[llvm-dev] contributing llvm-lipo

Alexander Shaposhnikov via llvm-dev llvm-dev at lists.llvm.org
Thu May 9 18:02:17 PDT 2019

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,

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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