compiler: revert `for package-scope "a = b; b = x" just set "a = x"`

[official-gcc.git] / libgo / go / runtime / mpagealloc.go

// 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.

// Page allocator.
//
// The page allocator manages mapped pages (defined by pageSize, NOT
// physPageSize) for allocation and re-use. It is embedded into mheap.
//
// Pages are managed using a bitmap that is sharded into chunks.
// In the bitmap, 1 means in-use, and 0 means free. The bitmap spans the
// process's address space. Chunks are managed in a sparse-array-style structure
// similar to mheap.arenas, since the bitmap may be large on some systems.
//
// The bitmap is efficiently searched by using a radix tree in combination
// with fast bit-wise intrinsics. Allocation is performed using an address-ordered
// first-fit approach.
//
// Each entry in the radix tree is a summary that describes three properties of
// a particular region of the address space: the number of contiguous free pages
// at the start and end of the region it represents, and the maximum number of
// contiguous free pages found anywhere in that region.
//
// Each level of the radix tree is stored as one contiguous array, which represents
// a different granularity of subdivision of the processes' address space. Thus, this
// radix tree is actually implicit in these large arrays, as opposed to having explicit
// dynamically-allocated pointer-based node structures. Naturally, these arrays may be
// quite large for system with large address spaces, so in these cases they are mapped
// into memory as needed. The leaf summaries of the tree correspond to a bitmap chunk.
//
// The root level (referred to as L0 and index 0 in pageAlloc.summary) has each
// summary represent the largest section of address space (16 GiB on 64-bit systems),
// with each subsequent level representing successively smaller subsections until we
// reach the finest granularity at the leaves, a chunk.
//
// More specifically, each summary in each level (except for leaf summaries)
// represents some number of entries in the following level. For example, each
// summary in the root level may represent a 16 GiB region of address space,
// and in the next level there could be 8 corresponding entries which represent 2
// GiB subsections of that 16 GiB region, each of which could correspond to 8
// entries in the next level which each represent 256 MiB regions, and so on.
//
// Thus, this design only scales to heaps so large, but can always be extended to
// larger heaps by simply adding levels to the radix tree, which mostly costs
// additional virtual address space. The choice of managing large arrays also means
// that a large amount of virtual address space may be reserved by the runtime.

package runtime

import (
        "runtime/internal/atomic"
        "unsafe"
)

const (
        // The size of a bitmap chunk, i.e. the amount of bits (that is, pages) to consider
        // in the bitmap at once.
        pallocChunkPages    = 1 << logPallocChunkPages
        pallocChunkBytes    = pallocChunkPages * pageSize
        logPallocChunkPages = 9
        logPallocChunkBytes = logPallocChunkPages + pageShift

        // The number of radix bits for each level.
        //
        // The value of 3 is chosen such that the block of summaries we need to scan at
        // each level fits in 64 bytes (2^3 summaries * 8 bytes per summary), which is
        // close to the L1 cache line width on many systems. Also, a value of 3 fits 4 tree
        // levels perfectly into the 21-bit pallocBits summary field at the root level.
        //
        // The following equation explains how each of the constants relate:
        // summaryL0Bits + (summaryLevels-1)*summaryLevelBits + logPallocChunkBytes = heapAddrBits
        //
        // summaryLevels is an architecture-dependent value defined in mpagealloc_*.go.
        summaryLevelBits = 3
        summaryL0Bits    = heapAddrBits - logPallocChunkBytes - (summaryLevels-1)*summaryLevelBits

        // pallocChunksL2Bits is the number of bits of the chunk index number
        // covered by the second level of the chunks map.
        //
        // See (*pageAlloc).chunks for more details. Update the documentation
        // there should this change.
        pallocChunksL2Bits  = heapAddrBits - logPallocChunkBytes - pallocChunksL1Bits
        pallocChunksL1Shift = pallocChunksL2Bits
)

// Maximum searchAddr value, which indicates that the heap has no free space.
//
// We alias maxOffAddr just to make it clear that this is the maximum address
// for the page allocator's search space. See maxOffAddr for details.
func maxSearchAddr() offAddr {
        return maxOffAddr
}

// Global chunk index.
//
// Represents an index into the leaf level of the radix tree.
// Similar to arenaIndex, except instead of arenas, it divides the address
// space into chunks.
type chunkIdx uint

// chunkIndex returns the global index of the palloc chunk containing the
// pointer p.
func chunkIndex(p uintptr) chunkIdx {
        return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes)
}

// chunkIndex returns the base address of the palloc chunk at index ci.
func chunkBase(ci chunkIdx) uintptr {
        return uintptr(ci)*pallocChunkBytes + arenaBaseOffset
}

