runtime: redo heap bitmap

[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 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting at "start",
// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
// through start+63*ptrSize, ha.bitmap[1] holds the entries for
// start+64*ptrSize through start+127*ptrSize, and so on.
// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
// (For 32-bit platforms, s/64/32/.)
//
// We also keep a noMorePtrs bitmap which allows us to stop scanning
// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
// has no more pointers beyond those described by ha.bitmap[8*i+j].
// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
// beyond must all be zero until the start of the next object.
//
// The bitmap for noscan spans is not maintained (can be junk). 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.
//
// The bitmap for unallocated objects is also not maintained.

package runtime

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

// 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.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nowritebarrier
//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))
}

// 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++
}

// clobberdeadPtr is a special value that is used by the compiler to
// clobber dead stack slots, when -clobberdead flag is set.
const clobberdeadPtr = uintptr(0xdeaddead | 0xdeaddead<<((^uintptr(0)>>63)*32))

// 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))
        if s != nil {
                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)
        }
        print("\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 {
                if (GOARCH == "amd64" || GOARCH == "arm64") && p == clobberdeadPtr && debug.invalidptr != 0 {
                        // Crash if clobberdeadPtr is seen. Only on AMD64 and ARM64 for now,
                        // as they are the only platform where compiler's clobberdead mode is
                        // implemented. On these platforms clobberdeadPtr cannot be a valid address.
                        badPointer(s, p, refBase, refOff)
                }
                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
}

// verifyNotInHeapPtr reports whether converting the not-in-heap pointer into a unsafe.Pointer is ok.
//
//go:linkname reflect_verifyNotInHeapPtr reflect.verifyNotInHeapPtr
func reflect_verifyNotInHeapPtr(p uintptr) bool {
        // Conversion to a pointer is ok as long as findObject above does not call badPointer.
        // Since we're already promised that p doesn't point into the heap, just disallow heap
        // pointers and the special clobbered pointer.
        return spanOf(p) == nil && p != clobberdeadPtr
}

const ptrBits = 8 * goarch.PtrSize

// 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 {
        // heapBits will report on pointers in the range [addr,addr+size).
        // The low bit of mask contains the pointerness of the word at addr
        // (assuming valid>0).
        addr, size uintptr

        // The next few pointer bits representing words starting at addr.
        // Those bits already returned by next() are zeroed.
        mask uintptr
        // Number of bits in mask that are valid. mask is always less than 1<<valid.
        valid uintptr
}

// heapBitsForAddr returns the heapBits for the address addr.
// The caller must ensure [addr,addr+size) 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, size uintptr) heapBits {
        // Find arena
        ai := arenaIndex(addr)
        ha := mheap_.arenas[ai.l1()][ai.l2()]

        // Word index in arena.
        word := addr / goarch.PtrSize % heapArenaWords

        // Word index and bit offset in bitmap array.
        idx := word / ptrBits
        off := word % ptrBits

        // Grab relevant bits of bitmap.
        mask := ha.bitmap[idx] >> off
        valid := ptrBits - off

        // Process depending on where the object ends.
        nptr := size / goarch.PtrSize
        if nptr < valid {
                // Bits for this object end before the end of this bitmap word.
                // Squash bits for the following objects.
                mask &= 1<<(nptr&(ptrBits-1)) - 1
                valid = nptr
        } else if nptr == valid {
                // Bits for this object end at exactly the end of this bitmap word.
                // All good.
        } else {
                // Bits for this object extend into the next bitmap word. See if there
                // may be any pointers recorded there.
                if uintptr(ha.noMorePtrs[idx/8])>>(idx%8)&1 != 0 {
                        // No more pointers in this object after this bitmap word.
                        // Update size so we know not to look there.
                        size = valid * goarch.PtrSize
                }
        }

        return heapBits{addr: addr, size: size, mask: mask, valid: valid}
}

// Returns the (absolute) address of the next known pointer and
// a heapBits iterator representing any remaining pointers.
// If there are no more pointers, returns address 0.
// 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, uintptr) {
        for {
                if h.mask != 0 {
                        var i int
                        if goarch.PtrSize == 8 {
                                i = sys.Ctz64(uint64(h.mask))
                        } else {
                                i = sys.Ctz32(uint32(h.mask))
                        }
                        h.mask ^= uintptr(1) << (i & (ptrBits - 1))
                        return h, h.addr + uintptr(i)*goarch.PtrSize
                }

                // Skip words that we've already processed.
                h.addr += h.valid * goarch.PtrSize
                h.size -= h.valid * goarch.PtrSize
                if h.size == 0 {
                        return h, 0 // no more pointers
                }

                // Grab more bits and try again.
                h = heapBitsForAddr(h.addr, h.size)
        }
}

