Share the empty-array singleton for zero-length Array results - #20388
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Share the empty-array singleton for zero-length Array results#20388T-Gro wants to merge 4 commits into
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Add a central Basics.Array.zeroCreateUncheckedOrEmpty primitive that returns the shared System.Array.Empty<_>() singleton (via the [||] literal) when count = 0, and route the reachable-zero allocation sites through it: subUnchecked (all slicing), concatArrays (concat + collect output), collect/map/mapi/indexed and their Array.Parallel counterparts. Existing 'if length = 0' guards now return [||] too. This removes the redundant zero-length array allocations reported in Array.collect (intermediate 'U[][] plus concat output) and Array.map with empty inputs, without adding per-function length checks (the single check lives in the primitive; fill loops naturally no-op at length 0). Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Route the remaining reachable-zero allocation sites through zeroCreateUncheckedOrEmpty: append, distinct, map2/mapi2/map3, partition, partitionWith (+ scatterPartitioned), zip/zip3, allPairs, unzip/unzip3, rev, removeAt/removeManyAt, transpose, and the Array.Parallel choose/mapi/zip/filter/ partition/partitionWith/groupBy builders. Count-based constructors (zeroCreate/ create/init/replicate/random*), the sort helpers and provably-nonzero sites keep the branch-free zeroCreateUnchecked. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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August 27, 2026 22:00
Invert the primitive: zeroCreateUnchecked now shares the empty-array singleton when count = 0 (the improve-by-default behavior), and a new zeroCreateUncheckedNonEmpty keeps the branch-free newarr for the ~18 sites where the length is provably > 0 (countBy/groupBy/choose chunks, filter, windowed, chunkBySize, insert/update, reduceBy, scanSubRight, splitInto, ...). This shrinks the diff and makes every reachable-zero site - including the count-based constructors (zeroCreate/create/init/replicate) - share the singleton for free, instead of each caller opting in. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Shouldn't the same optimization be done for empty list as well? |
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I think there's a few other places that could possibly do the same. e.g. Seq.toArray appears to be already special casing an empty array in the general fallback path, but not in the |
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Fixes #20382
Array.collect/Array.mapand their kin allocated a fresh zero-length array for empty results —Array.collecton empty input allocated two (the intermediate'U[][]and the concat output). Now the internal allocation primitivezeroCreateUncheckedhands back the sharedSystem.Array.Empty<_>()singleton when the length is 0, so empty results acrossArrayandArray.Parallel— and evenzeroCreate/create/initat length 0 — allocate nothing.Empty-sharing is the default: the handful of sites where the length is provably
> 0opt out viazeroCreateUncheckedNonEmptyto skip the branch. Non-empty results are unaffected.