github.com/c0deoo1/golang1.5@v0.0.0-20220525150107-c87c805d4593/src/runtime/mbitmap.go (about)

     1  // Copyright 2009 The Go Authors.  All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Garbage collector: type and heap bitmaps.
     6  //
     7  // Stack, data, and bss bitmaps
     8  //
     9  // Stack frames and global variables in the data and bss sections are described
    10  // by 1-bit bitmaps in which 0 means uninteresting and 1 means live pointer
    11  // to be visited during GC. The bits in each byte are consumed starting with
    12  // the low bit: 1<<0, 1<<1, and so on.
    13  //
    14  // Heap bitmap
    15  //
    16  // The allocated heap comes from a subset of the memory in the range [start, used),
    17  // where start == mheap_.arena_start and used == mheap_.arena_used.
    18  // The heap bitmap comprises 2 bits for each pointer-sized word in that range,
    19  // stored in bytes indexed backward in memory from start.
    20  // That is, the byte at address start-1 holds the 2-bit entries for the four words
    21  // start through start+3*ptrSize, the byte at start-2 holds the entries for
    22  // start+4*ptrSize through start+7*ptrSize, and so on.
    23  //
    24  // In each 2-bit entry, the lower bit holds the same information as in the 1-bit
    25  // bitmaps: 0 means uninteresting and 1 means live pointer to be visited during GC.
    26  // The meaning of the high bit depends on the position of the word being described
    27  // in its allocated object. In the first word, the high bit is the GC ``marked'' bit.
    28  // In the second word, the high bit is the GC ``checkmarked'' bit (see below).
    29  // In the third and later words, the high bit indicates that the object is still
    30  // being described. In these words, if a bit pair with a high bit 0 is encountered,
    31  // the low bit can also be assumed to be 0, and the object description is over.
    32  // This 00 is called the ``dead'' encoding: it signals that the rest of the words
    33  // in the object are uninteresting to the garbage collector.
    34  //
    35  // The 2-bit entries are split when written into the byte, so that the top half
    36  // of the byte contains 4 mark bits and the bottom half contains 4 pointer bits.
    37  // This form allows a copy from the 1-bit to the 4-bit form to keep the
    38  // pointer bits contiguous, instead of having to space them out.
    39  //
    40  // The code makes use of the fact that the zero value for a heap bitmap
    41  // has no live pointer bit set and is (depending on position), not marked,
    42  // not checkmarked, and is the dead encoding.
    43  // These properties must be preserved when modifying the encoding.
    44  //
    45  // Checkmarks
    46  //
    47  // In a concurrent garbage collector, one worries about failing to mark
    48  // a live object due to mutations without write barriers or bugs in the
    49  // collector implementation. As a sanity check, the GC has a 'checkmark'
    50  // mode that retraverses the object graph with the world stopped, to make
    51  // sure that everything that should be marked is marked.
    52  // In checkmark mode, in the heap bitmap, the high bit of the 2-bit entry
    53  // for the second word of the object holds the checkmark bit.
    54  // When not in checkmark mode, this bit is set to 1.
    55  //
    56  // The smallest possible allocation is 8 bytes. On a 32-bit machine, that
    57  // means every allocated object has two words, so there is room for the
    58  // checkmark bit. On a 64-bit machine, however, the 8-byte allocation is
    59  // just one word, so the second bit pair is not available for encoding the
    60  // checkmark. However, because non-pointer allocations are combined
    61  // into larger 16-byte (maxTinySize) allocations, a plain 8-byte allocation
    62  // must be a pointer, so the type bit in the first word is not actually needed.
    63  // It is still used in general, except in checkmark the type bit is repurposed
    64  // as the checkmark bit and then reinitialized (to 1) as the type bit when
    65  // finished.
    66  
    67  package runtime
    68  
    69  import "unsafe"
    70  
    71  const (
    72  	bitPointer = 1 << 0
    73  	bitMarked  = 1 << 4
    74  
    75  	heapBitsShift   = 1                 // shift offset between successive bitPointer or bitMarked entries
    76  	heapBitmapScale = ptrSize * (8 / 2) // number of data bytes described by one heap bitmap byte
    77  
    78  	// all mark/pointer bits in a byte
    79  	bitMarkedAll  = bitMarked | bitMarked<<heapBitsShift | bitMarked<<(2*heapBitsShift) | bitMarked<<(3*heapBitsShift)
    80  	bitPointerAll = bitPointer | bitPointer<<heapBitsShift | bitPointer<<(2*heapBitsShift) | bitPointer<<(3*heapBitsShift)
    81  )
    82  
    83  // addb returns the byte pointer p+n.
    84  //go:nowritebarrier
    85  func addb(p *byte, n uintptr) *byte {
    86  	// Note: wrote out full expression instead of calling add(p, n)
    87  	// to reduce the number of temporaries generated by the
    88  	// compiler for this trivial expression during inlining.
    89  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
    90  }
    91  
    92  // subtractb returns the byte pointer p-n.
    93  //go:nowritebarrier
    94  func subtractb(p *byte, n uintptr) *byte {
    95  	// Note: wrote out full expression instead of calling add(p, -n)
    96  	// to reduce the number of temporaries generated by the
    97  	// compiler for this trivial expression during inlining.
    98  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
    99  }
   100  
   101  // add1 returns the byte pointer p+1.
   102  //go:nowritebarrier
   103  func add1(p *byte) *byte {
   104  	// Note: wrote out full expression instead of calling addb(p, 1)
   105  	// to reduce the number of temporaries generated by the
   106  	// compiler for this trivial expression during inlining.
   107  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
   108  }
   109  
   110  // subtract1 returns the byte pointer p-1.
   111  //go:nowritebarrier
   112  func subtract1(p *byte) *byte {
   113  	// Note: wrote out full expression instead of calling subtractb(p, 1)
   114  	// to reduce the number of temporaries generated by the
   115  	// compiler for this trivial expression during inlining.
   116  	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
   117  }
   118  
   119  // mHeap_MapBits is called each time arena_used is extended.
   120  // It maps any additional bitmap memory needed for the new arena memory.
   121  // It must be called with the expected new value of arena_used,
   122  // *before* h.arena_used has been updated.
   123  // Waiting to update arena_used until after the memory has been mapped
   124  // avoids faults when other threads try access the bitmap immediately
   125  // after observing the change to arena_used.
   126  //
   127  //go:nowritebarrier
   128  func mHeap_MapBits(h *mheap, arena_used uintptr) {
   129  	// Caller has added extra mappings to the arena.
   130  	// Add extra mappings of bitmap words as needed.
   131  	// We allocate extra bitmap pieces in chunks of bitmapChunk.
   132  	const bitmapChunk = 8192
   133  
   134  	n := (arena_used - mheap_.arena_start) / heapBitmapScale
   135  	n = round(n, bitmapChunk)
   136  	n = round(n, _PhysPageSize)
   137  	if h.bitmap_mapped >= n {
   138  		return
   139  	}
   140  
   141  	sysMap(unsafe.Pointer(h.arena_start-n), n-h.bitmap_mapped, h.arena_reserved, &memstats.gc_sys)
   142  	h.bitmap_mapped = n
   143  }
   144  
   145  // heapBits provides access to the bitmap bits for a single heap word.
   146  // The methods on heapBits take value receivers so that the compiler
   147  // can more easily inline calls to those methods and registerize the
   148  // struct fields independently.
   149  type heapBits struct {
   150  	bitp  *uint8
   151  	shift uint32
   152  }
   153  
   154  // heapBitsForAddr returns the heapBits for the address addr.
   155  // The caller must have already checked that addr is in the range [mheap_.arena_start, mheap_.arena_used).
   156  //
   157  // nosplit because it is used during write barriers and must not be preempted.
   158  //go:nosplit
   159  func heapBitsForAddr(addr uintptr) heapBits {
   160  	// 2 bits per work, 4 pairs per byte, and a mask is hard coded.
   161  	off := (addr - mheap_.arena_start) / ptrSize
   162  	return heapBits{(*uint8)(unsafe.Pointer(mheap_.arena_start - off/4 - 1)), uint32(off & 3)}
   163  }
   164  
   165  // heapBitsForSpan returns the heapBits for the span base address base.
   166  func heapBitsForSpan(base uintptr) (hbits heapBits) {
   167  	if base < mheap_.arena_start || base >= mheap_.arena_used {
   168  		throw("heapBitsForSpan: base out of range")
   169  	}
   170  	hbits = heapBitsForAddr(base)
   171  	if hbits.shift != 0 {
   172  		throw("heapBitsForSpan: unaligned start")
   173  	}
   174  	return hbits
   175  }
   176  
   177  // heapBitsForObject returns the base address for the heap object
   178  // containing the address p, along with the heapBits for base.
   179  // If p does not point into a heap object,
   180  // return base == 0
   181  // otherwise return the base of the object.
   182  func heapBitsForObject(p uintptr) (base uintptr, hbits heapBits, s *mspan) {
   183  	arenaStart := mheap_.arena_start
   184  	if p < arenaStart || p >= mheap_.arena_used {
   185  		return
   186  	}
   187  	off := p - arenaStart
   188  	idx := off >> _PageShift
   189  	// p points into the heap, but possibly to the middle of an object.
   190  	// Consult the span table to find the block beginning.
