Eliminate most CharTable range checks

Motivation ---------- The index into the CharTable is completely safe when indexed via a byte, so we can eliminate it in hot paths safely. Also, we can use RVA statics with ushort on newer versions of C#. Modifications ------------- Use ref and Unsafe.Add to index into the char table in DecompressAllTags, and make CharTable an RVA static. Results ------- BenchmarkDotNet v0.13.10, Windows 11 (10.0.22621.2861/22H2/2022Update/SunValley2) 12th Gen Intel Core i7-1270P, 1 CPU, 16 logical and 12 physical cores .NET SDK 8.0.100 [Host] : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 Job-IVITYO : .NET Framework 4.8.1 (4.8.9181.0), X64 RyuJIT VectorSize=256 Job-SVSNXE : .NET Framework 4.8.1 (4.8.9181.0), X64 RyuJIT VectorSize=256 Job-UZLHWZ : .NET 6.0.25 (6.0.2523.51912), X64 RyuJIT AVX2 Job-VTIJJN : .NET 6.0.25 (6.0.2523.51912), X64 RyuJIT AVX2 Job-BDCQZW : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 Job-WMAPZR : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 Job-KAZCEW : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 Job-QKBVXP : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 | Method | Runtime | BuildConfiguration | PGO | Mean | Error | StdDev | Ratio | Rank | |----------- |------------------- |------------------- |---- |---------:|---------:|---------:|------:|-----:| | Decompress | .NET Framework 4.8 | Previous | N | 14.61 us | 0.104 us | 0.092 us | 1.00 | 1 | | Decompress | .NET Framework 4.8 | Default | N | 14.68 us | 0.099 us | 0.093 us | 1.01 | 1 | | | | | | | | | | | | Decompress | .NET 6.0 | Previous | N | 12.90 us | 0.063 us | 0.049 us | 1.00 | 1 | | Decompress | .NET 6.0 | Default | N | 12.80 us | 0.114 us | 0.106 us | 0.99 | 1 | | | | | | | | | | | | Decompress | .NET 8.0 | Previous | N | 10.41 us | 0.089 us | 0.083 us | 1.00 | 1 | | Decompress | .NET 8.0 | Default | N | 10.29 us | 0.075 us | 0.070 us | 0.99 | 1 | | | | | | | | | | | | Decompress | .NET 8.0 | Previous | Y | 10.50 us | 0.108 us | 0.101 us | 1.00 | 2 | | Decompress | .NET 8.0 | Default | Y | 10.28 us | 0.085 us | 0.075 us | 0.98 | 1 |
brantburnett · Dec 24, 2023 · 2f7ffab · 2f7ffab
1 parent 425a50b
commit 2f7ffab
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Showing 4 changed files with 23 additions and 20 deletions.
diff --git a/Snappier.Benchmarks/Configuration/VersionComparisonConfig.cs b/Snappier.Benchmarks/Configuration/VersionComparisonConfig.cs
@@ -20,23 +20,23 @@ public VersionComparisonConfig(Job baseJob)
 
             var jobBefore48 = jobBefore.WithRuntime(ClrRuntime.Net48).AsBaseline();
             var jobBefore60 = jobBefore.WithRuntime(CoreRuntime.Core60).AsBaseline();
-            var jobBefore70 = jobBefore.WithRuntime(CoreRuntime.Core70).AsBaseline();
-            var jobBefore70Pgo = jobBefore70.WithPgo();
+            var jobBefore80 = jobBefore.WithRuntime(CoreRuntime.Core80).AsBaseline();
+            var jobBefore80Pgo = jobBefore80.WithPgo();
 
             var jobAfter48 = baseJob.WithRuntime(ClrRuntime.Net48);
             var jobAfter60 = baseJob.WithRuntime(CoreRuntime.Core60);
-            var jobAfter70 = baseJob.WithRuntime(CoreRuntime.Core70);
-            var jobAfter70Pgo = jobAfter70.WithPgo();
+            var jobAfter80 = baseJob.WithRuntime(CoreRuntime.Core80);
+            var jobAfter80Pgo = jobAfter80.WithPgo();
 
