From 8a631cec77fe6a03f3dbc15c21855205cb350b1d Mon Sep 17 00:00:00 2001
From: Brant Burnett <bburnett@centeredgesoftware.com>
Date: Mon, 6 Feb 2023 08:34:10 -0500
Subject: [PATCH] Rewrite decompressor RefillTagFromScatch method to use refs
 (#49)

Motivation
----------
This is a step towards code that doesn't require pinning which can help
with GC when compression/decompression is run a lot. GC will be able to
move memory even when in the middle of a compression or decompression
run and update the ref pointers.

Modifications
-------------
Rewrite RefillTagFromScratch to use refs, returning a ValueTuple so that
the pointer may be incremented the correct amount by the caller.

Results
-------
.NET 6 shows a per improvement, .NET 7 is neutral. .NET 4.8 shows a
slight regression, I'm assuming due to poor enregistration.

BenchmarkDotNet=v0.13.4, OS=Windows 11 (10.0.22000.1455/21H2) Intel Core
i7-10850H CPU 2.70GHz, 1 CPU, 12 logical and 6 physical cores .NET
SDK=7.0.102
[Host] : .NET 7.0.2 (7.0.222.60605), X64 RyuJIT AVX2
MediumRun-.NET 6.0 : .NET 6.0.13 (6.0.1322.58009), X64 RyuJIT AVX2
MediumRun-.NET 7.0 : .NET 7.0.2 (7.0.222.60605), X64 RyuJIT AVX2
MediumRun-.NET Framework 4.8 : .NET Framework 4.8 (4.8.4515.0), X64
RyuJIT VectorSize=256

IterationCount=15  LaunchCount=2  WarmupCount=10

| Method | Job | Runtime | Mean | Error | StdDev | Median | Ratio |
RatioSD |
|-------- |----------------------------- |-------------------
|----------:|---------:|---------:|----------:|------:|--------:|
| Pointer | MediumRun-.NET 6.0 | .NET 6.0 | 109.06 us | 1.150 us | 1.686
us | 108.60 us | 1.00 | 0.00 |
| Ref | MediumRun-.NET 6.0 | .NET 6.0 | 106.20 us | 0.863 us | 1.265 us
| 106.47 us | 0.97 | 0.02 |
| | | | | | | | | |
| Pointer | MediumRun-.NET 7.0 | .NET 7.0 | 93.34 us | 0.877 us | 1.257
us | 93.13 us | 1.00 | 0.00 |
| Ref | MediumRun-.NET 7.0 | .NET 7.0 | 94.00 us | 1.161 us | 1.738 us |
95.01 us | 1.01 | 0.03 |
| | | | | | | | | |
| Pointer | MediumRun-.NET Framework 4.8 | .NET Framework 4.8 | 106.36
us | 0.357 us | 0.501 us | 106.42 us | 1.00 | 0.00 |
| Ref | MediumRun-.NET Framework 4.8 | .NET Framework 4.8 | 111.34 us |
1.041 us | 1.558 us | 110.50 us | 1.05 | 0.02 |
---
 Snappier/Internal/SnappyDecompressor.cs | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/Snappier/Internal/SnappyDecompressor.cs b/Snappier/Internal/SnappyDecompressor.cs
index 15ae694..ee00b24 100644
--- a/Snappier/Internal/SnappyDecompressor.cs
+++ b/Snappier/Internal/SnappyDecompressor.cs
@@ -210,7 +210,10 @@ internal unsafe void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
                                 // and any literal data from the _input buffer
 
                                 // scratch will be the scratch buffer with only the tag if true is returned
-                                if (!RefillTagFromScratch(ref input, inputEnd, scratch))
+                                (bool sufficientData, uint inputUsed) = RefillTagFromScratch(ref Unsafe.AsRef<byte>(input),
+                                    ref Unsafe.AsRef<byte>(inputEnd), ref Unsafe.AsRef<byte>(scratch));
+                                input += inputUsed;
+                                if (!sufficientData)
                                 {
                                     return;
                                 }
@@ -416,33 +419,31 @@ internal unsafe void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
             }
         }
 
-        private unsafe bool RefillTagFromScratch(ref byte* input, byte* inputEnd, byte* scratch)
+        private (bool sufficientData, uint inputUsed) RefillTagFromScratch(ref byte input, ref byte inputEnd, ref byte scratch)
         {
             Debug.Assert(_scratchLength > 0);
 
-            if (input >= inputEnd)
+            if (!Unsafe.IsAddressLessThan(ref input, ref inputEnd))
             {
-                return false;
+                return (false, 0);
             }
 
             // Read the tag character
-            byte c = *scratch;
-            uint entry = Constants.CharTable[c];
+            uint entry = Constants.CharTable[scratch];
             uint needed = (entry >> 11) + 1; // +1 byte for 'c'
 
-            uint toCopy = Math.Min(unchecked((uint)(inputEnd - input)), needed - _scratchLength);
-            Unsafe.CopyBlockUnaligned(scratch + _scratchLength, input, toCopy);
+            uint toCopy = Math.Min((uint)Unsafe.ByteOffset(ref input, ref inputEnd), needed - _scratchLength);
+            Unsafe.CopyBlockUnaligned(ref Unsafe.Add(ref scratch, _scratchLength), ref input, toCopy);
 
             _scratchLength += toCopy;
-            input += toCopy;
 
             if (_scratchLength < needed)
             {
                 // Still insufficient
-                return false;
+                return (false, toCopy);
             }
 
-            return true;
+            return (true, toCopy);
         }
 
         private unsafe bool RefillTag(ref byte* input, ref byte* inputEnd, byte* scratch)