runtime: avoid a bit of unneeded work when MemProfileRate==1

[gostls13.git] / src / runtime / malloc.go

// Copyright 2014 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.

// Memory allocator.
//
// This was originally based on tcmalloc, but has diverged quite a bit.
// http://goog-perftools.sourceforge.net/doc/tcmalloc.html

// The main allocator works in runs of pages.
// Small allocation sizes (up to and including 32 kB) are
// rounded to one of about 70 size classes, each of which
// has its own free set of objects of exactly that size.
// Any free page of memory can be split into a set of objects
// of one size class, which are then managed using a free bitmap.
//
// The allocator's data structures are:
//
//      fixalloc: a free-list allocator for fixed-size off-heap objects,
//              used to manage storage used by the allocator.
//      mheap: the malloc heap, managed at page (8192-byte) granularity.
//      mspan: a run of in-use pages managed by the mheap.
//      mcentral: collects all spans of a given size class.
//      mcache: a per-P cache of mspans with free space.
//      mstats: allocation statistics.
//
// Allocating a small object proceeds up a hierarchy of caches:
//
//      1. Round the size up to one of the small size classes
//         and look in the corresponding mspan in this P's mcache.
//         Scan the mspan's free bitmap to find a free slot.
//         If there is a free slot, allocate it.
//         This can all be done without acquiring a lock.
//
//      2. If the mspan has no free slots, obtain a new mspan
//         from the mcentral's list of mspans of the required size
//         class that have free space.
//         Obtaining a whole span amortizes the cost of locking
//         the mcentral.
//
//      3. If the mcentral's mspan list is empty, obtain a run
//         of pages from the mheap to use for the mspan.
//
//      4. If the mheap is empty or has no page runs large enough,
//         allocate a new group of pages (at least 1MB) from the
//         operating system. Allocating a large run of pages
//         amortizes the cost of talking to the operating system.
//
// Sweeping an mspan and freeing objects on it proceeds up a similar
// hierarchy:
//
//      1. If the mspan is being swept in response to allocation, it
//         is returned to the mcache to satisfy the allocation.
//
//      2. Otherwise, if the mspan still has allocated objects in it,
//         it is placed on the mcentral free list for the mspan's size
//         class.
//
//      3. Otherwise, if all objects in the mspan are free, the mspan's
//         pages are returned to the mheap and the mspan is now dead.
//
// Allocating and freeing a large object uses the mheap
// directly, bypassing the mcache and mcentral.
//
// If mspan.needzero is false, then free object slots in the mspan are
// already zeroed. Otherwise if needzero is true, objects are zeroed as
// they are allocated. There are various benefits to delaying zeroing
// this way:
//
//      1. Stack frame allocation can avoid zeroing altogether.
//
//      2. It exhibits better temporal locality, since the program is
//         probably about to write to the memory.
//
//      3. We don't zero pages that never get reused.

// Virtual memory layout
//
// The heap consists of a set of arenas, which are 64MB on 64-bit and
// 4MB on 32-bit (heapArenaBytes). Each arena's start address is also
// aligned to the arena size.
//
// Each arena has an associated heapArena object that stores the
// metadata for that arena: the heap bitmap for all words in the arena
// and the span map for all pages in the arena. heapArena objects are
// themselves allocated off-heap.
//
// Since arenas are aligned, the address space can be viewed as a
// series of arena frames. The arena map (mheap_.arenas) maps from
// arena frame number to *heapArena, or nil for parts of the address
// space not backed by the Go heap. The arena map is structured as a
// two-level array consisting of a "L1" arena map and many "L2" arena
// maps; however, since arenas are large, on many architectures, the
// arena map consists of a single, large L2 map.
//
// The arena map covers the entire possible address space, allowing
// the Go heap to use any part of the address space. The allocator
// attempts to keep arenas contiguous so that large spans (and hence
// large objects) can cross arenas.

package runtime

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

const (
        debugMalloc = false

        maxTinySize   = _TinySize
        tinySizeClass = _TinySizeClass
        maxSmallSize  = _MaxSmallSize

        pageShift = _PageShift
        pageSize  = _PageSize
        pageMask  = _PageMask
        // By construction, single page spans of the smallest object class
        // have the most objects per span.
        maxObjsPerSpan = pageSize / 8

        concurrentSweep = _ConcurrentSweep

        _PageSize = 1 << _PageShift
        _PageMask = _PageSize - 1

        // _64bit = 1 on 64-bit systems, 0 on 32-bit systems
        _64bit = 1 << (^uintptr(0) >> 63) / 2

        // Tiny allocator parameters, see "Tiny allocator" comment in malloc.go.
        _TinySize      = 16
        _TinySizeClass = int8(2)

        _FixAllocChunk = 16 << 10 // Chunk size for FixAlloc

        // Per-P, per order stack segment cache size.
        _StackCacheSize = 32 * 1024

        // Number of orders that get caching. Order 0 is FixedStack
        // and each successive order is twice as large.
        // We want to cache 2KB, 4KB, 8KB, and 16KB stacks. Larger stacks
        // will be allocated directly.
        // Since FixedStack is different on different systems, we
        // must vary NumStackOrders to keep the same maximum cached size.
        //   OS               | FixedStack | NumStackOrders
        //   -----------------+------------+---------------
        //   linux/darwin/bsd | 2KB        | 4
        //   windows/32       | 4KB        | 3
        //   windows/64       | 8KB        | 2
        //   plan9            | 4KB        | 3
        _NumStackOrders = 4 - sys.PtrSize/4*sys.GoosWindows - 1*sys.GoosPlan9

