RFC: Deprecate Makefile build

[elliottslaughter]

We recently had an issue in Regent which basically came down to the Terra Makefile being broken on Ubuntu 19.04. This is not the first system where I've seen the original Makefile break, there are certain machines where I just can't build with it at all. I haven't spent the time to debug the Ubuntu 19.04 issue yet, but at least on Cori the problem is that gcc's output is quite different from what we usually get---it doesn't seem like an issue we'll be able to fix easily.

I'm wondering if we should deprecate the Makefile and officially designate CMake as the primary build system for Terra (at least on Linux/macOS). The main downside I'm aware of is that LLVM CMake export files have been broken on certain older distros (e.g. Ubuntu prior to 18.04), so until those distros become obsolete we'd have to recommend people download official LLVM builds or else build it from source. On the flip side, there are certain LLVM versions (== 7) that we can only build with CMake, so we may be forced to go in this direction anyway in the future.

I want to assess the impact of this before making any changes. If anyone has thoughts, please comment on this issue.

Blockers:

• [ ] ~REEXPORT_LLVM_COMPONENTS #342~ (I think this is no longer mandatory)
• [x] Repeated rebuilds with CUDA #415 (optional but nice to have)
• [x] Documentation

What this means:

• The Makefile build will continue to be tested in CI but if configurations break in non-obvious ways I'm going to reduce effort to keep them working.
• The Makefile build will not be tested for any new LLVM versions moving forward. We will keep tests for older LLVM versions as long as those are supported.
• Documentation etc. will be updated point users towards the CMake build flow.
• A formal deprecation notice in the release notes.
• At some point in the eventual future (but not for a long time, post-1.0), the Makefile will be completely removed. I will do a minor version bump for this but not a major version bump, since it will be formally deprecated prior to 1.0 and this policy will have been established prior to that release.

[aiverson]

I would suggest not deprecating it until we get the new build-box system up and running, then switching everything over to CMake. The build box system should produce fairly portable builds of the binaries and be runnable on most linux systems. I'm really looking forward to getting Terra ported to LLVM 8 because that version takes WASM target support off of experimental and adds nice new stuff for JIT compilation, so that needs CMake.

[ErikMcClure]

I feel obligated to point out that I cannot currently figure out how to build terra using cmake on Ubuntu 19 for unknown reasons.

I think we need a work item to document how to use cmake for all platforms before this can happen, because currently the only way to figure out how you're actually supposed to invoke cmake is by looking at travis.sh or appveyor.yml.

[elliottslaughter]

Point well taken, I've added it to the list.

[elliottslaughter]

Please check https://github.com/terralang/terra#building-terra and see if those instructions seem reasonable or not.

[elliottslaughter]

@ErikMcClure I was able to make the following script work on all of Ubuntu 16.04, 18.04 and 19.10.

apt-get update -qq && apt-get install -qq build-essential cmake git llvm-6.0-dev libclang-6.0-dev clang-6.0 libedit-dev libncurses5-dev zlib1g-dev
git clone https://github.com/terralang/terra.git
cd terra/build
cmake -DCMAKE_INSTALL_PREFIX=$PWD/../install ..
make install -j4 # tune this for how many cores you have
make test

It may be worth actually adding this as a test in Travis (via Docker) so we have a "minimal viable installation" test.

[elliottslaughter]

This has been formalized in https://github.com/terralang/terra/pull/423, and we can now include Docker in our test suite. I believe I'm now quite convinced Terra should build out of the box on all modern Ubuntu versions.

[elliottslaughter]

Note: the Makefile build is now broken on macOS Mojave and newer, because of a failure in the step where we run a LuaJIT script to unpack the .a files (see #435). I do not have time to debug this. I am inclined to move forward with the deprecation because no one is screaming for #342, but at any rate anyone who cares can come along and add support for that later if they are so inclined.

[elliottslaughter]

As noted in #342, I think that can be accomplished other ways now, so I'm going to consider this not a blocker for deprecation. Unless anyone objects I'm going to move forward with the deprecation. I've updated the top-level issue to specify what exactly this means.

[elliottslaughter]

I'm going to consider Make hereby deprecated. I pushed a formal deprecation notice to CHANGES.md.