   191  	k := p >> _PageShift
   192  	s = h_spans[idx]
   193  	if s == nil || pageID(k) < s.start || p >= s.limit || s.state != mSpanInUse {
   194  		if s == nil || s.state == _MSpanStack {
   195  			// If s is nil, the virtual address has never been part of the heap.
   196  			// This pointer may be to some mmap'd region, so we allow it.
   197  			// Pointers into stacks are also ok, the runtime manages these explicitly.
   198  			return
   199  		}
   200  
   201  		// The following ensures that we are rigorous about what data
   202  		// structures hold valid pointers.
   203  		// TODO(rsc): Check if this still happens.
   204  		if debug.invalidptr != 0 {
   205  			// Still happens sometimes. We don't know why.
   206  			printlock()
   207  			print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))
   208  			if s == nil {
   209  				print(" s=nil\n")
   210  			} else {
   211  				print(" s.start=", hex(s.start<<_PageShift), " s.limit=", hex(s.limit), " s.state=", s.state, "\n")
   212  			}
   213  			printunlock()
   214  			throw("objectstart: bad pointer in unexpected span")
   215  		}
   216  		return
   217  	}
   218  	// If this span holds object of a power of 2 size, just mask off the bits to
   219  	// the interior of the object. Otherwise use the size to get the base.
   220  	if s.baseMask != 0 {
   221  		// optimize for power of 2 sized objects.
   222  		base = s.base()
   223  		base = base + (p-base)&s.baseMask
   224  		// base = p & s.baseMask is faster for small spans,
   225  		// but doesn't work for large spans.
   226  		// Overall, it's faster to use the more general computation above.
   227  	} else {
   228  		base = s.base()
   229  		if p-base >= s.elemsize {
   230  			// n := (p - base) / s.elemsize, using division by multiplication
   231  			n := uintptr(uint64(p-base) >> s.divShift * uint64(s.divMul) >> s.divShift2)
   232  			base += n * s.elemsize
   233  		}
   234  	}
   235  	// Now that we know the actual base, compute heapBits to return to caller.
   236  	hbits = heapBitsForAddr(base)
   237  	return
   238  }
   239  
   240  // prefetch the bits.
   241  func (h heapBits) prefetch() {
   242  	prefetchnta(uintptr(unsafe.Pointer((h.bitp))))
   243  }
   244  
   245  // next returns the heapBits describing the next pointer-sized word in memory.
   246  // That is, if h describes address p, h.next() describes p+ptrSize.
   247  // Note that next does not modify h. The caller must record the result.
   248  func (h heapBits) next() heapBits {
   249  	if h.shift < 3*heapBitsShift {
   250  		return heapBits{h.bitp, h.shift + heapBitsShift}
   251  	}
   252  	return heapBits{subtract1(h.bitp), 0}
   253  }
   254  
   255  // forward returns the heapBits describing n pointer-sized words ahead of h in memory.
   256  // That is, if h describes address p, h.forward(n) describes p+n*ptrSize.
   257  // h.forward(1) is equivalent to h.next(), just slower.
   258  // Note that forward does not modify h. The caller must record the result.
   259  // bits returns the heap bits for the current word.
   260  func (h heapBits) forward(n uintptr) heapBits {
   261  	n += uintptr(h.shift) / heapBitsShift
   262  	return heapBits{subtractb(h.bitp, n/4), uint32(n%4) * heapBitsShift}
   263  }
   264  
   265  // The caller can test isMarked and isPointer by &-ing with bitMarked and bitPointer.
   266  // The result includes in its higher bits the bits for subsequent words
   267  // described by the same bitmap byte.
   268  func (h heapBits) bits() uint32 {
   269  	return uint32(*h.bitp) >> h.shift
   270  }
   271  
   272  // isMarked reports whether the heap bits have the marked bit set.
   273  // h must describe the initial word of the object.
   274  func (h heapBits) isMarked() bool {
   275  	return *h.bitp&(bitMarked<<h.shift) != 0
   276  }
   277  
   278  // setMarked sets the marked bit in the heap bits, atomically.
   279  // h must describe the initial word of the object.
   280  func (h heapBits) setMarked() {
   281  	// Each byte of GC bitmap holds info for four words.
   282  	// Might be racing with other updates, so use atomic update always.
   283  	// We used to be clever here and use a non-atomic update in certain
   284  	// cases, but it's not worth the risk.
   285  	atomicor8(h.bitp, bitMarked<<h.shift)
   286  }
   287  
   288  // setMarkedNonAtomic sets the marked bit in the heap bits, non-atomically.
   289  // h must describe the initial word of the object.
   290  func (h heapBits) setMarkedNonAtomic() {
   291  	*h.bitp |= bitMarked << h.shift
   292  }
   293  
   294  // isPointer reports whether the heap bits describe a pointer word.
   295  // h must describe the initial word of the object.
   296  func (h heapBits) isPointer() bool {
   297  	return (*h.bitp>>h.shift)&bitPointer != 0
   298  }
   299  
   300  // hasPointers reports whether the given object has any pointers.
   301  // It must be told how large the object at h is, so that it does not read too
   302  // far into the bitmap.
   303  // h must describe the initial word of the object.
   304  func (h heapBits) hasPointers(size uintptr) bool {
   305  	if size == ptrSize { // 1-word objects are always pointers
   306  		return true
   307  	}
   308  	// Otherwise, at least a 2-word object, and at least 2-word aligned,
   309  	// so h.shift is either 0 or 4, so we know we can get the bits for the
   310  	// first two words out of *h.bitp.
   311  	// If either of the first two words is a pointer, not pointer free.
   312  	b := uint32(*h.bitp >> h.shift)
   313  	if b&(bitPointer|bitPointer<<heapBitsShift) != 0 {
   314  		return true
   315  	}
   316  	if size == 2*ptrSize {
   317  		return false
   318  	}
   319  	// At least a 4-word object. Check scan bit (aka marked bit) in third word.
   320  	if h.shift == 0 {
   321  		return b&(bitMarked<<(2*heapBitsShift)) != 0
   322  	}
   323  	return uint32(*subtract1(h.bitp))&bitMarked != 0
   324  }
   325  
   326  // isCheckmarked reports whether the heap bits have the checkmarked bit set.
   327  // It must be told how large the object at h is, because the encoding of the
   328  // checkmark bit varies by size.
   329  // h must describe the initial word of the object.
   330  func (h heapBits) isCheckmarked(size uintptr) bool {
   331  	if size == ptrSize {
   332  		return (*h.bitp>>h.shift)&bitPointer != 0
   333  	}
   334  	// All multiword objects are 2-word aligned,
   335  	// so we know that the initial word's 2-bit pair
   336  	// and the second word's 2-bit pair are in the
   337  	// same heap bitmap byte, *h.bitp.
   338  	return (*h.bitp>>(heapBitsShift+h.shift))&bitMarked != 0
   339  }
   340  
   341  // setCheckmarked sets the checkmarked bit.
   342  // It must be told how large the object at h is, because the encoding of the
   343  // checkmark bit varies by size.
   344  // h must describe the initial word of the object.
   345  func (h heapBits) setCheckmarked(size uintptr) {
   346  	if size == ptrSize {
   347  		atomicor8(h.bitp, bitPointer<<h.shift)
   348  		return
   349  	}
   350  	atomicor8(h.bitp, bitMarked<<(heapBitsShift+h.shift))
   351  }
   352  
   353  // heapBitsBulkBarrier executes writebarrierptr_nostore
   354  // for every pointer slot in the memory range [p, p+size),
   355  // using the heap bitmap to locate those pointer slots.
   356  // This executes the write barriers necessary after a memmove.
   357  // Both p and size must be pointer-aligned.
   358  // The range [p, p+size) must lie within a single allocation.
   359  //
   360  // Callers should call heapBitsBulkBarrier immediately after
   361  // calling memmove(p, src, size). This function is marked nosplit
   362  // to avoid being preempted; the GC must not stop the goroutine
   363  // between the memmove and the execution of the barriers.
   364  //
   365  // The heap bitmap is not maintained for allocations containing
   366  // no pointers at all; any caller of heapBitsBulkBarrier must first
   367  // make sure the underlying allocation contains pointers, usually
   368  // by checking typ.kind&kindNoPointers.
   369  //
   370  //go:nosplit
   371  func heapBitsBulkBarrier(p, size uintptr) {
   372  	if (p|size)&(ptrSize-1) != 0 {
   373  		throw("heapBitsBulkBarrier: unaligned arguments")
   374  	}
   375  	if !writeBarrierEnabled {
   376  		return
   377  	}
   378  	if !inheap(p) {
   379  		// If p is on the stack and in a higher frame than the
   380  		// caller, we either need to execute write barriers on
   381  		// it (which is what happens for normal stack writes
   382  		// through pointers to higher frames), or we need to
   383  		// force the mark termination stack scan to scan the
   384  		// frame containing p.
   385  		//
   386  		// Executing write barriers on p is complicated in the
   387  		// general case because we either need to unwind the
   388  		// stack to get the stack map, or we need the type's
   389  		// bitmap, which may be a GC program.
   390  		//
   391  		// Hence, we opt for forcing the re-scan to scan the
   392  		// frame containing p, which we can do by simply
   393  		// unwinding the stack barriers between the current SP
   394  		// and p's frame.
   395  		gp := getg().m.curg
   396  		if gp != nil && gp.stack.lo <= p && p < gp.stack.hi {
   397  			// Run on the system stack to give it more
   398  			// stack space.
