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From: James Grosbach <grosbach <at> apple.com>
Subject: Contributing the Apple ARM64 compiler backend
Newsgroups: gmane.comp.compilers.llvm.devel
Date: Friday 28th March 2014 19:35:52 UTC (over 3 years ago)

Attached below are the patches that make up the Apple ARM64 compiler
backend (in addition to compiler_rt, libc++ and lldb support), and we'd
like to start the process of integrating them into mainline LLVM.  We and
ARM have discussed a general approach to integrating them into mainline and
look forward to working through this with the community at large.

First a bit of context to help jump-start the discussion. The ARM64 backend
is a complete production quality implementation for ARM’s 64-bit
architecture, AArch64. It supports both iOS and Linux as target platforms.
In the source tree, the backend co-exists with the AArch64 backend without

As a first step, we'd like to add this as a peer of the AArch64 backend. 
While having two AArch64 backends in the tree long term doesn’t make
sense, this approach worked well when the Sparc backends were merged and
for AMDIL. This will allow us to work quickly towards merging the ports to
create a backend that is truly the best of both. The public AArch64 backend
has attracted an excellent core group of contributors all of whom have done
great work and we are very much looking forward to collaborating with them
and the rest of the community to make this happen.

Getting to that idillic end-state of one AArch64 backend in the tree which
encompasses the best of both is going to take a fair bit of work and a lot
of discussion amongst the interested parties. To get there, I think a rough
roadmap like the following makes sense:

1. Import the ARM64 backend into the public tree so it’s easily
accessible for everyone to investigate.
2. Test it. Benchmark it. Explore it. Get data for the community to work
with about the state of the back end. ARM has some excellent data that will
help guide us here.
3. Identify the core backend to build on and to merge features and tuning
from the other to. The data I have seen so far leads me to believe ARM64 is
the correct choice here, but that’s a decision primarily for the
contributors above.
4. Clean up the codebase (C++11-ify it, for example), fix any regressions
and test failures identified in benchmarking.

This will give us a backend that is a superset of both ARM64 and AArch64 in
terms both of performance and functionality. We can then consolidate to a
single backend, named AArch64 for consistency with the current public tree.

Jim Grosbach
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