runtime: Span.CopyTo is 10-20% slower than Array.CopyTo

How to run the benchmarks:

git clone https://github.com/dotnet/performance.git
# if you have .NET Core 3.0 installed
dotnet run -c Release -f netcoreapp3.0 -p .\performance\src\benchmarks\micro\MicroBenchmarks.csproj --filter *CopyTo<String>*.Array *CopyTo<String>.Span --join --affinity 2
# if you don't have .NET Core 3.0 installed
py .\performance\scripts\benchmarks_ci.py -f netcoreapp3.0 --filter *CopyTo<String>*.Array *CopyTo<String>.Span --bdn-arguments="--join true --affinity 2"

Method	Size	Mean
Array	2048	511.1 ns
Span	2048	609.9 ns

Full docs for the new benchmarking workflow: https://github.com/dotnet/performance/blob/master/docs/benchmarking-workflow-corefx.md

category:cq theme:optimization skill-level:expert cost:medium

About this issue

Original URL
State: open
Created 5 years ago
Reactions: 3
Comments: 21 (21 by maintainers)

Most upvoted comments

The jit doesn’t see string here, it sees __Canon. If we inline enough we can sometimes get all this into unshared code and figure out the actual types, but not always…

AndyAyersMS on Feb 24, 2020

Contrast this against the codegen for the generic CopyTo test:

; CopyTo<string>.Array

00007ff9`b6fab690 48894c2408      mov     qword ptr [rsp+8],rcx
00007ff9`b6fab695 488b4908        mov     rcx,qword ptr [rcx+8]
00007ff9`b6fab699 4c8b442408      mov     r8,qword ptr [rsp+8]
00007ff9`b6fab69e 498b5018        mov     rdx,qword ptr [r8+18h]
00007ff9`b6fab6a2 458b4020        mov     r8d,dword ptr [r8+20h]
00007ff9`b6fab6a6 e95d66d8ff      jmp     <Array.CopyTo(Array, Array, int)>

; CopyTo<string>.Span

00007ff9`b6fdbb40 57              push    rdi
00007ff9`b6fdbb41 56              push    rsi
00007ff9`b6fdbb42 4883ec68        sub     rsp,68h
00007ff9`b6fdbb46 488bf1          mov     rsi,rcx
00007ff9`b6fdbb49 488d7c2420      lea     rdi,[rsp+20h]
00007ff9`b6fdbb4e b910000000      mov     ecx,10h
00007ff9`b6fdbb53 33c0            xor     eax,eax
00007ff9`b6fdbb55 f3ab            rep stos dword ptr [rdi]
00007ff9`b6fdbb57 488bce          mov     rcx,rsi
00007ff9`b6fdbb5a 48894c2460      mov     qword ptr [rsp+60h],rcx
00007ff9`b6fdbb5f 488bf1          mov     rsi,rcx
00007ff9`b6fdbb62 488b0e          mov     rcx,qword ptr [rsi] ; classHnd
00007ff9`b6fdbb65 48ba80f407b7f97f0000 mov rdx,7FF9B707F480h  ; signature
00007ff9`b6fdbb6f e88c88825f      call    CoreCLR!JIT_GenericHandleClass (00007ffa`16804400)
00007ff9`b6fdbb74 488bf8          mov     rdi,rax
00007ff9`b6fdbb77 488bd7          mov     rdx,rdi
00007ff9`b6fdbb7a 4c8b4608        mov     r8,qword ptr [rsi+8]
00007ff9`b6fdbb7e 488d4c2440      lea     rcx,[rsp+40h]
00007ff9`b6fdbb83 e810daffff      call    <Span<__Canon>.ctor(__Canon[])>
00007ff9`b6fdbb88 488b542440      mov     rdx,qword ptr [rsp+40h]
00007ff9`b6fdbb8d 4889542450      mov     qword ptr [rsp+50h],rdx
00007ff9`b6fdbb92 8b542448        mov     edx,dword ptr [rsp+48h]
00007ff9`b6fdbb96 89542458        mov     dword ptr [rsp+58h],edx
00007ff9`b6fdbb9a 488bd7          mov     rdx,rdi
00007ff9`b6fdbb9d 4c8b4618        mov     r8,qword ptr [rsi+18h]
00007ff9`b6fdbba1 488d4c2430      lea     rcx,[rsp+30h]
00007ff9`b6fdbba6 e8edd9ffff      call    CLRStub[MethodDescPrestub]@7ff9b6fd9598 (00007ff9`b6fd9598)
00007ff9`b6fdbbab 488d4c2450      lea     rcx,[rsp+50h]
00007ff9`b6fdbbb0 488bd7          mov     rdx,rdi
00007ff9`b6fdbbb3 4c8d442420      lea     r8,[rsp+20h]
00007ff9`b6fdbbb8 488b442430      mov     rax,qword ptr [rsp+30h]
00007ff9`b6fdbbbd 498900          mov     qword ptr [r8],rax
00007ff9`b6fdbbc0 8b442438        mov     eax,dword ptr [rsp+38h]
00007ff9`b6fdbbc4 41894008        mov     dword ptr [r8+8],eax
00007ff9`b6fdbbc8 4c8d442420      lea     r8,[rsp+20h]
00007ff9`b6fdbbcd e85edaffff      call    CLRStub[MethodDescPrestub]@7ff9b6fd9630 (00007ff9`b6fd9630)
00007ff9`b6fdbbd2 90              nop
00007ff9`b6fdbbd3 4883c468        add     rsp,68h
00007ff9`b6fdbbd7 5e              pop     rsi
00007ff9`b6fdbbd8 5f              pop     rdi
00007ff9`b6fdbbd9 c3              ret

