// Copyright 2009 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. // Page heap. // // See malloc.go for overview. package runtime import ( "runtime/internal/atomic" "runtime/internal/sys" "unsafe" ) // minPhysPageSize is a lower-bound on the physical page size. The // true physical page size may be larger than this. In contrast, // sys.PhysPageSize is an upper-bound on the physical page size. const minPhysPageSize = 4096 // Main malloc heap. // The heap itself is the "free[]" and "large" arrays, // but all the other global data is here too. // // mheap must not be heap-allocated because it contains mSpanLists, // which must not be heap-allocated. // //go:notinheap type mheap struct { lock mutex free [_MaxMHeapList]mSpanList // free lists of given length up to _MaxMHeapList freelarge mTreap // free treap of length >= _MaxMHeapList busy [_MaxMHeapList]mSpanList // busy lists of large spans of given length busylarge mSpanList // busy lists of large spans length >= _MaxMHeapList sweepgen uint32 // sweep generation, see comment in mspan sweepdone uint32 // all spans are swept sweepers uint32 // number of active sweepone calls // allspans is a slice of all mspans ever created. Each mspan // appears exactly once. // // The memory for allspans is manually managed and can be // reallocated and move as the heap grows. // // In general, allspans is protected by mheap_.lock, which // prevents concurrent access as well as freeing the backing // store. Accesses during STW might not hold the lock, but // must ensure that allocation cannot happen around the // access (since that may free the backing store). allspans []*mspan // all spans out there // sweepSpans contains two mspan stacks: one of swept in-use // spans, and one of unswept in-use spans. These two trade // roles on each GC cycle. Since the sweepgen increases by 2 // on each cycle, this means the swept spans are in // sweepSpans[sweepgen/2%2] and the unswept spans are in // sweepSpans[1-sweepgen/2%2]. Sweeping pops spans from the // unswept stack and pushes spans that are still in-use on the // swept stack. Likewise, allocating an in-use span pushes it // on the swept stack. sweepSpans [2]gcSweepBuf //_ uint32 // align uint64 fields on 32-bit for atomics // Proportional sweep // // These parameters represent a linear function from heap_live // to page sweep count. The proportional sweep system works to // stay in the black by keeping the current page sweep count // above this line at the current heap_live. // // The line has slope sweepPagesPerByte and passes through a // basis point at (sweepHeapLiveBasis, pagesSweptBasis). At // any given time, the system is at (memstats.heap_live, // pagesSwept) in this space. // // It's important that the line pass through a point we // control rather than simply starting at a (0,0) origin // because that lets us adjust sweep pacing at any time while // accounting for current progress. If we could only adjust // the slope, it would create a discontinuity in debt if any // progress has already been made. pagesInUse uint64 // pages of spans in stats _MSpanInUse; R/W with mheap.lock pagesSwept uint64 // pages swept this cycle; updated atomically pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without // TODO(austin): pagesInUse should be a uintptr, but the 386 // compiler can't 8-byte align fields. // Malloc stats. largealloc uint64 // bytes allocated for large objects nlargealloc uint64 // number of large object allocations largefree uint64 // bytes freed for large objects (>maxsmallsize) nlargefree uint64 // number of frees for large objects (>maxsmallsize) nsmallfree [_NumSizeClasses]uint64 // number of frees for small objects (<=maxsmallsize) // arenas is the heap arena map. It points to the metadata for // the heap for every arena frame of the entire usable virtual // address space. // // Use arenaIndex to compute indexes into this array. // // For regions of the address space that are not backed by the // Go heap, the arena map contains nil. // // Modifications are protected by mheap_.lock. Reads can be // performed without locking; however, a given entry can // transition from nil to non-nil at any time when the lock // isn't held. (Entries never transitions back to nil.) // // In general, this is a two-level mapping consisting of an L1 // map and possibly many L2 maps. This saves space when there // are a huge number of arena frames. However, on many // platforms (even 64-bit), arenaL1Bits is 0, making this // effectively a single-level map. In this case, arenas[0] // will never be nil. arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena // heapArenaAlloc is pre-reserved space for allocating heapArena // objects. This is only used on 32-bit, where we pre-reserve // this space to avoid interleaving it with the heap itself. heapArenaAlloc linearAlloc // arenaHints is a list of addresses at which to attempt to // add more heap arenas. This is initially populated with a // set of general hint addresses, and grown with the bounds of // actual heap arena ranges. arenaHints *arenaHint // arena is a pre-reserved space for allocating heap arenas // (the actual arenas). This is only used on 32-bit. arena linearAlloc //_ uint32 // ensure 64-bit alignment of central // central free lists for small size classes. // the padding makes sure that the MCentrals are // spaced CacheLineSize bytes apart, so that each MCentral.lock // gets its own cache line. // central is indexed by spanClass. central [numSpanClasses]struct { mcentral mcentral pad [sys.CacheLineSize - unsafe.Sizeof(mcentral{})%sys.CacheLineSize]byte } spanalloc fixalloc // allocator for span* cachealloc fixalloc // allocator for mcache* treapalloc fixalloc // allocator for treapNodes* used by large objects specialfinalizeralloc fixalloc // allocator for specialfinalizer* specialprofilealloc fixalloc // allocator for specialprofile* speciallock mutex // lock for special record allocators. arenaHintAlloc fixalloc // allocator for arenaHints unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF } var mheap_ mheap // A heapArena stores metadata for a heap arena. heapArenas are stored // outside of the Go heap and accessed