Fix Lion to use decoupled weight decay in the Triton 32-bit kernel#2001
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matthewdouglas merged 2 commits intoJul 16, 2026
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Follow-up to bitsandbytes-foundation#1993, which fixed the same coupled-vs-decoupled weight-decay bug for Lion in the default backend and the CUDA 32-bit kernel. matthewdouglas invited a Triton follow-up when approving bitsandbytes-foundation#1993. _optimizer_update_1state_32bit_triton_kernel folded weight decay into the gradient (coupled L2) for every 1-state optimizer, Lion included. Lion requires *decoupled* (AdamW-style) decay applied to the param directly (p *= 1 - lr*wd), outside the sign update (Chen et al. 2023); folding decay into the gradient corrupts the input to sign(), changing the update direction, not just its magnitude. This mirrors the CUDA 32-bit fix exactly: exclude LION (id 4) from the gradient fold, and shrink the param in the LION branch before the sign update. The existing test_lion32bit_weight_decay (added in bitsandbytes-foundation#1993) is parametrized over get_available_devices(), so it already exercises this kernel on XPU hardware in CI. Like bitsandbytes-foundation#1993's CUDA change, the Triton kernel is XPU-only and cannot be built or run on the available hardware. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
matthewdouglas
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@jiqing-feng Would you mind checking the optimizer tests for this on XPU? At first glance it looks correct to me. Thanks in advance! |
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The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. |
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Verified it's okay on XPU with the command |
matthewdouglas
merged commit Jul 16, 2026
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Follow-up to #1993, which fixed the same coupled-vs-decoupled weight-decay bug for Lion in the default backend and the CUDA 32-bit kernel. matthewdouglas invited a Triton follow-up when approving #1993.
_optimizer_update_1state_32bit_triton_kernel folded weight decay into the gradient (coupled L2) for every 1-state optimizer, Lion included. Lion requires decoupled (AdamW-style) decay applied to the param directly (p = 1 - lrwd), outside the sign update (Chen et al. 2023); folding decay into the gradient corrupts the input to sign(), changing the update direction, not just its magnitude. This mirrors the CUDA 32-bit fix exactly: exclude LION (id 4) from the gradient fold, and shrink the param in the LION branch before the sign update.
The existing test_lion32bit_weight_decay (added in #1993) is parametrized over get_available_devices(), so it already exercises this kernel on XPU hardware in CI. Like #1993's CUDA change, the Triton kernel is XPU-only and cannot be built or run on the available hardware.