Iteration fast path, flat update frames, and flattened combinators - #21545
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The UpdatingVM walks every updating opcode of every list item on every render: cache groups (JumpIfNotModifiedOpcode) exist only at component boundaries, so a list of plain template rows revalidates every binding even when nothing in a row changed. Collect each item's consumed tags in a tracking frame (via a new frame-finalizer hook on UpdatingVMFrame) and skip the item's entire subtree while that combined tag validates. Trivial items opt out: for a text node or two, validating a combined tag costs as much as updating, so collection would be pure overhead. An item is trivial when it has <= 2 opcodes and no nested block -- a nested block child means an arbitrarily large subtree hides behind a small top-level count. dbmon-style workloads (fat rows, sparse changes): ~1.6x fps at 8x CPU throttle, ~6x (rAF-capped) at 4x. Dense-change / tiny-item workloads and the krausest bench: neutral. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Isolated same-batch measurement (today's
Reading the increment over #21544 alone (cross-batch ratios): dbmon is carried almost entirely by the subtree-skip; the fast path's own contribution shows on the random-update benches (5%: 1.5x → 2.2x; 25%: 1.8x → 2.4x vs main), where same-order streaming compare does the most work. The +5.5% on incrementing is borderline noise but worth a re-measure before merge if it matters — happy to run a deeper sample. 🤖 Generated with Claude Code |
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Three allocation/dispatch levers for the update path, measured together with subtree-skipping as the difference between ~14 and ~27 fps on an 8x-throttled dbmon: - streaming same-order list compare: when a list's order is unchanged (the overwhelmingly common case), items are matched in a single streaming pass writing into a scratch item (nextInto) instead of allocating an IterationItem per step, falling back to the general diff via a reconstructed prefix iterator on first mismatch - flat frame stack: the updating VM keeps parallel arrays indexed by depth instead of allocating an UpdatingVMFrame per block per render - combinator flattening: combine() flattens nested combinators and drops constants (capped) so validating a combined tag is one flat loop instead of a pointer-chasing tree walk Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Converting to draft as this seems like it is not ready |
kategengler
marked this pull request as draft
August 11, 2026 16:13
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Superseded by #21557 |
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Stacked on #21544 (its first commit is that PR; review the
Iteration fast path…commit here). Full suite green locally with both commits: 9443 tests, 0 failures.Three allocation/dispatch levers for the update path, extracted from the #21520 spike:
nextIntoonArrayIterator, allocation-free) instead of allocating anIterationItemper step; on first mismatch it falls back to the general diff via a reconstructed-prefix iterator, so reorders behave exactly as today.UpdatingVMFrameper block per render.combine()flattens nested combinators and drops constants (capped at 64) so validating a combined tag is one flat loop instead of a pointer-chasing tree walk.Measured together with #21544's subtree-skip as the difference between ~14 and ~27 fps on 8x-throttled dbmon during the spike campaign; no observable behavior change.
Related: #21520 (spike), #21543 (pools), #21544 (subtree-skip, base of this stack).
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