        // heapAddrBits is the number of bits in a heap address. On
        // amd64, addresses are sign-extended beyond heapAddrBits. On
        // other arches, they are zero-extended.
        //
        // On most 64-bit platforms, we limit this to 48 bits based on a
        // combination of hardware and OS limitations.
        //
        // amd64 hardware limits addresses to 48 bits, sign-extended
        // to 64 bits. Addresses where the top 16 bits are not either
        // all 0 or all 1 are "non-canonical" and invalid. Because of
        // these "negative" addresses, we offset addresses by 1<<47
        // (arenaBaseOffset) on amd64 before computing indexes into
        // the heap arenas index. In 2017, amd64 hardware added
        // support for 57 bit addresses; however, currently only Linux
        // supports this extension and the kernel will never choose an
        // address above 1<<47 unless mmap is called with a hint
        // address above 1<<47 (which we never do).
        //
        // arm64 hardware (as of ARMv8) limits user addresses to 48
        // bits, in the range [0, 1<<48).
        //
        // ppc64, mips64, and s390x support arbitrary 64 bit addresses
        // in hardware. On Linux, Go leans on stricter OS limits. Based
        // on Linux's processor.h, the user address space is limited as
        // follows on 64-bit architectures:
        //
        // Architecture  Name              Maximum Value (exclusive)
        // ---------------------------------------------------------------------
        // amd64         TASK_SIZE_MAX     0x007ffffffff000 (47 bit addresses)
        // arm64         TASK_SIZE_64      0x01000000000000 (48 bit addresses)
        // ppc64{,le}    TASK_SIZE_USER64  0x00400000000000 (46 bit addresses)
        // mips64{,le}   TASK_SIZE64       0x00010000000000 (40 bit addresses)
        // s390x         TASK_SIZE         1<<64 (64 bit addresses)
        //
        // These limits may increase over time, but are currently at
        // most 48 bits except on s390x. On all architectures, Linux
        // starts placing mmap'd regions at addresses that are
        // significantly below 48 bits, so even if it's possible to
        // exceed Go's 48 bit limit, it's extremely unlikely in
        // practice.
        //
        // On 32-bit platforms, we accept the full 32-bit address
        // space because doing so is cheap.
        // mips32 only has access to the low 2GB of virtual memory, so
        // we further limit it to 31 bits.
        //
        // On ios/arm64, although 64-bit pointers are presumably
        // available, pointers are truncated to 33 bits. Furthermore,
        // only the top 4 GiB of the address space are actually available
        // to the application, but we allow the whole 33 bits anyway for
        // simplicity.
        // TODO(mknyszek): Consider limiting it to 32 bits and using
        // arenaBaseOffset to offset into the top 4 GiB.
        //
        // WebAssembly currently has a limit of 4GB linear memory.
        heapAddrBits = (_64bit*(1-sys.GoarchWasm)*(1-sys.GoosIos*sys.GoarchArm64))*48 + (1-_64bit+sys.GoarchWasm)*(32-(sys.GoarchMips+sys.GoarchMipsle)) + 33*sys.GoosIos*sys.GoarchArm64

        // maxAlloc is the maximum size of an allocation. On 64-bit,
        // it's theoretically possible to allocate 1<<heapAddrBits bytes. On
        // 32-bit, however, this is one less than 1<<32 because the
        // number of bytes in the address space doesn't actually fit
        // in a uintptr.
        maxAlloc = (1 << heapAddrBits) - (1-_64bit)*1

        // The number of bits in a heap address, the size of heap
        // arenas, and the L1 and L2 arena map sizes are related by
        //
        //   (1 << addr bits) = arena size * L1 entries * L2 entries
        //
        // Currently, we balance these as follows:
        //
        //       Platform  Addr bits  Arena size  L1 entries   L2 entries
        // --------------  ---------  ----------  ----------  -----------
        //       */64-bit         48        64MB           1    4M (32MB)
        // windows/64-bit         48         4MB          64    1M  (8MB)
        //       */32-bit         32         4MB           1  1024  (4KB)
        //     */mips(le)         31         4MB           1   512  (2KB)

        // heapArenaBytes is the size of a heap arena. The heap
        // consists of mappings of size heapArenaBytes, aligned to
        // heapArenaBytes. The initial heap mapping is one arena.
        //
        // This is currently 64MB on 64-bit non-Windows and 4MB on
        // 32-bit and on Windows. We use smaller arenas on Windows
        // because all committed memory is charged to the process,
        // even if it's not touched. Hence, for processes with small
        // heaps, the mapped arena space needs to be commensurate.
        // This is particularly important with the race detector,
        // since it significantly amplifies the cost of committed
        // memory.
        heapArenaBytes = 1 << logHeapArenaBytes

        // logHeapArenaBytes is log_2 of heapArenaBytes. For clarity,
        // prefer using heapArenaBytes where possible (we need the
        // constant to compute some other constants).
        logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoarchWasm)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (2+20)*sys.GoarchWasm

        // heapArenaBitmapBytes is the size of each heap arena's bitmap.
        heapArenaBitmapBytes = heapArenaBytes / (sys.PtrSize * 8 / 2)

        pagesPerArena = heapArenaBytes / pageSize

        // arenaL1Bits is the number of bits of the arena number
        // covered by the first level arena map.
        //
        // This number should be small, since the first level arena
        // map requires PtrSize*(1<<arenaL1Bits) of space in the
        // binary's BSS. It can be zero, in which case the first level
        // index is effectively unused. There is a performance benefit
        // to this, since the generated code can be more efficient,
        // but comes at the cost of having a large L2 mapping.
        //
        // We use the L1 map on 64-bit Windows because the arena size
        // is small, but the address space is still 48 bits, and
        // there's a high cost to having a large L2.
        arenaL1Bits = 6 * (_64bit * sys.GoosWindows)

        // arenaL2Bits is the number of bits of the arena number
        // covered by the second level arena index.
        //
        // The size of each arena map allocation is proportional to
        // 1<<arenaL2Bits, so it's important that this not be too
        // large. 48 bits leads to 32MB arena index allocations, which
        // is about the practical threshold.
        arenaL2Bits = heapAddrBits - logHeapArenaBytes - arenaL1Bits

        // arenaL1Shift is the number of bits to shift an arena frame
        // number by to compute an index into the first level arena map.
        arenaL1Shift = arenaL2Bits

        // arenaBits is the total bits in a combined arena map index.
        // This is split between the index into the L1 arena map and
        // the L2 arena map.
        arenaBits = arenaL1Bits + arenaL2Bits

        // arenaBaseOffset is the pointer value that corresponds to
        // index 0 in the heap arena map.
        //
        // On amd64, the address space is 48 bits, sign extended to 64
        // bits. This offset lets us handle "negative" addresses (or
        // high addresses if viewed as unsigned).
        //
        // On aix/ppc64, this offset allows to keep the heapAddrBits to
        // 48. Otherwize, it would be 60 in order to handle mmap addresses
        // (in range 0x0a00000000000000 - 0x0afffffffffffff). But in this
        // case, the memory reserved in (s *pageAlloc).init for chunks
        // is causing important slowdowns.
        //
        // On other platforms, the user address space is contiguous
        // and starts at 0, so no offset is necessary.
        arenaBaseOffset = 0xffff800000000000*sys.GoarchAmd64 + 0x0a00000000000000*sys.GoosAix
        // A typed version of this constant that will make it into DWARF (for viewcore).
        arenaBaseOffsetUintptr = uintptr(arenaBaseOffset)

        // Max number of threads to run garbage collection.
        // 2, 3, and 4 are all plausible maximums depending
        // on the hardware details of the machine. The garbage
        // collector scales well to 32 cpus.
        _MaxGcproc = 32

