runtime: simplify divmagic for span calculations

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

// 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.

// Garbage collector: type and heap bitmaps.
//
// Stack, data, and bss bitmaps
//
// Stack frames and global variables in the data and bss sections are
// described by bitmaps with 1 bit per pointer-sized word. A "1" bit
// means the word is a live pointer to be visited by the GC (referred to
// as "pointer"). A "0" bit means the word should be ignored by GC
// (referred to as "scalar", though it could be a dead pointer value).
//
// Heap bitmap
//
// The heap bitmap comprises 2 bits for each pointer-sized word in the heap,
// stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting a start,
// then ha.bitmap[0] holds the 2-bit entries for the four words start
// through start+3*ptrSize, ha.bitmap[1] holds the entries for
// start+4*ptrSize through start+7*ptrSize, and so on.
//
// In each 2-bit entry, the lower bit is a pointer/scalar bit, just
// like in the stack/data bitmaps described above. The upper bit
// indicates scan/dead: a "1" value ("scan") indicates that there may
// be pointers in later words of the allocation, and a "0" value
// ("dead") indicates there are no more pointers in the allocation. If
// the upper bit is 0, the lower bit must also be 0, and this
// indicates scanning can ignore the rest of the allocation.
//
// The 2-bit entries are split when written into the byte, so that the top half
// of the byte contains 4 high (scan) bits and the bottom half contains 4 low
// (pointer) bits. This form allows a copy from the 1-bit to the 4-bit form to
// keep the pointer bits contiguous, instead of having to space them out.
//
// The code makes use of the fact that the zero value for a heap
// bitmap means scalar/dead. This property must be preserved when
// modifying the encoding.
//
// The bitmap for noscan spans is not maintained. Code must ensure
// that an object is scannable before consulting its bitmap by
// checking either the noscan bit in the span or by consulting its
// type's information.

package runtime

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

const (
        bitPointer = 1 << 0
        bitScan    = 1 << 4

        heapBitsShift      = 1     // shift offset between successive bitPointer or bitScan entries
        wordsPerBitmapByte = 8 / 2 // heap words described by one bitmap byte

        // all scan/pointer bits in a byte
        bitScanAll    = bitScan | bitScan<<heapBitsShift | bitScan<<(2*heapBitsShift) | bitScan<<(3*heapBitsShift)
        bitPointerAll = bitPointer | bitPointer<<heapBitsShift | bitPointer<<(2*heapBitsShift) | bitPointer<<(3*heapBitsShift)
)

// addb returns the byte pointer p+n.
//go:nowritebarrier
//go:nosplit
func addb(p *byte, n uintptr) *byte {
        // Note: wrote out full expression instead of calling add(p, n)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}

// subtractb returns the byte pointer p-n.
//go:nowritebarrier
//go:nosplit
func subtractb(p *byte, n uintptr) *byte {
        // Note: wrote out full expression instead of calling add(p, -n)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
}

// add1 returns the byte pointer p+1.
//go:nowritebarrier
//go:nosplit
func add1(p *byte) *byte {
        // Note: wrote out full expression instead of calling addb(p, 1)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
}

// subtract1 returns the byte pointer p-1.
//go:nowritebarrier
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func subtract1(p *byte) *byte {
        // Note: wrote out full expression instead of calling subtractb(p, 1)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
}

// heapBits provides access to the bitmap bits for a single heap word.
// The methods on heapBits take value receivers so that the compiler
// can more easily inline calls to those methods and registerize the
// struct fields independently.
type heapBits struct {
        bitp  *uint8
        shift uint32
        arena uint32 // Index of heap arena containing bitp
        last  *uint8 // Last byte arena's bitmap
}

// Make the compiler check that heapBits.arena is large enough to hold
// the maximum arena frame number.
var _ = heapBits{arena: (1<<heapAddrBits)/heapArenaBytes - 1}

// markBits provides access to the mark bit for an object in the heap.
// bytep points to the byte holding the mark bit.
// mask is a byte with a single bit set that can be &ed with *bytep
// to see if the bit has been set.
// *m.byte&m.mask != 0 indicates the mark bit is set.
// index can be used along with span information to generate
// the address of the object in the heap.
// We maintain one set of mark bits for allocation and one for
// marking purposes.
type markBits struct {
        bytep *uint8
        mask  uint8
        index uintptr
}

//go:nosplit
func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
        bytep, mask := s.allocBits.bitp(allocBitIndex)
        return markBits{bytep, mask, allocBitIndex}
}

// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
// and negates them so that ctz (count trailing zeros) instructions
// can be used. It then places these 8 bytes into the cached 64 bit
// s.allocCache.
func (s *mspan) refillAllocCache(whichByte uintptr) {
        bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(whichByte)))
        aCache := uint64(0)
        aCache |= uint64(bytes[0])
        aCache |= uint64(bytes[1]) << (1 * 8)
        aCache |= uint64(bytes[2]) << (2 * 8)
        aCache |= uint64(bytes[3]) << (3 * 8)
        aCache |= uint64(bytes[4]) << (4 * 8)
        aCache |= uint64(bytes[5]) << (5 * 8)
        aCache |= uint64(bytes[6]) << (6 * 8)
        aCache |= uint64(bytes[7]) << (7 * 8)
        s.allocCache = ^aCache
}

// nextFreeIndex returns the index of the next free object in s at
// or after s.freeindex.
// There are hardware instructions that can be used to make this
// faster if profiling warrants it.
func (s *mspan) nextFreeIndex() uintptr {
        sfreeindex := s.freeindex
        snelems := s.nelems
        if sfreeindex == snelems {
                return sfreeindex
        }
        if sfreeindex > snelems {
                throw("s.freeindex > s.nelems")
        }

        aCache := s.allocCache

        bitIndex := sys.Ctz64(aCache)
        for bitIndex == 64 {
                // Move index to start of next cached bits.
                sfreeindex = (sfreeindex + 64) &^ (64 - 1)
                if sfreeindex >= snelems {
                        s.freeindex = snelems
                        return snelems
                }
                whichByte := sfreeindex / 8
                // Refill s.allocCache with the next 64 alloc bits.
                s.refillAllocCache(whichByte)
                aCache = s.allocCache
                bitIndex = sys.Ctz64(aCache)
                // nothing available in cached bits
                // grab the next 8 bytes and try again.
        }
        result := sfreeindex + uintptr(bitIndex)
        if result >= snelems {
                s.freeindex = snelems
                return snelems
        }

        s.allocCache >>= uint(bitIndex + 1)
        sfreeindex = result + 1

        if sfreeindex%64 == 0 && sfreeindex != snelems {
                // We just incremented s.freeindex so it isn't 0.
                // As each 1 in s.allocCache was encountered and used for allocation
                // it was shifted away. At this point s.allocCache contains all 0s.
                // Refill s.allocCache so that it corresponds
                // to the bits at s.allocBits starting at s.freeindex.
                whichByte := sfreeindex / 8
                s.refillAllocCache(whichByte)
        }
        s.freeindex = sfreeindex
        return result
}

// isFree reports whether the index'th object in s is unallocated.
//
// The caller must ensure s.state is mSpanInUse, and there must have
// been no preemption points since ensuring this (which could allow a
// GC transition, which would allow the state to change).
func (s *mspan) isFree(index uintptr) bool {
        if index < s.freeindex {
                return false
        }
        bytep, mask := s.allocBits.bitp(index)
        return *bytep&mask == 0
}

// divideByElemSize returns n/s.elemsize.
// n must be within [0, s.npages*_PageSize),
// or may be exactly s.npages*_PageSize
// if s.elemsize is from sizeclasses.go.
func (s *mspan) divideByElemSize(n uintptr) uintptr {
        const doubleCheck = false

        // See explanation in mksizeclasses.go's computeDivMagic.
        q := uintptr((uint64(n) * uint64(s.divMul)) >> 32)

        if doubleCheck && q != n/s.elemsize {
                println(n, "/", s.elemsize, "should be", n/s.elemsize, "but got", q)
                throw("bad magic division")
        }
        return q
}

func (s *mspan) objIndex(p uintptr) uintptr {
        return s.divideByElemSize(p - s.base())
}

func markBitsForAddr(p uintptr) markBits {
        s := spanOf(p)
        objIndex := s.objIndex(p)
        return s.markBitsForIndex(objIndex)
}

func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
        bytep, mask := s.gcmarkBits.bitp(objIndex)
        return markBits{bytep, mask, objIndex}
}

func (s *mspan) markBitsForBase() markBits {
        return markBits{(*uint8)(s.gcmarkBits), uint8(1), 0}
}

// isMarked reports whether mark bit m is set.
func (m markBits) isMarked() bool {
        return *m.bytep&m.mask != 0
}

// setMarked sets the marked bit in the markbits, atomically.
func (m markBits) setMarked() {
        // Might be racing with other updates, so use atomic update always.
        // We used to be clever here and use a non-atomic update in certain
        // cases, but it's not worth the risk.
        atomic.Or8(m.bytep, m.mask)
}

