Re: [PATCH RFC 00/13] improve constexpr handling

[josh@joshtriplett.org]

On Thu, Jul 23, 2015 at 11:13:57AM +0200, Nicolai Stange wrote:
> josh@joshtriplett.org writes:
> >> Since the additional information on expressions obtained through the
> >> first two parts is rather pointless without making any use of it, I
> >> implemented part three, the checking of static storage duration
> >> objects' initializers for constness.
> >> This part is the reason why there is a 'RFC' tag in the subject.
> >> It is up to you to decide whether letting sparse check for C99
> >> conformity is a valuable thing to have or whether being stricter than
> >> GCC is counter-productive/completely idiotic.
> >
> > I think it's absolutely a valuable thing to have.  It may or may not be
> > the right *default* behavior, but having an appropriate -W option to
> > enable it would be a good start.
> 
> My next resend will contain such a -Wcheck-static-initializers then.

<bikeshed>Shouldn't it be something like -Wnon-constant-initializer,
since that's what it checks for?</bikeshed>

> However, I will delay that resend in order to be able to incorporate
> other reviews arriving in the meantime.

Sounds reasonable.

> > On Thu, Jul 23, 2015 at 12:54:25AM +0200, Nicolai Stange wrote:
> >> sparse now finds 519 occurences of non-const initializers of static
> >> storage duration objects, 474 of which are located in drivers/acpi
> >> and stem from this subsystem's custom offsetof macro implemented by
> >> means of taking pointer differences.
> >
> > Ideally, I'd suggest that ACPICA should add a translation from
> > ACPI_OFFSET to offsetof as part of its Linux-izing scripts.
> >
> > That said, I also can't think of an obvious reason why ACPI_OFFSET
> > *should* be considered non-constant.  Perhaps there's a detail in the
> > C99 spec that explains why what it does isn't OK, but it *seems* like it
> > should be a compile-time constant expression.  I've CCed Al Viro, who
> > knows the C99 constant expression rules very well; Al, could you provide
> > some clarity here?  The ACPI_OFFSET macro in question expands to this:
> >
> > (acpi_size) (((u8 *) (void *) ((&(((struct some_struct *) 0)->fieldname)))) - ((u8 *) (void *) (((void *) (void *) 0))))
> >
> > Does the -> make this non-constant?
> 
> No, it is the pointer difference. At least to my interpretion of C99
> [6.6(9)] which might be arguable.
> Upon your request, I could relax these constraints as I have did already for
> some special cases of conditionals in [11/13].

Ah, I see.  I don't know whether relaxing that makes sense or not; Al?

- Josh Triplett