src/runtime/mpallocbits.go - go - Git at Google

 // Copyright 2019 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package runtime

 import (
 	"runtime/internal/sys"
 )

 // pageBits is a bitmap representing one bit per page in a palloc chunk.
 type pageBits [pallocChunkPages / 64]uint64

 // get returns the value of the i'th bit in the bitmap.
 func (b *pageBits) get(i uint) uint {
 	return uint((b[i/64] >> (i % 64)) & 1)
 }

 // block64 returns the 64-bit aligned block of bits containing the i'th bit.
 func (b *pageBits) block64(i uint) uint64 {
 	return b[i/64]
 }

 // set sets bit i of pageBits.
 func (b *pageBits) set(i uint) {
 	b[i/64] |= 1 << (i % 64)
 }

 // setRange sets bits in the range [i, i+n).
 func (b *pageBits) setRange(i, n uint) {
 	_ = b[i/64]
 	if n == 1 {
 		// Fast path for the n == 1 case.
 		b.set(i)
 		return
 	}
 	// Set bits [i, j].
 	j := i + n - 1
 	if i/64 == j/64 {
 		b[i/64] |= ((uint64(1) << n) - 1) << (i % 64)
 		return
 	}
 	_ = b[j/64]
 	// Set leading bits.
 	b[i/64] |= ^uint64(0) << (i % 64)
 	for k := i/64 + 1; k < j/64; k++ {
 		b[k] = ^uint64(0)
 	}
 	// Set trailing bits.
 	b[j/64] |= (uint64(1) << (j%64 + 1)) - 1
 }

 // setAll sets all the bits of b.
 func (b *pageBits) setAll() {
 	for i := range b {
 		b[i] = ^uint64(0)
 	}
 }

 // clear clears bit i of pageBits.
 func (b *pageBits) clear(i uint) {
 	b[i/64] &^= 1 << (i % 64)
 }

 // clearRange clears bits in the range [i, i+n).
 func (b *pageBits) clearRange(i, n uint) {
 	_ = b[i/64]
 	if n == 1 {
 		// Fast path for the n == 1 case.
 		b.clear(i)
 		return
 	}
 	// Clear bits [i, j].
 	j := i + n - 1
 	if i/64 == j/64 {
 		b[i/64] &^= ((uint64(1) << n) - 1) << (i % 64)
 		return
 	}
 	_ = b[j/64]
 	// Clear leading bits.
 	b[i/64] &^= ^uint64(0) << (i % 64)
 	for k := i/64 + 1; k < j/64; k++ {
 		b[k] = 0
 	}
 	// Clear trailing bits.
 	b[j/64] &^= (uint64(1) << (j%64 + 1)) - 1
 }

 // clearAll frees all the bits of b.
 func (b *pageBits) clearAll() {
 	for i := range b {
 		b[i] = 0
 	}
 }

 // popcntRange counts the number of set bits in the
 // range [i, i+n).
 func (b *pageBits) popcntRange(i, n uint) (s uint) {
 	if n == 1 {
 		return uint((b[i/64] >> (i % 64)) & 1)
 	}
 	_ = b[i/64]
 	j := i + n - 1
 	if i/64 == j/64 {
 		return uint(sys.OnesCount64((b[i/64] >> (i % 64)) & ((1 << n) - 1)))
 	}
 	_ = b[j/64]
 	s += uint(sys.OnesCount64(b[i/64] >> (i % 64)))
 	for k := i/64 + 1; k < j/64; k++ {
 		s += uint(sys.OnesCount64(b[k]))
 	}
 	s += uint(sys.OnesCount64(b[j/64] & ((1 << (j%64 + 1)) - 1)))
 	return
 }

 // pallocBits is a bitmap that tracks page allocations for at most one
 // palloc chunk.
 //
 // The precise representation is an implementation detail, but for the
 // sake of documentation, 0s are free pages and 1s are allocated pages.
 type pallocBits pageBits

