[cfe-dev] Clang Modules: modularization of standard headers required?

[Raphael Isemann via cfe-dev]

1) If you don't modularize STL, then you just won't get any
performance benefits when parsing the STL headers. Also some headers
are sometimes incompatible with modules unless they're in their own
[sub]module. E.g. you could have a header that is doing `class vector
{ MEM_TYPE a; }` and in one inclusion context we have `#define
MEM_TYPE long` and in another `#define MEM_TYPE unsigned`. This breaks
ODR checking but usually is prevented by modularizing the header that
contains `vector`.

2)  It's probably not a regression, but IIRC this ODR check that
recognizes the issue was introduced between those two revisions.
Either it's a code issue like in my example above or it's a bug in
this check. Try filing a bug report and you can usually workaround for
now by modularizing those headers.

By the way, you can mount the modulemap in the right spot on your
filesystem with clang's filesystem overlays. E.g. write a JSON file
like this [1] and then mount modulemaps for libc[2] and STL[3] by
writing the right paths into the file (or let the build system do that
for you) and then use "-ivfsoverlay<FILE>".

[1] https://github.com/root-project/root/blob/master/build/unix/modulemap.overlay.yaml.in
[2] https://github.com/root-project/root/blob/master/build/unix/libc.modulemap
[3] https://github.com/root-project/root/blob/master/build/unix/stl.cppmap

- Raphael


2017-07-01 7:23 GMT+02:00 Dmitry Panin via cfe-dev <cfe-dev at lists.llvm.org>:
> I am trying to enable Clang Modules using latest version of clang on a tiny
> example, but getting the following error:
>
> error: 'std::vector' has different definitions in different modules
>
> 1) Documentation from
> http://clang.llvm.org/docs/Modules.html#modularizing-a-platform says
>
>>> To get any benefit out of modules, one needs to introduce module maps for
>>> software libraries starting at the bottom of the stack.
>
> Does it mean that modularization of system/std headers is **required**
> before going upper levels?
> I am not using modulemaps on standard headers and would like to avoid that
> for now.
>
> 2) When I try some older version of clang (say from April 2017) it doesn't
> show this error on the same input. Does it mean there was regression
> happened in between? Or the current behavior is actually expected?
>
>
> Below comes more details and reproduction.
> Full message:
>
> In file included from main.cpp:1:
> In file included from
> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/vector:64:
> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_vector.h:210:11:
> error: 'std::vector' has different definitions in
>       different modules; defined here
>     class vector : protected _Vector_base<_Tp, _Alloc>
>           ^
> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_vector.h:210:11:
> note: definition in module 'experimental_Range_h' is here
>     class vector : protected _Vector_base<_Tp, _Alloc>
>           ^
> 1 error generated.
>
>
> The files I am using:
> // -- main.cpp:
> #include <vector>
> #include "Range.h"
>
> int main () {
>   return 0;
> }
>
> // -- Range.h:
> #pragma once
>
> #include <string>
> #include "Bits.h"
> #include <vector>
>
> // -- Bit.h:
> #pragma once
> #include <iterator>
>
> // -- module.modulemap:
> module experimental_Bits_h {
>    header "Bits.h"
>    export *
> }
>
> module experimental_Range_h {
>    header "Range.h"
>    export *
> }
>
> Commandline:
> $ clang++ --std=c++14 -I .  -fmodules -fcxx-modules
> -fmodules-cache-path=./modules_out/_module_cache -c main.cpp -o main.o
>
>
> $ clang++ --version
> clang version 5.0.0 (http://llvm.org/git/clang.git
> 742c4c842393005d91a6f805f665b077014d061a) (http://llvm.org/git/llvm.git
> a98fd55665d422389d69d94efe631fac93d11173)
>
> --
> Thanks,
> Dmitry
>
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