c3e513c4cb
The checker works by keeping a worklist of blocks to process, and adds a block to the worklist when its entry state changes. Every entry state is initially `Top` (in a lattice). The entry block is explicitly added to the worklist to kick off the processing. In ordinary cases, the entry block has some instructions that change state from `Top` to something else (lower in the lattice), and this is propagated to its successors; its successors are added to the worklist; and so on. No other state is `Top` from then on (because of monotonicity) so every reachable block is processed. However, if the entry block is completely empty except for the terminating branch, the state remains `Top`; then the entry state of its successors, even when updated, is still `Top`; and the state didn't change so the blocks are not added to the worklist. (Nevermind that they were not processed in the first place!) The bug is that the invariant "has been processed already with current state" is not true initially, when the current state is set to `Top` but nothing has been processed. This PR makes a simple fix: it adds every block to the worklist initially to be processed, in input order (which is usually RPO order in practice) as a good first heuristic; then if after processing the input state changes again, it can be reprocessed until fixpoint as always. Fixes bytecodealliance/wasmtime#5791.
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. (However, non-SSA should be considered deprecated; we want to move to SSA-only in the future, to enable some performance improvements. See #4.)
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.