// setMarkedNonAtomic sets the marked bit in the markbits, non-atomically.
func (m markBits) setMarkedNonAtomic() {
        *m.bytep |= m.mask
}

// clearMarked clears the marked bit in the markbits, atomically.
func (m markBits) clearMarked() {
        // Might be racing with other updates, so use atomic update always.
        // We used to be clever here and use a non-atomic update in certain
        // cases, but it's not worth the risk.
        atomic.And8(m.bytep, ^m.mask)
}

// markBitsForSpan returns the markBits for the span base address base.
func markBitsForSpan(base uintptr) (mbits markBits) {
        mbits = markBitsForAddr(base)
        if mbits.mask != 1 {
                throw("markBitsForSpan: unaligned start")
        }
        return mbits
}

// advance advances the markBits to the next object in the span.
func (m *markBits) advance() {
        if m.mask == 1<<7 {
                m.bytep = (*uint8)(unsafe.Pointer(uintptr(unsafe.Pointer(m.bytep)) + 1))
                m.mask = 1
        } else {
                m.mask = m.mask << 1
        }
        m.index++
}

// heapBitsForAddr returns the heapBits for the address addr.
// The caller must ensure addr is in an allocated span.
// In particular, be careful not to point past the end of an object.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func heapBitsForAddr(addr uintptr) (h heapBits) {
        // 2 bits per word, 4 pairs per byte, and a mask is hard coded.
        arena := arenaIndex(addr)
        ha := mheap_.arenas[arena.l1()][arena.l2()]
        // The compiler uses a load for nil checking ha, but in this
        // case we'll almost never hit that cache line again, so it
        // makes more sense to do a value check.
        if ha == nil {
                // addr is not in the heap. Return nil heapBits, which
                // we expect to crash in the caller.
                return
        }
        h.bitp = &ha.bitmap[(addr/(sys.PtrSize*4))%heapArenaBitmapBytes]
        h.shift = uint32((addr / sys.PtrSize) & 3)
        h.arena = uint32(arena)
        h.last = &ha.bitmap[len(ha.bitmap)-1]
        return
}

// badPointer throws bad pointer in heap panic.
func badPointer(s *mspan, p, refBase, refOff uintptr) {
        // Typically this indicates an incorrect use
        // of unsafe or cgo to store a bad pointer in
        // the Go heap. It may also indicate a runtime
        // bug.
        //
        // TODO(austin): We could be more aggressive
        // and detect pointers to unallocated objects
        // in allocated spans.
        printlock()
        print("runtime: pointer ", hex(p))
        state := s.state.get()
        if state != mSpanInUse {
                print(" to unallocated span")
        } else {
                print(" to unused region of span")
        }
        print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", state, "\n")
        if refBase != 0 {
                print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
                gcDumpObject("object", refBase, refOff)
        }
        getg().m.traceback = 2
        throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
}

// findObject returns the base address for the heap object containing
// the address p, the object's span, and the index of the object in s.
// If p does not point into a heap object, it returns base == 0.
//
// If p points is an invalid heap pointer and debug.invalidptr != 0,
// findObject panics.
//
// refBase and refOff optionally give the base address of the object
// in which the pointer p was found and the byte offset at which it
// was found. These are used for error reporting.
//
// It is nosplit so it is safe for p to be a pointer to the current goroutine's stack.
// Since p is a uintptr, it would not be adjusted if the stack were to move.
//go:nosplit
func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex uintptr) {
        s = spanOf(p)
        // If s is nil, the virtual address has never been part of the heap.
        // This pointer may be to some mmap'd region, so we allow it.
        if s == nil {
                return
        }
        // If p is a bad pointer, it may not be in s's bounds.
        //
        // Check s.state to synchronize with span initialization
        // before checking other fields. See also spanOfHeap.
        if state := s.state.get(); state != mSpanInUse || p < s.base() || p >= s.limit {
                // Pointers into stacks are also ok, the runtime manages these explicitly.
                if state == mSpanManual {
                        return
                }
                // The following ensures that we are rigorous about what data
                // structures hold valid pointers.
                if debug.invalidptr != 0 {
                        badPointer(s, p, refBase, refOff)
                }
                return
        }

        objIndex = s.objIndex(p)
        base = s.base() + objIndex*s.elemsize
        return
}

// next returns the heapBits describing the next pointer-sized word in memory.
// That is, if h describes address p, h.next() describes p+ptrSize.
// Note that next does not modify h. The caller must record the result.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) next() heapBits {
        if h.shift < 3*heapBitsShift {
                h.shift += heapBitsShift
        } else if h.bitp != h.last {
                h.bitp, h.shift = add1(h.bitp), 0
        } else {
                // Move to the next arena.
                return h.nextArena()
        }
        return h
}

// nextArena advances h to the beginning of the next heap arena.
//
// This is a slow-path helper to next. gc's inliner knows that
// heapBits.next can be inlined even though it calls this. This is
// marked noinline so it doesn't get inlined into next and cause next
// to be too big to inline.
//
//go:nosplit
//go:noinline
func (h heapBits) nextArena() heapBits {
        h.arena++
        ai := arenaIdx(h.arena)
        l2 := mheap_.arenas[ai.l1()]
        if l2 == nil {
                // We just passed the end of the object, which
                // was also the end of the heap. Poison h. It
                // should never be dereferenced at this point.
                return heapBits{}
        }
        ha := l2[ai.l2()]
        if ha == nil {
                return heapBits{}
        }
        h.bitp, h.shift = &ha.bitmap[0], 0
        h.last = &ha.bitmap[len(ha.bitmap)-1]
        return h
}

// forward returns the heapBits describing n pointer-sized words ahead of h in memory.
// That is, if h describes address p, h.forward(n) describes p+n*ptrSize.
// h.forward(1) is equivalent to h.next(), just slower.
// Note that forward does not modify h. The caller must record the result.
// bits returns the heap bits for the current word.
//go:nosplit
func (h heapBits) forward(n uintptr) heapBits {
        n += uintptr(h.shift) / heapBitsShift
        nbitp := uintptr(unsafe.Pointer(h.bitp)) + n/4
        h.shift = uint32(n%4) * heapBitsShift
        if nbitp <= uintptr(unsafe.Pointer(h.last)) {
                h.bitp = (*uint8)(unsafe.Pointer(nbitp))
                return h
        }

        // We're in a new heap arena.
        past := nbitp - (uintptr(unsafe.Pointer(h.last)) + 1)
        h.arena += 1 + uint32(past/heapArenaBitmapBytes)
        ai := arenaIdx(h.arena)
        if l2 := mheap_.arenas[ai.l1()]; l2 != nil && l2[ai.l2()] != nil {
                a := l2[ai.l2()]
                h.bitp = &a.bitmap[past%heapArenaBitmapBytes]
                h.last = &a.bitmap[len(a.bitmap)-1]
        } else {
                h.bitp, h.last = nil, nil
        }
        return h
}

// forwardOrBoundary is like forward, but stops at boundaries between
// contiguous sections of the bitmap. It returns the number of words
// advanced over, which will be <= n.
func (h heapBits) forwardOrBoundary(n uintptr) (heapBits, uintptr) {
        maxn := 4 * ((uintptr(unsafe.Pointer(h.last)) + 1) - uintptr(unsafe.Pointer(h.bitp)))
        if n > maxn {
                n = maxn
        }
        return h.forward(n), n
}

// The caller can test morePointers and isPointer by &-ing with bitScan and bitPointer.
// The result includes in its higher bits the bits for subsequent words
// described by the same bitmap byte.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) bits() uint32 {
        // The (shift & 31) eliminates a test and conditional branch
        // from the generated code.
        return uint32(*h.bitp) >> (h.shift & 31)
}

// morePointers reports whether this word and all remaining words in this object
// are scalars.
// h must not describe the second word of the object.
func (h heapBits) morePointers() bool {
        return h.bits()&bitScan != 0
}

// isPointer reports whether the heap bits describe a pointer word.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) isPointer() bool {
        return h.bits()&bitPointer != 0
}

