[llvm-dev] contributing llvm-lipo

Alexander Shaposhnikov via llvm-dev llvm-dev at lists.llvm.org
Fri May 10 06:02:02 PDT 2019

Hi James,
that's perfectly fine,
thanks for letting me know!
Regarding llvm-objcopy (librarifying & MachO) - sounds great, please, keep
me in the loop.
Regarding llvm-lipo - I'm looking at it right now (just want to have some
time for thinking),
if things are fairly independent from llvm-objcopy
I will probably send an update / proposal soon (~1-2 days) for review (and
if everything goes well then features can be added to the tool
is it OK with you / am I missing anything?

Kind regards,

On Fri, May 10, 2019 at 2:25 AM James Henderson <
jh7370.2008 at my.bristol.ac.uk> wrote:

> 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!
> James
> 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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