[Tooling] [isocpp-modules] Dependency format with module details implementation

Gabriel Dos Reis gdr at microsoft.com
Wed Apr 10 23:36:25 CEST 2019


Right, but wouldn't the build system would have set up a step to build the BMI for '<foo.h>' because foo.cpp has a dependency on the BMI of <foo.h>?
Why is the information required for this to work any different from if you had used #include of a header that contained an import-declaration?

I guess I am trying to understand why we need to generalize this to header units...

-- Gaby

| -----Original Message-----
| From: Ben Boeckel <ben.boeckel at kitware.com>
| Sent: Wednesday, April 10, 2019 1:14 PM
| To: Gabriel Dos Reis <gdr at microsoft.com>
| Cc: modules at lists.isocpp.org; brad.king at kitware.com; WG21 Tooling Study
| Group SG15 <tooling at open-std.org>
| Subject: Re: [isocpp-modules] Dependency format with module details
| implementation
| 
| On Wed, Apr 10, 2019 at 18:32:49 +0000, Gabriel Dos Reis wrote:
| > It is not clear to me why we would want to have this apply to header
| > unit.  By the definitions, they are not modules.  They don't have
| > names other than the name of the header or header file.  They don't
| > provide anything other than their content as a whole.  The only
| > dependencies they have are exactly the same that their headers or
| > header files have and therefore can be extracted via the usual
| > pre-processing scan.  Why would their BMIs be required for
| > dependencies?
| 
| I'm OK with this as well. However, GCC currently expects a BMI for
| header units during compilation:
| 
| foo.h:
| 
|     const int foo = 0;
| 
| foo.cpp:
| 
|     import <foo.h>;
| 
|     int main(int argc, char* argv[]) {
|         return foo;
|     }
| 
| ```
| % g++ -fmodules-ts -std=c++2a -I. -o foo foo.cpp
| In module imported at foo.cpp:1:1:
| ./foo.h: error: failed to read module ‘!/foo.h.gchm’: No such file or directory
| ./foo.h: fatal error: jumping off the crazy train to crashville
| compilation terminated.
| ```
| 
| Using `"foo.h"` instead:
| 
| ```
| foo.cpp:1:8: internal compiler error: in module_state, at cp/module.cc:3221
|     1 | import "foo.h";
|       |        ^~~~~~~
| We are damaged, This is broken.
| Partial device,
| Pliable design,
| Hunt through the rubble for what once was.
| See
| <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.
| gnu.org%2Fwiki%2Fcxx-
| modules%23Bugs&amp;data=02%7C01%7Cgdr%40microsoft.com%7C5cb436
| 4f8fe749470ba108d6bdf11f38%7C72f988bf86f141af91ab2d7cd011db47%7C1
| %7C0%7C636905240721176468&amp;sdata=gZXBSOZfqK9b%2FXFX8OnJkWvv
| Yg%2FWHe9lpsGISkweboY%3D&amp;reserved=0> for instructions.
| ```
| 
| --Ben


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