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

[David Laight]

From: Martin Uecker
> Sent: 07 December 2024 21:06
> 
> Am Samstag, dem 07.12.2024 um 21:00 +0000 schrieb David Laight:
> > From: Martin Uecker
> > > Sent: 07 December 2024 19:52
> ...
> 
> >
> > > There exist proposals along those lines for C2Y.
> > >
> > > From a more near-term solution, I wonder if making it possible (or
> > > easier) to return integer constant expressions from statement
> > > expressions and allowing a restricted form of statement expressions
> > > at file scope would help?
> >
> > It would help a lot if a #define that just used local variables
> > to avoid arguments being re-expanded and for CSE could still
> > generate a constant value.
> > Does need to be a #define - to get token pasting and 'stringify'.
> > Although you would need something for reporting detected errors,
> > and builtin compiler support for const_true() for the detection
> > itself.
> 
> We are super close:
> 
> https://godbolt.org/z/Tarq89bha

(The preprocess output is about 500 bytes for each line.)

> (if we ignore the grotesque hacks to get there, but this would go
> away if the compiler does it internally)

Some of those hacks look excessive.
Isn't IF_CONST(x, y, z) just
	_Generic(0 ? (void *)((x) ? 0L : 0L) : (char *)0, char *: y, void *: z)
and that gets rid some of the grossness.
Just having that as a builtin would simplify some things.
Although you could use:
	__builtin_choose_expr(IS_CONST(x), y, z)
if you need y and z to have different types, and just:
	IS_CONST(x) ? y : z
otherwise.
Since AFAICT they are otherwise equivalent.

	David

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