[PATCH] D139168: [C++20] [Modules] [ClangScanDeps] Enable to print make-style dependency file within P1689 format (4/4)

[Chuanqi Xu via Phabricator via cfe-commits]

ChuanqiXu added a comment.

In D139168#4026799 <https://reviews.llvm.org/D139168#4026799>, @ben.boeckel wrote:

> In D139168#4025277 <https://reviews.llvm.org/D139168#4025277>, @ChuanqiXu wrote:
>
>> Currently we will detect `-MF` in the command line and we will write the make-format dependency output to the corresponding file once we find `-MF`.
>
> Which is fine, but docs need to mention that some clang-looking flags are actually for `clang-scan-deps` and that, as such, these flags cannot just be grabbed blindly from a compilation database.

Sure, agreed. I'll add docs for the series patches once they got accepted (otherwise we might do many useless works).

> When using clang-scan-deps with a compdb with multiple command lines, which depfile path will it use? Or must all agree on the same path and options? Because there should be a single one for all scanning that is performed (e.g., in batch mode).

Currently, clang-scan-deps won't check for this. If we have multiple command lines with different `-MF` value, the make-style dependency information will be written to these different depfiles. If all the command lines use the same depfile, then the make-style dependency information will be written to the depfile. (BTW, We have a lock for this so we don't need to worry about data racing).

> Is this really simpler than just adding -MF and friends to clang-scan-deps directly?

Personally, I feel the complexity of both methods are acceptable. I rewrite the patch since it looks like @jansvoboda11 don't like to add new command line options to clang-scan-deps.

I feel like we have 2 (or 3) options

1. (The current way) Extract `-MF` in the command line of clang (from compilation database or from the args after `--`)
2. (The original way) Specify `-MF` in the command line of clang-scan-deps.
3. (Not good) Do nothing. I feel like it is possible for build systems to get the make-style dependency information by scanning twice. One for P1689 <https://reviews.llvm.org/P1689> format and one for make-format. It may be workable but it sounds a little bit silly.

Personally, I feel the first (current) style is cleaner.

> I'll note that I'm becoming more and more convinced that `compilation_database.json` is being abused for this as we are quite contorting it from its intended use case: listing the command lines to compile sources. Instead, we are using it as a source of *related* information that differs from the *real* command line in some important ways. Namely:
>
> - dependency information extraction is re-used and therefore must be different from the *real* command
> - module information must be missing from the scanning command line as it cannot be known at this point as scanning is used to discover those flags in the first place (usually a response file)

Agreed. But this may beyond the scope of the current patch.


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