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Improve Span.Reverse fast path performance #70944

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merged 11 commits into from
Nov 18, 2022
Merged

Improve Span.Reverse fast path performance #70944

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7a678a8
Improve Span.Reverse fast path performance
yesmey Jun 17, 2022
0b7954d
Merge branch 'main' into reverse_span
yesmey Jun 22, 2022
cc4d784
Merge branch 'main' into reverse_span
yesmey Sep 3, 2022
fc6faf0
Merge branch 'main' into reverse_span
yesmey Sep 3, 2022
7e2071c
Merge mistake for Vector128 long shuffle
yesmey Sep 3, 2022
f54b51a
Revert tuple swap
yesmey Sep 4, 2022
2b16052
Merge branch 'main' into reverse_span
yesmey Nov 13, 2022
58067bb
Allow first two vectors to overlap in byte/char. Update contract for …
yesmey Nov 16, 2022
901c4dd
Simplify loop as suggested by saucecontrol
yesmey Nov 16, 2022
fda790a
More improvements, again thanks to saucecontrol
yesmey Nov 16, 2022
4fafb0e
Change to using offsets instead of reference iteration
yesmey Nov 18, 2022
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109 changes: 77 additions & 32 deletions src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Byte.cs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Buffers.Binary;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Numerics;
Expand Down Expand Up @@ -1129,21 +1130,23 @@ private static unsafe nuint UnalignedCountVector128(ref byte searchSpace)

public static void Reverse(ref byte buf, nuint length)
{
if (Avx2.IsSupported && (nuint)Vector256<byte>.Count * 2 <= length)
Debug.Assert(length > 1);

nint remainder = (nint)length;
nint offset = 0;

if (Avx2.IsSupported && remainder >= Vector256<byte>.Count)
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The trick with taking the vectorized path for sizes smaller than 2*Vector256.Count is neat, but it hurts performance in number of cases. There can be a lot of redundant work done for certain sizes with this PR.

I have opened #78604 on this.

{
Vector256<byte> reverseMask = Vector256.Create(
(byte)15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, // first 128-bit lane
15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0); // second 128-bit lane
nuint numElements = (nuint)Vector256<byte>.Count;
nuint numIters = (length / numElements) / 2;
for (nuint i = 0; i < numIters; i++)
{
nuint firstOffset = i * numElements;
nuint lastOffset = length - ((1 + i) * numElements);

// Load in values from beginning and end of the array.
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref buf, firstOffset);
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref buf, lastOffset);
nint lastOffset = remainder - Vector256<byte>.Count;
do
{
// Load the values into vectors
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref buf, (nuint)offset);
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref buf, (nuint)lastOffset);

// Avx2 operates on two 128-bit lanes rather than the full 256-bit vector.
// Perform a shuffle to reverse each 128-bit lane, then permute to finish reversing the vector:
Expand All @@ -1170,24 +1173,23 @@ public static void Reverse(ref byte buf, nuint length)
tempLast = Avx2.Permute2x128(tempLast, tempLast, 0b00_01);

// Store the reversed vectors
tempLast.StoreUnsafe(ref buf, firstOffset);
tempFirst.StoreUnsafe(ref buf, lastOffset);
}
buf = ref Unsafe.Add(ref buf, numIters * numElements);
length -= numIters * numElements * 2;
tempLast.StoreUnsafe(ref buf, (nuint)offset);
tempFirst.StoreUnsafe(ref buf, (nuint)lastOffset);

offset += Vector256<byte>.Count;
lastOffset -= Vector256<byte>.Count;
} while (lastOffset >= offset);

remainder = lastOffset + Vector256<byte>.Count - offset;
}
else if (Vector128.IsHardwareAccelerated && (nuint)Vector128<byte>.Count * 2 <= length)
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the removal of Vector128 code path causes a 10% regression for larger inputs on arm64. This needs to be addressed. I can run the benchmarks for you on an arm64 machine, just ping me here.

