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Source file src/runtime/mheap.go

Documentation: runtime

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Page heap.
     6  //
     7  // See malloc.go for overview.
     8  
     9  package runtime
    10  
    11  import (
    12  	"internal/cpu"
    13  	"runtime/internal/atomic"
    14  	"runtime/internal/sys"
    15  	"unsafe"
    16  )
    17  
    18  const (
    19  	// minPhysPageSize is a lower-bound on the physical page size. The
    20  	// true physical page size may be larger than this. In contrast,
    21  	// sys.PhysPageSize is an upper-bound on the physical page size.
    22  	minPhysPageSize = 4096
    23  
    24  	// maxPhysPageSize is the maximum page size the runtime supports.
    25  	maxPhysPageSize = 512 << 10
    26  
    27  	// maxPhysHugePageSize sets an upper-bound on the maximum huge page size
    28  	// that the runtime supports.
    29  	maxPhysHugePageSize = pallocChunkBytes
    30  
    31  	// pagesPerReclaimerChunk indicates how many pages to scan from the
    32  	// pageInUse bitmap at a time. Used by the page reclaimer.
    33  	//
    34  	// Higher values reduce contention on scanning indexes (such as
    35  	// h.reclaimIndex), but increase the minimum latency of the
    36  	// operation.
    37  	//
    38  	// The time required to scan this many pages can vary a lot depending
    39  	// on how many spans are actually freed. Experimentally, it can
    40  	// scan for pages at ~300 GB/ms on a 2.6GHz Core i7, but can only
    41  	// free spans at ~32 MB/ms. Using 512 pages bounds this at
    42  	// roughly 100┬Ás.
    43  	//
    44  	// Must be a multiple of the pageInUse bitmap element size and
    45  	// must also evenly divide pagesPerArena.
    46  	pagesPerReclaimerChunk = 512
    47  
    48  	// physPageAlignedStacks indicates whether stack allocations must be
    49  	// physical page aligned. This is a requirement for MAP_STACK on
    50  	// OpenBSD.
    51  	physPageAlignedStacks = GOOS == "openbsd"
    52  )
    53  
    54  // Main malloc heap.
    55  // The heap itself is the "free" and "scav" treaps,
    56  // but all the other global data is here too.
    57  //
    58  // mheap must not be heap-allocated because it contains mSpanLists,
    59  // which must not be heap-allocated.
    60  //
    61  //go:notinheap
    62  type mheap struct {
    63  	// lock must only be acquired on the system stack, otherwise a g
    64  	// could self-deadlock if its stack grows with the lock held.
    65  	lock      mutex
    66  	pages     pageAlloc // page allocation data structure
    67  	sweepgen  uint32    // sweep generation, see comment in mspan; written during STW
    68  	sweepdone uint32    // all spans are swept
    69  	sweepers  uint32    // number of active sweepone calls
    70  
    71  	// allspans is a slice of all mspans ever created. Each mspan
    72  	// appears exactly once.
    73  	//
    74  	// The memory for allspans is manually managed and can be
    75  	// reallocated and move as the heap grows.
    76  	//
    77  	// In general, allspans is protected by mheap_.lock, which
    78  	// prevents concurrent access as well as freeing the backing
    79  	// store. Accesses during STW might not hold the lock, but
    80  	// must ensure that allocation cannot happen around the
    81  	// access (since that may free the backing store).
    82  	allspans []*mspan // all spans out there
    83  
    84  	_ uint32 // align uint64 fields on 32-bit for atomics
    85  
    86  	// Proportional sweep
    87  	//
    88  	// These parameters represent a linear function from heap_live
    89  	// to page sweep count. The proportional sweep system works to
    90  	// stay in the black by keeping the current page sweep count
    91  	// above this line at the current heap_live.
    92  	//
    93  	// The line has slope sweepPagesPerByte and passes through a
    94  	// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
    95  	// any given time, the system is at (memstats.heap_live,
    96  	// pagesSwept) in this space.
    97  	//
    98  	// It's important that the line pass through a point we
    99  	// control rather than simply starting at a (0,0) origin
   100  	// because that lets us adjust sweep pacing at any time while
   101  	// accounting for current progress. If we could only adjust
   102  	// the slope, it would create a discontinuity in debt if any
   103  	// progress has already been made.
   104  	pagesInUse         uint64  // pages of spans in stats mSpanInUse; updated atomically
   105  	pagesSwept         uint64  // pages swept this cycle; updated atomically
   106  	pagesSweptBasis    uint64  // pagesSwept to use as the origin of the sweep ratio; updated atomically
   107  	sweepHeapLiveBasis uint64  // value of heap_live to use as the origin of sweep ratio; written with lock, read without
   108  	sweepPagesPerByte  float64 // proportional sweep ratio; written with lock, read without
   109  	// TODO(austin): pagesInUse should be a uintptr, but the 386
   110  	// compiler can't 8-byte align fields.
   111  
   112  	// scavengeGoal is the amount of total retained heap memory (measured by
   113  	// heapRetained) that the runtime will try to maintain by returning memory
   114  	// to the OS.
   115  	scavengeGoal uint64
   116  
   117  	// Page reclaimer state
   118  
   119  	// reclaimIndex is the page index in allArenas of next page to
   120  	// reclaim. Specifically, it refers to page (i %
   121  	// pagesPerArena) of arena allArenas[i / pagesPerArena].
   122  	//
   123  	// If this is >= 1<<63, the page reclaimer is done scanning
   124  	// the page marks.
   125  	//
   126  	// This is accessed atomically.
   127  	reclaimIndex uint64
   128  	// reclaimCredit is spare credit for extra pages swept. Since
   129  	// the page reclaimer works in large chunks, it may reclaim
   130  	// more than requested. Any spare pages released go to this
   131  	// credit pool.
   132  	//
   133  	// This is accessed atomically.
   134  	reclaimCredit uintptr
   135  
   136  	// arenas is the heap arena map. It points to the metadata for
   137  	// the heap for every arena frame of the entire usable virtual
   138  	// address space.
   139  	//
   140  	// Use arenaIndex to compute indexes into this array.
   141  	//
   142  	// For regions of the address space that are not backed by the
   143  	// Go heap, the arena map contains nil.
   144  	//
   145  	// Modifications are protected by mheap_.lock. Reads can be
   146  	// performed without locking; however, a given entry can
   147  	// transition from nil to non-nil at any time when the lock
   148  	// isn't held. (Entries never transitions back to nil.)
   149  	//
   150  	// In general, this is a two-level mapping consisting of an L1
   151  	// map and possibly many L2 maps. This saves space when there
   152  	// are a huge number of arena frames. However, on many
   153  	// platforms (even 64-bit), arenaL1Bits is 0, making this
   154  	// effectively a single-level map. In this case, arenas[0]
   155  	// will never be nil.
   156  	arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena
   157  
   158  	// heapArenaAlloc is pre-reserved space for allocating heapArena
   159  	// objects. This is only used on 32-bit, where we pre-reserve
   160  	// this space to avoid interleaving it with the heap itself.
   161  	heapArenaAlloc linearAlloc
   162  
   163  	// arenaHints is a list of addresses at which to attempt to
   164  	// add more heap arenas. This is initially populated with a
   165  	// set of general hint addresses, and grown with the bounds of
   166  	// actual heap arena ranges.
   167  	arenaHints *arenaHint
   168  
   169  	// arena is a pre-reserved space for allocating heap arenas
   170  	// (the actual arenas). This is only used on 32-bit.
   171  	arena linearAlloc
   172  
   173  	// allArenas is the arenaIndex of every mapped arena. This can
   174  	// be used to iterate through the address space.
   175  	//
   176  	// Access is protected by mheap_.lock. However, since this is
   177  	// append-only and old backing arrays are never freed, it is
   178  	// safe to acquire mheap_.lock, copy the slice header, and
   179  	// then release mheap_.lock.
   180  	allArenas []arenaIdx
   181  
   182  	// sweepArenas is a snapshot of allArenas taken at the
   183  	// beginning of the sweep cycle. This can be read safely by
   184  	// simply blocking GC (by disabling preemption).
   185  	sweepArenas []arenaIdx
   186  
   187  	// markArenas is a snapshot of allArenas taken at the beginning
   188  	// of the mark cycle. Because allArenas is append-only, neither
   189  	// this slice nor its contents will change during the mark, so
   190  	// it can be read safely.
   191  	markArenas []arenaIdx
   192  
   193  	// curArena is the arena that the heap is currently growing
   194  	// into. This should always be physPageSize-aligned.
   195  	curArena struct {
   196  		base, end uintptr
   197  	}
   198  
   199  	_ uint32 // ensure 64-bit alignment of central
   200  
   201  	// central free lists for small size classes.
   202  	// the padding makes sure that the mcentrals are
   203  	// spaced CacheLinePadSize bytes apart, so that each mcentral.lock
   204  	// gets its own cache line.
   205  	// central is indexed by spanClass.
   206  	central [numSpanClasses]struct {
   207  		mcentral mcentral
   208  		pad      [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
   209  	}
   210  
   211  	spanalloc             fixalloc // allocator for span*
   212  	cachealloc            fixalloc // allocator for mcache*
   213  	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
   214  	specialprofilealloc   fixalloc // allocator for specialprofile*
   215  	speciallock           mutex    // lock for special record allocators.
   216  	arenaHintAlloc        fixalloc // allocator for arenaHints
   217  
   218  	unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
   219  }
   220  
   221  var mheap_ mheap
   222  
   223  // A heapArena stores metadata for a heap arena. heapArenas are stored
   224  // outside of the Go heap and accessed via the mheap_.arenas index.
   225  //
   226  //go:notinheap
   227  type heapArena struct {
   228  	// bitmap stores the pointer/scalar bitmap for the words in
   229  	// this arena. See mbitmap.go for a description. Use the
   230  	// heapBits type to access this.
   231  	bitmap [heapArenaBitmapBytes]byte
   232  
   233  	// spans maps from virtual address page ID within this arena to *mspan.
   234  	// For allocated spans, their pages map to the span itself.
   235  	// For free spans, only the lowest and highest pages map to the span itself.
   236  	// Internal pages map to an arbitrary span.
   237  	// For pages that have never been allocated, spans entries are nil.
   238  	//
   239  	// Modifications are protected by mheap.lock. Reads can be
   240  	// performed without locking, but ONLY from indexes that are
   241  	// known to contain in-use or stack spans. This means there
   242  	// must not be a safe-point between establishing that an
   243  	// address is live and looking it up in the spans array.
