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

docs/release-notes/.FSharp.Core/11.0.100.md

@@ -23,6 +23,7 @@
 
 * Deduplicate repeated XML documentation (sort stability/complexity, `Parallel.For`, dynamic entry points) via the compile-time `<include>` tag; generated FSharp.Core.xml is unchanged. ([PR #20231](https://github.com/dotnet/fsharp/pull/20231))
 * Added modules for `Async`, `Task` and `ValueTask` with consistent `result`, `map`, `bind`, `ignore`, `catchWith`, `catch`, and `empty` functions ([LanguageSuggestion #1466](https://github.com/fsharp/fslang-suggestions/issues/1466), [PR #19844](https://github.com/dotnet/fsharp/pull/19844))
+* Most `Array` and `Array.Parallel` functions now return the shared empty-array singleton instead of allocating a fresh zero-length array when the result is empty (`collect`, `map`, `concat`, `append`, `zip`/`unzip`, `partition`, `filter`, `distinct`, the slicing functions, `zeroCreate`/`create`/`init` at length 0, and more). ([Issue #20382](https://github.com/dotnet/fsharp/issues/20382), [PR #20388](https://github.com/dotnet/fsharp/pull/20388))
 * Added conversion functions `Task.ofValueTask` and `ValueTask.ofTask`. ([LanguageSuggestion #1466](https://github.com/fsharp/fslang-suggestions/issues/1466), [PR #19844](https://github.com/dotnet/fsharp/pull/19844))
 * Add `Async.StartTaskImmediate`: passes the ambient `Async.CancellationToken` to a task factory, then await the result using `Async.Await` semantics. Overloads for `Task`, `Task<'T>`, `ValueTask`, `ValueTask<'T>` and task-like `.GetAwaiter()` (via SRTP). ([Language Suggestion #1284](https://github.com/fsharp/fslang-suggestions/issues/1284), [PR #20258](https://github.com/dotnet/fsharp/pull/20258))
 * Add `Async.RunSynchronouslyImmediate`: runs work on the calling thread until the first asynchronous suspension (as opposed to `RunSynchronously`, which immediately offloads if not on a background and/or threadpool thread). ([Issue #1042](https://github.com/fsharp/fslang-suggestions/issues/1042), [PR #19804](https://github.com/dotnet/fsharp/pull/19804))

src/FSharp.Core/array.fs

@@ -641,7 +641,7 @@ module Array =
         if i <> array.Length then
 
             let chunk1: 'U array =
-                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked ((array.Length >>> 2) + 1)
+                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty ((array.Length >>> 2) + 1)
 
             chunk1.[0] <- first
             let mutable count = 1
@@ -660,7 +660,7 @@ module Array =
 
             if i < array.Length then
                 let chunk2: 'U array =
-                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (array.Length - i)
+                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty (array.Length - i)
 
                 count <- 0
 
@@ -676,7 +676,7 @@ module Array =
                     i <- i + 1
 
                 let res: 'U array =
-                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (chunk1.Length + count)
+                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty (chunk1.Length + count)
 
                 Array.Copy(chunk1, res, chunk1.Length)
                 Array.Copy(chunk2, 0, res, chunk1.Length, count)
@@ -848,7 +848,7 @@ module Array =
                 if maskArrayLength = 0 then
                     null
                 else
-                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked<uint32> maskArrayLength
+                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty<uint32> maskArrayLength
 
             let mutable count =
                 match maskArray with
@@ -1032,7 +1032,7 @@ module Array =
 
         let private filterViaMask (maskArray: uint32 array | null) (leftoverMask: uint32) (count: int) (src: _ array) =
 
-            let dst = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked count
+            let dst = Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty count
 
             let mutable dstIdx = 0
 
@@ -1244,7 +1244,7 @@ module Array =
             empty
         else
             let res: 'T array array =
-                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked (len - windowSize + 1)
+                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty (len - windowSize + 1)
 
             for i = 0 to len - windowSize do
                 res.[i] <- Microsoft.FSharp.Primitives.Basics.Array.subUnchecked i windowSize array
@@ -1268,7 +1268,7 @@ module Array =
             let chunkCount = (len - 1) / chunkSize + 1
 
             let res: 'T array array =
-                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked chunkCount
+                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty chunkCount
 