// nextFast is like next, but can return 0 even when there are more pointers
// to be found. Callers should call next if nextFast returns 0 as its second
// return value.
//     if addr, h = h.nextFast(); addr == 0 {
//         if addr, h = h.next(); addr == 0 {
//             ... no more pointers ...
//         }
//     }
//     ... process pointer at addr ...
// nextFast is designed to be inlineable.
//
//go:nosplit
func (h heapBits) nextFast() (heapBits, uintptr) {
        // TESTQ/JEQ
        if h.mask == 0 {
                return h, 0
        }
        // BSFQ
        var i int
        if goarch.PtrSize == 8 {
                i = sys.Ctz64(uint64(h.mask))
        } else {
                i = sys.Ctz32(uint32(h.mask))
        }
        // BTCQ
        h.mask ^= uintptr(1) << (i & (ptrBits - 1))
        // LEAQ (XX)(XX*8)
        return h, h.addr + uintptr(i)*goarch.PtrSize
}

// 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)&(goarch.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, size)
        if src == 0 {
                for {
                        var addr uintptr
                        if h, addr = h.next(); addr == 0 {
                                break
                        }
                        dstx := (*uintptr)(unsafe.Pointer(addr))
                        if !buf.putFast(*dstx, 0) {
                                wbBufFlush(nil, 0)
                        }
                }
        } else {
                for {
                        var addr uintptr
                        if h, addr = h.next(); addr == 0 {
                                break
                        }
                        dstx := (*uintptr)(unsafe.Pointer(addr))
                        srcx := (*uintptr)(unsafe.Pointer(src + (addr - dst)))
                        if !buf.putFast(*dstx, *srcx) {
                                wbBufFlush(nil, 0)
                        }
                }
        }
}

// 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)&(goarch.PtrSize-1) != 0 {
                throw("bulkBarrierPreWrite: unaligned arguments")
        }
        if !writeBarrier.needed {
                return
        }
        buf := &getg().m.p.ptr().wbBuf
        h := heapBitsForAddr(dst, size)
        for {
                var addr uintptr
                if h, addr = h.next(); addr == 0 {
                        break
                }
                srcx := (*uintptr)(unsafe.Pointer(addr - dst + src))
                if !buf.putFast(0, *srcx) {
                        wbBufFlush(nil, 0)
                }
        }
}

// 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 / goarch.PtrSize
        bits = addb(bits, word/8)
        mask := uint8(1) << (word % 8)

        buf := &getg().m.p.ptr().wbBuf
        for i := uintptr(0); i < size; i += goarch.PtrSize {
                if mask == 0 {
                        bits = addb(bits, 1)
                        if *bits == 0 {
                                // Skip 8 words.
                                i += 7 * goarch.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 += goarch.PtrSize {
                if i&(goarch.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)
                        }
                }
        }
}

// initHeapBits initializes the heap bitmap for a span.
// If this is a span of single pointer allocations, it initializes all
// words to pointer.
func (s *mspan) initHeapBits() {
        isPtrs := goarch.PtrSize == 8 && s.elemsize == goarch.PtrSize
        if !isPtrs {
                return // nothing to do
        }
        h := writeHeapBitsForAddr(s.base())
        size := s.npages * pageSize
        nptrs := size / goarch.PtrSize
        for i := uintptr(0); i < nptrs; i += ptrBits {
                h = h.write(^uintptr(0), ptrBits)
        }
        h.flush(s.base(), size)
}

// 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
}

type writeHeapBits struct {
        addr  uintptr // address that the low bit of mask represents the pointer state of.
        mask  uintptr // some pointer bits starting at the address addr.
        valid uintptr // number of bits in buf that are valid (including low)
        low   uintptr // number of low-order bits to not overwrite
}

func writeHeapBitsForAddr(addr uintptr) (h writeHeapBits) {
        // We start writing bits maybe in the middle of a heap bitmap word.
        // Remember how many bits into the word we started, so we can be sure
        // not to overwrite the previous bits.
        h.low = addr / goarch.PtrSize % ptrBits

        // round down to heap word that starts the bitmap word.
        h.addr = addr - h.low*goarch.PtrSize

        // We don't have any bits yet.
        h.mask = 0
        h.valid = h.low

        return
}

// write appends the pointerness of the next valid pointer slots
// using the low valid bits of bits. 1=pointer, 0=scalar.
func (h writeHeapBits) write(bits, valid uintptr) writeHeapBits {
        if h.valid+valid <= ptrBits {
                // Fast path - just accumulate the bits.
                h.mask |= bits << h.valid
                h.valid += valid
                return h
        }
        // Too many bits to fit in this word. Write the current word
        // out and move on to the next word.