        // minLegalPointer is the smallest possible legal pointer.
        // This is the smallest possible architectural page size,
        // since we assume that the first page is never mapped.
        //
        // This should agree with minZeroPage in the compiler.
        minLegalPointer uintptr = 4096
)

// physPageSize is the size in bytes of the OS's physical pages.
// Mapping and unmapping operations must be done at multiples of
// physPageSize.
//
// This must be set by the OS init code (typically in osinit) before
// mallocinit.
var physPageSize uintptr

// physHugePageSize is the size in bytes of the OS's default physical huge
// page size whose allocation is opaque to the application. It is assumed
// and verified to be a power of two.
//
// If set, this must be set by the OS init code (typically in osinit) before
// mallocinit. However, setting it at all is optional, and leaving the default
// value is always safe (though potentially less efficient).
//
// Since physHugePageSize is always assumed to be a power of two,
// physHugePageShift is defined as physHugePageSize == 1 << physHugePageShift.
// The purpose of physHugePageShift is to avoid doing divisions in
// performance critical functions.
var (
        physHugePageSize  uintptr
        physHugePageShift uint
)

// OS memory management abstraction layer
//
// Regions of the address space managed by the runtime may be in one of four
// states at any given time:
// 1) None - Unreserved and unmapped, the default state of any region.
// 2) Reserved - Owned by the runtime, but accessing it would cause a fault.
//               Does not count against the process' memory footprint.
// 3) Prepared - Reserved, intended not to be backed by physical memory (though
//               an OS may implement this lazily). Can transition efficiently to
//               Ready. Accessing memory in such a region is undefined (may
//               fault, may give back unexpected zeroes, etc.).
// 4) Ready - may be accessed safely.
//
// This set of states is more than is strictly necessary to support all the
// currently supported platforms. One could get by with just None, Reserved, and
// Ready. However, the Prepared state gives us flexibility for performance
// purposes. For example, on POSIX-y operating systems, Reserved is usually a
// private anonymous mmap'd region with PROT_NONE set, and to transition
// to Ready would require setting PROT_READ|PROT_WRITE. However the
// underspecification of Prepared lets us use just MADV_FREE to transition from
// Ready to Prepared. Thus with the Prepared state we can set the permission
// bits just once early on, we can efficiently tell the OS that it's free to
// take pages away from us when we don't strictly need them.
//
// For each OS there is a common set of helpers defined that transition
// memory regions between these states. The helpers are as follows:
//
// sysAlloc transitions an OS-chosen region of memory from None to Ready.
// More specifically, it obtains a large chunk of zeroed memory from the
// operating system, typically on the order of a hundred kilobytes
// or a megabyte. This memory is always immediately available for use.
//
// sysFree transitions a memory region from any state to None. Therefore, it
// returns memory unconditionally. It is used if an out-of-memory error has been
// detected midway through an allocation or to carve out an aligned section of
// the address space. It is okay if sysFree is a no-op only if sysReserve always
// returns a memory region aligned to the heap allocator's alignment
// restrictions.
//
// sysReserve transitions a memory region from None to Reserved. It reserves
// address space in such a way that it would cause a fatal fault upon access
// (either via permissions or not committing the memory). Such a reservation is
// thus never backed by physical memory.
// If the pointer passed to it is non-nil, the caller wants the
// reservation there, but sysReserve can still choose another
// location if that one is unavailable.
// NOTE: sysReserve returns OS-aligned memory, but the heap allocator
// may use larger alignment, so the caller must be careful to realign the
// memory obtained by sysReserve.
//
// sysMap transitions a memory region from Reserved to Prepared. It ensures the
// memory region can be efficiently transitioned to Ready.
//
// sysUsed transitions a memory region from Prepared to Ready. It notifies the
// operating system that the memory region is needed and ensures that the region
// may be safely accessed. This is typically a no-op on systems that don't have
// an explicit commit step and hard over-commit limits, but is critical on
// Windows, for example.
//
// sysUnused transitions a memory region from Ready to Prepared. It notifies the
// operating system that the physical pages backing this memory region are no
// longer needed and can be reused for other purposes. The contents of a
// sysUnused memory region are considered forfeit and the region must not be
// accessed again until sysUsed is called.
//
// sysFault transitions a memory region from Ready or Prepared to Reserved. It
// marks a region such that it will always fault if accessed. Used only for
// debugging the runtime.

func mallocinit() {
        if class_to_size[_TinySizeClass] != _TinySize {
                throw("bad TinySizeClass")
        }

        testdefersizes()

        if heapArenaBitmapBytes&(heapArenaBitmapBytes-1) != 0 {
                // heapBits expects modular arithmetic on bitmap
                // addresses to work.
                throw("heapArenaBitmapBytes not a power of 2")
        }

        // Copy class sizes out for statistics table.
        for i := range class_to_size {
                memstats.by_size[i].size = uint32(class_to_size[i])
        }

        // Check physPageSize.
        if physPageSize == 0 {
                // The OS init code failed to fetch the physical page size.
                throw("failed to get system page size")
        }
        if physPageSize > maxPhysPageSize {
                print("system page size (", physPageSize, ") is larger than maximum page size (", maxPhysPageSize, ")\n")
                throw("bad system page size")
        }
        if physPageSize < minPhysPageSize {
                print("system page size (", physPageSize, ") is smaller than minimum page size (", minPhysPageSize, ")\n")
                throw("bad system page size")
        }
        if physPageSize&(physPageSize-1) != 0 {
                print("system page size (", physPageSize, ") must be a power of 2\n")
                throw("bad system page size")
        }
        if physHugePageSize&(physHugePageSize-1) != 0 {
                print("system huge page size (", physHugePageSize, ") must be a power of 2\n")
                throw("bad system huge page size")
        }
        if physHugePageSize > maxPhysHugePageSize {
                // physHugePageSize is greater than the maximum supported huge page size.
                // Don't throw here, like in the other cases, since a system configured
                // in this way isn't wrong, we just don't have the code to support them.
                // Instead, silently set the huge page size to zero.
                physHugePageSize = 0
        }
        if physHugePageSize != 0 {
                // Since physHugePageSize is a power of 2, it suffices to increase
                // physHugePageShift until 1<<physHugePageShift == physHugePageSize.
                for 1<<physHugePageShift != physHugePageSize {
                        physHugePageShift++
                }
        }
        if pagesPerArena%pagesPerSpanRoot != 0 {
                print("pagesPerArena (", pagesPerArena, ") is not divisible by pagesPerSpanRoot (", pagesPerSpanRoot, ")\n")
                throw("bad pagesPerSpanRoot")
        }
        if pagesPerArena%pagesPerReclaimerChunk != 0 {
                print("pagesPerArena (", pagesPerArena, ") is not divisible by pagesPerReclaimerChunk (", pagesPerReclaimerChunk, ")\n")
                throw("bad pagesPerReclaimerChunk")
        }