             for i = 0 to len / chunkSize - 1 do
                 res.[i] <- Microsoft.FSharp.Primitives.Basics.Array.subUnchecked (i * chunkSize) chunkSize array
@@ -1860,7 +1860,7 @@ module Array =
                 Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked lenInner
 
             for i in 0 .. lenInner - 1 do
-                result.[i] <- Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked len
+                result.[i] <- Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty len
 
                 for j in 0 .. len - 1 do
                     result.[i].[j] <- array.[j].[i]
@@ -2397,7 +2397,7 @@ module Array =
             let chunks = createPartitionsUpToWithMinChunkSize array.Length 2 array // We need at least 2 elements/chunk for 'reduction'
 
             let chunkResults =
-                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked chunks.Length
+                Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty chunks.Length
 
             Parallel.For(
                 0,
@@ -2543,7 +2543,7 @@ module Array =
 
             for kvp in counts do
                 let arrayForThisGroup =
-                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUnchecked kvp.Value.Value
+                    Microsoft.FSharp.Primitives.Basics.Array.zeroCreateUncheckedNonEmpty kvp.Value.Value
 
                 finalResults.[finalIdx] <- getKey kvp.Key, arrayForThisGroup
                 finalResultsLookup[kvp.Key] <- kvp.Value, arrayForThisGroup

src/FSharp.Core/local.fs

@@ -987,10 +987,20 @@ module internal Array =
     let inline fastComparerForArraySort<'t when 't : comparison> () =
         LanguagePrimitives.FastGenericComparerCanBeNull<'t>
 
-    // The input parameter should be checked by callers if necessary
-    let inline zeroCreateUnchecked (count:int) =
+    // Allocates a new array of the given length without validating the count, always returning
+    // a freshly allocated array (even for count = 0). Prefer zeroCreateUnchecked unless the
+    // length is provably > 0; this variant exists only to skip the empty-array branch on hot,
+    // known-non-empty paths.
+    let inline zeroCreateUncheckedNonEmpty (count: int) =
         (# "newarr !0" type ('T) count : 'T array #)
 
+    // Allocates a new array of the given length without validating the count, but shares the
+    // empty-array singleton (System.Array.Empty<_>(), which the compiler emits for the [||]
+    // literal) when count = 0, so empty results allocate nothing. This is the default; use
+    // zeroCreateUncheckedNonEmpty only where the length is provably > 0 to skip the branch.
+    let inline zeroCreateUnchecked (count: int) : 'T array =
+        if count = 0 then [||] else zeroCreateUncheckedNonEmpty count
+
     let inline init (count:int) ([<InlineIfLambda>] f: int -> 'T) =
         if count < 0 then invalidArgInputMustBeNonNegative "count" count
         let arr = (zeroCreateUnchecked count : 'T array)
@@ -1069,7 +1079,7 @@ module internal Array =
     let scanSubRight f (array: _ array) start fin initState =
         let f = OptimizedClosures.FSharpFunc<_, _, _>.Adapt(f)
         let mutable state = initState
-        let res = zeroCreateUnchecked (fin-start+2)
+        let res = zeroCreateUncheckedNonEmpty (fin-start+2)
         res.[fin - start + 1] <- state
         for i = fin downto start do
             state <- f.Invoke(array.[i], state)
@@ -1166,7 +1176,7 @@ module internal Array =
             [| |]
         else
             let count = min count len
-            let res = zeroCreateUnchecked count : 'T array array
+            let res = zeroCreateUncheckedNonEmpty count : 'T array array
             let minChunkSize = len / count
             let mutable startIndex = 0
             for i = 0 to len % count - 1 do

src/FSharp.Core/local.fsi

@@ -84,7 +84,10 @@ module internal List =
     val tryLastV: 'T list -> 'T ValueOption
 
 module internal Array =
-    // The input parameter should be checked by callers if necessary
+    // Allocates without validating the count; always a fresh array. Use only where count > 0.
+    val inline zeroCreateUncheckedNonEmpty: int -> 'T array
+
+    // Allocates without validating the count, sharing the empty-array singleton when count = 0.
     val inline zeroCreateUnchecked: int -> 'T array
 
     val inline init: int -> (int -> 'T) -> 'T array

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/ArrayModule.fs

@@ -461,6 +461,63 @@ type ArrayModule() =
     member this.``Parallel.Collect`` () =
         this.CollectTester Array.Parallel.collect Array.Parallel.collect
 