 // consec8tab is a table containing the number of consecutive
 // zero bits for any uint8 value.
 //
 // The table is generated by calling consec8(i) for each
 // possible uint8 value, which is defined as:
 //
 // // consec8 counts the maximum number of consecutive 0 bits
 // // in a uint8.
 // func consec8(n uint8) int {
 // 	n = ^n
 // 	i := 0
 // 	for n != 0 {
 // 		n &= (n << 1)
 // 		i++
 // 	}
 // 	return i
 // }
 var consec8tab = [256]uint{
 	8, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
 	4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
 	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
 	4, 3, 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
 	6, 5, 4, 4, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2,
 	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
 	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
 	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
 	7, 6, 5, 5, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3,
 	4, 3, 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
 	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
 	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
 	6, 5, 4, 4, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2,
 	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
 	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
 	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 0,
 }

 // summarize returns a packed summary of the bitmap in pallocBits.
 func (b *pallocBits) summarize() pallocSum {
 	// TODO(mknyszek): There may be something more clever to be done
 	// here to make the summarize operation more efficient. For example,
 	// we can compute start and end with 64-bit wide operations easily,
 	// but max is a bit more complex. Perhaps there exists some way to
 	// leverage the 64-bit start and end to our advantage?
 	var start, max, end uint
 	for i := 0; i < len(b); i++ {
 		a := b[i]
 		for j := 0; j < 64; j += 8 {
 			k := uint8(a >> j)

 			// Compute start.
 			si := uint(sys.TrailingZeros8(k))
 			if start == uint(i*64+j) {
 				start += si
 			}

 			// Compute max.
 			if end+si > max {
 				max = end + si
 			}
 			if mi := consec8tab[k]; mi > max {
 				max = mi
 			}

 			// Compute end.
 			if k == 0 {
 				end += 8
 			} else {
 				end = uint(sys.LeadingZeros8(k))
 			}
 		}
 	}
 	return packPallocSum(start, max, end)
 }

 // find searches for npages contiguous free pages in pallocBits and returns
 // the index where that run starts, as well as the index of the first free page
 // it found in the search. searchIdx represents the first known free page and
 // where to begin the search from.
 //
 // If find fails to find any free space, it returns an index of ^uint(0) and
 // the new searchIdx should be ignored.
 //
 // The returned searchIdx is always the index of the first free page found
 // in this bitmap during the search, except if npages == 1, in which
 // case it will be the index just after the first free page, because the
 // index returned as the first result is assumed to be allocated and so
 // represents a minor optimization for that case.
 func (b *pallocBits) find(npages uintptr, searchIdx uint) (uint, uint) {
 	if npages == 1 {
 		addr := b.find1(searchIdx)
 		// Return a searchIdx of addr + 1 since we assume addr will be
 		// allocated.
 		return addr, addr + 1
 	} else if npages <= 64 {
 		return b.findSmallN(npages, searchIdx)
 	}
 	return b.findLargeN(npages, searchIdx)
 }

 // find1 is a helper for find which searches for a single free page
 // in the pallocBits and returns the index.
 //
 // See find for an explanation of the searchIdx parameter.
 func (b *pallocBits) find1(searchIdx uint) uint {
 	for i := searchIdx / 64; i < uint(len(b)); i++ {
 		x := b[i]
 		if x == ^uint64(0) {
 			continue
 		}
 		return i*64 + uint(sys.TrailingZeros64(^x))
 	}
 	return ^uint(0)
 }

 // findSmallN is a helper for find which searches for npages contiguous free pages
 // in this pallocBits and returns the index where that run of contiguous pages
 // starts as well as the index of the first free page it finds in its search.
 //
 // See find for an explanation of the searchIdx parameter.
 //
 // Returns a ^uint(0) index on failure and the new searchIdx should be ignored.
 //
 // findSmallN assumes npages <= 64, where any such contiguous run of pages
 // crosses at most one aligned 64-bit boundary in the bits.
 func (b *pallocBits) findSmallN(npages uintptr, searchIdx uint) (uint, uint) {
 	end, newSearchIdx := uint(0), ^uint(0)
 	for i := searchIdx / 64; i < uint(len(b)); i++ {
 		bi := b[i]
 		if bi == ^uint64(0) {
 			end = 0
 			continue
 		}
 		// First see if we can pack our allocation in the trailing
 		// zeros plus the end of the last 64 bits.
 		start := uint(sys.TrailingZeros64(bi))
 		if newSearchIdx == ^uint(0) {
 			// The new searchIdx is going to be at these 64 bits after any
 			// 1s we file, so count trailing 1s.
 			newSearchIdx = i*64 + uint(sys.TrailingZeros64(^bi))
 		}
 		if end+start >= uint(npages) {
 			return i*64 - end, newSearchIdx
 		}
 		// Next, check the interior of the 64-bit chunk.
 		j := findBitRange64(^bi, uint(npages))
 		if j < 64 {
 			return i*64 + j, newSearchIdx
 		}
 		end = uint(sys.LeadingZeros64(bi))
 	}
 	return ^uint(0), newSearchIdx
 }

