llvm dev - Jul 2019 - 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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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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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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