   244  	spans [pagesPerArena]*mspan
   245  
   246  	// pageInUse is a bitmap that indicates which spans are in
   247  	// state mSpanInUse. This bitmap is indexed by page number,
   248  	// but only the bit corresponding to the first page in each
   249  	// span is used.
   250  	//
   251  	// Reads and writes are atomic.
   252  	pageInUse [pagesPerArena / 8]uint8
   253  
   254  	// pageMarks is a bitmap that indicates which spans have any
   255  	// marked objects on them. Like pageInUse, only the bit
   256  	// corresponding to the first page in each span is used.
   257  	//
   258  	// Writes are done atomically during marking. Reads are
   259  	// non-atomic and lock-free since they only occur during
   260  	// sweeping (and hence never race with writes).
   261  	//
   262  	// This is used to quickly find whole spans that can be freed.
   263  	//
   264  	// TODO(austin): It would be nice if this was uint64 for
   265  	// faster scanning, but we don't have 64-bit atomic bit
   266  	// operations.
   267  	pageMarks [pagesPerArena / 8]uint8
   268  
   269  	// pageSpecials is a bitmap that indicates which spans have
   270  	// specials (finalizers or other). Like pageInUse, only the bit
   271  	// corresponding to the first page in each span is used.
   272  	//
   273  	// Writes are done atomically whenever a special is added to
   274  	// a span and whenever the last special is removed from a span.
   275  	// Reads are done atomically to find spans containing specials
   276  	// during marking.
   277  	pageSpecials [pagesPerArena / 8]uint8
   278  
   279  	// checkmarks stores the debug.gccheckmark state. It is only
   280  	// used if debug.gccheckmark > 0.
   281  	checkmarks *checkmarksMap
   282  
   283  	// zeroedBase marks the first byte of the first page in this
   284  	// arena which hasn't been used yet and is therefore already
   285  	// zero. zeroedBase is relative to the arena base.
   286  	// Increases monotonically until it hits heapArenaBytes.
   287  	//
   288  	// This field is sufficient to determine if an allocation
   289  	// needs to be zeroed because the page allocator follows an
   290  	// address-ordered first-fit policy.
   291  	//
   292  	// Read atomically and written with an atomic CAS.
   293  	zeroedBase uintptr
   294  }
   295  
   296  // arenaHint is a hint for where to grow the heap arenas. See
   297  // mheap_.arenaHints.
   298  //
   299  //go:notinheap
   300  type arenaHint struct {
   301  	addr uintptr
   302  	down bool
   303  	next *arenaHint
   304  }
   305  
   306  // An mspan is a run of pages.
   307  //
   308  // When a mspan is in the heap free treap, state == mSpanFree
   309  // and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span.
   310  // If the mspan is in the heap scav treap, then in addition to the
   311  // above scavenged == true. scavenged == false in all other cases.
   312  //
   313  // When a mspan is allocated, state == mSpanInUse or mSpanManual
   314  // and heapmap(i) == span for all s->start <= i < s->start+s->npages.
   315  
   316  // Every mspan is in one doubly-linked list, either in the mheap's
   317  // busy list or one of the mcentral's span lists.
   318  
   319  // An mspan representing actual memory has state mSpanInUse,
   320  // mSpanManual, or mSpanFree. Transitions between these states are
   321  // constrained as follows:
   322  //
   323  // * A span may transition from free to in-use or manual during any GC
   324  //   phase.
   325  //
   326  // * During sweeping (gcphase == _GCoff), a span may transition from
   327  //   in-use to free (as a result of sweeping) or manual to free (as a
   328  //   result of stacks being freed).
   329  //
   330  // * During GC (gcphase != _GCoff), a span *must not* transition from
   331  //   manual or in-use to free. Because concurrent GC may read a pointer
   332  //   and then look up its span, the span state must be monotonic.
   333  //
   334  // Setting mspan.state to mSpanInUse or mSpanManual must be done
   335  // atomically and only after all other span fields are valid.
   336  // Likewise, if inspecting a span is contingent on it being
   337  // mSpanInUse, the state should be loaded atomically and checked
   338  // before depending on other fields. This allows the garbage collector
   339  // to safely deal with potentially invalid pointers, since resolving
   340  // such pointers may race with a span being allocated.
   341  type mSpanState uint8
   342  
   343  const (
   344  	mSpanDead   mSpanState = iota
   345  	mSpanInUse             // allocated for garbage collected heap
   346  	mSpanManual            // allocated for manual management (e.g., stack allocator)
   347  )
   348  
   349  // mSpanStateNames are the names of the span states, indexed by
   350  // mSpanState.
   351  var mSpanStateNames = []string{
   352  	"mSpanDead",
   353  	"mSpanInUse",
   354  	"mSpanManual",
   355  	"mSpanFree",
   356  }
   357  
   358  // mSpanStateBox holds an mSpanState and provides atomic operations on
   359  // it. This is a separate type to disallow accidental comparison or
   360  // assignment with mSpanState.
   361  type mSpanStateBox struct {
   362  	s mSpanState
   363  }
   364  
   365  func (b *mSpanStateBox) set(s mSpanState) {
   366  	atomic.Store8((*uint8)(&b.s), uint8(s))
   367  }
   368  
   369  func (b *mSpanStateBox) get() mSpanState {
   370  	return mSpanState(atomic.Load8((*uint8)(&b.s)))
   371  }
   372  
   373  // mSpanList heads a linked list of spans.
   374  //
   375  //go:notinheap
   376  type mSpanList struct {
   377  	first *mspan // first span in list, or nil if none
   378  	last  *mspan // last span in list, or nil if none
   379  }
   380  
   381  //go:notinheap
   382  type mspan struct {
   383  	next *mspan     // next span in list, or nil if none
   384  	prev *mspan     // previous span in list, or nil if none
   385  	list *mSpanList // For debugging. TODO: Remove.
   386  
   387  	startAddr uintptr // address of first byte of span aka s.base()
   388  	npages    uintptr // number of pages in span
   389  
   390  	manualFreeList gclinkptr // list of free objects in mSpanManual spans
   391  
   392  	// freeindex is the slot index between 0 and nelems at which to begin scanning
   393  	// for the next free object in this span.
   394  	// Each allocation scans allocBits starting at freeindex until it encounters a 0
   395  	// indicating a free object. freeindex is then adjusted so that subsequent scans begin
   396  	// just past the newly discovered free object.
   397  	//
   398  	// If freeindex == nelem, this span has no free objects.
   399  	//
   400  	// allocBits is a bitmap of objects in this span.
   401  	// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
   402  	// then object n is free;
   403  	// otherwise, object n is allocated. Bits starting at nelem are
   404  	// undefined and should never be referenced.
   405  	//
   406  	// Object n starts at address n*elemsize + (start << pageShift).
   407  	freeindex uintptr
   408  	// TODO: Look up nelems from sizeclass and remove this field if it
   409  	// helps performance.
   410  	nelems uintptr // number of object in the span.
   411  
   412  	// Cache of the allocBits at freeindex. allocCache is shifted
   413  	// such that the lowest bit corresponds to the bit freeindex.
   414  	// allocCache holds the complement of allocBits, thus allowing
   415  	// ctz (count trailing zero) to use it directly.
   416  	// allocCache may contain bits beyond s.nelems; the caller must ignore
   417  	// these.
   418  	allocCache uint64
   419  
   420  	// allocBits and gcmarkBits hold pointers to a span's mark and
   421  	// allocation bits. The pointers are 8 byte aligned.
   422  	// There are three arenas where this data is held.
   423  	// free: Dirty arenas that are no longer accessed
   424  	//       and can be reused.
   425  	// next: Holds information to be used in the next GC cycle.
   426  	// current: Information being used during this GC cycle.
   427  	// previous: Information being used during the last GC cycle.
   428  	// A new GC cycle starts with the call to finishsweep_m.
   429  	// finishsweep_m moves the previous arena to the free arena,
   430  	// the current arena to the previous arena, and
   431  	// the next arena to the current arena.
   432  	// The next arena is populated as the spans request
   433  	// memory to hold gcmarkBits for the next GC cycle as well
   434  	// as allocBits for newly allocated spans.
   435  	//
   436  	// The pointer arithmetic is done "by hand" instead of using
   437  	// arrays to avoid bounds checks along critical performance
   438  	// paths.
   439  	// The sweep will free the old allocBits and set allocBits to the
   440  	// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
   441  	// out memory.
   442  	allocBits  *gcBits
   443  	gcmarkBits *gcBits
   444  
   445  	// sweep generation:
   446  	// if sweepgen == h->sweepgen - 2, the span needs sweeping
   447  	// if sweepgen == h->sweepgen - 1, the span is currently being swept
   448  	// if sweepgen == h->sweepgen, the span is swept and ready to use
   449  	// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
   450  	// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
   451  	// h->sweepgen is incremented by 2 after every GC
   452  
   453  	sweepgen    uint32
   454  	divMul      uint16        // for divide by elemsize - divMagic.mul
   455  	baseMask    uint16        // if non-0, elemsize is a power of 2, & this will get object allocation base
   456  	allocCount  uint16        // number of allocated objects
   457  	spanclass   spanClass     // size class and noscan (uint8)
   458  	state       mSpanStateBox // mSpanInUse etc; accessed atomically (get/set methods)
   459  	needzero    uint8         // needs to be zeroed before allocation
   460  	divShift    uint8         // for divide by elemsize - divMagic.shift
   461  	divShift2   uint8         // for divide by elemsize - divMagic.shift2
   462  	elemsize    uintptr       // computed from sizeclass or from npages
   463  	limit       uintptr       // end of data in span
   464  	speciallock mutex         // guards specials list
   465  	specials    *special      // linked list of special records sorted by offset.
   466  }
   467  
   468  func (s *mspan) base() uintptr {
   469  	return s.startAddr
   470  }
   471  
   472  func (s *mspan) layout() (size, n, total uintptr) {
   473  	total = s.npages << _PageShift
   474  	size = s.elemsize
   475  	if size > 0 {
   476  		n = total / size
   477  	}
   478  	return
   479  }
   480  
   481  // recordspan adds a newly allocated span to h.allspans.
   482  //
   483  // This only happens the first time a span is allocated from
   484  // mheap.spanalloc (it is not called when a span is reused).
