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

[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.

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?

            Linus