| name | jit-overview |
| description | Orientation to facet-format JIT deserialization (tiering, fallbacks, key types/entry points) and where to look when changing or debugging JIT code Use when this capability is needed. |
| metadata | {"author":"facet-rs"} |
JIT deserialization overview (facet-format)
Facet’s JIT lives in facet-format and is used by format crates like facet-json.
When to read this
- You’re touching anything under
facet-format/src/jit/ or enabling the jit feature.
- You’re investigating performance changes in deserialization (especially “tier2”/JIT benchmarks).
- You’re debugging a JIT crash (SIGSEGV/UB-ish symptoms).
Mental model
facet-format defines a format-neutral deserialization pipeline:
- A
FormatParser (implemented by each format crate) produces a stream of ParseEvents.
- A shape-driven layer consumes those events and writes to an output value (often via
facet-reflect).
The JIT accelerates this by compiling deserialization code specialized for:
- the target type (
T / its Shape), and sometimes
- the format parser (
P).
Two-tier architecture (high level)
Tier 1 (shape JIT)
- Compiles code that consumes
ParseEvents and writes directly into the output’s memory at known offsets.
- Works with any format that implements
FormatParser (JSON/YAML/TOML/…).
Tier 2 (format JIT)
- For the “entire input slice is available” case, a format crate can provide a
FormatJitParser + JitFormat implementation.
- Tier 2 emits Cranelift IR to parse bytes directly, bypassing the
ParseEvent stream for maximum throughput.
Fallbacks are part of the design
- Tier 2 may return “unsupported” for shapes/input it can’t handle, and must be side-effect-free in that case.
- Callers typically try tier 2, then tier 1, then reflection.
Entry points & where to look
- Main docs and contracts:
facet-format/src/jit/mod.rs
- JIT-enabled parser trait:
facet-format/src/parser.rs (FormatJitParser)
- JIT usage in a format crate:
facet-json/Cargo.toml feature jit = ["facet-format/jit"]- Example:
facet-json/examples/profile_jit_vec (requires jit)
- Windows crash debugging notes:
.claude/skills/windbg-jit.md
- Memory debugging:
.claude/skills/debug-with-valgrind/SKILL.md (uses nextest profiles)
Debugging checklist (practical)
- Reproduce with a minimal type + input (see
.claude/skills/reproduce-reduce-regress/SKILL.md).
- Run the failing test under:
- valgrind:
cargo nextest run --profile valgrind … (configured in .config/nextest.toml)
- or Miri when applicable (
just miri) for UB/provenance issues outside the JIT itself.
- If the crash is in JIT codegen/execution:
- Prefer isolating the smallest shape that triggers tier selection and failure.
- Look for tier selection diagnostics and caching behavior in
facet-format/src/jit/mod.rs.
Common pitfalls
- Assuming tier 2 supports “all shapes”: it intentionally supports a performance-focused subset.
- Forgetting that “unsupported” must not advance the parser cursor or partially initialize output.
- Introducing new unsafe paths without tests that exercise drop/cleanup on error paths.
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