        // Initialize the heap.
        mheap_.init()
        mcache0 = allocmcache()
        lockInit(&gcBitsArenas.lock, lockRankGcBitsArenas)
        lockInit(&proflock, lockRankProf)
        lockInit(&globalAlloc.mutex, lockRankGlobalAlloc)

        // Create initial arena growth hints.
        if sys.PtrSize == 8 {
                // On a 64-bit machine, we pick the following hints
                // because:
                //
                // 1. Starting from the middle of the address space
                // makes it easier to grow out a contiguous range
                // without running in to some other mapping.
                //
                // 2. This makes Go heap addresses more easily
                // recognizable when debugging.
                //
                // 3. Stack scanning in gccgo is still conservative,
                // so it's important that addresses be distinguishable
                // from other data.
                //
                // Starting at 0x00c0 means that the valid memory addresses
                // will begin 0x00c0, 0x00c1, ...
                // In little-endian, that's c0 00, c1 00, ... None of those are valid
                // UTF-8 sequences, and they are otherwise as far away from
                // ff (likely a common byte) as possible. If that fails, we try other 0xXXc0
                // addresses. An earlier attempt to use 0x11f8 caused out of memory errors
                // on OS X during thread allocations.  0x00c0 causes conflicts with
                // AddressSanitizer which reserves all memory up to 0x0100.
                // These choices reduce the odds of a conservative garbage collector
                // not collecting memory because some non-pointer block of memory
                // had a bit pattern that matched a memory address.
                //
                // However, on arm64, we ignore all this advice above and slam the
                // allocation at 0x40 << 32 because when using 4k pages with 3-level
                // translation buffers, the user address space is limited to 39 bits
                // On ios/arm64, the address space is even smaller.
                //
                // On AIX, mmaps starts at 0x0A00000000000000 for 64-bit.
                // processes.
                for i := 0x7f; i >= 0; i-- {
                        var p uintptr
                        switch {
                        case raceenabled:
                                // The TSAN runtime requires the heap
                                // to be in the range [0x00c000000000,
                                // 0x00e000000000).
                                p = uintptr(i)<<32 | uintptrMask&(0x00c0<<32)
                                if p >= uintptrMask&0x00e000000000 {
                                        continue
                                }
                        case GOARCH == "arm64" && GOOS == "ios":
                                p = uintptr(i)<<40 | uintptrMask&(0x0013<<28)
                        case GOARCH == "arm64":
                                p = uintptr(i)<<40 | uintptrMask&(0x0040<<32)
                        case GOOS == "aix":
                                if i == 0 {
                                        // We don't use addresses directly after 0x0A00000000000000
                                        // to avoid collisions with others mmaps done by non-go programs.
                                        continue
                                }
                                p = uintptr(i)<<40 | uintptrMask&(0xa0<<52)
                        default:
                                p = uintptr(i)<<40 | uintptrMask&(0x00c0<<32)
                        }
                        hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
                        hint.addr = p
                        hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
                }
        } else {
                // On a 32-bit machine, we're much more concerned
                // about keeping the usable heap contiguous.
                // Hence:
                //
                // 1. We reserve space for all heapArenas up front so
                // they don't get interleaved with the heap. They're
                // ~258MB, so this isn't too bad. (We could reserve a
                // smaller amount of space up front if this is a
                // problem.)
                //
                // 2. We hint the heap to start right above the end of
                // the binary so we have the best chance of keeping it
                // contiguous.
                //
                // 3. We try to stake out a reasonably large initial
                // heap reservation.

                const arenaMetaSize = (1 << arenaBits) * unsafe.Sizeof(heapArena{})
                meta := uintptr(sysReserve(nil, arenaMetaSize))
                if meta != 0 {
                        mheap_.heapArenaAlloc.init(meta, arenaMetaSize)
                }

                // We want to start the arena low, but if we're linked
                // against C code, it's possible global constructors
                // have called malloc and adjusted the process' brk.
                // Query the brk so we can avoid trying to map the
                // region over it (which will cause the kernel to put
                // the region somewhere else, likely at a high
                // address).
                procBrk := sbrk0()

                // If we ask for the end of the data segment but the
                // operating system requires a little more space
                // before we can start allocating, it will give out a
                // slightly higher pointer. Except QEMU, which is
                // buggy, as usual: it won't adjust the pointer
                // upward. So adjust it upward a little bit ourselves:
                // 1/4 MB to get away from the running binary image.
                p := firstmoduledata.end
                if p < procBrk {
                        p = procBrk
                }
                if mheap_.heapArenaAlloc.next <= p && p < mheap_.heapArenaAlloc.end {
                        p = mheap_.heapArenaAlloc.end
                }
                p = alignUp(p+(256<<10), heapArenaBytes)
                // Because we're worried about fragmentation on
                // 32-bit, we try to make a large initial reservation.
                arenaSizes := []uintptr{
                        512 << 20,
                        256 << 20,
                        128 << 20,
                }
                for _, arenaSize := range arenaSizes {
                        a, size := sysReserveAligned(unsafe.Pointer(p), arenaSize, heapArenaBytes)
                        if a != nil {
                                mheap_.arena.init(uintptr(a), size)
                                p = mheap_.arena.end // For hint below
                                break
                        }
                }
                hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
                hint.addr = p
                hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
        }
}

// sysAlloc allocates heap arena space for at least n bytes. The
// returned pointer is always heapArenaBytes-aligned and backed by
// h.arenas metadata. The returned size is always a multiple of
// heapArenaBytes. sysAlloc returns nil on failure.
// There is no corresponding free function.
//
// sysAlloc returns a memory region in the Prepared state. This region must
// be transitioned to Ready before use.
//
// h must be locked.
func (h *mheap) sysAlloc(n uintptr) (v unsafe.Pointer, size uintptr) {
        assertLockHeld(&h.lock)

        n = alignUp(n, heapArenaBytes)

        // First, try the arena pre-reservation.
        v = h.arena.alloc(n, heapArenaBytes, &memstats.heap_sys)
        if v != nil {
                size = n
                goto mapped
        }

