Re: [modules] Preprocessing requires compiled header unit modules

[Ben Boeckel <ben.boeckel@kitware.com>]

On Thu, Apr 21, 2022 at 18:59:56 +0100, Iain Sandoe wrote:
> Hi Ben,
> 
> > On 21 Apr 2022, at 13:05, Ben Boeckel via Gcc <gcc@gcc.gnu.org> wrote:
> > 
> > On Thu, Apr 21, 2022 at 06:05:52 +0200, Boris Kolpackov wrote:
> >> I don't think it is. A header unit (unlike a named module) may export
> >> macros which could affect further dependencies. Consider:
> >> 
> >> import "header-unit.hpp"; // May or may not export macro FOO.
> 
> 1.  If you know how this was built, then you could do an -E -fdirectives-only build (both
>    GCC and clang support this now) to obtain the macros.

My understanding is that how it gets used determines how it should be
made for Clang (because the consumer's `-D`, `-W`, etc. flags matter). I
do not yet know how I am to support this in CMake.

> 2.  I suppose we could invent a tool (or FE mode) to dump the macros exported by a HU ***

Fun considerations:

  - are `-D` flags exported? `-U`?
  - how about this if `value` is the same as or different from the
    at-start expansion:

```c++
#undef SOME_MACRO
#define SOME_MACRO value
```

  - how about `#undef FOO`?

> >> #ifdef FOO
> >> import "header-unit2.hpp"
> >> #endif
> > 
> > I agree that the header needs to be *found*, but scanning cannot require
> > a pre-existing BMI for that header. A new mode likely needs to be laid
> > down to get the information necessary (instead of just piggy-backing on
> > `-E` behavior to get what I want).
> 
> perhaps that means (2)?

Can't it just read the header as if it wasn't imported? AFAIU, that's
what GCC did in Jan 2019. I understand that CPP state is probably not
easy, but something to consider.

> *** it’s kinda frustrating that this is hard infomation to get as a developer, so
>     perhaps we can anticipate users wanting such output.

I think cacheing and distributed build tools are the most likely
consumers of such information.

--Ben