// chunkPageIndex computes the index of the page that contains p,
// relative to the chunk which contains p.
func chunkPageIndex(p uintptr) uint {
        return uint(p % pallocChunkBytes / pageSize)
}

// l1 returns the index into the first level of (*pageAlloc).chunks.
func (i chunkIdx) l1() uint {
        if pallocChunksL1Bits == 0 {
                // Let the compiler optimize this away if there's no
                // L1 map.
                return 0
        } else {
                return uint(i) >> pallocChunksL1Shift
        }
}

// l2 returns the index into the second level of (*pageAlloc).chunks.
func (i chunkIdx) l2() uint {
        if pallocChunksL1Bits == 0 {
                return uint(i)
        } else {
                return uint(i) & (1<<pallocChunksL2Bits - 1)
        }
}

// offAddrToLevelIndex converts an address in the offset address space
// to the index into summary[level] containing addr.
func offAddrToLevelIndex(level int, addr offAddr) int {
        return int((addr.a - arenaBaseOffset) >> levelShift[level])
}

// levelIndexToOffAddr converts an index into summary[level] into
// the corresponding address in the offset address space.
func levelIndexToOffAddr(level, idx int) offAddr {
        return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset}
}

// addrsToSummaryRange converts base and limit pointers into a range
// of entries for the given summary level.
//
// The returned range is inclusive on the lower bound and exclusive on
// the upper bound.
func addrsToSummaryRange(level int, base, limit uintptr) (lo int, hi int) {
        // This is slightly more nuanced than just a shift for the exclusive
        // upper-bound. Note that the exclusive upper bound may be within a
        // summary at this level, meaning if we just do the obvious computation
        // hi will end up being an inclusive upper bound. Unfortunately, just
        // adding 1 to that is too broad since we might be on the very edge
        // of a summary's max page count boundary for this level
        // (1 << levelLogPages[level]). So, make limit an inclusive upper bound
        // then shift, then add 1, so we get an exclusive upper bound at the end.
        lo = int((base - arenaBaseOffset) >> levelShift[level])
        hi = int(((limit-1)-arenaBaseOffset)>>levelShift[level]) + 1
        return
}

// blockAlignSummaryRange aligns indices into the given level to that
// level's block width (1 << levelBits[level]). It assumes lo is inclusive
// and hi is exclusive, and so aligns them down and up respectively.
func blockAlignSummaryRange(level int, lo, hi int) (int, int) {
        e := uintptr(1) << levelBits[level]
        return int(alignDown(uintptr(lo), e)), int(alignUp(uintptr(hi), e))
}

type pageAlloc struct {
        // Radix tree of summaries.
        //
        // Each slice's cap represents the whole memory reservation.
        // Each slice's len reflects the allocator's maximum known
        // mapped heap address for that level.
        //
        // The backing store of each summary level is reserved in init
        // and may or may not be committed in grow (small address spaces
        // may commit all the memory in init).
        //
        // The purpose of keeping len <= cap is to enforce bounds checks
        // on the top end of the slice so that instead of an unknown
        // runtime segmentation fault, we get a much friendlier out-of-bounds
        // error.
        //
        // To iterate over a summary level, use inUse to determine which ranges
        // are currently available. Otherwise one might try to access
        // memory which is only Reserved which may result in a hard fault.
        //
        // We may still get segmentation faults < len since some of that
        // memory may not be committed yet.
        summary [summaryLevels][]pallocSum

        // chunks is a slice of bitmap chunks.
        //
        // The total size of chunks is quite large on most 64-bit platforms
        // (O(GiB) or more) if flattened, so rather than making one large mapping
        // (which has problems on some platforms, even when PROT_NONE) we use a
        // two-level sparse array approach similar to the arena index in mheap.
        //
        // To find the chunk containing a memory address `a`, do:
        //   chunkOf(chunkIndex(a))
        //
        // Below is a table describing the configuration for chunks for various
        // heapAddrBits supported by the runtime.
        //
        // heapAddrBits | L1 Bits | L2 Bits | L2 Entry Size
        // ------------------------------------------------
        // 32           | 0       | 10      | 128 KiB
        // 33 (iOS)     | 0       | 11      | 256 KiB
        // 48           | 13      | 13      | 1 MiB
        //
        // There's no reason to use the L1 part of chunks on 32-bit, the
        // address space is small so the L2 is small. For platforms with a
        // 48-bit address space, we pick the L1 such that the L2 is 1 MiB
        // in size, which is a good balance between low granularity without
        // making the impact on BSS too high (note the L1 is stored directly
        // in pageAlloc).
        //
        // To iterate over the bitmap, use inUse to determine which ranges
        // are currently available. Otherwise one might iterate over unused
        // ranges.
        //
        // Protected by mheapLock.
        //
        // TODO(mknyszek): Consider changing the definition of the bitmap
        // such that 1 means free and 0 means in-use so that summaries and
        // the bitmaps align better on zero-values.
        chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData

