onnxruntime

0066c380 - [WebGPU EP] fixes fp16 accumulation bug in gemm kernel (#33057)

[WebGPU EP] fixes fp16 accumulation bug in gemm kernel (#33057) # 🚀 WebGPU FP16 MatMul Accuracy Fix ## 🐛 Bug The Qwen 2.5 0.5B GenAI WebGPU test produced logits that disagreed with the CPU reference. The issue was isolated to the generic packed FP16 MatMul kernel with shape `M=3, K=896, N=4864`. The shader stored its accumulator as `vec4<f16>`. Each addition was therefore rounded to FP16, causing significant error over the long `K=896` reduction. The worst single-node WebGPU-to-FP32 error was `0.7760754`. ## 🔧 Fix Packed MatMul now supports FP32 accumulation. FP16 inputs and workgroup storage remain unchanged, cached operands are widened to FP32 for multiplication and accumulation, and the result is converted back to the output type when written. ## 🎯 Scope This PR fixes FP16 accumulation in the generic packed ONNX `MatMul` implementation, covering both vec4 and scalar paths. FP16 atomic Split-K is disabled for this implementation to avoid rounding or overflowing partial sums before the final reduction. ONNX `Gemm` is outside this PR's scope. Its packed caller retains the default `use_f32_accumulation = false`, so FP16 `Gemm` accumulation is unchanged. The shared helpers remain opt-in; this PR does not enable FP32 accumulation for every caller or change vendor-specific MatMul kernels. ## ✅ Validation 1. **🧪 ONNX Runtime unit test** Added `MathOpTest.MatMulPackedFp16LongReductionUsesFloat32Accumulator`. Before the fix, it failed with expected `8.953125`, actual `9.140625`, and error `0.1875`. It passes after the fix. 2. **🤖 GenAI regression test** Ran `test_webgpu_paged_export_runs_prefill_and_decode`, the Qwen WebGPU test that exposed the issue. It passed with the rebuilt plugin package. The standalone full-model repro also passed with matching argmax (`4`) and a maximum logit difference of `0.101529`. --------- Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com> Co-authored-by: Prathik Rao <prathikrao@microsoft.com> Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>