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

// Copyright 2023 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.

//go:build !goexperiment.allocheaders

// 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 set to all zero at span allocation time.
//
// The bitmap for unallocated objects in scannable spans is not maintained
// (can be junk).

package runtime

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

// heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC.
type heapArenaPtrScalar struct {
        // bitmap stores the pointer/scalar bitmap for the words in
        // this arena. See mbitmap.go for a description.
        // This array uses 1 bit per word of heap, or 1.6% of the heap size (for 64-bit).
        bitmap [heapArenaBitmapWords]uintptr

        // If the ith bit of noMorePtrs is true, then there are no more
        // pointers for the object containing the word described by the
        // high bit of bitmap[i].
        // In that case, bitmap[i+1], ... must be zero until the start
        // of the next object.
        // We never operate on these entries using bit-parallel techniques,
        // so it is ok if they are small. Also, they can't be bigger than
        // uint16 because at that size a single noMorePtrs entry
        // represents 8K of memory, the minimum size of a span. Any larger
        // and we'd have to worry about concurrent updates.
        // This array uses 1 bit per word of bitmap, or .024% of the heap size (for 64-bit).
        noMorePtrs [heapArenaBitmapWords / 8]uint8
}

// 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.TrailingZeros64(uint64(h.mask))
                        } else {
                                i = sys.TrailingZeros32(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.TrailingZeros64(uint64(h.mask))
        } else {
                i = sys.TrailingZeros32(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.PtrBytes.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
        if (dst|src|size)&(goarch.PtrSize-1) != 0 {
                throw("bulkBarrierPreWrite: unaligned arguments")
        }
        if !writeBarrier.enabled {
                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))
                        p := buf.get1()
                        p[0] = *dstx
                }
        } 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)))
                        p := buf.get2()
                        p[0] = *dstx
                        p[1] = *srcx
                }
        }
}

// 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.enabled {
                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))
                p := buf.get1()
                p[0] = *srcx
        }
}

// initHeapBits initializes the heap bitmap for a span.
// If this is a span of single pointer allocations, it initializes all
// words to pointer. If force is true, clears all bits.
func (s *mspan) initHeapBits(forceClear bool) {
        if forceClear || s.spanclass.noscan() {
                // Set all the pointer bits to zero. We do this once
                // when the span is allocated so we don't have to do it
                // for each object allocation.
                base := s.base()
                size := s.npages * pageSize
                h := writeHeapBitsForAddr(base)
                h.flush(base, size)
                return
        }
        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)
}

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 noscan spans, 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 = false // 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),
                // (*mspan).initHeapBits 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.PtrBytes is the number of words in the prefix
        // of the object that contains pointers. That is, the final
        // typ.Size - typ.PtrBytes 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.PtrBytes 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".

        ptrs := typ.PtrBytes / goarch.PtrSize
        if typ.Size_ == dataSize { // Single element
                if ptrs <= ptrBits { // Single small element
                        m := readUintptr(typ.GCData)
                        h = h.write(m, ptrs)
                } else { // Single large element
                        p := typ.GCData
                        for {
                                h = h.write(readUintptr(p), ptrBits)
                                p = addb(p, ptrBits/8)
                                ptrs -= ptrBits
                                if ptrs <= ptrBits {
                                        break
                                }
                        }
                        m := readUintptr(p)
                        h = h.write(m, ptrs)
                }
        } else { // Repeated element
                words := typ.Size_ / goarch.PtrSize // total words, including scalar tail
                if words <= ptrBits {               // Repeated small element
                        n := dataSize / typ.Size_
                        m := readUintptr(typ.GCData)
                        // Make larger unit to repeat
                        for words <= ptrBits/2 {
                                if n&1 != 0 {
                                        h = h.write(m, words)
                                }
                                n /= 2
                                m |= m << words
                                ptrs += words
                                words *= 2
                                if n == 1 {
                                        break
                                }
                        }
                        for n > 1 {
                                h = h.write(m, words)
                                n--
                        }
                        h = h.write(m, ptrs)
                } else { // Repeated large element
                        for i := uintptr(0); true; i += typ.Size_ {
                                p := typ.GCData
                                j := ptrs
                                for j > ptrBits {
                                        h = h.write(readUintptr(p), ptrBits)
                                        p = addb(p, ptrBits/8)
                                        j -= ptrBits
                                }
                                m := readUintptr(p)
                                h = h.write(m, j)
                                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.PtrBytes)
                        }
                }
        }
        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.PtrBytes {
                                        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")
                }
        }
}

// Testing.

