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Rethinking Cross-Layer Information Routing in Diffusion Transformer
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Diagnoses residual-stream routing in DiTs (magnitude inflation, gradient decay, block redundancy) and proposes DAR, a timestep-adaptive softmax attention over preceding sublayer outputs that replaces the fixed residual sum, improving SiT-XL/2 on ImageNet 256 and stacking with REPA.
Problem
Standard pre-norm residual routing in Diffusion Transformers is fixed, time-agnostic and uniformly weighted, leading to PreNorm dilution-like symptoms across depth, and it may be suboptimal for the time-varying denoising process.
Approach
Replace the unit-weight residual sum with a learnable softmax attention over all preceding sublayer outputs, with queries (static, dynamic, or timestep-injected) that make routing timestep-adaptive, using chunked summaries to bound memory.
Key results
- DAR static c4 reaches FID 6.92 (SDE, no CFG) on ImageNet 256 at 600K iterations versus 9.67 ODE for SiT baseline at 1.75M iterations.
- DAR matches the SiT baseline's quality (FID 9.67) in roughly 8.75x fewer iterations, and DAR+REPA reaches FID 7.09 at 100K vs 9.89 for REPA.
- Under SDE with CFG, DAR static c4 outperforms U-DiT-L by 0.77 FID with 0.83x parameters.
Surprising findings
- Dynamic query variant gives a sharp departure from the LLM AttnRes finding, where dynamic queries improve only marginally over static ones.
- Counterfactual gate gradients on the unmodified SiT baseline already vary with timestep, suggesting latent timestep-dependent source preferences.
Limitations
- Experiments mostly on SiT-XL/2 at 256x256 ImageNet; scaling to large MM-DiT/video DiT is left to future work.
- Large-scale T2I evaluation is limited to a preliminary DMD experiment with qualitative and small quantitative comparison.
- Chunk size analysis relies on a simplified rate-distortion model with an alpha parameter fit only qualitatively.
Experimental setup
Datasets: ImageNet-1K GenEval2 prompts
Benchmarks: ImageNet 256x256
Metrics: FID sFID IS Precision Recall LPIPS RAPSD high-frequency ratio deviation
Baselines: DiT SiT SiT-Plus U-ViT U-DiT REPA Attention Residuals
Builds on: Attention Residuals SiT REPA DiT U-ViT
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