Re: [PATCH] c: Implement C23 nullptr (N3042)

[Marek Polacek <polacek@redhat.com>]

On Mon, Aug 15, 2022 at 04:03:13PM -0400, Jason Merrill wrote:
> On 8/13/22 14:35, Marek Polacek wrote:
> > This patch implements the C23 nullptr literal:
> > <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm>, which is
> > intended to replace the problematic definition of NULL which might be
> > either of integer type or void*.
> > 
> > Since C++ has had nullptr for over a decade now, it was relatively easy
> > to just move the built-in node definitions from the C++ FE to the C/C++
> > common code.  Also, our DWARF emitter already handles NULLPTR_TYPE by
> > emitting DW_TAG_unspecified_type.  However, I had to handle a lot of
> > contexts such as ?:, comparison, conversion, etc.
> > 
> > There are some minor differences, e.g. in C you can do
> > 
> >    bool b = nullptr;
> > 
> > but in C++ you have to use direct-initialization:
> > 
> >    bool b{nullptr};
> > 
> > And I think that
> > 
> >    nullptr_t n = 0;
> > 
> > is only valid in C++.
> > 
> > Of course, C doesn't have to handle mangling, RTTI, substitution,
> > overloading, ...
> > 
> > This patch also defines nullptr_t in <stddef.h>.  I'm uncertain about
> > the __STDC_VERSION__ version I should be checking.  Also, I'm not
> > defining __STDC_VERSION_STDDEF_H__ yet, because I don't know what value
> > it should be defined to.  Do we know yet?
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> The C++ changes are OK, but you probably want a comment in
> c_common_nodes_and_builtins that we aren't setting the alignment there for
> C++ backward ABI bug compatibility.  Or perhaps set it there and then break
> it in the C++ front end when abi < 9.

Thanks!  I added a comment to that effect in the v2 patch I just posted.

Marek