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Remove an unnecessary branch from Log2Floor
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Neither of the callsites using Helpers.Log2Floor ever pass a 0, so the
special case to return -1 in the 0 case is unnecessary.

BenchmarkDotNet v0.14.0, Windows 11 (10.0.26100.2314)
Unknown processor
.NET SDK 9.0.100
  [Host]     : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  Job-JLJFKR : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256
  Job-PXVNHY : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256
  Job-DVRAUN : .NET 6.0.36 (6.0.3624.51421), X64 RyuJIT AVX2
  Job-GQEZIF : .NET 6.0.36 (6.0.3624.51421), X64 RyuJIT AVX2
  Job-PUVQSP : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
  Job-YPVVHF : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
  Job-MKMULB : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  Job-PYOQFK : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2

| Method    | Runtime            | BuildConfiguration | Mean      | Error     | StdDev    | Median    | Ratio | RatioSD | Rank | Code Size |
|---------- |------------------- |------------------- |----------:|----------:|----------:|----------:|------:|--------:|-----:|----------:|
| Log2Floor | .NET Framework 4.8 | Previous           | 8.2904 ns | 0.1383 ns | 0.1155 ns | 8.2723 ns |  1.00 |    0.02 |    2 |     186 B |
| Log2Floor | .NET Framework 4.8 | Default            | 7.8699 ns | 0.0244 ns | 0.0204 ns | 7.8673 ns |  0.95 |    0.01 |    1 |     170 B |
|           |                    |                    |           |           |           |           |       |         |      |           |
| Log2Floor | .NET 6.0           | Previous           | 0.3634 ns | 0.0273 ns | 0.0256 ns | 0.3612 ns |  1.00 |    0.10 |    2 |      25 B |
| Log2Floor | .NET 6.0           | Default            | 0.0527 ns | 0.0175 ns | 0.0164 ns | 0.0534 ns |  0.15 |    0.05 |    1 |      14 B |
|           |                    |                    |           |           |           |           |       |         |      |           |
| Log2Floor | .NET 8.0           | Previous           | 0.1953 ns | 0.0041 ns | 0.0037 ns | 0.1947 ns |  1.00 |    0.03 |    2 |      25 B |
| Log2Floor | .NET 8.0           | Default            | 0.0030 ns | 0.0064 ns | 0.0053 ns | 0.0000 ns |  0.02 |    0.03 |    1 |      14 B |
|           |                    |                    |           |           |           |           |       |         |      |           |
| Log2Floor | .NET 9.0           | Previous           | 0.2028 ns | 0.0030 ns | 0.0028 ns | 0.2025 ns |  1.00 |    0.02 |    2 |      25 B |
| Log2Floor | .NET 9.0           | Default            | 0.0147 ns | 0.0064 ns | 0.0057 ns | 0.0127 ns |  0.07 |    0.03 |    1 |      14 B |
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@brantburnett
brantburnett committed Nov 30, 2024
1 parent 74847fd commit 4854273
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Showing 6 changed files with 36 additions and 35 deletions.
11 changes: 6 additions & 5 deletions Snappier.Benchmarks/Configuration/FrameworkCompareConfig.cs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -13,14 +13,15 @@ public FrameworkCompareConfig(Job baseJob)
.WithRuntime(ClrRuntime.Net48));
AddJob(baseJob
.WithRuntime(CoreRuntime.Core60));

var job80 = baseJob.WithRuntime(CoreRuntime.Core80);
AddJob(job80.WithPgo(false));
AddJob(job80.WithPgo(true));
AddJob(baseJob
.WithRuntime(CoreRuntime.Core80)
.WithPgo(true));
AddJob(baseJob
.WithRuntime(CoreRuntime.Core90)
.WithPgo(true));

AddLogicalGroupRules(BenchmarkLogicalGroupRule.ByJob);