        // Try to grow the heap at a hint address.
        for h.arenaHints != nil {
                hint := h.arenaHints
                p := hint.addr
                if hint.down {
                        p -= n
                }
                if p+n < p {
                        // We can't use this, so don't ask.
                        v = nil
                } else if arenaIndex(p+n-1) >= 1<<arenaBits {
                        // Outside addressable heap. Can't use.
                        v = nil
                } else {
                        v = sysReserve(unsafe.Pointer(p), n)
                }
                if p == uintptr(v) {
                        // Success. Update the hint.
                        if !hint.down {
                                p += n
                        }
                        hint.addr = p
                        size = n
                        break
                }
                // Failed. Discard this hint and try the next.
                //
                // TODO: This would be cleaner if sysReserve could be
                // told to only return the requested address. In
                // particular, this is already how Windows behaves, so
                // it would simplify things there.
                if v != nil {
                        sysFree(v, n, nil)
                }
                h.arenaHints = hint.next
                h.arenaHintAlloc.free(unsafe.Pointer(hint))
        }

        if size == 0 {
                if raceenabled {
                        // The race detector assumes the heap lives in
                        // [0x00c000000000, 0x00e000000000), but we
                        // just ran out of hints in this region. Give
                        // a nice failure.
                        throw("too many address space collisions for -race mode")
                }

                // All of the hints failed, so we'll take any
                // (sufficiently aligned) address the kernel will give
                // us.
                v, size = sysReserveAligned(nil, n, heapArenaBytes)
                if v == nil {
                        return nil, 0
                }

                // Create new hints for extending this region.
                hint := (*arenaHint)(h.arenaHintAlloc.alloc())
                hint.addr, hint.down = uintptr(v), true
                hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
                hint = (*arenaHint)(h.arenaHintAlloc.alloc())
                hint.addr = uintptr(v) + size
                hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
        }

        // Check for bad pointers or pointers we can't use.
        {
                var bad string
                p := uintptr(v)
                if p+size < p {
                        bad = "region exceeds uintptr range"
                } else if arenaIndex(p) >= 1<<arenaBits {
                        bad = "base outside usable address space"
                } else if arenaIndex(p+size-1) >= 1<<arenaBits {
                        bad = "end outside usable address space"
                }
                if bad != "" {
                        // This should be impossible on most architectures,
                        // but it would be really confusing to debug.
                        print("runtime: memory allocated by OS [", hex(p), ", ", hex(p+size), ") not in usable address space: ", bad, "\n")
                        throw("memory reservation exceeds address space limit")
                }
        }

        if uintptr(v)&(heapArenaBytes-1) != 0 {
                throw("misrounded allocation in sysAlloc")
        }

        // Transition from Reserved to Prepared.
        sysMap(v, size, &memstats.heap_sys)

mapped:
        // Create arena metadata.
        for ri := arenaIndex(uintptr(v)); ri <= arenaIndex(uintptr(v)+size-1); ri++ {
                l2 := h.arenas[ri.l1()]
                if l2 == nil {
                        // Allocate an L2 arena map.
                        l2 = (*[1 << arenaL2Bits]*heapArena)(persistentalloc(unsafe.Sizeof(*l2), sys.PtrSize, nil))
                        if l2 == nil {
                                throw("out of memory allocating heap arena map")
                        }
                        atomic.StorepNoWB(unsafe.Pointer(&h.arenas[ri.l1()]), unsafe.Pointer(l2))
                }

                if l2[ri.l2()] != nil {
                        throw("arena already initialized")
                }
                var r *heapArena
                r = (*heapArena)(h.heapArenaAlloc.alloc(unsafe.Sizeof(*r), sys.PtrSize, &memstats.gcMiscSys))
                if r == nil {
                        r = (*heapArena)(persistentalloc(unsafe.Sizeof(*r), sys.PtrSize, &memstats.gcMiscSys))
                        if r == nil {
                                throw("out of memory allocating heap arena metadata")
                        }
                }

                // Add the arena to the arenas list.
                if len(h.allArenas) == cap(h.allArenas) {
                        size := 2 * uintptr(cap(h.allArenas)) * sys.PtrSize
                        if size == 0 {
                                size = physPageSize
                        }
                        newArray := (*notInHeap)(persistentalloc(size, sys.PtrSize, &memstats.gcMiscSys))
                        if newArray == nil {
                                throw("out of memory allocating allArenas")
                        }
                        oldSlice := h.allArenas
                        *(*notInHeapSlice)(unsafe.Pointer(&h.allArenas)) = notInHeapSlice{newArray, len(h.allArenas), int(size / sys.PtrSize)}
                        copy(h.allArenas, oldSlice)
                        // Do not free the old backing array because
                        // there may be concurrent readers. Since we
                        // double the array each time, this can lead
                        // to at most 2x waste.
                }
                h.allArenas = h.allArenas[:len(h.allArenas)+1]
                h.allArenas[len(h.allArenas)-1] = ri

                // Store atomically just in case an object from the
                // new heap arena becomes visible before the heap lock
                // is released (which shouldn't happen, but there's
                // little downside to this).
                atomic.StorepNoWB(unsafe.Pointer(&l2[ri.l2()]), unsafe.Pointer(r))
        }

        // Tell the race detector about the new heap memory.
        if raceenabled {
                racemapshadow(v, size)
        }

        return
}

// sysReserveAligned is like sysReserve, but the returned pointer is
// aligned to align bytes. It may reserve either n or n+align bytes,
// so it returns the size that was reserved.
func sysReserveAligned(v unsafe.Pointer, size, align uintptr) (unsafe.Pointer, uintptr) {
        // Since the alignment is rather large in uses of this
        // function, we're not likely to get it by chance, so we ask
        // for a larger region and remove the parts we don't need.
        retries := 0
retry:
        p := uintptr(sysReserve(v, size+align))
        switch {
        case p == 0:
                return nil, 0
        case p&(align-1) == 0:
                // We got lucky and got an aligned region, so we can
                // use the whole thing.
                return unsafe.Pointer(p), size + align
        case GOOS == "windows":
                // On Windows we can't release pieces of a
                // reservation, so we release the whole thing and
                // re-reserve the aligned sub-region. This may race,
                // so we may have to try again.
                sysFree(unsafe.Pointer(p), size+align, nil)
                p = alignUp(p, align)
                p2 := sysReserve(unsafe.Pointer(p), size)
                if p != uintptr(p2) {
                        // Must have raced. Try again.
                        sysFree(p2, size, nil)
                        if retries++; retries == 100 {
                                throw("failed to allocate aligned heap memory; too many retries")
                        }
                        goto retry
                }
                // Success.
                return p2, size
        default:
                // Trim off the unaligned parts.
                pAligned := alignUp(p, align)
                sysFree(unsafe.Pointer(p), pAligned-p, nil)
                end := pAligned + size
                endLen := (p + size + align) - end
                if endLen > 0 {
                        sysFree(unsafe.Pointer(end), endLen, nil)
                }
                return unsafe.Pointer(pAligned), size
        }
}