             AddJob(jobBefore48);
             AddJob(jobBefore60);
-            AddJob(jobBefore70);
-            AddJob(jobBefore70Pgo);
+            AddJob(jobBefore80);
+            AddJob(jobBefore80Pgo);
 
             AddJob(jobAfter48);
             AddJob(jobAfter60);
-            AddJob(jobAfter70);
-            AddJob(jobAfter70Pgo);
+            AddJob(jobAfter80);
+            AddJob(jobAfter80Pgo);
 
             WithOrderer(VersionComparisonOrderer.Default);
 
@@ -53,8 +53,8 @@ public IEnumerable<BenchmarkCase> GetExecutionOrder(ImmutableArray<BenchmarkCase
                 benchmarksCase
                     .OrderBy(p => p.Job.Environment.Runtime.MsBuildMoniker)
                     .ThenBy(p => PgoColumn.IsPgo(p) ? 1 : 0)
-                    .ThenBy(p => p.DisplayInfo)
-                    .ThenBy(p => !p.Descriptor.Baseline);
+                    .ThenBy(p => !p.Descriptor.Baseline)
+                    .ThenBy(p => p.DisplayInfo);
 
             public IEnumerable<BenchmarkCase> GetSummaryOrder(ImmutableArray<BenchmarkCase> benchmarksCases,
                 Summary summary) =>

diff --git a/Snappier.Benchmarks/Snappier.Benchmarks.csproj b/Snappier.Benchmarks/Snappier.Benchmarks.csproj
@@ -33,7 +33,7 @@
     <ProjectReference Include="..\Snappier\Snappier.csproj" />
   </ItemGroup>
   <ItemGroup Condition=" '$(Configuration)' == 'Previous' ">
-    <PackageReference Include="Snappier" Version="1.0.0" />
+    <PackageReference Include="Snappier" Version="1.1.3" />
 
     <Compile Remove="FindMatchLength.cs" />
     <Compile Remove="IncrementalCopy.cs" />

diff --git a/Snappier/Internal/Constants.cs b/Snappier/Internal/Constants.cs
@@ -1,4 +1,6 @@
-namespace Snappier.Internal
+using System;
+
+namespace Snappier.Internal
 {
     internal static class Constants
     {
@@ -35,8 +37,8 @@ public enum ChunkType : byte
         ///      (1) Extracting a byte is faster than a bit-field
         ///      (2) It properly aligns copy offset so we do not need a &lt;&lt;8
         /// </summary>
-        public static readonly ushort[] CharTable =
-        {
+        public static ReadOnlySpan<ushort> CharTable =>
+        [
             0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
             0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
             0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
@@ -69,6 +71,6 @@ public enum ChunkType : byte
             0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
             0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
             0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
-        };
+        ];
     }
 }
diff --git a/Snappier/Internal/SnappyDecompressor.cs b/Snappier/Internal/SnappyDecompressor.cs
@@ -3,6 +3,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace Snappier.Internal
 {
@@ -178,10 +179,10 @@ public static int ReadUncompressedLength(ReadOnlySpan<byte> input)
 
         internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
         {
-            // Put Constants.CharTable on the stack to simplify lookups within the loops below.
-            // Slicing with length 256 here allows the JIT compiler to recognize the size is greater than
-            // the size of the byte we're indexing with and optimize out range checks.
-            ReadOnlySpan<ushort> charTable = Constants.CharTable.AsSpan(0, 256);
+            // We only index into this array with a byte, and the list is 256 long, so it's safe to skip range checks.
+            // JIT doesn't seem to recognize this currently, so we'll use a ref and Unsafe.Add to avoid the checks.
+            Debug.Assert(Constants.CharTable.Length >= 256);
+            ref ushort charTable = ref MemoryMarshal.GetReference(Constants.CharTable);
 
             unchecked
             {
@@ -323,7 +324,7 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
                         }
                         else
                         {
-                            ushort entry = charTable[c];
+                            ushort entry = Unsafe.Add(ref charTable, c);
 
                             // We don't use BitConverter to read because we might be reading past the end of the span
                             // But we know that's safe because we'll be doing it in _scratch with extra data on the end.