// bulkBarrierPreWrite executes a write barrier
// for every pointer slot in the memory range [src, src+size),
// using pointer/scalar information from [dst, dst+size).
// This executes the write barriers necessary before a memmove.
// src, dst, and size must be pointer-aligned.
// The range [dst, dst+size) must lie within a single object.
// It does not perform the actual writes.
//
// As a special case, src == 0 indicates that this is being used for a
// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
// barrier.
//
// Callers should call bulkBarrierPreWrite immediately before
// calling memmove(dst, src, size). This function is marked nosplit
// to avoid being preempted; the GC must not stop the goroutine
// between the memmove and the execution of the barriers.
// The caller is also responsible for cgo pointer checks if this
// may be writing Go pointers into non-Go memory.
//
// The pointer bitmap is not maintained for allocations containing
// no pointers at all; any caller of bulkBarrierPreWrite must first
// make sure the underlying allocation contains pointers, usually
// by checking typ.ptrdata.
//
// Callers must perform cgo checks if writeBarrier.cgo.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
        if (dst|src|size)&(sys.PtrSize-1) != 0 {
                throw("bulkBarrierPreWrite: unaligned arguments")
        }
        if !writeBarrier.needed {
                return
        }
        if s := spanOf(dst); s == nil {
                // If dst is a global, use the data or BSS bitmaps to
                // execute write barriers.
                for _, datap := range activeModules() {
                        if datap.data <= dst && dst < datap.edata {
                                bulkBarrierBitmap(dst, src, size, dst-datap.data, datap.gcdatamask.bytedata)
                                return
                        }
                }
                for _, datap := range activeModules() {
                        if datap.bss <= dst && dst < datap.ebss {
                                bulkBarrierBitmap(dst, src, size, dst-datap.bss, datap.gcbssmask.bytedata)
                                return
                        }
                }
                return
        } else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
                // dst was heap memory at some point, but isn't now.
                // It can't be a global. It must be either our stack,
                // or in the case of direct channel sends, it could be
                // another stack. Either way, no need for barriers.
                // This will also catch if dst is in a freed span,
                // though that should never have.
                return
        }

        buf := &getg().m.p.ptr().wbBuf
        h := heapBitsForAddr(dst)
        if src == 0 {
                for i := uintptr(0); i < size; i += sys.PtrSize {
                        if h.isPointer() {
                                dstx := (*uintptr)(unsafe.Pointer(dst + i))
                                if !buf.putFast(*dstx, 0) {
                                        wbBufFlush(nil, 0)
                                }
                        }
                        h = h.next()
                }
        } else {
                for i := uintptr(0); i < size; i += sys.PtrSize {
                        if h.isPointer() {
                                dstx := (*uintptr)(unsafe.Pointer(dst + i))
                                srcx := (*uintptr)(unsafe.Pointer(src + i))
                                if !buf.putFast(*dstx, *srcx) {
                                        wbBufFlush(nil, 0)
                                }
                        }
                        h = h.next()
                }
        }
}

// bulkBarrierPreWriteSrcOnly is like bulkBarrierPreWrite but
// does not execute write barriers for [dst, dst+size).
//
// In addition to the requirements of bulkBarrierPreWrite
// callers need to ensure [dst, dst+size) is zeroed.
//
// This is used for special cases where e.g. dst was just
// created and zeroed with malloc.
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
        if (dst|src|size)&(sys.PtrSize-1) != 0 {
                throw("bulkBarrierPreWrite: unaligned arguments")
        }
        if !writeBarrier.needed {
                return
        }
        buf := &getg().m.p.ptr().wbBuf
        h := heapBitsForAddr(dst)
        for i := uintptr(0); i < size; i += sys.PtrSize {
                if h.isPointer() {
                        srcx := (*uintptr)(unsafe.Pointer(src + i))
                        if !buf.putFast(0, *srcx) {
                                wbBufFlush(nil, 0)
                        }
                }
                h = h.next()
        }
}

// bulkBarrierBitmap executes write barriers for copying from [src,
// src+size) to [dst, dst+size) using a 1-bit pointer bitmap. src is
// assumed to start maskOffset bytes into the data covered by the
// bitmap in bits (which may not be a multiple of 8).
//
// This is used by bulkBarrierPreWrite for writes to data and BSS.
//
//go:nosplit
func bulkBarrierBitmap(dst, src, size, maskOffset uintptr, bits *uint8) {
        word := maskOffset / sys.PtrSize
        bits = addb(bits, word/8)
        mask := uint8(1) << (word % 8)

        buf := &getg().m.p.ptr().wbBuf
        for i := uintptr(0); i < size; i += sys.PtrSize {
                if mask == 0 {
                        bits = addb(bits, 1)
                        if *bits == 0 {
                                // Skip 8 words.
                                i += 7 * sys.PtrSize
                                continue
                        }
                        mask = 1
                }
                if *bits&mask != 0 {
                        dstx := (*uintptr)(unsafe.Pointer(dst + i))
                        if src == 0 {
                                if !buf.putFast(*dstx, 0) {
                                        wbBufFlush(nil, 0)
                                }
                        } else {
                                srcx := (*uintptr)(unsafe.Pointer(src + i))
                                if !buf.putFast(*dstx, *srcx) {
                                        wbBufFlush(nil, 0)
                                }
                        }
                }
                mask <<= 1
        }
}

// typeBitsBulkBarrier executes a write barrier for every
// pointer that would be copied from [src, src+size) to [dst,
// dst+size) by a memmove using the type bitmap to locate those
// pointer slots.
//
// The type typ must correspond exactly to [src, src+size) and [dst, dst+size).
// dst, src, and size must be pointer-aligned.
// The type typ must have a plain bitmap, not a GC program.
// The only use of this function is in channel sends, and the
// 64 kB channel element limit takes care of this for us.
//
// Must not be preempted because it typically runs right before memmove,
// and the GC must observe them as an atomic action.
//
// Callers must perform cgo checks if writeBarrier.cgo.
//
//go:nosplit
func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
        if typ == nil {
                throw("runtime: typeBitsBulkBarrier without type")
        }
        if typ.size != size {
                println("runtime: typeBitsBulkBarrier with type ", typ.string(), " of size ", typ.size, " but memory size", size)
                throw("runtime: invalid typeBitsBulkBarrier")
        }
        if typ.kind&kindGCProg != 0 {
                println("runtime: typeBitsBulkBarrier with type ", typ.string(), " with GC prog")
                throw("runtime: invalid typeBitsBulkBarrier")
        }
        if !writeBarrier.needed {
                return
        }
        ptrmask := typ.gcdata
        buf := &getg().m.p.ptr().wbBuf
        var bits uint32
        for i := uintptr(0); i < typ.ptrdata; i += sys.PtrSize {
                if i&(sys.PtrSize*8-1) == 0 {
                        bits = uint32(*ptrmask)
                        ptrmask = addb(ptrmask, 1)
                } else {
                        bits = bits >> 1
                }
                if bits&1 != 0 {
                        dstx := (*uintptr)(unsafe.Pointer(dst + i))
                        srcx := (*uintptr)(unsafe.Pointer(src + i))
                        if !buf.putFast(*dstx, *srcx) {
                                wbBufFlush(nil, 0)
                        }
                }
        }
}

// The methods operating on spans all require that h has been returned
// by heapBitsForSpan and that size, n, total are the span layout description
// returned by the mspan's layout method.
// If total > size*n, it means that there is extra leftover memory in the span,
// usually due to rounding.
//
// TODO(rsc): Perhaps introduce a different heapBitsSpan type.

// initSpan initializes the heap bitmap for a span.
// If this is a span of pointer-sized objects, it initializes all
// words to pointer/scan.
// Otherwise, it initializes all words to scalar/dead.
func (h heapBits) initSpan(s *mspan) {
        // Clear bits corresponding to objects.
        nw := (s.npages << _PageShift) / sys.PtrSize
        if nw%wordsPerBitmapByte != 0 {
                throw("initSpan: unaligned length")
        }
        if h.shift != 0 {
                throw("initSpan: unaligned base")
        }
        isPtrs := sys.PtrSize == 8 && s.elemsize == sys.PtrSize
        for nw > 0 {
                hNext, anw := h.forwardOrBoundary(nw)
                nbyte := anw / wordsPerBitmapByte
                if isPtrs {
                        bitp := h.bitp
                        for i := uintptr(0); i < nbyte; i++ {
                                *bitp = bitPointerAll | bitScanAll
                                bitp = add1(bitp)
                        }
                } else {
                        memclrNoHeapPointers(unsafe.Pointer(h.bitp), nbyte)
                }
                h = hNext
                nw -= anw
        }
}

// countAlloc returns the number of objects allocated in span s by
// scanning the allocation bitmap.
func (s *mspan) countAlloc() int {
        count := 0
        bytes := divRoundUp(s.nelems, 8)
        // Iterate over each 8-byte chunk and count allocations
        // with an intrinsic. Note that newMarkBits guarantees that
        // gcmarkBits will be 8-byte aligned, so we don't have to
        // worry about edge cases, irrelevant bits will simply be zero.
        for i := uintptr(0); i < bytes; i += 8 {
                // Extract 64 bits from the byte pointer and get a OnesCount.
                // Note that the unsafe cast here doesn't preserve endianness,
                // but that's OK. We only care about how many bits are 1, not
                // about the order we discover them in.
                mrkBits := *(*uint64)(unsafe.Pointer(s.gcmarkBits.bytep(i)))
                count += sys.OnesCount64(mrkBits)
        }
        return count
}

