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Apache License
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97
README.md
97
README.md
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@ -1 +1,96 @@
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WAFFLE: Wasm Analysis Framework for Lightweight Experimentation
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# WAFFLE: Wasm Analysis Framework for Lightweight Experimentation
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Synopsis: an SSA IR compiler framework for Wasm-to-Wasm transforms, in Rust.
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## Status: incomplete with known bugs
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The transform from Wasm to IR works well, and has been fuzzed.
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The transform from IR to Wasm works to roundtrip a simple WASI "hello world"
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written in C, but appears to have some bugs still. There are a few fuzzing
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oracles (differential execution of roundtripped code, double-roundtripping)
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that I have tried to use to shake out bugs, but it's not quite there yet.
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Nothing in a middle-end has been designed or written, and the IR traversal APIs
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could use work. I'm trying to get roundtripping working with the right basic
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abstraction (CFG of SSA) first.
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This is a *hobby side project*; please do not expect support or rely on this
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for anything critical! I'm happy to accept PRs that make this more
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production-ready, of course.
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## Architecture
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The IR is a CFG of blocks, containing operators that correspond 1-to-1 to Wasm
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operators. Dataflow is via SSA, and blocks have blockparams (rather than
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phi-nodes). Wasm locals are not used in the IR (they are converted to SSA).
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The frontend converts Wasm into this IR by building SSA as it goes, inserting
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blockparams when it discovers multiple reaching definitions for a local.
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Multivalue Wasm (parameters and results for every control-flow block) is fully
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supported, and converted to SSA. This process more or less works like
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Cranelift's does, except that memory, table, etc. operations remain at the Wasm
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abstraction layer (are not lowered into implementation details), and arithmetic
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operators mirror Wasm's exactly.
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The backend operates in several stages:
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* [Structured control flow recovery](src/backend/structured.rs), which computes
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a loop nest then adds blocks for noncontiguous forward edges, mirroring the
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[Stackifier](https://medium.com/leaningtech/solving-the-structured-control-flow-problem-once-and-for-all-5123117b1ee2)
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algorithm.
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* [Serialization](src/backend/serialize.rs), converting the CFG itself into a
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linear sequence of Wasm operators, with resolved block targets and explicit
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use of the stack. References to locals lowered from SSA are still virtual
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(not allocated yet).
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* [Location assignment ("regalloc")](src/backend/locations.rs), allocating Wasm
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locals to store values as they flow from operator to operator.
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We use Wasm locals rather than the stack for most dataflow. We can use the
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stack in a limited way, opportunistically, but doing more requires
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sophisticated scheduling (moving operators so they occur at the right time),
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which we haven't yet implemented.
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We could allocate a local per SSA value, but this is very wasteful. First,
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there is a limit to the number of locals supported by most engines. Second,
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"baseline" compilation strategies typically just allocate a stackslot per
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local, and so this would create huge, sparsely-used frames.
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Instead, we reuse locals by tracking live-ranges of each SSA value and
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assigning non-overlapping ranges to the same local. This is actually a very
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simple register allocator. It remains simpler than other production
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allocators by (i) only tracking a single span for each SSA value, rather than
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a discontiguous set of spans, and (ii) freely reaching for new locals when
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needed rather than "spilling" (it behaves as if the register file is
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infinite).
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* [Finalization](src/backend/final.rs), doing a few final rewrite steps to get
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actual Wasm bytecode, produced using
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[wasm-encoder](https://github.com/bytecodealliance/wasm-tools/blob/main/crates/wasm-encoder/README.md).
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## Comparisons / Related Work
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- Like [Binaryen](https://github.com/WebAssembly/binaryen) but with an SSA IR,
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rather than an AST-based IR. Dataflow analyses are much easier when one
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doesn't have to handle arbitrary reads and writes to locals. Binaryen is able
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to stackify/reloop arbitrary control flow (CFG to Wasm) but does not
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implement the other direction (Wasm to CFG), and it has only a C/C++ API, not
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Rust.
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- Like [Walrus](https://github.com/rustwasm/walrus) but also with an SSA IR.
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Walrus is in Rust and designed for Wasm-to-Wasm transforms as well, but its
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IR mirrors the Wasm bytecode closely and thus presents the same difficulties
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as Binaryen for traditional CFG-of-SSA-style compiler analyses and
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transforms.
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- Halfway like
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[Cranelift](https://github.com/bytecodealliance/wasmtime/tree/main/cranelift/),
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in that the IR is similar to Cranelift's (a CFG of SSA IR with blockparams),
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but with the Wasm backend as well (Cranelift only does Wasm-to-IR). WAFFLE's
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IR also deliberately remains at the Wasm abstraction level, maintaining
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1-to-1 correspondence with all operators and maintaining the concepts of
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memories, tables, etc., while Cranelift lowers operations and storage
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abstractions into runtime/embedding-specific implementation details in the
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IR.
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