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I changed it back to the previous behavior, can you run the benchmarks again when you got time, please?

else if (Vector128.IsHardwareAccelerated && remainder >= Vector128<byte>.Count)
{
nuint numElements = (nuint)Vector128<byte>.Count;
nuint numIters = (length / numElements) / 2;
for (nuint i = 0; i < numIters; i++)
nint lastOffset = remainder - Vector128<byte>.Count;
do
{
nuint firstOffset = i * numElements;
nuint lastOffset = length - ((1 + i) * numElements);

// Load in values from beginning and end of the array.
Vector128<byte> tempFirst = Vector128.LoadUnsafe(ref buf, firstOffset);
Vector128<byte> tempLast = Vector128.LoadUnsafe(ref buf, lastOffset);
// Load the values into vectors
Vector128<byte> tempFirst = Vector128.LoadUnsafe(ref buf, (nuint)offset);
Vector128<byte> tempLast = Vector128.LoadUnsafe(ref buf, (nuint)lastOffset);

// Shuffle to reverse each vector:
// +---------------------------------------------------------------+
Expand All @@ -1203,15 +1205,58 @@ public static void Reverse(ref byte buf, nuint length)
(byte)15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0));

// Store the reversed vectors
tempLast.StoreUnsafe(ref buf, firstOffset);
tempFirst.StoreUnsafe(ref buf, lastOffset);
}
buf = ref Unsafe.Add(ref buf, numIters * numElements);
length -= numIters * numElements * 2;
tempLast.StoreUnsafe(ref buf, (nuint)offset);
tempFirst.StoreUnsafe(ref buf, (nuint)lastOffset);

offset += Vector128<byte>.Count;
lastOffset -= Vector128<byte>.Count;
} while (lastOffset >= offset);

remainder = lastOffset + Vector128<byte>.Count - offset;
}

if (remainder >= sizeof(long))
{
nint lastOffset = (nint)length - offset - sizeof(long);
do
{
long tempFirst = Unsafe.ReadUnaligned<long>(ref Unsafe.Add(ref buf, offset));
long tempLast = Unsafe.ReadUnaligned<long>(ref Unsafe.Add(ref buf, lastOffset));

// swap and store in reversed position
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, offset), BinaryPrimitives.ReverseEndianness(tempLast));
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, lastOffset), BinaryPrimitives.ReverseEndianness(tempFirst));

offset += sizeof(long);
lastOffset -= sizeof(long);
} while (lastOffset >= offset);

remainder = lastOffset + sizeof(long) - offset;
}

// Store any remaining values one-by-one
ReverseInner(ref buf, length);
if (remainder >= sizeof(int))
{
nint lastOffset = (nint)length - offset - sizeof(int);
do
{
int tempFirst = Unsafe.ReadUnaligned<int>(ref Unsafe.Add(ref buf, offset));
int tempLast = Unsafe.ReadUnaligned<int>(ref Unsafe.Add(ref buf, lastOffset));

// swap and store in reversed position
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, offset), BinaryPrimitives.ReverseEndianness(tempLast));
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, lastOffset), BinaryPrimitives.ReverseEndianness(tempFirst));

offset += sizeof(int);
lastOffset -= sizeof(int);
} while (lastOffset >= offset);

remainder = lastOffset + sizeof(int) - offset;
}

if (remainder > 1)
{
ReverseInner(ref Unsafe.Add(ref buf, offset), (nuint)remainder);
}
}
}
}
82 changes: 44 additions & 38 deletions src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Char.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -733,23 +733,25 @@ private static unsafe nint UnalignedCountVector128(ref char searchSpace)

public static void Reverse(ref char buf, nuint length)
{
if (Avx2.IsSupported && (nuint)Vector256<short>.Count * 2 <= length)
Debug.Assert(length > 1);

nint remainder = (nint)length;
nint offset = 0;

if (Avx2.IsSupported && remainder >= Vector256<ushort>.Count)
{
ref byte bufByte = ref Unsafe.As<char, byte>(ref buf);
nuint byteLength = length * sizeof(char);
Vector256<byte> reverseMask = Vector256.Create(
(byte)14, 15, 12, 13, 10, 11, 8, 9, 6, 7, 4, 5, 2, 3, 0, 1, // first 128-bit lane
14, 15, 12, 13, 10, 11, 8, 9, 6, 7, 4, 5, 2, 3, 0, 1); // second 128-bit lane
nuint numElements = (nuint)Vector256<byte>.Count;
nuint numIters = (byteLength / numElements) / 2;
for (nuint i = 0; i < numIters; i++)

nint lastOffset = remainder - Vector256<ushort>.Count;
do
{
nuint firstOffset = i * numElements;
nuint lastOffset = byteLength - ((1 + i) * numElements);
ref byte first = ref Unsafe.As<char, byte>(ref Unsafe.Add(ref buf, offset));
ref byte last = ref Unsafe.As<char, byte>(ref Unsafe.Add(ref buf, lastOffset));

