[llvm-dev] RFC: auto-generating build dependency file from lld

[Rui Ueyama via llvm-dev]

On Wed, Jul 31, 2019 at 4:04 PM Fāng-ruì Sòng <maskray at google.com> wrote:

> On the COFF side, I think /verbose already gives `Reading ...` logs. Users
> can parse the filenames.
> It probably makes sense to add a fine-grained option /trace as per ELF (-t
> --trace).
>

That is also true for lld/ELF and perhaps even for the compilers. If you
enable logging, it must contain input filenames, and you can create a dep
file from it. So the point is convenience -- you don't have to parse linker
logs to create a dep file (and that's true for compiler's -MD option, I
think).

On Wed, Jul 31, 2019 at 2:03 AM Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Yes, it's a little bit off-topic, but it is also planned. The data
>> structure that the linker handles can be considered a large graph where
>> vertices are file sections and edges are symbol names. You can say that
>> file A depends on file B if and only if in the graph a section in file A
>> has an edge to a section in file B. There is a plan to dump the graph in a
>> machine-readable format such as JSON so that you can run arbitrary graph
>> analysis algorithms on it.
>>
>> On Wed, Jul 31, 2019 at 10:55 AM Shi, Steven <steven.shi at intel.com>
>> wrote:
>>
>>> Very nice. It can directly help me know which lib and obj file is
>>> redundant in my linker script.
>>>
>>>
>>>
>>> BTW, besides the lib and file level dependency, is it possible to
>>> further print the function and global variable level dependency? E.g. the
>>> really linked symbols before any link level optimization. The fine
>>> granularity dependency could help me clean the redundant code and more
>>> accurately select regression test case in CI.
>>>
>>>
>>>
>>>
>>>
>>> Thanks
>>>
>>> Steven
>>>
>>>
>>>
>>> *From:* Rui Ueyama [mailto:ruiu at google.com]
>>> *Sent:* Wednesday, July 31, 2019 9:29 AM
>>> *To:* Shi, Steven <steven.shi at intel.com>
>>> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
>>> *Subject:* Re: [llvm-dev] RFC: auto-generating build dependency file
>>> from lld
>>>
>>>
>>>
>>> Yeah, I think there's no reason to not add this to lld/COFF if people
>>> find it useful.
>>>
>>>
>>>
>>> On Wed, Jul 31, 2019 at 10:25 AM Shi, Steven <steven.shi at intel.com>
>>> wrote:
>>>
>>> I love this feature. Does it plan to support COFF as well?
>>>
>>>
>>>
>>>
>>>
>>> Thanks
>>>
>>> Steven
>>>
>>>
>>>
>>> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of
>>> *Rui Ueyama via llvm-dev
>>> *Sent:* Wednesday, July 31, 2019 8:54 AM
>>> *To:* llvm-dev <llvm-dev at lists.llvm.org>
>>> *Subject:* [llvm-dev] RFC: auto-generating build dependency file from
>>> lld
>>>
>>>
>>>
>>> Hi,
>>>
>>>
>>>
>>> I'd like to propose a new feature and a flag
>>> (`--write-dependencies=<path>`) for lld so that the linker can generate a
>>> dependency file (.d file). This is analogous to `-MD` compiler flag.
>>>
>>>
>>>
>>> Background:
>>>
>>> Clang and GCC have a feature (`-MD` flag) to create a dependency file in
>>> a format that `make` and other build tools can read, so that you don't have
>>> to manually maintain dependencies between .c files and .h files. There's no
>>> similar feature for the linker, even though it seems useful in some
>>> situations.
>>>
>>>
>>>
>>> In particular, if a compiler driver automatically appends a static
>>> library to the final executable but you don't know the exact path of the
>>> library, there's currently no way to keep track of that dependency. A
>>> typical example of it is `-fsanitizer=asan` which adds libasan to the
>>> linker command line. If libasan is updated, you may want to rebuild your
>>> program, but you don't want to manually write its path to a build file
>>> because that may change.
>>>
>>>
>>>
>>> Proposal:
>>>
>>> Add a new command line flag `--write-dependencies=<path>` to lld. If the
>>> flag is given, lld creates a file at a given path with the following
>>> contents:
>>>
>>>
>>>
>>>   <output-file>: <input-file> ...
>>>
>>>
>>>
>>> where <output-file> is a pathname of an output file and <input-file> ...
>>> is a list of pathnames of all input files. This file format is the same as
>>> the `-MD` compiler flag output.
>>>
>>>
>>>
>>> Here is a change to implement the above feature:
>>> https://reviews.llvm.org/D65430
>>>
>>>
>>>
>>> Any comments?
>>>
>>>
>>>
>>> Thanks,
>>>
>>> Rui
>>>
>>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> 宋方睿
>
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