On Mon, Jun 25, 2018 at 12:48 PM Nathan Sidwell <nat...@acm.org> wrote:
>
> NO_IMPLICIT_EXTERN_C was introduced to tell the compiler that it didn't
> need to fake up 'extern "C" { ... }' around system header files. Over
> the years more and more system headers have become C++-aware, leading to
> more targets defining this macro.
>
> Unfortunately because of the sense of this macro, and that the
> requirement is based on the target-OS, whereas we partition the config
> directory by target-ARCH, it's become hard to know which targets still
> require the older functionality.
>
> There have been a few questions over the past 2 decades to figure this
> out, but they didn;t progress.
>
> This patch replaces the negative NO_IMPLICIT_EXTERN_C with the positive
> SYSTEM_IMPLICIT_EXTERN_C. Targets that previously did not define
> NO_IMPLICIT_EXTERN_C now need to define SYSTEM_IMPLICIT_EXTERN_C. I
> know of one such target -- AIX, and I'd be grateful this patch could be
> tried there.
>
> Going through the config files was tricky, and I may well have missed
> something. One suspicious file is config/sparc/openbsd64.h which did
> explicitly undef the macro, with the comment:
>
> /* Inherited from sp64-elf. */
>
> sp64-elf.h does define the macro, but the other bsd's also define it,
> which leaves me wondering if openbsd.h has bit rotted here. Which leads
> me to another observation:
>
> It's quite possible the extern "C" functionality is enabled on targets
> that no longer need it, because their observed behaviour would not be
> broken. On the other hand, the failure mode of not defining its
> replacement (or alternatively mistakenly defining NO_IMPLICIT_EXTERN_C),
> would be immediate and obvious. And the fix is also simple.
>
> So, if you have a target that you think has C++-unaware system headers,
> please give this patch a spin and report. Blessing from a GM after a
> few days out there would be nice :)
>
> The lesson here is that when one has a transition, chose an enablement
> mechanism that makes it easy to tell when the transition is complete.
I tried the subset of the patch that directly affects AIX and saw no
ill effects.
Thanks, David
