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

[Martin Uecker]

Am Samstag, dem 07.12.2024 um 10:19 -0800 schrieb Linus Torvalds:
> On Sat, 7 Dec 2024 at 04:24, Vincent Mailhol <vincent.mailhol at gmail.com> wrote:
> > 
> > > No good - expands everything twice.
> > 
> > And? __is_const_zero() does not evaluate its arguments, so no side effect:
> 
> No, the problem is literally the expansion.
> 
> Double expansion of these fundamental helpers gets exponential,
> because they are used in various nested ways in other fundamental
> helpers.
> 
> That's why we then spent so much effort on trying to clean up the
> min/max macros, because a single line of code would expand to
> literally tens of megabytes of horrific expansions.
> 
> And the problem with these things is that you can't make them inline
> functions, so they have to be macros, and then you build up other
> macros using them (like that "clamp()" macro), and it really gets
> horrendous and affects the build time.
> 
> And yes, it is very sad. Particularly since a compiler would have a
> really easy time with some nice helper builtins.
> 
> Of course, often the compiler *does* have helper builtins, but we
> can't use them, because they aren't *quite* the right thing. Like that
> "__builtin_constant_p()" not actually working for some situations
> where we absolutely need not just a constant value, but a constant
> _expression_ due to C parsing rules.
> 
> Quite a lot of the pain we tend to have with these things is directly
> related to the fact that we often want to do these tests in contexts
> like global array initializers etc.
> 
> If there is one feature of C I would have liked it is "allow inline
> functions and statement expressions with constant arguments as
> constant expressions". Other languages have done that, and it really
> does help. And yes, it means that you have to basically have a
> language interpreter in the compiler (you do want to allow loop
> constructions etc), but it really is very useful.
> 
> Oh well. Even if compilers added that today, it would be years until
> we could take advantage of it.
> 
> At one point I literally was thinking I'd do 'sparse' as a
> pre-processor for kernel code, in order to have extended language
> facilities like that.

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?


Martin