skl/arena.go
func (s *Arena) putNode(height int) uint32 {
	// Compute the amount of the tower that will never be used, since the height
	// is less than maxHeight.
	unusedSize := (maxHeight - height) * offsetSize

	// Pad the allocation with enough bytes to ensure pointer alignment.
	l := uint32(MaxNodeSize - unusedSize + nodeAlign)
	n := atomic.AddUint32(&s.n, l)
	y.AssertTruef(int(n) <= len(s.buf),
		"Arena too small, toWrite:%d newTotal:%d limit:%d",
		l, n, len(s.buf))

	// Return the aligned offset.
	m := (n - l + uint32(nodeAlign)) & ^uint32(nodeAlign)
	return m
}
func (s *Arena) putNode(height int) uint32 {
	// Compute the amount of the tower that will never be used, since the height
	// is less than maxHeight.
	unusedSize := (maxHeight - height) * offsetSize

	// Pad the allocation with enough bytes to ensure pointer alignment.
	l := uint32(MaxNodeSize - unusedSize + nodeAlign)
	n := atomic.AddUint32(&s.n, l)
	y.AssertTruef(int(n) <= len(s.buf),
		"Arena too small, toWrite:%d newTotal:%d limit:%d",
		l, n, len(s.buf))

	// Return the aligned offset.
	m := (n - l + uint32(nodeAlign)) & ^uint32(nodeAlign)
	return m
}
// putNode allocates a node in the arena. The node is aligned on a pointer-sized
// boundary. The arena offset of the node is returned.