// heapBitsSetType records that the new allocation [x, x+size)
// holds in [x, x+dataSize) one or more values of type typ.
// (The number of values is given by dataSize / typ.size.)
// If dataSize < size, the fragment [x+dataSize, x+size) is
// recorded as non-pointer data.
// It is known that the type has pointers somewhere;
// malloc does not call heapBitsSetType when there are no pointers,
// because all free objects are marked as noscan during
// heapBitsSweepSpan.
//
// There can only be one allocation from a given span active at a time,
// and the bitmap for a span always falls on byte boundaries,
// so there are no write-write races for access to the heap bitmap.
// Hence, heapBitsSetType can access the bitmap without atomics.
//
// There can be read-write races between heapBitsSetType and things
// that read the heap bitmap like scanobject. However, since
// heapBitsSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// bits that belong to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
        const doubleCheck = false // slow but helpful; enable to test modifications to this code

        const (
                mask1 = bitPointer | bitScan                        // 00010001
                mask2 = bitPointer | bitScan | mask1<<heapBitsShift // 00110011
                mask3 = bitPointer | bitScan | mask2<<heapBitsShift // 01110111
        )

        // dataSize is always size rounded up to the next malloc size class,
        // except in the case of allocating a defer block, in which case
        // size is sizeof(_defer{}) (at least 6 words) and dataSize may be
        // arbitrarily larger.
        //
        // The checks for size == sys.PtrSize and size == 2*sys.PtrSize can therefore
        // assume that dataSize == size without checking it explicitly.

        if sys.PtrSize == 8 && size == sys.PtrSize {
                // It's one word and it has pointers, it must be a pointer.
                // Since all allocated one-word objects are pointers
                // (non-pointers are aggregated into tinySize allocations),
                // initSpan sets the pointer bits for us. Nothing to do here.
                if doubleCheck {
                        h := heapBitsForAddr(x)
                        if !h.isPointer() {
                                throw("heapBitsSetType: pointer bit missing")
                        }
                        if !h.morePointers() {
                                throw("heapBitsSetType: scan bit missing")
                        }
                }
                return
        }

        h := heapBitsForAddr(x)
        ptrmask := typ.gcdata // start of 1-bit pointer mask (or GC program, handled below)

        // 2-word objects only have 4 bitmap bits and 3-word objects only have 6 bitmap bits.
        // Therefore, these objects share a heap bitmap byte with the objects next to them.
        // These are called out as a special case primarily so the code below can assume all
        // objects are at least 4 words long and that their bitmaps start either at the beginning
        // of a bitmap byte, or half-way in (h.shift of 0 and 2 respectively).

        if size == 2*sys.PtrSize {
                if typ.size == sys.PtrSize {
                        // We're allocating a block big enough to hold two pointers.
                        // On 64-bit, that means the actual object must be two pointers,
                        // or else we'd have used the one-pointer-sized block.
                        // On 32-bit, however, this is the 8-byte block, the smallest one.
                        // So it could be that we're allocating one pointer and this was
                        // just the smallest block available. Distinguish by checking dataSize.
                        // (In general the number of instances of typ being allocated is
                        // dataSize/typ.size.)
                        if sys.PtrSize == 4 && dataSize == sys.PtrSize {
                                // 1 pointer object. On 32-bit machines clear the bit for the
                                // unused second word.
                                *h.bitp &^= (bitPointer | bitScan | (bitPointer|bitScan)<<heapBitsShift) << h.shift
                                *h.bitp |= (bitPointer | bitScan) << h.shift
                        } else {
                                // 2-element array of pointer.
                                *h.bitp |= (bitPointer | bitScan | (bitPointer|bitScan)<<heapBitsShift) << h.shift
                        }
                        return
                }
                // Otherwise typ.size must be 2*sys.PtrSize,
                // and typ.kind&kindGCProg == 0.
                if doubleCheck {
                        if typ.size != 2*sys.PtrSize || typ.kind&kindGCProg != 0 {
                                print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, " gcprog=", typ.kind&kindGCProg != 0, "\n")
                                throw("heapBitsSetType")
                        }
                }
                b := uint32(*ptrmask)
                hb := b & 3
                hb |= bitScanAll & ((bitScan << (typ.ptrdata / sys.PtrSize)) - 1)
                // Clear the bits for this object so we can set the
                // appropriate ones.
                *h.bitp &^= (bitPointer | bitScan | ((bitPointer | bitScan) << heapBitsShift)) << h.shift
                *h.bitp |= uint8(hb << h.shift)
                return
        } else if size == 3*sys.PtrSize {
                b := uint8(*ptrmask)
                if doubleCheck {
                        if b == 0 {
                                println("runtime: invalid type ", typ.string())
                                throw("heapBitsSetType: called with non-pointer type")
                        }
                        if sys.PtrSize != 8 {
                                throw("heapBitsSetType: unexpected 3 pointer wide size class on 32 bit")
                        }
                        if typ.kind&kindGCProg != 0 {
                                throw("heapBitsSetType: unexpected GC prog for 3 pointer wide size class")
                        }
                        if typ.size == 2*sys.PtrSize {
                                print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, "\n")
                                throw("heapBitsSetType: inconsistent object sizes")
                        }
                }
                if typ.size == sys.PtrSize {
                        // The type contains a pointer otherwise heapBitsSetType wouldn't have been called.
                        // Since the type is only 1 pointer wide and contains a pointer, its gcdata must be exactly 1.
                        if doubleCheck && *typ.gcdata != 1 {
                                print("runtime: heapBitsSetType size=", size, " typ.size=", typ.size, "but *typ.gcdata", *typ.gcdata, "\n")
                                throw("heapBitsSetType: unexpected gcdata for 1 pointer wide type size in 3 pointer wide size class")
                        }
                        // 3 element array of pointers. Unrolling ptrmask 3 times into p yields 00000111.
                        b = 7
                }

                hb := b & 7
                // Set bitScan bits for all pointers.
                hb |= hb << wordsPerBitmapByte
                // First bitScan bit is always set since the type contains pointers.
                hb |= bitScan
                // Second bitScan bit needs to also be set if the third bitScan bit is set.
                hb |= hb & (bitScan << (2 * heapBitsShift)) >> 1

                // For h.shift > 1 heap bits cross a byte boundary and need to be written part
                // to h.bitp and part to the next h.bitp.
                switch h.shift {
                case 0:
                        *h.bitp &^= mask3 << 0
                        *h.bitp |= hb << 0
                case 1:
                        *h.bitp &^= mask3 << 1
                        *h.bitp |= hb << 1
                case 2:
                        *h.bitp &^= mask2 << 2
                        *h.bitp |= (hb & mask2) << 2
                        // Two words written to the first byte.
                        // Advance two words to get to the next byte.
                        h = h.next().next()
                        *h.bitp &^= mask1
                        *h.bitp |= (hb >> 2) & mask1
                case 3:
                        *h.bitp &^= mask1 << 3
                        *h.bitp |= (hb & mask1) << 3
                        // One word written to the first byte.
                        // Advance one word to get to the next byte.
                        h = h.next()
                        *h.bitp &^= mask2
                        *h.bitp |= (hb >> 1) & mask2
                }
                return
        }

        // Copy from 1-bit ptrmask into 2-bit bitmap.
        // The basic approach is to use a single uintptr as a bit buffer,
        // alternating between reloading the buffer and writing bitmap bytes.
        // In general, one load can supply two bitmap byte writes.
        // This is a lot of lines of code, but it compiles into relatively few
        // machine instructions.

        outOfPlace := false
        if arenaIndex(x+size-1) != arenaIdx(h.arena) || (doubleCheck && fastrand()%2 == 0) {
                // This object spans heap arenas, so the bitmap may be
                // discontiguous. Unroll it into the object instead
                // and then copy it out.
                //
                // In doubleCheck mode, we randomly do this anyway to
                // stress test the bitmap copying path.
                outOfPlace = true
                h.bitp = (*uint8)(unsafe.Pointer(x))
                h.last = nil
        }

        var (
                // Ptrmask input.
                p     *byte   // last ptrmask byte read
                b     uintptr // ptrmask bits already loaded
                nb    uintptr // number of bits in b at next read
                endp  *byte   // final ptrmask byte to read (then repeat)
                endnb uintptr // number of valid bits in *endp
                pbits uintptr // alternate source of bits

                // Heap bitmap output.
                w     uintptr // words processed
                nw    uintptr // number of words to process
                hbitp *byte   // next heap bitmap byte to write
                hb    uintptr // bits being prepared for *hbitp
        )

        hbitp = h.bitp

        // Handle GC program. Delayed until this part of the code
        // so that we can use the same double-checking mechanism
        // as the 1-bit case. Nothing above could have encountered
        // GC programs: the cases were all too small.
        if typ.kind&kindGCProg != 0 {
                heapBitsSetTypeGCProg(h, typ.ptrdata, typ.size, dataSize, size, addb(typ.gcdata, 4))
                if doubleCheck {
                        // Double-check the heap bits written by GC program
                        // by running the GC program to create a 1-bit pointer mask
                        // and then jumping to the double-check code below.
                        // This doesn't catch bugs shared between the 1-bit and 4-bit
                        // GC program execution, but it does catch mistakes specific
                        // to just one of those and bugs in heapBitsSetTypeGCProg's
                        // implementation of arrays.
                        lock(&debugPtrmask.lock)
                        if debugPtrmask.data == nil {
                                debugPtrmask.data = (*byte)(persistentalloc(1<<20, 1, &memstats.other_sys))
                        }
                        ptrmask = debugPtrmask.data
                        runGCProg(addb(typ.gcdata, 4), nil, ptrmask, 1)
                }
                goto Phase4
        }

