[PATCH] D121098: [C++20][Modules][HU 4/5] Handle pre-processed header units.

Tue Mar 15 19:21:11 PDT 2022

ChuanqiXu added a comment.

In D121098#3383835 <https://reviews.llvm.org/D121098#3383835>, @iains wrote:

>> I am a little bit confused for the intuition. Couldn't we just import the pre-processed header? What's the problem? Could you elaborate on this?
>
> We cannot import a pre-processed file; the preprocessed file is still source code, not a CMI.

But if I understand correctly, we are able to import a pre-processed file as a header unit. Couldn't we?

For example:

  // cc -E foo.h > foo.preprocessed
  import "foo.preprocessed"
  ...

Is this not acceptable?

> In addition, the pre-processor output for a header unit requires further pre-processing on read.
> This is because a header unit actually preserves some of the pre-processor information [macro definitions] (which would normally be discarded after phase 4).
>
> Header units are identifiable by a name (in common with GCC, we make the name == the path by which the header is specified) -- that name is intentionally not a  legal named module name (neither is it, in general, a legal identifier).

So it doesn't eliminate my confusion. I guess there is some workflows that we need to import a preprocessed header as header unit (and we need to find the position of original file). Or in what circumstances, do we need to import a pre-processed header as header unit?

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