[Utf8Array] AsSpanの最適化

[finphie]

https://sharplab.io/#v2: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

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public class C {
    readonly byte[] _x;
    
    public C(Byte[] x) => _x = x;
    
    public ReadOnlySpan<byte> A1() => _x;
    
    public ReadOnlySpan<byte> A2()
    {
        ref var xs = ref MemoryMarshal.GetArrayDataReference(_x);
        return MemoryMarshal.CreateReadOnlySpan(ref xs, _x.Length);
    }
    
    public ReadOnlySpan<byte> A3() {
        var span = new ReadOnlySpan<byte>(_x, 0, _x.Length);
        return span;
    }
   
    
    public ReadOnlySpan<byte> B1(int start) => _x.AsSpan(start);
    
    public ReadOnlySpan<byte> B2(int start)
    {
        ref var xs = ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(_x), (nint)(uint)start);
        return MemoryMarshal.CreateReadOnlySpan(ref xs, _x.Length);
    }
    
    public ReadOnlySpan<byte> B3(int start) {
        var span = new ReadOnlySpan<byte>(_x, start, _x.Length);
        return span;
    }
}

C.A1()
    L0000: mov rax, [rcx+8]
    L0004: test rax, rax
    L0007: jne short L0010
    L0009: xor ecx, ecx
    L000b: xor r8d, r8d
    L000e: jmp short L0018
    L0010: lea rcx, [rax+0x10]
    L0014: mov r8d, [rax+8]
    L0018: mov [rdx], rcx
    L001b: mov [rdx+8], r8d
    L001f: mov rax, rdx
    L0022: ret

C.A2()
    L0000: mov rax, [rcx+8]
    L0004: mov rcx, rax
    L0007: cmp [rcx], ecx
    L0009: add rcx, 0x10
    L000d: mov eax, [rax+8]
    L0010: mov [rdx], rcx
    L0013: mov [rdx+8], eax
    L0016: mov rax, rdx
    L0019: ret

C.A3()
    L0000: sub rsp, 0x28
    L0004: mov rax, [rcx+8]
    L0008: mov rcx, rax
    L000b: mov eax, [rax+8]
    L000e: mov r8d, [rcx+8]
    L0012: mov r9d, eax
    L0015: cmp r8, r9
    L0018: jb short L002c
    L001a: add rcx, 0x10
    L001e: mov [rdx], rcx
    L0021: mov [rdx+8], eax
    L0024: mov rax, rdx
    L0027: add rsp, 0x28
    L002b: ret
    L002c: call 0x00007ffc8f6d5b28
    L0031: int3

C.B1(Int32)
    L0000: sub rsp, 0x28
    L0004: mov rax, [rcx+8]
    L0008: test rax, rax
    L000b: jne short L0019
    L000d: test r8d, r8d
    L0010: jne short L003e
    L0012: xor ecx, ecx
    L0014: xor r9d, r9d
    L0017: jmp short L002f
    L0019: mov r9d, [rax+8]
    L001d: cmp r9d, r8d
    L0020: jb short L003e
    L0022: add rax, 0x10
    L0026: sub r9d, r8d
    L0029: movsxd rcx, r8d
    L002c: add rcx, rax
    L002f: mov [rdx], rcx
    L0032: mov [rdx+8], r9d
    L0036: mov rax, rdx
    L0039: add rsp, 0x28
    L003d: ret
    L003e: call 0x00007ffc8f6d5b28
    L0043: int3

C.B2(Int32)
    L0000: mov rax, [rcx+8]
    L0004: mov rcx, rax
    L0007: cmp [rcx], ecx
    L0009: add rcx, 0x10
    L000d: mov eax, [rax+8]
    L0010: mov r8d, r8d
    L0013: add rcx, r8
    L0016: mov [rdx], rcx
    L0019: mov [rdx+8], eax
    L001c: mov rax, rdx
    L001f: ret

C.B3(Int32)
    L0000: sub rsp, 0x28
    L0004: mov rax, [rcx+8]
    L0008: mov rcx, rax
    L000b: mov eax, [rax+8]
    L000e: mov r9d, r8d
    L0011: mov r10d, eax
    L0014: add r9, r10
    L0017: mov r10d, [rcx+8]
    L001b: cmp r9, r10
    L001e: ja short L0038
    L0020: add rcx, 0x10
    L0024: movsxd r8, r8d
    L0027: add rcx, r8
    L002a: mov [rdx], rcx
    L002d: mov [rdx+8], eax
    L0030: mov rax, rdx
    L0033: add rsp, 0x28
    L0037: ret
    L0038: call 0x00007ffc8f6d5b28
    L003d: int3