        // Note about sizes:
        //
        // typ.size is the number of words in the object,
        // and typ.ptrdata is the number of words in the prefix
        // of the object that contains pointers. That is, the final
        // typ.size - typ.ptrdata words contain no pointers.
        // This allows optimization of a common pattern where
        // an object has a small header followed by a large scalar
        // buffer. If we know the pointers are over, we don't have
        // to scan the buffer's heap bitmap at all.
        // The 1-bit ptrmasks are sized to contain only bits for
        // the typ.ptrdata prefix, zero padded out to a full byte
        // of bitmap. This code sets nw (below) so that heap bitmap
        // bits are only written for the typ.ptrdata prefix; if there is
        // more room in the allocated object, the next heap bitmap
        // entry is a 00, indicating that there are no more pointers
        // to scan. So only the ptrmask for the ptrdata bytes is needed.
        //
        // Replicated copies are not as nice: if there is an array of
        // objects with scalar tails, all but the last tail does have to
        // be initialized, because there is no way to say "skip forward".
        // However, because of the possibility of a repeated type with
        // size not a multiple of 4 pointers (one heap bitmap byte),
        // the code already must handle the last ptrmask byte specially
        // by treating it as containing only the bits for endnb pointers,
        // where endnb <= 4. We represent large scalar tails that must
        // be expanded in the replication by setting endnb larger than 4.
        // This will have the effect of reading many bits out of b,
        // but once the real bits are shifted out, b will supply as many
        // zero bits as we try to read, which is exactly what we need.

        p = ptrmask
        if typ.size < dataSize {
                // Filling in bits for an array of typ.
                // Set up for repetition of ptrmask during main loop.
                // Note that ptrmask describes only a prefix of
                const maxBits = sys.PtrSize*8 - 7
                if typ.ptrdata/sys.PtrSize <= maxBits {
                        // Entire ptrmask fits in uintptr with room for a byte fragment.
                        // Load into pbits and never read from ptrmask again.
                        // This is especially important when the ptrmask has
                        // fewer than 8 bits in it; otherwise the reload in the middle
                        // of the Phase 2 loop would itself need to loop to gather
                        // at least 8 bits.

                        // Accumulate ptrmask into b.
                        // ptrmask is sized to describe only typ.ptrdata, but we record
                        // it as describing typ.size bytes, since all the high bits are zero.
                        nb = typ.ptrdata / sys.PtrSize
                        for i := uintptr(0); i < nb; i += 8 {
                                b |= uintptr(*p) << i
                                p = add1(p)
                        }
                        nb = typ.size / sys.PtrSize

                        // Replicate ptrmask to fill entire pbits uintptr.
                        // Doubling and truncating is fewer steps than
                        // iterating by nb each time. (nb could be 1.)
                        // Since we loaded typ.ptrdata/sys.PtrSize bits
                        // but are pretending to have typ.size/sys.PtrSize,
                        // there might be no replication necessary/possible.
                        pbits = b
                        endnb = nb
                        if nb+nb <= maxBits {
                                for endnb <= sys.PtrSize*8 {
                                        pbits |= pbits << endnb
                                        endnb += endnb
                                }
                                // Truncate to a multiple of original ptrmask.
                                // Because nb+nb <= maxBits, nb fits in a byte.
                                // Byte division is cheaper than uintptr division.
                                endnb = uintptr(maxBits/byte(nb)) * nb
                                pbits &= 1<<endnb - 1
                                b = pbits
                                nb = endnb
                        }

                        // Clear p and endp as sentinel for using pbits.
                        // Checked during Phase 2 loop.
                        p = nil
                        endp = nil
                } else {
                        // Ptrmask is larger. Read it multiple times.
                        n := (typ.ptrdata/sys.PtrSize+7)/8 - 1
                        endp = addb(ptrmask, n)
                        endnb = typ.size/sys.PtrSize - n*8
                }
        }
        if p != nil {
                b = uintptr(*p)
                p = add1(p)
                nb = 8
        }

        if typ.size == dataSize {
                // Single entry: can stop once we reach the non-pointer data.
                nw = typ.ptrdata / sys.PtrSize
        } else {
                // Repeated instances of typ in an array.
                // Have to process first N-1 entries in full, but can stop
                // once we reach the non-pointer data in the final entry.
                nw = ((dataSize/typ.size-1)*typ.size + typ.ptrdata) / sys.PtrSize
        }
        if nw == 0 {
                // No pointers! Caller was supposed to check.
                println("runtime: invalid type ", typ.string())
                throw("heapBitsSetType: called with non-pointer type")
                return
        }

        // Phase 1: Special case for leading byte (shift==0) or half-byte (shift==2).
        // The leading byte is special because it contains the bits for word 1,
        // which does not have the scan bit set.
        // The leading half-byte is special because it's a half a byte,
        // so we have to be careful with the bits already there.
        switch {
        default:
                throw("heapBitsSetType: unexpected shift")

        case h.shift == 0:
                // Ptrmask and heap bitmap are aligned.
                //
                // This is a fast path for small objects.
                //
                // The first byte we write out covers the first four
                // words of the object. The scan/dead bit on the first
                // word must be set to scan since there are pointers
                // somewhere in the object.
                // In all following words, we set the scan/dead
                // appropriately to indicate that the object continues
                // to the next 2-bit entry in the bitmap.
                //
                // We set four bits at a time here, but if the object
                // is fewer than four words, phase 3 will clear
                // unnecessary bits.
                hb = b & bitPointerAll
                hb |= bitScanAll
                if w += 4; w >= nw {
                        goto Phase3
                }
                *hbitp = uint8(hb)
                hbitp = add1(hbitp)
                b >>= 4
                nb -= 4

        case h.shift == 2:
                // Ptrmask and heap bitmap are misaligned.
                //
                // On 32 bit architectures only the 6-word object that corresponds
                // to a 24 bytes size class can start with h.shift of 2 here since
                // all other non 16 byte aligned size classes have been handled by
                // special code paths at the beginning of heapBitsSetType on 32 bit.
                //
                // Many size classes are only 16 byte aligned. On 64 bit architectures
                // this results in a heap bitmap position starting with a h.shift of 2.
                //
                // The bits for the first two words are in a byte shared
                // with another object, so we must be careful with the bits
                // already there.
                //
                // We took care of 1-word, 2-word, and 3-word objects above,
                // so this is at least a 6-word object.
                hb = (b & (bitPointer | bitPointer<<heapBitsShift)) << (2 * heapBitsShift)
                hb |= bitScan << (2 * heapBitsShift)
                if nw > 1 {
                        hb |= bitScan << (3 * heapBitsShift)
                }
                b >>= 2
                nb -= 2
                *hbitp &^= uint8((bitPointer | bitScan | ((bitPointer | bitScan) << heapBitsShift)) << (2 * heapBitsShift))
                *hbitp |= uint8(hb)
                hbitp = add1(hbitp)
                if w += 2; w >= nw {
                        // We know that there is more data, because we handled 2-word and 3-word objects above.
                        // This must be at least a 6-word object. If we're out of pointer words,
                        // mark no scan in next bitmap byte and finish.
                        hb = 0
                        w += 4
                        goto Phase3
                }
        }

        // Phase 2: Full bytes in bitmap, up to but not including write to last byte (full or partial) in bitmap.
        // The loop computes the bits for that last write but does not execute the write;
        // it leaves the bits in hb for processing by phase 3.
        // To avoid repeated adjustment of nb, we subtract out the 4 bits we're going to
        // use in the first half of the loop right now, and then we only adjust nb explicitly
        // if the 8 bits used by each iteration isn't balanced by 8 bits loaded mid-loop.
        nb -= 4
        for {
                // Emit bitmap byte.
                // b has at least nb+4 bits, with one exception:
                // if w+4 >= nw, then b has only nw-w bits,
                // but we'll stop at the break and then truncate
                // appropriately in Phase 3.
                hb = b & bitPointerAll
                hb |= bitScanAll
                if w += 4; w >= nw {
                        break
                }
                *hbitp = uint8(hb)
                hbitp = add1(hbitp)
                b >>= 4

