Share the empty-array singleton for zero-length Array results by T-Gro · Pull Request #20388 · dotnet/fsharp

T-Gro · 2026-08-27T14:21:14Z

Fixes #20382

Array.collect/Array.map and their kin allocated a fresh zero-length array for empty results — Array.collect on empty input allocated two (the intermediate 'U[][] and the concat output). Now the internal allocation primitive zeroCreateUnchecked hands back the shared System.Array.Empty<_>() singleton when the length is 0, so empty results across Array and Array.Parallel — and even zeroCreate/create/init at length 0 — allocate nothing.

Empty-sharing is the default: the handful of sites where the length is provably > 0 opt out via zeroCreateUncheckedNonEmpty to skip the branch. Non-empty results are unaffected.

github-actions · 2026-08-27T14:21:28Z

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Lanayx · 2026-08-28T04:10:53Z

Shouldn't the same optimization be done for empty list as well?

Numpsy · 2026-08-28T08:24:04Z

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 ICollection special case

T-Gro · 2026-09-01T08:50:44Z

@Lanayx : List already flows trough the shared singleton.
@Numpsy : Let's tackle this as well by changing the central helpers for array creation.

Array.collect/Array.map and many sibling builders allocated a fresh zero-length array for empty results (Array.collect on empty input allocated two). Make the internal allocation primitives Basics.Array.zeroCreateUnchecked and Basics.List.arrayZeroCreate return the shared System.Array.Empty<_>() singleton (via the [||] literal) when count = 0, so empty results across Array, Array.Parallel and the toArray conversions (List.toArray, Array.ofList/ofSeq, Seq.toArray, Set/Map.toArray) allocate nothing. Array.copy and Seq.toArray no longer clone an empty source array, and Array.insertManyAt reuses Array.copy. The count = 0 check is a single well-predicted branch; provably-non-empty callers pay only that. Empty F# lists are already the interned FSharpList.Empty singleton, so there is no list-result change. The empty-guard inlines into FSharp.Core's methods, growing the trimmed FSharp.Core.dll by 512 bytes; trimming baselines updated accordingly. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…mpty-perf

T-Gro

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro · 2026-09-10T13:03:19Z

All done.

…mpty-perf

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

T-Gro · 2026-09-16T09:18:54Z

@copilot resolve the merge conflicts in this pull request

…mpty-perf # Conflicts: # tests/AheadOfTime/Trimming/check.ps1 Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Copilot · 2026-09-16T09:30:27Z

@copilot resolve the merge conflicts in this pull request

Merged origin/main in 07f5988. The only conflict was tests/AheadOfTime/Trimming/check.ps1: main lowered the net9.0 baselines by 1024 bytes while this branch raised them by 512, so the merged expectations are the combined deltas (FSharp.Core.dll 310784, StaticLinkedFSharpCore_Trimming_Test.dll 9174016). src/FSharp.Core/array.fs/.fsi auto-merged with main's new inline iter2/iteri/forall2 etc., and FSharp.Core builds clean in Release.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…#20388) * Share the empty-array singleton for zero-length results (dotnet#20382) Array.collect/Array.map and many sibling builders allocated a fresh zero-length array for empty results (Array.collect on empty input allocated two). Make the internal allocation primitives Basics.Array.zeroCreateUnchecked and Basics.List.arrayZeroCreate return the shared System.Array.Empty<_>() singleton (via the [||] literal) when count = 0, so empty results across Array, Array.Parallel and the toArray conversions (List.toArray, Array.ofList/ofSeq, Seq.toArray, Set/Map.toArray) allocate nothing. Array.copy and Seq.toArray no longer clone an empty source array, and Array.insertManyAt reuses Array.copy. The count = 0 check is a single well-predicted branch; provably-non-empty callers pay only that. Empty F# lists are already the interned FSharpList.Empty singleton, so there is no list-result change. The empty-guard inlines into FSharp.Core's methods, growing the trimmed FSharp.Core.dll by 512 bytes; trimming baselines updated accordingly. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> * Address review feedback Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> * Clarify empty-array sharing in copy and insertManyAt docs Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> * Fix CI failures Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: perf-bundle <perf@local> Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Co-authored-by: Copilot <copilot@github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

T-Gro requested a review from a team as a code owner August 27, 2026 14:21

github-project-automation Bot added this to F# Compiler and Tooling Aug 27, 2026

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T-Gro marked this pull request as draft August 27, 2026 22:00

T-Gro force-pushed the t-gro-array-collect-empty-perf branch from 5ce67ee to a0cb3f5 Compare August 27, 2026 23:16

T-Gro force-pushed the t-gro-array-collect-empty-perf branch from 677538e to 12c7027 Compare September 1, 2026 08:21

T-Gro force-pushed the t-gro-array-collect-empty-perf branch 3 times, most recently from 9290046 to 3f5f8ce Compare September 1, 2026 12:25

T-Gro force-pushed the t-gro-array-collect-empty-perf branch from 3f5f8ce to aadb137 Compare September 1, 2026 12:58

Merge remote-tracking branch 'origin/main' into t-gro-array-collect-e…

1748e9e

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T-Gro marked this pull request as ready for review September 1, 2026 14:42

T-Gro added the NO_RELEASE_NOTES Label for pull requests which signals, that user opted-out of providing release notes label Sep 1, 2026

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T-Gro enabled auto-merge (squash) September 1, 2026 14:42

T-Gro commented Sep 9, 2026

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Copilot added 2 commits September 10, 2026 18:08

Merge remote-tracking branch 'origin/main' into t-gro-array-collect-e…

39644e8

…mpty-perf

Merge remote-tracking branch 'origin/main' into t-gro-array-collect-e…

74763d7

…mpty-perf

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Merge remote-tracking branch 'origin/main' into t-gro-array-collect-e…

068debd

…mpty-perf

Clarify empty-array sharing in copy and insertManyAt docs

9af1127

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot started work on behalf of T-Gro September 16, 2026 09:19 View session

Merge remote-tracking branch 'origin/main' into t-gro-array-collect-e…

07f5988

…mpty-perf # Conflicts: # tests/AheadOfTime/Trimming/check.ps1 Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

auto-merge was automatically disabled September 16, 2026 09:30
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abonie approved these changes Sep 16, 2026

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Fix CI failures

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro merged commit 1636ef4 into main Sep 16, 2026
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T-Gro deleted the t-gro-array-collect-empty-perf branch September 16, 2026 19:20

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[release/11.0.1xx] Source code updates from dotnet/fsharp dotnet/dotnet#9425

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Share the empty-array singleton for zero-length Array results - #20388

Share the empty-array singleton for zero-length Array results#20388
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