Optimize Array Slicing Performance (dotnet#18778)

Author: albert-du

docs/release-notes/.FSharp.Core/10.0.100.md

@@ -7,5 +7,8 @@
 ### Changed
 
 * Random functions support for zero element chosen/sampled ([PR #18568](https://github.com/dotnet/fsharp/pull/18568))
+* Optimize array slicing performance. ([PR #18778](https://github.com/dotnet/fsharp/pull/18778))
 
 ### Breaking Changes
+
+* 1D array slicing now returns an empty array singleton instead of allocating a new array when the result is empty. ([PR #18778](https://github.com/dotnet/fsharp/pull/18778))

src/FSharp.Core/prim-types.fs

@@ -779,17 +779,17 @@ namespace Microsoft.FSharp.Core
 
             let inline SetArray (target: 'T array) (index:int) (value:'T) =  (# "stelem.any !0" type ('T) target index value #)  
 
-            let inline GetArraySub arr (start:int) (len:int) =
-                let len = if len < 0 then 0 else len
-                let dst = zeroCreate len   
-                for i = 0 to len - 1 do 
-                    SetArray dst i (GetArray arr (start + i))
-                dst
-
-            let inline SetArraySub arr (start:int) (len:int) (src:_ array) =
-                for i = 0 to len - 1 do 
-                    SetArray arr (start+i) (GetArray src i)
+            let inline GetArraySub (arr: 'a array) (start:int) (len:int) : 'a array =
+                if len <= 0 then
+                    [||]
+                else
+                    let dst = zeroCreate len
+                    Array.Copy(arr, start, dst, 0, len)
+                    dst
 
+            let inline SetArraySub (arr: 'T array) (start:int) (len:int) (src: 'T array) =
+                if len > 0 then
+                    Array.Copy(src, 0, arr, start, len)
 
             let inline GetArray2D (source: 'T[,]) (index1: int) (index2: int) = (# "ldelem.multi 2 !0" type ('T) source index1 index2 : 'T #)  
 

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

@@ -1962,6 +1962,24 @@ type ArrayModule() =
         Assert.AreEqual(arr.[-4..(-3)], ([||]: int array))
 
 
+    [<Fact>]
+    member _.``Slicing creates new non-empty arrays`` () =
+        let arr = [|1;2;3;4;5;6|]
+
+        Assert.False(Object.ReferenceEquals(arr[1..3], arr[1..3]))
+        Assert.False(Object.ReferenceEquals(arr[0..], arr[0..]))
+        Assert.False(Object.ReferenceEquals(arr[..^1], arr[..^1]))
+        
+    [<Fact>]
+    member _.``Empty slices are referentially equivalent`` () =
+        let arr = [|1;2;3;4;5;6|]
+
+        Assert.AreEqual(arr[1..-1].Length, 0)
+        Assert.True(Object.ReferenceEquals(arr[1..-1], arr[1..-1]))
+        
+        Assert.True(Object.ReferenceEquals(arr[1..-1], ([||]: int array)))
+
+
     [<Fact>]
     member _.RandomShuffle() =
         let arr = [| 1..20 |]

tests/benchmarks/CompiledCodeBenchmarks/MicroPerf/ArraySlicing.fs

@@ -0,0 +1,15 @@
+module ArraySlicing
+
+open BenchmarkDotNet.Attributes
+open System
+
+[<SimpleJob(launchCount = 2, warmupCount = 1, iterationCount = 2)>]
+[<GcServer(true)>]
+[<MemoryDiagnoser>]
+[<MarkdownExporterAttribute.GitHub>]
+type ArraySlicingBenchmark() =
+    let b = [| for _ in 1 .. 100_000 -> byte DateTime.Now.Ticks |]
+
+    [<Benchmark>]
+    member _.x () =
+        b[ 20 .. 4999 ]

tests/benchmarks/CompiledCodeBenchmarks/MicroPerf/MicroPerf.fsproj

@@ -26,6 +26,7 @@
     <Compile Include="Equality\OptionsAndCo.fs" />
     <Compile Include="Equality\ExactEquals.fs" />
     <Compile Include="Async.fs" />
+    <Compile Include="ArraySlicing.fs" />
     <Compile Include="Conditions.fs" />
     <Compile Include="Collections.fs" />
     <Compile Include="Program.fs" />