                // Load more bits. b has nb right now.
                if p != endp {
                        // Fast path: keep reading from ptrmask.
                        // nb unmodified: we just loaded 8 bits,
                        // and the next iteration will consume 8 bits,
                        // leaving us with the same nb the next time we're here.
                        if nb < 8 {
                                b |= uintptr(*p) << nb
                                p = add1(p)
                        } else {
                                // Reduce the number of bits in b.
                                // This is important if we skipped
                                // over a scalar tail, since nb could
                                // be larger than the bit width of b.
                                nb -= 8
                        }
                } else if p == nil {
                        // Almost as fast path: track bit count and refill from pbits.
                        // For short repetitions.
                        if nb < 8 {
                                b |= pbits << nb
                                nb += endnb
                        }
                        nb -= 8 // for next iteration
                } else {
                        // Slow path: reached end of ptrmask.
                        // Process final partial byte and rewind to start.
                        b |= uintptr(*p) << nb
                        nb += endnb
                        if nb < 8 {
                                b |= uintptr(*ptrmask) << nb
                                p = add1(ptrmask)
                        } else {
                                nb -= 8
                                p = ptrmask
                        }
                }

                // Emit bitmap byte.
                hb = b & bitPointerAll
                hb |= bitScanAll
                if w += 4; w >= nw {
                        break
                }
                *hbitp = uint8(hb)
                hbitp = add1(hbitp)
                b >>= 4
        }

Phase3:
        // Phase 3: Write last byte or partial byte and zero the rest of the bitmap entries.
        if w > nw {
                // Counting the 4 entries in hb not yet written to memory,
                // there are more entries than possible pointer slots.
                // Discard the excess entries (can't be more than 3).
                mask := uintptr(1)<<(4-(w-nw)) - 1
                hb &= mask | mask<<4 // apply mask to both pointer bits and scan bits
        }

        // Change nw from counting possibly-pointer words to total words in allocation.
        nw = size / sys.PtrSize

        // Write whole bitmap bytes.
        // The first is hb, the rest are zero.
        if w <= nw {
                *hbitp = uint8(hb)
                hbitp = add1(hbitp)
                hb = 0 // for possible final half-byte below
                for w += 4; w <= nw; w += 4 {
                        *hbitp = 0
                        hbitp = add1(hbitp)
                }
        }

        // Write final partial bitmap byte if any.
        // We know w > nw, or else we'd still be in the loop above.
        // It can be bigger only due to the 4 entries in hb that it counts.
        // If w == nw+4 then there's nothing left to do: we wrote all nw entries
        // and can discard the 4 sitting in hb.
        // But if w == nw+2, we need to write first two in hb.
        // The byte is shared with the next object, so be careful with
        // existing bits.
        if w == nw+2 {
                *hbitp = *hbitp&^(bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift) | uint8(hb)
        }

Phase4:
        // Phase 4: Copy unrolled bitmap to per-arena bitmaps, if necessary.
        if outOfPlace {
                // TODO: We could probably make this faster by
                // handling [x+dataSize, x+size) specially.
                h := heapBitsForAddr(x)
                // cnw is the number of heap words, or bit pairs
                // remaining (like nw above).
                cnw := size / sys.PtrSize
                src := (*uint8)(unsafe.Pointer(x))
                // We know the first and last byte of the bitmap are
                // not the same, but it's still possible for small
                // objects span arenas, so it may share bitmap bytes
                // with neighboring objects.
                //
                // Handle the first byte specially if it's shared. See
                // Phase 1 for why this is the only special case we need.
                if doubleCheck {
                        if !(h.shift == 0 || h.shift == 2) {
                                print("x=", x, " size=", size, " cnw=", h.shift, "\n")
                                throw("bad start shift")
                        }
                }
                if h.shift == 2 {
                        *h.bitp = *h.bitp&^((bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift)<<(2*heapBitsShift)) | *src
                        h = h.next().next()
                        cnw -= 2
                        src = addb(src, 1)
                }
                // We're now byte aligned. Copy out to per-arena
                // bitmaps until the last byte (which may again be
                // partial).
                for cnw >= 4 {
                        // This loop processes four words at a time,
                        // so round cnw down accordingly.
                        hNext, words := h.forwardOrBoundary(cnw / 4 * 4)

                        // n is the number of bitmap bytes to copy.
                        n := words / 4
                        memmove(unsafe.Pointer(h.bitp), unsafe.Pointer(src), n)
                        cnw -= words
                        h = hNext
                        src = addb(src, n)
                }
                if doubleCheck && h.shift != 0 {
                        print("cnw=", cnw, " h.shift=", h.shift, "\n")
                        throw("bad shift after block copy")
                }
                // Handle the last byte if it's shared.
                if cnw == 2 {
                        *h.bitp = *h.bitp&^(bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift) | *src
                        src = addb(src, 1)
                        h = h.next().next()
                }
                if doubleCheck {
                        if uintptr(unsafe.Pointer(src)) > x+size {
                                throw("copy exceeded object size")
                        }
                        if !(cnw == 0 || cnw == 2) {
                                print("x=", x, " size=", size, " cnw=", cnw, "\n")
                                throw("bad number of remaining words")
                        }
                        // Set up hbitp so doubleCheck code below can check it.
                        hbitp = h.bitp
                }
                // Zero the object where we wrote the bitmap.
                memclrNoHeapPointers(unsafe.Pointer(x), uintptr(unsafe.Pointer(src))-x)
        }

        // Double check the whole bitmap.
        if doubleCheck {
                // x+size may not point to the heap, so back up one
                // word and then advance it the way we do above.
                end := heapBitsForAddr(x + size - sys.PtrSize)
                if outOfPlace {
                        // In out-of-place copying, we just advance
                        // using next.
                        end = end.next()
                } else {
                        // Don't use next because that may advance to
                        // the next arena and the in-place logic
                        // doesn't do that.
                        end.shift += heapBitsShift
                        if end.shift == 4*heapBitsShift {
                                end.bitp, end.shift = add1(end.bitp), 0
                        }
                }
                if typ.kind&kindGCProg == 0 && (hbitp != end.bitp || (w == nw+2) != (end.shift == 2)) {
                        println("ended at wrong bitmap byte for", typ.string(), "x", dataSize/typ.size)
                        print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
                        print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
                        h0 := heapBitsForAddr(x)
                        print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
                        print("ended at hbitp=", hbitp, " but next starts at bitp=", end.bitp, " shift=", end.shift, "\n")
                        throw("bad heapBitsSetType")
                }

                // Double-check that bits to be written were written correctly.
                // Does not check that other bits were not written, unfortunately.
                h := heapBitsForAddr(x)
                nptr := typ.ptrdata / sys.PtrSize
                ndata := typ.size / sys.PtrSize
                count := dataSize / typ.size
                totalptr := ((count-1)*typ.size + typ.ptrdata) / sys.PtrSize
                for i := uintptr(0); i < size/sys.PtrSize; i++ {
                        j := i % ndata
                        var have, want uint8
                        have = (*h.bitp >> h.shift) & (bitPointer | bitScan)
                        if i >= totalptr {
                                if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 {
                                        // heapBitsSetTypeGCProg always fills
                                        // in full nibbles of bitScan.
                                        want = bitScan
                                }
                        } else {
                                if j < nptr && (*addb(ptrmask, j/8)>>(j%8))&1 != 0 {
                                        want |= bitPointer
                                }
                                want |= bitScan
                        }
                        if have != want {
                                println("mismatch writing bits for", typ.string(), "x", dataSize/typ.size)
                                print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
                                print("kindGCProg=", typ.kind&kindGCProg != 0, " outOfPlace=", outOfPlace, "\n")
                                print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
                                h0 := heapBitsForAddr(x)
                                print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
                                print("current bits h.bitp=", h.bitp, " h.shift=", h.shift, " *h.bitp=", hex(*h.bitp), "\n")
                                print("ptrmask=", ptrmask, " p=", p, " endp=", endp, " endnb=", endnb, " pbits=", hex(pbits), " b=", hex(b), " nb=", nb, "\n")
                                println("at word", i, "offset", i*sys.PtrSize, "have", hex(have), "want", hex(want))
                                if typ.kind&kindGCProg != 0 {
                                        println("GC program:")
                                        dumpGCProg(addb(typ.gcdata, 4))
                                }
                                throw("bad heapBitsSetType")
                        }
                        h = h.next()
                }
                if ptrmask == debugPtrmask.data {
                        unlock(&debugPtrmask.lock)
                }
        }
}

var debugPtrmask struct {
        lock mutex
        data *byte
}

// heapBitsSetTypeGCProg implements heapBitsSetType using a GC program.
// progSize is the size of the memory described by the program.
// elemSize is the size of the element that the GC program describes (a prefix of).
// dataSize is the total size of the intended data, a multiple of elemSize.
// allocSize is the total size of the allocated memory.
//
// GC programs are only used for large allocations.
// heapBitsSetType requires that allocSize is a multiple of 4 words,
// so that the relevant bitmap bytes are not shared with surrounding
// objects.
func heapBitsSetTypeGCProg(h heapBits, progSize, elemSize, dataSize, allocSize uintptr, prog *byte) {
        if sys.PtrSize == 8 && allocSize%(4*sys.PtrSize) != 0 {
                // Alignment will be wrong.
                throw("heapBitsSetTypeGCProg: small allocation")
        }
        var totalBits uintptr
        if elemSize == dataSize {
                totalBits = runGCProg(prog, nil, h.bitp, 2)
                if totalBits*sys.PtrSize != progSize {
                        println("runtime: heapBitsSetTypeGCProg: total bits", totalBits, "but progSize", progSize)
                        throw("heapBitsSetTypeGCProg: unexpected bit count")
                }
        } else {
                count := dataSize / elemSize