// base address for all 0-byte allocations
var zerobase uintptr

// nextFreeFast returns the next free object if one is quickly available.
// Otherwise it returns 0.
func nextFreeFast(s *mspan) gclinkptr {
        theBit := sys.Ctz64(s.allocCache) // Is there a free object in the allocCache?
        if theBit < 64 {
                result := s.freeindex + uintptr(theBit)
                if result < s.nelems {
                        freeidx := result + 1
                        if freeidx%64 == 0 && freeidx != s.nelems {
                                return 0
                        }
                        s.allocCache >>= uint(theBit + 1)
                        s.freeindex = freeidx
                        s.allocCount++
                        return gclinkptr(result*s.elemsize + s.base())
                }
        }
        return 0
}

// nextFree returns the next free object from the cached span if one is available.
// Otherwise it refills the cache with a span with an available object and
// returns that object along with a flag indicating that this was a heavy
// weight allocation. If it is a heavy weight allocation the caller must
// determine whether a new GC cycle needs to be started or if the GC is active
// whether this goroutine needs to assist the GC.
//
// Must run in a non-preemptible context since otherwise the owner of
// c could change.
func (c *mcache) nextFree(spc spanClass) (v gclinkptr, s *mspan, shouldhelpgc bool) {
        s = c.alloc[spc]
        shouldhelpgc = false
        freeIndex := s.nextFreeIndex()
        if freeIndex == s.nelems {
                // The span is full.
                if uintptr(s.allocCount) != s.nelems {
                        println("runtime: s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
                        throw("s.allocCount != s.nelems && freeIndex == s.nelems")
                }
                c.refill(spc)
                shouldhelpgc = true
                s = c.alloc[spc]

                freeIndex = s.nextFreeIndex()
        }

        if freeIndex >= s.nelems {
                throw("freeIndex is not valid")
        }

        v = gclinkptr(freeIndex*s.elemsize + s.base())
        s.allocCount++
        if uintptr(s.allocCount) > s.nelems {
                println("s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
                throw("s.allocCount > s.nelems")
        }
        return
}

// Allocate an object of size bytes.
// Small objects are allocated from the per-P cache's free lists.
// Large objects (> 32 kB) are allocated straight from the heap.
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
        if gcphase == _GCmarktermination {
                throw("mallocgc called with gcphase == _GCmarktermination")
        }

        if size == 0 {
                return unsafe.Pointer(&zerobase)
        }

        if debug.malloc {
                if debug.sbrk != 0 {
                        align := uintptr(16)
                        if typ != nil {
                                // TODO(austin): This should be just
                                //   align = uintptr(typ.align)
                                // but that's only 4 on 32-bit platforms,
                                // even if there's a uint64 field in typ (see #599).
                                // This causes 64-bit atomic accesses to panic.
                                // Hence, we use stricter alignment that matches
                                // the normal allocator better.
                                if size&7 == 0 {
                                        align = 8
                                } else if size&3 == 0 {
                                        align = 4
                                } else if size&1 == 0 {
                                        align = 2
                                } else {
                                        align = 1
                                }
                        }
                        return persistentalloc(size, align, &memstats.other_sys)
                }

                if inittrace.active && inittrace.id == getg().goid {
                        // Init functions are executed sequentially in a single Go routine.
                        inittrace.allocs += 1
                }
        }

        // assistG is the G to charge for this allocation, or nil if
        // GC is not currently active.
        var assistG *g
        if gcBlackenEnabled != 0 {
                // Charge the current user G for this allocation.
                assistG = getg()
                if assistG.m.curg != nil {
                        assistG = assistG.m.curg
                }
                // Charge the allocation against the G. We'll account
                // for internal fragmentation at the end of mallocgc.
                assistG.gcAssistBytes -= int64(size)

                if assistG.gcAssistBytes < 0 {
                        // This G is in debt. Assist the GC to correct
                        // this before allocating. This must happen
                        // before disabling preemption.
                        gcAssistAlloc(assistG)
                }
        }

        // Set mp.mallocing to keep from being preempted by GC.
        mp := acquirem()
        if mp.mallocing != 0 {
                throw("malloc deadlock")
        }
        if mp.gsignal == getg() {
                throw("malloc during signal")
        }
        mp.mallocing = 1

        shouldhelpgc := false
        dataSize := size
        c := getMCache()
        if c == nil {
                throw("mallocgc called without a P or outside bootstrapping")
        }
        var span *mspan
        var x unsafe.Pointer
        noscan := typ == nil || typ.ptrdata == 0
        if size <= maxSmallSize {
                if noscan && size < maxTinySize {
                        // Tiny allocator.
                        //
                        // Tiny allocator combines several tiny allocation requests
                        // into a single memory block. The resulting memory block
                        // is freed when all subobjects are unreachable. The subobjects
                        // must be noscan (don't have pointers), this ensures that
                        // the amount of potentially wasted memory is bounded.
                        //
                        // Size of the memory block used for combining (maxTinySize) is tunable.
                        // Current setting is 16 bytes, which relates to 2x worst case memory
                        // wastage (when all but one subobjects are unreachable).
                        // 8 bytes would result in no wastage at all, but provides less
                        // opportunities for combining.
                        // 32 bytes provides more opportunities for combining,
                        // but can lead to 4x worst case wastage.
                        // The best case winning is 8x regardless of block size.
                        //
                        // Objects obtained from tiny allocator must not be freed explicitly.
                        // So when an object will be freed explicitly, we ensure that
                        // its size >= maxTinySize.
                        //
                        // SetFinalizer has a special case for objects potentially coming
                        // from tiny allocator, it such case it allows to set finalizers
                        // for an inner byte of a memory block.
                        //
                        // The main targets of tiny allocator are small strings and
                        // standalone escaping variables. On a json benchmark
                        // the allocator reduces number of allocations by ~12% and
                        // reduces heap size by ~20%.
                        off := c.tinyoffset
                        // Align tiny pointer for required (conservative) alignment.
                        if size&7 == 0 {
                                off = alignUp(off, 8)
                        } else if sys.PtrSize == 4 && size == 12 {
                                // Conservatively align 12-byte objects to 8 bytes on 32-bit
                                // systems so that objects whose first field is a 64-bit
                                // value is aligned to 8 bytes and does not cause a fault on
                                // atomic access. See issue 37262.
                                // TODO(mknyszek): Remove this workaround if/when issue 36606
                                // is resolved.
                                off = alignUp(off, 8)
                        } else if size&3 == 0 {
                                off = alignUp(off, 4)
                        } else if size&1 == 0 {
                                off = alignUp(off, 2)
                        }
                        if off+size <= maxTinySize && c.tiny != 0 {
                                // The object fits into existing tiny block.
                                x = unsafe.Pointer(c.tiny + off)
                                c.tinyoffset = off + size
                                c.tinyAllocs++
                                mp.mallocing = 0
                                releasem(mp)
                                return x
                        }
                        // Allocate a new maxTinySize block.
                        span = c.alloc[tinySpanClass]
                        v := nextFreeFast(span)
                        if v == 0 {
                                v, span, shouldhelpgc = c.nextFree(tinySpanClass)
                        }
                        x = unsafe.Pointer(v)
                        (*[2]uint64)(x)[0] = 0
                        (*[2]uint64)(x)[1] = 0
                        // See if we need to replace the existing tiny block with the new one
                        // based on amount of remaining free space.
                        if size < c.tinyoffset || c.tiny == 0 {
                                c.tiny = uintptr(x)
                                c.tinyoffset = size
                        }
                        size = maxTinySize
                } else {
                        var sizeclass uint8
                        if size <= smallSizeMax-8 {
                                sizeclass = size_to_class8[divRoundUp(size, smallSizeDiv)]
                        } else {
                                sizeclass = size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]
                        }
                        size = uintptr(class_to_size[sizeclass])
                        spc := makeSpanClass(sizeclass, noscan)
                        span = c.alloc[spc]
                        v := nextFreeFast(span)
                        if v == 0 {
                                v, span, shouldhelpgc = c.nextFree(spc)
                        }
                        x = unsafe.Pointer(v)
                        if needzero && span.needzero != 0 {
                                memclrNoHeapPointers(unsafe.Pointer(v), size)
                        }
                }
        } else {
                shouldhelpgc = true
                span = c.allocLarge(size, needzero, noscan)
                span.freeindex = 1
                span.allocCount = 1
                x = unsafe.Pointer(span.base())
                size = span.elemsize
        }

