[llvm-dev] Porting libxar to LLVM

[Jez via llvm-dev]

+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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