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Source file src/runtime/mbitmap.go

Documentation: runtime

  // 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 1-bit bitmaps in which 0 means uninteresting and 1 means live pointer
  // to be visited during GC. The bits in each byte are consumed starting with
  // the low bit: 1<<0, 1<<1, and so on.
  //
  // Heap bitmap
  //
  // The allocated heap comes from a subset of the memory in the range [start, used),
  // where start == mheap_.arena_start and used == mheap_.arena_used.
  // The heap bitmap comprises 2 bits for each pointer-sized word in that range,
  // stored in bytes indexed backward in memory from start.
  // That is, the byte at address start-1 holds the 2-bit entries for the four words
  // start through start+3*ptrSize, the byte at start-2 holds the entries for
  // start+4*ptrSize through start+7*ptrSize, and so on.
  //
  // In each 2-bit entry, the lower bit holds the same information as in the 1-bit
  // bitmaps: 0 means uninteresting and 1 means live pointer to be visited during GC.
  // The meaning of the high bit depends on the position of the word being described
  // in its allocated object. In all words *except* the second word, the
  // high bit indicates that the object is still being described. In
  // these words, if a bit pair with a high bit 0 is encountered, the
  // low bit can also be assumed to be 0, and the object description is
  // over. This 00 is called the ``dead'' encoding: it signals that the
  // rest of the words in the object are uninteresting to the garbage
  // collector.
  //
  // In the second word, the high bit is the GC ``checkmarked'' bit (see below).
  //
  // The 2-bit entries are split when written into the byte, so that the top half
  // of the byte contains 4 high bits and the bottom half contains 4 low (pointer)
  // bits.
  // This form allows a copy from the 1-bit to the 4-bit form to keep the
  // pointer bits contiguous, instead of having to space them out.
  //
  // The code makes use of the fact that the zero value for a heap bitmap
  // has no live pointer bit set and is (depending on position), not used,
  // not checkmarked, and is the dead encoding.
  // These properties must be preserved when modifying the encoding.
  //
  // Checkmarks
  //
  // In a concurrent garbage collector, one worries about failing to mark
  // a live object due to mutations without write barriers or bugs in the
  // collector implementation. As a sanity check, the GC has a 'checkmark'
  // mode that retraverses the object graph with the world stopped, to make
  // sure that everything that should be marked is marked.
  // In checkmark mode, in the heap bitmap, the high bit of the 2-bit entry
  // for the second word of the object holds the checkmark bit.
  // When not in checkmark mode, this bit is set to 1.
  //
  // The smallest possible allocation is 8 bytes. On a 32-bit machine, that
  // means every allocated object has two words, so there is room for the
  // checkmark bit. On a 64-bit machine, however, the 8-byte allocation is
  // just one word, so the second bit pair is not available for encoding the
  // checkmark. However, because non-pointer allocations are combined
  // into larger 16-byte (maxTinySize) allocations, a plain 8-byte allocation
  // must be a pointer, so the type bit in the first word is not actually needed.
  // It is still used in general, except in checkmark the type bit is repurposed
  // as the checkmark bit and then reinitialized (to 1) as the type bit when
  // finished.
  //
  
  package runtime
  
  import (
  	"runtime/internal/atomic"
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  const (
  	bitPointer = 1 << 0
  	bitScan    = 1 << 4
  
  	heapBitsShift   = 1                     // shift offset between successive bitPointer or bitScan entries
  	heapBitmapScale = sys.PtrSize * (8 / 2) // number of data bytes described by one heap bitmap byte
  
  	// all scan/pointer bits in a byte
  	bitScanAll    = bitScan | bitScan<<heapBitsShift | bitScan<<(2*heapBitsShift) | bitScan<<(3*heapBitsShift)
  	bitPointerAll = bitPointer | bitPointer<<heapBitsShift | bitPointer<<(2*heapBitsShift) | bitPointer<<(3*heapBitsShift)
  )
  
  // addb returns the byte pointer p+n.
  //go:nowritebarrier
  //go:nosplit
  func addb(p *byte, n uintptr) *byte {
  	// Note: wrote out full expression instead of calling add(p, n)
  	// to reduce the number of temporaries generated by the
  	// compiler for this trivial expression during inlining.
  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
  }
  
  // subtractb returns the byte pointer p-n.
  // subtractb is typically used when traversing the pointer tables referred to by hbits
  // which are arranged in reverse order.
  //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.
  // subtract1 is typically used when traversing the pointer tables referred to by hbits
  // which are arranged in reverse order.
  //go:nowritebarrier
  //
  // nosplit because it is used during write barriers and must not be preempted.
  //go:nosplit
  func subtract1(p *byte) *byte {
  	// Note: wrote out full expression instead of calling subtractb(p, 1)
  	// to reduce the number of temporaries generated by the
  	// compiler for this trivial expression during inlining.
  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
  }
  
  // mHeap_MapBits is called each time arena_used is extended.
  // It maps any additional bitmap memory needed for the new arena memory.
  // It must be called with the expected new value of arena_used,
  // *before* h.arena_used has been updated.
  // Waiting to update arena_used until after the memory has been mapped
  // avoids faults when other threads try access the bitmap immediately
  // after observing the change to arena_used.
  //
  //go:nowritebarrier
  func (h *mheap) mapBits(arena_used uintptr) {
  	// Caller has added extra mappings to the arena.
  	// Add extra mappings of bitmap words as needed.
  	// We allocate extra bitmap pieces in chunks of bitmapChunk.
  	const bitmapChunk = 8192
  
  	n := (arena_used - mheap_.arena_start) / heapBitmapScale
  	n = round(n, bitmapChunk)
  	n = round(n, physPageSize)
  	if h.bitmap_mapped >= n {
  		return
  	}
  
  	sysMap(unsafe.Pointer(h.bitmap-n), n-h.bitmap_mapped, h.arena_reserved, &memstats.gc_sys)
  	h.bitmap_mapped = n
  }
  
  // heapBits provides access to the bitmap bits for a single heap word.
  // The methods on heapBits take value receivers so that the compiler
  // can more easily inline calls to those methods and registerize the
  // struct fields independently.
  type heapBits struct {
  	bitp  *uint8
  	shift uint32
  }
  
  // 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 {
  	whichByte := allocBitIndex / 8
  	whichBit := allocBitIndex % 8
  	bytePtr := addb(s.allocBits, whichByte)
  	return markBits{bytePtr, uint8(1 << whichBit), allocBitIndex}
  }
  
  // refillaCache 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(addb(s.allocBits, 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 >>= (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 returns whether the index'th object in s is unallocated.
  func (s *mspan) isFree(index uintptr) bool {
  	if index < s.freeindex {
  		return false
  	}
  	whichByte := index / 8
  	whichBit := index % 8
  	byteVal := *addb(s.allocBits, whichByte)
  	return byteVal&uint8(1<<whichBit) == 0
  }
  
  func (s *mspan) objIndex(p uintptr) uintptr {
  	byteOffset := p - s.base()
  	if byteOffset == 0 {
  		return 0
  	}
  	if s.baseMask != 0 {
  		// s.baseMask is 0, elemsize is a power of two, so shift by s.divShift
  		return byteOffset >> s.divShift
  	}
  	return uintptr(((uint64(byteOffset) >> s.divShift) * uint64(s.divMul)) >> s.divShift2)
  }
  
  func markBitsForAddr(p uintptr) markBits {
  	s := spanOf(p)
  	objIndex := s.objIndex(p)
  	return s.markBitsForIndex(objIndex)
  }
  
  func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
  	whichByte := objIndex / 8
  	bitMask := uint8(1 << (objIndex % 8)) // low 3 bits hold the bit index
  	bytePtr := addb(s.gcmarkBits, whichByte)
  	return markBits{bytePtr, bitMask, objIndex}
  }
  
  func (s *mspan) markBitsForBase() markBits {
  	return markBits{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. Some compilers
  // are not able to inline atomic.Or8 function so if it appears as a hot spot consider
  // inlining it manually.
  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)
  }
  