        data := h.mask | bits<<h.valid       // mask for this word
        h.mask = bits >> (ptrBits - h.valid) // leftover for next word
        h.valid += valid - ptrBits           // have h.valid+valid bits, writing ptrBits of them

        // Flush mask to the memory bitmap.
        // TODO: figure out how to cache arena lookup.
        ai := arenaIndex(h.addr)
        ha := mheap_.arenas[ai.l1()][ai.l2()]
        idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
        m := uintptr(1)<<h.low - 1
        ha.bitmap[idx] = ha.bitmap[idx]&m | data
        // Note: no synchronization required for this write because
        // the allocator has exclusive access to the page, and the bitmap
        // entries are all for a single page. Also, visibility of these
        // writes is guaranteed by the publication barrier in mallocgc.

        // Clear noMorePtrs bit, since we're going to be writing bits
        // into the following word.
        ha.noMorePtrs[idx/8] &^= uint8(1) << (idx % 8)
        // Note: same as above

        // Move to next word of bitmap.
        h.addr += ptrBits * goarch.PtrSize
        h.low = 0
        return h
}

// Add padding of size bytes.
func (h writeHeapBits) pad(size uintptr) writeHeapBits {
        if size == 0 {
                return h
        }
        words := size / goarch.PtrSize
        for words > ptrBits {
                h = h.write(0, ptrBits)
                words -= ptrBits
        }
        return h.write(0, words)
}

// Flush the bits that have been written, and add zeros as needed
// to cover the full object [addr, addr+size).
func (h writeHeapBits) flush(addr, size uintptr) {
        // zeros counts the number of bits needed to represent the object minus the
        // number of bits we've already written. This is the number of 0 bits
        // that need to be added.
        zeros := (addr+size-h.addr)/goarch.PtrSize - h.valid

        // Add zero bits up to the bitmap word boundary
        if zeros > 0 {
                z := ptrBits - h.valid
                if z > zeros {
                        z = zeros
                }
                h.valid += z
                zeros -= z
        }

        // Find word in bitmap that we're going to write.
        ai := arenaIndex(h.addr)
        ha := mheap_.arenas[ai.l1()][ai.l2()]
        idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords

        // Write remaining bits.
        if h.valid != h.low {
                m := uintptr(1)<<h.low - 1      // don't clear existing bits below "low"
                m |= ^(uintptr(1)<<h.valid - 1) // don't clear existing bits above "valid"
                ha.bitmap[idx] = ha.bitmap[idx]&m | h.mask
        }
        if zeros == 0 {
                return
        }

        // Record in the noMorePtrs map that there won't be any more 1 bits,
        // so readers can stop early.
        ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)

        // Advance to next bitmap word.
        h.addr += ptrBits * goarch.PtrSize

        // Continue on writing zeros for the rest of the object.
        // For standard use of the ptr bits this is not required, as
        // the bits are read from the beginning of the object. Some uses,
        // like oblets, bulk write barriers, and cgocheck, might
        // start mid-object, so these writes are still required.
        for {
                // Write zero bits.
                ai := arenaIndex(h.addr)
                ha := mheap_.arenas[ai.l1()][ai.l2()]
                idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
                if zeros < ptrBits {
                        ha.bitmap[idx] &^= uintptr(1)<<zeros - 1
                        break
                } else if zeros == ptrBits {
                        ha.bitmap[idx] = 0
                        break
                } else {
                        ha.bitmap[idx] = 0
                        zeros -= ptrBits
                }
                ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
                h.addr += ptrBits * goarch.PtrSize
        }
}

// 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 word 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 = true // slow but helpful; enable to test modifications to this code

        if doubleCheck && dataSize%typ.size != 0 {
                throw("heapBitsSetType: dataSize not a multiple of typ.size")
        }

        if goarch.PtrSize == 8 && size == goarch.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, addr := heapBitsForAddr(x, size).next()
                        if addr != x {
                                throw("heapBitsSetType: pointer bit missing")
                        }
                        _, addr = h.next()
                        if addr != 0 {
                                throw("heapBitsSetType: second pointer bit found")
                        }
                }
                return
        }

        h := writeHeapBitsForAddr(x)

        // Handle GC program.
        if typ.kind&kindGCProg != 0 {
                // Expand the gc program into the storage we're going to use for the actual object.
                obj := (*uint8)(unsafe.Pointer(x))
                n := runGCProg(addb(typ.gcdata, 4), obj)
                // Use the expanded program to set the heap bits.
                for i := uintptr(0); true; i += typ.size {
                        // Copy expanded program to heap bitmap.
                        p := obj
                        j := n
                        for j > 8 {
                                h = h.write(uintptr(*p), 8)
                                p = add1(p)
                                j -= 8
                        }
                        h = h.write(uintptr(*p), j)

