Re: [PATCH 1/3] rust: add static_call support

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 07, 2024 at 09:43:29AM +0000, Alice Ryhl wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Jun 06, 2024 at 09:09:00PM +0200, Miguel Ojeda wrote:
> > > On Thu, Jun 6, 2024 at 7:19 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > This is absolutely unreadable gibberish -- how am I supposed to keep
> > > > this in sync with the rest of the static_call infrastructure?
> > > 
> > > Yeah, they are macros, which look different from "normal" Rust code.
> > 
> > Macros like CPP ?
> 
> Yes, this patch series uses declarative macros, which are the closest
> that Rust has to the C preprocessor. They are powerful, but just like
> CPP, they can become pretty complicated and hard to read if you are
> doing something non-trivial.
> 
> The macro_rules! block is how you define a new declarative macro.

I'm sorry, but 30+ years of reading ! as NOT (or factorial) isn't going
to go away. So I'm reading your macros do NOT rule.

> The ($name:ident($($args:expr),* $(,)?)) part defines the arguments to
> the declarative macro. This syntax means:
> 
> 1. The input starts with any identifier, which we call $name.
> 2. Then there must be a ( token.

The above exaple fails, because the next token is :ident, whatever the
heck that might be. Also, extra points for line-noise due to lack of
whitespace.

> So for example, you might invoke the macro like this:
> 
> static_call!(tp_func_my_tracepoint(__data, &mut my_task_struct));

static_call NOT (blah dog blah);

> Inside the macro, you will see things such as:
> $crate::macros::paste! { $crate::bindings:: [<__SCK__ $name >]; }
> 
> The Rust syntax for invoking a macro has an exclamation mark after the

Like I said before, the creator of Rust must've been an esoteric
language freak and must've wanted to make this unreadable on purpose :/

Also, why the white space beteen the :: scope operator and the [< thing?
that's just weird. I would then expect the output to be:

  ...::bindings:: __SCK__my_static_key

> name, so you know that $crate::macros::paste is a macro. The `paste`
> macro just emits its input unchanged, except that any identifiers
> between [< and >] are concatenated into a single identifier. So if $name
> is my_static_key, then the above invocation of paste! emits:
> 
> 	$crate::bindings::__SCK__my_static_key;

But it doesn't, so it isn't unmodified, it seems to strip whitespace.

> The $crate::bindings module is where the output of bindgen goes, so this
> should correspond to the C symbol called __SCK__my_static_key.
> 
> > > Is there something we could do to help here? I think Alice and others
> > > would be happy to explain how it works and/or help maintain it in the
> > > future if you prefer.
> > 
> > Write a new language that looks more like C -- pretty please ? :-)
> > 
> > Mostly I would just really like you to just use arm/jump_label.h,
> > they're inline functions and should be doable with IR, no weirdo CPP
> > involved in this case.
> 
> I assume that you're referring to static_key_false here? I don't think
> that function can be exposed using IR because it passes the function
> argument called key as an "i" argument to an inline assembly block. Any
> attempt to compile static_key_false without knowing the value of key at
> compile time will surely fail to compile with the
> 
> 	invalid operand for inline asm constraint 'i'
> 
> error.

You can have clang read the header files and compile them into
Intermediate-Representation, and have it splice the lot into the Rust
crap's IR and voila, compile time.

You just need to extend the rust thing to be able to consume C header
files.