Performance

Execution tiers

zwasm uses tiered execution:

Interpreter: All functions start as register IR, executed by a dispatch loop. Fast startup, no compilation overhead.
JIT (ARM64/x86_64): Hot functions are compiled to native code when call count or back-edge count exceeds a threshold.

When JIT kicks in

Call threshold: After ~8 calls to the same function
Back-edge counting: Hot loops trigger JIT faster (loop iterations count toward the threshold)
Adaptive: The threshold adjusts based on function characteristics

Once JIT-compiled, all subsequent calls to that function execute native machine code directly, bypassing the interpreter.

Metric	Value
Binary size (ReleaseSafe)	~1.4 MB
Runtime memory (fib benchmark)	~3.5 MB RSS
wasmtime binary for comparison	56.3 MB

Benchmark results

Representative benchmarks comparing zwasm against wasmtime 41.0.1, Bun 1.3.8, and Node v24.13.0 on Apple M4 Pro. 16 of 29 benchmarks match or beat wasmtime. 25/29 within 1.5x.

Benchmark	zwasm	wasmtime	Bun	Node
nqueens(8)	2 ms	5 ms	14 ms	23 ms
nbody(1M)	22 ms	22 ms	32 ms	36 ms
gcd(12K,67K)	2 ms	5 ms	14 ms	23 ms
tak(24,16,8)	5 ms	9 ms	17 ms	29 ms
sieve(1M)	5 ms	7 ms	17 ms	29 ms
fib(35)	46 ms	51 ms	36 ms	52 ms
st_fib2	900 ms	674 ms	353 ms	389 ms

zwasm uses 3-4x less memory than wasmtime and 8-10x less than Bun/Node.

Full results (29 benchmarks): bench/runtime_comparison.yaml

SIMD operations are functionally complete (256 opcodes, 100% spec) but run on the stack interpreter, not the register IR or JIT. This results in ~22x slower SIMD execution vs wasmtime. Planned improvement: extend register IR for v128, then selective JIT NEON/SSE.

Benchmark methodology

All measurements use hyperfine with ReleaseSafe builds:

# Quick check (1 run, no warmup)
bash bench/run_bench.sh --quick

# Full measurement (3 runs, 1 warmup)
bash bench/run_bench.sh

# Record to history
bash bench/record.sh --id="X" --reason="description"

Benchmark layers

Layer	Count	Description
WAT micro	5	Hand-written: fib, tak, sieve, nbody, nqueens
TinyGo	11	TinyGo compiler output: same algorithms + string ops
Shootout	5	Sightglass shootout suite (WASI)
Real-world	6	Rust, C, C++ compiled to Wasm (matrix, math, string, sort)
GC	2	GC proposal: struct allocation, tree traversal

CI regression detection

PRs are automatically checked for performance regressions:

6 representative benchmarks run on both base and PR branch
Fails if any benchmark regresses by more than 20%
Same runner ensures fair comparison

Performance tips

ReleaseSafe: Always use for production. Debug is 5-10x slower.
Hot functions: Functions called frequently will be JIT-compiled automatically.
Fuel limit: --fuel adds overhead per instruction. Only use for untrusted code.
Memory: Wasm modules with linear memory allocate guard pages. Initial RSS is ~3.5 MB regardless of module size.

zwasm