  // clearMarkedNonAtomic clears the marked bit non-atomically.
  func (m markBits) clearMarkedNonAtomic() {
  	*m.bytep ^= m.mask
  }
  
  // markBitsForSpan returns the markBits for the span base address base.
  func markBitsForSpan(base uintptr) (mbits markBits) {
  	if base < mheap_.arena_start || base >= mheap_.arena_used {
  		throw("markBitsForSpan: base out of range")
  	}
  	mbits = markBitsForAddr(base)
  	if mbits.mask != 1 {
  		throw("markBitsForSpan: unaligned start")
  	}
  	return mbits
  }
  
  // advance advances the markBits to the next object in the span.
  func (m *markBits) advance() {
  	if m.mask == 1<<7 {
  		m.bytep = (*uint8)(unsafe.Pointer(uintptr(unsafe.Pointer(m.bytep)) + 1))
  		m.mask = 1
  	} else {
  		m.mask = m.mask << 1
  	}
  	m.index++
  }
  
  // heapBitsForAddr returns the heapBits for the address addr.
  // The caller must have already checked that addr is in the range [mheap_.arena_start, mheap_.arena_used).
  //
  // nosplit because it is used during write barriers and must not be preempted.
  //go:nosplit
  func heapBitsForAddr(addr uintptr) heapBits {
  	// 2 bits per work, 4 pairs per byte, and a mask is hard coded.
  	off := (addr - mheap_.arena_start) / sys.PtrSize
  	return heapBits{(*uint8)(unsafe.Pointer(mheap_.bitmap - off/4 - 1)), uint32(off & 3)}
  }
  
  // heapBitsForSpan returns the heapBits for the span base address base.
  func heapBitsForSpan(base uintptr) (hbits heapBits) {
  	if base < mheap_.arena_start || base >= mheap_.arena_used {
  		print("runtime: base ", hex(base), " not in range [", hex(mheap_.arena_start), ",", hex(mheap_.arena_used), ")\n")
  		throw("heapBitsForSpan: base out of range")
  	}
  	return heapBitsForAddr(base)
  }
  
  // heapBitsForObject returns the base address for the heap object
  // containing the address p, the heapBits for base,
  // the object's span, and of the index of the object in s.
  // If p does not point into a heap object,
  // return base == 0
  // otherwise return the base of the object.
  //
  // 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.
  func heapBitsForObject(p, refBase, refOff uintptr) (base uintptr, hbits heapBits, s *mspan, objIndex uintptr) {
  	arenaStart := mheap_.arena_start
  	if p < arenaStart || p >= mheap_.arena_used {
  		return
  	}
  	off := p - arenaStart
  	idx := off >> _PageShift
  	// p points into the heap, but possibly to the middle of an object.
  	// Consult the span table to find the block beginning.
  	s = mheap_.spans[idx]
  	if s == nil || p < s.base() || p >= s.limit || s.state != mSpanInUse {
  		if s == nil || s.state == _MSpanStack {
  			// 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.
  			// Pointers into stacks are also ok, the runtime manages these explicitly.
  			return
  		}
  
  		// The following ensures that we are rigorous about what data
  		// structures hold valid pointers.
  		if debug.invalidptr != 0 {
  			// 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.state != mSpanInUse {
  				print(" to unallocated span")
  			} else {
  				print(" to unused region of span")
  			}
  			print(" idx=", hex(idx), " span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", s.state, "\n")
  			if refBase != 0 {
  				print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
  				gcDumpObject("object", refBase, refOff)
  			}
  			throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
  		}
  		return
  	}
  	// If this span holds object of a power of 2 size, just mask off the bits to
  	// the interior of the object. Otherwise use the size to get the base.
  	if s.baseMask != 0 {
  		// optimize for power of 2 sized objects.
  		base = s.base()
  		base = base + (p-base)&uintptr(s.baseMask)
  		objIndex = (base - s.base()) >> s.divShift
  		// base = p & s.baseMask is faster for small spans,
  		// but doesn't work for large spans.
  		// Overall, it's faster to use the more general computation above.
  	} else {
  		base = s.base()
  		if p-base >= s.elemsize {
  			// n := (p - base) / s.elemsize, using division by multiplication
  			objIndex = uintptr(p-base) >> s.divShift * uintptr(s.divMul) >> s.divShift2
  			base += objIndex * s.elemsize
  		}
  	}
  	// Now that we know the actual base, compute heapBits to return to caller.
  	hbits = heapBitsForAddr(base)
  	return
  }
  
  // prefetch the bits.
  func (h heapBits) prefetch() {
  	prefetchnta(uintptr(unsafe.Pointer((h.bitp))))
  }
  
  // next returns the heapBits describing the next pointer-sized word in memory.
  // That is, if h describes address p, h.next() describes p+ptrSize.
  // Note that next does not modify h. The caller must record the result.
  //
  // nosplit because it is used during write barriers and must not be preempted.
  //go:nosplit
  func (h heapBits) next() heapBits {
  	if h.shift < 3*heapBitsShift {
  		return heapBits{h.bitp, h.shift + heapBitsShift}
  	}
  	return heapBits{subtract1(h.bitp), 0}
  }
  
  // forward returns the heapBits describing n pointer-sized words ahead of h in memory.
  // That is, if h describes address p, h.forward(n) describes p+n*ptrSize.
  // h.forward(1) is equivalent to h.next(), just slower.
  // Note that forward does not modify h. The caller must record the result.
  // bits returns the heap bits for the current word.
  func (h heapBits) forward(n uintptr) heapBits {
  	n += uintptr(h.shift) / heapBitsShift
  	return heapBits{subtractb(h.bitp, n/4), uint32(n%4) * heapBitsShift}
  }
  
  // The caller can test morePointers and isPointer by &-ing with bitScan and bitPointer.
  // The result includes in its higher bits the bits for subsequent words
  // described by the same bitmap byte.
  func (h heapBits) bits() uint32 {
  	// The (shift & 31) eliminates a test and conditional branch
  	// from the generated code.
  	return uint32(*h.bitp) >> (h.shift & 31)
  }
  
  // morePointers returns true if this word and all remaining words in this object
  // are scalars.
  // h must not describe the second word of the object.
  func (h heapBits) morePointers() bool {
  	return h.bits()&bitScan != 0
  }
  
  // isPointer reports whether the heap bits describe a pointer word.
  //
  // nosplit because it is used during write barriers and must not be preempted.
  //go:nosplit
  func (h heapBits) isPointer() bool {
  	return h.bits()&bitPointer != 0
  }
  
  // hasPointers reports whether the given object has any pointers.
  // It must be told how large the object at h is for efficiency.
  // h must describe the initial word of the object.
  func (h heapBits) hasPointers(size uintptr) bool {
  	if size == sys.PtrSize { // 1-word objects are always pointers
  		return true
  	}
  	return (*h.bitp>>h.shift)&bitScan != 0
  }
  
  // isCheckmarked reports whether the heap bits have the checkmarked bit set.
  // It must be told how large the object at h is, because the encoding of the
  // checkmark bit varies by size.
  // h must describe the initial word of the object.
  func (h heapBits) isCheckmarked(size uintptr) bool {
  	if size == sys.PtrSize {
  		return (*h.bitp>>h.shift)&bitPointer != 0
  	}
  	// All multiword objects are 2-word aligned,
  	// so we know that the initial word's 2-bit pair
  	// and the second word's 2-bit pair are in the
  	// same heap bitmap byte, *h.bitp.
  	return (*h.bitp>>(heapBitsShift+h.shift))&bitScan != 0
  }
  
  // setCheckmarked sets the checkmarked bit.
  // It must be told how large the object at h is, because the encoding of the
  // checkmark bit varies by size.
  // h must describe the initial word of the object.
  func (h heapBits) setCheckmarked(size uintptr) {
  	if size == sys.PtrSize {
  		atomic.Or8(h.bitp, bitPointer<<h.shift)
  		return
  	}
  	atomic.Or8(h.bitp, bitScan<<(heapBitsShift+h.shift))
  }
  