        var scanSize uintptr
        if !noscan {
                // If allocating a defer+arg block, now that we've picked a malloc size
                // large enough to hold everything, cut the "asked for" size down to
                // just the defer header, so that the GC bitmap will record the arg block
                // as containing nothing at all (as if it were unused space at the end of
                // a malloc block caused by size rounding).
                // The defer arg areas are scanned as part of scanstack.
                if typ == deferType {
                        dataSize = unsafe.Sizeof(_defer{})
                }
                heapBitsSetType(uintptr(x), size, dataSize, typ)
                if dataSize > typ.size {
                        // Array allocation. If there are any
                        // pointers, GC has to scan to the last
                        // element.
                        if typ.ptrdata != 0 {
                                scanSize = dataSize - typ.size + typ.ptrdata
                        }
                } else {
                        scanSize = typ.ptrdata
                }
                c.scanAlloc += scanSize
        }

        // Ensure that the stores above that initialize x to
        // type-safe memory and set the heap bits occur before
        // the caller can make x observable to the garbage
        // collector. Otherwise, on weakly ordered machines,
        // the garbage collector could follow a pointer to x,
        // but see uninitialized memory or stale heap bits.
        publicationBarrier()

        // Allocate black during GC.
        // All slots hold nil so no scanning is needed.
        // This may be racing with GC so do it atomically if there can be
        // a race marking the bit.
        if gcphase != _GCoff {
                gcmarknewobject(span, uintptr(x), size, scanSize)
        }

        if raceenabled {
                racemalloc(x, size)
        }

        if msanenabled {
                msanmalloc(x, size)
        }

        mp.mallocing = 0
        releasem(mp)

        if debug.malloc {
                if debug.allocfreetrace != 0 {
                        tracealloc(x, size, typ)
                }

                if inittrace.active && inittrace.id == getg().goid {
                        // Init functions are executed sequentially in a single Go routine.
                        inittrace.bytes += uint64(size)
                }
        }

        if rate := MemProfileRate; rate > 0 {
                if rate != 1 && size < c.nextSample {
                        c.nextSample -= size
                } else {
                        mp := acquirem()
                        profilealloc(mp, x, size)
                        releasem(mp)
                }
        }

        if assistG != nil {
                // Account for internal fragmentation in the assist
                // debt now that we know it.
                assistG.gcAssistBytes -= int64(size - dataSize)
        }

        if shouldhelpgc {
                if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
                        gcStart(t)
                }
        }

        return x
}

// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function
func newobject(typ *_type) unsafe.Pointer {
        return mallocgc(typ.size, typ, true)
}

//go:linkname reflect_unsafe_New reflect.unsafe_New
func reflect_unsafe_New(typ *_type) unsafe.Pointer {
        return mallocgc(typ.size, typ, true)
}

//go:linkname reflectlite_unsafe_New internal/reflectlite.unsafe_New
func reflectlite_unsafe_New(typ *_type) unsafe.Pointer {
        return mallocgc(typ.size, typ, true)
}

// newarray allocates an array of n elements of type typ.
func newarray(typ *_type, n int) unsafe.Pointer {
        if n == 1 {
                return mallocgc(typ.size, typ, true)
        }
        mem, overflow := math.MulUintptr(typ.size, uintptr(n))
        if overflow || mem > maxAlloc || n < 0 {
                panic(plainError("runtime: allocation size out of range"))
        }
        return mallocgc(mem, typ, true)
}

//go:linkname reflect_unsafe_NewArray reflect.unsafe_NewArray
func reflect_unsafe_NewArray(typ *_type, n int) unsafe.Pointer {
        return newarray(typ, n)
}

func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
        c := getMCache()
        if c == nil {
                throw("profilealloc called without a P or outside bootstrapping")
        }
        c.nextSample = nextSample()
        mProf_Malloc(x, size)
}

// nextSample returns the next sampling point for heap profiling. The goal is
// to sample allocations on average every MemProfileRate bytes, but with a
// completely random distribution over the allocation timeline; this
// corresponds to a Poisson process with parameter MemProfileRate. In Poisson
// processes, the distance between two samples follows the exponential
// distribution (exp(MemProfileRate)), so the best return value is a random
// number taken from an exponential distribution whose mean is MemProfileRate.
func nextSample() uintptr {
        if MemProfileRate == 1 {
                // Callers assign our return value to
                // mcache.next_sample, but next_sample is not used
                // when the rate is 1. So avoid the math below and
                // just return something.
                return 0
        }
        if GOOS == "plan9" {
                // Plan 9 doesn't support floating point in note handler.
                if g := getg(); g == g.m.gsignal {
                        return nextSampleNoFP()
                }
        }

        return uintptr(fastexprand(MemProfileRate))
}

// fastexprand returns a random number from an exponential distribution with
// the specified mean.
func fastexprand(mean int) int32 {
        // Avoid overflow. Maximum possible step is
        // -ln(1/(1<<randomBitCount)) * mean, approximately 20 * mean.
        switch {
        case mean > 0x7000000:
                mean = 0x7000000
        case mean == 0:
                return 0
        }