via the mheap_.arenas index. // // This gets allocated directly from the OS, so ideally it should be a // multiple of the system page size. For example, avoid adding small // fields. // //go:notinheap type heapArena struct { // bitmap stores the pointer/scalar bitmap for the words in // this arena. See mbitmap.go for a description. Use the // heapBits type to access this. bitmap [heapArenaBitmapBytes]byte // spans maps from virtual address page ID within this arena to *mspan. // For allocated spans, their pages map to the span itself. // For free spans, only the lowest and highest pages map to the span itself. // Internal pages map to an arbitrary span. // For pages that have never been allocated, spans entries are nil. // // Modifications are protected by mheap.lock. Reads can be // performed without locking, but ONLY from indexes that are // known to contain in-use or stack spans. This means there // must not be a safe-point between establishing that an // address is live and looking it up in the spans array. spans [pagesPerArena]*mspan } // arenaHint is a hint for where to grow the heap arenas. See // mheap_.arenaHints. // //go:notinheap type arenaHint struct { addr uintptr down bool next *arenaHint } // An MSpan is a run of pages. // // When a MSpan is in the heap free list, state == MSpanFree // and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span. // // When a MSpan is allocated, state == MSpanInUse or MSpanManual // and heapmap(i) == span for all s->start <= i < s->start+s->npages. // Every MSpan is in one doubly-linked list, // either one of the MHeap's free lists or one of the // MCentral's span lists. // An MSpan representing actual memory has state _MSpanInUse, // _MSpanManual, or _MSpanFree. Transitions between these states are // constrained as follows: // // * A span may transition from free to in-use or manual during any GC // phase. // // * During sweeping (gcphase == _GCoff), a span may transition from // in-use to free (as a result of sweeping) or manual to free (as a // result of stacks being freed). // // * During GC (gcphase != _GCoff), a span *must not* transition from // manual or in-use to free. Because concurrent GC may read a pointer // and then look up its span, the span state must be monotonic. type mSpanState uint8 const ( _MSpanDead mSpanState = iota _MSpanInUse // allocated for garbage collected heap _MSpanManual // allocated for manual management (e.g., stack allocator) _MSpanFree ) // mSpanStateNames are the names of the span states, indexed by // mSpanState. var mSpanStateNames = []string{ "_MSpanDead", "_MSpanInUse", "_MSpanManual", "_MSpanFree", } // mSpanList heads a linked list of spans. // //go:notinheap type mSpanList struct { first *mspan // first span in list, or nil if none last *mspan // last span in list, or nil if none } //go:notinheap type mspan struct { next *mspan // next span in list, or nil if none prev *mspan // previous span in list, or nil if none list *mSpanList // For debugging. TODO: Remove. startAddr uintptr // address of first byte of span aka s.base() npages uintptr // number of pages in span manualFreeList gclinkptr // list of free objects in _MSpanManual spans // freeindex is the slot index between 0 and nelems at which to begin scanning // for the next free object in this span. // Each allocation scans allocBits starting at freeindex until it encounters a 0 // indicating a free object. freeindex is then adjusted so that subsequent scans begin // just past the newly discovered free object. // // If freeindex == nelem, this span has no free objects. // // allocBits is a bitmap of objects in this span. // If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0 // then object n is free; // otherwise, object n is allocated. Bits starting at nelem are // undefined and should never be referenced. // // Object n starts at address n*elemsize + (start << pageShift). freeindex uintptr // TODO: Look up nelems from sizeclass and remove this field if it // helps performance. nelems uintptr // number of object in the span. // Cache of the allocBits at freeindex. allocCache is shifted // such that the lowest bit corresponds to the bit freeindex. // allocCache holds the complement of allocBits, thus allowing // ctz (count trailing zero) to use it directly. // allocCache may contain bits beyond s.nelems; the caller must ignore // these. allocCache uint64 // allocBits and gcmarkBits hold pointers to a span's mark and // allocation bits. The pointers are 8 byte aligned. // There are three arenas where this data is held. // free: Dirty arenas that are no longer accessed // and can be reused. // next: Holds information to be used in the next GC cycle. // current: Information being used during this GC cycle. // previous: Information being used during the last GC cycle. // A new GC cycle starts with the call to finishsweep_m. // finishsweep_m moves the previous arena to the free arena, // the current arena to the previous arena, and // the next arena to the current arena. // The next arena is populated as the spans request // memory to hold gcmarkBits for the next GC cycle as well // as allocBits for newly allocated spans. // // The pointer arithmetic is done "by hand" instead of using // arrays to avoid bounds checks along critical performance // paths. // The sweep will free the old allocBits and set allocBits to the // gcmarkBits. The gcmarkBits are replaced with a fresh zeroed // out memory. allocBits *gcBits gcmarkBits *gcBits // sweep generation: // if sweepgen == h->sweepgen - 2, the span needs sweeping // if sweepgen == h->sweepgen - 1, the span is currently being swept // if sweepgen == h->sweepgen, the span is swept and ready to use // h->sweepgen is incremented by 2 after every GC sweepgen uint32 divMul uint16 // for divide by elemsize - divMagic.mul baseMask uint16 // if non-0, elemsize is a power of 2, & this will get object allocation base allocCount uint16 // number of allocated objects spanclass spanClass // size class and noscan (uint8) incache bool // being used by an mcache state mSpanState // mspaninuse etc needzero uint8 // needs to be zeroed before allocation divShift uint8 // for divide by elemsize - divMagic.shift divShift2 uint8 // for divide by elemsize - divMagic.shift2 elemsize uintptr // computed from sizeclass or from npages unusedsince int64 // first time spotted by gc in mspanfree state npreleased uintptr // number of pages released to the os limit uintptr // end of data in span speciallock mutex // guards specials list specials *special // linked list of special records sorted by offset. } func (s *mspan) base() uintptr { return s.startAddr } func (s *mspan) layout() (size, n, total uintptr) { total = s.npages << _PageShift size = s.elemsize if size > 0 { n = total / size } return } // recordspan adds a newly allocated span to h.allspans. // // This only happens the first time a span is allocated from // mheap.spanalloc (it is not called when a span is reused). // // Write barriers are disallowed here because it can be called from // gcWork when allocating new workbufs. However, because it's an // indirect call from the fixalloc initializer, the compiler can't see // this. // //go:nowritebarrierrec func recordspan(vh unsafe.Pointer, p unsafe.Pointer) { h := (*mheap)(vh) s := (*mspan)(p) if len(h.allspans) >= cap(h.allspans) { n := 64 * 1024 / sys.PtrSize if n < cap(h.allspans)*3/2 { n = cap(h.allspans) * 3 / 2 } var new []*mspan sp := (*slice)(unsafe.Pointer(&new)) sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys) if sp.array == nil { throw("runtime: cannot allocate memory") } sp.len = len(h.allspans) sp.cap = n if len(h.allspans) > 0 { copy(new, h.allspans) } oldAllspans := h.allspans *(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new)) if len(oldAllspans) != 0 { sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys) } } h.allspans = h.allspans[:len(h.allspans)+1] h.allspans[len(h.allspans)-1] = s } // A spanClass represents the size class and noscan-ness of a span. // // Each size class has a noscan spanClass and a scan spanClass. The // noscan spanClass contains only noscan objects, which do not contain // pointers and thus do not need to be scanned by the garbage // collector. type spanClass uint8 const ( numSpanClasses = _NumSizeClasses << 1 tinySpanClass = spanClass(tinySizeClass<<1 | 1) ) func makeSpanClass(sizeclass uint8, noscan bool) spanClass { return spanClass(sizeclass<<1) | spanClass(bool2int(noscan)) } func (sc spanClass) sizeclass() int8 { return int8(sc >> 1) } func (sc spanClass) noscan() bool { return sc&1 != 0 } // arenaIndex returns the index into mheap_.arenas of the arena // containing metadata for p. This index combines of an index into the // L1 map and an index into the L2 map and should be used as // mheap_.arenas[ai.l1()][ai.l2()]. // // If p is outside the range of valid heap addresses, either l1() or // l2() will be out of bounds. // // It is nosplit because it's called by spanOf and several other // nosplit functions. // //go:nosplit func arenaIndex(p uintptr) arenaIdx { return arenaIdx((p + arenaBaseOffset) / heapArenaBytes) } // arenaBase returns the low address of the region covered by heap // arena i. func arenaBase(i arenaIdx) uintptr { return uintptr(i)*heapArenaBytes - arenaBaseOffset } type arenaIdx uint func (i arenaIdx) l1() uint { if arenaL1Bits == 0 { // Let the compiler optimize this away if there's no // L1 map. return 0 } else { return uint(i) >> arenaL1Shift } } func (i arenaIdx) l2() uint { if arenaL1Bits == 0 { return uint(i) } else { return uint(i) & (1<<arenaL2Bits - 1) } } // inheap reports whether b is a pointer into a (potentially dead) heap object. // It returns false for pointers into _MSpanManual spans. // Non-preemptible because it is used by write barriers. //go:nowritebarrier //go:nosplit func inheap(b uintptr) bool { return spanOfHeap(b) != nil } // inHeapOrStack is a variant of inheap that returns true for pointers // into any allocated heap span. // //go:nowritebarrier //go:nosplit func inHeapOrStack(b uintptr) bool { s := spanOf(b) if s == nil || b < s.base() { return false } switch s.state { case mSpanInUse, _MSpanManual: return b < s.limit default: return false } } // spanOf returns the span of p. If p does not point into the heap // arena or no span has ever contained p, spanOf returns nil. // // If p does not point to allocated memory, this may return a non-nil // span that does *not* contain p. If this is a possibility, the // caller should either call spanOfHeap or check the span bounds // explicitly. // // Must be nosplit because it has callers that are nosplit. // //go:nosplit func spanOf(p uintptr) *mspan { // This function looks big, but we use a lot of constant // folding around arenaL1Bits to get it under the inlining // budget. Also, many of the checks here are safety checks // that Go needs to do anyway, so the generated code is quite // short. ri := arenaIndex(p) if arenaL1Bits == 0 { // If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can. if ri.l2() >= uint(len(mheap_.arenas[0])) { return nil } } else { // If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't. if ri.l1() >= uint(len(mheap_.arenas)) { return nil } } l2 := mheap_.arenas[ri.l1()] if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1. return nil } ha := l2[ri.l2()] if ha == nil { return nil } return ha.spans[(p/pageSize)%pagesPerArena] } // spanOfUnchecked is equivalent to spanOf, but the caller must ensure // that p points into an allocated heap arena. // // Must be nosplit because it has callers that are nosplit. // //go:nosplit func spanOfUnchecked(p uintptr) *mspan { ai := arenaIndex(p) return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena] } // spanOfHeap is like spanOf, but returns nil if p does not point to a // heap object. // // Must be nosplit because it has callers that are nosplit. // //go:nosplit func spanOfHeap(p uintptr) *mspan { s := spanOf(p) // If p is not allocated, it may point to a stale span, so we // have to check the span's bounds and state. if s == nil || p < s.base() || p >= s.limit || s.state != mSpanInUse { return nil } return s } // Initialize the heap. func (h *mheap) init() { h.treapalloc.init(unsafe.Sizeof(treapNode{}), nil, nil, &memstats.other_sys) h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys) h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys) h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys) h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys) h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys) // Don't zero mspan allocations. Background sweeping can // inspect a span concurrently with allocating it, so it's // important that the span's sweepgen survive across freeing // and re-allocating a span to prevent background sweeping // from improperly cas'ing it from 0. // // This is safe because mspan contains no heap pointers. h.spanalloc.zero = false // h->mapcache needs no init for i := range h.free { h.free[i].init() h.busy[i].init() } h.busylarge.init() for i := range h.central { h.central[i].mcentral.init(spanClass(i)) } } // Sweeps spans in list until reclaims at least npages into heap. // Returns the actual number of pages reclaimed. func (h *mheap) reclaimList(list *mSpanList, npages uintptr) uintptr { n := uintptr(0) sg := mheap_.sweepgen retry: for s := list.first; s != nil; s = s.next { if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) { list.remove(s) // swept spans are at the end of the list list.insertBack(s) // Puts it back on a busy list. s is not in the treap at this point. unlock(&h.lock) snpages := s.npages if s.sweep(false) { n += snpages } lock(&h.lock) if n >= npages { return n } // the span could have been moved elsewhere goto retry } if s.sweepgen == sg-1 { // the span is being swept by background sweeper, skip continue } // already swept empty span, // all subsequent ones must also be either swept or in process of sweeping break } return n } // Sweeps and reclaims at least npage pages into heap. // Called before allocating npage pages. func (h *mheap) reclaim(npage uintptr) { // First try to sweep busy spans with large objects of size >= npage, // this has good chances of reclaiming the necessary space. for i := int(npage); i < len(h.busy); i++ { if h.reclaimList(&h.busy[i], npage) != 0 { return // Bingo! } } // Then -- even larger objects. if h.reclaimList(&h.busylarge, npage) != 0 { return // Bingo! } // Now try smaller objects. // One such object is not enough, so we need to reclaim several of them. reclaimed := uintptr(0) for i := 0; i < int(npage) && i < len(h.busy); i++ { reclaimed += h.reclaimList(&h.busy[i], npage-reclaimed) if reclaimed >= npage { return } } // Now sweep everything that is not yet swept. unlock(&h.lock) for { n := sweepone() if n == ^uintptr(0) { // all spans are swept break } reclaimed += n if reclaimed >= npage { break } } lock(&h.lock) } // Allocate a new span of npage pages from the heap for GC'd memory // and record its size class in the HeapMap and HeapMapCache. func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan { _g_ := getg() if _g_ != _g_.m.g0 { throw("_mheap_alloc not on g0 stack") } lock(&h.lock) // To prevent excessive heap growth, before allocating n pages // we need to sweep and reclaim at least n pages. if h.sweepdone == 0 { // TODO(austin): This tends to sweep a large number of // spans in order to find a few completely free spans // (for example, in the garbage benchmark, this sweeps // ~30x the number of pages its trying to allocate). // If GC kept a bit for whether there were any marks // in a span, we could release these free spans // at the end of GC and eliminate this entirely. if trace.enabled { traceGCSweepStart() } h.reclaim(npage) if trace.enabled { traceGCSweepDone() } } // transfer stats from cache to global memstats.heap_scan += uint64(_g_.m.mcache.local_scan) _g_.m.mcache.local_scan = 0 memstats.tinyallocs += uint64(_g_.m.mcache.local_tinyallocs) _g_.m.mcache.local_tinyallocs = 0 s := h.allocSpanLocked(npage, &memstats.heap_inuse) if s != nil { // Record span info, because gc needs to be // able to map interior pointer to containing span. atomic.Store(&s.sweepgen, h.sweepgen) h.sweepSpans[h.sweepgen/2%2].push(s) // Add to swept in-use list. s.state = _MSpanInUse s.allocCount = 0 s.spanclass = spanclass if sizeclass := spanclass.sizeclass(); sizeclass == 0 { s.elemsize = s.npages << _PageShift s.divShift = 0 s.divMul = 0 s.divShift2 = 0 s.baseMask = 0 } else { s.elemsize = uintptr(class_to_size[sizeclass]) m := &class_to_divmagic[sizeclass] s.divShift = m.shift s.divMul = m.mul s.divShift2 = m.shift2 s.baseMask = m.baseMask } // update stats, sweep lists h.pagesInUse += uint64(npage) if large { memstats.heap_objects++ mheap_.largealloc += uint64(s.elemsize) mheap_.nlargealloc++ atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift)) // Swept spans are at the end of lists. if s.npages < uintptr(len(h.busy)) { h.busy[s.npages].insertBack(s) } else { h.busylarge.insertBack(s) } } } // heap_scan and heap_live were updated. if gcBlackenEnabled != 0 { gcController.revise() } if trace.enabled { traceHeapAlloc() } // h.spans is accessed concurrently without synchronization // from other threads. Hence, there must be a store/store // barrier here to ensure the writes to h.spans above happen // before the caller can publish a pointer p to an object // allocated from s. As soon as this happens, the garbage // collector running on another processor could read p and // look up s in h.spans. The unlock acts as the barrier to // order these writes. On the read side, the data dependency // between p and the index in h.spans orders the reads. unlock(&h.lock) return s } func (h *mheap) alloc(npage uintptr, spanclass spanClass, large bool, needzero bool) *mspan { // Don't do any operations that lock the heap on the G stack. // It might trigger stack growth, and the stack growth code needs // to be able to allocate heap. var s *mspan systemstack(func() { s = h.alloc_m(npage, spanclass, large) }) if s != nil { if needzero && s.needzero != 0 { memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift) } s.needzero = 0 } return s } // allocManual allocates a manually-managed span of npage pages. // allocManual returns nil if allocation fails. // // allocManual adds the bytes used to *stat, which should be a // memstats in-use field. Unlike allocations in the GC'd heap, the // allocation does *not* count toward heap_inuse or heap_sys. // // The memory backing the returned span may not be zeroed if // span.needzero is set. // // allocManual must be called on the system stack to prevent stack // growth. Since this is used by the stack allocator, stack growth // during allocManual would self-deadlock. // //go:systemstack func (h *mheap) allocManual(npage uintptr, stat *uint64) *mspan { lock(&h.lock) s := h.allocSpanLocked(npage, stat) if s != nil { s.state = _MSpanManual s.manualFreeList = 0 s.allocCount = 0 s.spanclass = 0 s.nelems = 0 s.elemsize = 0 s.limit = s.base() + s.npages<<_PageShift // Manually managed memory doesn't count toward heap_sys. memstats.heap_sys -= uint64(s.npages << _PageShift) } // This unlock acts as a release barrier. See mheap.alloc_m. unlock(&h.lock) return s } // setSpan modifies the span map so spanOf(base) is s. func (h *mheap) setSpan(base uintptr, s *mspan) { ai := arenaIndex(base) h.arenas[ai.l1()][ai.l2()].spans[(base/pageSize)%pagesPerArena] = s } // setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize)) // is s. func (h *mheap) setSpans(base, npage uintptr, s *mspan) { p := base / pageSize ai := arenaIndex(base) ha := h.arenas[ai.l1()][ai.l2()] for n := uintptr(0); n < npage; n++ { i := (p + n) % pagesPerArena if i == 0 { ai = arenaIndex(base + n*pageSize) ha = h.arenas[ai.l1()][ai.l2()] } ha.spans[i] = s } } // Allocates a span of the given size. h must be locked. // The returned span has been removed from the // free list, but its state is still MSpanFree. func (h *mheap) allocSpanLocked(npage uintptr, stat *uint64) *mspan { var list *mSpanList var s *mspan // Try in fixed-size lists up to max. for i := int(npage); i < len(h.free); i++ { list = &h.free[i] if !list.isEmpty() { s = list.first list.remove(s) goto HaveSpan } } // Best fit in list of large spans. s = h.allocLarge(npage) // allocLarge removed s from h.freelarge for us if s == nil { if !h.grow(npage) { return nil } s = h.allocLarge(npage) if s == nil { return nil } } HaveSpan: // Mark span in use. if s.state != _MSpanFree { throw("MHeap_AllocLocked - MSpan not free") } if s.npages < npage { throw("MHeap_AllocLocked - bad npages") } if s.npreleased > 0 { sysUsed(unsafe.Pointer(s.base()), s.npages<<_PageShift) memstats.heap_released -= uint64(s.npreleased << _PageShift) s.npreleased = 0 } if s.npages > npage { // Trim extra and put it back in the heap. t := (*mspan)(h.spanalloc.alloc()) t.init(s.base()+npage<<_PageShift, s.npages-npage) s.npages = npage h.setSpan(t.base()-1, s) h.setSpan(t.base(), t) h.setSpan(t.base()+t.npages*pageSize-1, t) t.needzero = s.needzero s.state = _MSpanManual // prevent coalescing with s t.state = _MSpanManual h.freeSpanLocked(t, false, false, s.unusedsince) s.state = _MSpanFree } s.unusedsince = 0 h.setSpans(s.base(), npage, s) *stat += uint64(npage << _PageShift) memstats.heap_idle -= uint64(npage << _PageShift) //println("spanalloc", hex(s.start<<_PageShift)) if s.inList() { throw("still in list") } return s } // Large spans have a minimum size of 1MByte. The maximum number of large spans to support // 1TBytes is 1 million, experimentation using random sizes indicates that the depth of // the tree is less that 2x that of a perfectly balanced tree. For 1TByte can be referenced // by a perfectly balanced tree with a depth of 20. Twice that is an acceptable 40. func (h *mheap) isLargeSpan(npages uintptr) bool { return npages >= uintptr(len(h.free)) } // allocLarge allocates a span of at least npage pages from the treap of large spans. // Returns nil if no such span currently exists. func (h *mheap) allocLarge(npage uintptr) *mspan { // Search treap for smallest span with >= npage pages. return h.freelarge.remove(npage) } // Try to add at least npage pages of memory to the heap, // returning whether it worked. // // h must be locked. func (h *mheap) grow(npage uintptr) bool { ask := npage << _PageShift v, size := h.sysAlloc(ask) if v == nil { print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n") return false } // Create a fake "in use" span and free it, so that the // right coalescing happens. s := (*mspan)(h.spanalloc.alloc()) s.init(uintptr(v), size/pageSize) h.setSpans(s.base(), s.npages, s) atomic.Store(&s.sweepgen, h.sweepgen) s.state = _MSpanInUse h.pagesInUse += uint64(s.npages) h.freeSpanLocked(s, false, true, 0) return true } // Free the span back into the heap. func (h *mheap) freeSpan(s *mspan, acct int32) { systemstack(func() { mp := getg().m lock(&h.lock) memstats.heap_scan += uint64(mp.mcache.local_scan) mp.mcache.local_scan = 0 memstats.tinyallocs += uint64(mp.mcache.local_tinyallocs) mp.mcache.local_tinyallocs = 0 if msanenabled { // Tell msan that this entire span is no longer in use. base := unsafe.Pointer(s.base()) bytes := s.npages << _PageShift msanfree(base, bytes) } if acct != 0 { memstats.heap_objects-- } if gcBlackenEnabled != 0 { // heap_scan changed. gcController.revise() } h.freeSpanLocked(s, true, true, 0) unlock(&h.lock) }) } // freeManual frees a manually-managed span returned by allocManual. // stat must be the same as the stat passed to the allocManual that // allocated s. // // This must only be called when gcphase == _GCoff. See mSpanState for // an explanation. // // freeManual must be called on the system stack to prevent stack // growth, just like allocManual. // //go:systemstack func (h *mheap) freeManual(s *mspan, stat *uint64) { s.needzero = 1 lock(&h.lock) *stat -= uint64(s.npages << _PageShift) memstats.heap_sys += uint64(s.npages << _PageShift) h.freeSpanLocked(s, false, true, 0) unlock(&h.lock) } // s must be on a busy list (h.busy or h.busylarge) or unlinked. func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool, unusedsince int64) { switch s.state { case _MSpanManual: if s.allocCount != 0 { throw("MHeap_FreeSpanLocked - invalid stack free") } case _MSpanInUse: if s.allocCount != 0 || s.sweepgen != h.sweepgen { print("MHeap_FreeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n") throw("MHeap_FreeSpanLocked - invalid free") } h.pagesInUse -= uint64(s.npages) default: throw("MHeap_FreeSpanLocked - invalid span state") } if acctinuse { memstats.heap_inuse -= uint64(s.npages << _PageShift) } if acctidle { memstats.heap_idle += uint64(s.npages << _PageShift) } s.state = _MSpanFree if