                // Piece together program trailer to run after prog that does:
                //      literal(0)
                //      repeat(1, elemSize-progSize-1) // zeros to fill element size
                //      repeat(elemSize, count-1) // repeat that element for count
                // This zero-pads the data remaining in the first element and then
                // repeats that first element to fill the array.
                var trailer [40]byte // 3 varints (max 10 each) + some bytes
                i := 0
                if n := elemSize/sys.PtrSize - progSize/sys.PtrSize; n > 0 {
                        // literal(0)
                        trailer[i] = 0x01
                        i++
                        trailer[i] = 0
                        i++
                        if n > 1 {
                                // repeat(1, n-1)
                                trailer[i] = 0x81
                                i++
                                n--
                                for ; n >= 0x80; n >>= 7 {
                                        trailer[i] = byte(n | 0x80)
                                        i++
                                }
                                trailer[i] = byte(n)
                                i++
                        }
                }
                // repeat(elemSize/ptrSize, count-1)
                trailer[i] = 0x80
                i++
                n := elemSize / sys.PtrSize
                for ; n >= 0x80; n >>= 7 {
                        trailer[i] = byte(n | 0x80)
                        i++
                }
                trailer[i] = byte(n)
                i++
                n = count - 1
                for ; n >= 0x80; n >>= 7 {
                        trailer[i] = byte(n | 0x80)
                        i++
                }
                trailer[i] = byte(n)
                i++
                trailer[i] = 0
                i++

                runGCProg(prog, &trailer[0], h.bitp, 2)

                // Even though we filled in the full array just now,
                // record that we only filled in up to the ptrdata of the
                // last element. This will cause the code below to
                // memclr the dead section of the final array element,
                // so that scanobject can stop early in the final element.
                totalBits = (elemSize*(count-1) + progSize) / sys.PtrSize
        }
        endProg := unsafe.Pointer(addb(h.bitp, (totalBits+3)/4))
        endAlloc := unsafe.Pointer(addb(h.bitp, allocSize/sys.PtrSize/wordsPerBitmapByte))
        memclrNoHeapPointers(endProg, uintptr(endAlloc)-uintptr(endProg))
}

// progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
// size the size of the region described by prog, in bytes.
// The resulting bitvector will have no more than size/sys.PtrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
        n := (size/sys.PtrSize + 7) / 8
        x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
        x[len(x)-1] = 0xa1 // overflow check sentinel
        n = runGCProg(prog, nil, &x[0], 1)
        if x[len(x)-1] != 0xa1 {
                throw("progToPointerMask: overflow")
        }
        return bitvector{int32(n), &x[0]}
}

// Packed GC pointer bitmaps, aka GC programs.
//
// For large types containing arrays, the type information has a
// natural repetition that can be encoded to save space in the
// binary and in the memory representation of the type information.
//
// The encoding is a simple Lempel-Ziv style bytecode machine
// with the following instructions:
//
//      00000000: stop
//      0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
//      10000000 n c: repeat the previous n bits c times; n, c are varints
//      1nnnnnnn c: repeat the previous n bits c times; c is a varint

// runGCProg executes the GC program prog, and then trailer if non-nil,
// writing to dst with entries of the given size.
// If size == 1, dst is a 1-bit pointer mask laid out moving forward from dst.
// If size == 2, dst is the 2-bit heap bitmap, and writes move backward
// starting at dst (because the heap bitmap does). In this case, the caller guarantees
// that only whole bytes in dst need to be written.
//
// runGCProg returns the number of 1- or 2-bit entries written to memory.
func runGCProg(prog, trailer, dst *byte, size int) uintptr {
        dstStart := dst

        // Bits waiting to be written to memory.
        var bits uintptr
        var nbits uintptr

        p := prog
Run:
        for {
                // Flush accumulated full bytes.
                // The rest of the loop assumes that nbits <= 7.
                for ; nbits >= 8; nbits -= 8 {
                        if size == 1 {
                                *dst = uint8(bits)
                                dst = add1(dst)
                                bits >>= 8
                        } else {
                                v := bits&bitPointerAll | bitScanAll
                                *dst = uint8(v)
                                dst = add1(dst)
                                bits >>= 4
                                v = bits&bitPointerAll | bitScanAll
                                *dst = uint8(v)
                                dst = add1(dst)
                                bits >>= 4
                        }
                }

                // Process one instruction.
                inst := uintptr(*p)
                p = add1(p)
                n := inst & 0x7F
                if inst&0x80 == 0 {
                        // Literal bits; n == 0 means end of program.
                        if n == 0 {
                                // Program is over; continue in trailer if present.
                                if trailer != nil {
                                        p = trailer
                                        trailer = nil
                                        continue
                                }
                                break Run
                        }
                        nbyte := n / 8
                        for i := uintptr(0); i < nbyte; i++ {
                                bits |= uintptr(*p) << nbits
                                p = add1(p)
                                if size == 1 {
                                        *dst = uint8(bits)
                                        dst = add1(dst)
                                        bits >>= 8
                                } else {
                                        v := bits&0xf | bitScanAll
                                        *dst = uint8(v)
                                        dst = add1(dst)
                                        bits >>= 4
                                        v = bits&0xf | bitScanAll
                                        *dst = uint8(v)
                                        dst = add1(dst)
                                        bits >>= 4
                                }
                        }
                        if n %= 8; n > 0 {
                                bits |= uintptr(*p) << nbits
                                p = add1(p)
                                nbits += n
                        }
                        continue Run
                }

                // Repeat. If n == 0, it is encoded in a varint in the next bytes.
                if n == 0 {
                        for off := uint(0); ; off += 7 {
                                x := uintptr(*p)
                                p = add1(p)
                                n |= (x & 0x7F) << off
                                if x&0x80 == 0 {
                                        break
                                }
                        }
                }

                // Count is encoded in a varint in the next bytes.
                c := uintptr(0)
                for off := uint(0); ; off += 7 {
                        x := uintptr(*p)
                        p = add1(p)
                        c |= (x & 0x7F) << off
                        if x&0x80 == 0 {
                                break
                        }
                }
                c *= n // now total number of bits to copy

                // If the number of bits being repeated is small, load them
                // into a register and use that register for the entire loop
                // instead of repeatedly reading from memory.
                // Handling fewer than 8 bits here makes the general loop simpler.
                // The cutoff is sys.PtrSize*8 - 7 to guarantee that when we add
                // the pattern to a bit buffer holding at most 7 bits (a partial byte)
                // it will not overflow.
                src := dst
                const maxBits = sys.PtrSize*8 - 7
                if n <= maxBits {
                        // Start with bits in output buffer.
                        pattern := bits
                        npattern := nbits

                        // If we need more bits, fetch them from memory.
                        if size == 1 {
                                src = subtract1(src)
                                for npattern < n {
                                        pattern <<= 8
                                        pattern |= uintptr(*src)
                                        src = subtract1(src)
                                        npattern += 8
                                }
                        } else {
                                src = subtract1(src)
                                for npattern < n {
                                        pattern <<= 4
                                        pattern |= uintptr(*src) & 0xf
                                        src = subtract1(src)
                                        npattern += 4
                                }
                        }

                        // We started with the whole bit output buffer,
                        // and then we loaded bits from whole bytes.
                        // Either way, we might now have too many instead of too few.
                        // Discard the extra.
                        if npattern > n {
                                pattern >>= npattern - n
                                npattern = n
                        }

                        // Replicate pattern to at most maxBits.
                        if npattern == 1 {
                                // One bit being repeated.
                                // If the bit is 1, make the pattern all 1s.
                                // If the bit is 0, the pattern is already all 0s,
                                // but we can claim that the number of bits
                                // in the word is equal to the number we need (c),
                                // because right shift of bits will zero fill.
                                if pattern == 1 {
                                        pattern = 1<<maxBits - 1
                                        npattern = maxBits
                                } else {
                                        npattern = c
                                }
                        } else {
                                b := pattern
                                nb := npattern
                                if nb+nb <= maxBits {
                                        // Double pattern until the whole uintptr is filled.
                                        for nb <= sys.PtrSize*8 {
                                                b |= b << nb
                                                nb += nb
                                        }
                                        // Trim away incomplete copy of original pattern in high bits.
                                        // TODO(rsc): Replace with table lookup or loop on systems without divide?
                                        nb = maxBits / npattern * npattern
                                        b &= 1<<nb - 1
                                        pattern = b
                                        npattern = nb
                                }
                        }

