Re: [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions

[Andy Shevchenko]

On Mon, Sep 19, 2022 at 08:23:00AM -0700, Linus Torvalds wrote:
> On Mon, Sep 19, 2022 at 6:46 AM Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> >
> > > +#define FIND_NEXT_BIT(FETCH, MUNGE, size, start)                             \
> > > +({                                                                           \
> [..]
> > > +out:                                                                         \
> >
> > I dunno if GCC expression limits the scope of goto labels
> 
> No. Labels are function-global by default. If you need block-scope for
> them, you need to declare them explicitly in tha block before use with
> "__label__".
> 
> > but on the safe side you can add a prefix to it, so it becomes:
> >
> > FIND_NEXT_BIT_out:
> 
> That doesn't really help, since if you were to use the macro twice,
> you'd still get a name clash.
> 
> That said, I'm not convinced any of this matters, since these macros
> aren't supposed to be used anywhere else, and in fact, they aren't
> even in any header file that would allow anybody else to use them.
> 
> So I think all the normal "make macros safe" rules are simply
> irrelevant for these cases - despite the readable name, these macros
> are local special cases for code generation and avoiding duplication,
> not generic "use this macro to find a bit".
> 
> So it's one thing if a macro is in a header file to be used by random
> code. It's a different thing entirely if it's a specialized local
> macro for a local issue, that nobody else is ever going to even see.
> 
> So I don't think it would be wrong to use __label__ to block-scope it,
> or to use a longer name, but I also don't think it's really required.
> 
> It's not exactly super-common, but we have various cases of macros
> that generate full (or partial) function bodies in various places,
> where the macro does various things that should *never* be done in a
> "regular" macro that gets used by normal code.
> 
> You can see one class of this with something like
> 
>    git grep '^static.*##.*(.*\\$' -- '*.c'
> 
> but to *really* go blind, see the "SYSCALL_DEFINE*()" macros in
> <linux/syscalls.h>.
> 
> Those will mess with your mind, and go "whoever wrote those macros
> needs to be institutionalized". They do some impressive things,
> including creating local functions _and_ starting a new function
> definition where the actual code then isn't part of the macro, but
> actually just continues where the macro was used.
> 
> Which is all very natural and actually looks quite nice and readable
> in the places that use it, with users looking like
> 
>   SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
>   {
>         int fd;
>         struct pid *p;
>    ...
> 
> which is all pretty legible. But there's no question that that macro
> violates every single "normal" macro rule.
> 
> The FIND_NEXT_BIT() macro in comparison is pretty tame.

Thanks for elaboration. It makes a lot of sense and something TIL material
to me.

-- 
With Best Regards,
Andy Shevchenko