internal/atlas: reintroduce a byte pool

The old byte pool depnded on lifetimes of command queues, and this
was tricky.

The new byte pool uses runtime.SetFinalizer so this should be much
easier to use.

Updates #1681

internal/atlas/bytes.go

@@ -0,0 +1,93 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package atlas
+
+import (
+	"runtime"
+	"sync"
+	"unsafe"
+)
+
+// allocBytesFromPool returns a byte slice with the given size.
+// The slice might be obtained from a cache, and might not be zero-cleared.
+func allocBytesFromPool(size int) []byte {
+	return theBytesPool.get(size)
+}
+
+type bytesPool struct {
+	pool [][]byte
+
+	m sync.Mutex
+}
+
+var theBytesPool bytesPool
+
+func (b *bytesPool) get(size int) []byte {
+	if size == 0 {
+		return nil
+	}
+
+	if bs := b.getFromCache(size); bs != nil {
+		return bs
+	}
+
+	bs := make([]byte, size)
+	b.setFinalizer(bs)
+	return bs
+}
+
+func (b *bytesPool) getFromCache(size int) []byte {
+	b.m.Lock()
+	defer b.m.Unlock()
+
+	for i, bs := range b.pool {
+		if cap(bs) < size {
+			continue
+		}
+
+		copy(b.pool[i:], b.pool[i+1:])
+		b.pool[len(b.pool)-1] = nil
+		b.pool = b.pool[:len(b.pool)-1]
+
+		bs = bs[:size]
+		b.setFinalizer(bs)
+		return bs
+	}
+
+	return nil
+}
+
+func (b *bytesPool) setFinalizer(bs []byte) {
+	c := cap(bs)
+	runtime.SetFinalizer(&bs[0], func(ptr *byte) {
+		b.m.Lock()
+		defer b.m.Unlock()
+
+		b.pool = append(b.pool, unsafe.Slice(ptr, c))
+
+		// GC the pool. The size limitation is arbitrary.
+		for len(b.pool) >= 32 || b.totalSize() >= 1024*1024*1024 {
+			b.pool = b.pool[1:]
+		}
+	})
+}
+
+func (b *bytesPool) totalSize() int {
+	var s int
+	for _, bs := range b.pool {
+		s += len(bs)
+	}
+	return s
+}

internal/atlas/image.go

@@ -482,15 +482,13 @@ func (i *Image) writePixels(pix []byte, region image.Rectangle) {
 		}
 
 		// Copy pixels in the case when pix is modified before the graphics command is executed.
-		// TODO: Create byte slices from a pool.
-		pix2 := make([]byte, len(pix))
+		pix2 := allocBytesFromPool(len(pix))
 		copy(pix2, pix)
 		i.backend.restorable.WritePixels(pix2, region)
 		return
 	}
 
-	// TODO: Create byte slices from a pool.
-	pixb := make([]byte, 4*r.Dx()*r.Dy())
+	pixb := allocBytesFromPool(4 * r.Dx() * r.Dy())
 
 	// Clear the edges. pixb might not be zero-cleared.
 	// TODO: These loops assume that paddingSize is 1.