[llvm-dev] contributing llvm-lipo

James Henderson via llvm-dev llvm-dev at lists.llvm.org
Fri May 10 02:25:04 PDT 2019

Hi all,

Before we get too stuck into llvm-objcopy library discussions, I'd like to
point out that Alex Brachet-Mialot (CC'ed) has been accepted this year as a
GSOC student with his project of librarifying llvm-objcopy. Hopefully he'll
introduce himself and his proposal a bit more in the coming days. Jordan
and I are mentoring him and another GSOC student (Seiya Nuta, who is
planning on working on the Mach-O side of things too). I'd like Alex to
have a chance to put forward his thoughts on this, and I'm sure we can all
help him in the design decisions.

I'll have more to say on this a bit later, I'm sure, but I just wanted to
bring this to your attention before things progress too far without him!


On Fri, 10 May 2019 at 02:16, Alexander Shaposhnikov <
alexander.v.shaposhnikov at gmail.com> wrote:

> Every case is different,
> but yes, as I said - I would like to take a closer look at the problem
> again,
> it might be the case that we don't need this complexity in this particular
> case,
> but want to double check.
> But yeah, in general I agree with you!
> On Thu, May 9, 2019 at 6:09 PM Jake Ehrlich <jakehehrlich at google.com>
> wrote:
>> 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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