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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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99 changes: 66 additions & 33 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
Expand Up @@ -1132,22 +1132,21 @@ public static void Reverse(ref byte buf, nuint length)
{
Debug.Assert(length > 1);

ref byte first = ref buf;
ref byte last = ref Unsafe.Add(ref first, length);
nint remainder = (nint)length;
nint offset = 0;

if (Avx2.IsSupported && length >= (nuint)Vector256<byte>.Count)
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

last = ref Unsafe.Subtract(ref last, Vector256<byte>.Count);
nint lastOffset = remainder - Vector256<byte>.Count;
do
{
// Load the values into vectors
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref first);
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref last);
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 @@ -1174,55 +1173,89 @@ public static void Reverse(ref byte buf, nuint length)
tempLast = Avx2.Permute2x128(tempLast, tempLast, 0b00_01);

// Store the reversed vectors
tempLast.StoreUnsafe(ref first);
tempFirst.StoreUnsafe(ref last);
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 && remainder >= Vector128<byte>.Count)
{
nint lastOffset = remainder - Vector128<byte>.Count;
do
{
// 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:
// +---------------------------------------------------------------+
// | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P |
// +---------------------------------------------------------------+
// --->
// +---------------------------------------------------------------+
// | P | O | N | M | L | K | J | I | H | G | F | E | D | C | B | A |
// +---------------------------------------------------------------+
tempFirst = Vector128.Shuffle(tempFirst, Vector128.Create(
(byte)15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0));
tempLast = Vector128.Shuffle(tempLast, Vector128.Create(
(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, (nuint)offset);
tempFirst.StoreUnsafe(ref buf, (nuint)lastOffset);

first = ref Unsafe.Add(ref first, Vector256<byte>.Count);
last = ref Unsafe.Subtract(ref last, Vector256<byte>.Count);
} while (!Unsafe.IsAddressGreaterThan(ref first, ref last));
offset += Vector128<byte>.Count;
lastOffset -= Vector128<byte>.Count;
} while (lastOffset >= offset);

remainder = Unsafe.ByteOffset(ref first, ref Unsafe.Add(ref last, Vector256<byte>.Count));
remainder = lastOffset + Vector128<byte>.Count - offset;
}
else if (remainder >= sizeof(long))

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

first = ref Unsafe.Add(ref first, sizeof(long));
last = ref Unsafe.Subtract(ref last, sizeof(long));
} while (!Unsafe.IsAddressGreaterThan(ref first, ref last));
offset += sizeof(long);
lastOffset -= sizeof(long);
} while (lastOffset >= offset);

remainder = Unsafe.ByteOffset(ref first, ref Unsafe.Add(ref last, sizeof(long)));
remainder = lastOffset + sizeof(long) - offset;
}
else if (remainder >= sizeof(int))

if (remainder >= sizeof(int))
{
last = ref Unsafe.Subtract(ref last, sizeof(int));
nint lastOffset = (nint)length - offset - sizeof(int);
do
{
int tempFirst = Unsafe.ReadUnaligned<int>(ref first);
int tempLast = Unsafe.ReadUnaligned<int>(ref last);
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 first, BinaryPrimitives.ReverseEndianness(tempLast));
Unsafe.WriteUnaligned(ref last, BinaryPrimitives.ReverseEndianness(tempFirst));
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, offset), BinaryPrimitives.ReverseEndianness(tempLast));
Unsafe.WriteUnaligned(ref Unsafe.Add(ref buf, lastOffset), BinaryPrimitives.ReverseEndianness(tempFirst));

first = ref Unsafe.Add(ref first, sizeof(int));
last = ref Unsafe.Subtract(ref last, sizeof(int));
} while (!Unsafe.IsAddressGreaterThan(ref first, ref last));
offset += sizeof(int);
lastOffset -= sizeof(int);
} while (lastOffset >= offset);

remainder = Unsafe.ByteOffset(ref first, ref Unsafe.Add(ref last, sizeof(int)));
remainder = lastOffset + sizeof(int) - offset;
}

