborn <rikard.falkeborn@...il.com>
Subject: Re: [PATCH v4 1/2] compiler.h: add const_true()

On Sun, 17 Nov 2024 at 09:42, David Laight <David.Laight@...lab.com> wrote:
>
> #define const_true(x) __if_constexpr(x, x, 0)

No, let's not do this "double expansion" thing again.

If we actually want to make things smarter, the trick to use is to
know that only a constant _zero_ turns into a void pointer (aka NULL).

IOW, something like this:

   /*
    * iff 'x' is a non-zero constant integer expression,
    * then '!(x)' will be a zero constant integer expression,
    * and casting that to 'void *' will result in a NULL
    * pointer. Otherwise casting it to 'void *' will be just
    * a regular 'void *'.
    *
    * The type of '0 ? NULL : (char *)' is 'char *'
    * The type of '0 ? (void *) : (char *) is 'void *'
    */
    #define const_true(x) \
        _Generic(0 ? (void *)((long)!(x)) : (char *)0, char *: 1, void *: 0)

should work, and doesn't do any double expansion of complex arguments.

           Linus