[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[Martin Uecker]

Am Donnerstag, dem 05.12.2024 um 22:14 -0800 schrieb Linus Torvalds:
> On Thu, 5 Dec 2024 at 18:26, David Laight <David.Laight at aculab.com> wrote:
> > 
> > From: Vincent Mailhol
> > > ACK. Would adding a suggested--by Linus tag solve your concern?
> 
> I'm genberally the one person who doesn't need any more credit ;)
> 
> > I actually suspect the first patches to change __is_constexpr() to
> > use _Generic were from myself.
> 
> Yes. And David was also I think the one who suggested something else
> than "!!" originally too.
> 
> I may have liked "!!" for being very idiomatic and traditional C, but
> there were those pesky compilers that warn about "integer in bool
> context" or whatever the annoying warning was when then doing the
> "multiply by zero" to turn a constant expression into a constant zero
> expression.
> 
> So that
> 
>   #define is_const(x) __is_const_zero(0 * (x))
> 
> causes issues when 'x' is not an integer expression (think
> "is_const(NULL)" or "is_const(1 == 2)".
> 
> Side note: I think "(x) == 0" will make sparse unhappy when 'x' is a
> pointer, because it results that horrid "use integer zero as NULL
> without a cast" thing when the plain zero gets implicitly cast to a
> pointer. Which is a really nasty and broken C pattern and should never
> have been silent.


BTW: I added '-Wzero-as-null-pointer-constant' to GCC 15.

Hopefully we can also remove / deprecate this for C2Y.

Martin

> 
> I think David suggested using ((x)?0:0) at some point. Silly
> nonsensical and complex expression, but maybe that finally gets rid of
> all the warnings:
> 
>      #define is_const(x) __is_const_zero((x)?0:0)
> 
> might work regardless of the type of 'x'.
> 
> Or does that trigger some odd case too?