// 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 (
	"internal/goarch"
	"internal/goexperiment"
	"internal/goos"
	"internal/runtime/atomic"
	"internal/runtime/gc"
	"internal/runtime/math"
	"internal/runtime/sys"
	"unsafe"
)

const (
	maxTinySize   = _TinySize
	tinySizeClass = _TinySizeClass
	maxSmallSize  = gc.MaxSmallSize
	pageSize      = 1 << gc.PageShift
	pageMask      = pageSize - 1

	// Unused. Left for viewcore.
	_PageSize              = pageSize
	minSizeForMallocHeader = gc.MinSizeForMallocHeader
	mallocHeaderSize       = gc.MallocHeaderSize

	// _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      = gc.TinySize
	_TinySizeClass = int8(gc.TinySizeClass)

	_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 - goarch.PtrSize/4*goos.IsWindows - 1*goos.IsPlan9

	// 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 in iOS <14.
	// Furthermore, only the top 4 GiB of the address space are
	// actually available to the application. In iOS >=14, more
	// of the address space is available, and the OS can now
	// provide addresses outside of those 33 bits. Pick 40 bits
	// as a reasonable balance between address space usage by the
	// page allocator, and flexibility for what mmap'd regions
	// we'll accept for the heap. We can't just move to the full
	// 48 bits because this uses too much address space for older
	// iOS versions.
	// TODO(mknyszek): Once iOS <14 is deprecated, promote ios/arm64
	// to a 48-bit address space like every other arm64 platform.
	//
	// WebAssembly currently has a limit of 4GB linear memory.
	heapAddrBits = (_64bit*(1-goarch.IsWasm)*(1-goos.IsIos*goarch.IsArm64))*48 + (1-_64bit+goarch.IsWasm)*(32-(goarch.IsMips+goarch.IsMipsle)) + 40*goos.IsIos*goarch.IsArm64

	// 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)
	//      ios/arm64         40         4MB           1  256K  (2MB)
	//       */32-bit         32         4MB           1  1024  (4KB)
	//     */mips(le)         31         4MB           1   512  (2KB)
	//           wasm         32       512KB           1  8192 (64KB)

	// 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, 4MB on
	// 32-bit and on Windows, and 512KB on Wasm. 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. We use smaller arenas on Wasm
	// because some Wasm programs have very small heap, and
	// everything in the Wasm linear memory is charged.
	heapArenaBytes = 1 << logHeapArenaBytes

	heapArenaWords = heapArenaBytes / goarch.PtrSize

	// 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-goos.IsWindows)*(1-goarch.IsWasm)*(1-goos.IsIos*goarch.IsArm64)) + (2+20)*(_64bit*goos.IsWindows) + (2+20)*(1-_64bit) + (9+10)*goarch.IsWasm + (2+20)*goos.IsIos*goarch.IsArm64

	// heapArenaBitmapWords is the size of each heap arena's bitmap in uintptrs.
	heapArenaBitmapWords = heapArenaWords / (8 * goarch.PtrSize)

	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 * goos.IsWindows)

	// 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. Otherwise, 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*goarch.IsAmd64 + 0x0a00000000000000*goos.IsAix
	// 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

	// minHeapForMetadataHugePages sets a threshold on when certain kinds of
	// heap metadata, currently the arenas map L2 entries and page alloc bitmap
	// mappings, are allowed to be backed by huge pages. If the heap goal ever
	// exceeds this threshold, then huge pages are enabled.
	//
	// These numbers are chosen with the assumption that huge pages are on the
	// order of a few MiB in size.
	//
	// The kind of metadata this applies to has a very low overhead when compared
	// to address space used, but their constant overheads for small heaps would
	// be very high if they were to be backed by huge pages (e.g. a few MiB makes
	// a huge difference for an 8 MiB heap, but barely any difference for a 1 GiB
	// heap). The benefit of huge pages is also not worth it for small heaps,
	// because only a very, very small part of the metadata is used for small heaps.
	//
	// N.B. If the heap goal exceeds the threshold then shrinks to a very small size
	// again, then huge pages will still be enabled for this mapping. The reason is that
	// there's no point unless we're also returning the physical memory for these
	// metadata mappings back to the OS. That would be quite complex to do in general
	// as the heap is likely fragmented after a reduction in heap size.
	minHeapForMetadataHugePages = 1 << 30

	// randomizeHeapBase indicates if the heap base address should be randomized.
	// See comment in mallocinit for how the randomization is performed.
	randomizeHeapBase = goexperiment.RandomizedHeapBase64 && goarch.PtrSize == 8 && !isSbrkPlatform && !raceenabled && !msanenabled && !asanenabled

	// randHeapAddrBits is the number of address bits usable by the randomized
	// heap base. heapAddrBits is 48 on most platforms, but we only use 47 of
	// those bits in order to provide a good amount of room for the heap to
	// grow contiguously. On amd64, there are 48 bits, but the top bit is sign
	// extended, so we throw away another bit, just to be safe.
	randHeapAddrBits = heapAddrBits - 1 - goarch.IsAmd64

	// randHeapBasePrefixMask clears the top byte of the randomized heap base
	// address -- the byte hint generation replaces with randHeapBasePrefix+i.
	// The prefix occupies bits [randHeapAddrBits-8, randHeapAddrBits), so the
	// mask must be defined from randHeapAddrBits, not heapAddrBits: a wider
	// mask would let stray randHeapBase bits overlap the prefix byte in the
	// OR that hint generation performs, and wherever such a stray bit is 1,
	// the corresponding bit of every generated prefix is forced to 1,
	// collapsing distinct prefixes into duplicate hint addresses.
	randHeapBasePrefixMask = ^uintptr(0xff << (randHeapAddrBits - 8))
)

// 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
)

var (
	// heapRandSeed is a random value that is populated in mallocinit if
	// randomizeHeapBase is set. It is used in mallocinit, and mheap.grow, to
	// randomize the base heap address.
	heapRandSeed              uintptr
	heapRandSeedBitsRemaining int
)

func nextHeapRandBits(bits int) uintptr {
	if bits > heapRandSeedBitsRemaining {
		throw("not enough heapRandSeed bits remaining")
	}
	r := heapRandSeed >> (64 - bits)
	heapRandSeed <<= bits
	heapRandSeedBitsRemaining -= bits
	return r
}

func mallocinit() {
	if gc.SizeClassToSize[tinySizeClass] != maxTinySize {
		throw("bad TinySizeClass")
	}

