1379c65a6a
Currently, we unconditionally trim the ends of liveranges around a split when we do a split, including splits due to conflicts in a liverange/bundle's requirements (e.g., a liverange with both a register and a stack use). These trimmed ends, if they exist, go to the spill bundle, and the spill bundle may receive a register during second-chance allocation or otherwise will receive a stack slot. This was previously measured to reduce contention significantly, because it reduces the sizes of liveranges that participate in the first-chance competition for allocations. When a split has to occur, we might as well relegate the "connecting pieces" to a process that comes later, with a hint to try to get the right register if possible but no hard connection to either end. However, in the case of a split arising from a reg-to-stack / stack-to-reg conflict, as happens when references are used or def'd as registers and then cross safepoints, this extra step in the connectivity (normal LR with register use, then spill bundle, then normal LR with stack use) can lead to extra moves. Additionally, when one of the LRs has a stack constraint, contention is far less important; so it doesn't hurt to skip the trimming step. In fact, it's likely much better to put the "connecting piece" together with the stack side of the conflict. Ideally we would handle this with the same move-cost logic we use for conflicts detected during backtracking, but the requirements-related splitting happens separately and that logic would need to be generalized further. For now, this is sufficient to eliminate redundant moves as seen in e.g. bytecodealliance/wasmtime#3785.
regalloc2: another register allocator
This is a register allocator that started life as, and is about 50% still, a port of IonMonkey's backtracking register allocator to Rust. In many regards, it has been generalized, optimized, and improved since the initial port, and now supports both SSA and non-SSA use-cases.
In addition, it contains substantial amounts of testing infrastructure (fuzzing harnesses and checkers) that does not exist in the original IonMonkey allocator.
See the design overview for (much!) more detail on how the allocator works.
License
This crate is licensed under the Apache 2.0 License with LLVM
Exception. This license text can be found in the file LICENSE.
Parts of the code are derived from regalloc.rs: in particular,
src/checker.rs and src/domtree.rs. This crate has the same license
as regalloc.rs, so the license on these files does not differ.