   485  //
   486  // Write barriers are disallowed here because it can be called from
   487  // gcWork when allocating new workbufs. However, because it's an
   488  // indirect call from the fixalloc initializer, the compiler can't see
   489  // this.
   490  //
   491  // The heap lock must be held.
   492  //
   493  //go:nowritebarrierrec
   494  func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
   495  	h := (*mheap)(vh)
   496  	s := (*mspan)(p)
   497  
   498  	assertLockHeld(&h.lock)
   499  
   500  	if len(h.allspans) >= cap(h.allspans) {
   501  		n := 64 * 1024 / sys.PtrSize
   502  		if n < cap(h.allspans)*3/2 {
   503  			n = cap(h.allspans) * 3 / 2
   504  		}
   505  		var new []*mspan
   506  		sp := (*slice)(unsafe.Pointer(&new))
   507  		sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys)
   508  		if sp.array == nil {
   509  			throw("runtime: cannot allocate memory")
   510  		}
   511  		sp.len = len(h.allspans)
   512  		sp.cap = n
   513  		if len(h.allspans) > 0 {
   514  			copy(new, h.allspans)
   515  		}
   516  		oldAllspans := h.allspans
   517  		*(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new))
   518  		if len(oldAllspans) != 0 {
   519  			sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys)
   520  		}
   521  	}
   522  	h.allspans = h.allspans[:len(h.allspans)+1]
   523  	h.allspans[len(h.allspans)-1] = s
   524  }
   525  
   526  // A spanClass represents the size class and noscan-ness of a span.
   527  //
   528  // Each size class has a noscan spanClass and a scan spanClass. The
   529  // noscan spanClass contains only noscan objects, which do not contain
   530  // pointers and thus do not need to be scanned by the garbage
   531  // collector.
   532  type spanClass uint8
   533  
   534  const (
   535  	numSpanClasses = _NumSizeClasses << 1
   536  	tinySpanClass  = spanClass(tinySizeClass<<1 | 1)
   537  )
   538  
   539  func makeSpanClass(sizeclass uint8, noscan bool) spanClass {
   540  	return spanClass(sizeclass<<1) | spanClass(bool2int(noscan))
   541  }
   542  
   543  func (sc spanClass) sizeclass() int8 {
   544  	return int8(sc >> 1)
   545  }
   546  
   547  func (sc spanClass) noscan() bool {
   548  	return sc&1 != 0
   549  }
   550  
   551  // arenaIndex returns the index into mheap_.arenas of the arena
   552  // containing metadata for p. This index combines of an index into the
   553  // L1 map and an index into the L2 map and should be used as
   554  // mheap_.arenas[ai.l1()][ai.l2()].
   555  //
   556  // If p is outside the range of valid heap addresses, either l1() or
   557  // l2() will be out of bounds.
   558  //
   559  // It is nosplit because it's called by spanOf and several other
   560  // nosplit functions.
   561  //
   562  //go:nosplit
   563  func arenaIndex(p uintptr) arenaIdx {
   564  	return arenaIdx((p - arenaBaseOffset) / heapArenaBytes)
   565  }
   566  
   567  // arenaBase returns the low address of the region covered by heap
   568  // arena i.
   569  func arenaBase(i arenaIdx) uintptr {
   570  	return uintptr(i)*heapArenaBytes + arenaBaseOffset
   571  }
   572  
   573  type arenaIdx uint
   574  
   575  func (i arenaIdx) l1() uint {
   576  	if arenaL1Bits == 0 {
   577  		// Let the compiler optimize this away if there's no
   578  		// L1 map.
   579  		return 0
   580  	} else {
   581  		return uint(i) >> arenaL1Shift
   582  	}
   583  }
   584  
   585  func (i arenaIdx) l2() uint {
   586  	if arenaL1Bits == 0 {
   587  		return uint(i)
   588  	} else {
   589  		return uint(i) & (1<<arenaL2Bits - 1)
   590  	}
   591  }
   592  
   593  // inheap reports whether b is a pointer into a (potentially dead) heap object.
   594  // It returns false for pointers into mSpanManual spans.
   595  // Non-preemptible because it is used by write barriers.
   596  //go:nowritebarrier
   597  //go:nosplit
   598  func inheap(b uintptr) bool {
   599  	return spanOfHeap(b) != nil
   600  }
   601  
   602  // inHeapOrStack is a variant of inheap that returns true for pointers
   603  // into any allocated heap span.
   604  //
   605  //go:nowritebarrier
   606  //go:nosplit
   607  func inHeapOrStack(b uintptr) bool {
   608  	s := spanOf(b)
   609  	if s == nil || b < s.base() {
   610  		return false
   611  	}
   612  	switch s.state.get() {
   613  	case mSpanInUse, mSpanManual:
   614  		return b < s.limit
   615  	default:
   616  		return false
   617  	}
   618  }
   619  
   620  // spanOf returns the span of p. If p does not point into the heap
   621  // arena or no span has ever contained p, spanOf returns nil.
   622  //
   623  // If p does not point to allocated memory, this may return a non-nil
   624  // span that does *not* contain p. If this is a possibility, the
   625  // caller should either call spanOfHeap or check the span bounds
   626  // explicitly.
   627  //
   628  // Must be nosplit because it has callers that are nosplit.
   629  //
   630  //go:nosplit
   631  func spanOf(p uintptr) *mspan {
   632  	// This function looks big, but we use a lot of constant
   633  	// folding around arenaL1Bits to get it under the inlining
   634  	// budget. Also, many of the checks here are safety checks
   635  	// that Go needs to do anyway, so the generated code is quite
   636  	// short.
   637  	ri := arenaIndex(p)
   638  	if arenaL1Bits == 0 {
   639  		// If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can.
   640  		if ri.l2() >= uint(len(mheap_.arenas[0])) {
   641  			return nil
   642  		}
   643  	} else {
   644  		// If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't.
   645  		if ri.l1() >= uint(len(mheap_.arenas)) {
   646  			return nil
   647  		}
   648  	}
   649  	l2 := mheap_.arenas[ri.l1()]
   650  	if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1.
   651  		return nil
   652  	}
   653  	ha := l2[ri.l2()]
   654  	if ha == nil {
   655  		return nil
   656  	}
   657  	return ha.spans[(p/pageSize)%pagesPerArena]
   658  }
   659  
   660  // spanOfUnchecked is equivalent to spanOf, but the caller must ensure
   661  // that p points into an allocated heap arena.
   662  //
   663  // Must be nosplit because it has callers that are nosplit.
   664  //
   665  //go:nosplit
   666  func spanOfUnchecked(p uintptr) *mspan {
   667  	ai := arenaIndex(p)
   668  	return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena]
   669  }
   670  
   671  // spanOfHeap is like spanOf, but returns nil if p does not point to a
   672  // heap object.
   673  //
   674  // Must be nosplit because it has callers that are nosplit.
   675  //
   676  //go:nosplit
   677  func spanOfHeap(p uintptr) *mspan {
   678  	s := spanOf(p)
   679  	// s is nil if it's never been allocated. Otherwise, we check
   680  	// its state first because we don't trust this pointer, so we
   681  	// have to synchronize with span initialization. Then, it's
   682  	// still possible we picked up a stale span pointer, so we
   683  	// have to check the span's bounds.
   684  	if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
   685  		return nil
   686  	}
   687  	return s
   688  }
   689  
   690  // pageIndexOf returns the arena, page index, and page mask for pointer p.
   691  // The caller must ensure p is in the heap.
   692  func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) {
   693  	ai := arenaIndex(p)
   694  	arena = mheap_.arenas[ai.l1()][ai.l2()]
   695  	pageIdx = ((p / pageSize) / 8) % uintptr(len(arena.pageInUse))
   696  	pageMask = byte(1 << ((p / pageSize) % 8))
   697  	return
   698  }
   699  
   700  // Initialize the heap.
   701  func (h *mheap) init() {
   702  	lockInit(&h.lock, lockRankMheap)
   703  	lockInit(&h.speciallock, lockRankMheapSpecial)
   704  
   705  	h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys)
   706  	h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
   707  	h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
   708  	h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
   709  	h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
   710  
   711  	// Don't zero mspan allocations. Background sweeping can
   712  	// inspect a span concurrently with allocating it, so it's
   713  	// important that the span's sweepgen survive across freeing
   714  	// and re-allocating a span to prevent background sweeping
   715  	// from improperly cas'ing it from 0.
   716  	//
   717  	// This is safe because mspan contains no heap pointers.
   718  	h.spanalloc.zero = false
   719  
   720  	// h->mapcache needs no init
   721  
   722  	for i := range h.central {
   723  		h.central[i].mcentral.init(spanClass(i))
   724  	}
   725  
   726  	h.pages.init(&h.lock, &memstats.gcMiscSys)
   727  }
   728  
   729  // reclaim sweeps and reclaims at least npage pages into the heap.
   730  // It is called before allocating npage pages to keep growth in check.
   731  //
   732  // reclaim implements the page-reclaimer half of the sweeper.
   733  //
   734  // h.lock must NOT be held.
   735  func (h *mheap) reclaim(npage uintptr) {
   736  	// TODO(austin): Half of the time spent freeing spans is in
   737  	// locking/unlocking the heap (even with low contention). We
   738  	// could make the slow path here several times faster by
   739  	// batching heap frees.
   740  
   741  	// Bail early if there's no more reclaim work.
   742  	if atomic.Load64(&h.reclaimIndex) >= 1<<63 {
   743  		return
   744  	}
   745  
   746  	// Disable preemption so the GC can't start while we're
   747  	// sweeping, so we can read h.sweepArenas, and so
   748  	// traceGCSweepStart/Done pair on the P.
   749  	mp := acquirem()
   750  
   751  	if trace.enabled {
   752  		traceGCSweepStart()
   753  	}
   754  
   755  	arenas := h.sweepArenas
   756  	locked := false
   757  	for npage > 0 {
   758  		// Pull from accumulated credit first.
   759  		if credit := atomic.Loaduintptr(&h.reclaimCredit); credit > 0 {
   760  			take := credit
   761  			if take > npage {
   762  				// Take only what we need.
   763  				take = npage
   764  			}
   765  			if atomic.Casuintptr(&h.reclaimCredit, credit, credit-take) {
   766  				npage -= take
   767  			}
   768  			continue
   769  		}
   770  
   771  		// Claim a chunk of work.
