Add GPU bitmask expansion for constrained sampling - #141
Merged
Conversation
MPSGraph subgraph that expands packed Int32 xgrammar bitmasks into Float16 logits masks entirely on GPU. Blocked tokens get -65504 (Float16 min), ensuring they are never selected by argmax or topK. Both MPSGraphArgmaxSampler and MPSGraphCompositeSampler gain: - bitmaskBuffer (lazy): shared-memory MTLBuffer, allocated on first use - constrainedExecutable (lazy): compiled graph with bitmask input, built on first applyBitmask: true call - encode(..., applyBitmask:) overloads on all encode paths Zero overhead for unconstrained generation — no bitmask buffer or constrained graph is compiled until the first constrained call. Existing protocol methods and behavior unchanged. Part 2 of 4 for GPU-based constrained sampling (apple#114).
stikves
requested review from
alejandro-isaza,
carinapeng,
kevchengcodes and
tjia1818
August 3, 2026 17:34
tjia1818
approved these changes
Aug 4, 2026
2 tasks
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
MPSGraph subgraph that expands packed Int32 xgrammar bitmasks into Float16 logits masks on Metal GPU.
Blocked tokens get -65504 (Float16 min), ensuring they are never selected by argmax or topK.
Both MPSGraphArgmaxSampler and MPSGraphCompositeSampler gain:
Existing protocol methods and behavior unchanged.
Part 2 of 4 for GPU-based constrained sampling (#114).