   399  			systemstack(func() {
   400  				gcUnwindBarriers(gp, p)
   401  			})
   402  		}
   403  		return
   404  	}
   405  
   406  	h := heapBitsForAddr(p)
   407  	for i := uintptr(0); i < size; i += ptrSize {
   408  		if h.isPointer() {
   409  			x := (*uintptr)(unsafe.Pointer(p + i))
   410  			writebarrierptr_nostore(x, *x)
   411  		}
   412  		h = h.next()
   413  	}
   414  }
   415  
   416  // typeBitsBulkBarrier executes writebarrierptr_nostore
   417  // for every pointer slot in the memory range [p, p+size),
   418  // using the type bitmap to locate those pointer slots.
   419  // The type typ must correspond exactly to [p, p+size).
   420  // This executes the write barriers necessary after a copy.
   421  // Both p and size must be pointer-aligned.
   422  // The type typ must have a plain bitmap, not a GC program.
   423  // The only use of this function is in channel sends, and the
   424  // 64 kB channel element limit takes care of this for us.
   425  //
   426  // Must not be preempted because it typically runs right after memmove,
   427  // and the GC must not complete between those two.
   428  //
   429  //go:nosplit
   430  func typeBitsBulkBarrier(typ *_type, p, size uintptr) {
   431  	if typ == nil {
   432  		throw("runtime: typeBitsBulkBarrier without type")
   433  	}
   434  	if typ.size != size {
   435  		println("runtime: typeBitsBulkBarrier with type ", *typ._string, " of size ", typ.size, " but memory size", size)
   436  		throw("runtime: invalid typeBitsBulkBarrier")
   437  	}
   438  	if typ.kind&kindGCProg != 0 {
   439  		println("runtime: typeBitsBulkBarrier with type ", *typ._string, " with GC prog")
   440  		throw("runtime: invalid typeBitsBulkBarrier")
   441  	}
   442  	if !writeBarrierEnabled {
   443  		return
   444  	}
   445  	ptrmask := typ.gcdata
   446  	var bits uint32
   447  	for i := uintptr(0); i < typ.ptrdata; i += ptrSize {
   448  		if i&(ptrSize*8-1) == 0 {
   449  			bits = uint32(*ptrmask)
   450  			ptrmask = addb(ptrmask, 1)
   451  		} else {
   452  			bits = bits >> 1
   453  		}
   454  		if bits&1 != 0 {
   455  			x := (*uintptr)(unsafe.Pointer(p + i))
   456  			writebarrierptr_nostore(x, *x)
   457  		}
   458  	}
   459  }
   460  
   461  // The methods operating on spans all require that h has been returned
   462  // by heapBitsForSpan and that size, n, total are the span layout description
   463  // returned by the mspan's layout method.
   464  // If total > size*n, it means that there is extra leftover memory in the span,
   465  // usually due to rounding.
   466  //
   467  // TODO(rsc): Perhaps introduce a different heapBitsSpan type.
   468  
   469  // initSpan initializes the heap bitmap for a span.
   470  func (h heapBits) initSpan(size, n, total uintptr) {
   471  	if total%heapBitmapScale != 0 {
   472  		throw("initSpan: unaligned length")
   473  	}
   474  	nbyte := total / heapBitmapScale
   475  	if ptrSize == 8 && size == ptrSize {
   476  		end := h.bitp
   477  		bitp := subtractb(end, nbyte-1)
   478  		for {
   479  			*bitp = bitPointerAll
   480  			if bitp == end {
   481  				break
   482  			}
   483  			bitp = add1(bitp)
   484  		}
   485  		return
   486  	}
   487  	memclr(unsafe.Pointer(subtractb(h.bitp, nbyte-1)), nbyte)
   488  }
   489  
   490  // initCheckmarkSpan initializes a span for being checkmarked.
   491  // It clears the checkmark bits, which are set to 1 in normal operation.
   492  func (h heapBits) initCheckmarkSpan(size, n, total uintptr) {
   493  	// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
   494  	if ptrSize == 8 && size == ptrSize {
   495  		// Checkmark bit is type bit, bottom bit of every 2-bit entry.
   496  		// Only possible on 64-bit system, since minimum size is 8.
   497  		// Must clear type bit (checkmark bit) of every word.
   498  		// The type bit is the lower of every two-bit pair.
   499  		bitp := h.bitp
   500  		for i := uintptr(0); i < n; i += 4 {
   501  			*bitp &^= bitPointerAll
   502  			bitp = subtract1(bitp)
   503  		}
   504  		return
   505  	}
   506  	for i := uintptr(0); i < n; i++ {
   507  		*h.bitp &^= bitMarked << (heapBitsShift + h.shift)
   508  		h = h.forward(size / ptrSize)
   509  	}
   510  }
   511  
   512  // clearCheckmarkSpan undoes all the checkmarking in a span.
   513  // The actual checkmark bits are ignored, so the only work to do
   514  // is to fix the pointer bits. (Pointer bits are ignored by scanobject
   515  // but consulted by typedmemmove.)
   516  func (h heapBits) clearCheckmarkSpan(size, n, total uintptr) {
   517  	// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
   518  	if ptrSize == 8 && size == ptrSize {
   519  		// Checkmark bit is type bit, bottom bit of every 2-bit entry.
   520  		// Only possible on 64-bit system, since minimum size is 8.
   521  		// Must clear type bit (checkmark bit) of every word.
   522  		// The type bit is the lower of every two-bit pair.
   523  		bitp := h.bitp
   524  		for i := uintptr(0); i < n; i += 4 {
   525  			*bitp |= bitPointerAll
   526  			bitp = subtract1(bitp)
   527  		}
   528  	}
   529  }
   530  
   531  // heapBitsSweepSpan coordinates the sweeping of a span by reading
   532  // and updating the corresponding heap bitmap entries.
   533  // For each free object in the span, heapBitsSweepSpan sets the type
   534  // bits for the first two words (or one for single-word objects) to typeDead
   535  // and then calls f(p), where p is the object's base address.
   536  // f is expected to add the object to a free list.
   537  // For non-free objects, heapBitsSweepSpan turns off the marked bit.
   538  func heapBitsSweepSpan(base, size, n uintptr, f func(uintptr)) {
   539  	h := heapBitsForSpan(base)
   540  	switch {
   541  	default:
   542  		throw("heapBitsSweepSpan")
   543  	case ptrSize == 8 && size == ptrSize:
   544  		// Consider mark bits in all four 2-bit entries of each bitmap byte.
   545  		bitp := h.bitp
   546  		for i := uintptr(0); i < n; i += 4 {
   547  			x := uint32(*bitp)
   548  			// Note that unlike the other size cases, we leave the pointer bits set here.
   549  			// These are initialized during initSpan when the span is created and left
   550  			// in place the whole time the span is used for pointer-sized objects.
   551  			// That lets heapBitsSetType avoid an atomic update to set the pointer bit
   552  			// during allocation.
   553  			if x&bitMarked != 0 {
   554  				x &^= bitMarked
   555  			} else {
   556  				f(base + i*ptrSize)
   557  			}
   558  			if x&(bitMarked<<heapBitsShift) != 0 {
   559  				x &^= bitMarked << heapBitsShift
   560  			} else {
   561  				f(base + (i+1)*ptrSize)
   562  			}
   563  			if x&(bitMarked<<(2*heapBitsShift)) != 0 {
   564  				x &^= bitMarked << (2 * heapBitsShift)
   565  			} else {
   566  				f(base + (i+2)*ptrSize)
   567  			}
   568  			if x&(bitMarked<<(3*heapBitsShift)) != 0 {
   569  				x &^= bitMarked << (3 * heapBitsShift)
   570  			} else {
   571  				f(base + (i+3)*ptrSize)
   572  			}
   573  			*bitp = uint8(x)
   574  			bitp = subtract1(bitp)
   575  		}
   576  
   577  	case size%(4*ptrSize) == 0:
   578  		// Mark bit is in first word of each object.
   579  		// Each object starts at bit 0 of a heap bitmap byte.
   580  		bitp := h.bitp
   581  		step := size / heapBitmapScale
   582  		for i := uintptr(0); i < n; i++ {
   583  			x := uint32(*bitp)
   584  			if x&bitMarked != 0 {
   585  				x &^= bitMarked
   586  			} else {
   587  				x = 0
   588  				f(base + i*size)
   589  			}
   590  			*bitp = uint8(x)
   591  			bitp = subtractb(bitp, step)
   592  		}
   593  
   594  	case size%(4*ptrSize) == 2*ptrSize:
   595  		// Mark bit is in first word of each object,
   596  		// but every other object starts halfway through a heap bitmap byte.
   597  		// Unroll loop 2x to handle alternating shift count and step size.