 // findLargeN is a helper for find which searches for npages contiguous free pages
 // in this pallocBits and returns the index where that run starts, as well as the
 // index of the first free page it found it its search.
 //
 // See alloc for an explanation of the searchIdx parameter.
 //
 // Returns a ^uint(0) index on failure and the new searchIdx should be ignored.
 //
 // findLargeN assumes npages > 64, where any such run of free pages
 // crosses at least one aligned 64-bit boundary in the bits.
 func (b *pallocBits) findLargeN(npages uintptr, searchIdx uint) (uint, uint) {
 	start, size, newSearchIdx := ^uint(0), uint(0), ^uint(0)
 	for i := searchIdx / 64; i < uint(len(b)); i++ {
 		x := b[i]
 		if x == ^uint64(0) {
 			size = 0
 			continue
 		}
 		if newSearchIdx == ^uint(0) {
 			// The new searchIdx is going to be at these 64 bits after any
 			// 1s we file, so count trailing 1s.
 			newSearchIdx = i*64 + uint(sys.TrailingZeros64(^x))
 		}
 		if size == 0 {
 			size = uint(sys.LeadingZeros64(x))
 			start = i*64 + 64 - size
 			continue
 		}
 		s := uint(sys.TrailingZeros64(x))
 		if s+size >= uint(npages) {
 			size += s
 			return start, newSearchIdx
 		}
 		if s < 64 {
 			size = uint(sys.LeadingZeros64(x))
 			start = i*64 + 64 - size
 			continue
 		}
 		size += 64
 	}
 	if size < uint(npages) {
 		return ^uint(0), newSearchIdx
 	}
 	return start, newSearchIdx
 }

 // allocRange allocates the range [i, i+n).
 func (b *pallocBits) allocRange(i, n uint) {
 	(*pageBits)(b).setRange(i, n)
 }

 // allocAll allocates all the bits of b.
 func (b *pallocBits) allocAll() {
 	(*pageBits)(b).setAll()
 }

 // free1 frees a single page in the pallocBits at i.
 func (b *pallocBits) free1(i uint) {
 	(*pageBits)(b).clear(i)
 }

 // free frees the range [i, i+n) of pages in the pallocBits.
 func (b *pallocBits) free(i, n uint) {
 	(*pageBits)(b).clearRange(i, n)
 }

 // freeAll frees all the bits of b.
 func (b *pallocBits) freeAll() {
 	(*pageBits)(b).clearAll()
 }

 // pages64 returns a 64-bit bitmap representing a block of 64 pages aligned
 // to 64 pages. The returned block of pages is the one containing the i'th
 // page in this pallocBits. Each bit represents whether the page is in-use.
 func (b *pallocBits) pages64(i uint) uint64 {
 	return (*pageBits)(b).block64(i)
 }

 // findBitRange64 returns the bit index of the first set of
 // n consecutive 1 bits. If no consecutive set of 1 bits of
 // size n may be found in c, then it returns an integer >= 64.
 func findBitRange64(c uint64, n uint) uint {
 	i := uint(0)
 	cont := uint(sys.TrailingZeros64(^c))
 	for cont < n && i < 64 {
 		i += cont
 		i += uint(sys.TrailingZeros64(c >> i))
 		cont = uint(sys.TrailingZeros64(^(c >> i)))
 	}
 	return i
 }

 // pallocData encapsulates pallocBits and a bitmap for
 // whether or not a given page is scavenged in a single
 // structure. It's effectively a pallocBits with
 // additional functionality.
 type pallocData struct {
 	pallocBits
 	scavenged pageBits
 }

 // allocRange sets bits [i, i+n) in the bitmap to 1 and
 // updates the scavenged bits appropriately.
 func (m *pallocData) allocRange(i, n uint) {
 	// Clear the scavenged bits when we alloc the range.
 	m.pallocBits.allocRange(i, n)
 	m.scavenged.clearRange(i, n)
 }

