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

[David Laight]

From: Martin Uecker
> Sent: 07 December 2024 19:52
...
> > 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.

In many cases the C language says 'constant integer expression'
but is reality something that passes __builtin_constant_p()
is likely to be fine - because the actual value isn't needed
until late on in the compilation.

...
> > 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.

It the output needs to be a constant the loop would have to be unrollable.
In those cases the result will be 'constant enough' for __builtin_constant_p()
and the compiler is actually already capable doing 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.

You could use m4 :-) and make it entirely unreadable.

> 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.



	David

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