                        if i+typ.size == dataSize {
                                break // no padding after last element
                        }

                        // Pad with zeros to the start of the next element.
                        h = h.pad(typ.size - n*goarch.PtrSize)
                }

                h.flush(x, size)

                // Erase the expanded GC program.
                memclrNoHeapPointers(unsafe.Pointer(obj), (n+7)/8)
                return
        }

        // 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. If there is more room in the allocated object,
        // that space is pointerless. The noMorePtrs bitmap will prevent
        // scanning large pointerless tails of an object.
        //
        // 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".

        for i := uintptr(0); true; i += typ.size {
                p := typ.gcdata
                var j uintptr
                for j = 0; j+8*goarch.PtrSize < typ.ptrdata; j += 8 * goarch.PtrSize {
                        h = h.write(uintptr(*p), 8)
                        p = add1(p)
                }
                h = h.write(uintptr(*p), (typ.ptrdata-j)/goarch.PtrSize)
                if i+typ.size == dataSize {
                        break // don't need the trailing nonptr bits on the last element.
                }
                // Pad with zeros to the start of the next element.
                h = h.pad(typ.size - typ.ptrdata)
        }
        h.flush(x, size)

        if doubleCheck {
                h := heapBitsForAddr(x, size)
                for i := uintptr(0); i < size; i += goarch.PtrSize {
                        // Compute the pointer bit we want at offset i.
                        want := false
                        if i < dataSize {
                                off := i % typ.size
                                if off < typ.ptrdata {
                                        j := off / goarch.PtrSize
                                        want = *addb(typ.gcdata, j/8)>>(j%8)&1 != 0
                                }
                        }
                        if want {
                                var addr uintptr
                                h, addr = h.next()
                                if addr != x+i {
                                        throw("heapBitsSetType: pointer entry not correct")
                                }
                        }
                }
                if _, addr := h.next(); addr != 0 {
                        throw("heapBitsSetType: extra pointer")
                }
        }
}

var debugPtrmask struct {
        lock mutex
        data *byte
}

// 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/goarch.PtrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
        n := (size/goarch.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, &x[0])
        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 returns the number of 1-bit entries written to memory.
func runGCProg(prog, dst *byte) 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 {
                        *dst = uint8(bits)
                        dst = add1(dst)
                        bits >>= 8
                }

                // 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.
                                break Run
                        }
                        nbyte := n / 8
                        for i := uintptr(0); i < nbyte; i++ {
                                bits |= uintptr(*p) << nbits
                                p = add1(p)
                                *dst = uint8(bits)
                                dst = add1(dst)
                                bits >>= 8
                        }
                        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 goarch.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 = goarch.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.
                        src = subtract1(src)
                        for npattern < n {
                                pattern <<= 8
                                pattern |= uintptr(*src)
                                src = subtract1(src)
                                npattern += 8
                        }

                        // 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 <= goarch.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
                                for nbits >= 8 {
                                        *dst = uint8(bits)
                                        dst = add1(dst)
                                        bits >>= 8
                                        nbits -= 8
                                }
                        }

                        // 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
                // 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
                }
        }

        // Write any final bits out, using full-byte writes, even for the final byte.
        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
        }
        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*goarch.PtrSize)
        // Compute the number of pages needed for bitmapBytes.
        pages := divRoundUp(bitmapBytes, pageSize)
        s := mheap_.allocManual(pages, spanAllocPtrScalarBits)
        runGCProg(addb(prog, 4), (*byte)(unsafe.Pointer(s.startAddr)))
        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 any) []byte {
        return getgcmask(x)
}

// 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 any) (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/goarch.PtrSize)
                        for i := uintptr(0); i < n; i += goarch.PtrSize {
                                off := (uintptr(p) + i - datap.data) / goarch.PtrSize
                                mask[i/goarch.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/goarch.PtrSize)
                        for i := uintptr(0); i < n; i += goarch.PtrSize {
                                off := (uintptr(p) + i - datap.bss) / goarch.PtrSize
                                mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
                        }
                        return
                }
        }

        // heap
        if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
                if s.spanclass.noscan() {
                        return nil
                }
                n := s.elemsize
                hbits := heapBitsForAddr(base, n)
                mask = make([]byte, n/goarch.PtrSize)
                for {
                        var addr uintptr
                        if hbits, addr = hbits.next(); addr == 0 {
                                break
                        }
                        mask[(addr-base)/goarch.PtrSize] = 1
                }
                // Callers expect this mask to end at the last pointer.
                for len(mask) > 0 && mask[len(mask)-1] == 0 {
                        mask = mask[:len(mask)-1]
                }
                return
        }

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

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