onnx_runtime_dart 0.3.1

A pure-Dart ONNX inference runtime for transformer-style models — no FFI, runs on every target including web and WebAssembly.

onnx_runtime_dart

A small, dependency-light pure-Dart ONNX inference runtime. No FFI and no native onnxruntime — the graph is interpreted in plain Dart, so the same code runs on every Dart/Flutter target, including the web and WebAssembly.

It implements the operator set used by transformer / attention style models (BERT-family text embedders, rerankers, and RoPE / ALiBi variants), the convolution / pooling family used by CNN vision models, recurrent ops (LSTM/GRU/RNN), control flow (If/Loop/Scan with subgraph execution) and quantized models (both QDQ and QOperator formats). It is still not a complete ONNX runtime — see the operator list below for exactly what is covered.

Verified to cosine-1.0 parity against ONNX Runtime (via ort), max abs diff ~1e-6 (float32 rounding), on: jina-embeddings-v2-base-en (BERT + ALiBi), bge-small-en-v1.5, all-MiniLM-L6-v2, ms-marco-MiniLM (cross-encoder reranker), the nllb-200-600M encoder (seq2seq / mBART), a 0.6B RoPE embedder (external-data weights), the vision CNNs MobileNetV2 and ResNet18, Silero VAD (Conv1D + LSTM + If + reflect-Pad), AECMOS (both echo-MOS models: Conv + MaxPool + bidirectional GRU + ReduceMax — with a complete pure-Dart scoring pipeline in example/aecmos/), CAM++ (speaker-embedding x-vector: 225 convs + BatchNorm/AveragePool/Pad/ReduceProd), Maia3-5M (chess transformer, policy + WDL value heads, Einsum attention), and from the Parakeet-TDT 0.6B ASR stack: the NeMo mel featurizer (STFT op + float64 weights), the RNN-T decoder/joint (LSTM + Split), and the int8 conformer encoder (ConvInteger 1-D/2-D + MatMulInteger + Tile; dynamic-quant model, so judged by the intrinsic-band criterion — our deviation from ORT-int8, cosine 0.997, is far below that export's own quantization error vs fp32, cosine 0.63). Also verified: the CosyVoice3 speech tokenizer (all 25 discrete speech tokens exactly equal to ORT's) and the llama-nemotron-rerank-1B int4 reranker (logit within 1.7e-5).

One known precision-mode gap: exports that run regions in fp16 compute (115 Cast-to-fp16 pairs in zerank-1-small int4) execute here in float32 between the cast points (values are rounded through fp16 at each cast). ORT computes those ops in true half precision, so results agree only to the model's fp16 sensitivity (~2% on zerank's score) — ours is the more precise of the two, but not bit-matching. Every op is additionally covered by generated per-op parity fixtures against native ONNX Runtime (test/fixtures/, see tool/gen_fixtures.py).

Quantized MobileNetV2 (QDQ) classifies identically to ORT (same top-5, in order). Quantized models have no bitwise logit parity to target: tiny float-ordering differences flip quantization buckets, and ORT's own optimized vs unoptimized execution of the same model only agrees with itself to cosine ≈0.993 — our output lands in the same band (≈0.991).

Octen-0.6B int4 (MatMulNBits, 1.8 GB packed weights) runs at cosine-1.0 vs ORT — static block quantization has no runtime quantization boundaries, so int4 models reproduce exactly, unlike dynamic int8 below.

Octen-0.6B int8 (QOperator dynamic quantization, 196 MatMulInteger, 1 GB of int8 weights running in ~1 GB of RAM via compact storage) runs end-to-end; the runtime-induced deviation from ORT-int8 (cosine 0.96 on the pooled embedding) is far smaller than that model's own quantization error vs its fp32 original (cosine 0.73) — dynamic quantization amplifies transcendental-function rounding differences across layers, so int8-vs-int8 bitwise parity is unattainable for any independent implementation.

Execution uses a packed, register-tiled Float32x4 SIMD GEMM kernel on native targets (scalar fallback on web), im2col convolution, load-time constant folding and weight prepacking; see BENCHMARKS.md for current numbers vs native ONNX Runtime.