   772  		idx := uintptr(atomic.Xadd64(&h.reclaimIndex, pagesPerReclaimerChunk) - pagesPerReclaimerChunk)
   773  		if idx/pagesPerArena >= uintptr(len(arenas)) {
   774  			// Page reclaiming is done.
   775  			atomic.Store64(&h.reclaimIndex, 1<<63)
   776  			break
   777  		}
   778  
   779  		if !locked {
   780  			// Lock the heap for reclaimChunk.
   781  			lock(&h.lock)
   782  			locked = true
   783  		}
   784  
   785  		// Scan this chunk.
   786  		nfound := h.reclaimChunk(arenas, idx, pagesPerReclaimerChunk)
   787  		if nfound <= npage {
   788  			npage -= nfound
   789  		} else {
   790  			// Put spare pages toward global credit.
   791  			atomic.Xadduintptr(&h.reclaimCredit, nfound-npage)
   792  			npage = 0
   793  		}
   794  	}
   795  	if locked {
   796  		unlock(&h.lock)
   797  	}
   798  
   799  	if trace.enabled {
   800  		traceGCSweepDone()
   801  	}
   802  	releasem(mp)
   803  }
   804  
   805  // reclaimChunk sweeps unmarked spans that start at page indexes [pageIdx, pageIdx+n).
   806  // It returns the number of pages returned to the heap.
   807  //
   808  // h.lock must be held and the caller must be non-preemptible. Note: h.lock may be
   809  // temporarily unlocked and re-locked in order to do sweeping or if tracing is
   810  // enabled.
   811  func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr {
   812  	// The heap lock must be held because this accesses the
   813  	// heapArena.spans arrays using potentially non-live pointers.
   814  	// In particular, if a span were freed and merged concurrently
   815  	// with this probing heapArena.spans, it would be possible to
   816  	// observe arbitrary, stale span pointers.
   817  	assertLockHeld(&h.lock)
   818  
   819  	n0 := n
   820  	var nFreed uintptr
   821  	sg := h.sweepgen
   822  	for n > 0 {
   823  		ai := arenas[pageIdx/pagesPerArena]
   824  		ha := h.arenas[ai.l1()][ai.l2()]
   825  
   826  		// Get a chunk of the bitmap to work on.
   827  		arenaPage := uint(pageIdx % pagesPerArena)
   828  		inUse := ha.pageInUse[arenaPage/8:]
   829  		marked := ha.pageMarks[arenaPage/8:]
   830  		if uintptr(len(inUse)) > n/8 {
   831  			inUse = inUse[:n/8]
   832  			marked = marked[:n/8]
   833  		}
   834  
   835  		// Scan this bitmap chunk for spans that are in-use
   836  		// but have no marked objects on them.
   837  		for i := range inUse {
   838  			inUseUnmarked := atomic.Load8(&inUse[i]) &^ marked[i]
   839  			if inUseUnmarked == 0 {
   840  				continue
   841  			}
   842  
   843  			for j := uint(0); j < 8; j++ {
   844  				if inUseUnmarked&(1<<j) != 0 {
   845  					s := ha.spans[arenaPage+uint(i)*8+j]
   846  					if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
   847  						npages := s.npages
   848  						unlock(&h.lock)
   849  						if s.sweep(false) {
   850  							nFreed += npages
   851  						}
   852  						lock(&h.lock)
   853  						// Reload inUse. It's possible nearby
   854  						// spans were freed when we dropped the
   855  						// lock and we don't want to get stale
   856  						// pointers from the spans array.
   857  						inUseUnmarked = atomic.Load8(&inUse[i]) &^ marked[i]
   858  					}
   859  				}
   860  			}
   861  		}
   862  
   863  		// Advance.
   864  		pageIdx += uintptr(len(inUse) * 8)
   865  		n -= uintptr(len(inUse) * 8)
   866  	}
   867  	if trace.enabled {
   868  		unlock(&h.lock)
   869  		// Account for pages scanned but not reclaimed.
   870  		traceGCSweepSpan((n0 - nFreed) * pageSize)
   871  		lock(&h.lock)
   872  	}
   873  
   874  	assertLockHeld(&h.lock) // Must be locked on return.
   875  	return nFreed
   876  }
   877  
   878  // spanAllocType represents the type of allocation to make, or
   879  // the type of allocation to be freed.
   880  type spanAllocType uint8
   881  
   882  const (
   883  	spanAllocHeap          spanAllocType = iota // heap span
   884  	spanAllocStack                              // stack span
   885  	spanAllocPtrScalarBits                      // unrolled GC prog bitmap span
   886  	spanAllocWorkBuf                            // work buf span
   887  )
   888  
   889  // manual returns true if the span allocation is manually managed.
   890  func (s spanAllocType) manual() bool {
   891  	return s != spanAllocHeap
   892  }
   893  
   894  // alloc allocates a new span of npage pages from the GC'd heap.
   895  //
   896  // spanclass indicates the span's size class and scannability.
   897  //
   898  // If needzero is true, the memory for the returned span will be zeroed.
   899  func (h *mheap) alloc(npages uintptr, spanclass spanClass, needzero bool) *mspan {
   900  	// Don't do any operations that lock the heap on the G stack.
   901  	// It might trigger stack growth, and the stack growth code needs
   902  	// to be able to allocate heap.
   903  	var s *mspan
   904  	systemstack(func() {
   905  		// To prevent excessive heap growth, before allocating n pages
   906  		// we need to sweep and reclaim at least n pages.
   907  		if h.sweepdone == 0 {
   908  			h.reclaim(npages)
   909  		}
   910  		s = h.allocSpan(npages, spanAllocHeap, spanclass)
   911  	})
   912  
   913  	if s != nil {
   914  		if needzero && s.needzero != 0 {
   915  			memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift)
   916  		}
   917  		s.needzero = 0
   918  	}
   919  	return s
   920  }
   921  
   922  // allocManual allocates a manually-managed span of npage pages.
   923  // allocManual returns nil if allocation fails.
   924  //
   925  // allocManual adds the bytes used to *stat, which should be a
   926  // memstats in-use field. Unlike allocations in the GC'd heap, the
   927  // allocation does *not* count toward heap_inuse or heap_sys.
   928  //
   929  // The memory backing the returned span may not be zeroed if
   930  // span.needzero is set.
   931  //
   932  // allocManual must be called on the system stack because it may
   933  // acquire the heap lock via allocSpan. See mheap for details.
   934  //
   935  // If new code is written to call allocManual, do NOT use an
   936  // existing spanAllocType value and instead declare a new one.
   937  //
   938  //go:systemstack
   939  func (h *mheap) allocManual(npages uintptr, typ spanAllocType) *mspan {
   940  	if !typ.manual() {
   941  		throw("manual span allocation called with non-manually-managed type")
   942  	}
   943  	return h.allocSpan(npages, typ, 0)
   944  }
   945  
   946  // setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize))
   947  // is s.
   948  func (h *mheap) setSpans(base, npage uintptr, s *mspan) {
   949  	p := base / pageSize
   950  	ai := arenaIndex(base)
   951  	ha := h.arenas[ai.l1()][ai.l2()]
   952  	for n := uintptr(0); n < npage; n++ {
   953  		i := (p + n) % pagesPerArena
   954  		if i == 0 {
   955  			ai = arenaIndex(base + n*pageSize)
   956  			ha = h.arenas[ai.l1()][ai.l2()]
   957  		}
   958  		ha.spans[i] = s
   959  	}
   960  }
   961  
   962  // allocNeedsZero checks if the region of address space [base, base+npage*pageSize),
   963  // assumed to be allocated, needs to be zeroed, updating heap arena metadata for
   964  // future allocations.
   965  //
   966  // This must be called each time pages are allocated from the heap, even if the page
   967  // allocator can otherwise prove the memory it's allocating is already zero because
   968  // they're fresh from the operating system. It updates heapArena metadata that is
   969  // critical for future page allocations.
   970  //
   971  // There are no locking constraints on this method.
   972  func (h *mheap) allocNeedsZero(base, npage uintptr) (needZero bool) {
   973  	for npage > 0 {
   974  		ai := arenaIndex(base)
   975  		ha := h.arenas[ai.l1()][ai.l2()]
   976  
   977  		zeroedBase := atomic.Loaduintptr(&ha.zeroedBase)
   978  		arenaBase := base % heapArenaBytes
   979  		if arenaBase < zeroedBase {
   980  			// We extended into the non-zeroed part of the
   981  			// arena, so this region needs to be zeroed before use.
   982  			//
   983  			// zeroedBase is monotonically increasing, so if we see this now then
   984  			// we can be sure we need to zero this memory region.
   985  			//
   986  			// We still need to update zeroedBase for this arena, and
   987  			// potentially more arenas.
   988  			needZero = true
   989  		}
   990  		// We may observe arenaBase > zeroedBase if we're racing with one or more
   991  		// allocations which are acquiring memory directly before us in the address
   992  		// space. But, because we know no one else is acquiring *this* memory, it's
   993  		// still safe to not zero.
   994  
   995  		// Compute how far into the arena we extend into, capped
   996  		// at heapArenaBytes.
   997  		arenaLimit := arenaBase + npage*pageSize
   998  		if arenaLimit > heapArenaBytes {
   999  			arenaLimit = heapArenaBytes
  1000  		}
  1001  		// Increase ha.zeroedBase so it's >= arenaLimit.
  1002  		// We may be racing with other updates.
  1003  		for arenaLimit > zeroedBase {
  1004  			if atomic.Casuintptr(&ha.zeroedBase, zeroedBase, arenaLimit) {
  1005  				break
  1006  			}
  1007  			zeroedBase = atomic.Loaduintptr(&ha.zeroedBase)
  1008  			// Sanity check zeroedBase.
  1009  			if zeroedBase <= arenaLimit && zeroedBase > arenaBase {
  1010  				// The zeroedBase moved into the space we were trying to
  1011  				// claim. That's very bad, and indicates someone allocated
  1012  				// the same region we did.
  1013  				throw("potentially overlapping in-use allocations detected")
  1014  			}
  1015  		}
  1016  
  1017  		// Move base forward and subtract from npage to move into
  1018  		// the next arena, or finish.
  1019  		base += arenaLimit - arenaBase
  1020  		npage -= (arenaLimit - arenaBase) / pageSize
  1021  	}
  1022  	return
  1023  }
  1024  
  1025  // tryAllocMSpan attempts to allocate an mspan object from
  1026  // the P-local cache, but may fail.
  1027  //
  1028  // h.lock need not be held.