   598  		bitp := h.bitp
   599  		step := size / heapBitmapScale
   600  		var i uintptr
   601  		for i = uintptr(0); i < n; i += 2 {
   602  			x := uint32(*bitp)
   603  			if x&bitMarked != 0 {
   604  				x &^= bitMarked
   605  			} else {
   606  				x &^= bitMarked | bitPointer | (bitMarked|bitPointer)<<heapBitsShift
   607  				f(base + i*size)
   608  				if size > 2*ptrSize {
   609  					x = 0
   610  				}
   611  			}
   612  			*bitp = uint8(x)
   613  			if i+1 >= n {
   614  				break
   615  			}
   616  			bitp = subtractb(bitp, step)
   617  			x = uint32(*bitp)
   618  			if x&(bitMarked<<(2*heapBitsShift)) != 0 {
   619  				x &^= bitMarked << (2 * heapBitsShift)
   620  			} else {
   621  				x &^= (bitMarked|bitPointer)<<(2*heapBitsShift) | (bitMarked|bitPointer)<<(3*heapBitsShift)
   622  				f(base + (i+1)*size)
   623  				if size > 2*ptrSize {
   624  					*subtract1(bitp) = 0
   625  				}
   626  			}
   627  			*bitp = uint8(x)
   628  			bitp = subtractb(bitp, step+1)
   629  		}
   630  	}
   631  }
   632  
   633  // heapBitsSetType records that the new allocation [x, x+size)
   634  // holds in [x, x+dataSize) one or more values of type typ.
   635  // (The number of values is given by dataSize / typ.size.)
   636  // If dataSize < size, the fragment [x+dataSize, x+size) is
   637  // recorded as non-pointer data.
   638  // It is known that the type has pointers somewhere;
   639  // malloc does not call heapBitsSetType when there are no pointers,
   640  // because all free objects are marked as noscan during
   641  // heapBitsSweepSpan.
   642  // There can only be one allocation from a given span active at a time,
   643  // so this code is not racing with other instances of itself,
   644  // and we don't allocate from a span until it has been swept,
   645  // so this code is not racing with heapBitsSweepSpan.
   646  // It is, however, racing with the concurrent GC mark phase,
   647  // which can be setting the mark bit in the leading 2-bit entry
   648  // of an allocated block. The block we are modifying is not quite
   649  // allocated yet, so the GC marker is not racing with updates to x's bits,
   650  // but if the start or end of x shares a bitmap byte with an adjacent
   651  // object, the GC marker is racing with updates to those object's mark bits.
   652  func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
   653  	const doubleCheck = false // slow but helpful; enable to test modifications to this code
   654  
   655  	// dataSize is always size rounded up to the next malloc size class,
   656  	// except in the case of allocating a defer block, in which case
   657  	// size is sizeof(_defer{}) (at least 6 words) and dataSize may be
   658  	// arbitrarily larger.
   659  	//
   660  	// The checks for size == ptrSize and size == 2*ptrSize can therefore
   661  	// assume that dataSize == size without checking it explicitly.
   662  
   663  	if ptrSize == 8 && size == ptrSize {
   664  		// It's one word and it has pointers, it must be a pointer.
   665  		// In general we'd need an atomic update here if the
   666  		// concurrent GC were marking objects in this span,
   667  		// because each bitmap byte describes 3 other objects
   668  		// in addition to the one being allocated.
   669  		// However, since all allocated one-word objects are pointers
   670  		// (non-pointers are aggregated into tinySize allocations),
   671  		// initSpan sets the pointer bits for us. Nothing to do here.
   672  		if doubleCheck {
   673  			h := heapBitsForAddr(x)
   674  			if !h.isPointer() {
   675  				throw("heapBitsSetType: pointer bit missing")
   676  			}
   677  		}
   678  		return
   679  	}
   680  
   681  	h := heapBitsForAddr(x)
   682  	ptrmask := typ.gcdata // start of 1-bit pointer mask (or GC program, handled below)
   683  
   684  	// Heap bitmap bits for 2-word object are only 4 bits,
   685  	// so also shared with objects next to it; use atomic updates.
   686  	// This is called out as a special case primarily for 32-bit systems,
   687  	// so that on 32-bit systems the code below can assume all objects
   688  	// are 4-word aligned (because they're all 16-byte aligned).
   689  	if size == 2*ptrSize {
   690  		if typ.size == ptrSize {
   691  			// We're allocating a block big enough to hold two pointers.
   692  			// On 64-bit, that means the actual object must be two pointers,
   693  			// or else we'd have used the one-pointer-sized block.
   694  			// On 32-bit, however, this is the 8-byte block, the smallest one.
   695  			// So it could be that we're allocating one pointer and this was
   696  			// just the smallest block available. Distinguish by checking dataSize.
   697  			// (In general the number of instances of typ being allocated is
   698  			// dataSize/typ.size.)
   699  			if ptrSize == 4 && dataSize == ptrSize {
   700  				// 1 pointer.
   701  				if gcphase == _GCoff {
   702  					*h.bitp |= bitPointer << h.shift
   703  				} else {
   704  					atomicor8(h.bitp, bitPointer<<h.shift)
   705  				}
   706  			} else {
   707  				// 2-element slice of pointer.
   708  				if gcphase == _GCoff {
   709  					*h.bitp |= (bitPointer | bitPointer<<heapBitsShift) << h.shift
   710  				} else {
   711  					atomicor8(h.bitp, (bitPointer|bitPointer<<heapBitsShift)<<h.shift)
   712  				}
   713  			}
   714  			return
   715  		}
   716  		// Otherwise typ.size must be 2*ptrSize, and typ.kind&kindGCProg == 0.
   717  		if doubleCheck {
   718  			if typ.size != 2*ptrSize || typ.kind&kindGCProg != 0 {
   719  				print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, " gcprog=", typ.kind&kindGCProg != 0, "\n")
   720  				throw("heapBitsSetType")
   721  			}
   722  		}
   723  		b := uint32(*ptrmask)
   724  		hb := b & 3
   725  		if gcphase == _GCoff {
   726  			*h.bitp |= uint8(hb << h.shift)
   727  		} else {
   728  			atomicor8(h.bitp, uint8(hb<<h.shift))
   729  		}
   730  		return
   731  	}
   732  
   733  	// Copy from 1-bit ptrmask into 2-bit bitmap.
   734  	// The basic approach is to use a single uintptr as a bit buffer,
   735  	// alternating between reloading the buffer and writing bitmap bytes.
   736  	// In general, one load can supply two bitmap byte writes.
   737  	// This is a lot of lines of code, but it compiles into relatively few
   738  	// machine instructions.
   739  
   740  	var (
   741  		// Ptrmask input.
   742  		p     *byte   // last ptrmask byte read
   743  		b     uintptr // ptrmask bits already loaded
   744  		nb    uintptr // number of bits in b at next read
   745  		endp  *byte   // final ptrmask byte to read (then repeat)
   746  		endnb uintptr // number of valid bits in *endp
   747  		pbits uintptr // alternate source of bits
   748  
   749  		// Heap bitmap output.
   750  		w     uintptr // words processed
   751  		nw    uintptr // number of words to process
   752  		hbitp *byte   // next heap bitmap byte to write
   753  		hb    uintptr // bits being prepared for *hbitp
   754  	)
   755  
   756  	hbitp = h.bitp
   757  
   758  	// Handle GC program. Delayed until this part of the code
   759  	// so that we can use the same double-checking mechanism
   760  	// as the 1-bit case. Nothing above could have encountered
   761  	// GC programs: the cases were all too small.
   762  	if typ.kind&kindGCProg != 0 {
   763  		heapBitsSetTypeGCProg(h, typ.ptrdata, typ.size, dataSize, size, addb(typ.gcdata, 4))
   764  		if doubleCheck {
   765  			// Double-check the heap bits written by GC program
   766  			// by running the GC program to create a 1-bit pointer mask
   767  			// and then jumping to the double-check code below.
   768  			// This doesn't catch bugs shared between the 1-bit and 4-bit
   769  			// GC program execution, but it does catch mistakes specific
   770  			// to just one of those and bugs in heapBitsSetTypeGCProg's
   771  			// implementation of arrays.
   772  			lock(&debugPtrmask.lock)
   773  			if debugPtrmask.data == nil {
   774  				debugPtrmask.data = (*byte)(persistentalloc(1<<20, 1, &memstats.other_sys))
   775  			}
   776  			ptrmask = debugPtrmask.data
   777  			runGCProg(addb(typ.gcdata, 4), nil, ptrmask, 1)
   778  			goto Phase4
   779  		}
   780  		return
   781  	}
   782  
   783  	// Note about sizes:
   784  	//
   785  	// typ.size is the number of words in the object,
   786  	// and typ.ptrdata is the number of words in the prefix
   787  	// of the object that contains pointers. That is, the final
   788  	// typ.size - typ.ptrdata words contain no pointers.
   789  	// This allows optimization of a common pattern where
   790  	// an object has a small header followed by a large scalar
   791  	// buffer. If we know the pointers are over, we don't have
   792  	// to scan the buffer's heap bitmap at all.
   793  	// The 1-bit ptrmasks are sized to contain only bits for
   794  	// the typ.ptrdata prefix, zero padded out to a full byte
   795  	// of bitmap. This code sets nw (below) so that heap bitmap
   796  	// bits are only written for the typ.ptrdata prefix; if there is
   797  	// more room in the allocated object, the next heap bitmap
   798  	// entry is a 00, indicating that there are no more pointers
   799  	// to scan. So only the ptrmask for the ptrdata bytes is needed.
   800  	//
   801  	// Replicated copies are not as nice: if there is an array of
   802  	// objects with scalar tails, all but the last tail does have to
   803  	// be initialized, because there is no way to say "skip forward".
   804  	// However, because of the possibility of a repeated type with
   805  	// size not a multiple of 4 pointers (one heap bitmap byte),
   806  	// the code already must handle the last ptrmask byte specially
   807  	// by treating it as containing only the bits for endnb pointers,
   808  	// where endnb <= 4. We represent large scalar tails that must
   809  	// be expanded in the replication by setting endnb larger than 4.
