RavenDB-22986 - use Vector256 for finding the first set bit

src/Voron/Impl/FreeSpace/StreamBitArray.cs

@@ -38,6 +38,8 @@
 using Sparrow.Server;
 using System.Numerics;
 using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.X86;
 
 namespace Voron.Impl.FreeSpace
 {
@@ -116,6 +118,24 @@ public bool Get(int index)
 
         private int? FirstSetBit()
         {
+            if (Avx2.IsSupported)
+            {
+                for (int i = 0; i < _inner.Length; i += Vector256<uint>.Count)
+                {
+                    var a = Vector256.LoadUnsafe(ref _inner[i]).AsUInt32();
+                    var gt = Vector256.GreaterThan(a, Vector256<uint>.Zero);
+                    if (gt == Vector256<uint>.Zero)
+                        continue;
+
+                    var mask = gt.ExtractMostSignificantBits();
+                    var idx = BitOperations.TrailingZeroCount(mask) + i;
+                    var item = _inner[idx];
+                    return idx * BitsInWord + BitOperations.TrailingZeroCount(item);
+                }
+
+                return null;
+            }
+
             // finding a single set bit
             for (var i = 0; i < _inner.Length; i++)
             {