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

	// 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")
	}
	// Check that the minimum size (exclusive) for a malloc header is also
	// a size class boundary. This is important to making sure checks align
	// across different parts of the runtime.
	//
	// While we're here, also check to make sure all these size classes'
	// span sizes are one page. Some code relies on this.
	minSizeForMallocHeaderIsSizeClass := false
	sizeClassesUpToMinSizeForMallocHeaderAreOnePage := true
	for i := 0; i < len(gc.SizeClassToSize); i++ {
		if gc.SizeClassToNPages[i] > 1 {
			sizeClassesUpToMinSizeForMallocHeaderAreOnePage = false
		}
		if gc.MinSizeForMallocHeader == uintptr(gc.SizeClassToSize[i]) {
			minSizeForMallocHeaderIsSizeClass = true
			break
		}
	}
	if !minSizeForMallocHeaderIsSizeClass {
		throw("min size of malloc header is not a size class boundary")
	}
	if !sizeClassesUpToMinSizeForMallocHeaderAreOnePage {
		throw("expected all size classes up to min size for malloc header to fit in one-page spans")
	}
	// Check that the pointer bitmap for all small sizes without a malloc header
	// fits in a word.
	if gc.MinSizeForMallocHeader/goarch.PtrSize > 8*goarch.PtrSize {
		throw("max pointer/scan bitmap size for headerless objects is too large")
	}

	if minTagBits > tagBits {
		throw("tagBits too small")
	}

	// Initialize the heap.
	mheap_.init()
	mcache0 = allocmcache()
	lockInit(&gcBitsArenas.lock, lockRankGcBitsArenas)
	lockInit(&profInsertLock, lockRankProfInsert)
	lockInit(&profBlockLock, lockRankProfBlock)
	lockInit(&profMemActiveLock, lockRankProfMemActive)
	for i := range profMemFutureLock {
		lockInit(&profMemFutureLock[i], lockRankProfMemFuture)
	}
	lockInit(&globalAlloc.mutex, lockRankGlobalAlloc)

	// Create initial arena growth hints.
	if isSbrkPlatform {
		// Don't generate hints on sbrk platforms. We can
		// only grow the break sequentially.
	} else if goarch.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.
		//
		// Space mapped for user arenas comes immediately after the range
		// originally reserved for the regular heap when race mode is not
		// enabled because user arena chunks can never be used for regular heap
		// allocations and we want to avoid fragmenting the address space.
		//
		// In race mode we have no choice but to just use the same hints because
		// the race detector requires that the heap be mapped contiguously.
		//
		// If randomizeHeapBase is set, we attempt to randomize the base address
		// as much as possible. We do this by generating a random uint64 via
		// bootstrapRand and using it's bits to randomize portions of the base
		// address as follows:
		//   * We first generate a random heapArenaBytes aligned address that we use for
		//     generating the hints.
		//   * On the first call to mheap.grow, we then generate a random PallocChunkBytes
		//     aligned offset into the mmap'd heap region, which we use as the base for
		//     the heap region.
		//   * We then select a page offset in that PallocChunkBytes region to start the
		//     heap at, and mark all the pages up to that offset as allocated.
		//
		// Our final randomized "heap base address" becomes the first byte of
		// the first available page returned by the page allocator. This results
		// in an address with at least heapAddrBits-gc.PageShift-2-(1*goarch.IsAmd64)
		// bits of entropy.

		var randHeapBase uintptr
		var randHeapBasePrefix byte
		if randomizeHeapBase {
			// Generate a random value, and take the bottom heapAddrBits-logHeapArenaBytes
			// bits, using them as the top bits for randHeapBase.
			heapRandSeed, heapRandSeedBitsRemaining = uintptr(bootstrapRand()), 64

			topBits := (randHeapAddrBits - logHeapArenaBytes)
			randHeapBase = nextHeapRandBits(topBits) << (randHeapAddrBits - topBits)
			randHeapBase = alignUp(randHeapBase, heapArenaBytes)
			randHeapBasePrefix = byte(randHeapBase >> (randHeapAddrBits - 8))
		}

		var vmaSize int
		if GOARCH == "riscv64" {
			// Identify which memory layout is in use based on the system
			// stack address, knowing that the bottom half of virtual memory
			// is user space. This should result in 39, 48 or 57. It may be
			// possible to use RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS at some
			// point in the future - for now use the system stack address.
			vmaSize = sys.Len64(uint64(getg().m.g0.stack.hi)) + 1
			if raceenabled && vmaSize != 39 && vmaSize != 48 {
				println("vma size = ", vmaSize)
				throw("riscv64 vma size is unknown and race mode is enabled")
			}
		}

		for i := 0x7f; i >= 0; i-- {
			var p uintptr
			switch {
			case raceenabled && GOARCH == "riscv64" && vmaSize == 39:
				p = uintptr(i)<<28 | uintptrMask&(0x0013<<28)
				if p >= uintptrMask&0x000f00000000 {
					continue
				}
			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 randomizeHeapBase:
				prefix := uintptr(randHeapBasePrefix+byte(i)) << (randHeapAddrBits - 8)
				p = prefix | (randHeapBase & randHeapBasePrefixMask)
			case GOARCH == "arm64" && GOOS == "ios":
				p = uintptr(i)<<40 | uintptrMask&(0x0013<<28)
			case GOARCH == "arm64":
				p = uintptr(i)<<40 | uintptrMask&(0x0040<<32)
			case GOARCH == "riscv64" && vmaSize == 39:
				p = uintptr(i)<<32 | uintptrMask&(0x0013<<28)
			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)
			}
			// Switch to generating hints for user arenas if we've gone
			// through about half the hints. In race mode, take only about
			// a quarter; we don't have very much space to work with.
			hintList := &mheap_.arenaHints
			if (!raceenabled && i > 0x3f) || (raceenabled && i > 0x5f) {
				hintList = &mheap_.userArena.arenaHints
			}
			hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
			hint.addr = p
			hint.next, *hintList = *hintList, 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, "heap reservation"))
		if meta != 0 {
			mheap_.heapArenaAlloc.init(meta, arenaMetaSize, true)
		}

		// 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, "heap reservation")
			if a != nil {
				mheap_.arena.init(uintptr(a), size, false)
				p = mheap_.arena.end // For hint below
				break
			}
		}
		hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
		hint.addr = p
		hint.next, mheap_.arenaHints = mheap_.arenaHints, hint

		// Place the hint for user arenas just after the large reservation.
		//
		// While this potentially competes with the hint above, in practice we probably
		// aren't going to be getting this far anyway on 32-bit platforms.
		userArenaHint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
		userArenaHint.addr = p
		userArenaHint.next, mheap_.userArena.arenaHints = mheap_.userArena.arenaHints, userArenaHint
	}
	// Initialize the memory limit here because the allocator is going to look at it
	// but we haven't called gcinit yet and we're definitely going to allocate memory before then.
	gcController.memoryLimit.Store(math.MaxInt64)
}

// 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.
//
// hintList is a list of hint addresses for where to allocate new
// heap arenas. It must be non-nil.
//
// sysAlloc returns a memory region in the Reserved state. This region must
// be transitioned to Prepared and then Ready before use.
//
// arenaList is the list the arena should be added to.
//
// h must be locked.
func (h *mheap) sysAlloc(n uintptr, hintList **arenaHint, arenaList *[]arenaIdx) (v unsafe.Pointer, size uintptr) {
	assertLockHeld(&h.lock)

	n = alignUp(n, heapArenaBytes)

	if hintList == &h.arenaHints {
		// First, try the arena pre-reservation.
		// Newly-used mappings are considered released.
		//
		// Only do this if we're using the regular heap arena hints.
		// This behavior is only for the heap.
		v = h.arena.alloc(n, heapArenaBytes, &gcController.heapReleased, "heap")
		if v != nil {
			size = n
			goto mapped
		}
	}