        // The address to start an allocation search with. It must never
        // point to any memory that is not contained in inUse, i.e.
        // inUse.contains(searchAddr.addr()) must always be true. The one
        // exception to this rule is that it may take on the value of
        // maxOffAddr to indicate that the heap is exhausted.
        //
        // We guarantee that all valid heap addresses below this value
        // are allocated and not worth searching.
        searchAddr offAddr

        // start and end represent the chunk indices
        // which pageAlloc knows about. It assumes
        // chunks in the range [start, end) are
        // currently ready to use.
        start, end chunkIdx

        // inUse is a slice of ranges of address space which are
        // known by the page allocator to be currently in-use (passed
        // to grow).
        //
        // This field is currently unused on 32-bit architectures but
        // is harmless to track. We care much more about having a
        // contiguous heap in these cases and take additional measures
        // to ensure that, so in nearly all cases this should have just
        // 1 element.
        //
        // All access is protected by the mheapLock.
        inUse addrRanges

        // scav stores the scavenger state.
        scav struct {
                lock mutex

                // inUse is a slice of ranges of address space which have not
                // yet been looked at by the scavenger.
                //
                // Protected by lock.
                inUse addrRanges

                // gen is the scavenge generation number.
                //
                // Protected by lock.
                gen uint32

                // reservationBytes is how large of a reservation should be made
                // in bytes of address space for each scavenge iteration.
                //
                // Protected by lock.
                reservationBytes uintptr

                // released is the amount of memory released this generation.
                //
                // Updated atomically.
                released uintptr

                // scavLWM is the lowest (offset) address that the scavenger reached this
                // scavenge generation.
                //
                // Protected by lock.
                scavLWM offAddr

                // freeHWM is the highest (offset) address of a page that was freed to
                // the page allocator this scavenge generation.
                //
                // Protected by mheapLock.
                freeHWM offAddr
        }

        // mheap_.lock. This level of indirection makes it possible
        // to test pageAlloc indepedently of the runtime allocator.
        mheapLock *mutex

        // sysStat is the runtime memstat to update when new system
        // memory is committed by the pageAlloc for allocation metadata.
        sysStat *sysMemStat

        // Whether or not this struct is being used in tests.
        test bool
}

func (p *pageAlloc) init(mheapLock *mutex, sysStat *sysMemStat) {
        if levelLogPages[0] > logMaxPackedValue {
                // We can't represent 1<<levelLogPages[0] pages, the maximum number
                // of pages we need to represent at the root level, in a summary, which
                // is a big problem. Throw.
                print("runtime: root level max pages = ", 1<<levelLogPages[0], "\n")
                print("runtime: summary max pages = ", maxPackedValue, "\n")
                throw("root level max pages doesn't fit in summary")
        }
        p.sysStat = sysStat

        // Initialize p.inUse.
        p.inUse.init(sysStat)

        // System-dependent initialization.
        p.sysInit()

        // Start with the searchAddr in a state indicating there's no free memory.
        p.searchAddr = maxSearchAddr()

        // Set the mheapLock.
        p.mheapLock = mheapLock

        // Initialize scavenge tracking state.
        p.scav.scavLWM = maxSearchAddr()
}

// tryChunkOf returns the bitmap data for the given chunk.
//
// Returns nil if the chunk data has not been mapped.
func (p *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData {
        l2 := p.chunks[ci.l1()]
        if l2 == nil {
                return nil
        }
        return &l2[ci.l2()]
}

// chunkOf returns the chunk at the given chunk index.
//
// The chunk index must be valid or this method may throw.
func (p *pageAlloc) chunkOf(ci chunkIdx) *pallocData {
        return &p.chunks[ci.l1()][ci.l2()]
}

// grow sets up the metadata for the address range [base, base+size).
// It may allocate metadata, in which case *p.sysStat will be updated.
//
// p.mheapLock must be held.
func (p *pageAlloc) grow(base, size uintptr) {
        assertLockHeld(p.mheapLock)

        // Round up to chunks, since we can't deal with increments smaller
        // than chunks. Also, sysGrow expects aligned values.
        limit := alignUp(base+size, pallocChunkBytes)
        base = alignDown(base, pallocChunkBytes)

        // Grow the summary levels in a system-dependent manner.
        // We just update a bunch of additional metadata here.
        p.sysGrow(base, limit)