// Load in values from beginning and end of the array.
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref bufByte, firstOffset);
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref bufByte, lastOffset);
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref first);
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref last);

// Avx2 operates on two 128-bit lanes rather than the full 256-bit vector.
// Perform a shuffle to reverse each 128-bit lane, then permute to finish reversing the vector:
Expand All @@ -770,27 +772,25 @@ public static void Reverse(ref char buf, nuint length)
tempLast = Avx2.Permute2x128(tempLast, tempLast, 0b00_01);

// Store the reversed vectors
tempLast.StoreUnsafe(ref bufByte, firstOffset);
tempFirst.StoreUnsafe(ref bufByte, lastOffset);
}
bufByte = ref Unsafe.Add(ref bufByte, numIters * numElements);
length -= numIters * (nuint)Vector256<short>.Count * 2;
// Store any remaining values one-by-one
buf = ref Unsafe.As<byte, char>(ref bufByte);
tempLast.StoreUnsafe(ref first);
tempFirst.StoreUnsafe(ref last);

offset += Vector256<ushort>.Count;
lastOffset -= Vector256<ushort>.Count;
} while (lastOffset >= offset);

remainder = (lastOffset + Vector256<ushort>.Count - offset);
}
else if (Vector128.IsHardwareAccelerated && (nuint)Vector128<short>.Count * 2 <= length)
else if (Vector128.IsHardwareAccelerated && remainder >= Vector128<ushort>.Count)
{
ref short bufShort = ref Unsafe.As<char, short>(ref buf);
nuint numElements = (nuint)Vector128<short>.Count;
nuint numIters = (length / numElements) / 2;
for (nuint i = 0; i < numIters; i++)
nint lastOffset = remainder - Vector128<ushort>.Count;
do
{
nuint firstOffset = i * numElements;
nuint lastOffset = length - ((1 + i) * numElements);
ref ushort first = ref Unsafe.As<char, ushort>(ref Unsafe.Add(ref buf, offset));
ref ushort last = ref Unsafe.As<char, ushort>(ref Unsafe.Add(ref buf, lastOffset));

// Load in values from beginning and end of the array.
Vector128<short> tempFirst = Vector128.LoadUnsafe(ref bufShort, firstOffset);
Vector128<short> tempLast = Vector128.LoadUnsafe(ref bufShort, lastOffset);
Vector128<ushort> tempFirst = Vector128.LoadUnsafe(ref first);
Vector128<ushort> tempLast = Vector128.LoadUnsafe(ref last);

// Shuffle to reverse each vector:
// +-------------------------------+
Expand All @@ -800,19 +800,25 @@ public static void Reverse(ref char buf, nuint length)
// +-------------------------------+
// | H | G | F | E | D | C | B | A |
// +-------------------------------+
tempFirst = Vector128.Shuffle(tempFirst, Vector128.Create(7, 6, 5, 4, 3, 2, 1, 0));
tempLast = Vector128.Shuffle(tempLast, Vector128.Create(7, 6, 5, 4, 3, 2, 1, 0));
tempFirst = Vector128.Shuffle(tempFirst, Vector128.Create((ushort)7, 6, 5, 4, 3, 2, 1, 0));
tempLast = Vector128.Shuffle(tempLast, Vector128.Create((ushort)7, 6, 5, 4, 3, 2, 1, 0));

// Store the reversed vectors
tempLast.StoreUnsafe(ref bufShort, firstOffset);
tempFirst.StoreUnsafe(ref bufShort, lastOffset);
}
bufShort = ref Unsafe.Add(ref bufShort, numIters * numElements);
length -= numIters * (nuint)Vector128<short>.Count * 2;
// Store any remaining values one-by-one
buf = ref Unsafe.As<short, char>(ref bufShort);
tempLast.StoreUnsafe(ref first);
tempFirst.StoreUnsafe(ref last);

offset += Vector128<ushort>.Count;
lastOffset -= Vector128<ushort>.Count;
} while (lastOffset >= offset);

remainder = (lastOffset + Vector128<ushort>.Count - offset);
}

// Store any remaining values one-by-one
if (remainder > 1)
{
ReverseInner(ref Unsafe.Add(ref buf, offset), (nuint)remainder);
}
ReverseInner(ref buf, length);
}
}
}
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