	// Try to grow the heap at a hint address.
	for *hintList != nil {
		hint := *hintList
		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, "heap reservation")
		}
		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 {
			sysUnreserve(v, n)
		}
		*hintList = 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, "heap")
		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")
	}

mapped:
	if valgrindenabled {
		valgrindCreateMempool(v)
		valgrindMakeMemNoAccess(v, size)
	}

	// 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.
			//
			// Use sysAllocOS instead of sysAlloc or persistentalloc because there's no
			// statistic we can comfortably account for this space in. With this structure,
			// we rely on demand paging to avoid large overheads, but tracking which memory
			// is paged in is too expensive. Trying to account for the whole region means
			// that it will appear like an enormous memory overhead in statistics, even though
			// it is not.
			l2 = (*[1 << arenaL2Bits]*heapArena)(sysAllocOS(unsafe.Sizeof(*l2), "heap index"))
			if l2 == nil {
				throw("out of memory allocating heap arena map")
			}
			if h.arenasHugePages {
				sysHugePage(unsafe.Pointer(l2), unsafe.Sizeof(*l2))
			} else {
				sysNoHugePage(unsafe.Pointer(l2), unsafe.Sizeof(*l2))
			}
			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), goarch.PtrSize, &memstats.gcMiscSys, "heap metadata"))
		if r == nil {
			r = (*heapArena)(persistentalloc(unsafe.Sizeof(*r), goarch.PtrSize, &memstats.gcMiscSys))
			if r == nil {
				throw("out of memory allocating heap arena metadata")
			}
		}

		// Register the arena in allArenas if requested.
		if len((*arenaList)) == cap((*arenaList)) {
			size := 2 * uintptr(cap((*arenaList))) * goarch.PtrSize
			if size == 0 {
				size = physPageSize
			}
			newArray := (*notInHeap)(persistentalloc(size, goarch.PtrSize, &memstats.gcMiscSys))
			if newArray == nil {
				throw("out of memory allocating allArenas")
			}
			oldSlice := (*arenaList)
			*(*notInHeapSlice)(unsafe.Pointer(&(*arenaList))) = notInHeapSlice{newArray, len((*arenaList)), int(size / goarch.PtrSize)}
			copy((*arenaList), 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.
		}
		(*arenaList) = (*arenaList)[:len((*arenaList))+1]
		(*arenaList)[len((*arenaList))-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
}

// enableMetadataHugePages enables huge pages for various sources of heap metadata.
//
// A note on latency: for sufficiently small heaps (<10s of GiB) this function will take constant
// time, but may take time proportional to the size of the mapped heap beyond that.
//
// This function is idempotent.
//
// The heap lock must not be held over this operation, since it will briefly acquire
// the heap lock.
//
// Must be called on the system stack because it acquires the heap lock.
//
//go:systemstack
func (h *mheap) enableMetadataHugePages() {
	// Enable huge pages for page structure.
	h.pages.enableChunkHugePages()

	// Grab the lock and set arenasHugePages if it's not.
	//
	// Once arenasHugePages is set, all new L2 entries will be eligible for
	// huge pages. We'll set all the old entries after we release the lock.
	lock(&h.lock)
	if h.arenasHugePages {
		unlock(&h.lock)
		return
	}
	h.arenasHugePages = true
	unlock(&h.lock)

	// N.B. The arenas L1 map is quite small on all platforms, so it's fine to
	// just iterate over the whole thing.
	for i := range h.arenas {
		l2 := (*[1 << arenaL2Bits]*heapArena)(atomic.Loadp(unsafe.Pointer(&h.arenas[i])))
		if l2 == nil {
			continue
		}
		sysHugePage(unsafe.Pointer(l2), unsafe.Sizeof(*l2))
	}
}

// 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.TrailingZeros64(s.allocCache) // Is there a free object in the allocCache?
	if theBit < 64 {
		result := s.freeindex + uint16(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(uintptr(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, checkGCTrigger bool) {
	s = c.alloc[spc]
	checkGCTrigger = false
	freeIndex := s.nextFreeIndex()
	if freeIndex == s.nelems {
		// The span is full.
		if 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)
		checkGCTrigger = true
		s = c.alloc[spc]

		freeIndex = s.nextFreeIndex()
	}

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

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

// doubleCheckMalloc enables a bunch of extra checks to malloc to double-check
// that various invariants are upheld.
//
// We might consider turning these on by default; many of them previously were.
// They account for a few % of mallocgc's cost though, which does matter somewhat
// at scale. (When testing changes to malloc, consider enabling this, and also
// some function-local 'doubleCheck' consts such as in mbitmap.go currently.)
const doubleCheckMalloc = false

// sizeSpecializedMallocEnabled is the set of conditions where we enable the size-specialized
// mallocgc implementation: none of the sanitizers should be enabled. The tables used to select
// the size-specialized malloc function do not compile properly on plan9, so
// size-specialized malloc is also disabled on plan9.
const sizeSpecializedMallocEnabled = GOOS != "plan9" && !asanenabled && !raceenabled && !msanenabled && !valgrindenabled

// runtimeFreegcEnabled is the set of conditions where we enable the runtime.freegc
// implementation and the corresponding allocation-related changes: the experiment must be
// enabled, and none of the memory sanitizers should be enabled. We allow the race detector,
// in contrast to sizeSpecializedMallocEnabled.
// TODO(thepudds): it would be nice to check Valgrind integration, though there are some hints
// there might not be any canned tests in tree for Go's integration with Valgrind.
const runtimeFreegcEnabled = goexperiment.RuntimeFreegc && !asanenabled && !msanenabled && !valgrindenabled

// 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.
//
// mallocgc should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/bytedance/gopkg
//   - github.com/bytedance/sonic
//   - github.com/cloudwego/frugal
//   - github.com/cockroachdb/cockroach
//   - github.com/cockroachdb/pebble
//   - github.com/ugorji/go/codec
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname mallocgc
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
	if doubleCheckMalloc {
		if gcphase == _GCmarktermination {
			throw("mallocgc called with gcphase == _GCmarktermination")
		}
	}

	// Short-circuit zero-sized allocation requests.
	if size == 0 {
		return unsafe.Pointer(&zerobase)
	}

	if sizeSpecializedMallocEnabled && size < uintptr(len(mallocNoScanTable)) {
		if typ == nil || !typ.Pointers() {
			if size >= maxTinySize {
				return mallocNoScanTable[size](size, typ, needzero)
			}
			return mallocgcTinySC2(size, typ, needzero)
		} else {
			if !needzero {
				throw("objects with pointers must be zeroed")
			}
			return mallocScanTable[size](size, typ, needzero)
		}
	}

	// It's possible for any malloc to trigger sweeping, which may in
	// turn queue finalizers. Record this dynamic lock edge.
	// N.B. Compiled away if lockrank experiment is not enabled.
	lockRankMayQueueFinalizer()