        // Update p.start and p.end.
        // If no growth happened yet, start == 0. This is generally
        // safe since the zero page is unmapped.
        firstGrowth := p.start == 0
        start, end := chunkIndex(base), chunkIndex(limit)
        if firstGrowth || start < p.start {
                p.start = start
        }
        if end > p.end {
                p.end = end
        }
        // Note that [base, limit) will never overlap with any existing
        // range inUse because grow only ever adds never-used memory
        // regions to the page allocator.
        p.inUse.add(makeAddrRange(base, limit))

        // A grow operation is a lot like a free operation, so if our
        // chunk ends up below p.searchAddr, update p.searchAddr to the
        // new address, just like in free.
        if b := (offAddr{base}); b.lessThan(p.searchAddr) {
                p.searchAddr = b
        }

        // Add entries into chunks, which is sparse, if needed. Then,
        // initialize the bitmap.
        //
        // Newly-grown memory is always considered scavenged.
        // Set all the bits in the scavenged bitmaps high.
        for c := chunkIndex(base); c < chunkIndex(limit); c++ {
                if p.chunks[c.l1()] == nil {
                        // Create the necessary l2 entry.
                        //
                        // Store it atomically to avoid races with readers which
                        // don't acquire the heap lock.
                        r := sysAlloc(unsafe.Sizeof(*p.chunks[0]), p.sysStat)
                        if r == nil {
                                throw("pageAlloc: out of memory")
                        }
                        atomic.StorepNoWB(unsafe.Pointer(&p.chunks[c.l1()]), r)
                }
                p.chunkOf(c).scavenged.setRange(0, pallocChunkPages)
        }

        // Update summaries accordingly. The grow acts like a free, so
        // we need to ensure this newly-free memory is visible in the
        // summaries.
        p.update(base, size/pageSize, true, false)
}

// update updates heap metadata. It must be called each time the bitmap
// is updated.
//
// If contig is true, update does some optimizations assuming that there was
// a contiguous allocation or free between addr and addr+npages. alloc indicates
// whether the operation performed was an allocation or a free.
//
// p.mheapLock must be held.
func (p *pageAlloc) update(base, npages uintptr, contig, alloc bool) {
        assertLockHeld(p.mheapLock)

        // base, limit, start, and end are inclusive.
        limit := base + npages*pageSize - 1
        sc, ec := chunkIndex(base), chunkIndex(limit)

        // Handle updating the lowest level first.
        if sc == ec {
                // Fast path: the allocation doesn't span more than one chunk,
                // so update this one and if the summary didn't change, return.
                x := p.summary[len(p.summary)-1][sc]
                y := p.chunkOf(sc).summarize()
                if x == y {
                        return
                }
                p.summary[len(p.summary)-1][sc] = y
        } else if contig {
                // Slow contiguous path: the allocation spans more than one chunk
                // and at least one summary is guaranteed to change.
                summary := p.summary[len(p.summary)-1]

                // Update the summary for chunk sc.
                summary[sc] = p.chunkOf(sc).summarize()

                // Update the summaries for chunks in between, which are
                // either totally allocated or freed.
                whole := p.summary[len(p.summary)-1][sc+1 : ec]
                if alloc {
                        // Should optimize into a memclr.
                        for i := range whole {
                                whole[i] = 0
                        }
                } else {
                        for i := range whole {
                                whole[i] = freeChunkSum
                        }
                }

                // Update the summary for chunk ec.
                summary[ec] = p.chunkOf(ec).summarize()
        } else {
                // Slow general path: the allocation spans more than one chunk
                // and at least one summary is guaranteed to change.
                //
                // We can't assume a contiguous allocation happened, so walk over
                // every chunk in the range and manually recompute the summary.
                summary := p.summary[len(p.summary)-1]
                for c := sc; c <= ec; c++ {
                        summary[c] = p.chunkOf(c).summarize()
                }
        }

        // Walk up the radix tree and update the summaries appropriately.
        changed := true
        for l := len(p.summary) - 2; l >= 0 && changed; l-- {
                // Update summaries at level l from summaries at level l+1.
                changed = false

                // "Constants" for the previous level which we
                // need to compute the summary from that level.
                logEntriesPerBlock := levelBits[l+1]
                logMaxPages := levelLogPages[l+1]

                // lo and hi describe all the parts of the level we need to look at.
                lo, hi := addrsToSummaryRange(l, base, limit+1)

                // Iterate over each block, updating the corresponding summary in the less-granular level.
                for i := lo; i < hi; i++ {
                        children := p.summary[l+1][i<<logEntriesPerBlock : (i+1)<<logEntriesPerBlock]
                        sum := mergeSummaries(children, logMaxPages)
                        old := p.summary[l][i]
                        if old != sum {
                                changed = true
                                p.summary[l][i] = sum
                        }
                }
        }
}