; Span<__Canon>.ctor(__Canon[])

00007ff9`b6fdba50 57              push    rdi
00007ff9`b6fdba51 56              push    rsi
00007ff9`b6fdba52 55              push    rbp
00007ff9`b6fdba53 53              push    rbx
00007ff9`b6fdba54 4883ec28        sub     rsp,28h
00007ff9`b6fdba58 c5f877          vzeroupper
00007ff9`b6fdba5b 4889542420      mov     qword ptr [rsp+20h],rdx
00007ff9`b6fdba60 488bd9          mov     rbx,rcx
00007ff9`b6fdba63 488bfa          mov     rdi,rdx
00007ff9`b6fdba66 498bf0          mov     rsi,r8
00007ff9`b6fdba69 4885f6          test    rsi,rsi
00007ff9`b6fdba6c 7511            jne     System_Private_CoreLib!System.Span`1[[System.__Canon, System.Private.CoreLib]]..ctor(System.__Canon[])+0xffffffff`a3af5ddf (00007ff9`b6fdba7f)
00007ff9`b6fdba6e c5f857c0        vxorps  xmm0,xmm0,xmm0
00007ff9`b6fdba72 c5fa7f03        vmovdqu xmmword ptr [rbx],xmm0
00007ff9`b6fdba76 4883c428        add     rsp,28h
00007ff9`b6fdba7a 5b              pop     rbx
00007ff9`b6fdba7b 5d              pop     rbp
00007ff9`b6fdba7c 5e              pop     rsi
00007ff9`b6fdba7d 5f              pop     rdi
00007ff9`b6fdba7e c3              ret
00007ff9`b6fdba7f 488bce          mov     rcx,rsi
00007ff9`b6fdba82 e889988d5f      call    CoreCLR!ObjectNative::GetClass (00007ffa`168b5310)
00007ff9`b6fdba87 488be8          mov     rbp,rax
00007ff9`b6fdba8a 488b4f30        mov     rcx,qword ptr [rdi+30h]
00007ff9`b6fdba8e 488b09          mov     rcx,qword ptr [rcx]
00007ff9`b6fdba91 488b4908        mov     rcx,qword ptr [rcx+8]
00007ff9`b6fdba95 4885c9          test    rcx,rcx
00007ff9`b6fdba98 7402            je      System_Private_CoreLib!System.Span`1[[System.__Canon, System.Private.CoreLib]]..ctor(System.__Canon[])+0xffffffff`a3af5dfc (00007ff9`b6fdba9c)
00007ff9`b6fdba9a eb15            jmp     System_Private_CoreLib!System.Span`1[[System.__Canon, System.Private.CoreLib]]..ctor(System.__Canon[])+0xffffffff`a3af5e11 (00007ff9`b6fdbab1)
00007ff9`b6fdba9c 488bcf          mov     rcx,rdi
00007ff9`b6fdba9f 48ba40060ab7f97f0000 mov rdx,7FF9B70A0640h
00007ff9`b6fdbaa9 e85289825f      call    CoreCLR!JIT_GenericHandleClass (00007ffa`16804400)
00007ff9`b6fdbaae 488bc8          mov     rcx,rax
00007ff9`b6fdbab1 e8eaed825f      call    CoreCLR!JIT_GetRuntimeType (00007ffa`1680a8a0)