	// Pre-malloc debug hooks.
	if debug.malloc {
		if x := preMallocgcDebug(size, typ); x != nil {
			return x
		}
	}

	// For ASAN, we allocate extra memory around each allocation called the "redzone."
	// These "redzones" are marked as unaddressable.
	var asanRZ uintptr
	if asanenabled {
		asanRZ = redZoneSize(size)
		size += asanRZ
	}

	// Assist the GC if needed. (On the reuse path, we currently compensate for this;
	// changes here might require changes there.)
	if gcBlackenEnabled != 0 {
		deductAssistCredit(size)
	}

	// Actually do the allocation.
	var x unsafe.Pointer
	var elemsize uintptr
	if sizeSpecializedMallocEnabled {
		if size <= maxSmallSize-gc.MallocHeaderSize {
			if typ == nil || !typ.Pointers() {
				x, elemsize = mallocgcSmallNoscan(size, typ, needzero)
			} else {
				if !needzero {
					throw("objects with pointers must be zeroed")
				}
				if heapBitsInSpan(size) {
					x, elemsize = mallocgcSmallScanNoHeader(size, typ)
				} else {
					x, elemsize = mallocgcSmallScanHeader(size, typ)
				}
			}
		} else {
			x, elemsize = mallocgcLarge(size, typ, needzero)
		}
	} else {
		if size <= maxSmallSize-gc.MallocHeaderSize {
			if typ == nil || !typ.Pointers() {
				// tiny allocations might be kept alive by other co-located values.
				// Make sure secret allocations get zeroed by avoiding the tiny allocator
				// See go.dev/issue/76356
				gp := getg()
				if size < maxTinySize && gp.secret == 0 {
					x, elemsize = mallocgcTiny(size, typ)
				} else {
					x, elemsize = mallocgcSmallNoscan(size, typ, needzero)
				}
			} else {
				if !needzero {
					throw("objects with pointers must be zeroed")
				}
				if heapBitsInSpan(size) {
					x, elemsize = mallocgcSmallScanNoHeader(size, typ)
				} else {
					x, elemsize = mallocgcSmallScanHeader(size, typ)
				}
			}
		} else {
			x, elemsize = mallocgcLarge(size, typ, needzero)
		}
	}

	gp := getg()
	if goexperiment.RuntimeSecret && gp.secret > 0 {
		// Mark any object allocated while in secret mode as secret.
		// This ensures we zero it immediately when freeing it.
		addSecret(x, size)
	}

	// Notify sanitizers, if enabled.
	if raceenabled {
		racemalloc(x, size-asanRZ)
	}
	if msanenabled {
		msanmalloc(x, size-asanRZ)
	}
	if asanenabled {
		// Poison the space between the end of the requested size of x
		// and the end of the slot. Unpoison the requested allocation.
		asanpoison(unsafe.Add(x, size-asanRZ), asanRZ)
		asanunpoison(x, size-asanRZ)
	}
	if valgrindenabled {
		valgrindMalloc(x, size-asanRZ)
	}

	// Adjust our GC assist debt to account for internal fragmentation.
	if gcBlackenEnabled != 0 && elemsize != 0 {
		if assistG := getg().m.curg; assistG != nil {
			assistG.gcAssistBytes -= int64(elemsize - size)
		}
	}

	// Post-malloc debug hooks.
	if debug.malloc {
		postMallocgcDebug(x, elemsize, typ)
	}
	return x
}

func mallocgcTiny(size uintptr, typ *_type) (unsafe.Pointer, uintptr) {
	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if doubleCheckMalloc {
		if mp.mallocing != 0 {
			throw("malloc deadlock")
		}
		if mp.gsignal == getg() {
			throw("malloc during signal")
		}
		if typ != nil && typ.Pointers() {
			throw("expected noscan for tiny alloc")
		}
	}
	mp.mallocing = 1

	// 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%.
	c := getMCache(mp)
	off := c.tinyoffset
	// Align tiny pointer for required (conservative) alignment.
	if size&7 == 0 {
		off = alignUp(off, 8)
	} else if goarch.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, 0
	}
	// Allocate a new maxTinySize block.
	checkGCTrigger := false
	span := c.alloc[tinySpanClass]
	v := nextFreeFast(span)
	if v == 0 {
		v, span, checkGCTrigger = c.nextFree(tinySpanClass)
	}
	x := unsafe.Pointer(v)
	(*[2]uint64)(x)[0] = 0 // Always zero
	(*[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 !raceenabled && (size < c.tinyoffset || c.tiny == 0) {
		// Note: disabled when race detector is on, see comment near end of this function.
		c.tiny = uintptr(x)
		c.tinyoffset = size
	}

	// 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()

	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	} else {
		// Track the last free index before the mark phase. This field
		// is only used by the garbage collector. During the mark phase
		// this is used by the conservative scanner to filter out objects
		// that are both free and recently-allocated. It's safe to do that
		// because we allocate-black if the GC is enabled. The conservative
		// scanner produces pointers out of thin air, so without additional
		// synchronization it might otherwise observe a partially-initialized
		// object, which could crash the program.
		span.freeIndexForScan = span.freeindex
	}

	// Note cache c only valid while m acquired; see #47302
	//
	// N.B. Use the full size because that matches how the GC
	// will update the mem profile on the "free" side.
	//
	// TODO(mknyszek): We should really count the header as part
	// of gc_sys or something. The code below just pretends it is
	// internal fragmentation and matches the GC's accounting by
	// using the whole allocation slot.
	c.nextSample -= int64(span.elemsize)
	if c.nextSample < 0 || MemProfileRate != c.memProfRate {
		profilealloc(mp, x, span.elemsize)
	}
	mp.mallocing = 0
	releasem(mp)

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

	if raceenabled {
		// Pad tinysize allocations so they are aligned with the end
		// of the tinyalloc region. This ensures that any arithmetic
		// that goes off the top end of the object will be detectable
		// by checkptr (issue 38872).
		// Note that we disable tinyalloc when raceenabled for this to work.
		// TODO: This padding is only performed when the race detector
		// is enabled. It would be nice to enable it if any package
		// was compiled with checkptr, but there's no easy way to
		// detect that (especially at compile time).
		// TODO: enable this padding for all allocations, not just
		// tinyalloc ones. It's tricky because of pointer maps.
		// Maybe just all noscan objects?
		x = add(x, span.elemsize-size)
	}
	return x, span.elemsize
}

func mallocgcSmallNoscan(size uintptr, typ *_type, needzero bool) (unsafe.Pointer, uintptr) {
	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if doubleCheckMalloc {
		if mp.mallocing != 0 {
			throw("malloc deadlock")
		}
		if mp.gsignal == getg() {
			throw("malloc during signal")
		}
		if typ != nil && typ.Pointers() {
			throw("expected noscan type for noscan alloc")
		}
	}
	mp.mallocing = 1

	checkGCTrigger := false
	c := getMCache(mp)
	var sizeclass uint8
	if size <= gc.SmallSizeMax-8 {
		sizeclass = gc.SizeToSizeClass8[divRoundUp(size, gc.SmallSizeDiv)]
	} else {
		sizeclass = gc.SizeToSizeClass128[divRoundUp(size-gc.SmallSizeMax, gc.LargeSizeDiv)]
	}
	size = uintptr(gc.SizeClassToSize[sizeclass])
	spc := makeSpanClass(sizeclass, true)
	span := c.alloc[spc]