  1029  //
  1030  // This caller must ensure that its P won't change underneath
  1031  // it during this function. Currently to ensure that we enforce
  1032  // that the function is run on the system stack, because that's
  1033  // the only place it is used now. In the future, this requirement
  1034  // may be relaxed if its use is necessary elsewhere.
  1035  //
  1036  //go:systemstack
  1037  func (h *mheap) tryAllocMSpan() *mspan {
  1038  	pp := getg().m.p.ptr()
  1039  	// If we don't have a p or the cache is empty, we can't do
  1040  	// anything here.
  1041  	if pp == nil || pp.mspancache.len == 0 {
  1042  		return nil
  1043  	}
  1044  	// Pull off the last entry in the cache.
  1045  	s := pp.mspancache.buf[pp.mspancache.len-1]
  1046  	pp.mspancache.len--
  1047  	return s
  1048  }
  1049  
  1050  // allocMSpanLocked allocates an mspan object.
  1051  //
  1052  // h.lock must be held.
  1053  //
  1054  // allocMSpanLocked must be called on the system stack because
  1055  // its caller holds the heap lock. See mheap for details.
  1056  // Running on the system stack also ensures that we won't
  1057  // switch Ps during this function. See tryAllocMSpan for details.
  1058  //
  1059  //go:systemstack
  1060  func (h *mheap) allocMSpanLocked() *mspan {
  1061  	assertLockHeld(&h.lock)
  1062  
  1063  	pp := getg().m.p.ptr()
  1064  	if pp == nil {
  1065  		// We don't have a p so just do the normal thing.
  1066  		return (*mspan)(h.spanalloc.alloc())
  1067  	}
  1068  	// Refill the cache if necessary.
  1069  	if pp.mspancache.len == 0 {
  1070  		const refillCount = len(pp.mspancache.buf) / 2
  1071  		for i := 0; i < refillCount; i++ {
  1072  			pp.mspancache.buf[i] = (*mspan)(h.spanalloc.alloc())
  1073  		}
  1074  		pp.mspancache.len = refillCount
  1075  	}
  1076  	// Pull off the last entry in the cache.
  1077  	s := pp.mspancache.buf[pp.mspancache.len-1]
  1078  	pp.mspancache.len--
  1079  	return s
  1080  }
  1081  
  1082  // freeMSpanLocked free an mspan object.
  1083  //
  1084  // h.lock must be held.
  1085  //
  1086  // freeMSpanLocked must be called on the system stack because
  1087  // its caller holds the heap lock. See mheap for details.
  1088  // Running on the system stack also ensures that we won't
  1089  // switch Ps during this function. See tryAllocMSpan for details.
  1090  //
  1091  //go:systemstack
  1092  func (h *mheap) freeMSpanLocked(s *mspan) {
  1093  	assertLockHeld(&h.lock)
  1094  
  1095  	pp := getg().m.p.ptr()
  1096  	// First try to free the mspan directly to the cache.
  1097  	if pp != nil && pp.mspancache.len < len(pp.mspancache.buf) {
  1098  		pp.mspancache.buf[pp.mspancache.len] = s
  1099  		pp.mspancache.len++
  1100  		return
  1101  	}
  1102  	// Failing that (or if we don't have a p), just free it to
  1103  	// the heap.
  1104  	h.spanalloc.free(unsafe.Pointer(s))
  1105  }
  1106  
  1107  // allocSpan allocates an mspan which owns npages worth of memory.
  1108  //
  1109  // If typ.manual() == false, allocSpan allocates a heap span of class spanclass
  1110  // and updates heap accounting. If manual == true, allocSpan allocates a
  1111  // manually-managed span (spanclass is ignored), and the caller is
  1112  // responsible for any accounting related to its use of the span. Either
  1113  // way, allocSpan will atomically add the bytes in the newly allocated
  1114  // span to *sysStat.
  1115  //
  1116  // The returned span is fully initialized.
  1117  //
  1118  // h.lock must not be held.
  1119  //
  1120  // allocSpan must be called on the system stack both because it acquires
  1121  // the heap lock and because it must block GC transitions.
  1122  //
  1123  //go:systemstack
  1124  func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass) (s *mspan) {
  1125  	// Function-global state.
  1126  	gp := getg()
  1127  	base, scav := uintptr(0), uintptr(0)
  1128  
  1129  	// On some platforms we need to provide physical page aligned stack
  1130  	// allocations. Where the page size is less than the physical page
  1131  	// size, we already manage to do this by default.
  1132  	needPhysPageAlign := physPageAlignedStacks && typ == spanAllocStack && pageSize < physPageSize
  1133  
  1134  	// If the allocation is small enough, try the page cache!
  1135  	// The page cache does not support aligned allocations, so we cannot use
  1136  	// it if we need to provide a physical page aligned stack allocation.
  1137  	pp := gp.m.p.ptr()
  1138  	if !needPhysPageAlign && pp != nil && npages < pageCachePages/4 {
  1139  		c := &pp.pcache
  1140  
  1141  		// If the cache is empty, refill it.
  1142  		if c.empty() {
  1143  			lock(&h.lock)
  1144  			*c = h.pages.allocToCache()
  1145  			unlock(&h.lock)
  1146  		}
  1147  
  1148  		// Try to allocate from the cache.
  1149  		base, scav = c.alloc(npages)
  1150  		if base != 0 {
  1151  			s = h.tryAllocMSpan()
  1152  			if s != nil {
  1153  				goto HaveSpan
  1154  			}
  1155  			// We have a base but no mspan, so we need
  1156  			// to lock the heap.
  1157  		}
  1158  	}
  1159  
  1160  	// For one reason or another, we couldn't get the
  1161  	// whole job done without the heap lock.
  1162  	lock(&h.lock)
  1163  
  1164  	if needPhysPageAlign {
  1165  		// Overallocate by a physical page to allow for later alignment.
  1166  		npages += physPageSize / pageSize
  1167  	}
  1168  
  1169  	if base == 0 {
  1170  		// Try to acquire a base address.
  1171  		base, scav = h.pages.alloc(npages)
  1172  		if base == 0 {
  1173  			if !h.grow(npages) {
  1174  				unlock(&h.lock)
  1175  				return nil
  1176  			}
  1177  			base, scav = h.pages.alloc(npages)
  1178  			if base == 0 {
  1179  				throw("grew heap, but no adequate free space found")
  1180  			}
  1181  		}
  1182  	}
  1183  	if s == nil {
  1184  		// We failed to get an mspan earlier, so grab
  1185  		// one now that we have the heap lock.
  1186  		s = h.allocMSpanLocked()
  1187  	}
  1188  
  1189  	if needPhysPageAlign {
  1190  		allocBase, allocPages := base, npages
  1191  		base = alignUp(allocBase, physPageSize)
  1192  		npages -= physPageSize / pageSize
  1193  
  1194  		// Return memory around the aligned allocation.
  1195  		spaceBefore := base - allocBase
  1196  		if spaceBefore > 0 {
  1197  			h.pages.free(allocBase, spaceBefore/pageSize)
  1198  		}
  1199  		spaceAfter := (allocPages-npages)*pageSize - spaceBefore
  1200  		if spaceAfter > 0 {
  1201  			h.pages.free(base+npages*pageSize, spaceAfter/pageSize)
  1202  		}
  1203  	}
  1204  
  1205  	unlock(&h.lock)
  1206  
  1207  HaveSpan:
  1208  	// At this point, both s != nil and base != 0, and the heap
  1209  	// lock is no longer held. Initialize the span.
  1210  	s.init(base, npages)
  1211  	if h.allocNeedsZero(base, npages) {
  1212  		s.needzero = 1
  1213  	}
  1214  	nbytes := npages * pageSize
  1215  	if typ.manual() {
  1216  		s.manualFreeList = 0
  1217  		s.nelems = 0
  1218  		s.limit = s.base() + s.npages*pageSize
  1219  		s.state.set(mSpanManual)
  1220  	} else {
  1221  		// We must set span properties before the span is published anywhere
  1222  		// since we're not holding the heap lock.
  1223  		s.spanclass = spanclass
  1224  		if sizeclass := spanclass.sizeclass(); sizeclass == 0 {
  1225  			s.elemsize = nbytes
  1226  			s.nelems = 1
  1227  
  1228  			s.divShift = 0
  1229  			s.divMul = 0
  1230  			s.divShift2 = 0
  1231  			s.baseMask = 0
  1232  		} else {
  1233  			s.elemsize = uintptr(class_to_size[sizeclass])
  1234  			s.nelems = nbytes / s.elemsize
  1235  
  1236  			m := &class_to_divmagic[sizeclass]
  1237  			s.divShift = m.shift
  1238  			s.divMul = m.mul
  1239  			s.divShift2 = m.shift2
  1240  			s.baseMask = m.baseMask
  1241  		}
  1242  
  1243  		// Initialize mark and allocation structures.
  1244  		s.freeindex = 0
  1245  		s.allocCache = ^uint64(0) // all 1s indicating all free.
  1246  		s.gcmarkBits = newMarkBits(s.nelems)
  1247  		s.allocBits = newAllocBits(s.nelems)
  1248  
  1249  		// It's safe to access h.sweepgen without the heap lock because it's
  1250  		// only ever updated with the world stopped and we run on the
  1251  		// systemstack which blocks a STW transition.
  1252  		atomic.Store(&s.sweepgen, h.sweepgen)
  1253  
  1254  		// Now that the span is filled in, set its state. This
  1255  		// is a publication barrier for the other fields in
  1256  		// the span. While valid pointers into this span
  1257  		// should never be visible until the span is returned,
  1258  		// if the garbage collector finds an invalid pointer,
  1259  		// access to the span may race with initialization of
  1260  		// the span. We resolve this race by atomically
  1261  		// setting the state after the span is fully
  1262  		// initialized, and atomically checking the state in
  1263  		// any situation where a pointer is suspect.
  1264  		s.state.set(mSpanInUse)
  1265  	}
  1266  
  1267  	// Commit and account for any scavenged memory that the span now owns.
  1268  	if scav != 0 {
  1269  		// sysUsed all the pages that are actually available
  1270  		// in the span since some of them might be scavenged.
  1271  		sysUsed(unsafe.Pointer(base), nbytes)
  1272  		atomic.Xadd64(&memstats.heap_released, -int64(scav))
  1273  	}
  1274  	// Update stats.
