llvm dev - Jul 2019 - 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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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<mailto: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<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<mailto: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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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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