nim-ffi/tests/bench

FFI wire-format codec benchmark

bench_codec.nim is a single-process Nim microbenchmark comparing the two FFI wire-format codecs head-to-head on identical payloads:

• cbor — cborEncode / cborDecode, self-describing bytes over seq[byte]. The codec the cbor ABI uses on every boundary crossing.
• c (cwire) — cwirePack / cwireUnpack / cwireFree, flat C-struct shared-memory packing. The codec the c ABI uses, emitted for every {.ffi: "abi = c".} type as its <T>_CWire companion.

Both paths run in the same process on the same values, so the numbers isolate codec cost only — no thread hop, no callback dispatch, no chronos work. The full FFI round-trip (thread channel + callback) is identical for both ABIs, so the codec is where the ABI difference actually lives.

Running

nimble bench_codec
# or directly, with the size sweep extended to 1 MiB:
nim c -r --mm:orc -d:danger tests/bench/bench_codec.nim --include-1mib

Build with -d:danger (the nimble task does) so the figures reflect optimized codegen rather than a debug build.

Payload shapes covered

Type	Shape
EchoRequest	1 string + 1 int (small struct)
EchoResponse	2 strings
ComplexRequest	seq[EchoRequest], seq[string], Option[...]
BytesPayload	seq[byte], swept 100 B → 150 KiB (--include-1mib adds 1 MiB)

Interpreting

Small structs are dominated by CBOR's per-field tag/length framing, so cwire wins by a large factor. As payloads grow into big seq[byte] blobs, both codecs become memcpy-bound and the ratio converges toward ~1×. The byte-blob sweep also reports throughput (MiB/s) for each codec.

Note: this benchmark exercises the c ABI codec (the cwire companions on the Nim side). Wiring the c ABI through the full proc-dispatch path and the foreign (C++/Rust) generators is tracked separately; only cbor currently generates working end-to-end bindings.