Decoupled Contrastive Decoding via Expert-Aligned Drafting
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Computer Science > Computation and Language
Title:Decoupled Contrastive Decoding via Expert-Aligned Drafting
Abstract:Contrastive Decoding (CD) improves generation quality, but its amateur-model pass makes decoding expensive. Accelerating CD with speculative decoding raises a proposal-alignment question: should the contrastive signal shape the drafter, or should it remain only in verification? We study this question in the lightweight feature-level drafter regime. Two controlled diagnostics, matched Cross-alpha training and an Approximate Dual-Drafter decomposition, give the same diagnosis: contrastive-aware drafting does not consistently improve over expert-aligned drafting because the contrastive correction is usually weaker than drafter error, and reconstruction can amplify that error. We introduce Decoupled Contrastive Decoding (DCD), which drafts with an expert-aligned lightweight proposer and applies the amateur only in unchanged CD verification. Standard speculative verification preserves the vanilla-CD output distribution. Across the main 8B settings, EAGLE3-based DCD achieves average greedy speedups of 1.65 to 1.95x over vanilla CD and reduces MMLU proposal-path latency by about 5 to 12x relative to amateur-coupled proposal paths.
| Comments: | 28 pages, 11 figures, 20 tables. Code: this https URL |
| Subjects: | Computation and Language (cs.CL) |
| Cite as: | arXiv:2608.12913 [cs.CL] |
| (or arXiv:2608.12913v1 [cs.CL] for this version) | |
| https://doi.org/10.48550/arXiv.2608.12913
arXiv-issued DOI via DataCite (pending registration)
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