   810  	// This will have the effect of reading many bits out of b,
   811  	// but once the real bits are shifted out, b will supply as many
   812  	// zero bits as we try to read, which is exactly what we need.
   813  
   814  	p = ptrmask
   815  	if typ.size < dataSize {
   816  		// Filling in bits for an array of typ.
   817  		// Set up for repetition of ptrmask during main loop.
   818  		// Note that ptrmask describes only a prefix of
   819  		const maxBits = ptrSize*8 - 7
   820  		if typ.ptrdata/ptrSize <= maxBits {
   821  			// Entire ptrmask fits in uintptr with room for a byte fragment.
   822  			// Load into pbits and never read from ptrmask again.
   823  			// This is especially important when the ptrmask has
   824  			// fewer than 8 bits in it; otherwise the reload in the middle
   825  			// of the Phase 2 loop would itself need to loop to gather
   826  			// at least 8 bits.
   827  
   828  			// Accumulate ptrmask into b.
   829  			// ptrmask is sized to describe only typ.ptrdata, but we record
   830  			// it as describing typ.size bytes, since all the high bits are zero.
   831  			nb = typ.ptrdata / ptrSize
   832  			for i := uintptr(0); i < nb; i += 8 {
   833  				b |= uintptr(*p) << i
   834  				p = add1(p)
   835  			}
   836  			nb = typ.size / ptrSize
   837  
   838  			// Replicate ptrmask to fill entire pbits uintptr.
   839  			// Doubling and truncating is fewer steps than
   840  			// iterating by nb each time. (nb could be 1.)
   841  			// Since we loaded typ.ptrdata/ptrSize bits
   842  			// but are pretending to have typ.size/ptrSize,
   843  			// there might be no replication necessary/possible.
   844  			pbits = b
   845  			endnb = nb
   846  			if nb+nb <= maxBits {
   847  				for endnb <= ptrSize*8 {
   848  					pbits |= pbits << endnb
   849  					endnb += endnb
   850  				}
   851  				// Truncate to a multiple of original ptrmask.
   852  				endnb = maxBits / nb * nb
   853  				pbits &= 1<<endnb - 1
   854  				b = pbits
   855  				nb = endnb
   856  			}
   857  
   858  			// Clear p and endp as sentinel for using pbits.
   859  			// Checked during Phase 2 loop.
   860  			p = nil
   861  			endp = nil
   862  		} else {
   863  			// Ptrmask is larger. Read it multiple times.
   864  			n := (typ.ptrdata/ptrSize+7)/8 - 1
   865  			endp = addb(ptrmask, n)
   866  			endnb = typ.size/ptrSize - n*8
   867  		}
   868  	}
   869  	if p != nil {
   870  		b = uintptr(*p)
   871  		p = add1(p)
   872  		nb = 8
   873  	}
   874  
   875  	if typ.size == dataSize {
   876  		// Single entry: can stop once we reach the non-pointer data.
   877  		nw = typ.ptrdata / ptrSize
   878  	} else {
   879  		// Repeated instances of typ in an array.
   880  		// Have to process first N-1 entries in full, but can stop
   881  		// once we reach the non-pointer data in the final entry.
   882  		nw = ((dataSize/typ.size-1)*typ.size + typ.ptrdata) / ptrSize
   883  	}
   884  	if nw == 0 {
   885  		// No pointers! Caller was supposed to check.
   886  		println("runtime: invalid type ", *typ._string)
   887  		throw("heapBitsSetType: called with non-pointer type")
   888  		return
   889  	}
   890  	if nw < 2 {
   891  		// Must write at least 2 words, because the "no scan"
   892  		// encoding doesn't take effect until the third word.
   893  		nw = 2
   894  	}
   895  
   896  	// Phase 1: Special case for leading byte (shift==0) or half-byte (shift==4).
   897  	// The leading byte is special because it contains the bits for words 0 and 1,
   898  	// which do not have the marked bits set.
   899  	// The leading half-byte is special because it's a half a byte and must be
   900  	// manipulated atomically.
   901  	switch {
   902  	default:
   903  		throw("heapBitsSetType: unexpected shift")
   904  
   905  	case h.shift == 0:
   906  		// Ptrmask and heap bitmap are aligned.
   907  		// Handle first byte of bitmap specially.
   908  		// The first byte we write out contains the first two words of the object.
   909  		// In those words, the mark bits are mark and checkmark, respectively,
   910  		// and must not be set. In all following words, we want to set the mark bit
   911  		// as a signal that the object continues to the next 2-bit entry in the bitmap.
   912  		hb = b & bitPointerAll
   913  		hb |= bitMarked<<(2*heapBitsShift) | bitMarked<<(3*heapBitsShift)
   914  		if w += 4; w >= nw {
   915  			goto Phase3
   916  		}
   917  		*hbitp = uint8(hb)
   918  		hbitp = subtract1(hbitp)
   919  		b >>= 4
   920  		nb -= 4
   921  
   922  	case ptrSize == 8 && h.shift == 2:
   923  		// Ptrmask and heap bitmap are misaligned.
   924  		// The bits for the first two words are in a byte shared with another object
   925  		// and must be updated atomically.
   926  		// NOTE(rsc): The atomic here may not be necessary.
   927  		// We took care of 1-word and 2-word objects above,
   928  		// so this is at least a 6-word object, so our start bits
   929  		// are shared only with the type bits of another object,
   930  		// not with its mark bit. Since there is only one allocation
   931  		// from a given span at a time, we should be able to set
   932  		// these bits non-atomically. Not worth the risk right now.
   933  		hb = (b & 3) << (2 * heapBitsShift)
   934  		b >>= 2
   935  		nb -= 2
   936  		// Note: no bitMarker in hb because the first two words don't get markers from us.
   937  		if gcphase == _GCoff {
   938  			*hbitp |= uint8(hb)
   939  		} else {
   940  			atomicor8(hbitp, uint8(hb))
   941  		}
   942  		hbitp = subtract1(hbitp)
   943  		if w += 2; w >= nw {
   944  			// We know that there is more data, because we handled 2-word objects above.
   945  			// This must be at least a 6-word object. If we're out of pointer words,
   946  			// mark no scan in next bitmap byte and finish.
   947  			hb = 0
   948  			w += 4
   949  			goto Phase3
   950  		}
   951  	}
   952  
   953  	// Phase 2: Full bytes in bitmap, up to but not including write to last byte (full or partial) in bitmap.
   954  	// The loop computes the bits for that last write but does not execute the write;
   955  	// it leaves the bits in hb for processing by phase 3.
   956  	// To avoid repeated adjustment of nb, we subtract out the 4 bits we're going to
   957  	// use in the first half of the loop right now, and then we only adjust nb explicitly
   958  	// if the 8 bits used by each iteration isn't balanced by 8 bits loaded mid-loop.
   959  	nb -= 4
   960  	for {
   961  		// Emit bitmap byte.
   962  		// b has at least nb+4 bits, with one exception:
   963  		// if w+4 >= nw, then b has only nw-w bits,
   964  		// but we'll stop at the break and then truncate
   965  		// appropriately in Phase 3.
   966  		hb = b & bitPointerAll
   967  		hb |= bitMarkedAll
   968  		if w += 4; w >= nw {
   969  			break
   970  		}
   971  		*hbitp = uint8(hb)
   972  		hbitp = subtract1(hbitp)
   973  		b >>= 4
   974  
   975  		// Load more bits. b has nb right now.
   976  		if p != endp {
   977  			// Fast path: keep reading from ptrmask.
   978  			// nb unmodified: we just loaded 8 bits,
   979  			// and the next iteration will consume 8 bits,
   980  			// leaving us with the same nb the next time we're here.
   981  			if nb < 8 {
   982  				b |= uintptr(*p) << nb
   983  				p = add1(p)
   984  			} else {
   985  				// Reduce the number of bits in b.
   986  				// This is important if we skipped
   987  				// over a scalar tail, since nb could
   988  				// be larger than the bit width of b.
   989  				nb -= 8
   990  			}
   991  		} else if p == nil {
   992  			// Almost as fast path: track bit count and refill from pbits.
   993  			// For short repetitions.
   994  			if nb < 8 {
   995  				b |= pbits << nb
   996  				nb += endnb
   997  			}
   998  			nb -= 8 // for next iteration
   999  		} else {
  1000  			// Slow path: reached end of ptrmask.
  1001  			// Process final partial byte and rewind to start.
  1002  			b |= uintptr(*p) << nb
  1003  			nb += endnb
  1004  			if nb < 8 {
  1005  				b |= uintptr(*ptrmask) << nb
  1006  				p = add1(ptrmask)
  1007  			} else {
  1008  				nb -= 8
  1009  				p = ptrmask
  1010  			}
  1011  		}
  1012  
  1013  		// Emit bitmap byte.
  1014  		hb = b & bitPointerAll
  1015  		hb |= bitMarkedAll
  1016  		if w += 4; w >= nw {
  1017  			break
  1018  		}
  1019  		*hbitp = uint8(hb)
  1020  		hbitp = subtract1(hbitp)
  1021  		b >>= 4
  1022  	}
  1023  
  1024  Phase3:
  1025  	// Phase 3: Write last byte or partial byte and zero the rest of the bitmap entries.
