Description

In a simple copy-loop Span is a lot slower than an array-version. I’d expect that Span and array have similar perf.

Note: Span_CopyTo is just for reference included.

Benchmark

Results

BenchmarkDotNet=v0.10.13, OS=Windows 10 Redstone 3 [1709, Fall Creators Update] (10.0.16299.309)
Intel Core i7-7700HQ CPU 2.80GHz (Kaby Lake), 1 CPU, 8 logical cores and 4 physical cores
Frequency=2742189 Hz, Resolution=364.6722 ns, Timer=TSC
.NET Core SDK=2.1.300-preview3-008384
  [Host]     : .NET Core 2.1.0-preview2-26313-01 (CoreCLR 4.6.26310.01, CoreFX 4.6.26313.01), 64bit RyuJIT
  DefaultJob : .NET Core 2.1.0-preview2-26313-01 (CoreCLR 4.6.26310.01, CoreFX 4.6.26313.01), 64bit RyuJIT

Code

public class Benchmarks
{
    private readonly int[] _source;
    private readonly int[] _destination;

    public Benchmarks()
    {
        _source = Enumerable.Range(0, 10_000).ToArray();
        _destination = new int[_source.Length];
    }

    [Benchmark(Baseline = true)]
    public void Array()
    {
        Copy(_source, _destination);
    }

    [Benchmark]
    public void Span()
    {
        Copy(_source.AsSpan(), _destination.AsSpan());
    }

    [Benchmark]
    public void Span_CopyTo()
    {
        _source.AsSpan().CopyTo(_destination.AsSpan());
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static void Copy(int[] src, int[] dst)
    {
        for (int i = 0; i < src.Length; ++i)
            dst[i] = src[i];
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static void Copy(ReadOnlySpan<int> src, Span<int> dst)
    {
        for (int i = 0; i < src.Length; ++i)
            dst[i] = src[i];
    }
}

Disassembly

Note: The JitDisasm is produced of fresh coreclr-build on ubuntu.

.NET Core SDK (reflecting any global.json):
 Version:   2.1.300-preview3-008384
 Commit:    4343118151

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  16.04
 OS Platform: Linux
 RID:         ubuntu.16.04-x64
 Base Path:   /usr/share/dotnet/sdk/2.1.300-preview3-008384/

Host (useful for support):
  Version: 2.1.0-preview3-26319-04
  Commit:  939333dbc8

.NET Core SDKs installed:
  2.1.4 [/usr/share/dotnet/sdk]
  2.1.300-preview3-008384 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0-preview2-30338 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0-preview2-30338 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.5 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0-preview2-26313-01 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0-preview3-26319-04 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Also only the loops are shown.

Array-variant

G_M16768_IG04:
       4863C8               movsxd   rcx, eax
       448B448F10           mov      r8d, dword ptr [rdi+4*rcx+16]
       4489448E10           mov      dword ptr [rsi+4*rcx+16], r8d
       FFC0                 inc      eax
       3BD0                 cmp      edx, eax
       7FED                 jg       SHORT G_M16768_IG04

Span-variant

G_M16768_IG03:
       413BDE               cmp      ebx, r14d
       732D                 jae      SHORT G_M16768_IG05
       4863FB               movsxd   rdi, ebx
       3B5DE0               cmp      ebx, dword ptr [rbp-20H]
       7325                 jae      SHORT G_M16768_IG05
       488B45D8             mov      rax, bword ptr [rbp-28H]
       8B04B8               mov      eax, dword ptr [rax+4*rdi]
       418904BF             mov      dword ptr [r15+4*rdi], eax
       FFC3                 inc      ebx
       488D7DD8             lea      rdi, bword ptr [rbp-28H]
       E8278F5CFF           call     System.ReadOnlySpan`1[Int32][System.Int32]:get_Length():int:this
       3BC3                 cmp      eax, ebx
       7FD9                 jg       SHORT G_M16768_IG03

category:cq theme:loop-opt skill-level:expert cost:medium