  // bulkBarrierPreWrite executes writebarrierptr_prewrite1
  // 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.
  //
  // 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.kind&kindNoPointers.
  //
  //go:nosplit
  func bulkBarrierPreWrite(dst, src, size uintptr) {
  	if (dst|src|size)&(sys.PtrSize-1) != 0 {
  		throw("bulkBarrierPreWrite: unaligned arguments")
  	}
  	if !writeBarrier.needed {
  		return
  	}
  	if !inheap(dst) {
  		// If dst is on the stack and in a higher frame than the
  		// caller, we either need to execute write barriers on
  		// it (which is what happens for normal stack writes
  		// through pointers to higher frames), or we need to
  		// force the mark termination stack scan to scan the
  		// frame containing dst.
  		//
  		// Executing write barriers on dst is complicated in the
  		// general case because we either need to unwind the
  		// stack to get the stack map, or we need the type's
  		// bitmap, which may be a GC program.
  		//
  		// Hence, we opt for forcing the re-scan to scan the
  		// frame containing dst, which we can do by simply
  		// unwinding the stack barriers between the current SP
  		// and dst's frame.
  		gp := getg().m.curg
  		if gp != nil && gp.stack.lo <= dst && dst < gp.stack.hi {
  			// Run on the system stack to give it more
  			// stack space.
  			systemstack(func() {
  				gcUnwindBarriers(gp, dst)
  			})
  			return
  		}
  
  		// 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
  	}
  
  	h := heapBitsForAddr(dst)
  	if src == 0 {
  		for i := uintptr(0); i < size; i += sys.PtrSize {
  			if h.isPointer() {
  				dstx := (*uintptr)(unsafe.Pointer(dst + i))
  				writebarrierptr_prewrite1(dstx, 0)
  			}
  			h = h.next()
  		}
  	} else {
  		for i := uintptr(0); i < size; i += sys.PtrSize {
  			if h.isPointer() {
  				dstx := (*uintptr)(unsafe.Pointer(dst + i))
  				srcx := (*uintptr)(unsafe.Pointer(src + i))
  				writebarrierptr_prewrite1(dstx, *srcx)
  			}
  			h = h.next()
  		}
  	}
  }
  
  // bulkBarrierBitmap executes write barriers for copying from [src,
  // src+size) to [dst, dst+size) using a 1-bit pointer bitmap. src is
  // assumed to start maskOffset bytes into the data covered by the
  // bitmap in bits (which may not be a multiple of 8).
  //
  // This is used by bulkBarrierPreWrite for writes to data and BSS.
  //
  //go:nosplit
  func bulkBarrierBitmap(dst, src, size, maskOffset uintptr, bits *uint8) {
  	word := maskOffset / sys.PtrSize
  	bits = addb(bits, word/8)
  	mask := uint8(1) << (word % 8)
  
  	for i := uintptr(0); i < size; i += sys.PtrSize {
  		if mask == 0 {
  			bits = addb(bits, 1)
  			if *bits == 0 {
  				// Skip 8 words.
  				i += 7 * sys.PtrSize
  				continue
  			}
  			mask = 1
  		}
  		if *bits&mask != 0 {
  			dstx := (*uintptr)(unsafe.Pointer(dst + i))
  			if src == 0 {
  				writebarrierptr_prewrite1(dstx, 0)
  			} else {
  				srcx := (*uintptr)(unsafe.Pointer(src + i))
  				writebarrierptr_prewrite1(dstx, *srcx)
  			}
  		}
  		mask <<= 1
  	}
  }
  
  // typeBitsBulkBarrier executes writebarrierptr_prewrite 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.
  //
  //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
  	var bits uint32
  	for i := uintptr(0); i < typ.ptrdata; i += sys.PtrSize {
  		if i&(sys.PtrSize*8-1) == 0 {
  			bits = uint32(*ptrmask)
  			ptrmask = addb(ptrmask, 1)
  		} else {
  			bits = bits >> 1
  		}
  		if bits&1 != 0 {
  			dstx := (*uintptr)(unsafe.Pointer(dst + i))
  			srcx := (*uintptr)(unsafe.Pointer(src + i))
  			writebarrierptr_prewrite(dstx, *srcx)
  		}
  	}
  }
  
  // The methods operating on spans all require that h has been returned
  // by heapBitsForSpan and that size, n, total are the span layout description
  // returned by the mspan's layout method.
  // If total > size*n, it means that there is extra leftover memory in the span,
  // usually due to rounding.
  //
  // TODO(rsc): Perhaps introduce a different heapBitsSpan type.
  
  // initSpan initializes the heap bitmap for a span.
  // It clears all checkmark bits.
  // If this is a span of pointer-sized objects, it initializes all
  // words to pointer/scan.
  // Otherwise, it initializes all words to scalar/dead.
  func (h heapBits) initSpan(s *mspan) {
  	size, n, total := s.layout()
  
  	// Init the markbit structures
  	s.freeindex = 0
  	s.allocCache = ^uint64(0) // all 1s indicating all free.
  	s.nelems = n
  	s.allocBits = nil
  	s.gcmarkBits = nil
  	s.gcmarkBits = newMarkBits(s.nelems)
  	s.allocBits = newAllocBits(s.nelems)
  
  	// Clear bits corresponding to objects.
  	if total%heapBitmapScale != 0 {
  		throw("initSpan: unaligned length")
  	}
  	nbyte := total / heapBitmapScale
  	if sys.PtrSize == 8 && size == sys.PtrSize {
  		end := h.bitp
  		bitp := subtractb(end, nbyte-1)
  		for {
  			*bitp = bitPointerAll | bitScanAll
  			if bitp == end {
  				break
  			}
  			bitp = add1(bitp)
  		}
  		return
  	}
  	memclrNoHeapPointers(unsafe.Pointer(subtractb(h.bitp, nbyte-1)), nbyte)
  }
  
  // initCheckmarkSpan initializes a span for being checkmarked.
  // It clears the checkmark bits, which are set to 1 in normal operation.
  func (h heapBits) initCheckmarkSpan(size, n, total uintptr) {
  	// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
  	if sys.PtrSize == 8 && size == sys.PtrSize {
  		// Checkmark bit is type bit, bottom bit of every 2-bit entry.
  		// Only possible on 64-bit system, since minimum size is 8.
  		// Must clear type bit (checkmark bit) of every word.
  		// The type bit is the lower of every two-bit pair.
  		bitp := h.bitp
  		for i := uintptr(0); i < n; i += 4 {
  			*bitp &^= bitPointerAll
  			bitp = subtract1(bitp)
  		}
  		return
  	}
  	for i := uintptr(0); i < n; i++ {
  		*h.bitp &^= bitScan << (heapBitsShift + h.shift)
  		h = h.forward(size / sys.PtrSize)
  	}
  }
  
  // clearCheckmarkSpan undoes all the checkmarking in a span.
  // The actual checkmark bits are ignored, so the only work to do
  // is to fix the pointer bits. (Pointer bits are ignored by scanobject
  // but consulted by typedmemmove.)
  func (h heapBits) clearCheckmarkSpan(size, n, total uintptr) {
  	// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
  	if sys.PtrSize == 8 && size == sys.PtrSize {
  		// Checkmark bit is type bit, bottom bit of every 2-bit entry.
  		// Only possible on 64-bit system, since minimum size is 8.
  		// Must clear type bit (checkmark bit) of every word.
  		// The type bit is the lower of every two-bit pair.
  		bitp := h.bitp
  		for i := uintptr(0); i < n; i += 4 {
  			*bitp |= bitPointerAll
  			bitp = subtract1(bitp)
  		}
  	}
  }
  
