[PATCH] D158709: [Headers][Modules] Make separate headers for the stdarg.h and stddef.h pieces so that they can be modularized

[Ian Anderson via Phabricator via cfe-commits]

iana added a comment.

In D158709#4617295 <https://reviews.llvm.org/D158709#4617295>, @aaron.ballman wrote:

> I'm a bit concerned about the impact on (non-modules) build time performance. This splits stddef.h and stdarg.h into needing to open 14 different files instead of 2. Hopefully that's not a lot of overhead, but given that stddef.h is included in approximately every TU and that sometimes external forces cause files to be slow to open (network drives, AV software, etc), I think we should be cautious and measure the impact. I also really do not like how poorly this scales -- the fact that this is only needed for these two files helps a bit, but this is a lot of C++-specific hoops to jump through for C standard library headers.

This is in support of clang modules, not C++ modules. The plan is to make modules like this.

  module _Builtin_stddef [system] {
    textual header "stddef.h"
  
    explicit module max_align_t {
      header "__stddef_max_align_t.h"
      export *
    }
  
    explicit module null {
      header "__stddef_null.h"
      export *
    }
    ...
  }

I do agree it's a bit of a file explosion, but I can't really think of any better alternatives. It's tough to measure build time performance impact. If you have a slow enough file system (and the fs on our builders at Apple is quite slow), and you aren't using modules or pch's, and you have quite a lot of source files, then probably this would be noticeable? I still don't know what we could really do about it besides have `#if !__has_feature(modules)` and inline the contents in that case. That seems like a maintenance headache and a half though. I would hope that most sizable projects either use clang modules or pch's?


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