// 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 {
                found := false
                var u unwinder
                for u.initAt(gp.m.curg.sched.pc, gp.m.curg.sched.sp, 0, gp.m.curg, 0); u.valid(); u.next() {
                        if u.frame.sp <= uintptr(p) && uintptr(p) < u.frame.varp {
                                found = true
                                break
                        }
                }
                if found {
                        locals, _, _ := u.frame.getStackMap(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 - u.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
}

// userArenaHeapBitsSetType is the equivalent of heapBitsSetType but for
// non-slice-backing-store Go values allocated in a user arena chunk. It
// sets up the heap bitmap for the value with type typ allocated at address ptr.
// base is the base address of the arena chunk.
func userArenaHeapBitsSetType(typ *_type, ptr unsafe.Pointer, base uintptr) {
        h := writeHeapBitsForAddr(uintptr(ptr))

        // Our last allocation might have ended right at a noMorePtrs mark,
        // which we would not have erased. We need to erase that mark here,
        // because we're going to start adding new heap bitmap bits.
        // We only need to clear one mark, because below we make sure to
        // pad out the bits with zeroes and only write one noMorePtrs bit
        // for each new object.
        // (This is only necessary at noMorePtrs boundaries, as noMorePtrs
        // marks within an object allocated with newAt will be erased by
        // the normal writeHeapBitsForAddr mechanism.)
        //
        // Note that we skip this if this is the first allocation in the
        // arena because there's definitely no previous noMorePtrs mark
        // (in fact, we *must* do this, because we're going to try to back
        // up a pointer to fix this up).
        if uintptr(ptr)%(8*goarch.PtrSize*goarch.PtrSize) == 0 && uintptr(ptr) != base {
                // Back up one pointer and rewrite that pointer. That will
                // cause the writeHeapBits implementation to clear the
                // noMorePtrs bit we need to clear.
                r := heapBitsForAddr(uintptr(ptr)-goarch.PtrSize, goarch.PtrSize)
                _, p := r.next()
                b := uintptr(0)
                if p == uintptr(ptr)-goarch.PtrSize {
                        b = 1
                }
                h = writeHeapBitsForAddr(uintptr(ptr) - goarch.PtrSize)
                h = h.write(b, 1)
        }

        p := typ.GCData // start of 1-bit pointer mask (or GC program)
        var gcProgBits uintptr
        if typ.Kind_&kindGCProg != 0 {
                // Expand gc program, using the object itself for storage.
                gcProgBits = runGCProg(addb(p, 4), (*byte)(ptr))
                p = (*byte)(ptr)
        }
        nb := typ.PtrBytes / goarch.PtrSize

        for i := uintptr(0); i < nb; i += ptrBits {
                k := nb - i
                if k > ptrBits {
                        k = ptrBits
                }
                h = h.write(readUintptr(addb(p, i/8)), k)
        }
        // Note: we call pad here to ensure we emit explicit 0 bits
        // for the pointerless tail of the object. This ensures that
        // there's only a single noMorePtrs mark for the next object
        // to clear. We don't need to do this to clear stale noMorePtrs
        // markers from previous uses because arena chunk pointer bitmaps
        // are always fully cleared when reused.
        h = h.pad(typ.Size_ - typ.PtrBytes)
        h.flush(uintptr(ptr), typ.Size_)

        if typ.Kind_&kindGCProg != 0 {
                // Zero out temporary ptrmask buffer inside object.
                memclrNoHeapPointers(ptr, (gcProgBits+7)/8)
        }

        // Double-check that the bitmap was written out correctly.
        //
        // Derived from heapBitsSetType.
        const doubleCheck = false
        if doubleCheck {
                size := typ.Size_
                x := uintptr(ptr)
                h := heapBitsForAddr(x, size)
                for i := uintptr(0); i < size; i += goarch.PtrSize {
                        // Compute the pointer bit we want at offset i.
                        want := false
                        off := i % typ.Size_
                        if off < typ.PtrBytes {
                                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("userArenaHeapBitsSetType: pointer entry not correct")
                                }
                        }
                }
                if _, addr := h.next(); addr != 0 {
                        throw("userArenaHeapBitsSetType: extra pointer")
                }
        }
}