00007ff9`b6fdbab6 483bc5          cmp     rax,rbp
00007ff9`b6fdbab9 7516            jne     System_Private_CoreLib!System.Span`1[[System.__Canon, System.Private.CoreLib]]..ctor(System.__Canon[])+0xffffffff`a3af5e31 (00007ff9`b6fdbad1)
00007ff9`b6fdbabb 488d4610        lea     rax,[rsi+10h]
00007ff9`b6fdbabf 488903          mov     qword ptr [rbx],rax
00007ff9`b6fdbac2 8b4608          mov     eax,dword ptr [rsi+8]
00007ff9`b6fdbac5 894308          mov     dword ptr [rbx+8],eax
00007ff9`b6fdbac8 4883c428        add     rsp,28h
00007ff9`b6fdbacc 5b              pop     rbx
00007ff9`b6fdbacd 5d              pop     rbp
00007ff9`b6fdbace 5e              pop     rsi
00007ff9`b6fdbacf 5f              pop     rdi
00007ff9`b6fdbad0 c3              ret
00007ff9`b6fdbad1 e8a2e2d9ff      call    CLRStub[MethodDescPrestub]@7ff9b6d79d78 (00007ff9`b6d79d78)
00007ff9`b6fdbad6 cc              int     3

I didn’t even dump all the relevant codegen since it’s so big, but a few things immediately stand out:

Methods like the Span<T>.ctor(T[]) and Buffer.Memmove(__Canon&, __Canon&, ulong) aren’t being inlined into their caller.
There’s hydration of a RuntimeType instance from the type handle, possibly due to the array.GetType() != typeof(T[]) check at the beginning of the ctor.

<< Partial stack trace showing memmove not inlined >>
00 0000009a`7337ce08 00007ff9`b6fdbc56 System_Private_CoreLib!System.Buffer.Memmove[[System.__Canon, System.Private.CoreLib]](System.__Canon ByRef, System.__Canon ByRef, UInt64)+0xffffffff`a3b81660 [C:\runtime\src\libraries\System.Private.CoreLib\src\System\Buffer.cs @ 338] 
01 0000009a`7337ce10 00007ff9`b6fdbbd2 System_Private_CoreLib!System.Span`1[[System.__Canon, System.Private.CoreLib]].CopyTo(System.Span`1<System.__Canon>)+0xffffffff`a3af5976 [C:\runtime\src\libraries\System.Private.CoreLib\src\System\Span.cs @ 363]

/cc @EgorBo In case he has any suggestions.

GrabYourPitchforks on Feb 24, 2020

There is no functional problem with it. If you look at what InlinedSetCardsAfterBulkCopyHelper does, it casts the Object** to BYTE* or size_t.

Maybe the argument of InlinedSetCardsAfterBulkCopyHelper should be typed void* to make this less confusing.

jkotas on Feb 21, 2020