  1026  	if w > nw {
  1027  		// Counting the 4 entries in hb not yet written to memory,
  1028  		// there are more entries than possible pointer slots.
  1029  		// Discard the excess entries (can't be more than 3).
  1030  		mask := uintptr(1)<<(4-(w-nw)) - 1
  1031  		hb &= mask | mask<<4 // apply mask to both pointer bits and mark bits
  1032  	}
  1033  
  1034  	// Change nw from counting possibly-pointer words to total words in allocation.
  1035  	nw = size / ptrSize
  1036  
  1037  	// Write whole bitmap bytes.
  1038  	// The first is hb, the rest are zero.
  1039  	if w <= nw {
  1040  		*hbitp = uint8(hb)
  1041  		hbitp = subtract1(hbitp)
  1042  		hb = 0 // for possible final half-byte below
  1043  		for w += 4; w <= nw; w += 4 {
  1044  			*hbitp = 0
  1045  			hbitp = subtract1(hbitp)
  1046  		}
  1047  	}
  1048  
  1049  	// Write final partial bitmap byte if any.
  1050  	// We know w > nw, or else we'd still be in the loop above.
  1051  	// It can be bigger only due to the 4 entries in hb that it counts.
  1052  	// If w == nw+4 then there's nothing left to do: we wrote all nw entries
  1053  	// and can discard the 4 sitting in hb.
  1054  	// But if w == nw+2, we need to write first two in hb.
  1055  	// The byte is shared with the next object so we may need an atomic.
  1056  	if w == nw+2 {
  1057  		if gcphase == _GCoff {
  1058  			*hbitp = *hbitp&^(bitPointer|bitMarked|(bitPointer|bitMarked)<<heapBitsShift) | uint8(hb)
  1059  		} else {
  1060  			atomicand8(hbitp, ^uint8(bitPointer|bitMarked|(bitPointer|bitMarked)<<heapBitsShift))
  1061  			atomicor8(hbitp, uint8(hb))
  1062  		}
  1063  	}
  1064  
  1065  Phase4:
  1066  	// Phase 4: all done, but perhaps double check.
  1067  	if doubleCheck {
  1068  		end := heapBitsForAddr(x + size)
  1069  		if typ.kind&kindGCProg == 0 && (hbitp != end.bitp || (w == nw+2) != (end.shift == 2)) {
  1070  			println("ended at wrong bitmap byte for", *typ._string, "x", dataSize/typ.size)
  1071  			print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
  1072  			print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
  1073  			h0 := heapBitsForAddr(x)
  1074  			print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
  1075  			print("ended at hbitp=", hbitp, " but next starts at bitp=", end.bitp, " shift=", end.shift, "\n")
  1076  			throw("bad heapBitsSetType")
  1077  		}
  1078  
  1079  		// Double-check that bits to be written were written correctly.
  1080  		// Does not check that other bits were not written, unfortunately.
  1081  		h := heapBitsForAddr(x)
  1082  		nptr := typ.ptrdata / ptrSize
  1083  		ndata := typ.size / ptrSize
  1084  		count := dataSize / typ.size
  1085  		totalptr := ((count-1)*typ.size + typ.ptrdata) / ptrSize
  1086  		for i := uintptr(0); i < size/ptrSize; i++ {
  1087  			j := i % ndata
  1088  			var have, want uint8
  1089  			have = (*h.bitp >> h.shift) & (bitPointer | bitMarked)
  1090  			if i >= totalptr {
  1091  				want = 0 // deadmarker
  1092  				if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 {
  1093  					want = bitMarked
  1094  				}
  1095  			} else {
  1096  				if j < nptr && (*addb(ptrmask, j/8)>>(j%8))&1 != 0 {
  1097  					want |= bitPointer
  1098  				}
  1099  				if i >= 2 {
  1100  					want |= bitMarked
  1101  				} else {
  1102  					have &^= bitMarked
  1103  				}
  1104  			}
  1105  			if have != want {
  1106  				println("mismatch writing bits for", *typ._string, "x", dataSize/typ.size)
  1107  				print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
  1108  				print("kindGCProg=", typ.kind&kindGCProg != 0, "\n")
  1109  				print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
  1110  				h0 := heapBitsForAddr(x)
  1111  				print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
  1112  				print("current bits h.bitp=", h.bitp, " h.shift=", h.shift, " *h.bitp=", hex(*h.bitp), "\n")
  1113  				print("ptrmask=", ptrmask, " p=", p, " endp=", endp, " endnb=", endnb, " pbits=", hex(pbits), " b=", hex(b), " nb=", nb, "\n")
  1114  				println("at word", i, "offset", i*ptrSize, "have", have, "want", want)
  1115  				if typ.kind&kindGCProg != 0 {
  1116  					println("GC program:")
  1117  					dumpGCProg(addb(typ.gcdata, 4))
  1118  				}
  1119  				throw("bad heapBitsSetType")
  1120  			}
  1121  			h = h.next()
  1122  		}
  1123  		if ptrmask == debugPtrmask.data {
  1124  			unlock(&debugPtrmask.lock)
  1125  		}
  1126  	}
  1127  }
  1128  
  1129  var debugPtrmask struct {
  1130  	lock mutex
  1131  	data *byte
  1132  }
  1133  
  1134  // heapBitsSetTypeGCProg implements heapBitsSetType using a GC program.
  1135  // progSize is the size of the memory described by the program.
  1136  // elemSize is the size of the element that the GC program describes (a prefix of).
  1137  // dataSize is the total size of the intended data, a multiple of elemSize.
  1138  // allocSize is the total size of the allocated memory.
  1139  //
  1140  // GC programs are only used for large allocations.
  1141  // heapBitsSetType requires that allocSize is a multiple of 4 words,
  1142  // so that the relevant bitmap bytes are not shared with surrounding
  1143  // objects and need not be accessed with atomic instructions.
  1144  func heapBitsSetTypeGCProg(h heapBits, progSize, elemSize, dataSize, allocSize uintptr, prog *byte) {
  1145  	if ptrSize == 8 && allocSize%(4*ptrSize) != 0 {
  1146  		// Alignment will be wrong.
  1147  		throw("heapBitsSetTypeGCProg: small allocation")
  1148  	}
  1149  	var totalBits uintptr
  1150  	if elemSize == dataSize {
  1151  		totalBits = runGCProg(prog, nil, h.bitp, 2)
  1152  		if totalBits*ptrSize != progSize {
  1153  			println("runtime: heapBitsSetTypeGCProg: total bits", totalBits, "but progSize", progSize)
  1154  			throw("heapBitsSetTypeGCProg: unexpected bit count")
  1155  		}
  1156  	} else {
  1157  		count := dataSize / elemSize
  1158  
  1159  		// Piece together program trailer to run after prog that does:
  1160  		//	literal(0)
  1161  		//	repeat(1, elemSize-progSize-1) // zeros to fill element size
  1162  		//	repeat(elemSize, count-1) // repeat that element for count
  1163  		// This zero-pads the data remaining in the first element and then
  1164  		// repeats that first element to fill the array.
  1165  		var trailer [40]byte // 3 varints (max 10 each) + some bytes
  1166  		i := 0
  1167  		if n := elemSize/ptrSize - progSize/ptrSize; n > 0 {
  1168  			// literal(0)
  1169  			trailer[i] = 0x01
  1170  			i++
  1171  			trailer[i] = 0
  1172  			i++
  1173  			if n > 1 {
  1174  				// repeat(1, n-1)
  1175  				trailer[i] = 0x81
  1176  				i++
  1177  				n--
  1178  				for ; n >= 0x80; n >>= 7 {
  1179  					trailer[i] = byte(n | 0x80)
  1180  					i++
  1181  				}
  1182  				trailer[i] = byte(n)
  1183  				i++
  1184  			}
  1185  		}
  1186  		// repeat(elemSize/ptrSize, count-1)
  1187  		trailer[i] = 0x80
  1188  		i++
  1189  		n := elemSize / ptrSize
  1190  		for ; n >= 0x80; n >>= 7 {
  1191  			trailer[i] = byte(n | 0x80)
  1192  			i++
  1193  		}
  1194  		trailer[i] = byte(n)
  1195  		i++
  1196  		n = count - 1
  1197  		for ; n >= 0x80; n >>= 7 {
  1198  			trailer[i] = byte(n | 0x80)
  1199  			i++
  1200  		}
  1201  		trailer[i] = byte(n)
  1202  		i++
  1203  		trailer[i] = 0
  1204  		i++
  1205  
  1206  		runGCProg(prog, &trailer[0], h.bitp, 2)
  1207  
  1208  		// Even though we filled in the full array just now,
  1209  		// record that we only filled in up to the ptrdata of the
  1210  		// last element. This will cause the code below to
  1211  		// memclr the dead section of the final array element,
  1212  		// so that scanobject can stop early in the final element.
  1213  		totalBits = (elemSize*(count-1) + progSize) / ptrSize
  1214  	}
  1215  	endProg := unsafe.Pointer(subtractb(h.bitp, (totalBits+3)/4))
  1216  	endAlloc := unsafe.Pointer(subtractb(h.bitp, allocSize/heapBitmapScale))
  1217  	memclr(add(endAlloc, 1), uintptr(endProg)-uintptr(endAlloc))
  1218  }
  1219  
  1220  // progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
  1221  // size the size of the region described by prog, in bytes.
