Re: [modules] Preprocessing requires compiled header unit modules

[Boris Kolpackov <boris@codesynthesis.com>]

Ben Boeckel <ben.boeckel@kitware.com> writes:

> 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.
> > 
> > #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.

Well, if scanning cannot require a pre-existing BMI but a pre-existing
BMI is required to get accurate dependency information, then something
has to give.

You hint at a potential solution in your subsequent email:

> 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 problem with this approach is that a header import and a header
include have subtly different semantics around macros. In particular,
the header import does not "see" macros defined by the importer while
the header include does. Here is an example:

// file: header-unit.hpp
//
#ifdef BAR
#define FOO
#endif

// file: importer.cpp
//
#define BAR
import "header-unit.hpp";    // Should not "see" BAR.
//#include "header-unit.hpp" // Should "see" BAR.

#ifdef FOO
import "header-unit2.hpp";
#endif

In this example, if you treat import of header-unit.hpp as
include, you will get incorrect dependency information.

So to make this work correctly we will need to re-create the
macro isolation semantics of import for include.

Even if we manage to do this, there are some implications I
am not sure we will like: the isolated macros will contain
inclusion guards, which means we will keep re-scanning the
same files potentially many many time. Here is an example,
assume each header-unitN.hpp includes or imports <functional>:

// file: importer.cpp
//
import <functional>; // Defined _GLIBCXX_FUNCTIONAL include

import "header-unit1.hpp"; // Ignores _GLIBCXX_FUNCTIONAL
import "header-unit2.hpp"; // Ditto.
import "header-unit3.hpp"; // Ditto.
import "header-unit4.hpp"; // Ditto.