What is company-clang--guess-pch-file trying to matching?

[xuchunyang]

Hello,

I got a question about the regexp pattern used in the following:

(defsubst company-clang--guess-pch-file (file)
  (let ((dir (directory-file-name (file-name-directory file))))
    (when (equal (file-name-nondirectory dir) "Classes")
      (setq dir (file-name-directory dir)))
    (car (directory-files dir t "\\([^.]h\\|[^h]\\).pch\\'" t))))

according to https://clang.llvm.org/docs/UsersManual.html#generating-a-pch-file, the pch file looks like test.h.pch, shouldn't the filename be matched? this pattern doesn't match test.h.pch

(string-match "\\([^.]h\\|[^h]\\).pch\\'" "test.h.pch")
;; => nil

[dgutov]

Good question. I don't have a good answer, and this code is 10 years old now. Maybe @nschum could remember, but probably not.

Were you looking into support for precompiled headers?

[dgutov]

Judging by the commentary, though, the code is trying to distinguish between "precompiled headers" and "prefix files".

[xuchunyang]

Were you looking into support for precompiled headers?

No, I notice this when I read the code to figure out how company-clang works, the initial cause is that I find company-clang can't complete symbols from cairo.h.

distinguish between "precompiled headers" and "prefix files".

I know "precompiled headers", but I don't know what's "prefix files", what the "prefix" stands for? I couldn't find the term on https://clang.llvm.org/docs/UsersManual.html, and M-x company-clang-set-prefix's docstring suggests it's for -include, however man clang's doc doesn't mention "prefix".

   -include <filename>
          Adds an implicit #include into the predefines buffer which is read before the source file is preprocessed

[dgutov]

I don't really use Clang myself, so I don't have an answer to that unfortunately.

Does the documented feature related to -include make sense? Does it work, it does it need fixing?

[xuchunyang]

I'm not familiar with Clang too. I don't understand the documentation.

company-clang-set-prefix is an interactive compiled Lisp function in
`company-clang.el'.

(company-clang-set-prefix &optional PREFIX)

Use PREFIX as a prefix (-include ...) file for clang completion.

maybe "a prefix file" means a c header file? I have not used this command, however, since Clang does have the -include option, so company-clang-set-prefix probably works.

[dgutov]

Maybe it does, maybe it doesn't. But the intention seems to be to looks for a file that the user probably generated manually, to speed up completion.

There is some very similar logic in https://github.com/Golevka/emacs-clang-complete-async/blob/master/auto-complete-clang-async.el#L86, except in the end it uses -include-pch, and not just -include like we do.

Some time ago, Vim's clang_complete included using it as a possible optimization as well: xavierd/clang_complete@cecca90. But not the current version.