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

[Fāng-ruì Sòng via llvm-dev]

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).

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
>>
>> _______________________________________________
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