[GPU] Track GPU write ticks per buffer range

src/SharpEmu.Libs/Gpu/Buffers/GpuBuffer.cs

@@ -172,16 +172,39 @@ public unsafe class GpuBuffer : IDisposable
     // A readback of it only has to wait for that tick, not for all queued work.
     public ulong LastGpuWriteTick { get; private set; }
 
+    private readonly WriteTickMap _writeTicks = new();
+    private int _writeTickPruneCount = 64;
+
     // Called by every path that records a GPU write into this buffer.
-    public void NoteGpuWrite()
+    public void NoteGpuWrite() => NoteGpuWrite(0, Size);
+
+    public void NoteGpuWrite(ulong offset, ulong size)
     {
         var tick = _scheduler.CurrentTick;
         if (tick > LastGpuWriteTick)
         {
             LastGpuWriteTick = tick;
         }
+
+        if (size == 0 || offset >= Size)
+        {
+            return;
+        }
+
+        _writeTicks.Assign(offset, Math.Min(size, Size - offset), tick);
+        if (_writeTicks.Count >= _writeTickPruneCount)
+        {
+            if (Volatile.Read(ref _foreignReads) == 0)
+            {
+                _writeTicks.Prune(_scheduler.Timeline.CompletedTick + 1);
+            }
+
+            _writeTickPruneCount = Math.Max(64, _writeTicks.Count * 2);
+        }
     }
 
+    public ulong LastGpuWriteTickIn(ulong offset, ulong size) => _writeTicks.MaxTick(offset, size);
+
     public void Write(ulong offset, ReadOnlySpan<byte> source)
     {
         if (_mapped == null || offset > Size || (ulong)source.Length > Size - offset)
@@ -243,7 +266,7 @@ public unsafe class GpuBuffer : IDisposable
         }
 
         command.EndRendering();
-        NoteGpuWrite();
+        NoteGpuWrite(destinationOffset, size);
         var vk = _device.Vk;
         var native = new CommandBuffer(command.Handle);
         var before = stackalloc BufferMemoryBarrier2[2];
@@ -271,7 +294,7 @@ public unsafe class GpuBuffer : IDisposable
 
         var command = _scheduler.Current;
         command.EndRendering();
-        NoteGpuWrite();
+        NoteGpuWrite(offset, size);
         var vk = _device.Vk;
         var native = new CommandBuffer(command.Handle);
         var before = CreateBarrier(offset, size, MemoryAccess, AccessFlags.TransferWriteBit);

src/SharpEmu.Libs/Gpu/Buffers/WriteTickMap.cs

@@ -0,0 +1,143 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.Libs.Gpu.Buffers;
+
+public sealed class WriteTickMap
+{
+    private readonly List<Entry> _entries = [];
+    private ulong _floor;
+
+    private readonly record struct Entry(ulong Begin, ulong End, ulong Tick);
+
+    public int Count => _entries.Count;
+
+    public ulong Floor => _floor;
+
+    public void Assign(ulong begin, ulong size, ulong tick)
+    {
+        if (size == 0)
+        {
+            return;
+        }
+
+        var end = begin + size;
+        var index = FirstEndingAfter(begin);
+        var removed = 0;
+        while (index + removed < _entries.Count && _entries[index + removed].Begin < end)
+        {
+            removed++;
+        }
+
+        Entry? left = null;
+        Entry? right = null;
+        if (removed != 0)
+        {
+            var first = _entries[index];
+            if (first.Begin < begin)
+            {
+                left = first with { End = begin };
+            }
+
+            var last = _entries[index + removed - 1];
+            if (last.End > end)
+            {
+                right = last with { Begin = end };
+            }
+
+            _entries.RemoveRange(index, removed);
+        }
+
+        var insert = index;
+        if (left is { } leftPiece)
+        {
+            _entries.Insert(insert++, leftPiece);
+        }
+
+        _entries.Insert(insert, new Entry(begin, end, tick));
+        var middle = insert++;
+        if (right is { } rightPiece)
+        {
+            _entries.Insert(insert, rightPiece);
+        }
+
+        if (middle + 1 < _entries.Count)
+        {
+            _ = TryMerge(middle, middle + 1);
+        }
+
+        if (middle > 0)
+        {
+            _ = TryMerge(middle - 1, middle);
+        }
+    }
+
+    public ulong MaxTick(ulong begin, ulong size)
+    {
+        var tick = _floor;
+        if (size == 0)
+        {
+            return tick;
+        }
+
+        var end = begin + size;
+        for (var index = FirstEndingAfter(begin); index < _entries.Count && _entries[index].Begin < end; index++)
+        {
+            tick = Math.Max(tick, _entries[index].Tick);
+        }
+
+        return tick;
+    }
+
+    public void Prune(ulong below)
+    {
+        var kept = 0;
+        for (var index = 0; index < _entries.Count; index++)
+        {
+            var entry = _entries[index];
+            if (entry.Tick < below)
+            {
+                _floor = Math.Max(_floor, entry.Tick);
+                continue;
+            }
+
+            _entries[kept++] = entry;
+        }
+
+        _entries.RemoveRange(kept, _entries.Count - kept);
+    }
+
+    private bool TryMerge(int left, int right)
+    {
+        var a = _entries[left];
+        var b = _entries[right];
+        if (a.End != b.Begin || a.Tick != b.Tick)
+        {
+            return false;
+        }
+
+        _entries[left] = a with { End = b.End };
+        _entries.RemoveAt(right);
+        return true;
+    }
+
+    private int FirstEndingAfter(ulong address)
+    {
+        var low = 0;
+        var high = _entries.Count;
+        while (low < high)
+        {
+            var middle = low + ((high - low) >> 1);
+            if (_entries[middle].End <= address)
+            {
+                low = middle + 1;
+            }
+            else
+            {
+                high = middle;
+            }
+        }
+
+        return low;
+    }
+}