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vllm - 💡(How to fix) Fix [Feature]: support unbacked in export [1 comments, 1 participants]

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vllm-project/vllm#38843•Fetched 2026-04-08 02:34:33
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laithsakkalaithsakka
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laithsakkalaithsakka
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Code Example

dynamic_shapes = {
   "x": {0:  Dim("batch1", min=1, max=100, unbacked=True)},
   "y": {0:  Dim("batch1", min=1, max=100,  unbacked=True)},
}
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🚀 The feature, motivation and pitch

some context https://docs.google.com/document/d/17VMPSBflbQ0zCl-r2WkCRCH4t06PlrVoruFR0TkTy5M/edit?tab=t.0#heading=h.gq9bcerm8ep1

TLDR:
We want to do the following:

  1. Support unbacked in export Named Dims someone shall be able to say :
dynamic_shapes = {
   "x": {0:  Dim("batch1", min=1, max=100, unbacked=True)},
   "y": {0:  Dim("batch1", min=1, max=100,  unbacked=True)},
}

  1. This shall elevate the user experience! since Named dims in export imposes restriction that all shapes restrictions (guards) are encoded in spec. with the unbacked we "auto learn" restrictions that are encoded with torch._checks and not require users to be explicit about them. (see the example in the doc above).

  2. Shall we enable by default for named dims? at least for export? probably yes as step2 i do not forsee any regression regressions due to that.

Note: We want to extend the export dynamic shapes specs to be supported in compile in the near future.

Alternatives

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extent analysis

TL;DR

Enable unbacked Named Dims in export by modifying the dynamic_shapes configuration to include unbacked=True for the desired dimensions.

Guidance

  • Review the provided Google Doc for context and examples of using unbacked=True with Named Dims.
  • Update the dynamic_shapes dictionary to include unbacked=True for the desired dimensions, such as "x" and "y".
  • Consider enabling unbacked Named Dims by default for export, as it may improve user experience without introducing regressions.
  • Plan to extend the export dynamic shapes specs to be supported in compile in the near future.

Notes

The issue lacks specific technical details about the current implementation, so the guidance is based on the provided context and examples.

Recommendation

Apply workaround: Enable unbacked=True for Named Dims in export, as it seems to be a straightforward way to improve user experience without introducing regressions.

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