 // allocAll sets every bit in the bitmap to 1 and updates
 // the scavenged bits appropriately.
 func (m *pallocData) allocAll() {
 	// Clear the scavenged bits when we alloc the range.
 	m.pallocBits.allocAll()
 	m.scavenged.clearAll()
 }
	// Copyright 2019 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package runtime

	import (
	"runtime/internal/sys"
	)

	// pageBits is a bitmap representing one bit per page in a palloc chunk.
	type pageBits [pallocChunkPages / 64]uint64

	// get returns the value of the i'th bit in the bitmap.
	func (b *pageBits) get(i uint) uint {
	return uint((b[i/64] >> (i % 64)) & 1)
	}

	// block64 returns the 64-bit aligned block of bits containing the i'th bit.
	func (b *pageBits) block64(i uint) uint64 {
	return b[i/64]
	}

	// set sets bit i of pageBits.
	func (b *pageBits) set(i uint) {
	b[i/64] \|= 1 << (i % 64)
	}

	// setRange sets bits in the range [i, i+n).
	func (b *pageBits) setRange(i, n uint) {
	_ = b[i/64]
	if n == 1 {
	// Fast path for the n == 1 case.
	b.set(i)
	return
	}
	// Set bits [i, j].
	j := i + n - 1
	if i/64 == j/64 {
	b[i/64] \|= ((uint64(1) << n) - 1) << (i % 64)
	return
	}
	_ = b[j/64]
	// Set leading bits.
	b[i/64] \|= ^uint64(0) << (i % 64)
	for k := i/64 + 1; k < j/64; k++ {
	b[k] = ^uint64(0)
	}
	// Set trailing bits.
	b[j/64] \|= (uint64(1) << (j%64 + 1)) - 1
	}

	// setAll sets all the bits of b.
	func (b *pageBits) setAll() {
	for i := range b {
	b[i] = ^uint64(0)
	}
	}

	// clear clears bit i of pageBits.
	func (b *pageBits) clear(i uint) {
	b[i/64] &^= 1 << (i % 64)
	}

	// clearRange clears bits in the range [i, i+n).
	func (b *pageBits) clearRange(i, n uint) {
	_ = b[i/64]
	if n == 1 {
	// Fast path for the n == 1 case.
	b.clear(i)
	return
	}
	// Clear bits [i, j].
	j := i + n - 1
	if i/64 == j/64 {
	b[i/64] &^= ((uint64(1) << n) - 1) << (i % 64)
	return
	}
	_ = b[j/64]
	// Clear leading bits.
	b[i/64] &^= ^uint64(0) << (i % 64)
	for k := i/64 + 1; k < j/64; k++ {
	b[k] = 0
	}
	// Clear trailing bits.
	b[j/64] &^= (uint64(1) << (j%64 + 1)) - 1
	}

	// clearAll frees all the bits of b.
	func (b *pageBits) clearAll() {
	for i := range b {
	b[i] = 0
	}
	}

	// popcntRange counts the number of set bits in the
	// range [i, i+n).
	func (b *pageBits) popcntRange(i, n uint) (s uint) {
	if n == 1 {
	return uint((b[i/64] >> (i % 64)) & 1)
	}
	_ = b[i/64]
	j := i + n - 1
	if i/64 == j/64 {
	return uint(sys.OnesCount64((b[i/64] >> (i % 64)) & ((1 << n) - 1)))
	}
	_ = b[j/64]
	s += uint(sys.OnesCount64(b[i/64] >> (i % 64)))
	for k := i/64 + 1; k < j/64; k++ {
	s += uint(sys.OnesCount64(b[k]))
	}
	s += uint(sys.OnesCount64(b[j/64] & ((1 << (j%64 + 1)) - 1)))
	return
	}

	// pallocBits is a bitmap that tracks page allocations for at most one
	// palloc chunk.
	//
	// The precise representation is an implementation detail, but for the
	// sake of documentation, 0s are free pages and 1s are allocated pages.
	type pallocBits pageBits