  1275  	if typ == spanAllocHeap {
  1276  		atomic.Xadd64(&memstats.heap_inuse, int64(nbytes))
  1277  	}
  1278  	if typ.manual() {
  1279  		// Manually managed memory doesn't count toward heap_sys.
  1280  		memstats.heap_sys.add(-int64(nbytes))
  1281  	}
  1282  	// Update consistent stats.
  1283  	stats := memstats.heapStats.acquire()
  1284  	atomic.Xaddint64(&stats.committed, int64(scav))
  1285  	atomic.Xaddint64(&stats.released, -int64(scav))
  1286  	switch typ {
  1287  	case spanAllocHeap:
  1288  		atomic.Xaddint64(&stats.inHeap, int64(nbytes))
  1289  	case spanAllocStack:
  1290  		atomic.Xaddint64(&stats.inStacks, int64(nbytes))
  1291  	case spanAllocPtrScalarBits:
  1292  		atomic.Xaddint64(&stats.inPtrScalarBits, int64(nbytes))
  1293  	case spanAllocWorkBuf:
  1294  		atomic.Xaddint64(&stats.inWorkBufs, int64(nbytes))
  1295  	}
  1296  	memstats.heapStats.release()
  1297  
  1298  	// Publish the span in various locations.
  1299  
  1300  	// This is safe to call without the lock held because the slots
  1301  	// related to this span will only ever be read or modified by
  1302  	// this thread until pointers into the span are published (and
  1303  	// we execute a publication barrier at the end of this function
  1304  	// before that happens) or pageInUse is updated.
  1305  	h.setSpans(s.base(), npages, s)
  1306  
  1307  	if !typ.manual() {
  1308  		// Mark in-use span in arena page bitmap.
  1309  		//
  1310  		// This publishes the span to the page sweeper, so
  1311  		// it's imperative that the span be completely initialized
  1312  		// prior to this line.
  1313  		arena, pageIdx, pageMask := pageIndexOf(s.base())
  1314  		atomic.Or8(&arena.pageInUse[pageIdx], pageMask)
  1315  
  1316  		// Update related page sweeper stats.
  1317  		atomic.Xadd64(&h.pagesInUse, int64(npages))
  1318  	}
  1319  
  1320  	// Make sure the newly allocated span will be observed
  1321  	// by the GC before pointers into the span are published.
  1322  	publicationBarrier()
  1323  
  1324  	return s
  1325  }
  1326  
  1327  // Try to add at least npage pages of memory to the heap,
  1328  // returning whether it worked.
  1329  //
  1330  // h.lock must be held.
  1331  func (h *mheap) grow(npage uintptr) bool {
  1332  	assertLockHeld(&h.lock)
  1333  
  1334  	// We must grow the heap in whole palloc chunks.
  1335  	ask := alignUp(npage, pallocChunkPages) * pageSize
  1336  
  1337  	totalGrowth := uintptr(0)
  1338  	// This may overflow because ask could be very large
  1339  	// and is otherwise unrelated to h.curArena.base.
  1340  	end := h.curArena.base + ask
  1341  	nBase := alignUp(end, physPageSize)
  1342  	if nBase > h.curArena.end || /* overflow */ end < h.curArena.base {
  1343  		// Not enough room in the current arena. Allocate more
  1344  		// arena space. This may not be contiguous with the
  1345  		// current arena, so we have to request the full ask.
  1346  		av, asize := h.sysAlloc(ask)
  1347  		if av == nil {
  1348  			print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n")
  1349  			return false
  1350  		}
  1351  
  1352  		if uintptr(av) == h.curArena.end {
  1353  			// The new space is contiguous with the old
  1354  			// space, so just extend the current space.
  1355  			h.curArena.end = uintptr(av) + asize
  1356  		} else {
  1357  			// The new space is discontiguous. Track what
  1358  			// remains of the current space and switch to
  1359  			// the new space. This should be rare.
  1360  			if size := h.curArena.end - h.curArena.base; size != 0 {
  1361  				h.pages.grow(h.curArena.base, size)
  1362  				totalGrowth += size
  1363  			}
  1364  			// Switch to the new space.
  1365  			h.curArena.base = uintptr(av)
  1366  			h.curArena.end = uintptr(av) + asize
  1367  		}
  1368  
  1369  		// The memory just allocated counts as both released
  1370  		// and idle, even though it's not yet backed by spans.
  1371  		//
  1372  		// The allocation is always aligned to the heap arena
  1373  		// size which is always > physPageSize, so its safe to
  1374  		// just add directly to heap_released.
  1375  		atomic.Xadd64(&memstats.heap_released, int64(asize))
  1376  		stats := memstats.heapStats.acquire()
  1377  		atomic.Xaddint64(&stats.released, int64(asize))
  1378  		memstats.heapStats.release()
  1379  
  1380  		// Recalculate nBase.
  1381  		// We know this won't overflow, because sysAlloc returned
  1382  		// a valid region starting at h.curArena.base which is at
  1383  		// least ask bytes in size.
  1384  		nBase = alignUp(h.curArena.base+ask, physPageSize)
  1385  	}
  1386  
  1387  	// Grow into the current arena.
  1388  	v := h.curArena.base
  1389  	h.curArena.base = nBase
  1390  	h.pages.grow(v, nBase-v)
  1391  	totalGrowth += nBase - v
  1392  
  1393  	// We just caused a heap growth, so scavenge down what will soon be used.
  1394  	// By scavenging inline we deal with the failure to allocate out of
  1395  	// memory fragments by scavenging the memory fragments that are least
  1396  	// likely to be re-used.
  1397  	if retained := heapRetained(); retained+uint64(totalGrowth) > h.scavengeGoal {
  1398  		todo := totalGrowth
  1399  		if overage := uintptr(retained + uint64(totalGrowth) - h.scavengeGoal); todo > overage {
  1400  			todo = overage
  1401  		}
  1402  		h.pages.scavenge(todo, false)
  1403  	}
  1404  	return true
  1405  }
  1406  
  1407  // Free the span back into the heap.
  1408  func (h *mheap) freeSpan(s *mspan) {
  1409  	systemstack(func() {
  1410  		lock(&h.lock)
  1411  		if msanenabled {
  1412  			// Tell msan that this entire span is no longer in use.
  1413  			base := unsafe.Pointer(s.base())
  1414  			bytes := s.npages << _PageShift
  1415  			msanfree(base, bytes)
  1416  		}
  1417  		h.freeSpanLocked(s, spanAllocHeap)
  1418  		unlock(&h.lock)
  1419  	})
  1420  }
  1421  
  1422  // freeManual frees a manually-managed span returned by allocManual.
  1423  // typ must be the same as the spanAllocType passed to the allocManual that
  1424  // allocated s.
  1425  //
  1426  // This must only be called when gcphase == _GCoff. See mSpanState for
  1427  // an explanation.
  1428  //
  1429  // freeManual must be called on the system stack because it acquires
  1430  // the heap lock. See mheap for details.
  1431  //
  1432  //go:systemstack
  1433  func (h *mheap) freeManual(s *mspan, typ spanAllocType) {
  1434  	s.needzero = 1
  1435  	lock(&h.lock)
  1436  	h.freeSpanLocked(s, typ)
  1437  	unlock(&h.lock)
  1438  }
  1439  
  1440  func (h *mheap) freeSpanLocked(s *mspan, typ spanAllocType) {
  1441  	assertLockHeld(&h.lock)
  1442  
  1443  	switch s.state.get() {
  1444  	case mSpanManual:
  1445  		if s.allocCount != 0 {
  1446  			throw("mheap.freeSpanLocked - invalid stack free")
  1447  		}
  1448  	case mSpanInUse:
  1449  		if s.allocCount != 0 || s.sweepgen != h.sweepgen {
  1450  			print("mheap.freeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n")
  1451  			throw("mheap.freeSpanLocked - invalid free")
  1452  		}
  1453  		atomic.Xadd64(&h.pagesInUse, -int64(s.npages))
  1454  
  1455  		// Clear in-use bit in arena page bitmap.
  1456  		arena, pageIdx, pageMask := pageIndexOf(s.base())
  1457  		atomic.And8(&arena.pageInUse[pageIdx], ^pageMask)
  1458  	default:
  1459  		throw("mheap.freeSpanLocked - invalid span state")
  1460  	}
  1461  
  1462  	// Update stats.
  1463  	//
  1464  	// Mirrors the code in allocSpan.
  1465  	nbytes := s.npages * pageSize
  1466  	if typ == spanAllocHeap {
  1467  		atomic.Xadd64(&memstats.heap_inuse, -int64(nbytes))
  1468  	}
  1469  	if typ.manual() {
  1470  		// Manually managed memory doesn't count toward heap_sys, so add it back.
  1471  		memstats.heap_sys.add(int64(nbytes))
  1472  	}
  1473  	// Update consistent stats.
  1474  	stats := memstats.heapStats.acquire()
  1475  	switch typ {
  1476  	case spanAllocHeap:
  1477  		atomic.Xaddint64(&stats.inHeap, -int64(nbytes))
  1478  	case spanAllocStack:
  1479  		atomic.Xaddint64(&stats.inStacks, -int64(nbytes))
  1480  	case spanAllocPtrScalarBits:
  1481  		atomic.Xaddint64(&stats.inPtrScalarBits, -int64(nbytes))
  1482  	case spanAllocWorkBuf:
  1483  		atomic.Xaddint64(&stats.inWorkBufs, -int64(nbytes))
  1484  	}
  1485  	memstats.heapStats.release()
  1486  
  1487  	// Mark the space as free.
  1488  	h.pages.free(s.base(), s.npages)
  1489  
  1490  	// Free the span structure. We no longer have a use for it.
  1491  	s.state.set(mSpanDead)
  1492  	h.freeMSpanLocked(s)
  1493  }
  1494  
  1495  // scavengeAll acquires the heap lock (blocking any additional
  1496  // manipulation of the page allocator) and iterates over the whole
  1497  // heap, scavenging every free page available.
  1498  func (h *mheap) scavengeAll() {
  1499  	// Disallow malloc or panic while holding the heap lock. We do
  1500  	// this here because this is a non-mallocgc entry-point to
  1501  	// the mheap API.
  1502  	gp := getg()
  1503  	gp.m.mallocing++
  1504  	lock(&h.lock)
  1505  	// Start a new scavenge generation so we have a chance to walk
  1506  	// over the whole heap.