  1222  // The resulting bitvector will have no more than size/ptrSize bits.
  1223  func progToPointerMask(prog *byte, size uintptr) bitvector {
  1224  	n := (size/ptrSize + 7) / 8
  1225  	x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
  1226  	x[len(x)-1] = 0xa1 // overflow check sentinel
  1227  	n = runGCProg(prog, nil, &x[0], 1)
  1228  	if x[len(x)-1] != 0xa1 {
  1229  		throw("progToPointerMask: overflow")
  1230  	}
  1231  	return bitvector{int32(n), &x[0]}
  1232  }
  1233  
  1234  // Packed GC pointer bitmaps, aka GC programs.
  1235  //
  1236  // For large types containing arrays, the type information has a
  1237  // natural repetition that can be encoded to save space in the
  1238  // binary and in the memory representation of the type information.
  1239  //
  1240  // The encoding is a simple Lempel-Ziv style bytecode machine
  1241  // with the following instructions:
  1242  //
  1243  //	00000000: stop
  1244  //	0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
  1245  //	10000000 n c: repeat the previous n bits c times; n, c are varints
  1246  //	1nnnnnnn c: repeat the previous n bits c times; c is a varint
  1247  
  1248  // runGCProg executes the GC program prog, and then trailer if non-nil,
  1249  // writing to dst with entries of the given size.
  1250  // If size == 1, dst is a 1-bit pointer mask laid out moving forward from dst.
  1251  // If size == 2, dst is the 2-bit heap bitmap, and writes move backward
  1252  // starting at dst (because the heap bitmap does). In this case, the caller guarantees
  1253  // that only whole bytes in dst need to be written.
  1254  //
  1255  // runGCProg returns the number of 1- or 2-bit entries written to memory.
  1256  func runGCProg(prog, trailer, dst *byte, size int) uintptr {
  1257  	dstStart := dst
  1258  
  1259  	// Bits waiting to be written to memory.
  1260  	var bits uintptr
  1261  	var nbits uintptr
  1262  
  1263  	p := prog
  1264  Run:
  1265  	for {
  1266  		// Flush accumulated full bytes.
  1267  		// The rest of the loop assumes that nbits <= 7.
  1268  		for ; nbits >= 8; nbits -= 8 {
  1269  			if size == 1 {
  1270  				*dst = uint8(bits)
  1271  				dst = add1(dst)
  1272  				bits >>= 8
  1273  			} else {
  1274  				v := bits&bitPointerAll | bitMarkedAll
  1275  				*dst = uint8(v)
  1276  				dst = subtract1(dst)
  1277  				bits >>= 4
  1278  				v = bits&bitPointerAll | bitMarkedAll
  1279  				*dst = uint8(v)
  1280  				dst = subtract1(dst)
  1281  				bits >>= 4
  1282  			}
  1283  		}
  1284  
  1285  		// Process one instruction.
  1286  		inst := uintptr(*p)
  1287  		p = add1(p)
  1288  		n := inst & 0x7F
  1289  		if inst&0x80 == 0 {
  1290  			// Literal bits; n == 0 means end of program.
  1291  			if n == 0 {
  1292  				// Program is over; continue in trailer if present.
  1293  				if trailer != nil {
  1294  					//println("trailer")
  1295  					p = trailer
  1296  					trailer = nil
  1297  					continue
  1298  				}
  1299  				//println("done")
  1300  				break Run
  1301  			}
  1302  			//println("lit", n, dst)
  1303  			nbyte := n / 8
  1304  			for i := uintptr(0); i < nbyte; i++ {
  1305  				bits |= uintptr(*p) << nbits
  1306  				p = add1(p)
  1307  				if size == 1 {
  1308  					*dst = uint8(bits)
  1309  					dst = add1(dst)
  1310  					bits >>= 8
  1311  				} else {
  1312  					v := bits&0xf | bitMarkedAll
  1313  					*dst = uint8(v)
  1314  					dst = subtract1(dst)
  1315  					bits >>= 4
  1316  					v = bits&0xf | bitMarkedAll
  1317  					*dst = uint8(v)
  1318  					dst = subtract1(dst)
  1319  					bits >>= 4
  1320  				}
  1321  			}
  1322  			if n %= 8; n > 0 {
  1323  				bits |= uintptr(*p) << nbits
  1324  				p = add1(p)
  1325  				nbits += n
  1326  			}
  1327  			continue Run
  1328  		}
  1329  
  1330  		// Repeat. If n == 0, it is encoded in a varint in the next bytes.
  1331  		if n == 0 {
  1332  			for off := uint(0); ; off += 7 {
  1333  				x := uintptr(*p)
  1334  				p = add1(p)
  1335  				n |= (x & 0x7F) << off
  1336  				if x&0x80 == 0 {
  1337  					break
  1338  				}
  1339  			}
  1340  		}
  1341  
  1342  		// Count is encoded in a varint in the next bytes.
  1343  		c := uintptr(0)
  1344  		for off := uint(0); ; off += 7 {
  1345  			x := uintptr(*p)
  1346  			p = add1(p)
  1347  			c |= (x & 0x7F) << off
  1348  			if x&0x80 == 0 {
  1349  				break
  1350  			}
  1351  		}
  1352  		c *= n // now total number of bits to copy
  1353  
  1354  		// If the number of bits being repeated is small, load them
  1355  		// into a register and use that register for the entire loop
  1356  		// instead of repeatedly reading from memory.
  1357  		// Handling fewer than 8 bits here makes the general loop simpler.
  1358  		// The cutoff is ptrSize*8 - 7 to guarantee that when we add
  1359  		// the pattern to a bit buffer holding at most 7 bits (a partial byte)
  1360  		// it will not overflow.
  1361  		src := dst
  1362  		const maxBits = ptrSize*8 - 7
  1363  		if n <= maxBits {
  1364  			// Start with bits in output buffer.
  1365  			pattern := bits
  1366  			npattern := nbits
  1367  
  1368  			// If we need more bits, fetch them from memory.
  1369  			if size == 1 {
  1370  				src = subtract1(src)
  1371  				for npattern < n {
  1372  					pattern <<= 8
  1373  					pattern |= uintptr(*src)
  1374  					src = subtract1(src)
  1375  					npattern += 8
  1376  				}
  1377  			} else {
  1378  				src = add1(src)
  1379  				for npattern < n {
  1380  					pattern <<= 4
  1381  					pattern |= uintptr(*src) & 0xf
  1382  					src = add1(src)
  1383  					npattern += 4
  1384  				}
  1385  			}
  1386  
  1387  			// We started with the whole bit output buffer,
  1388  			// and then we loaded bits from whole bytes.
  1389  			// Either way, we might now have too many instead of too few.
  1390  			// Discard the extra.
  1391  			if npattern > n {
  1392  				pattern >>= npattern - n
  1393  				npattern = n
  1394  			}
  1395  
  1396  			// Replicate pattern to at most maxBits.
  1397  			if npattern == 1 {
  1398  				// One bit being repeated.
  1399  				// If the bit is 1, make the pattern all 1s.
  1400  				// If the bit is 0, the pattern is already all 0s,
  1401  				// but we can claim that the number of bits
  1402  				// in the word is equal to the number we need (c),
  1403  				// because right shift of bits will zero fill.
  1404  				if pattern == 1 {
  1405  					pattern = 1<<maxBits - 1
  1406  					npattern = maxBits
  1407  				} else {
  1408  					npattern = c
  1409  				}
  1410  			} else {
  1411  				b := pattern
  1412  				nb := npattern
  1413  				if nb+nb <= maxBits {
  1414  					// Double pattern until the whole uintptr is filled.
  1415  					for nb <= ptrSize*8 {
  1416  						b |= b << nb
  1417  						nb += nb
  1418  					}
  1419  					// Trim away incomplete copy of original pattern in high bits.
  1420  					// TODO(rsc): Replace with table lookup or loop on systems without divide?
  1421  					nb = maxBits / npattern * npattern
  1422  					b &= 1<<nb - 1
  1423  					pattern = b
  1424  					npattern = nb
  1425  				}
  1426  			}
  1427  
  1428  			// Add pattern to bit buffer and flush bit buffer, c/npattern times.
  1429  			// Since pattern contains >8 bits, there will be full bytes to flush
  1430  			// on each iteration.
  1431  			for ; c >= npattern; c -= npattern {
  1432  				bits |= pattern << nbits
  1433  				nbits += npattern
  1434  				if size == 1 {
  1435  					for nbits >= 8 {
  1436  						*dst = uint8(bits)
  1437  						dst = add1(dst)
  1438  						bits >>= 8
  1439  						nbits -= 8
  1440  					}
  1441  				} else {
  1442  					for nbits >= 4 {
  1443  						*dst = uint8(bits&0xf | bitMarkedAll)
  1444  						dst = subtract1(dst)
  1445  						bits >>= 4
  1446  						nbits -= 4
  1447  					}
  1448  				}
  1449  			}
  1450  
  1451  			// Add final fragment to bit buffer.
  1452  			if c > 0 {
  1453  				pattern &= 1<<c - 1
  1454  				bits |= pattern << nbits
  1455  				nbits += c
  1456  			}
  1457  			continue Run
  1458  		}
  1459  
  1460  		// Repeat; n too large to fit in a register.
  1461  		// Since nbits <= 7, we know the first few bytes of repeated data
  1462  		// are already written to memory.