	// consec8tab is a table containing the number of consecutive
	// zero bits for any uint8 value.
	//
	// The table is generated by calling consec8(i) for each
	// possible uint8 value, which is defined as:
	//
	// // consec8 counts the maximum number of consecutive 0 bits
	// // in a uint8.
	// func consec8(n uint8) int {
	// n = ^n
	// i := 0
	// for n != 0 {
	// n &= (n << 1)
	// i++
	// }
	// return i
	// }
	var consec8tab = [256]uint{
	8, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
	4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
	4, 3, 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
	6, 5, 4, 4, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2,
	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
	7, 6, 5, 5, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3,
	4, 3, 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2,
	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
	6, 5, 4, 4, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2,
	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 1,
	5, 4, 3, 3, 2, 2, 2, 2, 3, 2, 1, 1, 2, 1, 1, 1,
	4, 3, 2, 2, 2, 1, 1, 1, 3, 2, 1, 1, 2, 1, 1, 0,
	}

	// summarize returns a packed summary of the bitmap in pallocBits.
	func (b *pallocBits) summarize() pallocSum {
	// TODO(mknyszek): There may be something more clever to be done
	// here to make the summarize operation more efficient. For example,
	// we can compute start and end with 64-bit wide operations easily,
	// but max is a bit more complex. Perhaps there exists some way to
	// leverage the 64-bit start and end to our advantage?
	var start, max, end uint
	for i := 0; i < len(b); i++ {
	a := b[i]
	for j := 0; j < 64; j += 8 {
	k := uint8(a >> j)

	// Compute start.
	si := uint(sys.TrailingZeros8(k))
	if start == uint(i*64+j) {
	start += si
	}

	// Compute max.
	if end+si > max {
	max = end + si
	}
	if mi := consec8tab[k]; mi > max {
	max = mi
	}

	// Compute end.
	if k == 0 {
	end += 8
	} else {
	end = uint(sys.LeadingZeros8(k))
	}
	}
	}
	return packPallocSum(start, max, end)
	}

	// find searches for npages contiguous free pages in pallocBits and returns
	// the index where that run starts, as well as the index of the first free page
	// it found in the search. searchIdx represents the first known free page and
	// where to begin the search from.
	//
	// If find fails to find any free space, it returns an index of ^uint(0) and
	// the new searchIdx should be ignored.
	//
	// The returned searchIdx is always the index of the first free page found
	// in this bitmap during the search, except if npages == 1, in which
	// case it will be the index just after the first free page, because the
	// index returned as the first result is assumed to be allocated and so
	// represents a minor optimization for that case.
	func (b *pallocBits) find(npages uintptr, searchIdx uint) (uint, uint) {
	if npages == 1 {
	addr := b.find1(searchIdx)
	// Return a searchIdx of addr + 1 since we assume addr will be
	// allocated.
	return addr, addr + 1
	} else if npages <= 64 {
	return b.findSmallN(npages, searchIdx)
	}
	return b.findLargeN(npages, searchIdx)
	}

	// find1 is a helper for find which searches for a single free page
	// in the pallocBits and returns the index.
	//
	// See find for an explanation of the searchIdx parameter.
	func (b *pallocBits) find1(searchIdx uint) uint {
	for i := searchIdx / 64; i < uint(len(b)); i++ {
	x := b[i]
	if x == ^uint64(0) {
	continue
	}
	return i*64 + uint(sys.TrailingZeros64(^x))
	}
	return ^uint(0)
	}

	// findSmallN is a helper for find which searches for npages contiguous free pages
	// in this pallocBits and returns the index where that run of contiguous pages
	// starts as well as the index of the first free page it finds in its search.
	//
	// See find for an explanation of the searchIdx parameter.
	//
	// Returns a ^uint(0) index on failure and the new searchIdx should be ignored.
	//
	// findSmallN assumes npages <= 64, where any such contiguous run of pages
	// crosses at most one aligned 64-bit boundary in the bits.
	func (b *pallocBits) findSmallN(npages uintptr, searchIdx uint) (uint, uint) {
	end, newSearchIdx := uint(0), ^uint(0)
	for i := searchIdx / 64; i < uint(len(b)); i++ {
	bi := b[i]
	if bi == ^uint64(0) {
	end = 0
	continue
	}
	// First see if we can pack our allocation in the trailing
	// zeros plus the end of the last 64 bits.
	start := uint(sys.TrailingZeros64(bi))
	if newSearchIdx == ^uint(0) {
	// The new searchIdx is going to be at these 64 bits after any
	// 1s we file, so count trailing 1s.
	newSearchIdx = i*64 + uint(sys.TrailingZeros64(^bi))
	}
	if end+start >= uint(npages) {
	return i*64 - end, newSearchIdx
	}
	// Next, check the interior of the 64-bit chunk.
	j := findBitRange64(^bi, uint(npages))
	if j < 64 {
	return i*64 + j, newSearchIdx
	}
	end = uint(sys.LeadingZeros64(bi))
	}
	return ^uint(0), newSearchIdx
	}