        // Take a random sample of the exponential distribution exp(-mean*x).
        // The probability distribution function is mean*exp(-mean*x), so the CDF is
        // p = 1 - exp(-mean*x), so
        // q = 1 - p == exp(-mean*x)
        // log_e(q) = -mean*x
        // -log_e(q)/mean = x
        // x = -log_e(q) * mean
        // x = log_2(q) * (-log_e(2)) * mean    ; Using log_2 for efficiency
        const randomBitCount = 26
        q := fastrand()%(1<<randomBitCount) + 1
        qlog := fastlog2(float64(q)) - randomBitCount
        if qlog > 0 {
                qlog = 0
        }
        const minusLog2 = -0.6931471805599453 // -ln(2)
        return int32(qlog*(minusLog2*float64(mean))) + 1
}

// nextSampleNoFP is similar to nextSample, but uses older,
// simpler code to avoid floating point.
func nextSampleNoFP() uintptr {
        // Set first allocation sample size.
        rate := MemProfileRate
        if rate > 0x3fffffff { // make 2*rate not overflow
                rate = 0x3fffffff
        }
        if rate != 0 {
                return uintptr(fastrand() % uint32(2*rate))
        }
        return 0
}

type persistentAlloc struct {
        base *notInHeap
        off  uintptr
}

var globalAlloc struct {
        mutex
        persistentAlloc
}

// persistentChunkSize is the number of bytes we allocate when we grow
// a persistentAlloc.
const persistentChunkSize = 256 << 10

// persistentChunks is a list of all the persistent chunks we have
// allocated. The list is maintained through the first word in the
// persistent chunk. This is updated atomically.
var persistentChunks *notInHeap

// Wrapper around sysAlloc that can allocate small chunks.
// There is no associated free operation.
// Intended for things like function/type/debug-related persistent data.
// If align is 0, uses default align (currently 8).
// The returned memory will be zeroed.
//
// Consider marking persistentalloc'd types go:notinheap.
func persistentalloc(size, align uintptr, sysStat *sysMemStat) unsafe.Pointer {
        var p *notInHeap
        systemstack(func() {
                p = persistentalloc1(size, align, sysStat)
        })
        return unsafe.Pointer(p)
}

// Must run on system stack because stack growth can (re)invoke it.
// See issue 9174.
//go:systemstack
func persistentalloc1(size, align uintptr, sysStat *sysMemStat) *notInHeap {
        const (
                maxBlock = 64 << 10 // VM reservation granularity is 64K on windows
        )

        if size == 0 {
                throw("persistentalloc: size == 0")
        }
        if align != 0 {
                if align&(align-1) != 0 {
                        throw("persistentalloc: align is not a power of 2")
                }
                if align > _PageSize {
                        throw("persistentalloc: align is too large")
                }
        } else {
                align = 8
        }

        if size >= maxBlock {
                return (*notInHeap)(sysAlloc(size, sysStat))
        }

        mp := acquirem()
        var persistent *persistentAlloc
        if mp != nil && mp.p != 0 {
                persistent = &mp.p.ptr().palloc
        } else {
                lock(&globalAlloc.mutex)
                persistent = &globalAlloc.persistentAlloc
        }
        persistent.off = alignUp(persistent.off, align)
        if persistent.off+size > persistentChunkSize || persistent.base == nil {
                persistent.base = (*notInHeap)(sysAlloc(persistentChunkSize, &memstats.other_sys))
                if persistent.base == nil {
                        if persistent == &globalAlloc.persistentAlloc {
                                unlock(&globalAlloc.mutex)
                        }
                        throw("runtime: cannot allocate memory")
                }

                // Add the new chunk to the persistentChunks list.
                for {
                        chunks := uintptr(unsafe.Pointer(persistentChunks))
                        *(*uintptr)(unsafe.Pointer(persistent.base)) = chunks
                        if atomic.Casuintptr((*uintptr)(unsafe.Pointer(&persistentChunks)), chunks, uintptr(unsafe.Pointer(persistent.base))) {
                                break
                        }
                }
                persistent.off = alignUp(sys.PtrSize, align)
        }
        p := persistent.base.add(persistent.off)
        persistent.off += size
        releasem(mp)
        if persistent == &globalAlloc.persistentAlloc {
                unlock(&globalAlloc.mutex)
        }

        if sysStat != &memstats.other_sys {
                sysStat.add(int64(size))
                memstats.other_sys.add(-int64(size))
        }
        return p
}

// inPersistentAlloc reports whether p points to memory allocated by
// persistentalloc. This must be nosplit because it is called by the
// cgo checker code, which is called by the write barrier code.
//go:nosplit
func inPersistentAlloc(p uintptr) bool {
        chunk := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&persistentChunks)))
        for chunk != 0 {
                if p >= chunk && p < chunk+persistentChunkSize {
                        return true
                }
                chunk = *(*uintptr)(unsafe.Pointer(chunk))
        }
        return false
}

// linearAlloc is a simple linear allocator that pre-reserves a region
// of memory and then maps that region into the Ready state as needed. The
// caller is responsible for locking.
type linearAlloc struct {
        next   uintptr // next free byte
        mapped uintptr // one byte past end of mapped space
        end    uintptr // end of reserved space
}

func (l *linearAlloc) init(base, size uintptr) {
        if base+size < base {
                // Chop off the last byte. The runtime isn't prepared
                // to deal with situations where the bounds could overflow.
                // Leave that memory reserved, though, so we don't map it
                // later.
                size -= 1
        }
        l.next, l.mapped = base, base
        l.end = base + size
}

func (l *linearAlloc) alloc(size, align uintptr, sysStat *sysMemStat) unsafe.Pointer {
        p := alignUp(l.next, align)
        if p+size > l.end {
                return nil
        }
        l.next = p + size
        if pEnd := alignUp(l.next-1, physPageSize); pEnd > l.mapped {
                // Transition from Reserved to Prepared to Ready.
                sysMap(unsafe.Pointer(l.mapped), pEnd-l.mapped, sysStat)
                sysUsed(unsafe.Pointer(l.mapped), pEnd-l.mapped)
                l.mapped = pEnd
        }
        return unsafe.Pointer(p)
}

// notInHeap is off-heap memory allocated by a lower-level allocator
// like sysAlloc or persistentAlloc.
//
// In general, it's better to use real types marked as go:notinheap,
// but this serves as a generic type for situations where that isn't
// possible (like in the allocators).
//
// TODO: Use this as the return type of sysAlloc, persistentAlloc, etc?
//
//go:notinheap
type notInHeap struct{}

func (p *notInHeap) add(bytes uintptr) *notInHeap {
        return (*notInHeap)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + bytes))
}