// allocRange marks the range of memory [base, base+npages*pageSize) as
// allocated. It also updates the summaries to reflect the newly-updated
// bitmap.
//
// Returns the amount of scavenged memory in bytes present in the
// allocated range.
//
// p.mheapLock must be held.
func (p *pageAlloc) allocRange(base, npages uintptr) uintptr {
        assertLockHeld(p.mheapLock)

        limit := base + npages*pageSize - 1
        sc, ec := chunkIndex(base), chunkIndex(limit)
        si, ei := chunkPageIndex(base), chunkPageIndex(limit)

        scav := uint(0)
        if sc == ec {
                // The range doesn't cross any chunk boundaries.
                chunk := p.chunkOf(sc)
                scav += chunk.scavenged.popcntRange(si, ei+1-si)
                chunk.allocRange(si, ei+1-si)
        } else {
                // The range crosses at least one chunk boundary.
                chunk := p.chunkOf(sc)
                scav += chunk.scavenged.popcntRange(si, pallocChunkPages-si)
                chunk.allocRange(si, pallocChunkPages-si)
                for c := sc + 1; c < ec; c++ {
                        chunk := p.chunkOf(c)
                        scav += chunk.scavenged.popcntRange(0, pallocChunkPages)
                        chunk.allocAll()
                }
                chunk = p.chunkOf(ec)
                scav += chunk.scavenged.popcntRange(0, ei+1)
                chunk.allocRange(0, ei+1)
        }
        p.update(base, npages, true, true)
        return uintptr(scav) * pageSize
}

// findMappedAddr returns the smallest mapped offAddr that is
// >= addr. That is, if addr refers to mapped memory, then it is
// returned. If addr is higher than any mapped region, then
// it returns maxOffAddr.
//
// p.mheapLock must be held.
func (p *pageAlloc) findMappedAddr(addr offAddr) offAddr {
        assertLockHeld(p.mheapLock)

        // If we're not in a test, validate first by checking mheap_.arenas.
        // This is a fast path which is only safe to use outside of testing.
        ai := arenaIndex(addr.addr())
        if p.test || mheap_.arenas[ai.l1()] == nil || mheap_.arenas[ai.l1()][ai.l2()] == nil {
                vAddr, ok := p.inUse.findAddrGreaterEqual(addr.addr())
                if ok {
                        return offAddr{vAddr}
                } else {
                        // The candidate search address is greater than any
                        // known address, which means we definitely have no
                        // free memory left.
                        return maxOffAddr
                }
        }
        return addr
}

// find searches for the first (address-ordered) contiguous free region of
// npages in size and returns a base address for that region.
//
// It uses p.searchAddr to prune its search and assumes that no palloc chunks
// below chunkIndex(p.searchAddr) contain any free memory at all.
//
// find also computes and returns a candidate p.searchAddr, which may or
// may not prune more of the address space than p.searchAddr already does.
// This candidate is always a valid p.searchAddr.
//
// find represents the slow path and the full radix tree search.
//
// Returns a base address of 0 on failure, in which case the candidate
// searchAddr returned is invalid and must be ignored.
//
// p.mheapLock must be held.
func (p *pageAlloc) find(npages uintptr) (uintptr, offAddr) {
        assertLockHeld(p.mheapLock)

        // Search algorithm.
        //
        // This algorithm walks each level l of the radix tree from the root level
        // to the leaf level. It iterates over at most 1 << levelBits[l] of entries
        // in a given level in the radix tree, and uses the summary information to
        // find either:
        //  1) That a given subtree contains a large enough contiguous region, at
        //     which point it continues iterating on the next level, or
        //  2) That there are enough contiguous boundary-crossing bits to satisfy
        //     the allocation, at which point it knows exactly where to start
        //     allocating from.
        //
        // i tracks the index into the current level l's structure for the
        // contiguous 1 << levelBits[l] entries we're actually interested in.
        //
        // NOTE: Technically this search could allocate a region which crosses
        // the arenaBaseOffset boundary, which when arenaBaseOffset != 0, is
        // a discontinuity. However, the only way this could happen is if the
        // page at the zero address is mapped, and this is impossible on
        // every system we support where arenaBaseOffset != 0. So, the
        // discontinuity is already encoded in the fact that the OS will never
        // map the zero page for us, and this function doesn't try to handle
        // this case in any way.