  1463  		off := n - nbits // n > nbits because n > maxBits and nbits <= 7
  1464  		if size == 1 {
  1465  			// Leading src fragment.
  1466  			src = subtractb(src, (off+7)/8)
  1467  			if frag := off & 7; frag != 0 {
  1468  				bits |= uintptr(*src) >> (8 - frag) << nbits
  1469  				src = add1(src)
  1470  				nbits += frag
  1471  				c -= frag
  1472  			}
  1473  			// Main loop: load one byte, write another.
  1474  			// The bits are rotating through the bit buffer.
  1475  			for i := c / 8; i > 0; i-- {
  1476  				bits |= uintptr(*src) << nbits
  1477  				src = add1(src)
  1478  				*dst = uint8(bits)
  1479  				dst = add1(dst)
  1480  				bits >>= 8
  1481  			}
  1482  			// Final src fragment.
  1483  			if c %= 8; c > 0 {
  1484  				bits |= (uintptr(*src) & (1<<c - 1)) << nbits
  1485  				nbits += c
  1486  			}
  1487  		} else {
  1488  			// Leading src fragment.
  1489  			src = addb(src, (off+3)/4)
  1490  			if frag := off & 3; frag != 0 {
  1491  				bits |= (uintptr(*src) & 0xf) >> (4 - frag) << nbits
  1492  				src = subtract1(src)
  1493  				nbits += frag
  1494  				c -= frag
  1495  			}
  1496  			// Main loop: load one byte, write another.
  1497  			// The bits are rotating through the bit buffer.
  1498  			for i := c / 4; i > 0; i-- {
  1499  				bits |= (uintptr(*src) & 0xf) << nbits
  1500  				src = subtract1(src)
  1501  				*dst = uint8(bits&0xf | bitMarkedAll)
  1502  				dst = subtract1(dst)
  1503  				bits >>= 4
  1504  			}
  1505  			// Final src fragment.
  1506  			if c %= 4; c > 0 {
  1507  				bits |= (uintptr(*src) & (1<<c - 1)) << nbits
  1508  				nbits += c
  1509  			}
  1510  		}
  1511  	}
  1512  
  1513  	// Write any final bits out, using full-byte writes, even for the final byte.
  1514  	var totalBits uintptr
  1515  	if size == 1 {
  1516  		totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
  1517  		nbits += -nbits & 7
  1518  		for ; nbits > 0; nbits -= 8 {
  1519  			*dst = uint8(bits)
  1520  			dst = add1(dst)
  1521  			bits >>= 8
  1522  		}
  1523  	} else {
  1524  		totalBits = (uintptr(unsafe.Pointer(dstStart))-uintptr(unsafe.Pointer(dst)))*4 + nbits
  1525  		nbits += -nbits & 3
  1526  		for ; nbits > 0; nbits -= 4 {
  1527  			v := bits&0xf | bitMarkedAll
  1528  			*dst = uint8(v)
  1529  			dst = subtract1(dst)
  1530  			bits >>= 4
  1531  		}
  1532  		// Clear the mark bits in the first two entries.
  1533  		// They are the actual mark and checkmark bits,
  1534  		// not non-dead markers. It simplified the code
  1535  		// above to set the marker in every bit written and
  1536  		// then clear these two as a special case at the end.
  1537  		*dstStart &^= bitMarked | bitMarked<<heapBitsShift
  1538  	}
  1539  	return totalBits
  1540  }
  1541  
  1542  func dumpGCProg(p *byte) {
  1543  	nptr := 0
  1544  	for {
  1545  		x := *p
  1546  		p = add1(p)
  1547  		if x == 0 {
  1548  			print("\t", nptr, " end\n")
  1549  			break
  1550  		}
  1551  		if x&0x80 == 0 {
  1552  			print("\t", nptr, " lit ", x, ":")
  1553  			n := int(x+7) / 8
  1554  			for i := 0; i < n; i++ {
  1555  				print(" ", hex(*p))
  1556  				p = add1(p)
  1557  			}
  1558  			print("\n")
  1559  			nptr += int(x)
  1560  		} else {
  1561  			nbit := int(x &^ 0x80)
  1562  			if nbit == 0 {
  1563  				for nb := uint(0); ; nb += 7 {
  1564  					x := *p
  1565  					p = add1(p)
  1566  					nbit |= int(x&0x7f) << nb
  1567  					if x&0x80 == 0 {
  1568  						break
  1569  					}
  1570  				}
  1571  			}
  1572  			count := 0
  1573  			for nb := uint(0); ; nb += 7 {
  1574  				x := *p
  1575  				p = add1(p)
  1576  				count |= int(x&0x7f) << nb
  1577  				if x&0x80 == 0 {
  1578  					break
  1579  				}
  1580  			}
  1581  			print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
  1582  			nptr += nbit * count
  1583  		}
  1584  	}
  1585  }
  1586  
  1587  // Testing.
  1588  
  1589  func getgcmaskcb(frame *stkframe, ctxt unsafe.Pointer) bool {
  1590  	target := (*stkframe)(ctxt)
  1591  	if frame.sp <= target.sp && target.sp < frame.varp {
  1592  		*target = *frame
  1593  		return false
  1594  	}
  1595  	return true
  1596  }
  1597  
  1598  // gcbits returns the GC type info for x, for testing.
  1599  // The result is the bitmap entries (0 or 1), one entry per byte.
  1600  //go:linkname reflect_gcbits reflect.gcbits
  1601  func reflect_gcbits(x interface{}) []byte {
  1602  	ret := getgcmask(x)
  1603  	typ := (*ptrtype)(unsafe.Pointer((*eface)(unsafe.Pointer(&x))._type)).elem
  1604  	nptr := typ.ptrdata / ptrSize
  1605  	for uintptr(len(ret)) > nptr && ret[len(ret)-1] == 0 {
  1606  		ret = ret[:len(ret)-1]
  1607  	}
  1608  	return ret
  1609  }
  1610  
  1611  // Returns GC type info for object p for testing.
  1612  func getgcmask(ep interface{}) (mask []byte) {
  1613  	e := *(*eface)(unsafe.Pointer(&ep))
  1614  	p := e.data
  1615  	t := e._type
  1616  	// data or bss
  1617  	for datap := &firstmoduledata; datap != nil; datap = datap.next {
  1618  		// data
  1619  		if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
  1620  			bitmap := datap.gcdatamask.bytedata
  1621  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  1622  			mask = make([]byte, n/ptrSize)
  1623  			for i := uintptr(0); i < n; i += ptrSize {
  1624  				off := (uintptr(p) + i - datap.data) / ptrSize
  1625  				mask[i/ptrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  1626  			}
  1627  			return
  1628  		}
  1629  
  1630  		// bss
  1631  		if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
  1632  			bitmap := datap.gcbssmask.bytedata
  1633  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  1634  			mask = make([]byte, n/ptrSize)
  1635  			for i := uintptr(0); i < n; i += ptrSize {
  1636  				off := (uintptr(p) + i - datap.bss) / ptrSize
  1637  				mask[i/ptrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  1638  			}
  1639  			return
  1640  		}
  1641  	}
  1642  
  1643  	// heap
  1644  	var n uintptr
  1645  	var base uintptr
  1646  	if mlookup(uintptr(p), &base, &n, nil) != 0 {
  1647  		mask = make([]byte, n/ptrSize)
  1648  		for i := uintptr(0); i < n; i += ptrSize {
  1649  			hbits := heapBitsForAddr(base + i)
  1650  			if hbits.isPointer() {
  1651  				mask[i/ptrSize] = 1
  1652  			}
  1653  			if i >= 2*ptrSize && !hbits.isMarked() {
  1654  				mask = mask[:i/ptrSize]
  1655  				break
  1656  			}
  1657  		}
  1658  		return
  1659  	}
  1660  
  1661  	// stack
  1662  	if _g_ := getg(); _g_.m.curg.stack.lo <= uintptr(p) && uintptr(p) < _g_.m.curg.stack.hi {
  1663  		var frame stkframe
  1664  		frame.sp = uintptr(p)
  1665  		_g_ := getg()
  1666  		gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
  1667  		if frame.fn != nil {
  1668  			f := frame.fn
  1669  			targetpc := frame.continpc
  1670  			if targetpc == 0 {
  1671  				return
  1672  			}
  1673  			if targetpc != f.entry {
  1674  				targetpc--
  1675  			}
  1676  			pcdata := pcdatavalue(f, _PCDATA_StackMapIndex, targetpc)
  1677  			if pcdata == -1 {
  1678  				return
  1679  			}
  1680  			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
  1681  			if stkmap == nil || stkmap.n <= 0 {
  1682  				return
  1683  			}
  1684  			bv := stackmapdata(stkmap, pcdata)
  1685  			size := uintptr(bv.n) * ptrSize
  1686  			n := (*ptrtype)(unsafe.Pointer(t)).elem.size
  1687  			mask = make([]byte, n/ptrSize)
  1688  			for i := uintptr(0); i < n; i += ptrSize {
  1689  				bitmap := bv.bytedata
  1690  				off := (uintptr(p) + i - frame.varp + size) / ptrSize
  1691  				mask[i/ptrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
  1692  			}
  1693  		}
  1694  		return
  1695  	}
  1696  
  1697  	// otherwise, not something the GC knows about.
  1698  	// possibly read-only data, like malloc(0).
  1699  	// must not have pointers
  1700  	return
  1701  }