  // oneBitCount is indexed by byte and produces the
  // number of 1 bits in that byte. For example 128 has 1 bit set
  // and oneBitCount[128] will holds 1.
  var oneBitCount = [256]uint8{
  	0, 1, 1, 2, 1, 2, 2, 3,
  	1, 2, 2, 3, 2, 3, 3, 4,
  	1, 2, 2, 3, 2, 3, 3, 4,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	1, 2, 2, 3, 2, 3, 3, 4,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	1, 2, 2, 3, 2, 3, 3, 4,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	4, 5, 5, 6, 5, 6, 6, 7,
  	1, 2, 2, 3, 2, 3, 3, 4,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	4, 5, 5, 6, 5, 6, 6, 7,
  	2, 3, 3, 4, 3, 4, 4, 5,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	4, 5, 5, 6, 5, 6, 6, 7,
  	3, 4, 4, 5, 4, 5, 5, 6,
  	4, 5, 5, 6, 5, 6, 6, 7,
  	4, 5, 5, 6, 5, 6, 6, 7,
  	5, 6, 6, 7, 6, 7, 7, 8}
  
  // countFree runs through the mark bits in a span and counts the number of free objects
  // in the span.
  // TODO:(rlh) Use popcount intrinsic.
  func (s *mspan) countFree() int {
  	count := 0
  	maxIndex := s.nelems / 8
  	for i := uintptr(0); i < maxIndex; i++ {
  		mrkBits := *addb(s.gcmarkBits, i)
  		count += int(oneBitCount[mrkBits])
  	}
  	if bitsInLastByte := s.nelems % 8; bitsInLastByte != 0 {
  		mrkBits := *addb(s.gcmarkBits, maxIndex)
  		mask := uint8((1 << bitsInLastByte) - 1)
  		bits := mrkBits & mask
  		count += int(oneBitCount[bits])
  	}
  	return int(s.nelems) - count
  }
  
  // heapBitsSetType records that the new allocation [x, x+size)
  // holds in [x, x+dataSize) one or more values of type typ.
  // (The number of values is given by dataSize / typ.size.)
  // If dataSize < size, the fragment [x+dataSize, x+size) is
  // recorded as non-pointer data.
  // It is known that the type has pointers somewhere;
  // malloc does not call heapBitsSetType when there are no pointers,
  // because all free objects are marked as noscan during
  // heapBitsSweepSpan.
  //
  // There can only be one allocation from a given span active at a time,
  // and the bitmap for a span always falls on byte boundaries,
  // so there are no write-write races for access to the heap bitmap.
  // Hence, heapBitsSetType can access the bitmap without atomics.
  //
  // There can be read-write races between heapBitsSetType and things
  // that read the heap bitmap like scanobject. However, since
  // heapBitsSetType is only used for objects that have not yet been
  // made reachable, readers will ignore bits being modified by this
  // function. This does mean this function cannot transiently modify
  // bits that belong to neighboring objects. Also, on weakly-ordered
  // machines, callers must execute a store/store (publication) barrier
  // between calling this function and making the object reachable.
  func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
  	const doubleCheck = false // slow but helpful; enable to test modifications to this code
  
  	// dataSize is always size rounded up to the next malloc size class,
  	// except in the case of allocating a defer block, in which case
  	// size is sizeof(_defer{}) (at least 6 words) and dataSize may be
  	// arbitrarily larger.
  	//
  	// The checks for size == sys.PtrSize and size == 2*sys.PtrSize can therefore
  	// assume that dataSize == size without checking it explicitly.
  
  	if sys.PtrSize == 8 && size == sys.PtrSize {
  		// It's one word and it has pointers, it must be a pointer.
  		// Since all allocated one-word objects are pointers
  		// (non-pointers are aggregated into tinySize allocations),
  		// initSpan sets the pointer bits for us. Nothing to do here.
  		if doubleCheck {
  			h := heapBitsForAddr(x)
  			if !h.isPointer() {
  				throw("heapBitsSetType: pointer bit missing")
  			}
  			if !h.morePointers() {
  				throw("heapBitsSetType: scan bit missing")
  			}
  		}
  		return
  	}
  
  	h := heapBitsForAddr(x)
  	ptrmask := typ.gcdata // start of 1-bit pointer mask (or GC program, handled below)
  
  	// Heap bitmap bits for 2-word object are only 4 bits,
  	// so also shared with objects next to it.
  	// This is called out as a special case primarily for 32-bit systems,
  	// so that on 32-bit systems the code below can assume all objects
  	// are 4-word aligned (because they're all 16-byte aligned).
  	if size == 2*sys.PtrSize {
  		if typ.size == sys.PtrSize {
  			// We're allocating a block big enough to hold two pointers.
  			// On 64-bit, that means the actual object must be two pointers,
  			// or else we'd have used the one-pointer-sized block.
  			// On 32-bit, however, this is the 8-byte block, the smallest one.
  			// So it could be that we're allocating one pointer and this was
  			// just the smallest block available. Distinguish by checking dataSize.
  			// (In general the number of instances of typ being allocated is
  			// dataSize/typ.size.)
  			if sys.PtrSize == 4 && dataSize == sys.PtrSize {
  				// 1 pointer object. On 32-bit machines clear the bit for the
  				// unused second word.
  				*h.bitp &^= (bitPointer | bitScan | ((bitPointer | bitScan) << heapBitsShift)) << h.shift
  				*h.bitp |= (bitPointer | bitScan) << h.shift
  			} else {
  				// 2-element slice of pointer.
  				*h.bitp |= (bitPointer | bitScan | bitPointer<<heapBitsShift) << h.shift
  			}
  			return
  		}
  		// Otherwise typ.size must be 2*sys.PtrSize,
  		// and typ.kind&kindGCProg == 0.
  		if doubleCheck {
  			if typ.size != 2*sys.PtrSize || typ.kind&kindGCProg != 0 {
  				print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, " gcprog=", typ.kind&kindGCProg != 0, "\n")
  				throw("heapBitsSetType")
  			}
  		}
  		b := uint32(*ptrmask)
  		hb := (b & 3) | bitScan
  		// bitPointer == 1, bitScan is 1 << 4, heapBitsShift is 1.
  		// 110011 is shifted h.shift and complemented.
  		// This clears out the bits that are about to be
  		// ored into *h.hbitp in the next instructions.
  		*h.bitp &^= (bitPointer | bitScan | ((bitPointer | bitScan) << heapBitsShift)) << h.shift
  		*h.bitp |= uint8(hb << h.shift)
  		return
  	}
  
  	// Copy from 1-bit ptrmask into 2-bit bitmap.
  	// The basic approach is to use a single uintptr as a bit buffer,
  	// alternating between reloading the buffer and writing bitmap bytes.
  	// In general, one load can supply two bitmap byte writes.
  	// This is a lot of lines of code, but it compiles into relatively few
  	// machine instructions.
  
  	var (
  		// Ptrmask input.
  		p     *byte   // last ptrmask byte read
  		b     uintptr // ptrmask bits already loaded
  		nb    uintptr // number of bits in b at next read
  		endp  *byte   // final ptrmask byte to read (then repeat)
  		endnb uintptr // number of valid bits in *endp
  		pbits uintptr // alternate source of bits
  
  		// Heap bitmap output.
  		w     uintptr // words processed
  		nw    uintptr // number of words to process
  		hbitp *byte   // next heap bitmap byte to write
  		hb    uintptr // bits being prepared for *hbitp
  	)
  
  	hbitp = h.bitp
  
  	// Handle GC program. Delayed until this part of the code
  	// so that we can use the same double-checking mechanism
  	// as the 1-bit case. Nothing above could have encountered
  	// GC programs: the cases were all too small.
  	if typ.kind&kindGCProg != 0 {
  		heapBitsSetTypeGCProg(h, typ.ptrdata, typ.size, dataSize, size, addb(typ.gcdata, 4))
  		if doubleCheck {
  			// Double-check the heap bits written by GC program
  			// by running the GC program to create a 1-bit pointer mask
  			// and then jumping to the double-check code below.
  			// This doesn't catch bugs shared between the 1-bit and 4-bit
  			// GC program execution, but it does catch mistakes specific
  			// to just one of those and bugs in heapBitsSetTypeGCProg's
  			// implementation of arrays.
  			lock(&debugPtrmask.lock)
  			if debugPtrmask.data == nil {
  				debugPtrmask.data = (*byte)(persistentalloc(1<<20, 1, &memstats.other_sys))
  			}
  			ptrmask = debugPtrmask.data
  			runGCProg(addb(typ.gcdata, 4), nil, ptrmask, 1)
  			goto Phase4
  		}
  		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. This code sets nw (below) so that heap bitmap
  	// bits are only written for the typ.ptrdata prefix; if there is
  	// more room in the allocated object, the next heap bitmap
  	// entry is a 00, indicating that there are no more pointers
  	// to scan. So only the ptrmask for the ptrdata bytes is needed.
  	//
  	// Replicated copies are not as nice: if there is an array of
  	// objects with scalar tails, all but the last tail does have to
  	// be initialized, because there is no way to say "skip forward".
  	// However, because of the possibility of a repeated type with
  	// size not a multiple of 4 pointers (one heap bitmap byte),
  	// the code already must handle the last ptrmask byte specially
  	// by treating it as containing only the bits for endnb pointers,
  	// where endnb <= 4. We represent large scalar tails that must
  	// be expanded in the replication by setting endnb larger than 4.
  	// This will have the effect of reading many bits out of b,
  	// but once the real bits are shifted out, b will supply as many
  	// zero bits as we try to read, which is exactly what we need.
  
