From 3cd673863b18d0afe5a9e9d4927093fa284d08c7 Mon Sep 17 00:00:00 2001 From: Chris Fallin Date: Thu, 30 Dec 2021 12:24:28 -0800 Subject: [PATCH] Add README and LICENSE. --- LICENSE | 220 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ README.md | 97 +++++++++++++++++++++++- 2 files changed, 316 insertions(+), 1 deletion(-) create mode 100644 LICENSE diff --git a/LICENSE b/LICENSE new file mode 100644 index 0000000..f9d8195 --- /dev/null +++ b/LICENSE @@ -0,0 +1,220 @@ + + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. 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There are a few fuzzing +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 +could use work. I'm trying to get roundtripping working with the right basic +abstraction (CFG of SSA) first. + +This is a *hobby side project*; please do not expect support or rely on this +for anything critical! I'm happy to accept PRs that make this more +production-ready, of course. + +## Architecture + +The IR is a CFG of blocks, containing operators that correspond 1-to-1 to Wasm +operators. Dataflow is via SSA, and blocks have blockparams (rather than +phi-nodes). Wasm locals are not used in the IR (they are converted to SSA). + +The frontend converts Wasm into this IR by building SSA as it goes, inserting +blockparams when it discovers multiple reaching definitions for a local. +Multivalue Wasm (parameters and results for every control-flow block) is fully +supported, and converted to SSA. This process more or less works like +Cranelift's does, except that memory, table, etc. operations remain at the Wasm +abstraction layer (are not lowered into implementation details), and arithmetic +operators mirror Wasm's exactly. + +The backend operates in several stages: + +* [Structured control flow recovery](src/backend/structured.rs), which computes + a loop nest then adds blocks for noncontiguous forward edges, mirroring the + [Stackifier](https://medium.com/leaningtech/solving-the-structured-control-flow-problem-once-and-for-all-5123117b1ee2) + algorithm. + +* [Serialization](src/backend/serialize.rs), converting the CFG itself into a + linear sequence of Wasm operators, with resolved block targets and explicit + use of the stack. References to locals lowered from SSA are still virtual + (not allocated yet). + +* [Location assignment ("regalloc")](src/backend/locations.rs), allocating Wasm + locals to store values as they flow from operator to operator. + + 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 + sophisticated scheduling (moving operators so they occur at the right time), + which we haven't yet implemented. + + We could allocate a local per SSA value, but this is very wasteful. First, + there is a limit to the number of locals supported by most engines. Second, + "baseline" compilation strategies typically just allocate a stackslot per + local, and so this would create huge, sparsely-used frames. + + Instead, we reuse locals by tracking live-ranges of each SSA value and + assigning non-overlapping ranges to the same local. This is actually a very + simple register allocator. It remains simpler than other production + allocators by (i) only tracking a single span for each SSA value, rather than + a discontiguous set of spans, and (ii) freely reaching for new locals when + needed rather than "spilling" (it behaves as if the register file is + 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 + +- Like [Binaryen](https://github.com/WebAssembly/binaryen) but with an SSA IR, + rather than an AST-based IR. Dataflow analyses are much easier when one + doesn't have to handle arbitrary reads and writes to locals. Binaryen is able + to stackify/reloop arbitrary control flow (CFG to Wasm) but does not + implement the other direction (Wasm to CFG), and it has only a C/C++ API, not + Rust. + +- Like [Walrus](https://github.com/rustwasm/walrus) but also with an SSA IR. + Walrus is in Rust and designed for Wasm-to-Wasm transforms as well, but its + IR mirrors the Wasm bytecode closely and thus presents the same difficulties + as Binaryen for traditional CFG-of-SSA-style compiler analyses and + transforms. + +- Halfway like + [Cranelift](https://github.com/bytecodealliance/wasmtime/tree/main/cranelift/), + in that the IR is similar to Cranelift's (a CFG of SSA IR with blockparams), + but with the Wasm backend as well (Cranelift only does Wasm-to-IR). WAFFLE's + IR also deliberately remains at the Wasm abstraction level, maintaining + 1-to-1 correspondence with all operators and maintaining the concepts of + memories, tables, etc., while Cranelift lowers operations and storage + abstractions into runtime/embedding-specific implementation details in the + IR.