        // i is the beginning of the block of entries we're searching at the
        // current level.
        i := 0

        // firstFree is the region of address space that we are certain to
        // find the first free page in the heap. base and bound are the inclusive
        // bounds of this window, and both are addresses in the linearized, contiguous
        // view of the address space (with arenaBaseOffset pre-added). At each level,
        // this window is narrowed as we find the memory region containing the
        // first free page of memory. To begin with, the range reflects the
        // full process address space.
        //
        // firstFree is updated by calling foundFree each time free space in the
        // heap is discovered.
        //
        // At the end of the search, base.addr() is the best new
        // searchAddr we could deduce in this search.
        firstFree := struct {
                base, bound offAddr
        }{
                base:  minOffAddr,
                bound: maxOffAddr,
        }
        // foundFree takes the given address range [addr, addr+size) and
        // updates firstFree if it is a narrower range. The input range must
        // either be fully contained within firstFree or not overlap with it
        // at all.
        //
        // This way, we'll record the first summary we find with any free
        // pages on the root level and narrow that down if we descend into
        // that summary. But as soon as we need to iterate beyond that summary
        // in a level to find a large enough range, we'll stop narrowing.
        foundFree := func(addr offAddr, size uintptr) {
                if firstFree.base.lessEqual(addr) && addr.add(size-1).lessEqual(firstFree.bound) {
                        // This range fits within the current firstFree window, so narrow
                        // down the firstFree window to the base and bound of this range.
                        firstFree.base = addr
                        firstFree.bound = addr.add(size - 1)
                } else if !(addr.add(size-1).lessThan(firstFree.base) || firstFree.bound.lessThan(addr)) {
                        // This range only partially overlaps with the firstFree range,
                        // so throw.
                        print("runtime: addr = ", hex(addr.addr()), ", size = ", size, "\n")
                        print("runtime: base = ", hex(firstFree.base.addr()), ", bound = ", hex(firstFree.bound.addr()), "\n")
                        throw("range partially overlaps")
                }
        }

        // lastSum is the summary which we saw on the previous level that made us
        // move on to the next level. Used to print additional information in the
        // case of a catastrophic failure.
        // lastSumIdx is that summary's index in the previous level.
        lastSum := packPallocSum(0, 0, 0)
        lastSumIdx := -1

nextLevel:
        for l := 0; l < len(p.summary); l++ {
                // For the root level, entriesPerBlock is the whole level.
                entriesPerBlock := 1 << levelBits[l]
                logMaxPages := levelLogPages[l]

                // We've moved into a new level, so let's update i to our new
                // starting index. This is a no-op for level 0.
                i <<= levelBits[l]

                // Slice out the block of entries we care about.
                entries := p.summary[l][i : i+entriesPerBlock]

                // Determine j0, the first index we should start iterating from.
                // The searchAddr may help us eliminate iterations if we followed the
                // searchAddr on the previous level or we're on the root leve, in which
                // case the searchAddr should be the same as i after levelShift.
                j0 := 0
                if searchIdx := offAddrToLevelIndex(l, p.searchAddr); searchIdx&^(entriesPerBlock-1) == i {
                        j0 = searchIdx & (entriesPerBlock - 1)
                }

                // Run over the level entries looking for
                // a contiguous run of at least npages either
                // within an entry or across entries.
                //
                // base contains the page index (relative to
                // the first entry's first page) of the currently
                // considered run of consecutive pages.
                //
                // size contains the size of the currently considered
                // run of consecutive pages.
                var base, size uint
                for j := j0; j < len(entries); j++ {
                        sum := entries[j]
                        if sum == 0 {
                                // A full entry means we broke any streak and
                                // that we should skip it altogether.
                                size = 0
                                continue
                        }

                        // We've encountered a non-zero summary which means
                        // free memory, so update firstFree.
                        foundFree(levelIndexToOffAddr(l, i+j), (uintptr(1)<<logMaxPages)*pageSize)

                        s := sum.start()
                        if size+s >= uint(npages) {
                                // If size == 0 we don't have a run yet,
                                // which means base isn't valid. So, set
                                // base to the first page in this block.
                                if size == 0 {
                                        base = uint(j) << logMaxPages
                                }
                                // We hit npages; we're done!
                                size += s
                                break
                        }
                        if sum.max() >= uint(npages) {
                                // The entry itself contains npages contiguous
                                // free pages, so continue on the next level
                                // to find that run.
                                i += j
                                lastSumIdx = i
                                lastSum = sum
                                continue nextLevel
                        }
                        if size == 0 || s < 1<<logMaxPages {
                                // We either don't have a current run started, or this entry
                                // isn't totally free (meaning we can't continue the current
                                // one), so try to begin a new run by setting size and base
                                // based on sum.end.
                                size = sum.end()
                                base = uint(j+1)<<logMaxPages - size
                                continue
                        }
                        // The entry is completely free, so continue the run.
                        size += 1 << logMaxPages
                }
                if size >= uint(npages) {
                        // We found a sufficiently large run of free pages straddling
                        // some boundary, so compute the address and return it.
                        addr := levelIndexToOffAddr(l, i).add(uintptr(base) * pageSize).addr()
                        return addr, p.findMappedAddr(firstFree.base)
                }
                if l == 0 {
                        // We're at level zero, so that means we've exhausted our search.
                        return 0, maxSearchAddr()
                }

