[llvm-dev] contributing llvm-lipo

Jake Ehrlich via llvm-dev llvm-dev at lists.llvm.org
Thu May 9 18:09:40 PDT 2019

I think that pretty much hits the nail on the head. The llvm-objcopy code
is for when you need to perform mutations primarily. A common mistake (well
a mistake in my opinion) I see is people wanting a one size fits all
solution when one doesn't exist (or perhaps rather, ELFTypes is as close as
we have). As long as we conclude that kind of mutative model fits well here
then we should proceed. That's what I left all the cryptic "if this really
fits into the llvm-objcopy model" messages.

*From: *Alexander Shaposhnikov <alexander.v.shaposhnikov at gmail.com>
*Date: *Thu, May 9, 2019, 6:02 PM
*To: *Jake Ehrlich
*Cc: *llvm-dev, <jh7370 at my.bristol.ac.uk>, Saleem Abdulrasool, Lang Hames, <
smeenai at fb.com>, James Henderson, Jordan Rupprecht

Hi Jake,
> many thanks,
> yea, I have very similar feelings / thoughts.
> After some thinking it seems to me that this discussion/problem which I
> have brought up is, in fact,
> more relevant to the tools which really need a robust mutable model of an
> object file (like objcopy, strip, install_name_tool, etc),
> but the particular case of "lipo" might be simpler, I need to double check
> that / will take a closer look again.
> What I mean - the tool "lipo" manipulates "fat" binaries by extracting,
> removing, replacing slices
> (slice = object file for a particular architecture),  but the slices
> themselves are unmodified, thus things can be simpler.
> I'd like to think a bit more about it to make sure I'm not missing
> anything important here.
> But anyway, I think it's very useful to discuss this problem, many thanks
> for your comments!
> Kind regards,
> Alex
> On Thu, May 9, 2019 at 4:34 PM Jake Ehrlich <jakehehrlich at google.com>
> wrote:
>> A) It sounds like you're pretty sure that llvm-objcopy style code could
>> be correctly reused here. Assuming that's the case this is probably not the
>> ideal option
>> B) This is something we want to do anyway and sounds like the most ideal
>> solution assuming llvm-objcopy style code is compatible
>> C) I have no ideas here
>> D) This seems strictly less ideal than B but, assuming this tool would
>> still fit generally into llvm-objcopy's general library format it would be
>> better than A. It sounds like the CopyConfig and all that logic is the
>> killer here. We could generalize all of that yet further but...yeah that
>> sounds like a losing battle and not as ideal than B.
>> An important thing to point out is that to proceed we *only* have to move
>> the MachO code into a library and we can follow up with the ELF code later
>> like we planned to anyway. I think that 1) goes in the correct direction
>> and 2) has virtually no overhead.
>> On Thu, May 9, 2019 at 4:22 PM Alexander Shaposhnikov <
>> alexander.v.shaposhnikov at gmail.com> wrote:
>>> Hey everyone!
>>> In October/November 2018 I started the implementation of llvm-objcopy
>>> for MachO with the long-term plan to build some popular binary-level tools
>>> on top of it. That effort stopped at the stage where some boilerplate code
>>> for reading/writing MachO files was reviewed & committed to
>>> LLVM/tools/llvm-objcopy.
>>> Later I started working on llvm-lipo (a drop-in replacing for the tool
>>> "lipo" for manipulating "fat" binaries), but that code has never been sent
>>> for code review. The original plan was to use the approach similar to
>>> llvm-strip, where the new tool is just another "driver" for llvm-objcopy.
>>> This approach worked well for llvm-strip (the command line interface of
>>> llvm-strip maps naturally onto the interface of llvm-objcopy) but turned
>>> out to be not flexible enough for llvm-lipo. The thing is that the tool
>>> lipo doesn't work "per object file", instead, it can have multiple input
>>> files & single output file, no output files at all, etc.
>>> So below is the proposal / request for suggestions how to move forward
>>> from here. It seems to me it might be a good time to try to factor out the
>>> reusable part of llvm-objcopy's codebase and build a library from it. At
>>> the beginning the library can contain : Reader + Object + Writer (for all
>>> the supported formats: ELF/COFF/MachO) and all the tool's specific logic
>>> will stay in place. There are several questions: what would be a good name
>>> for it and how would we want to organize the codebase.
>>> Some options which I can imagine:
>>> A) Leave the code where it is right now, create a library from it,
>>> put llvm-lipo.cpp into the folder llvm/tools/llvm-objcopy
>>> B) Move the code which belongs to the library
>>> out of the folder llvm/tools/llvm-objcopy
>>> and create a new folder llvm/tools/llvm-lipo for the lipo-specific code.
>>> C) Something else ?
>>> D) /* personally don't like */ Try to go with the approach when
>>> llvm-lipo is a symbolic link to llvm-objcopy. This kind of bloats the
>>> codebase of llvm-objcopy,
>>> I don't see many benefits on this path except some space savings.
>>> Any thoughts/suggestions/comments would be appreciated.
>>> Kind regards,
>>> Alex Shaposhnikov
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