                        // Add pattern to bit buffer and flush bit buffer, c/npattern times.
                        // Since pattern contains >8 bits, there will be full bytes to flush
                        // on each iteration.
                        for ; c >= npattern; c -= npattern {
                                bits |= pattern << nbits
                                nbits += npattern
                                if size == 1 {
                                        for nbits >= 8 {
                                                *dst = uint8(bits)
                                                dst = add1(dst)
                                                bits >>= 8
                                                nbits -= 8
                                        }
                                } else {
                                        for nbits >= 4 {
                                                *dst = uint8(bits&0xf | bitScanAll)
                                                dst = add1(dst)
                                                bits >>= 4
                                                nbits -= 4
                                        }
                                }
                        }

                        // Add final fragment to bit buffer.
                        if c > 0 {
                                pattern &= 1<<c - 1
                                bits |= pattern << nbits
                                nbits += c
                        }
                        continue Run
                }

                // Repeat; n too large to fit in a register.
                // Since nbits <= 7, we know the first few bytes of repeated data
                // are already written to memory.
                off := n - nbits // n > nbits because n > maxBits and nbits <= 7
                if size == 1 {
                        // Leading src fragment.
                        src = subtractb(src, (off+7)/8)
                        if frag := off & 7; frag != 0 {
                                bits |= uintptr(*src) >> (8 - frag) << nbits
                                src = add1(src)
                                nbits += frag
                                c -= frag
                        }
                        // Main loop: load one byte, write another.
                        // The bits are rotating through the bit buffer.
                        for i := c / 8; i > 0; i-- {
                                bits |= uintptr(*src) << nbits
                                src = add1(src)
                                *dst = uint8(bits)
                                dst = add1(dst)
                                bits >>= 8
                        }
                        // Final src fragment.
                        if c %= 8; c > 0 {
                                bits |= (uintptr(*src) & (1<<c - 1)) << nbits
                                nbits += c
                        }
                } else {
                        // Leading src fragment.
                        src = subtractb(src, (off+3)/4)
                        if frag := off & 3; frag != 0 {
                                bits |= (uintptr(*src) & 0xf) >> (4 - frag) << nbits
                                src = add1(src)
                                nbits += frag
                                c -= frag
                        }
                        // Main loop: load one byte, write another.
                        // The bits are rotating through the bit buffer.
                        for i := c / 4; i > 0; i-- {
                                bits |= (uintptr(*src) & 0xf) << nbits
                                src = add1(src)
                                *dst = uint8(bits&0xf | bitScanAll)
                                dst = add1(dst)
                                bits >>= 4
                        }
                        // Final src fragment.
                        if c %= 4; c > 0 {
                                bits |= (uintptr(*src) & (1<<c - 1)) << nbits
                                nbits += c
                        }
                }
        }

        // Write any final bits out, using full-byte writes, even for the final byte.
        var totalBits uintptr
        if size == 1 {
                totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
                nbits += -nbits & 7
                for ; nbits > 0; nbits -= 8 {
                        *dst = uint8(bits)
                        dst = add1(dst)
                        bits >>= 8
                }
        } else {
                totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*4 + nbits
                nbits += -nbits & 3
                for ; nbits > 0; nbits -= 4 {
                        v := bits&0xf | bitScanAll
                        *dst = uint8(v)
                        dst = add1(dst)
                        bits >>= 4
                }
        }
        return totalBits
}

// materializeGCProg allocates space for the (1-bit) pointer bitmask
// for an object of size ptrdata.  Then it fills that space with the
// pointer bitmask specified by the program prog.
// The bitmask starts at s.startAddr.
// The result must be deallocated with dematerializeGCProg.
func materializeGCProg(ptrdata uintptr, prog *byte) *mspan {
        // Each word of ptrdata needs one bit in the bitmap.
        bitmapBytes := divRoundUp(ptrdata, 8*sys.PtrSize)
        // Compute the number of pages needed for bitmapBytes.
        pages := divRoundUp(bitmapBytes, pageSize)
        s := mheap_.allocManual(pages, spanAllocPtrScalarBits)
        runGCProg(addb(prog, 4), nil, (*byte)(unsafe.Pointer(s.startAddr)), 1)
        return s
}
func dematerializeGCProg(s *mspan) {
        mheap_.freeManual(s, spanAllocPtrScalarBits)
}

func dumpGCProg(p *byte) {
        nptr := 0
        for {
                x := *p
                p = add1(p)
                if x == 0 {
                        print("\t", nptr, " end\n")
                        break
                }
                if x&0x80 == 0 {
                        print("\t", nptr, " lit ", x, ":")
                        n := int(x+7) / 8
                        for i := 0; i < n; i++ {
                                print(" ", hex(*p))
                                p = add1(p)
                        }
                        print("\n")
                        nptr += int(x)
                } else {
                        nbit := int(x &^ 0x80)
                        if nbit == 0 {
                                for nb := uint(0); ; nb += 7 {
                                        x := *p
                                        p = add1(p)
                                        nbit |= int(x&0x7f) << nb
                                        if x&0x80 == 0 {
                                                break
                                        }
                                }
                        }
                        count := 0
                        for nb := uint(0); ; nb += 7 {
                                x := *p
                                p = add1(p)
                                count |= int(x&0x7f) << nb
                                if x&0x80 == 0 {
                                        break
                                }
                        }
                        print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
                        nptr += nbit * count
                }
        }
}

// Testing.

func getgcmaskcb(frame *stkframe, ctxt unsafe.Pointer) bool {
        target := (*stkframe)(ctxt)
        if frame.sp <= target.sp && target.sp < frame.varp {
                *target = *frame
                return false
        }
        return true
}

// gcbits returns the GC type info for x, for testing.
// The result is the bitmap entries (0 or 1), one entry per byte.
//go:linkname reflect_gcbits reflect.gcbits
func reflect_gcbits(x interface{}) []byte {
        ret := getgcmask(x)
        typ := (*ptrtype)(unsafe.Pointer(efaceOf(&x)._type)).elem
        nptr := typ.ptrdata / sys.PtrSize
        for uintptr(len(ret)) > nptr && ret[len(ret)-1] == 0 {
                ret = ret[:len(ret)-1]
        }
        return ret
}

// Returns GC type info for the pointer stored in ep for testing.
// If ep points to the stack, only static live information will be returned
// (i.e. not for objects which are only dynamically live stack objects).
func getgcmask(ep interface{}) (mask []byte) {
        e := *efaceOf(&ep)
        p := e.data
        t := e._type
        // data or bss
        for _, datap := range activeModules() {
                // data
                if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
                        bitmap := datap.gcdatamask.bytedata
                        n := (*ptrtype)(unsafe.Pointer(t)).elem.size
                        mask = make([]byte, n/sys.PtrSize)
                        for i := uintptr(0); i < n; i += sys.PtrSize {
                                off := (uintptr(p) + i - datap.data) / sys.PtrSize
                                mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
                        }
                        return
                }

                // bss
                if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
                        bitmap := datap.gcbssmask.bytedata
                        n := (*ptrtype)(unsafe.Pointer(t)).elem.size
                        mask = make([]byte, n/sys.PtrSize)
                        for i := uintptr(0); i < n; i += sys.PtrSize {
                                off := (uintptr(p) + i - datap.bss) / sys.PtrSize
                                mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
                        }
                        return
                }
        }

        // heap
        if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
                hbits := heapBitsForAddr(base)
                n := s.elemsize
                mask = make([]byte, n/sys.PtrSize)
                for i := uintptr(0); i < n; i += sys.PtrSize {
                        if hbits.isPointer() {
                                mask[i/sys.PtrSize] = 1
                        }
                        if !hbits.morePointers() {
                                mask = mask[:i/sys.PtrSize]
                                break
                        }
                        hbits = hbits.next()
                }
                return
        }

        // stack
        if _g_ := getg(); _g_.m.curg.stack.lo <= uintptr(p) && uintptr(p) < _g_.m.curg.stack.hi {
                var frame stkframe
                frame.sp = uintptr(p)
                _g_ := getg()
                gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
                if frame.fn.valid() {
                        locals, _, _ := getStackMap(&frame, nil, false)
                        if locals.n == 0 {
                                return
                        }
                        size := uintptr(locals.n) * sys.PtrSize
                        n := (*ptrtype)(unsafe.Pointer(t)).elem.size
                        mask = make([]byte, n/sys.PtrSize)
                        for i := uintptr(0); i < n; i += sys.PtrSize {
                                off := (uintptr(p) + i - frame.varp + size) / sys.PtrSize
                                mask[i/sys.PtrSize] = locals.ptrbit(off)
                        }
                }
                return
        }

        // otherwise, not something the GC knows about.
        // possibly read-only data, like malloc(0).
        // must not have pointers
        return
}