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[ET-VK][test] Allow FP16 underflow in SwiGLU comparisons - #23719
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Pull Request resolved: #23716 ## Ulterior Motive Keep `VulkanSwiGLUTest` correctness checks usable on Vulkan devices that flush FP16 subnormal values to zero. ## Rationale **What**: Increase the absolute error allowances in `MatchesUnfusedAcrossResizes` to account for FP16 underflow. **Why**: On stable, all 28 FP16 configurations fail on Adreno 750 while FP32 passes. The first expected output is `5.30033e-6`, below FP16's minimum normal value, but the shader returns zero. The unfused path can also flush subnormal inputs and intermediate tensors before later multiplications amplify the difference. ## Details - Fused versus CPU reference: - **Before**: Absolute allowance `1e-7`. - **After**: Absolute allowance `2^-14`, FP16's minimum normal value; relative allowance remains `1e-3`. - Fused versus unfused graph: - **Before**: Absolute allowance `6e-8 * (abs(x*y) + abs(y) + 1)` assumes subnormal preservation. - **After**: Keep the original rounding allowance and add `(1 + 1e-3) * abs(reference)` only when an FP16 input is subnormal or a computed intermediate/output can round below the minimum normal value. Use the existing `3e-3` relative tolerance as the rounding margin at that boundary. - If the unfused result flushes to zero, its relative tolerance vanishes. `abs(reference)` covers the lost output, and the extra `1e-3` covers the fused result's relative error. - Cases without underflow keep the small original absolute allowance: `x=20, y=2` gives `2.58e-6`; the original tiny-output case gives about `6.63e-6`. - FP32 uses its existing assertions. Both FP16 comparisons keep `ASSERT_NEAR`. Differential Revision: [D124833698](https://our.internmc.facebook.com/intern/diff/D124833698/) ghstack-source-id: 446951678
🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/23719
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This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #23716 by @SS-JIA
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/SS-JIA/669/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/SS-JIA/669/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/SS-JIA/669/orig
Differential Revision: D124833698
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