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

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Message-ID: <CAHk-=wgfpLdt7SFFGcByTfHdkvv7AEa3MDu_s_W1kfOxQs49pw@mail.gmail.com>
Date: Sun, 17 Nov 2024 12:12:26 -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 11:23, David Laight <David.Laight@...lab.com> wrote:
>
> Since 99% will be 1,0 maybe saving the extra expansion is best anyway.
> So have is_const_zero(x) and add if_const_zero(x, if_z, if_nz) later.

Ok. So something like this seems to give us the relevant cases:

  #define __is_const_zero(x) \
        _Generic(0?(void *)(long)(x):(char *)0, char *:1, void *:0)

  #define is_const_zero(x) __is_const_zero(!!(x))
  #define is_const_true(x) __is_const_zero(!(x))
  #define is_const(x) __is_const_zero(0*!(x))

and should work with all scalar expressions that I can think of (ok,
technically 'void' is a scalar type and it obviously won't work with
that). And should work in all contexts.

It does want a comment (in addition to the comment about how NULL is
special for the ternary op that makes it work): the '(long)' cast is
so that there are no warnings for casting to 'void *' when it's *not*
a constant zero expression, and the '!' pattern is to turn pointers
and huge constants into 'int' without loss of information and without
warnings.

Compound types obviously will generate a warning. As they should.

The above looks reasonable to me, but I didn't actually test any of it
in the actual kernel build.

             Linus