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README update.

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Chris Fallin 2022-12-03 18:09:34 -08:00
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Synopsis: an SSA IR compiler framework for Wasm-to-Wasm transforms, in Rust. Synopsis: an SSA IR compiler framework for Wasm-to-Wasm transforms, in Rust.
## Status: incomplete with known bugs ## Status: working for Wasm MVP; roundtrips complex modules successfully
The transform from Wasm to IR works well, and has been fuzzed. The transforms from Wasm to IR and from IR to Wasm work well, and has been
fuzzed in various ways. In particular, waffle is fuzzed by roundtripping Wasm
through SSA IR and back, and differentially executing the original and
roundtripped Wasm under Wasmtime (with limits on execution time). At this time,
no correctness bugs have been found.
The transform from IR to Wasm works to roundtrip a simple WASI "hello world" Waffle is able to roundtrip (convert to IR, then compile back to Wasm) complex
written in C, but appears to have some bugs still. There are a few fuzzing modules such as the SpiderMonkey JS engine compiled to Wasm.
oracles (differential execution of roundtripped code, double-roundtripping)
that I have tried to use to shake out bugs, but it's not quite there yet.
Nothing in a middle-end has been designed or written, and the IR traversal APIs Waffle has some basic mid-end optimizations working, such as GVN and constant
could use work. I'm trying to get roundtripping working with the right basic propagation. Much more could be done on this.
abstraction (CFG of SSA) first.
This is a *hobby side project*; please do not expect support or rely on this There are various ways in which the generated Wasm bytecode could be improved;
for anything critical! I'm happy to accept PRs that make this more work is ongoing on this.
production-ready, of course.
## Architecture ## Architecture
@ -33,20 +33,21 @@ Cranelift's does, except that memory, table, etc. operations remain at the Wasm
abstraction layer (are not lowered into implementation details), and arithmetic abstraction layer (are not lowered into implementation details), and arithmetic
operators mirror Wasm's exactly. operators mirror Wasm's exactly.
The backend operates in several stages: The backend operates in three stages:
* [Structured control flow recovery](src/backend/structured.rs), which computes * [Structured control flow recovery](src/backend/stackify.rs), which uses
a loop nest then adds blocks for noncontiguous forward edges, mirroring the [Ramsey's algorithm](https://dl.acm.org/doi/abs/10.1145/3547621) to convert
[Stackifier](https://medium.com/leaningtech/solving-the-structured-control-flow-problem-once-and-for-all-5123117b1ee2) the CFG back into an AST of Wasm control-flow primitives (blocks, loops, and
algorithm. if-then AST nodes).
* [Serialization](src/backend/serialize.rs), converting the CFG itself into a * [Treeification](src/backend/treeify.rs), which computes whether some SSA
linear sequence of Wasm operators, with resolved block targets and explicit values are used only once and can be moved to just before their single
use of the stack. References to locals lowered from SSA are still virtual consumer, computing the value directly onto the Wasm stack without the need
(not allocated yet). for an intermediate local. This is a very simple form of code scheduling.
* [Location assignment ("regalloc")](src/backend/locations.rs), allocating Wasm * [Localification](src/backend/localify.rs), which performs a register
locals to store values as they flow from operator to operator. allocation (using a simple linear-scan algorithm) to assign all SSA values to
locals such that no live-ranges overlap in the same local.
We use Wasm locals rather than the stack for most dataflow. We can use the We use Wasm locals rather than the stack for most dataflow. We can use the
stack in a limited way, opportunistically, but doing more requires stack in a limited way, opportunistically, but doing more requires
@ -66,10 +67,6 @@ The backend operates in several stages:
needed rather than "spilling" (it behaves as if the register file is needed rather than "spilling" (it behaves as if the register file is
infinite). infinite).
* [Finalization](src/backend/final.rs), doing a few final rewrite steps to get
actual Wasm bytecode, produced using
[wasm-encoder](https://github.com/bytecodealliance/wasm-tools/blob/main/crates/wasm-encoder/README.md).
## Comparisons / Related Work ## Comparisons / Related Work
- Like [Binaryen](https://github.com/WebAssembly/binaryen) but with an SSA IR, - Like [Binaryen](https://github.com/WebAssembly/binaryen) but with an SSA IR,