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// 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 heap bitmap comprises 2 bits for each pointer-sized word in the heap,
// stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting a start,
// then ha.bitmap[0] holds the 2-bit entries for the four words start
// through start+3*ptrSize, ha.bitmap[1] holds the entries for
// start+4*ptrSize through start+7*ptrSize, and so on.
//
// In each 2-bit entry, the lower bit 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.
//
// The bitmap for noscan spans is not maintained. Code must ensure
// that an object is scannable before consulting its bitmap by
// checking either the noscan bit in the span or by consulting its
// type's information.
//
// 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
wordsPerBitmapByte = 8 / 2 // heap words described by one bitmap byte
// all scan/pointer bits in a byte
bitScanAll = bitScan | bitScan<<heapBitsShift | bitScan<<(2*heapBitsShift) | bitScan<<(3*heapBitsShift)
bitPointerAll = bitPointer | bitPointer<<heapBitsShift | bitPointer<<(2*heapBitsShift) | bitPointer<<(3*heapBitsShift)
)
// addb returns the byte pointer p+n.
//go:nowritebarrier
//go:nosplit
func addb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}
// subtractb returns the byte pointer p-n.
//go:nowritebarrier
//go:nosplit
func subtractb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, -n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
}
// add1 returns the byte pointer p+1.
//go:nowritebarrier
//go:nosplit
func add1(p *byte) *byte {
// Note: wrote out full expression instead of calling addb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
}
// subtract1 returns the byte pointer p-1.
//go:nowritebarrier
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func subtract1(p *byte) *byte {
// Note: wrote out full expression instead of calling subtractb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
}
// heapBits provides access to the bitmap bits for a single heap word.
// The methods on heapBits take value receivers so that the compiler
// can more easily inline calls to those methods and registerize the
// struct fields independently.
type heapBits struct {
bitp *uint8
shift uint32
arena uint32 // Index of heap arena containing bitp
last *uint8 // Last byte arena's bitmap
}
// Make the compiler check that heapBits.arena is large enough to hold
// the maximum arena frame number.
var _ = heapBits{arena: (1<<heapAddrBits)/heapArenaBytes - 1}
// markBits provides access to the mark bit for an object in the heap.
// bytep points to the byte holding the mark bit.
// mask is a byte with a single bit set that can be &ed with *bytep
// to see if the bit has been set.
// *m.byte&m.mask != 0 indicates the mark bit is set.
// index can be used along with span information to generate
// the address of the object in the heap.
// We maintain one set of mark bits for allocation and one for
// marking purposes.
type markBits struct {
bytep *uint8
mask uint8
index uintptr
}
//go:nosplit
func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
bytep, mask := s.allocBits.bitp(allocBitIndex)
return markBits{bytep, mask, allocBitIndex}
}
// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
// and negates them so that ctz (count trailing zeros) instructions
// can be used. It then places these 8 bytes into the cached 64 bit
// s.allocCache.
func (s *mspan) refillAllocCache(whichByte uintptr) {
bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(whichByte)))
aCache := uint64(0)
aCache |= uint64(bytes[0])
aCache |= uint64(bytes[1]) << (1 * 8)
aCache |= uint64(bytes[2]) << (2 * 8)
aCache |= uint64(bytes[3]) << (3 * 8)
aCache |= uint64(bytes[4]) << (4 * 8)
aCache |= uint64(bytes[5]) << (5 * 8)
aCache |= uint64(bytes[6]) << (6 * 8)
aCache |= uint64(bytes[7]) << (7 * 8)
s.allocCache = ^aCache
}
// nextFreeIndex returns the index of the next free object in s at
// or after s.freeindex.
// There are hardware instructions that can be used to make this
// faster if profiling warrants it.
func (s *mspan) nextFreeIndex() uintptr {
sfreeindex := s.freeindex
snelems := s.nelems
if sfreeindex == snelems {
return sfreeindex
}
if sfreeindex > snelems {
throw("s.freeindex > s.nelems")
}
aCache := s.allocCache
bitIndex := sys.Ctz64(aCache)
for bitIndex == 64 {
// Move index to start of next cached bits.
sfreeindex = (sfreeindex + 64) &^ (64 - 1)
if sfreeindex >= snelems {
s.freeindex = snelems
return snelems
}
whichByte := sfreeindex / 8
// Refill s.allocCache with the next 64 alloc bits.
s.refillAllocCache(whichByte)
aCache = s.allocCache
bitIndex = sys.Ctz64(aCache)
// nothing available in cached bits
// grab the next 8 bytes and try again.
}
result := sfreeindex + uintptr(bitIndex)
if result >= snelems {
s.freeindex = snelems
return snelems
}
s.allocCache >>= uint(bitIndex + 1)
sfreeindex = result + 1
if sfreeindex%64 == 0 && sfreeindex != snelems {
// We just incremented s.freeindex so it isn't 0.
// As each 1 in s.allocCache was encountered and used for allocation
// it was shifted away. At this point s.allocCache contains all 0s.
// Refill s.allocCache so that it corresponds
// to the bits at s.allocBits starting at s.freeindex.
whichByte := sfreeindex / 8
s.refillAllocCache(whichByte)
}
s.freeindex = sfreeindex
return result
}
// isFree reports whether the index'th object in s is unallocated.