On native targets you can additionally spread large matmuls across an isolate worker pool — each worker permanently owns a column slice of the big weights, so per-run messages carry only activations:

await model.parallelize(workers: 4);        // once, after loading
final out = await model.runAsync(inputs, ['output']); // bitwise == run()
model.dispose();                            // shuts the workers down

Install

dependencies:
  onnx_runtime_dart: ^0.1.0

Usage

import 'dart:typed_data';
import 'package:onnx_runtime_dart/onnx_runtime_dart.dart';
import 'package:onnx_runtime_dart/onnx_runtime_dart_io.dart'; // native only (dart:io)

void main() {
  // Resolves companion external-data files (large models) automatically.
  final model = loadOnnxModel('model.onnx');

  final out = model.run(
    {'input': Tensor.float(Float32List.fromList([1, 2, 3, 4]), [1, 4])},
    ['output'],
  );

  print(out['output']!.asFloatList());
}

On the web (no dart:io), use OnnxModel.fromBytes(bytes) directly; for external-data models pass an externalData resolver.

Weights load from float32, float16, int32, int64 and bool tensors, inline or from a companion .onnx.data file (read on demand, so multi-GB models don't load into memory all at once).

See example/onnx_runtime_dart_example.dart for a self-contained, runnable graph built with the protobuf types.

Supported operators

• Math: Add, Sub, Mul, Div, Pow, Sqrt, Reciprocal, Abs, Neg, Exp, Log, Relu, Erf, Sigmoid, Tanh, Cos, Sin, Clip.
• Compare / logic: Equal, Greater, Less, GreaterOrEqual, LessOrEqual, And, Or, Not, Where, Max, Min.
• Shape / index: Shape, Reshape, Transpose, Squeeze, Unsqueeze, Concat, Gather, GatherND, GatherElements, Expand, Slice, Range, Cast, Constant, ConstantOfShape.
• Reduce / linalg: ReduceMean, ReduceSum, ReduceMax, ReduceMin, ReduceProd, CumSum, Softmax, LayerNormalization, MatMul, Gemm, Einsum (bhi,oi->bho, bid,bjd->bij).
• Convolution / pooling: Conv (1–3 spatial dims, strides / pads / dilations / groups / depthwise / auto_pad, im2col+GEMM fast path), ConvTranspose, MaxPool, AveragePool, GlobalAveragePool, GlobalMaxPool, BatchNormalization, InstanceNormalization, Resize (nearest + linear), Flatten.
• Activations: LeakyRelu, Elu, PRelu, HardSigmoid, HardSwish, Softplus, Gelu (erf + tanh forms).
• Recurrent: LSTM (incl. peepholes), GRU (incl. linear_before_reset), RNN — forward / reverse / bidirectional, sequence_lens, initial_h/initial_c; default activations only.
• Control flow / misc: If, Loop, Scan (subgraphs capture the outer scope; scan outputs, non-default axes and reverse directions supported), Identity, Pad (constant / reflect / edge), Size.
• Quantization: QDQ format — QuantizeLinear, DequantizeLinear (per-tensor + per-axis, uint8/int8), DynamicQuantizeLinear; weight DequantizeLinear nodes constant-fold at load, so QDQ models run at float speed. QOperator format — MatMulInteger (scalar / per-row / per-column zero points, batched), ConvInteger, QLinearMatMul, QLinearConv (per-channel weight scales + int32 bias) with exact int32 accumulation and round-half-to-even requantization. int8/uint8 tensors use compact 1-byte storage end to end — a 1 GB int8 model needs ~1 GB, not the 8 GB an int64 widening would cost. MatMulNBits (com.microsoft, 4-bit block-quantized weights, packed zero points, partial blocks) runs int4 exports with weights kept packed (0.5 bytes/weight); dequantization streams through the SIMD GEMM per call.
• Fusion (load-time, automatic): the erf-GELU chain (0.5·x·(1+Erf(x/√2))) fuses to a single pass, and the attention epilogue MatMul(Softmax(MatMul(Q,K)·s + mask), V) fuses so scale + mask + softmax happen in one sweep over the attention matrix. Fusions only fire when the intermediate values have no other consumers; results stay within float rounding of the unfused graph.

Tensors are float32 or int64 (int32 and bool are widened to int64), row-major (matching ONNX's own layout). An unimplemented op throws UnsupportedError naming the op.

How it works

ONNX graphs are stored in topological order, so execution is a single linear pass over the nodes with a name -> Tensor value cache — initializers (weights) are decoded up front, runtime inputs are supplied to run, and the requested outputs are returned.

To build or inspect models programmatically, import package:onnx_runtime_dart/onnx_proto.dart for the ONNX message types (ModelProto, GraphProto, NodeProto, …).

Licensing

The runtime code is MIT. The generated protobuf bindings (lib/src/onnx.pb*.dart) derive from the ONNX project's onnx.proto3 and are used under Apache-2.0 — see NOTICE.md.