                // We're not at level zero, and we exhausted the level we were looking in.
                // This means that either our calculations were wrong or the level above
                // lied to us. In either case, dump some useful state and throw.
                print("runtime: summary[", l-1, "][", lastSumIdx, "] = ", lastSum.start(), ", ", lastSum.max(), ", ", lastSum.end(), "\n")
                print("runtime: level = ", l, ", npages = ", npages, ", j0 = ", j0, "\n")
                print("runtime: p.searchAddr = ", hex(p.searchAddr.addr()), ", i = ", i, "\n")
                print("runtime: levelShift[level] = ", levelShift[l], ", levelBits[level] = ", levelBits[l], "\n")
                for j := 0; j < len(entries); j++ {
                        sum := entries[j]
                        print("runtime: summary[", l, "][", i+j, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n")
                }
                throw("bad summary data")
        }

        // Since we've gotten to this point, that means we haven't found a
        // sufficiently-sized free region straddling some boundary (chunk or larger).
        // This means the last summary we inspected must have had a large enough "max"
        // value, so look inside the chunk to find a suitable run.
        //
        // After iterating over all levels, i must contain a chunk index which
        // is what the final level represents.
        ci := chunkIdx(i)
        j, searchIdx := p.chunkOf(ci).find(npages, 0)
        if j == ^uint(0) {
                // We couldn't find any space in this chunk despite the summaries telling
                // us it should be there. There's likely a bug, so dump some state and throw.
                sum := p.summary[len(p.summary)-1][i]
                print("runtime: summary[", len(p.summary)-1, "][", i, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n")
                print("runtime: npages = ", npages, "\n")
                throw("bad summary data")
        }

        // Compute the address at which the free space starts.
        addr := chunkBase(ci) + uintptr(j)*pageSize

        // Since we actually searched the chunk, we may have
        // found an even narrower free window.
        searchAddr := chunkBase(ci) + uintptr(searchIdx)*pageSize
        foundFree(offAddr{searchAddr}, chunkBase(ci+1)-searchAddr)
        return addr, p.findMappedAddr(firstFree.base)
}

// alloc allocates npages worth of memory from the page heap, returning the base
// address for the allocation and the amount of scavenged memory in bytes
// contained in the region [base address, base address + npages*pageSize).
//
// Returns a 0 base address on failure, in which case other returned values
// should be ignored.
//
// p.mheapLock must be held.
//
// Must run on the system stack because p.mheapLock must be held.
//
//go:systemstack
func (p *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) {
        assertLockHeld(p.mheapLock)

        // If the searchAddr refers to a region which has a higher address than
        // any known chunk, then we know we're out of memory.
        if chunkIndex(p.searchAddr.addr()) >= p.end {
                return 0, 0
        }

        // If npages has a chance of fitting in the chunk where the searchAddr is,
        // search it directly.
        searchAddr := minOffAddr
        if pallocChunkPages-chunkPageIndex(p.searchAddr.addr()) >= uint(npages) {
                // npages is guaranteed to be no greater than pallocChunkPages here.
                i := chunkIndex(p.searchAddr.addr())
                if max := p.summary[len(p.summary)-1][i].max(); max >= uint(npages) {
                        j, searchIdx := p.chunkOf(i).find(npages, chunkPageIndex(p.searchAddr.addr()))
                        if j == ^uint(0) {
                                print("runtime: max = ", max, ", npages = ", npages, "\n")
                                print("runtime: searchIdx = ", chunkPageIndex(p.searchAddr.addr()), ", p.searchAddr = ", hex(p.searchAddr.addr()), "\n")
                                throw("bad summary data")
                        }
                        addr = chunkBase(i) + uintptr(j)*pageSize
                        searchAddr = offAddr{chunkBase(i) + uintptr(searchIdx)*pageSize}
                        goto Found
                }
        }
        // We failed to use a searchAddr for one reason or another, so try
        // the slow path.
        addr, searchAddr = p.find(npages)
        if addr == 0 {
                if npages == 1 {
                        // We failed to find a single free page, the smallest unit
                        // of allocation. This means we know the heap is completely
                        // exhausted. Otherwise, the heap still might have free
                        // space in it, just not enough contiguous space to
                        // accommodate npages.
                        p.searchAddr = maxSearchAddr()
                }
                return 0, 0
        }
Found:
        // Go ahead and actually mark the bits now that we have an address.
        scav = p.allocRange(addr, npages)

        // If we found a higher searchAddr, we know that all the
        // heap memory before that searchAddr in an offset address space is
        // allocated, so bump p.searchAddr up to the new one.
        if p.searchAddr.lessThan(searchAddr) {
                p.searchAddr = searchAddr
        }
        return addr, scav
}