  	p = ptrmask
  	if typ.size < dataSize {
  		// Filling in bits for an array of typ.
  		// Set up for repetition of ptrmask during main loop.
  		// Note that ptrmask describes only a prefix of
  		const maxBits = sys.PtrSize*8 - 7
  		if typ.ptrdata/sys.PtrSize <= maxBits {
  			// Entire ptrmask fits in uintptr with room for a byte fragment.
  			// Load into pbits and never read from ptrmask again.
  			// This is especially important when the ptrmask has
  			// fewer than 8 bits in it; otherwise the reload in the middle
  			// of the Phase 2 loop would itself need to loop to gather
  			// at least 8 bits.
  
  			// Accumulate ptrmask into b.
  			// ptrmask is sized to describe only typ.ptrdata, but we record
  			// it as describing typ.size bytes, since all the high bits are zero.
  			nb = typ.ptrdata / sys.PtrSize
  			for i := uintptr(0); i < nb; i += 8 {
  				b |= uintptr(*p) << i
  				p = add1(p)
  			}
  			nb = typ.size / sys.PtrSize
  
  			// Replicate ptrmask to fill entire pbits uintptr.
  			// Doubling and truncating is fewer steps than
  			// iterating by nb each time. (nb could be 1.)
  			// Since we loaded typ.ptrdata/sys.PtrSize bits
  			// but are pretending to have typ.size/sys.PtrSize,
  			// there might be no replication necessary/possible.
  			pbits = b
  			endnb = nb
  			if nb+nb <= maxBits {
  				for endnb <= sys.PtrSize*8 {
  					pbits |= pbits << endnb
  					endnb += endnb
  				}
  				// Truncate to a multiple of original ptrmask.
  				endnb = maxBits / nb * nb
  				pbits &= 1<<endnb - 1
  				b = pbits
  				nb = endnb
  			}
  
  			// Clear p and endp as sentinel for using pbits.
  			// Checked during Phase 2 loop.
  			p = nil
  			endp = nil
  		} else {
  			// Ptrmask is larger. Read it multiple times.
  			n := (typ.ptrdata/sys.PtrSize+7)/8 - 1
  			endp = addb(ptrmask, n)
  			endnb = typ.size/sys.PtrSize - n*8
  		}
  	}
  	if p != nil {
  		b = uintptr(*p)
  		p = add1(p)
  		nb = 8
  	}
  
  	if typ.size == dataSize {
  		// Single entry: can stop once we reach the non-pointer data.
  		nw = typ.ptrdata / sys.PtrSize
  	} else {
  		// Repeated instances of typ in an array.
  		// Have to process first N-1 entries in full, but can stop
  		// once we reach the non-pointer data in the final entry.
  		nw = ((dataSize/typ.size-1)*typ.size + typ.ptrdata) / sys.PtrSize
  	}
  	if nw == 0 {
  		// No pointers! Caller was supposed to check.
  		println("runtime: invalid type ", typ.string())
  		throw("heapBitsSetType: called with non-pointer type")
  		return
  	}
  	if nw < 2 {
  		// Must write at least 2 words, because the "no scan"
  		// encoding doesn't take effect until the third word.
  		nw = 2
  	}
  
  	// Phase 1: Special case for leading byte (shift==0) or half-byte (shift==4).
  	// The leading byte is special because it contains the bits for word 1,
  	// which does not have the scan bit set.
  	// The leading half-byte is special because it's a half a byte,
  	// so we have to be careful with the bits already there.
  	switch {
  	default:
  		throw("heapBitsSetType: unexpected shift")
  
  	case h.shift == 0:
  		// Ptrmask and heap bitmap are aligned.
  		// Handle first byte of bitmap specially.
  		//
  		// The first byte we write out covers the first four
  		// words of the object. The scan/dead bit on the first
  		// word must be set to scan since there are pointers
  		// somewhere in the object. The scan/dead bit on the
  		// second word is the checkmark, so we don't set it.
  		// In all following words, we set the scan/dead
  		// appropriately to indicate that the object contains
  		// to the next 2-bit entry in the bitmap.
  		//
  		// TODO: It doesn't matter if we set the checkmark, so
  		// maybe this case isn't needed any more.
  		hb = b & bitPointerAll
  		hb |= bitScan | bitScan<<(2*heapBitsShift) | bitScan<<(3*heapBitsShift)
  		if w += 4; w >= nw {
  			goto Phase3
  		}
  		*hbitp = uint8(hb)
  		hbitp = subtract1(hbitp)
  		b >>= 4
  		nb -= 4
  
  	case sys.PtrSize == 8 && h.shift == 2:
  		// Ptrmask and heap bitmap are misaligned.
  		// The bits for the first two words are in a byte shared
  		// with another object, so we must be careful with the bits
  		// already there.
  		// We took care of 1-word and 2-word objects above,
  		// so this is at least a 6-word object.
  		hb = (b & (bitPointer | bitPointer<<heapBitsShift)) << (2 * heapBitsShift)
  		// This is not noscan, so set the scan bit in the
  		// first word.
  		hb |= bitScan << (2 * heapBitsShift)
  		b >>= 2
  		nb -= 2
  		// Note: no bitScan for second word because that's
  		// the checkmark.
  		*hbitp &^= uint8((bitPointer | bitScan | (bitPointer << heapBitsShift)) << (2 * heapBitsShift))
  		*hbitp |= uint8(hb)
  		hbitp = subtract1(hbitp)
  		if w += 2; w >= nw {
  			// We know that there is more data, because we handled 2-word objects above.
  			// This must be at least a 6-word object. If we're out of pointer words,
  			// mark no scan in next bitmap byte and finish.
  			hb = 0
  			w += 4
  			goto Phase3
  		}
  	}
  
  	// Phase 2: Full bytes in bitmap, up to but not including write to last byte (full or partial) in bitmap.
  	// The loop computes the bits for that last write but does not execute the write;
  	// it leaves the bits in hb for processing by phase 3.
  	// To avoid repeated adjustment of nb, we subtract out the 4 bits we're going to
  	// use in the first half of the loop right now, and then we only adjust nb explicitly
  	// if the 8 bits used by each iteration isn't balanced by 8 bits loaded mid-loop.
  	nb -= 4
  	for {
  		// Emit bitmap byte.
  		// b has at least nb+4 bits, with one exception:
  		// if w+4 >= nw, then b has only nw-w bits,
  		// but we'll stop at the break and then truncate
  		// appropriately in Phase 3.
  		hb = b & bitPointerAll
  		hb |= bitScanAll
  		if w += 4; w >= nw {
  			break
  		}
  		*hbitp = uint8(hb)
  		hbitp = subtract1(hbitp)
  		b >>= 4
  