  1507  	h.pages.scavengeStartGen()
  1508  	released := h.pages.scavenge(^uintptr(0), false)
  1509  	gen := h.pages.scav.gen
  1510  	unlock(&h.lock)
  1511  	gp.m.mallocing--
  1512  
  1513  	if debug.scavtrace > 0 {
  1514  		printScavTrace(gen, released, true)
  1515  	}
  1516  }
  1517  
  1518  //go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
  1519  func runtime_debug_freeOSMemory() {
  1520  	GC()
  1521  	systemstack(func() { mheap_.scavengeAll() })
  1522  }
  1523  
  1524  // Initialize a new span with the given start and npages.
  1525  func (span *mspan) init(base uintptr, npages uintptr) {
  1526  	// span is *not* zeroed.
  1527  	span.next = nil
  1528  	span.prev = nil
  1529  	span.list = nil
  1530  	span.startAddr = base
  1531  	span.npages = npages
  1532  	span.allocCount = 0
  1533  	span.spanclass = 0
  1534  	span.elemsize = 0
  1535  	span.speciallock.key = 0
  1536  	span.specials = nil
  1537  	span.needzero = 0
  1538  	span.freeindex = 0
  1539  	span.allocBits = nil
  1540  	span.gcmarkBits = nil
  1541  	span.state.set(mSpanDead)
  1542  	lockInit(&span.speciallock, lockRankMspanSpecial)
  1543  }
  1544  
  1545  func (span *mspan) inList() bool {
  1546  	return span.list != nil
  1547  }
  1548  
  1549  // Initialize an empty doubly-linked list.
  1550  func (list *mSpanList) init() {
  1551  	list.first = nil
  1552  	list.last = nil
  1553  }
  1554  
  1555  func (list *mSpanList) remove(span *mspan) {
  1556  	if span.list != list {
  1557  		print("runtime: failed mSpanList.remove span.npages=", span.npages,
  1558  			" span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n")
  1559  		throw("mSpanList.remove")
  1560  	}
  1561  	if list.first == span {
  1562  		list.first = span.next
  1563  	} else {
  1564  		span.prev.next = span.next
  1565  	}
  1566  	if list.last == span {
  1567  		list.last = span.prev
  1568  	} else {
  1569  		span.next.prev = span.prev
  1570  	}
  1571  	span.next = nil
  1572  	span.prev = nil
  1573  	span.list = nil
  1574  }
  1575  
  1576  func (list *mSpanList) isEmpty() bool {
  1577  	return list.first == nil
  1578  }
  1579  
  1580  func (list *mSpanList) insert(span *mspan) {
  1581  	if span.next != nil || span.prev != nil || span.list != nil {
  1582  		println("runtime: failed mSpanList.insert", span, span.next, span.prev, span.list)
  1583  		throw("mSpanList.insert")
  1584  	}
  1585  	span.next = list.first
  1586  	if list.first != nil {
  1587  		// The list contains at least one span; link it in.
  1588  		// The last span in the list doesn't change.
  1589  		list.first.prev = span
  1590  	} else {
  1591  		// The list contains no spans, so this is also the last span.
  1592  		list.last = span
  1593  	}
  1594  	list.first = span
  1595  	span.list = list
  1596  }
  1597  
  1598  func (list *mSpanList) insertBack(span *mspan) {
  1599  	if span.next != nil || span.prev != nil || span.list != nil {
  1600  		println("runtime: failed mSpanList.insertBack", span, span.next, span.prev, span.list)
  1601  		throw("mSpanList.insertBack")
  1602  	}
  1603  	span.prev = list.last
  1604  	if list.last != nil {
  1605  		// The list contains at least one span.
  1606  		list.last.next = span
  1607  	} else {
  1608  		// The list contains no spans, so this is also the first span.
  1609  		list.first = span
  1610  	}
  1611  	list.last = span
  1612  	span.list = list
  1613  }
  1614  
  1615  // takeAll removes all spans from other and inserts them at the front
  1616  // of list.
  1617  func (list *mSpanList) takeAll(other *mSpanList) {
  1618  	if other.isEmpty() {
  1619  		return
  1620  	}
  1621  
  1622  	// Reparent everything in other to list.
  1623  	for s := other.first; s != nil; s = s.next {
  1624  		s.list = list
  1625  	}
  1626  
  1627  	// Concatenate the lists.
  1628  	if list.isEmpty() {
  1629  		*list = *other
  1630  	} else {
  1631  		// Neither list is empty. Put other before list.
  1632  		other.last.next = list.first
  1633  		list.first.prev = other.last
  1634  		list.first = other.first
  1635  	}
  1636  
  1637  	other.first, other.last = nil, nil
  1638  }
  1639  
  1640  const (
  1641  	_KindSpecialFinalizer = 1
  1642  	_KindSpecialProfile   = 2
  1643  	// Note: The finalizer special must be first because if we're freeing
  1644  	// an object, a finalizer special will cause the freeing operation
  1645  	// to abort, and we want to keep the other special records around
  1646  	// if that happens.
  1647  )
  1648  
  1649  //go:notinheap
  1650  type special struct {
  1651  	next   *special // linked list in span
  1652  	offset uint16   // span offset of object
  1653  	kind   byte     // kind of special
  1654  }
  1655  
  1656  // spanHasSpecials marks a span as having specials in the arena bitmap.
  1657  func spanHasSpecials(s *mspan) {
  1658  	arenaPage := (s.base() / pageSize) % pagesPerArena
  1659  	ai := arenaIndex(s.base())
  1660  	ha := mheap_.arenas[ai.l1()][ai.l2()]
  1661  	atomic.Or8(&ha.pageSpecials[arenaPage/8], uint8(1)<<(arenaPage%8))
  1662  }
  1663  
  1664  // spanHasNoSpecials marks a span as having no specials in the arena bitmap.
  1665  func spanHasNoSpecials(s *mspan) {
  1666  	arenaPage := (s.base() / pageSize) % pagesPerArena
  1667  	ai := arenaIndex(s.base())
  1668  	ha := mheap_.arenas[ai.l1()][ai.l2()]
  1669  	atomic.And8(&ha.pageSpecials[arenaPage/8], ^(uint8(1) << (arenaPage % 8)))
  1670  }
  1671  
  1672  // Adds the special record s to the list of special records for
  1673  // the object p. All fields of s should be filled in except for
  1674  // offset & next, which this routine will fill in.
  1675  // Returns true if the special was successfully added, false otherwise.
  1676  // (The add will fail only if a record with the same p and s->kind
  1677  //  already exists.)
  1678  func addspecial(p unsafe.Pointer, s *special) bool {
  1679  	span := spanOfHeap(uintptr(p))
  1680  	if span == nil {
  1681  		throw("addspecial on invalid pointer")
  1682  	}
  1683  
  1684  	// Ensure that the span is swept.
  1685  	// Sweeping accesses the specials list w/o locks, so we have
  1686  	// to synchronize with it. And it's just much safer.
  1687  	mp := acquirem()
  1688  	span.ensureSwept()
  1689  
  1690  	offset := uintptr(p) - span.base()
  1691  	kind := s.kind
  1692  
  1693  	lock(&span.speciallock)
  1694  
  1695  	// Find splice point, check for existing record.
  1696  	t := &span.specials
  1697  	for {
  1698  		x := *t
  1699  		if x == nil {
  1700  			break
  1701  		}
  1702  		if offset == uintptr(x.offset) && kind == x.kind {
  1703  			unlock(&span.speciallock)
  1704  			releasem(mp)
  1705  			return false // already exists
  1706  		}
  1707  		if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) {
  1708  			break
  1709  		}
  1710  		t = &x.next
  1711  	}
  1712  
  1713  	// Splice in record, fill in offset.
  1714  	s.offset = uint16(offset)
  1715  	s.next = *t
  1716  	*t = s
  1717  	spanHasSpecials(span)
  1718  	unlock(&span.speciallock)
  1719  	releasem(mp)
  1720  
  1721  	return true
  1722  }
  1723  
  1724  // Removes the Special record of the given kind for the object p.
  1725  // Returns the record if the record existed, nil otherwise.
  1726  // The caller must FixAlloc_Free the result.
  1727  func removespecial(p unsafe.Pointer, kind uint8) *special {
  1728  	span := spanOfHeap(uintptr(p))
  1729  	if span == nil {
  1730  		throw("removespecial on invalid pointer")
  1731  	}
  1732  
  1733  	// Ensure that the span is swept.
  1734  	// Sweeping accesses the specials list w/o locks, so we have
  1735  	// to synchronize with it. And it's just much safer.
  1736  	mp := acquirem()
  1737  	span.ensureSwept()
  1738  
  1739  	offset := uintptr(p) - span.base()
  1740  
  1741  	var result *special
  1742  	lock(&span.speciallock)
  1743  	t := &span.specials
  1744  	for {
  1745  		s := *t
  1746  		if s == nil {
  1747  			break
  1748  		}
  1749  		// This function is used for finalizers only, so we don't check for
  1750  		// "interior" specials (p must be exactly equal to s->offset).
  1751  		if offset == uintptr(s.offset) && kind == s.kind {
  1752  			*t = s.next
  1753  			result = s
  1754  			break
  1755  		}
  1756  		t = &s.next
  1757  	}
  1758  	if span.specials == nil {
  1759  		spanHasNoSpecials(span)
  1760  	}
  1761  	unlock(&span.speciallock)
  1762  	releasem(mp)
  1763  	return result
  1764  }
  1765  
  1766  // The described object has a finalizer set for it.
  1767  //
  1768  // specialfinalizer is allocated from non-GC'd memory, so any heap
  1769  // pointers must be specially handled.
  1770  //
  1771  //go:notinheap
  1772  type specialfinalizer struct {
  1773  	special special
  1774  	fn      *funcval // May be a heap pointer.
  1775  	nret    uintptr
  1776  	fint    *_type   // May be a heap pointer, but always live.
  1777  	ot      *ptrtype // May be a heap pointer, but always live.
  1778  }
  1779  
  1780  // Adds a finalizer to the object p. Returns true if it succeeded.
  1781  func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
  1782  	lock(&mheap_.speciallock)
  1783  	s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc())
  1784  	unlock(&mheap_.speciallock)
  1785  	s.special.kind = _KindSpecialFinalizer
  1786  	s.fn = f
  1787  	s.nret = nret
  1788  	s.fint = fint
  1789  	s.ot = ot
  1790  	if addspecial(p, &s.special) {
  1791  		// This is responsible for maintaining the same
  1792  		// GC-related invariants as markrootSpans in any
  1793  		// situation where it's possible that markrootSpans
  1794  		// has already run but mark termination hasn't yet.