// free returns npages worth of memory starting at base back to the page heap.
//
// p.mheapLock must be held.
//
// Must run on the system stack because p.mheapLock must be held.
//
//go:systemstack
func (p *pageAlloc) free(base, npages uintptr, scavenged bool) {
        assertLockHeld(p.mheapLock)

        // If we're freeing pages below the p.searchAddr, update searchAddr.
        if b := (offAddr{base}); b.lessThan(p.searchAddr) {
                p.searchAddr = b
        }
        limit := base + npages*pageSize - 1
        if !scavenged {
                // Update the free high watermark for the scavenger.
                if offLimit := (offAddr{limit}); p.scav.freeHWM.lessThan(offLimit) {
                        p.scav.freeHWM = offLimit
                }
        }
        if npages == 1 {
                // Fast path: we're clearing a single bit, and we know exactly
                // where it is, so mark it directly.
                i := chunkIndex(base)
                p.chunkOf(i).free1(chunkPageIndex(base))
        } else {
                // Slow path: we're clearing more bits so we may need to iterate.
                sc, ec := chunkIndex(base), chunkIndex(limit)
                si, ei := chunkPageIndex(base), chunkPageIndex(limit)

                if sc == ec {
                        // The range doesn't cross any chunk boundaries.
                        p.chunkOf(sc).free(si, ei+1-si)
                } else {
                        // The range crosses at least one chunk boundary.
                        p.chunkOf(sc).free(si, pallocChunkPages-si)
                        for c := sc + 1; c < ec; c++ {
                                p.chunkOf(c).freeAll()
                        }
                        p.chunkOf(ec).free(0, ei+1)
                }
        }
        p.update(base, npages, true, false)
}

const (
        pallocSumBytes = unsafe.Sizeof(pallocSum(0))

        // maxPackedValue is the maximum value that any of the three fields in
        // the pallocSum may take on.
        maxPackedValue    = 1 << logMaxPackedValue
        logMaxPackedValue = logPallocChunkPages + (summaryLevels-1)*summaryLevelBits

        freeChunkSum = pallocSum(uint64(pallocChunkPages) |
                uint64(pallocChunkPages<<logMaxPackedValue) |
                uint64(pallocChunkPages<<(2*logMaxPackedValue)))
)

// pallocSum is a packed summary type which packs three numbers: start, max,
// 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 {
        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 {
        if uint64(p)&uint64(1<<63) != 0 {
                return maxPackedValue
        }
        return uint(uint64(p) & (maxPackedValue - 1))
}

// max extracts the max value from a packed sum.
func (p pallocSum) max() uint {
        if uint64(p)&uint64(1<<63) != 0 {
                return maxPackedValue
        }
        return uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1))
}

// end extracts the end value from a packed sum.
func (p pallocSum) end() uint {
        if uint64(p)&uint64(1<<63) != 0 {
                return maxPackedValue
        }
        return uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1))
}

// unpack unpacks all three values from the summary.
func (p pallocSum) unpack() (uint, uint, uint) {
        if uint64(p)&uint64(1<<63) != 0 {
                return maxPackedValue, maxPackedValue, maxPackedValue
        }
        return uint(uint64(p) & (maxPackedValue - 1)),
                uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)),
                uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1))
}

// mergeSummaries merges consecutive summaries which may each represent at
// most 1 << logMaxPagesPerSum pages each together into one.
func mergeSummaries(sums []pallocSum, logMaxPagesPerSum uint) pallocSum {
        // Merge the summaries in sums into one.
        //
        // We do this by keeping a running summary representing the merged
        // summaries of sums[:i] in start, max, and end.
        start, max, end := sums[0].unpack()
        for i := 1; i < len(sums); i++ {
                // Merge in sums[i].
                si, mi, ei := sums[i].unpack()

                // Merge in sums[i].start only if the running summary is
                // completely free, otherwise this summary's start
                // plays no role in the combined sum.
                if start == uint(i)<<logMaxPagesPerSum {
                        start += si
                }

                // Recompute the max value of the running sum by looking
                // across the boundary between the running sum and sums[i]
                // and at the max sums[i], taking the greatest of those two
                // and the max of the running sum.
                if end+si > max {
                        max = end + si
                }
                if mi > max {
                        max = mi
                }

                // Merge in end by checking if this new summary is totally
                // free. If it is, then we want to extend the running sum's
                // end by the new summary. If not, then we have some alloc'd
                // pages in there and we just want to take the end value in
                // sums[i].
                if ei == 1<<logMaxPagesPerSum {
                        end += 1 << logMaxPagesPerSum
                } else {
                        end = ei
                }
        }
        return packPallocSum(start, max, end)
}