+    [<Fact>]
+    member _.EmptyResultsShareSingleton () =
+        // A zero-length result should reuse the shared Array.empty singleton rather than
+        // allocating a fresh zero-length array (see issue #20382).
+        let isShared (a: 'a[]) = obj.ReferenceEquals(a, Array.empty<'a>)
+        let ints: int[] = [||]
+
+        Assert.True(isShared (Array.collect (fun x -> [| x |]) ints), "collect (empty input)")
+        Assert.True(isShared (Array.collect (fun _ -> [||]) [| 1; 2; 3 |]), "collect (all-empty results)")
+        Assert.True(isShared (Array.map (fun x -> x + 1) ints), "map")
+        Assert.True(isShared (Array.mapi (fun i x -> i + x) ints), "mapi")
+        Assert.True(isShared (Array.indexed ints), "indexed")
+        Assert.True(isShared (Array.concat ([]: int[] list)), "concat (empty)")
+        Assert.True(isShared (Array.concat [ [||]; [||] ]), "concat (of empties)")
+        Assert.True(isShared (Array.take 0 [| 1; 2; 3 |]), "take 0")
+        Assert.True(isShared (Array.skip 3 [| 1; 2; 3 |]), "skip all")
+        Assert.True(isShared (Array.sub [| 1; 2; 3 |] 1 0), "sub len 0")
+        Assert.True(isShared (Array.distinctBy id ints), "distinctBy")
+        Assert.True(isShared (Array.groupBy id ints), "groupBy")
+        Assert.True(isShared (Array.Parallel.map (fun x -> x + 1) ints), "Parallel.map")
+        Assert.True(isShared (Array.Parallel.collect (fun x -> [| x |]) ints), "Parallel.collect")
+
+        // Leaf builders that previously allocated a throwaway zero-length array for empty inputs.
+        Assert.True(isShared (Array.append ints ints), "append")
+        Assert.True(isShared (Array.map2 (fun a b -> a + b) ints ints), "map2")
+        Assert.True(isShared (Array.mapi2 (fun i a b -> i + a + b) ints ints), "mapi2")
+        Assert.True(isShared (Array.map3 (fun a b c -> a + b + c) ints ints ints), "map3")
+        Assert.True(isShared (Array.zip ints ints), "zip")
+        Assert.True(isShared (Array.zip3 ints ints ints), "zip3")
+        Assert.True(isShared (Array.allPairs ints ints), "allPairs")
+        Assert.True(isShared (Array.rev ints), "rev")
+        Assert.True(isShared (Array.distinct ints), "distinct")
+        Assert.True(isShared (Array.removeAt 0 [| 1 |]), "removeAt last")
+        Assert.True(isShared (Array.removeManyAt 0 1 [| 1 |]), "removeManyAt last")
+        Assert.True(isShared (Array.transpose [| ([||]: int[]) |]), "transpose (empty inner)")
+
+        let p1, p2 = Array.partition (fun _ -> true) ints
+        Assert.True(isShared p1 && isShared p2, "partition")
+        let u1, u2 = Array.unzip ([||]: (int * int)[])
+        Assert.True(isShared u1 && isShared u2, "unzip")
+        let t1, t2, t3 = Array.unzip3 ([||]: (int * int * int)[])
+        Assert.True(isShared t1 && isShared t2 && isShared t3, "unzip3")
+
+        Assert.True(isShared (Array.Parallel.mapi (fun i x -> i + x) ints), "Parallel.mapi")
+        Assert.True(isShared (Array.Parallel.choose Some ints), "Parallel.choose")
+        Assert.True(isShared (Array.Parallel.zip ints ints), "Parallel.zip")
+        Assert.True(isShared (Array.Parallel.filter (fun _ -> true) ints), "Parallel.filter")
+        Assert.True(isShared (Array.Parallel.groupBy id ints), "Parallel.groupBy")
+        let pp1, pp2 = Array.Parallel.partition (fun _ -> true) ints
+        Assert.True(isShared pp1 && isShared pp2, "Parallel.partition")
+
+        // Improve-by-default: even the count-based constructors share the singleton at length 0.
+        Assert.True(isShared (Array.zeroCreate 0: int[]), "zeroCreate 0")
+        Assert.True(isShared (Array.create 0 0), "create 0")
+        Assert.True(isShared (Array.init 0 id), "init 0")
+        Assert.True(isShared (Array.replicate 0 0), "replicate 0")
+
     [<Fact>]
     member this.compareWith() =
         // compareWith should work on empty arrays