	// First, check for a reusable object.
	if runtimeFreegcEnabled && c.hasReusableNoscan(spc) {
		// We have a reusable object, use it.
		x := mallocgcSmallNoscanReuse(c, span, spc, size, needzero)
		mp.mallocing = 0
		releasem(mp)
		return x, size
	}

	v := nextFreeFast(span)
	if v == 0 {
		v, span, checkGCTrigger = c.nextFree(spc)
	}
	x := unsafe.Pointer(v)
	if needzero && span.needzero != 0 {
		memclrNoHeapPointers(x, size)
	}

	// 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()

	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	} else {
		// Track the last free index before the mark phase. This field
		// is only used by the garbage collector. During the mark phase
		// this is used by the conservative scanner to filter out objects
		// that are both free and recently-allocated. It's safe to do that
		// because we allocate-black if the GC is enabled. The conservative
		// scanner produces pointers out of thin air, so without additional
		// synchronization it might otherwise observe a partially-initialized
		// object, which could crash the program.
		span.freeIndexForScan = span.freeindex
	}

	// Note cache c only valid while m acquired; see #47302
	//
	// N.B. Use the full size because that matches how the GC
	// will update the mem profile on the "free" side.
	//
	// TODO(mknyszek): We should really count the header as part
	// of gc_sys or something. The code below just pretends it is
	// internal fragmentation and matches the GC's accounting by
	// using the whole allocation slot.
	c.nextSample -= int64(size)
	if c.nextSample < 0 || MemProfileRate != c.memProfRate {
		profilealloc(mp, x, size)
	}
	mp.mallocing = 0
	releasem(mp)

	if checkGCTrigger {
		if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
			gcStart(t)
		}
	}
	return x, size
}

// mallocgcSmallNoscanReuse returns a previously freed noscan object after preparing it for reuse.
// It must only be called if hasReusableNoscan returned true.
func mallocgcSmallNoscanReuse(c *mcache, span *mspan, spc spanClass, size uintptr, needzero bool) unsafe.Pointer {
	// TODO(thepudds): could nextFreeFast, nextFree and nextReusable return unsafe.Pointer?
	// Maybe doesn't matter. gclinkptr might be for historical reasons.
	v, span := c.nextReusableNoScan(span, spc)
	x := unsafe.Pointer(v)

	// Compensate for the GC assist credit deducted in mallocgc (before calling us and
	// after we return) because this is not a newly allocated object. We use the full slot
	// size (elemsize) here because that's what mallocgc deducts overall. Note we only
	// adjust this when gcBlackenEnabled is true, which follows mallocgc behavior.
	// TODO(thepudds): a follow-up CL adds a more specific test of our assist credit
	// handling, including for validating internal fragmentation handling.
	if gcBlackenEnabled != 0 {
		addAssistCredit(size)
	}

	// This is a previously used object, so only check needzero (and not span.needzero)
	// for clearing.
	if needzero {
		memclrNoHeapPointers(x, size)
	}

	// See publicationBarrier comment in mallocgcSmallNoscan.
	publicationBarrier()

	// Finish and return. Note that we do not update span.freeIndexForScan, profiling info,
	// nor do we check gcTrigger.
	// TODO(thepudds): the current approach is viable for a GOEXPERIMENT, but
	// means we do not profile reused heap objects. Ultimately, we will need a better
	// approach for profiling, or at least ensure we are not introducing bias in the
	// profiled allocations.
	// TODO(thepudds): related, we probably want to adjust how allocs and frees are counted
	// in the existing stats. Currently, reused objects are not counted as allocs nor
	// frees, but instead roughly appear as if the original heap object lived on. We
	// probably will also want some additional runtime/metrics, and generally think about
	// user-facing observability & diagnostics, though all this likely can wait for an
	// official proposal.
	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	}
	return x
}

func mallocgcSmallScanNoHeader(size uintptr, typ *_type) (unsafe.Pointer, uintptr) {
	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if doubleCheckMalloc {
		if mp.mallocing != 0 {
			throw("malloc deadlock")
		}
		if mp.gsignal == getg() {
			throw("malloc during signal")
		}
		if typ == nil || !typ.Pointers() {
			throw("noscan allocated in scan-only path")
		}
		if !heapBitsInSpan(size) {
			throw("heap bits in not in span for non-header-only path")
		}
	}
	mp.mallocing = 1

	checkGCTrigger := false
	c := getMCache(mp)
	sizeclass := gc.SizeToSizeClass8[divRoundUp(size, gc.SmallSizeDiv)]
	spc := makeSpanClass(sizeclass, false)
	span := c.alloc[spc]
	v := nextFreeFast(span)
	if v == 0 {
		v, span, checkGCTrigger = c.nextFree(spc)
	}
	x := unsafe.Pointer(v)
	if span.needzero != 0 {
		memclrNoHeapPointers(x, size)
	}
	if goarch.PtrSize == 8 && sizeclass == 1 {
		// initHeapBits already set the pointer bits for the 8-byte sizeclass
		// on 64-bit platforms.
		c.scanAlloc += 8
	} else {
		c.scanAlloc += heapSetTypeNoHeader(uintptr(x), size, typ, span)
	}
	size = uintptr(gc.SizeClassToSize[sizeclass])

	// 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()

	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	} else {
		// Track the last free index before the mark phase. This field
		// is only used by the garbage collector. During the mark phase
		// this is used by the conservative scanner to filter out objects
		// that are both free and recently-allocated. It's safe to do that
		// because we allocate-black if the GC is enabled. The conservative
		// scanner produces pointers out of thin air, so without additional
		// synchronization it might otherwise observe a partially-initialized
		// object, which could crash the program.
		span.freeIndexForScan = span.freeindex
	}

	// Note cache c only valid while m acquired; see #47302
	//
	// N.B. Use the full size because that matches how the GC
	// will update the mem profile on the "free" side.
	//
	// TODO(mknyszek): We should really count the header as part
	// of gc_sys or something. The code below just pretends it is
	// internal fragmentation and matches the GC's accounting by
	// using the whole allocation slot.
	c.nextSample -= int64(size)
	if c.nextSample < 0 || MemProfileRate != c.memProfRate {
		profilealloc(mp, x, size)
	}
	mp.mallocing = 0
	releasem(mp)

	if checkGCTrigger {
		if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
			gcStart(t)
		}
	}
	return x, size
}

func mallocgcSmallScanHeader(size uintptr, typ *_type) (unsafe.Pointer, uintptr) {
	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if doubleCheckMalloc {
		if mp.mallocing != 0 {
			throw("malloc deadlock")
		}
		if mp.gsignal == getg() {
			throw("malloc during signal")
		}
		if typ == nil || !typ.Pointers() {
			throw("noscan allocated in scan-only path")
		}
		if heapBitsInSpan(size) {
			throw("heap bits in span for header-only path")
		}
	}
	mp.mallocing = 1

