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トップ > C# > 【C#】ゼロアロケーションバイト列

【C#】ゼロアロケーションバイト列

C#

【追記】
byte, sbyte以外にも対象が拡大されました。
【C#】ReadOnlySpan<T>最適化の確認 - てくメモ


バイト列はゼロアロケーションに最適化される場合がある。
参考：静的なデータの ReadOnlySpan 最適化 | ++C++; // 未確認飛行 C ブログ

// アロケーションされる
public IEnumerable<byte> HeaderAllocated = new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A };

// ゼロアロケーション
public ReadOnlySpan<byte> Header => new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A };
public ReadOnlySpan<byte> Utf8Header => "Hello, World"u8;

参考：IL 折りたたみ

// アロケーションされる
IL_0000: ldarg.0
IL_0001: ldc.i4.8
IL_0002: newarr [System.Runtime]System.Byte
IL_0007: dup
IL_0008: ldtoken field int64 '<PrivateImplementationDetails>'::'4C4B6A3BE1314AB86138BEF4314DDE022E600960D8689A2C8F8631802D20DAB6'
IL_000d: call void [System.Runtime]System.Runtime.CompilerServices.RuntimeHelpers::InitializeArray(class [System.Runtime]System.Array, valuetype [System.Runtime]System.RuntimeFieldHandle)
IL_0012: stfld class [System.Runtime]System.Collections.Generic.IEnumerable`1<uint8> C::HeaderAllocated
IL_0017: ldarg.0
IL_0018: call instance void [System.Runtime]System.Object::.ctor()
IL_001d: ret

// Header
IL_0000: ldsflda int64 '<PrivateImplementationDetails>'::'4C4B6A3BE1314AB86138BEF4314DDE022E600960D8689A2C8F8631802D20DAB6'
IL_0005: ldc.i4.8
IL_0006: newobj instance void valuetype [System.Runtime]System.ReadOnlySpan`1<uint8>::.ctor(void*, int32)
IL_000b: ret

// Utf8Header
IL_0000: ldsflda valuetype '<PrivateImplementationDetails>'/'__StaticArrayInitTypeSize=13' '<PrivateImplementationDetails>'::C1A6DBE96DF8BCEEEEC9B1E8E78E0AB0DDED1E51461CA0C89643D3C79B303FF6
IL_0005: ldc.i4.s 12
IL_0007: newobj instance void valuetype [System.Runtime]System.ReadOnlySpan`1<uint8>::.ctor(void*, int32)
IL_000c: ret


byteかsbyteのReadOnlySpanで受ける配列やUTF8リテラルは、実行ファイルのバイナリのアドレスから直接読み出され、アロケーションが発生しないということ。

普通の使い方ならアロケ量の差は微々たるものだと思うけれど、ゼロとあるとではGCの関係でやっぱり違うと思うので、固定のReadOnlySpan<byte>によるSequenceEqualsなどを使うなら利用したい。


ILを見れば瞭然かもしれないけれど、一応ベンチマークを見てアロケーションがないのを確認してみる。
なお、参考でstackallocも加えた。

[Benchmark]
public int HeapAllocatedByteArray()
{
    // 0-255
    var array = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255 };

    int sum = 0;
    foreach (var i in array)
    {
        sum += i;
    }

    return sum;
}

[Benchmark]
public int StackAllocatedByteArray()
{
    ReadOnlySpan<byte> array = stackalloc byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255 };

    int sum = 0;
    foreach (var i in array)
    {
        sum += i;
    }

    return sum;
}

[Benchmark]
public int NonAllocatedByteArray()
{
    ReadOnlySpan<byte> array = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255 };

    int sum = 0;
    foreach(var i in array)
    {
        sum += i;
    }

    return sum;
}

                 Method |     Mean |     Error |  StdDev |   Gen0 | Allocated |
----------------------- |---------:|----------:|--------:|-------:|----------:|
 HeapAllocatedByteArray | 188.7 ns | 100.12 ns | 5.49 ns | 0.0892 |     280 B |
    StackallocByteArray | 178.9 ns | 133.92 ns | 7.34 ns |      - |         - |
  NonAllocatedByteArray | 160.0 ns |   9.83 ns | 0.54 ns |      - |         - |


確かにゼロアロケ。