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

[David Laight]

From: David Laight
> Sent: 06 December 2024 02:26

(now it is no longer 2am...)

Linus suggested this a while back:

>  in https://lore.kernel.org/all/CAHk-=wiq=GUNWJwWh1CRAYchW73UmOaSkaCovLatfDKeveZctA@mail.gmail.com/
> 
>    /*
>     * 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)

But some of the things built on it has issues with compiler warnings.
I think there have several variations before and since with subtle differences.
Probably const_zero() const_true() const_false() and const_expr().

While the 'base' define is really const_zero() with just (long)(x) that
will mask high bits off 'long long'. 
A const_false() could 'fix' that using (long)!!(x) but even !(x) starts
giving compile errors.

If called for pointers (long)((x) == 0) is also problematic.
(Perhaps this is less likely now that min/max don't use it.)

So we may end up with (long)((x) ? 0 : 1) which really might as
well be written ((x) ? 0L : 1L).

The use is likely to be const_true(x > y) so perhaps there isn't a
reason to have const_false() since the boolean operator can be inverted.

const_expr() has relied on '* 0' to make all constant expressions zero.
But it has to handle pointers - so needs a conditional.
Since it is only one line, maybe just replicate the whole thing as:

#define const_true(x) _Generic(0 ? (void *)((x) ? 0L : 1L) : (char *)0, char *: 1, void *: 0)
#define const_expr(x) _Generic(0 ? (void *)((x) ? 0L : 0L) : (char *)0, char *: 1, void *: 0)

Or maybe (mostly because the lines are shorter):
#define const_NULL(x) _Generic(0 ? (x) : (char *)0, char *: 1, void *: 0)
#define const_true(x) const_NULL((void *)(x) ? 0L : 1L))
#define const_expr(x) const_NULL((void *)(x) ? 0L : 0L))

Or even:
#define const_true(x) const_NULL((x) ? NULL : (void *)1L))
#define const_expr(x) const_NULL((x) ? NULL : NULL))

	David

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