[llvm-dev] Porting libxar to LLVM

[Jez via llvm-dev]

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