	checkGCTrigger := false
	c := getMCache(mp)
	size += gc.MallocHeaderSize
	var sizeclass uint8
	if size <= gc.SmallSizeMax-8 {
		sizeclass = gc.SizeToSizeClass8[divRoundUp(size, gc.SmallSizeDiv)]
	} else {
		sizeclass = gc.SizeToSizeClass128[divRoundUp(size-gc.SmallSizeMax, gc.LargeSizeDiv)]
	}
	size = uintptr(gc.SizeClassToSize[sizeclass])
	spc := makeSpanClass(sizeclass, false)
	span := c.alloc[spc]
	v := nextFreeFast(span)
	if v == 0 {
		v, span, checkGCTrigger = c.nextFree(spc)
	}
	x := unsafe.Pointer(v)
	if span.needzero != 0 {
		memclrNoHeapPointers(x, size)
	}
	header := (**_type)(x)
	x = add(x, gc.MallocHeaderSize)
	c.scanAlloc += heapSetTypeSmallHeader(uintptr(x), size-gc.MallocHeaderSize, typ, header, span)

	// 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()

	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	} else {
		// Track the last free index before the mark phase. This field
		// is only used by the garbage collector. During the mark phase
		// this is used by the conservative scanner to filter out objects
		// that are both free and recently-allocated. It's safe to do that
		// because we allocate-black if the GC is enabled. The conservative
		// scanner produces pointers out of thin air, so without additional
		// synchronization it might otherwise observe a partially-initialized
		// object, which could crash the program.
		span.freeIndexForScan = span.freeindex
	}

	// Note cache c only valid while m acquired; see #47302
	//
	// N.B. Use the full size because that matches how the GC
	// will update the mem profile on the "free" side.
	//
	// TODO(mknyszek): We should really count the header as part
	// of gc_sys or something. The code below just pretends it is
	// internal fragmentation and matches the GC's accounting by
	// using the whole allocation slot.
	c.nextSample -= int64(size)
	if c.nextSample < 0 || MemProfileRate != c.memProfRate {
		profilealloc(mp, x, size)
	}
	mp.mallocing = 0
	releasem(mp)

	if checkGCTrigger {
		if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
			gcStart(t)
		}
	}
	return x, size
}

func mallocgcLarge(size uintptr, typ *_type, needzero bool) (unsafe.Pointer, uintptr) {
	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if doubleCheckMalloc {
		if mp.mallocing != 0 {
			throw("malloc deadlock")
		}
		if mp.gsignal == getg() {
			throw("malloc during signal")
		}
	}
	mp.mallocing = 1

	c := getMCache(mp)
	// For large allocations, keep track of zeroed state so that
	// bulk zeroing can be happen later in a preemptible context.
	span := c.allocLarge(size, typ == nil || !typ.Pointers())
	span.freeindex = 1
	span.allocCount = 1
	span.largeType = nil // Tell the GC not to look at this yet.
	size = span.elemsize
	x := unsafe.Pointer(span.base())

	// Ensure that the store above that sets largeType to
	// nil happens 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 a stale
	// largeType value.
	publicationBarrier()

	if writeBarrier.enabled {
		// 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.
		gcmarknewobject(span, uintptr(x))
	} else {
		// Track the last free index before the mark phase. This field
		// is only used by the garbage collector. During the mark phase
		// this is used by the conservative scanner to filter out objects
		// that are both free and recently-allocated. It's safe to do that
		// because we allocate-black if the GC is enabled. The conservative
		// scanner produces pointers out of thin air, so without additional
		// synchronization it might otherwise observe a partially-initialized
		// object, which could crash the program.
		span.freeIndexForScan = span.freeindex
	}

	// Note cache c only valid while m acquired; see #47302
	//
	// N.B. Use the full size because that matches how the GC
	// will update the mem profile on the "free" side.
	//
	// TODO(mknyszek): We should really count the header as part
	// of gc_sys or something. The code below just pretends it is
	// internal fragmentation and matches the GC's accounting by
	// using the whole allocation slot.
	c.nextSample -= int64(size)
	if c.nextSample < 0 || MemProfileRate != c.memProfRate {
		profilealloc(mp, x, size)
	}
	mp.mallocing = 0
	releasem(mp)

	// Check to see if we need to trigger the GC.
	if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
		gcStart(t)
	}

	// Objects can be zeroed late in a context where preemption can occur.
	//
	// x will keep the memory alive.
	if needzero && span.needzero != 0 {
		// N.B. size == fullSize always in this case.
		memclrNoHeapPointersChunked(size, x) // This is a possible preemption point: see #47302
	}

	// Set the type and run the publication barrier while non-preemptible. We need to make
	// sure that between heapSetTypeLarge and publicationBarrier we cannot get preempted,
	// otherwise the GC could potentially observe non-zeroed memory but largeType set on weak
	// memory architectures.
	//
	// The GC can also potentially observe non-zeroed memory if conservative scanning spuriously
	// observes a partially-allocated object, see the freeIndexForScan update above. This case is
	// handled by synchronization inside heapSetTypeLarge.
	mp = acquirem()
	if typ != nil && typ.Pointers() {
		// Finish storing the type information, now that we're certain the memory is zeroed.
		getMCache(mp).scanAlloc += heapSetTypeLarge(uintptr(x), size, typ, span)
	}
	// Publish the object again, now with zeroed memory and initialized type information.
	//
	// Even if we didn't update any type information, this is necessary to ensure that, for example,
	// x written to a global without any synchronization still results in other goroutines observing
	// zeroed memory.
	publicationBarrier()
	releasem(mp)
	return x, size
}

func preMallocgcDebug(size uintptr, typ *_type) unsafe.Pointer {
	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 goroutine.
		inittrace.allocs += 1
	}
	return nil
}

func postMallocgcDebug(x unsafe.Pointer, elemsize uintptr, typ *_type) {
	if inittrace.active && inittrace.id == getg().goid {
		// Init functions are executed sequentially in a single goroutine.
		inittrace.bytes += uint64(elemsize)
	}

	if traceAllocFreeEnabled() {
		trace := traceAcquire()
		if trace.ok() {
			trace.HeapObjectAlloc(uintptr(x), typ)
			traceRelease(trace)
		}
	}

	// N.B. elemsize == 0 indicates a tiny allocation, since no new slot was
	// allocated to fulfill this call to mallocgc. This means checkfinalizer
	// will only flag an error if there is actually any risk. If an allocation
	// has the tiny block to itself, it will not get flagged, because we won't
	// mark the block as a tiny block.
	if debug.checkfinalizers != 0 && elemsize == 0 {
		setTinyBlockContext(unsafe.Pointer(alignDown(uintptr(x), maxTinySize)))
	}
}