s.inList() { h.busyList(s.npages).remove(s) } // Stamp newly unused spans. The scavenger will use that // info to potentially give back some pages to the OS. s.unusedsince = unusedsince if unusedsince == 0 { s.unusedsince = nanotime() } s.npreleased = 0 // Coalesce with earlier, later spans. if before := spanOf(s.base() - 1); before != nil && before.state == _MSpanFree { // Now adjust s. s.startAddr = before.startAddr s.npages += before.npages s.npreleased = before.npreleased // absorb released pages s.needzero |= before.needzero h.setSpan(before.base(), s) // The size is potentially changing so the treap needs to delete adjacent nodes and // insert back as a combined node. if h.isLargeSpan(before.npages) { // We have a t, it is large so it has to be in the treap so we can remove it. h.freelarge.removeSpan(before) } else { h.freeList(before.npages).remove(before) } before.state = _MSpanDead h.spanalloc.free(unsafe.Pointer(before)) } // Now check to see if next (greater addresses) span is free and can be coalesced. if after := spanOf(s.base() + s.npages*pageSize); after != nil && after.state == _MSpanFree { s.npages += after.npages s.npreleased += after.npreleased s.needzero |= after.needzero h.setSpan(s.base()+s.npages*pageSize-1, s) if h.isLargeSpan(after.npages) { h.freelarge.removeSpan(after) } else { h.freeList(after.npages).remove(after) } after.state = _MSpanDead h.spanalloc.free(unsafe.Pointer(after)) } // Insert s into appropriate list or treap. if h.isLargeSpan(s.npages) { h.freelarge.insert(s) } else { h.freeList(s.npages).insert(s) } } func (h *mheap) freeList(npages uintptr) *mSpanList { return &h.free[npages] } func (h *mheap) busyList(npages uintptr) *mSpanList { if npages < uintptr(len(h.busy)) { return &h.busy[npages] } return &h.busylarge } func scavengeTreapNode(t *treapNode, now, limit uint64) uintptr { s := t.spanKey var sumreleased uintptr if (now-uint64(s.unusedsince)) > limit && s.npreleased != s.npages { start := s.base() end := start + s.npages<<_PageShift if physPageSize > _PageSize { // We can only release pages in // physPageSize blocks, so round start // and end in. (Otherwise, madvise // will round them *out* and release // more memory than we want.) start = (start + physPageSize - 1) &^ (physPageSize - 1) end &^= physPageSize - 1 if end <= start { // start and end don't span a // whole physical page. return sumreleased } } len := end - start released := len - (s.npreleased << _PageShift) if physPageSize > _PageSize && released == 0 { return sumreleased } memstats.heap_released += uint64(released) sumreleased += released s.npreleased = len >> _PageShift sysUnused(unsafe.Pointer(start), len) } return sumreleased } func scavengelist(list *mSpanList, now, limit uint64) uintptr { if list.isEmpty() { return 0 } var sumreleased uintptr for s := list.first; s != nil; s = s.next { if (now-uint64(s.unusedsince)) <= limit || s.npreleased == s.npages { continue } start := s.base() end := start + s.npages<<_PageShift if physPageSize > _PageSize { // We can only release pages in // physPageSize blocks, so round start // and end in. (Otherwise, madvise // will round them *out* and release // more memory than we want.) start = (start + physPageSize - 1) &^ (physPageSize - 1) end &^= physPageSize - 1 if end <= start { // start and end don't span a // whole physical page. continue } } len := end - start released := len - (s.npreleased << _PageShift) if physPageSize > _PageSize && released == 0 { continue } memstats.heap_released += uint64(released) sumreleased += released s.npreleased = len >> _PageShift sysUnused(unsafe.Pointer(start), len) } return sumreleased } func (h *mheap) scavenge(k int32, now, limit uint64) { // Disallow malloc or panic while holding the heap lock. We do // this here because this is an non-mallocgc entry-point to // the mheap API. gp := getg() gp.m.mallocing++ lock(&h.lock) var sumreleased uintptr for i := 0; i < len(h.free); i++ { sumreleased += scavengelist(&h.free[i], now, limit) } sumreleased += scavengetreap(h.freelarge.treap, now, limit) unlock(&h.lock) gp.m.mallocing-- if debug.gctrace > 0 { if sumreleased > 0 { print("scvg", k, ": ", sumreleased>>20, " MB released\n") } print("scvg", k, ": inuse: ", memstats.heap_inuse>>20, ", idle: ", memstats.heap_idle>>20, ", sys: ", memstats.heap_sys>>20, ", released: ", memstats.heap_released>>20, ", consumed: ", (memstats.heap_sys-memstats.heap_released)>>20, " (MB)\n") } } //go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory func runtime_debug_freeOSMemory() { GC() systemstack(func() { mheap_.scavenge(-1, ^uint64(0), 0) }) } // Initialize a new span with the given start and npages. func (span *mspan) init(base uintptr, npages uintptr) { // span is *not* zeroed. span.next = nil span.prev = nil span.list = nil span.startAddr = base span.npages = npages span.allocCount = 0 span.spanclass = 0 span.incache = false span.elemsize = 0 span.state = _MSpanDead span.unusedsince = 0 span.npreleased = 0 span.speciallock.key = 0 span.specials = nil span.needzero = 0 span.freeindex = 0 span.allocBits = nil span.gcmarkBits = nil } func (span *mspan) inList() bool { return span.list != nil } // Initialize an empty doubly-linked list. func (list *mSpanList) init() { list.first = nil list.last = nil } func (list *mSpanList) remove(span *mspan) { if span.list != list { print("runtime: failed MSpanList_Remove span.npages=", span.npages, " span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n") throw("MSpanList_Remove") } if list.first == span { list.first = span.next } else { span.prev.next = span.next } if list.last == span { list.last = span.prev } else { span.next.prev = span.prev } span.next = nil span.prev = nil span.list = nil } func (list *mSpanList) isEmpty() bool { return list.first == nil } func (list *mSpanList) insert(span *mspan) { if span.next != nil || span.prev != nil || span.list != nil { println("runtime: failed MSpanList_Insert", span, span.next, span.prev, span.list) throw("MSpanList_Insert") } span.next = list.first if list.first != nil { // The list contains at least one span; link it in. // The last span in the list doesn't change. list.first.prev = span } else { // The list