AddColumn(PgoColumn.Default);
HideColumns(Column.EnvironmentVariables);
}
}
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13 changes: 6 additions & 7 deletions Snappier.Benchmarks/Configuration/VersionComparisonConfig.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,27 +20,26 @@ public VersionComparisonConfig(Job baseJob)

var jobBefore48 = jobBefore.WithRuntime(ClrRuntime.Net48).AsBaseline();
var jobBefore60 = jobBefore.WithRuntime(CoreRuntime.Core60).AsBaseline();
var jobBefore80 = jobBefore.WithRuntime(CoreRuntime.Core80).AsBaseline();
var jobBefore80Pgo = jobBefore80.WithPgo();
var jobBefore80 = jobBefore.WithRuntime(CoreRuntime.Core80).WithPgo().AsBaseline();
var jobBefore90 = jobBefore.WithRuntime(CoreRuntime.Core90).WithPgo().AsBaseline();

var jobAfter48 = baseJob.WithRuntime(ClrRuntime.Net48);
var jobAfter60 = baseJob.WithRuntime(CoreRuntime.Core60);
var jobAfter80 = baseJob.WithRuntime(CoreRuntime.Core80);
var jobAfter80Pgo = jobAfter80.WithPgo();
var jobAfter80 = baseJob.WithRuntime(CoreRuntime.Core80).WithPgo();
var jobAfter90 = baseJob.WithRuntime(CoreRuntime.Core90).WithPgo();

AddJob(jobBefore48);
AddJob(jobBefore60);
AddJob(jobBefore80);
AddJob(jobBefore80Pgo);
AddJob(jobBefore90);

AddJob(jobAfter48);
AddJob(jobAfter60);
AddJob(jobAfter80);
AddJob(jobAfter80Pgo);
AddJob(jobAfter90);

WithOrderer(VersionComparisonOrderer.Default);

AddColumn(PgoColumn.Default);
HideColumns(Column.EnvironmentVariables, Column.Job);
}

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30 changes: 19 additions & 11 deletions Snappier.Tests/HelpersTests.cs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,4 @@
using System;
using System.Collections.Generic;
using System.Linq;
using Snappier.Internal;
using Xunit;

Expand Down Expand Up @@ -39,26 +37,36 @@ public void LeftShiftOverflows_False(byte value, int shift)
Assert.False(result);
}

#endregion

#region Log2FloorNonZero

public static IEnumerable<object[]> Log2FloorNonZeroValues() =>
Enumerable.Range(1, 31).Select(p => new object[] {p});
public static TheoryData<uint> Log2FloorValues() =>
[
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
];

[Theory]
[MemberData(nameof(Log2FloorNonZeroValues))]
public void Log2FloorNonZero(uint value)
[MemberData(nameof(Log2FloorValues))]
public void Log2Floor(uint value)
{
// Act

var result = Helpers.Log2FloorNonZero(value);
var result = Helpers.Log2Floor(value);

// Assert

Assert.Equal((int) Math.Floor(Math.Log(value, 2)), result);
}

[Fact]
public void Log2Floor_Zero()
{
// Act

var result = Helpers.Log2Floor(0);

// Assert

Assert.Equal(0, result);
}

#endregion
}
}
2 changes: 2 additions & 0 deletions Snappier/Internal/HashTable.cs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
using System;
using System.Buffers;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

Expand Down Expand Up @@ -67,6 +68,7 @@ private static int CalculateTableSize(int inputSize)
return MinHashTableSize;
}

Debug.Assert(inputSize > 1);
return 2 << Helpers.Log2Floor((uint)(inputSize - 1));
}

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14 changes: 2 additions & 12 deletions Snappier/Internal/Helpers.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -165,21 +165,11 @@ ref MemoryMarshal.GetReference(Log2DeBruijn),
#endif

/// <summary>
/// Return floor(log2(n)) for positive integer n. Returns -1 if n == 0.
/// Return floor(log2(n)) for positive integer n. Returns 0 for the special case n = 0.
/// </summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Log2Floor(uint n) =>
n == 0 ? -1 : Log2FloorNonZero(n);


/// <summary>
/// Return floor(log2(n)) for positive integer n.
/// </summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int Log2FloorNonZero(uint n)
public static int Log2Floor(uint n)
{
Debug.Assert(n != 0);

#if NET6_0_OR_GREATER
return BitOperations.Log2(n);
#else
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1 change: 1 addition & 0 deletions Snappier/Internal/SnappyCompressor.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -343,6 +343,7 @@ private static ref byte EmitLiteralSlow(ref byte op, ref byte literal, uint leng
}
else
{
Debug.Assert(n > 0);
int count = (Helpers.Log2Floor(n) >> 3) + 1;

Debug.Assert(count >= 1);
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