// and end into a single 8-byte value. Each of these values are a summary of
// a bitmap and are thus counts, each of which may have a maximum value of
// 2^21 - 1, or all three may be equal to 2^21. The latter case is represented
// by just setting the 64th bit.
type pallocSum uint64

// packPallocSum takes a start, max, and end value and produces a pallocSum.
func packPallocSum(start, max, end uint) pallocSum {