type test struct {
		free []BitRange // Ranges of free (zero) bits.
		hits []PallocSum
	}
	tests := make(map[string]test)
	tests["NoneFree"] = test{
		free: []BitRange{},
		hits: []PallocSum{
			PackPallocSum(0, 0, 0),
		},
	}
	tests["OnlyStart"] = test{
		free: []BitRange{{0, 10}},
		hits: []PallocSum{
			PackPallocSum(10, 10, 0),
		},
	}
	tests["OnlyEnd"] = test{

func packPallocSum(start, max, end uint) pallocSum {
	if max == maxPackedValue {
		return pallocSum(uint64(1 << 63))
	}
	return pallocSum((uint64(start) & (maxPackedValue - 1)) |
		((uint64(max) & (maxPackedValue - 1)) << logMaxPackedValue) |
		((uint64(end) & (maxPackedValue - 1)) << (2 * logMaxPackedValue)))
}

// start extracts the start value from a packed sum.
func (p pallocSum) start() uint {

	PallocSumBytes   = pallocSumBytes
)

// Expose pallocSum for testing.
type PallocSum pallocSum

func PackPallocSum(start, max, end uint) PallocSum { return PallocSum(packPallocSum(start, max, end)) }
func (m PallocSum) Start() uint                    { return pallocSum(m).start() }
func (m PallocSum) Max() uint                      { return pallocSum(m).max() }
func (m PallocSum) End() uint                      { return pallocSum(m).end() }

	for l, shift := range levelShift {
		entries := 1 << (heapAddrBits - shift)

		// Put this reservation into a slice.
		sl := notInHeapSlice{(*notInHeap)(reservation), 0, entries}
		p.summary[l] = *(*[]pallocSum)(unsafe.Pointer(&sl))

		reservation = add(reservation, uintptr(entries)*pallocSumBytes)
	}
}

// See mpagealloc_64bit.go for details.
func (p *pageAlloc) sysGrow(base, limit uintptr) {

		r := sysReserve(nil, b)
		if r == nil {
			throw("failed to reserve page summary memory")
		}

		// Put this reservation into a slice.
		sl := notInHeapSlice{(*notInHeap)(r), 0, entries}
		p.summary[l] = *(*[]pallocSum)(unsafe.Pointer(&sl))
	}
}

// sysGrow performs architecture-dependent operations on heap
// growth for the page allocator, such as mapping in new memory
// for summaries. It also updates the length of the slices in

// sake of documentation, 0s are free pages and 1s are allocated pages.
type pallocBits pageBits

// summarize returns a packed summary of the bitmap in pallocBits.
func (b *pallocBits) summarize() pallocSum {
	var start, most, cur uint
	const notSetYet = ^uint(0) // sentinel for start value
	start = notSetYet
	for i := 0; i < len(b); i++ {
		x := b[i]
		if x == 0 {
			cur += 64
			continue
		}