  		// Load more bits. b has nb right now.
  		if p != endp {
  			// Fast path: keep reading from ptrmask.
  			// nb unmodified: we just loaded 8 bits,
  			// and the next iteration will consume 8 bits,
  			// leaving us with the same nb the next time we're here.
  			if nb < 8 {
  				b |= uintptr(*p) << nb
  				p = add1(p)
  			} else {
  				// Reduce the number of bits in b.
  				// This is important if we skipped
  				// over a scalar tail, since nb could
  				// be larger than the bit width of b.
  				nb -= 8
  			}
  		} else if p == nil {
  			// Almost as fast path: track bit count and refill from pbits.
  			// For short repetitions.
  			if nb < 8 {
  				b |= pbits << nb
  				nb += endnb
  			}
  			nb -= 8 // for next iteration
  		} else {
  			// Slow path: reached end of ptrmask.
  			// Process final partial byte and rewind to start.
  			b |= uintptr(*p) << nb
  			nb += endnb
  			if nb < 8 {
  				b |= uintptr(*ptrmask) << nb
  				p = add1(ptrmask)
  			} else {
  				nb -= 8
  				p = ptrmask
  			}
  		}
  
  		// Emit bitmap byte.
  		hb = b & bitPointerAll
  		hb |= bitScanAll
  		if w += 4; w >= nw {
  			break
  		}
  		*hbitp = uint8(hb)
  		hbitp = subtract1(hbitp)
  		b >>= 4
  	}
  
  Phase3:
  	// Phase 3: Write last byte or partial byte and zero the rest of the bitmap entries.
  	if w > nw {
  		// Counting the 4 entries in hb not yet written to memory,
  		// there are more entries than possible pointer slots.
  		// Discard the excess entries (can't be more than 3).
  		mask := uintptr(1)<<(4-(w-nw)) - 1
  		hb &= mask | mask<<4 // apply mask to both pointer bits and scan bits
  	}
  
  	// Change nw from counting possibly-pointer words to total words in allocation.
  	nw = size / sys.PtrSize
  
  	// Write whole bitmap bytes.
  	// The first is hb, the rest are zero.
  	if w <= nw {
  		*hbitp = uint8(hb)
  		hbitp = subtract1(hbitp)
  		hb = 0 // for possible final half-byte below
  		for w += 4; w <= nw; w += 4 {
  			*hbitp = 0
  			hbitp = subtract1(hbitp)
  		}
  	}
  
  	// Write final partial bitmap byte if any.
  	// We know w > nw, or else we'd still be in the loop above.
  	// It can be bigger only due to the 4 entries in hb that it counts.
  	// If w == nw+4 then there's nothing left to do: we wrote all nw entries
  	// and can discard the 4 sitting in hb.
  	// But if w == nw+2, we need to write first two in hb.
  	// The byte is shared with the next object, so be careful with
  	// existing bits.
  	if w == nw+2 {
  		*hbitp = *hbitp&^(bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift) | uint8(hb)
  	}
  
  Phase4:
  	// Phase 4: all done, but perhaps double check.
  	if doubleCheck {
  		end := heapBitsForAddr(x + size)
  		if typ.kind&kindGCProg == 0 && (hbitp != end.bitp || (w == nw+2) != (end.shift == 2)) {
  			println("ended at wrong bitmap byte for", typ.string(), "x", dataSize/typ.size)
  			print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
  			print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
  			h0 := heapBitsForAddr(x)
  			print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
  			print("ended at hbitp=", hbitp, " but next starts at bitp=", end.bitp, " shift=", end.shift, "\n")
  			throw("bad heapBitsSetType")
  		}
  
  		// Double-check that bits to be written were written correctly.
  		// Does not check that other bits were not written, unfortunately.
  		h := heapBitsForAddr(x)
  		nptr := typ.ptrdata / sys.PtrSize
  		ndata := typ.size / sys.PtrSize
  		count := dataSize / typ.size
  		totalptr := ((count-1)*typ.size + typ.ptrdata) / sys.PtrSize
  		for i := uintptr(0); i < size/sys.PtrSize; i++ {
  			j := i % ndata
  			var have, want uint8
  			have = (*h.bitp >> h.shift) & (bitPointer | bitScan)
  			if i >= totalptr {
  				want = 0 // deadmarker
  				if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 {
  					want = bitScan
  				}
  			} else {
  				if j < nptr && (*addb(ptrmask, j/8)>>(j%8))&1 != 0 {
  					want |= bitPointer
  				}
  				if i != 1 {
  					want |= bitScan
  				} else {
  					have &^= bitScan
  				}
  			}
  			if have != want {
  				println("mismatch writing bits for", typ.string(), "x", dataSize/typ.size)
  				print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
  				print("kindGCProg=", typ.kind&kindGCProg != 0, "\n")
  				print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
  				h0 := heapBitsForAddr(x)
  				print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
  				print("current bits h.bitp=", h.bitp, " h.shift=", h.shift, " *h.bitp=", hex(*h.bitp), "\n")
  				print("ptrmask=", ptrmask, " p=", p, " endp=", endp, " endnb=", endnb, " pbits=", hex(pbits), " b=", hex(b), " nb=", nb, "\n")
  				println("at word", i, "offset", i*sys.PtrSize, "have", have, "want", want)
  				if typ.kind&kindGCProg != 0 {
  					println("GC program:")
  					dumpGCProg(addb(typ.gcdata, 4))
  				}
  				throw("bad heapBitsSetType")
  			}
  			h = h.next()
  		}
  		if ptrmask == debugPtrmask.data {
  			unlock(&debugPtrmask.lock)
  		}
  	}
  }
  
  // heapBitsSetTypeNoScan marks x as noscan by setting the first word
  // of x in the heap bitmap to scalar/dead.
  func heapBitsSetTypeNoScan(x uintptr) {
  	h := heapBitsForAddr(uintptr(x))
  	*h.bitp &^= (bitPointer | bitScan) << h.shift
  }
  
  var debugPtrmask struct {
  	lock mutex
  	data *byte
  }
  
  // heapBitsSetTypeGCProg implements heapBitsSetType using a GC program.
  // progSize is the size of the memory described by the program.
  // elemSize is the size of the element that the GC program describes (a prefix of).
  // dataSize is the total size of the intended data, a multiple of elemSize.
  // allocSize is the total size of the allocated memory.
  //
  // GC programs are only used for large allocations.
  // heapBitsSetType requires that allocSize is a multiple of 4 words,
  // so that the relevant bitmap bytes are not shared with surrounding
  // objects.
  func heapBitsSetTypeGCProg(h heapBits, progSize, elemSize, dataSize, allocSize uintptr, prog *byte) {
  	if sys.PtrSize == 8 && allocSize%(4*sys.PtrSize) != 0 {
  		// Alignment will be wrong.
  		throw("heapBitsSetTypeGCProg: small allocation")
  	}
  	var totalBits uintptr
  	if elemSize == dataSize {
  		totalBits = runGCProg(prog, nil, h.bitp, 2)
  		if totalBits*sys.PtrSize != progSize {
  			println("runtime: heapBitsSetTypeGCProg: total bits", totalBits, "but progSize", progSize)
  			throw("heapBitsSetTypeGCProg: unexpected bit count")
  		}
  	} else {
  		count := dataSize / elemSize
  
  		// Piece together program trailer to run after prog that does:
  		//	literal(0)
  		//	repeat(1, elemSize-progSize-1) // zeros to fill element size
  		//	repeat(elemSize, count-1) // repeat that element for count
  		// This zero-pads the data remaining in the first element and then
  		// repeats that first element to fill the array.
  		var trailer [40]byte // 3 varints (max 10 each) + some bytes
  		i := 0
  		if n := elemSize/sys.PtrSize - progSize/sys.PtrSize; n > 0 {
  			// literal(0)
  			trailer[i] = 0x01
  			i++
  			trailer[i] = 0
  			i++
  			if n > 1 {
  				// repeat(1, n-1)
  				trailer[i] = 0x81
  				i++
  				n--
  				for ; n >= 0x80; n >>= 7 {
  					trailer[i] = byte(n | 0x80)
  					i++
  				}
  				trailer[i] = byte(n)
  				i++
  			}
  		}
  		// repeat(elemSize/ptrSize, count-1)
  		trailer[i] = 0x80
  		i++
  		n := elemSize / sys.PtrSize
  		for ; n >= 0x80; n >>= 7 {
  			trailer[i] = byte(n | 0x80)
  			i++
  		}
  		trailer[i] = byte(n)
  		i++
  		n = count - 1
  		for ; n >= 0x80; n >>= 7 {
  			trailer[i] = byte(n | 0x80)
  			i++
  		}
  		trailer[i] = byte(n)
  		i++
  		trailer[i] = 0
  		i++
  
  		runGCProg(prog, &trailer[0], h.bitp, 2)
  