if (remainder > 1)
{
ReverseInner(ref first, (nuint)remainder);
ReverseInner(ref Unsafe.Add(ref buf, offset), (nuint)remainder);
}
}
}
Expand Down
53 changes: 28 additions & 25 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 @@ -735,21 +735,23 @@ public static void Reverse(ref char buf, nuint length)
{
Debug.Assert(length > 1);

ref char first = ref buf;
ref char last = ref Unsafe.Add(ref first, length);
nint remainder = (nint)length;
nint offset = 0;

if (Avx2.IsSupported && length >= (nuint)Vector256<ushort>.Count)
if (Avx2.IsSupported && remainder >= Vector256<ushort>.Count)
{
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

last = ref Unsafe.Subtract(ref last, Vector256<ushort>.Count);
nint lastOffset = remainder - Vector256<ushort>.Count;
do
{
Vector256<byte> tempFirst = Vector256.LoadUnsafe(ref Unsafe.As<char, byte>(ref first));
Vector256<byte> tempLast = Vector256.LoadUnsafe(ref Unsafe.As<char, byte>(ref last));
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));

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 @@ -770,23 +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 Unsafe.As<char, byte>(ref first));
tempFirst.StoreUnsafe(ref Unsafe.As<char, byte>(ref last));
tempLast.StoreUnsafe(ref first);
tempFirst.StoreUnsafe(ref last);

first = ref Unsafe.Add(ref first, Vector256<ushort>.Count);
last = ref Unsafe.Subtract(ref last, Vector256<ushort>.Count);
} while (!Unsafe.IsAddressGreaterThan(ref first, ref last));
offset += Vector256<ushort>.Count;
lastOffset -= Vector256<ushort>.Count;
} while (lastOffset >= offset);

// shift for division is fine here since we don't care about what any negative number end up being
remainder = Unsafe.ByteOffset(ref first, ref Unsafe.Add(ref last, Vector256<ushort>.Count)) >> 1;
remainder = (lastOffset + Vector256<ushort>.Count - offset);
}
else if (Vector128.IsHardwareAccelerated && length >= (nuint)Vector128<ushort>.Count)
else if (Vector128.IsHardwareAccelerated && remainder >= Vector128<ushort>.Count)
{
last = ref Unsafe.Subtract(ref last, Vector128<ushort>.Count);
nint lastOffset = remainder - Vector128<ushort>.Count;
do
{
Vector128<ushort> tempFirst = Vector128.LoadUnsafe(ref Unsafe.As<char, ushort>(ref first));
Vector128<ushort> tempLast = Vector128.LoadUnsafe(ref Unsafe.As<char, ushort>(ref last));
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));

Vector128<ushort> tempFirst = Vector128.LoadUnsafe(ref first);
Vector128<ushort> tempLast = Vector128.LoadUnsafe(ref last);

// Shuffle to reverse each vector:
// +-------------------------------+
Expand All @@ -800,21 +804,20 @@ public static void Reverse(ref char buf, nuint length)
tempLast = Vector128.Shuffle(tempLast, Vector128.Create((ushort)7, 6, 5, 4, 3, 2, 1, 0));

// Store the reversed vectors
tempLast.StoreUnsafe(ref Unsafe.As<char, ushort>(ref first));
tempFirst.StoreUnsafe(ref Unsafe.As<char, ushort>(ref last));
tempLast.StoreUnsafe(ref first);
tempFirst.StoreUnsafe(ref last);

first = ref Unsafe.Add(ref first, Vector128<ushort>.Count);
last = ref Unsafe.Subtract(ref last, Vector128<ushort>.Count);
} while (!Unsafe.IsAddressGreaterThan(ref first, ref last));
offset += Vector128<ushort>.Count;
lastOffset -= Vector128<ushort>.Count;
} while (lastOffset >= offset);

// shift for division is fine here since we don't care about what any negative number end up being
remainder = Unsafe.ByteOffset(ref first, ref Unsafe.Add(ref last, Vector128<ushort>.Count)) >> 1;
remainder = (lastOffset + Vector128<ushort>.Count - offset);
}

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