// addAssistCredit is like deductAssistCredit,
// but adds credit rather than removes,
// and never calls gcAssistAlloc.
func addAssistCredit(size uintptr) {
	// Credit the current user G.
	assistG := getg()
	if assistG.m.curg != nil { // TODO(thepudds): do we need to do this?
		assistG = assistG.m.curg
	}
	// Credit the size against the G.
	assistG.gcAssistBytes += int64(size)
}

const (
	// doubleCheckReusable enables some additional invariant checks for the
	// runtime.freegc and reusable objects. Note that some of these checks alter timing,
	// and it is good to test changes with and without this enabled.
	doubleCheckReusable = false

	// debugReusableLog enables some printlns for runtime.freegc and reusable objects.
	debugReusableLog = false
)

// freegc records that a heap object is reusable and available for
// immediate reuse in a subsequent mallocgc allocation, without
// needing to wait for the GC cycle to progress.
//
// The information is recorded in a free list stored in the
// current P's mcache. The caller must pass in the user size
// and whether the object has pointers, which allows a faster free
// operation.
//
// freegc must be called by the effective owner of ptr who knows
// the pointer is logically dead, with no possible aliases that might
// be used past that moment. In other words, ptr must be the
// last and only pointer to its referent.
//
// The intended caller is the compiler.
//
// Note: please do not send changes that attempt to add freegc calls
// to the standard library.
//
// ptr must point to a heap object or into the current g's stack,
// in which case freegc is a no-op. In particular, ptr must not point
// to memory in the data or bss sections, which is partially enforced.
// For objects with a malloc header, ptr should point mallocHeaderSize bytes
// past the base; otherwise, ptr should point to the base of the heap object.
// In other words, ptr should be the same pointer that was returned by mallocgc.
//
// In addition, the caller must know that ptr's object has no specials, such
// as might have been created by a call to SetFinalizer or AddCleanup.
// (Internally, the runtime deals appropriately with internally-created
// specials, such as specials for memory profiling).
//
// If the size of ptr's object is less than 16 bytes or greater than
// 32KiB - gc.MallocHeaderSize bytes, freegc is currently a no-op. It must only
// be called in alloc-safe places. It currently throws if noscan is false
// (support for which is implemented in a later CL in our stack).
//
// Note that freegc accepts an unsafe.Pointer and hence keeps the pointer
// alive. It therefore could be a pessimization in some cases (such
// as a long-lived function) if the caller does not call freegc before
// or roughly when the liveness analysis of the compiler
// would otherwise have determined ptr's object is reclaimable by the GC.
func freegc(ptr unsafe.Pointer, size uintptr, noscan bool) bool {
	if !runtimeFreegcEnabled || !reusableSize(size) {
		return false
	}
	if sizeSpecializedMallocEnabled && !noscan {
		// TODO(thepudds): temporarily disable freegc with SizeSpecializedMalloc for pointer types
		// until we finish integrating.
		return false
	}

	if ptr == nil {
		throw("freegc nil")
	}

	// Set mp.mallocing to keep from being preempted by GC.
	// Otherwise, the GC could flush our mcache or otherwise cause problems.
	mp := acquirem()
	if mp.mallocing != 0 {
		throw("freegc deadlock")
	}
	if mp.gsignal == getg() {
		throw("freegc during signal")
	}
	mp.mallocing = 1

	if mp.curg.stack.lo <= uintptr(ptr) && uintptr(ptr) < mp.curg.stack.hi {
		// This points into our stack, so free is a no-op.
		mp.mallocing = 0
		releasem(mp)
		return false
	}

	if doubleCheckReusable {
		// TODO(thepudds): we could enforce no free on globals in bss or data. Maybe by
		// checking span via spanOf or spanOfHeap, or maybe walk from firstmoduledata
		// like isGoPointerWithoutSpan, or activeModules, or something. If so, we might
		// be able to delay checking until reuse (e.g., check span just before reusing,
		// though currently we don't always need to lookup a span on reuse). If we think
		// no usage patterns could result in globals, maybe enforcement for globals could
		// be behind -d=checkptr=1 or similar. The compiler can have knowledge of where
		// a variable is allocated, but stdlib does not, although there are certain
		// usage patterns that cannot result in a global.
		// TODO(thepudds): separately, consider a local debugReusableMcacheOnly here
		// to ignore freed objects if not in mspan in mcache,  maybe when freeing and reading,
		// by checking something like s.base() <= uintptr(v) && uintptr(v) < s.limit. Or
		// maybe a GODEBUG or compiler debug flag.
		span := spanOf(uintptr(ptr))
		if span == nil {
			throw("nextReusable: nil span for pointer in free list")
		}
		if state := span.state.get(); state != mSpanInUse {
			throw("nextReusable: span is not in use")
		}
	}

	if debug.clobberfree != 0 {
		clobberfree(ptr, size)
	}

	// We first check if p is still in our per-P cache.
	// Get our per-P cache for small objects.
	c := getMCache(mp)
	if c == nil {
		throw("freegc called without a P or outside bootstrapping")
	}

	v := uintptr(ptr)
	if !noscan && !heapBitsInSpan(size) {
		// mallocgcSmallScanHeader expects to get the base address of the object back
		// from the findReusable funcs (as well as from nextFreeFast and nextFree), and
		// not mallocHeaderSize bytes into a object, so adjust that here.
		v -= mallocHeaderSize

		// The size class lookup wants size to be adjusted by mallocHeaderSize.
		size += mallocHeaderSize
	}

	// TODO(thepudds): should verify (behind doubleCheckReusable constant) that our calculated
	// sizeclass here matches what's in span found via spanOf(ptr) or findObject(ptr).
	var sizeclass uint8
	if size <= gc.SmallSizeMax-8 {
		sizeclass = gc.SizeToSizeClass8[divRoundUp(size, gc.SmallSizeDiv)]
	} else {
		sizeclass = gc.SizeToSizeClass128[divRoundUp(size-gc.SmallSizeMax, gc.LargeSizeDiv)]
	}

	spc := makeSpanClass(sizeclass, noscan)
	s := c.alloc[spc]

	if debugReusableLog {
		if s.base() <= uintptr(v) && uintptr(v) < s.limit {
			println("freegc [in mcache]:", hex(uintptr(v)), "sweepgen:", mheap_.sweepgen, "writeBarrier.enabled:", writeBarrier.enabled)
		} else {
			println("freegc [NOT in mcache]:", hex(uintptr(v)), "sweepgen:", mheap_.sweepgen, "writeBarrier.enabled:", writeBarrier.enabled)
		}
	}

	if noscan {
		c.addReusableNoscan(spc, uintptr(v))
	} else {
		// TODO(thepudds): implemented in later CL in our stack.
		throw("freegc called for object with pointers, not yet implemented")
	}

	// For stats, for now we leave allocCount alone, roughly pretending to the rest
	// of the system that this potential reuse never happened.

	mp.mallocing = 0
	releasem(mp)

	return true
}

// nextReusableNoScan returns the next reusable object for a noscan span,
// or 0 if no reusable object is found.
func (c *mcache) nextReusableNoScan(s *mspan, spc spanClass) (gclinkptr, *mspan) {
	if !runtimeFreegcEnabled {
		return 0, s
	}