  		// Even though we filled in the full array just now,
  		// record that we only filled in up to the ptrdata of the
  		// last element. This will cause the code below to
  		// memclr the dead section of the final array element,
  		// so that scanobject can stop early in the final element.
  		totalBits = (elemSize*(count-1) + progSize) / sys.PtrSize
  	}
  	endProg := unsafe.Pointer(subtractb(h.bitp, (totalBits+3)/4))
  	endAlloc := unsafe.Pointer(subtractb(h.bitp, allocSize/heapBitmapScale))
  	memclrNoHeapPointers(add(endAlloc, 1), uintptr(endProg)-uintptr(endAlloc))
  }
  
  // progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
  // size the size of the region described by prog, in bytes.
  // The resulting bitvector will have no more than size/sys.PtrSize bits.
  func progToPointerMask(prog *byte, size uintptr) bitvector {
  	n := (size/sys.PtrSize + 7) / 8
  	x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
  	x[len(x)-1] = 0xa1 // overflow check sentinel
  	n = runGCProg(prog, nil, &x[0], 1)
  	if x[len(x)-1] != 0xa1 {
  		throw("progToPointerMask: overflow")
  	}
  	return bitvector{int32(n), &x[0]}
  }
  
  // Packed GC pointer bitmaps, aka GC programs.
  //
  // For large types containing arrays, the type information has a
  // natural repetition that can be encoded to save space in the
  // binary and in the memory representation of the type information.
  //
  // The encoding is a simple Lempel-Ziv style bytecode machine
  // with the following instructions:
  //
  //	00000000: stop
  //	0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
  //	10000000 n c: repeat the previous n bits c times; n, c are varints
  //	1nnnnnnn c: repeat the previous n bits c times; c is a varint
  
  // runGCProg executes the GC program prog, and then trailer if non-nil,
  // writing to dst with entries of the given size.
  // If size == 1, dst is a 1-bit pointer mask laid out moving forward from dst.
  // If size == 2, dst is the 2-bit heap bitmap, and writes move backward
  // starting at dst (because the heap bitmap does). In this case, the caller guarantees
  // that only whole bytes in dst need to be written.
  //
  // runGCProg returns the number of 1- or 2-bit entries written to memory.
  func runGCProg(prog, trailer, dst *byte, size int) uintptr {
  	dstStart := dst
  
  	// Bits waiting to be written to memory.
  	var bits uintptr
  	var nbits uintptr
  
  	p := prog
  Run:
  	for {
  		// Flush accumulated full bytes.
  		// The rest of the loop assumes that nbits <= 7.
  		for ; nbits >= 8; nbits -= 8 {
  			if size == 1 {
  				*dst = uint8(bits)
  				dst = add1(dst)
  				bits >>= 8
  			} else {
  				v := bits&bitPointerAll | bitScanAll
  				*dst = uint8(v)
  				dst = subtract1(dst)
  				bits >>= 4
  				v = bits&bitPointerAll | bitScanAll
  				*dst = uint8(v)
  				dst = subtract1(dst)
  				bits >>= 4
  			}
  		}
  
  		// Process one instruction.
  		inst := uintptr(*p)
  		p = add1(p)
  		n := inst & 0x7F
  		if inst&0x80 == 0 {
  			// Literal bits; n == 0 means end of program.
  			if n == 0 {
  				// Program is over; continue in trailer if present.
  				if trailer != nil {
  					//println("trailer")
  					p = trailer
  					trailer = nil
  					continue
  				}
  				//println("done")
  				break Run
  			}
  			//println("lit", n, dst)
  			nbyte := n / 8
  			for i := uintptr(0); i < nbyte; i++ {
  				bits |= uintptr(*p) << nbits
  				p = add1(p)
  				if size == 1 {
  					*dst = uint8(bits)
  					dst = add1(dst)
  					bits >>= 8
  				} else {
  					v := bits&0xf | bitScanAll
  					*dst = uint8(v)
  					dst = subtract1(dst)
  					bits >>= 4
  					v = bits&0xf | bitScanAll
  					*dst = uint8(v)
  					dst = subtract1(dst)
  					bits >>= 4
  				}
  			}
  			if n %= 8; n > 0 {
  				bits |= uintptr(*p) << nbits
  				p = add1(p)
  				nbits += n
  			}
  			continue Run
  		}
  
  		// Repeat. If n == 0, it is encoded in a varint in the next bytes.
  		if n == 0 {
  			for off := uint(0); ; off += 7 {
  				x := uintptr(*p)
  				p = add1(p)
  				n |= (x & 0x7F) << off
  				if x&0x80 == 0 {
  					break
  				}
  			}
  		}
  
  		// Count is encoded in a varint in the next bytes.
  		c := uintptr(0)
  		for off := uint(0); ; off += 7 {
  			x := uintptr(*p)
  			p = add1(p)
  			c |= (x & 0x7F) << off
  			if x&0x80 == 0 {
  				break
  			}
  		}
  		c *= n // now total number of bits to copy
  
  		// If the number of bits being repeated is small, load them
  		// into a register and use that register for the entire loop
  		// instead of repeatedly reading from memory.
  		// Handling fewer than 8 bits here makes the general loop simpler.
  		// The cutoff is sys.PtrSize*8 - 7 to guarantee that when we add
  		// the pattern to a bit buffer holding at most 7 bits (a partial byte)
  		// it will not overflow.
  		src := dst
  		const maxBits = sys.PtrSize*8 - 7
  		if n <= maxBits {
  			// Start with bits in output buffer.
  			pattern := bits
  			npattern := nbits
  
  			// If we need more bits, fetch them from memory.
  			if size == 1 {
  				src = subtract1(src)
  				for npattern < n {
  					pattern <<= 8
  					pattern |= uintptr(*src)
  					src = subtract1(src)
  					npattern += 8
  				}
  			} else {
  				src = add1(src)
  				for npattern < n {
  					pattern <<= 4
  					pattern |= uintptr(*src) & 0xf
  					src = add1(src)
  					npattern += 4
  				}
  			}
  
  			// We started with the whole bit output buffer,
  			// and then we loaded bits from whole bytes.
  			// Either way, we might now have too many instead of too few.
  			// Discard the extra.
  			if npattern > n {
  				pattern >>= npattern - n
  				npattern = n
  			}
  
  			// Replicate pattern to at most maxBits.
  			if npattern == 1 {
  				// One bit being repeated.
  				// If the bit is 1, make the pattern all 1s.
  				// If the bit is 0, the pattern is already all 0s,
  				// but we can claim that the number of bits
  				// in the word is equal to the number we need (c),
  				// because right shift of bits will zero fill.
  				if pattern == 1 {
  					pattern = 1<<maxBits - 1
  					npattern = maxBits
  				} else {
  					npattern = c
  				}
  			} else {
  				b := pattern
  				nb := npattern
  				if nb+nb <= maxBits {
  					// Double pattern until the whole uintptr is filled.
  					for nb <= sys.PtrSize*8 {
  						b |= b << nb
  						nb += nb
  					}
  					// Trim away incomplete copy of original pattern in high bits.
  					// TODO(rsc): Replace with table lookup or loop on systems without divide?
  					nb = maxBits / npattern * npattern
  					b &= 1<<nb - 1
  					pattern = b
  					npattern = nb
  				}
  			}
  