contains no spans, so this is also the last span. list.last = span } list.first = span span.list = list } func (list *mSpanList) insertBack(span *mspan) { if span.next != nil || span.prev != nil || span.list != nil { println("runtime: failed MSpanList_InsertBack", span, span.next, span.prev, span.list) throw("MSpanList_InsertBack") } span.prev = list.last if list.last != nil { // The list contains at least one span. list.last.next = span } else { // The list contains no spans, so this is also the first span. list.first = span } list.last = span span.list = list } // takeAll removes all spans from other and inserts them at the front // of list. func (list *mSpanList) takeAll(other *mSpanList) { if other.isEmpty() { return } // Reparent everything in other to list. for s := other.first; s != nil; s = s.next { s.list = list } // Concatenate the lists. if list.isEmpty() { *list = *other } else { // Neither list is empty. Put other before list. other.last.next = list.first list.first.prev = other.last list.first = other.first } other.first, other.last = nil, nil } const ( _KindSpecialFinalizer = 1 _KindSpecialProfile = 2 // Note: The finalizer special must be first because if we're freeing // an object, a finalizer special will cause the freeing operation // to abort, and we want to keep the other special records around // if that happens. ) //go:notinheap type special struct { next *special // linked list in span offset uint16 // span offset of object kind byte // kind of special } // Adds the special record s to the list of special records for // the object p. All fields of s should be filled in except for // offset & next, which this routine will fill in. // Returns true if the special was successfully added, false otherwise. // (The add will fail only if a record with the same p and s->kind // already exists.) func addspecial(p unsafe.Pointer, s *special) bool { span := spanOfHeap(uintptr(p)) if span == nil { throw("addspecial on invalid pointer") } // Ensure that the span is swept. // Sweeping accesses the specials list w/o locks, so we have // to synchronize with it. And it's just much safer. mp := acquirem() span.ensureSwept() offset := uintptr(p) - span.base() kind := s.kind lock(&span.speciallock) // Find splice point, check for existing record. t := &span.specials for { x := *t if x == nil { break } if offset == uintptr(x.offset) && kind == x.kind { unlock(&span.speciallock) releasem(mp) return false // already exists } if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) { break } t = &x.next } // Splice in record, fill in offset. s.offset = uint16(offset) s.next = *t *t = s unlock(&span.speciallock) releasem(mp) return true } // Removes the Special record of the given kind for the object p. // Returns the record if the record existed, nil otherwise. // The caller must FixAlloc_Free the result. func removespecial(p unsafe.Pointer, kind uint8) *special { span := spanOfHeap(uintptr(p)) if span == nil { throw("removespecial on invalid pointer") } // Ensure that the span is swept. // Sweeping accesses the specials list w/o locks, so we have // to synchronize with it. And it's just much safer. mp := acquirem() span.ensureSwept() offset := uintptr(p) - span.base() lock(&span.speciallock) t := &span.specials for { s := *t if s == nil { break } // This function is used for finalizers only, so we don't check for // "interior" specials (p must be exactly equal to s->offset). if offset == uintptr(s.offset) && kind == s.kind { *t = s.next unlock(&span.speciallock) releasem(mp) return s } t = &s.next } unlock(&span.speciallock) releasem(mp) return nil } // The described object has a finalizer set for it. // // specialfinalizer is allocated from non-GC'd memory, so any heap // pointers must be specially handled. // //go:notinheap type specialfinalizer struct { special special fn *funcval // May be a heap pointer. nret uintptr fint *_type // May be a heap pointer, but always live. ot *ptrtype // May be a heap pointer, but always live. } // Adds a finalizer to the object p. Returns true if it succeeded. func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool { lock(&mheap_.speciallock) s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc()) unlock(&mheap_.speciallock) s.special.kind = _KindSpecialFinalizer s.fn = f s.nret = nret s.fint = fint s.ot = ot if addspecial(p, &s.special) { // This is responsible for maintaining the same // GC-related invariants as markrootSpans in any // situation where it's possible that markrootSpans // has already run but mark termination hasn't yet. if gcphase != _GCoff { base, _, _ := findObject(uintptr(p), 0, 0) mp := acquirem() gcw := &mp.p.ptr().gcw // Mark everything reachable from the object // so it's retained for the finalizer. scanobject(base, gcw) // Mark the finalizer itself, since the // special isn't part of the GC'd heap. scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw) if gcBlackenPromptly { gcw.dispose() } releasem(mp) } return true } // There was an old finalizer lock(&mheap_.speciallock) mheap_.specialfinalizeralloc.free(unsafe.Pointer(s)) unlock(&mheap_.speciallock) return false } // Removes the finalizer (if any) from the object p. func removefinalizer(p unsafe.Pointer) { s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer))) if s == nil { return // there wasn't a finalizer to remove } lock(&mheap_.speciallock) mheap_.specialfinalizeralloc.free(unsafe.Pointer(s)) unlock(&mheap_.speciallock) } // The described object is being heap profiled. // //go:notinheap type specialprofile struct { special special b *bucket } // Set the heap profile bucket associated with addr to b. func setprofilebucket(p unsafe.Pointer, b *bucket) { lock(&mheap_.speciallock) s := (*specialprofile)(mheap_.specialprofilealloc.alloc()) unlock(&mheap_.speciallock) s.special.kind = _KindSpecialProfile s.b = b if !addspecial(p, &s.special) { throw("setprofilebucket: profile already set") } } // Do whatever cleanup needs to be done to deallocate s. It has // already been unlinked from the MSpan specials list. func freespecial(s *special, p unsafe.Pointer, size uintptr) { switch s.kind { case _KindSpecialFinalizer: sf := (*specialfinalizer)(unsafe.Pointer(s)) queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot) lock(&mheap_.speciallock) mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf)) unlock(&mheap_.speciallock) case _KindSpecialProfile: sp := (*specialprofile)(unsafe.Pointer(s)) mProf_Free(sp.b, size) lock(&mheap_.speciallock) mheap_.specialprofilealloc.free(unsafe.Pointer(sp)) unlock(&mheap_.speciallock) default: throw("bad special kind") panic("not reached") } } // gcBits is an alloc/mark bitmap. This is always used as *gcBits. // //go:notinheap type gcBits uint8 // bytep returns a pointer to the n'th byte of b. func (b *gcBits) bytep(n uintptr) *uint8 { return addb((*uint8)(b), n) } // bitp returns a pointer to the byte containing bit n and a mask for // selecting that bit from *bytep. func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) { return b.bytep(n / 8), 1 << (n % 8) } const gcBitsChunkBytes = uintptr(64 << 10) const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{}) type gcBitsHeader struct { free uintptr // free is the index into bits of the next free byte. next uintptr // *gcBits triggers recursive type bug. (issue 14620) } //go:notinheap type gcBitsArena struct { // gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand. free uintptr // free is the index into bits of the next free byte; read/write atomically next *gcBitsArena bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits } var gcBitsArenas struct { lock mutex free *gcBitsArena next *gcBitsArena // Read atomically. Write atomically under lock. current *gcBitsArena previous *gcBitsArena } // tryAlloc allocates from b or returns nil if b does not have enough room. // This is safe to call concurrently. func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits { if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) { return nil } // Try to allocate from this block. end := atomic.Xadduintptr(&b.free, bytes) if end > uintptr(len(b.bits)) { return nil } // There was enough room. start := end - bytes return &b.bits[start] } // newMarkBits returns a pointer to 8 byte aligned bytes // to be used for a span's mark bits. func newMarkBits(nelems uintptr) *gcBits { blocksNeeded := uintptr((nelems + 63) / 64) bytesNeeded := blocksNeeded * 8 // Try directly allocating from the current head arena. head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next))) if p := head.tryAlloc(bytesNeeded); p != nil { return p } // There's not enough room in the head arena. We may need to // allocate a new arena. lock(&gcBitsArenas.lock) // Try the head arena again, since it may have changed. Now // that we hold the lock, the list head can't change, but its // free position still can. if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil { unlock(&gcBitsArenas.lock) return p } // Allocate a new arena. This may temporarily drop the lock. fresh := newArenaMayUnlock() // If newArenaMayUnlock dropped the lock, another thread may // have put a fresh arena on the "next" list. Try allocating // from next again. if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil { // Put fresh back on the free list. // TODO: Mark it "already zeroed" fresh.next = gcBitsArenas.free gcBitsArenas.free = fresh unlock(&gcBitsArenas.lock) return p } // Allocate from the fresh arena. We haven't linked it in yet, so // this cannot race and is guaranteed to succeed. p := fresh.tryAlloc(bytesNeeded) if p == nil { throw("markBits overflow") } // Add the fresh arena to the "next" list. fresh.next = gcBitsArenas.next atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh)) unlock(&gcBitsArenas.lock) return p } // newAllocBits returns a pointer to 8 byte aligned bytes // to be used for this span's alloc bits. // newAllocBits is used to provide newly initialized spans // allocation bits. For spans not being initialized the // mark bits are repurposed as allocation bits when // the span is swept. func newAllocBits(nelems uintptr) *gcBits { return newMarkBits(nelems) } // nextMarkBitArenaEpoch establishes a new epoch for the arenas // holding the mark bits. The arenas are named relative to the // current GC cycle which is demarcated by the call to finishweep_m. // // All current spans have been swept. // During that sweep each span allocated room for its gcmarkBits in // gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current // where the GC will mark objects and after each span is swept these bits // will be used to allocate objects. // gcBitsArenas.current becomes gcBitsArenas.previous where the span's // gcAllocBits live until all the spans have been swept during this GC cycle. // The span's sweep extinguishes all the references to gcBitsArenas.previous // by pointing gcAllocBits into the gcBitsArenas.current. // The gcBitsArenas.previous is released to the gcBitsArenas.free list. func nextMarkBitArenaEpoch() { lock(&gcBitsArenas.lock) if gcBitsArenas.previous != nil { if gcBitsArenas.free == nil { gcBitsArenas.free = gcBitsArenas.previous } else { // Find end of previous arenas. last := gcBitsArenas.previous for last = gcBitsArenas.previous; last.next != nil; last = last.next { } last.next = gcBitsArenas.free gcBitsArenas.free = gcBitsArenas.previous } } gcBitsArenas.previous = gcBitsArenas.current gcBitsArenas.current = gcBitsArenas.next atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed unlock(&gcBitsArenas.lock) } // newArenaMayUnlock allocates and zeroes a gcBits arena. // The caller must hold gcBitsArena.lock. This may temporarily release it. func newArenaMayUnlock() *gcBitsArena { var result *gcBitsArena if gcBitsArenas.free == nil { unlock(&gcBitsArenas.lock) result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gc_sys)) if result == nil { throw("runtime: cannot allocate memory") } lock(&gcBitsArenas.lock) } else { result = gcBitsArenas.free gcBitsArenas.free = gcBitsArenas.free.next memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes) } result.next = nil // If result.bits is not 8 byte aligned adjust index so // that &result.bits[result.free] is 8 byte aligned. if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 { result.free = 0 } else { result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7) } return result }