  1795  		if gcphase != _GCoff {
  1796  			base, _, _ := findObject(uintptr(p), 0, 0)
  1797  			mp := acquirem()
  1798  			gcw := &mp.p.ptr().gcw
  1799  			// Mark everything reachable from the object
  1800  			// so it's retained for the finalizer.
  1801  			scanobject(base, gcw)
  1802  			// Mark the finalizer itself, since the
  1803  			// special isn't part of the GC'd heap.
  1804  			scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
  1805  			releasem(mp)
  1806  		}
  1807  		return true
  1808  	}
  1809  
  1810  	// There was an old finalizer
  1811  	lock(&mheap_.speciallock)
  1812  	mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
  1813  	unlock(&mheap_.speciallock)
  1814  	return false
  1815  }
  1816  
  1817  // Removes the finalizer (if any) from the object p.
  1818  func removefinalizer(p unsafe.Pointer) {
  1819  	s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
  1820  	if s == nil {
  1821  		return // there wasn't a finalizer to remove
  1822  	}
  1823  	lock(&mheap_.speciallock)
  1824  	mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
  1825  	unlock(&mheap_.speciallock)
  1826  }
  1827  
  1828  // The described object is being heap profiled.
  1829  //
  1830  //go:notinheap
  1831  type specialprofile struct {
  1832  	special special
  1833  	b       *bucket
  1834  }
  1835  
  1836  // Set the heap profile bucket associated with addr to b.
  1837  func setprofilebucket(p unsafe.Pointer, b *bucket) {
  1838  	lock(&mheap_.speciallock)
  1839  	s := (*specialprofile)(mheap_.specialprofilealloc.alloc())
  1840  	unlock(&mheap_.speciallock)
  1841  	s.special.kind = _KindSpecialProfile
  1842  	s.b = b
  1843  	if !addspecial(p, &s.special) {
  1844  		throw("setprofilebucket: profile already set")
  1845  	}
  1846  }
  1847  
  1848  // Do whatever cleanup needs to be done to deallocate s. It has
  1849  // already been unlinked from the mspan specials list.
  1850  func freespecial(s *special, p unsafe.Pointer, size uintptr) {
  1851  	switch s.kind {
  1852  	case _KindSpecialFinalizer:
  1853  		sf := (*specialfinalizer)(unsafe.Pointer(s))
  1854  		queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot)
  1855  		lock(&mheap_.speciallock)
  1856  		mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf))
  1857  		unlock(&mheap_.speciallock)
  1858  	case _KindSpecialProfile:
  1859  		sp := (*specialprofile)(unsafe.Pointer(s))
  1860  		mProf_Free(sp.b, size)
  1861  		lock(&mheap_.speciallock)
  1862  		mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
  1863  		unlock(&mheap_.speciallock)
  1864  	default:
  1865  		throw("bad special kind")
  1866  		panic("not reached")
  1867  	}
  1868  }
  1869  
  1870  // gcBits is an alloc/mark bitmap. This is always used as *gcBits.
  1871  //
  1872  //go:notinheap
  1873  type gcBits uint8
  1874  
  1875  // bytep returns a pointer to the n'th byte of b.
  1876  func (b *gcBits) bytep(n uintptr) *uint8 {
  1877  	return addb((*uint8)(b), n)
  1878  }
  1879  
  1880  // bitp returns a pointer to the byte containing bit n and a mask for
  1881  // selecting that bit from *bytep.
  1882  func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) {
  1883  	return b.bytep(n / 8), 1 << (n % 8)
  1884  }
  1885  
  1886  const gcBitsChunkBytes = uintptr(64 << 10)
  1887  const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
  1888  
  1889  type gcBitsHeader struct {
  1890  	free uintptr // free is the index into bits of the next free byte.
  1891  	next uintptr // *gcBits triggers recursive type bug. (issue 14620)
  1892  }
  1893  
  1894  //go:notinheap
  1895  type gcBitsArena struct {
  1896  	// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
  1897  	free uintptr // free is the index into bits of the next free byte; read/write atomically
  1898  	next *gcBitsArena
  1899  	bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits
  1900  }
  1901  
  1902  var gcBitsArenas struct {
  1903  	lock     mutex
  1904  	free     *gcBitsArena
  1905  	next     *gcBitsArena // Read atomically. Write atomically under lock.
  1906  	current  *gcBitsArena
  1907  	previous *gcBitsArena
  1908  }
  1909  
  1910  // tryAlloc allocates from b or returns nil if b does not have enough room.
  1911  // This is safe to call concurrently.
  1912  func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits {
  1913  	if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) {
  1914  		return nil
  1915  	}
  1916  	// Try to allocate from this block.
  1917  	end := atomic.Xadduintptr(&b.free, bytes)
  1918  	if end > uintptr(len(b.bits)) {
  1919  		return nil
  1920  	}
  1921  	// There was enough room.
  1922  	start := end - bytes
  1923  	return &b.bits[start]
  1924  }
  1925  
  1926  // newMarkBits returns a pointer to 8 byte aligned bytes
  1927  // to be used for a span's mark bits.
  1928  func newMarkBits(nelems uintptr) *gcBits {
  1929  	blocksNeeded := uintptr((nelems + 63) / 64)
  1930  	bytesNeeded := blocksNeeded * 8
  1931  
  1932  	// Try directly allocating from the current head arena.
  1933  	head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next)))
  1934  	if p := head.tryAlloc(bytesNeeded); p != nil {
  1935  		return p
  1936  	}
  1937  
  1938  	// There's not enough room in the head arena. We may need to
  1939  	// allocate a new arena.
  1940  	lock(&gcBitsArenas.lock)
  1941  	// Try the head arena again, since it may have changed. Now
  1942  	// that we hold the lock, the list head can't change, but its
  1943  	// free position still can.
  1944  	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
  1945  		unlock(&gcBitsArenas.lock)
  1946  		return p
  1947  	}
  1948  
  1949  	// Allocate a new arena. This may temporarily drop the lock.
  1950  	fresh := newArenaMayUnlock()
  1951  	// If newArenaMayUnlock dropped the lock, another thread may
  1952  	// have put a fresh arena on the "next" list. Try allocating
  1953  	// from next again.
  1954  	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
  1955  		// Put fresh back on the free list.
  1956  		// TODO: Mark it "already zeroed"
  1957  		fresh.next = gcBitsArenas.free
  1958  		gcBitsArenas.free = fresh
  1959  		unlock(&gcBitsArenas.lock)
  1960  		return p
  1961  	}
  1962  
  1963  	// Allocate from the fresh arena. We haven't linked it in yet, so
  1964  	// this cannot race and is guaranteed to succeed.
  1965  	p := fresh.tryAlloc(bytesNeeded)
  1966  	if p == nil {
  1967  		throw("markBits overflow")
  1968  	}
  1969  
  1970  	// Add the fresh arena to the "next" list.
  1971  	fresh.next = gcBitsArenas.next
  1972  	atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh))
  1973  
  1974  	unlock(&gcBitsArenas.lock)
  1975  	return p
  1976  }
  1977  
  1978  // newAllocBits returns a pointer to 8 byte aligned bytes
  1979  // to be used for this span's alloc bits.
  1980  // newAllocBits is used to provide newly initialized spans
  1981  // allocation bits. For spans not being initialized the
  1982  // mark bits are repurposed as allocation bits when
  1983  // the span is swept.
  1984  func newAllocBits(nelems uintptr) *gcBits {
  1985  	return newMarkBits(nelems)
  1986  }
  1987  
  1988  // nextMarkBitArenaEpoch establishes a new epoch for the arenas
  1989  // holding the mark bits. The arenas are named relative to the
  1990  // current GC cycle which is demarcated by the call to finishweep_m.
  1991  //
  1992  // All current spans have been swept.
  1993  // During that sweep each span allocated room for its gcmarkBits in
  1994  // gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current
  1995  // where the GC will mark objects and after each span is swept these bits
  1996  // will be used to allocate objects.
  1997  // gcBitsArenas.current becomes gcBitsArenas.previous where the span's
  1998  // gcAllocBits live until all the spans have been swept during this GC cycle.
  1999  // The span's sweep extinguishes all the references to gcBitsArenas.previous
  2000  // by pointing gcAllocBits into the gcBitsArenas.current.
  2001  // The gcBitsArenas.previous is released to the gcBitsArenas.free list.
  2002  func nextMarkBitArenaEpoch() {
  2003  	lock(&gcBitsArenas.lock)
  2004  	if gcBitsArenas.previous != nil {
  2005  		if gcBitsArenas.free == nil {
  2006  			gcBitsArenas.free = gcBitsArenas.previous
  2007  		} else {
  2008  			// Find end of previous arenas.
  2009  			last := gcBitsArenas.previous
  2010  			for last = gcBitsArenas.previous; last.next != nil; last = last.next {
  2011  			}
  2012  			last.next = gcBitsArenas.free
  2013  			gcBitsArenas.free = gcBitsArenas.previous
  2014  		}
  2015  	}
  2016  	gcBitsArenas.previous = gcBitsArenas.current
  2017  	gcBitsArenas.current = gcBitsArenas.next
  2018  	atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed
  2019  	unlock(&gcBitsArenas.lock)
  2020  }
  2021  
  2022  // newArenaMayUnlock allocates and zeroes a gcBits arena.
  2023  // The caller must hold gcBitsArena.lock. This may temporarily release it.
  2024  func newArenaMayUnlock() *gcBitsArena {
  2025  	var result *gcBitsArena
  2026  	if gcBitsArenas.free == nil {
  2027  		unlock(&gcBitsArenas.lock)
  2028  		result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gcMiscSys))
  2029  		if result == nil {
  2030  			throw("runtime: cannot allocate memory")
  2031  		}
  2032  		lock(&gcBitsArenas.lock)
  2033  	} else {
  2034  		result = gcBitsArenas.free
  2035  		gcBitsArenas.free = gcBitsArenas.free.next
  2036  		memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes)
  2037  	}
  2038  	result.next = nil
  2039  	// If result.bits is not 8 byte aligned adjust index so
  2040  	// that &result.bits[result.free] is 8 byte aligned.
  2041  	if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 {
  2042  		result.free = 0
  2043  	} else {
  2044  		result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7)
  2045  	}
  2046  	return result
  2047  }
  2048  

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