Relicense fully to Apache-2.0 WITH LLVM-exception.

[cfallin]

Large parts of the code in regalloc2 are currently licensed under the Mozilla Public License (MPL) 2.0, because they derive in meaningful ways from the register allocator in IonMonkey, which is part of Firefox. The relevant source files are marked as such, with references to the files in the Firefox source tree.

The intent of the regalloc2 project was to port the register allocator from Firefox to use in Cranelift, borrowing good technology and improving on it in the spirit of open source.

However, Several use-cases of Cranelift require, or at least strongly prefer, the Apache-2.0 license with the LLVM exception (matching the license of Cranelift itself, and Bytecode Alliance projects generally). While using this license is not strictly necessary for
regalloc2 to be usable (The MPL is an excellent open-source license!), relicensing fully under this license to harmonize with the rest of Cranelift and Bytecode Alliance codebases significantly widens possibilities and reduces friction; then regalloc2 is "just another part of Cranelift" and doesn't have to be treated specially.

The source in src/ion/ specifically began as a fairly direct port of the algorithms in the following files in the mozilla-central repository (Firefox codebase):

• The bulk of the "backtracking allocator" algorithm:
  • js/src/jit/BacktrackingAllocator.{cpp,h}
• Helpers and definitions in the surrounding infrastructure:
  • js/src/jit/RegisterAllocator.h
  • js/src/jit/RegisterAllocator.cpp
  • js/src/jit/StackSlotAllocator.h
  • js/src/jit/LIR.h
• A few data structure implementations:
  • js/src/ds/SplayTree.h
  • js/src/ds/PriorityQueue.h

Subsequent work in improving regalloc2 has caused it to drift from the direct port -- for example, it no longer uses splay trees or the direct port of the priority queue above -- but it is of course very clearly still a derivative work.

Analysis of the contributors to these files indicates that we need signoff from the following folks:

• Mozilla Corp, for contributions made by Mozilla employees (the majority of the code). Communications with Mozilla (thanks @tschneidereit and @bholley for doing the work here!) indicate that @ekr is able to sign off when ready here.

• Andy Wingo, specifically for the work done in Bug 1620197 and Bug 1609057 to generalize the stack allocator for a Wasm feature (multiple returns).

Additionally, since the initial port, we have had three contributions from @Amanieu: #9, #11, #13.

So, if everyone applicable is happy with this relicensing, this PR removes the MPL-2.0 license in src/ion/ and marks all files as covered under Apache-2.0 WITH LLVM-exception. Please let us know if this is OK!

Signoffs:

• @ekr, for Mozilla's contributions
• @wingo, for contributions to original code in mozilla-central
• @Amanieu, for the three PRs linked above

Thanks!

(Fixes #7)

[Amanieu]

OK for me!

[wingo]

ok for me!

[ekr]

Approved by me.

[cfallin]

Thanks all! With all signoffs in, consider the code relicensed; merging now!

[bjorn3]

🎉