[llvm-dev] Porting libxar to LLVM

[Jez via llvm-dev]

> Is there any reason you want to replicate the same format on other platforms?

Of the 3 reasons listed, I guess running on Windows is the least
important. I think it's nice to be fully cross-platform given that
LLVM is a cross-platform project, but there's no pressing use case. I
think not having to write to disk is the biggest benefit here.

Ultimately I was thinking of this as a low-pri, nice-to-have, and
fairly self-contained feature that a beginner could use to get
familiarized with parts of LLVM. (Hence the mention of the eager
undergrad; I wasn't planning to do it myself.)

Jez

On Fri, Apr 30, 2021 at 12:22 PM Steven Wu <stevenwu at apple.com> wrote:
>
> I will say XAR is a very specific format for AppStore submission. Is there any reason you want to replicate the same format on other platforms? Is your goal to build and create AppStore submission from a Windows machine? That is the only reason I think of why you want to port libxar implementation. Otherwise, XAR is in the system for macOS and can probably be an optional configuration for other platforms.
>
> If you just want a general way to package up a bitcode bundle, there are probably better options now and may want to design something that is cross platform and can be easily used for different goals.
>
> Steven
>
>
> On Apr 30, 2021, at 8:56 AM, Davide Italiano <ditaliano at apple.com> wrote:
>
> + Wu
>
> On Apr 30, 2021, at 8:10 AM, pawel k. <pawel.kunio at gmail.com> wrote:
>
> Hello,
> Silly question but is xml the best format to be used? If so if it is fairly eparate lib, i could try to help with rewriting it.
>
> Best regards,
> Pawel Kunio
>
> pt., 30.04.2021, 15:23 użytkownik Jez via llvm-dev <llvm-dev at lists.llvm.org> napisał:
>>
>> +Davide -- do you know who (if anyone) at Apple maintains this?
>>
>> Jez
>>
>> On Fri, Apr 30, 2021 at 12:09 AM David Blaikie <dblaikie at gmail.com> wrote:
>> >
>> > Might want to CC any of the original authors from Apple - I expect
>> > they'd have some thoughts on this, maybe know something about the
>> > maintained-ness status of libxar, etc.
>> >
>> > If we do end up writing something of our own, I wonder if it has to be
>> > xar or whether there's something else we should entertain? (maybe the
>> > bitcode format could be reused/broadened/or the like)
>> >
>> > On Thu, Apr 29, 2021 at 7:51 PM Jez via llvm-dev
>> > <llvm-dev at lists.llvm.org> wrote:
>> > >
>> > > Hi,
>> > >
>> > > I want to float the idea of rewriting this external dependency as part
>> > > of LLVM. For context, Apple's `-bitcode_bundle` flag packages bitcode
>> > > within the `__LLVM` segment of the larger executable. The bitcode
>> > > files are packaged by serializing them into a XAR (XML Archive) file,
>> > > which then gets copied into `__LLVM`. llvm-objdump and LLD use libxar
>> > > to read and write these XARs.
>> > >
>> > > I think there are several good reasons for writing our own libxar:
>> > > 1. libxar's interface doesn't allow for *not* writing to disk (i.e. we
>> > > cannot write to an mmap'ed region), so it incurs unnecessary I/O
>> > > overhead
>> > > 2. It doesn't support Windows
>> > > 3. It exposes a rather primitive C interface (using raw ints to
>> > > indicate error status) that is out of place in LLVM's C++
>> > >
>> > > The goal would be to have both LLD and llvm-objdump use this
>> > > home-grown implementation of libxar, instead of the system one.
>> > >
>> > > One possibility is to contribute to the existing libxar, but I don't
>> > > know if it is really maintained; Apple releases open source tarball
>> > > drops every so often, and there's https://mackyle.github.io/xar/ which
>> > > appears to be a fork of the now-defunct http://xar.googlecode.com/,
>> > > but even that hasn't seen a commit since 2014.
>> > >
>> > > I may have an undergrad who's eager to take this work on, but before
>> > > they embark down that path I wanted to run it by y'all. Let me know if
>> > > you have thoughts.
>> > >
>> > > Cheers,
>> > > Jez
>> > > _______________________________________________
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>> > > llvm-dev at lists.llvm.org
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