.NET Core 3.1, IndexOf(char)

gistfile1.txt

	internal static unsafe class Intrinsics
	{
		private static ReadOnlySpan<byte> TrailingZeroCountDeBruijnTable => new byte[]
		{
			00, 01, 28, 02, 29, 14, 24, 03,
			30, 22, 20, 15, 25, 17, 04, 08,
			31, 27, 13, 23, 21, 19, 16, 07,
			26, 12, 18, 06, 11, 05, 10, 09
		};

		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
		public static int IndexOf(char* chars, int charCount, char c)
		{
			const int charsPerPass = 8; // Vector128<ushort>.Count

			var charsStart = chars;
			var stopChar = chars + (charCount & 0b_1111_1111_1111_1111_1111_1111_1111_1000);
			var endChar = stopChar + (charCount & 0b_0111);

			var vC = Vector128.Create(c);
			
			while (chars != stopChar)
			{
				var vChars = Sse2.LoadVector128((ushort*) chars);
				var vCompRes = Sse2.CompareEqual(vChars, vC);

				var hitsBitMask = Sse2.MoveMask(Sse2.ShiftRightLogical(vCompRes, 8).AsByte());
				if (hitsBitMask != 0)
				{
					if (Bmi1.IsSupported)
					{
						return (int) (chars - charsStart + (Bmi1.TrailingZeroCount((uint) hitsBitMask) >> 1));
					}

					return (int) (chars - charsStart) + (TrailingZeroCountDeBruijnTable[(int) ((uint)((hitsBitMask & -hitsBitMask) * 0x077CB531U) >> 27)] >> 1);
				}
				chars += charsPerPass;
			}

			while (chars != endChar)
			{
				if (*chars == c)
				{
					return (int) (chars - charsStart);
				}

				chars++;
			}
			return -1;
		}
	}