Re: [modules] Preprocessing requires compiled header unit modules

[Iain Sandoe <idsandoe@googlemail.com>]

> On 21 Apr 2022, at 19:08, Ben Boeckel <ben.boeckel@kitware.com> wrote:
> 
> 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.

I think that is what I mean by “if you know how it is built” - it means you can replicate those
conditions, but produce the fdirectives-only output.  Perhaps I’m missing a subtelty …

>> 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`?

AFAIU, that is defined by the standard - only the defined state of a macro is exported.

So if the file contains
#define FOO 1
….
….
#undef FOO

— there will be no mention of FOO in the exported macros (at least, that is what
my impl. does ;) ).

— and the output would contain 'SOME_MACRO value’ for your other case.

This is quite different from the behaviour of PCH where the macro history is preserved.

>>>> #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.

The BMIs (at least the two I’ve got some familiarity with) are quite complex - there
would have to be some mode that specifically extracted the macros only.

>> *** 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.

On an “industrial scale”, sure - but the ability for a user to check that what they think
is present _is_ present, is valuable - at least in debug.

Iain