	// Pop a reusable pointer from the free list for this span class.
	v := c.reusableNoscan[spc]
	if v == 0 {
		return 0, s
	}
	c.reusableNoscan[spc] = v.ptr().next

	if debugReusableLog {
		println("reusing from ptr free list:", hex(v), "sweepgen:", mheap_.sweepgen, "writeBarrier.enabled:", writeBarrier.enabled)
	}
	if doubleCheckReusable {
		doubleCheckNextReusable(v) // debug only sanity check
	}

	// For noscan spans, we only need the span if the write barrier is enabled (so that our caller
	// can call gcmarknewobject to allocate black). If the write barrier is enabled, we can skip
	// looking up the span when the pointer is in a span in the mcache.
	if !writeBarrier.enabled {
		return v, nil
	}
	if s.base() <= uintptr(v) && uintptr(v) < s.limit {
		// Return the original span.
		return v, s
	}

	// We must find and return the span.
	span := spanOf(uintptr(v))
	if span == nil {
		// TODO(thepudds): construct a test that triggers this throw.
		throw("nextReusableNoScan: nil span for pointer in reusable object free list")
	}

	return v, span
}

// doubleCheckNextReusable checks some invariants.
// TODO(thepudds): will probably delete some of this. Can mostly be ignored for review.
func doubleCheckNextReusable(v gclinkptr) {
	// TODO(thepudds): should probably take the spanClass as well to confirm expected
	// sizeclass match.
	_, span, objIndex := findObject(uintptr(v), 0, 0)
	if span == nil {
		throw("nextReusable: nil span for pointer in free list")
	}
	if state := span.state.get(); state != mSpanInUse {
		throw("nextReusable: span is not in use")
	}
	if uintptr(v) < span.base() || uintptr(v) >= span.limit {
		throw("nextReusable: span is not in range")
	}
	if span.objBase(uintptr(v)) != uintptr(v) {
		print("nextReusable: v=", hex(v), " base=", hex(span.objBase(uintptr(v))), "\n")
		throw("nextReusable: v is non-base-address for object found on pointer free list")
	}
	if span.isFree(objIndex) {
		throw("nextReusable: pointer on free list is free")
	}

	const debugReusableEnsureSwept = false
	if debugReusableEnsureSwept {
		// Currently disabled.
		// Note: ensureSwept here alters behavior (not just an invariant check).
		span.ensureSwept()
		if span.isFree(objIndex) {
			throw("nextReusable: pointer on free list is free after ensureSwept")
		}
	}
}

// reusableSize reports if size is a currently supported size for a reusable object.
func reusableSize(size uintptr) bool {
	if size < maxTinySize || size > maxSmallSize-mallocHeaderSize {
		return false
	}
	return true
}

// memclrNoHeapPointersChunked repeatedly calls memclrNoHeapPointers
// on chunks of the buffer to be zeroed, with opportunities for preemption
// along the way.  memclrNoHeapPointers contains no safepoints and also
// cannot be preemptively scheduled, so this provides a still-efficient
// block copy that can also be preempted on a reasonable granularity.
//
// Use this with care; if the data being cleared is tagged to contain
// pointers, this allows the GC to run before it is all cleared.
func memclrNoHeapPointersChunked(size uintptr, x unsafe.Pointer) {
	v := uintptr(x)
	// got this from benchmarking. 128k is too small, 512k is too large.
	const chunkBytes = 256 * 1024
	vsize := v + size
	for voff := v; voff < vsize; voff = voff + chunkBytes {
		if getg().preempt {
			// may hold locks, e.g., profiling
			goschedguarded()
		}
		// clear min(avail, lump) bytes
		n := vsize - voff
		if n > chunkBytes {
			n = chunkBytes
		}
		memclrNoHeapPointers(unsafe.Pointer(voff), n)
	}
}

// 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 maps_newobject internal/runtime/maps.newobject
func maps_newobject(typ *_type) unsafe.Pointer {
	return newobject(typ)
}

// reflect_unsafe_New is meant for package reflect,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gitee.com/quant1x/gox
//   - github.com/goccy/json
//   - github.com/modern-go/reflect2
//   - github.com/v2pro/plz
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//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.
//
// newarray should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/RomiChan/protobuf
//   - github.com/segmentio/encoding
//   - github.com/ugorji/go/codec
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname newarray
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)
}

// reflect_unsafe_NewArray is meant for package reflect,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - gitee.com/quant1x/gox
//   - github.com/bytedance/sonic
//   - github.com/goccy/json
//   - github.com/modern-go/reflect2
//   - github.com/segmentio/encoding
//   - github.com/segmentio/kafka-go
//   - github.com/v2pro/plz
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname reflect_unsafe_NewArray reflect.unsafe_NewArray
func reflect_unsafe_NewArray(typ *_type, n int) unsafe.Pointer {
	return newarray(typ, n)
}

//go:linkname maps_newarray internal/runtime/maps.newarray
func maps_newarray(typ *_type, n int) unsafe.Pointer {
	return newarray(typ, n)
}

// profilealloc resets the current mcache's nextSample counter and
// records a memory profile sample.
//
// The caller must be non-preemptible and have a P.
func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
	c := getMCache(mp)
	if c == nil {
		throw("profilealloc called without a P or outside bootstrapping")
	}
	c.memProfRate = MemProfileRate
	c.nextSample = nextSample()
	mProf_Malloc(mp, 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() int64 {
	if MemProfileRate == 0 {
		// Basically never sample.
		return math.MaxInt64
	}
	if MemProfileRate == 1 {
		// Sample immediately.
		return 0
	}
	return int64(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 := cheaprandn(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
}

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.
// sysStat must be non-nil.
//
// Consider marking persistentalloc'd types not in heap by embedding
// internal/runtime/sys.NotInHeap.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
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, "immortal metadata"))
	}

	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, "immortal metadata"))
		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(goarch.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 optionally 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

	mapMemory bool // transition memory from Reserved to Ready if true
}

func (l *linearAlloc) init(base, size uintptr, mapMemory bool) {
	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
	l.mapMemory = mapMemory
}

func (l *linearAlloc) alloc(size, align uintptr, sysStat *sysMemStat, vmaName string) 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 {
		if l.mapMemory {
			// Transition from Reserved to Prepared to Ready.
			n := pEnd - l.mapped
			sysMap(unsafe.Pointer(l.mapped), n, sysStat, vmaName)
			sysUsed(unsafe.Pointer(l.mapped), n, n)
		}
		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 which embed
// internal/runtime/sys.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?
type notInHeap struct{ _ sys.NotInHeap }

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

// redZoneSize computes the size of the redzone for a given allocation.
// Refer to the implementation of the compiler-rt.
func redZoneSize(userSize uintptr) uintptr {
	switch {
	case userSize <= (64 - 16):
		return 16 << 0
	case userSize <= (128 - 32):
		return 16 << 1
	case userSize <= (512 - 64):
		return 16 << 2
	case userSize <= (4096 - 128):
		return 16 << 3
	case userSize <= (1<<14)-256:
		return 16 << 4
	case userSize <= (1<<15)-512:
		return 16 << 5
	case userSize <= (1<<16)-1024:
		return 16 << 6
	default:
		return 16 << 7
	}
}