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Merged
stephentoub merged 1 commit into from
Oct 3, 2022
Merged

Improve performance of FormattingHelper.CountDigits(uint) #76519

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78 changes: 48 additions & 30 deletions ...libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
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Original file line number Diff line number Diff line change
@@ -1,9 +1,11 @@
// 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.Numerics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace System.Buffers.Text
{
Expand Down Expand Up @@ -109,36 +111,52 @@ public static int CountDigits(ulong value)
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int CountDigits(uint value)
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@jkotas jkotas Oct 3, 2022

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Update CountDigits(ulong value) above to take advantage of this?

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@mattzink mattzink Oct 3, 2022

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Update CountDigits(ulong value) above to take advantage of this?

FYI: There is a 64-bit version in the comments of Lemire's post

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@stephentoub stephentoub Oct 3, 2022

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FYI: There is a 64-bit version in the comments of Lemire's post

Which specifically? The variants I'd seen were only good up to a smaller number of bits, like 57.

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@EgorBo EgorBo Oct 3, 2022 •
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I assume Jan meant

if (longValue <= uint.MaxValue)
    return CountDigits((uint)longValue);

?

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@jkotas jkotas Oct 3, 2022

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More like:

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int CountDigits(ulong value)
        {
            int digits;
            uint part;
            if (value >= 1_000_000_000)
            {
                if (part >= 1_000_000_000_000_000_000)
                {
                    part = (uint)(value / 1_000_000_000_000_000_000);
                    digits = 18;
                }
                else
                {
                    part = (uint)(value / 1_000_000_000);
                    digits = 9;
                }
            }
            else
            {
                part = (uint)value;
                digits = 0;
            }
            
            return digits + CountDigits(part);
        }

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@stephentoub stephentoub Oct 4, 2022

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More like

Yeah, that's what I was planning on doing, but I want to experiment.

{
int digits = 1;
if (value >= 100000)
{
value /= 100000;
digits += 5;
}

if (value < 10)
{
// no-op
}
else if (value < 100)
{
digits++;
}
else if (value < 1000)
{
digits += 2;
}
else if (value < 10000)
{
digits += 3;
}
else
{
Debug.Assert(value < 100000);
digits += 4;
}

return digits;
// Algorithm based on https://lemire.me/blog/2021/06/03/computing-the-number-of-digits-of-an-integer-even-faster.
// TODO https://github.com/dotnet/runtime/issues/60948: Use ReadOnlySpan<long> instead of ReadOnlySpan<byte>.
ReadOnlySpan<byte> table = new byte[]
{
0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, // 4294967296
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x00, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x00, // 42949672960
0x00, 0x00, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x00, // 42949672960
};
Debug.Assert(table.Length == (32 * sizeof(long)), "Every result of uint.Log2(value) needs a long entry in the table.");

long tableValue = Unsafe.ReadUnaligned<long>(ref Unsafe.Add(ref MemoryMarshal.GetReference(table), uint.Log2(value) * sizeof(long)));
if (!BitConverter.IsLittleEndian)
{
tableValue = BinaryPrimitives.ReverseEndianness(tableValue);
}

return (int)((value + tableValue) >> 32);
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
Expand Down
14 changes: 6 additions & 8 deletions src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1593,15 +1593,14 @@ private static unsafe void UInt32ToNumber(uint value, ref NumberBuffer number)

internal static unsafe string UInt32ToDecStr(uint value)
{
// Intrinsified in mono interpreter
int bufferLength = FormattingHelpers.CountDigits(value);

// For single-digit values that are very common, especially 0 and 1, just return cached strings.
if (bufferLength == 1)
if (value < 10)
{
return s_singleDigitStringCache[value];
}

int bufferLength = FormattingHelpers.CountDigits(value);

string result = string.FastAllocateString(bufferLength);
fixed (char* buffer = result)
{
Expand Down Expand Up @@ -1935,15 +1934,14 @@ private static uint Int64DivMod1E9(ref ulong value)

internal static unsafe string UInt64ToDecStr(ulong value)
{
// Intrinsified in mono interpreter
int bufferLength = FormattingHelpers.CountDigits(value);

// For single-digit values that are very common, especially 0 and 1, just return cached strings.
if (bufferLength == 1)
if (value < 10)
{
return s_singleDigitStringCache[value];
}

int bufferLength = FormattingHelpers.CountDigits(value);

string result = string.FastAllocateString(bufferLength);
fixed (char* buffer = result)
{
Expand Down