Optimize System.SpanHelpers using arm64 intrinsics #33707

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1. Dotnet-GitSync-Bot

   added

   area-System.Memory

   untriagedNew issue has not been triaged by the area ownerNew issue has not been triaged by the area owner

   on Mar 18, 2020
2. BruceForstall

   added

   arch-arm64

   on Mar 18, 2020
3. BruceForstall

   added this to the 5.0 milestone on Mar 18, 2020
4. GrabYourPitchforks commented on Mar 18, 2020

   

   GrabYourPitchforks

   on Mar 18, 2020

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   A nit about these functions that might not be immediately obvious:

   SpanHelpers.IndexOf(ref byte, ...) and SpanHelpers.IndexOf(ref char, ...) are used as the workhorses for the strlen and wcslen implementations. The caller lies about the length parameter and assumes that if the null character first occurs at page n, the IndexOf method will never attempt to dereference page n + 1. We have some limited unit tests for this scenario as shown below, but I wanted to make whoever attempts to intrinsicify this method aware of this behavior.

   runtime/src/libraries/System.Runtime/tests/System/StringTests.cs

   Lines 48 to 68 in c0ddd1c

   	[Fact]
   	public static unsafe void Ctor_CharPtr_DoesNotAccessInvalidPage()
   	{
   	// Allocates a buffer of all 'x' followed by a null terminator,
   	// then attempts to create a string instance from this at various offsets.
   	const int MaxCharCount = 128;
   	using BoundedMemory<char> boundedMemory = BoundedMemory.Allocate<char>(MaxCharCount);
   	boundedMemory.Span.Fill('x');
   	boundedMemory.Span[MaxCharCount - 1] = '\0';
   	boundedMemory.MakeReadonly();
   	using MemoryHandle memoryHandle = boundedMemory.Memory.Pin();
   	for (int i = 0; i < MaxCharCount; i++)
   	{
   	string expectedString = new string('x', MaxCharCount - i - 1);
   	string actualString = new string((char*)memoryHandle.Pointer + i);
   	Assert.Equal(expectedString, actualString);
   	}
   	}
5. JulieLeeMSFT

   assigned

   kunalspathak

   on May 12, 2020
6. BruceForstall

   removed

   untriagedNew issue has not been triaged by the area ownerNew issue has not been triaged by the area owner

   on Jun 24, 2020
7. kunalspathak commented on Jun 25, 2020

   

   kunalspathak

   on Jun 25, 2020

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   IndexOf(char), IndexOf(byte) , IndexOfAny(byte) are now optimized with ARM64 intrinsics. We didn't optimize SequenceCompareTo(byte) and SequenceEqual(byte) because the existing SIMD vector implementation is fast enough and comparable with ARM64 intrinsics.
8. kunalspathak

   closed this as completedon Jun 25, 2020
9. ghost locked as resolved and limited conversation to collaborators on Dec 10, 2020