  			// Add pattern to bit buffer and flush bit buffer, c/npattern times.
  			// Since pattern contains >8 bits, there will be full bytes to flush
  			// on each iteration.
  			for ; c >= npattern; c -= npattern {
  				bits |= pattern << nbits
  				nbits += npattern
  				if size == 1 {
  					for nbits >= 8 {
  						*dst = uint8(bits)
  						dst = add1(dst)
  						bits >>= 8
  						nbits -= 8
  					}
  				} else {
  					for nbits >= 4 {
  						*dst = uint8(bits&0xf | bitScanAll)
  						dst = subtract1(dst)
  						bits >>= 4
  						nbits -= 4
  					}
  				}
  			}
  
  			// Add final fragment to bit buffer.
  			if c > 0 {
  				pattern &= 1<<c - 1
  				bits |= pattern << nbits
  				nbits += c
  			}
  			continue Run
  		}
  
  		// Repeat; n too large to fit in a register.
  		// Since nbits <= 7, we know the first few bytes of repeated data
  		// are already written to memory.
  		off := n - nbits // n > nbits because n > maxBits and nbits <= 7
  		if size == 1 {
  			// Leading src fragment.
  			src = subtractb(src, (off+7)/8)
  			if frag := off & 7; frag != 0 {
  				bits |= uintptr(*src) >> (8 - frag) << nbits
  				src = add1(src)
  				nbits += frag
  				c -= frag
  			}
  			// Main loop: load one byte, write another.
  			// The bits are rotating through the bit buffer.
  			for i := c / 8; i > 0; i-- {
  				bits |= uintptr(*src) << nbits
  				src = add1(src)
  				*dst = uint8(bits)
  				dst = add1(dst)
  				bits >>= 8
  			}
  			// Final src fragment.
  			if c %= 8; c > 0 {
  				bits |= (uintptr(*src) & (1<<c - 1)) << nbits
  				nbits += c
  			}
  		} else {
  			// Leading src fragment.
  			src = addb(src, (off+3)/4)
  			if frag := off & 3; frag != 0 {
  				bits |= (uintptr(*src) & 0xf) >> (4 - frag) << nbits
  				src = subtract1(src)
  				nbits += frag
  				c -= frag
  			}
  			// Main loop: load one byte, write another.
  			// The bits are rotating through the bit buffer.
  			for i := c / 4; i > 0; i-- {
  				bits |= (uintptr(*src) & 0xf) << nbits
  				src = subtract1(src)
  				*dst = uint8(bits&0xf | bitScanAll)
  				dst = subtract1(dst)
  				bits >>= 4
  			}
  			// Final src fragment.
  			if c %= 4; c > 0 {
  				bits |= (uintptr(*src) & (1<<c - 1)) << nbits
  				nbits += c
  			}
  		}
  	}
  
  	// Write any final bits out, using full-byte writes, even for the final byte.
  	var totalBits uintptr
  	if size == 1 {
  		totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
  		nbits += -nbits & 7
  		for ; nbits > 0; nbits -= 8 {
  			*dst = uint8(bits)
  			dst = add1(dst)
  			bits >>= 8
  		}
  	} else {
  		totalBits = (uintptr(unsafe.Pointer(dstStart))-uintptr(unsafe.Pointer(dst)))*4 + nbits
  		nbits += -nbits & 3
  		for ; nbits > 0; nbits -= 4 {
  			v := bits&0xf | bitScanAll
  			*dst = uint8(v)
  			dst = subtract1(dst)
  			bits >>= 4
  		}
  	}
  	return totalBits
  }
  
  func dumpGCProg(p *byte) {
  	nptr := 0
  	for {
  		x := *p
  		p = add1(p)
  		if x == 0 {
  			print("\t", nptr, " end\n")
  			break
  		}
  		if x&0x80 == 0 {
  			print("\t", nptr, " lit ", x, ":")
  			n := int(x+7) / 8
  			for i := 0; i < n; i++ {
  				print(" ", hex(*p))
  				p = add1(p)
  			}
  			print("\n")
  			nptr += int(x)
  		} else {
  			nbit := int(x &^ 0x80)
  			if nbit == 0 {
  				for nb := uint(0); ; nb += 7 {
  					x := *p
  					p = add1(p)
  					nbit |= int(x&0x7f) << nb
  					if x&0x80 == 0 {
  						break
  					}
  				}
  			}
  			count := 0
  			for nb := uint(0); ; nb += 7 {
  				x := *p
  				p = add1(p)
  				count |= int(x&0x7f) << nb
  				if x&0x80 == 0 {
  					break
  				}
  			}
  			print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
  			nptr += nbit * count
  		}
  	}
  }
  
  // Testing.
  
  func getgcmaskcb(frame *stkframe, ctxt unsafe.Pointer) bool {
  	target := (*stkframe)(ctxt)
  	if frame.sp <= target.sp && target.sp < frame.varp {
  		*target = *frame
  		return false
  	}
  	return true
  }
  
  // gcbits returns the GC type info for x, for testing.
  // The result is the bitmap entries (0 or 1), one entry per byte.
  //go:linkname reflect_gcbits reflect.gcbits
  func reflect_gcbits(x interface{}) []byte {
  	ret := getgcmask(x)
  	typ := (*ptrtype)(unsafe.Pointer(efaceOf(&x)._type)).elem
  	nptr := typ.ptrdata / sys.PtrSize
  	for uintptr(len(ret)) > nptr && ret[len(ret)-1] == 0 {
  		ret = ret[:len(ret)-1]
  	}
  	return ret
  }
  
  // Returns GC type info for object p for testing.
  func getgcmask(ep interface{}) (mask []byte) {
  	e := *efaceOf(&ep)
  	p := e.data
  	t := e._type
  	// data or bss
  	for _, datap := range activeModules() {
  		// data
  		if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
  			bitmap := datap.gcdatamask.bytedata
  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  			mask = make([]byte, n/sys.PtrSize)
  			for i := uintptr(0); i < n; i += sys.PtrSize {
  				off := (uintptr(p) + i - datap.data) / sys.PtrSize
  				mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  			}
  			return
  		}
  
  		// bss
  		if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
  			bitmap := datap.gcbssmask.bytedata
  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  			mask = make([]byte, n/sys.PtrSize)
  			for i := uintptr(0); i < n; i += sys.PtrSize {
  				off := (uintptr(p) + i - datap.bss) / sys.PtrSize
  				mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  			}
  			return
  		}
  	}
  
  	// heap
  	var n uintptr
  	var base uintptr
  	if mlookup(uintptr(p), &base, &n, nil) != 0 {
  		mask = make([]byte, n/sys.PtrSize)
  		for i := uintptr(0); i < n; i += sys.PtrSize {
  			hbits := heapBitsForAddr(base + i)
  			if hbits.isPointer() {
  				mask[i/sys.PtrSize] = 1
  			}
  			if i != 1*sys.PtrSize && !hbits.morePointers() {
  				mask = mask[:i/sys.PtrSize]
  				break
  			}
  		}
  		return
  	}
  
  	// stack
  	if _g_ := getg(); _g_.m.curg.stack.lo <= uintptr(p) && uintptr(p) < _g_.m.curg.stack.hi {
  		var frame stkframe
  		frame.sp = uintptr(p)
  		_g_ := getg()
  		gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
  		if frame.fn != nil {
  			f := frame.fn
  			targetpc := frame.continpc
  			if targetpc == 0 {
  				return
  			}
  			if targetpc != f.entry {
  				targetpc--
  			}
  			pcdata := pcdatavalue(f, _PCDATA_StackMapIndex, targetpc, nil)
  			if pcdata == -1 {
  				return
  			}
  			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
  			if stkmap == nil || stkmap.n <= 0 {
  				return
  			}
  			bv := stackmapdata(stkmap, pcdata)
  			size := uintptr(bv.n) * sys.PtrSize
  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  			mask = make([]byte, n/sys.PtrSize)
  			for i := uintptr(0); i < n; i += sys.PtrSize {
  				bitmap := bv.bytedata
  				off := (uintptr(p) + i - frame.varp + size) / sys.PtrSize
  				mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  			}
  		}
  		return
  	}
  
  	// otherwise, not something the GC knows about.
  	// possibly read-only data, like malloc(0).
  	// must not have pointers
  	return
  }
  

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