	// findLargeN is a helper for find which searches for npages contiguous free pages
	// in this pallocBits and returns the index where that run starts, as well as the
	// index of the first free page it found it its search.
	//
	// See alloc for an explanation of the searchIdx parameter.
	//
	// Returns a ^uint(0) index on failure and the new searchIdx should be ignored.
	//
	// findLargeN assumes npages > 64, where any such run of free pages
	// crosses at least one aligned 64-bit boundary in the bits.
	func (b *pallocBits) findLargeN(npages uintptr, searchIdx uint) (uint, uint) {
	start, size, newSearchIdx := ^uint(0), uint(0), ^uint(0)
	for i := searchIdx / 64; i < uint(len(b)); i++ {
	x := b[i]
	if x == ^uint64(0) {
	size = 0
	continue
	}
	if newSearchIdx == ^uint(0) {
	// The new searchIdx is going to be at these 64 bits after any
	// 1s we file, so count trailing 1s.
	newSearchIdx = i*64 + uint(sys.TrailingZeros64(^x))
	}
	if size == 0 {
	size = uint(sys.LeadingZeros64(x))
	start = i*64 + 64 - size
	continue
	}
	s := uint(sys.TrailingZeros64(x))
	if s+size >= uint(npages) {
	size += s
	return start, newSearchIdx
	}
	if s < 64 {
	size = uint(sys.LeadingZeros64(x))
	start = i*64 + 64 - size
	continue
	}
	size += 64
	}
	if size < uint(npages) {
	return ^uint(0), newSearchIdx
	}
	return start, newSearchIdx
	}

	// allocRange allocates the range [i, i+n).
	func (b *pallocBits) allocRange(i, n uint) {
	(*pageBits)(b).setRange(i, n)
	}

	// allocAll allocates all the bits of b.
	func (b *pallocBits) allocAll() {
	(*pageBits)(b).setAll()
	}

	// free1 frees a single page in the pallocBits at i.
	func (b *pallocBits) free1(i uint) {
	(*pageBits)(b).clear(i)
	}

	// free frees the range [i, i+n) of pages in the pallocBits.
	func (b *pallocBits) free(i, n uint) {
	(*pageBits)(b).clearRange(i, n)
	}

	// freeAll frees all the bits of b.
	func (b *pallocBits) freeAll() {
	(*pageBits)(b).clearAll()
	}

	// pages64 returns a 64-bit bitmap representing a block of 64 pages aligned
	// to 64 pages. The returned block of pages is the one containing the i'th
	// page in this pallocBits. Each bit represents whether the page is in-use.
	func (b *pallocBits) pages64(i uint) uint64 {
	return (*pageBits)(b).block64(i)
	}

	// findBitRange64 returns the bit index of the first set of
	// n consecutive 1 bits. If no consecutive set of 1 bits of
	// size n may be found in c, then it returns an integer >= 64.
	func findBitRange64(c uint64, n uint) uint {
	i := uint(0)
	cont := uint(sys.TrailingZeros64(^c))
	for cont < n && i < 64 {
	i += cont
	i += uint(sys.TrailingZeros64(c >> i))
	cont = uint(sys.TrailingZeros64(^(c >> i)))
	}
	return i
	}

	// pallocData encapsulates pallocBits and a bitmap for
	// whether or not a given page is scavenged in a single
	// structure. It's effectively a pallocBits with
	// additional functionality.
	type pallocData struct {
	pallocBits
	scavenged pageBits
	}

	// allocRange sets bits [i, i+n) in the bitmap to 1 and
	// updates the scavenged bits appropriately.
	func (m *pallocData) allocRange(i, n uint) {
	// Clear the scavenged bits when we alloc the range.
	m.pallocBits.allocRange(i, n)
	m.scavenged.clearRange(i, n)
	}

	// allocAll sets every bit in the bitmap to 1 and updates
	// the scavenged bits appropriately.
	func (m *pallocData) allocAll() {
	// Clear the scavenged bits when we alloc the range.
	m.pallocBits.allocAll()
	m.scavenged.clearAll()
	}