//
// The caller must ensure s.state is mSpanInUse, and there must have
// been no preemption points since ensuring this (which could allow a
// GC transition, which would allow the state to change).
func (s *mspan) isFree(index uintptr) bool {
if index < s.freeindex {
return false
}
bytep, mask := s.allocBits.bitp(index)
return *bytep&mask == 0
}
func (s *mspan) objIndex(p uintptr) uintptr {
byteOffset := p - s.base()
if byteOffset == 0 {
return 0
}
if s.baseMask != 0 {
// s.baseMask is non-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 {
bytep, mask := s.gcmarkBits.bitp(objIndex)
return markBits{bytep, mask, objIndex}
}
func (s *mspan) markBitsForBase() markBits {
return markBits{(*uint8)(s.gcmarkBits), uint8(1), 0}
}
// isMarked reports whether mark bit m is set.
func (m markBits) isMarked() bool {
return *m.bytep&m.mask != 0
}
// setMarked sets the marked bit in the markbits, atomically.
func (m markBits) setMarked() {
// Might be racing with other updates, so use atomic update always.
// We used to be clever here and use a non-atomic update in certain
// cases, but it's not worth the risk.
atomic.Or8(m.bytep, m.mask)
}
// setMarkedNonAtomic sets the marked bit in the markbits, non-atomically.
func (m markBits) setMarkedNonAtomic() {
*m.bytep |= m.mask
}
// clearMarked clears the marked bit in the markbits, atomically.
func (m markBits) clearMarked() {
// Might be racing with other updates, so use atomic update always.
// We used to be clever here and use a non-atomic update in certain
// cases, but it's not worth the risk.
atomic.And8(m.bytep, ^m.mask)
}
// markBitsForSpan returns the markBits for the span base address base.
func markBitsForSpan(base uintptr) (mbits markBits) {
mbits = markBitsForAddr(base)
if mbits.mask != 1 {
throw("markBitsForSpan: unaligned start")
}
return mbits
}
// advance advances the markBits to the next object in the span.
func (m *markBits) advance() {
if m.mask == 1<<7 {
m.bytep = (*uint8)(unsafe.Pointer(uintptr(unsafe.Pointer(m.bytep)) + 1))
m.mask = 1
} else {
m.mask = m.mask << 1
}
m.index++
}
// heapBitsForAddr returns the heapBits for the address addr.
// The caller must ensure addr is in an allocated span.
// In particular, be careful not to point past the end of an object.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func heapBitsForAddr(addr uintptr) (h heapBits) {
// 2 bits per word, 4 pairs per byte, and a mask is hard coded.
arena := arenaIndex(addr)
ha := mheap_.arenas[arena.l1()][arena.l2()]
// The compiler uses a load for nil checking ha, but in this
// case we'll almost never hit that cache line again, so it
// makes more sense to do a value check.
if ha == nil {
// addr is not in the heap. Return nil heapBits, which
// we expect to crash in the caller.
return
}
h.bitp = &ha.bitmap[(addr/(sys.PtrSize*4))%heapArenaBitmapBytes]
h.shift = uint32((addr / sys.PtrSize) & 3)
h.arena = uint32(arena)
h.last = &ha.bitmap[len(ha.bitmap)-1]
return
}
// badPointer throws bad pointer in heap panic.
func badPointer(s *mspan, p, refBase, refOff uintptr) {
// Typically this indicates an incorrect use
// of unsafe or cgo to store a bad pointer in
// the Go heap. It may also indicate a runtime
// bug.
//
// TODO(austin): We could be more aggressive
// and detect pointers to unallocated objects
// in allocated spans.
printlock()
print("runtime: pointer ", hex(p))
state := s.state.get()
if state != mSpanInUse {
print(" to unallocated span")
} else {
print(" to unused region of span")
}
print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", state, "\n")
if refBase != 0 {
print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
gcDumpObject("object", refBase, refOff)
}
getg().m.traceback = 2
throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
}
// findObject returns the base address for the heap object containing
// the address p, the object's span, and the index of the object in s.
// If p does not point into a heap object, it returns base == 0.
//
// If p points is an invalid heap pointer and debug.invalidptr != 0,
// findObject panics.
//
// refBase and refOff optionally give the base address of the object
// in which the pointer p was found and the byte offset at which it
// was found. These are used for error reporting.
//
// It is nosplit so it is safe for p to be a pointer to the current goroutine's stack.
// Since p is a uintptr, it would not be adjusted if the stack were to move.
//go:nosplit
func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex uintptr) {
s = spanOf(p)
// If s is nil, the virtual address has never been part of the heap.
// This pointer may be to some mmap'd region, so we allow it.
if s == nil {
return
}
// If p is a bad pointer, it may not be in s's bounds.
//
// Check s.state to synchronize with span initialization
// before checking other fields. See also spanOfHeap.
if state := s.state.get(); state != mSpanInUse || p < s.base() || p >= s.limit {
// Pointers into stacks are also ok, the runtime manages these explicitly.
if state == mSpanManual {
return
}
// The following ensures that we are rigorous about what data
// structures hold valid pointers.
if debug.invalidptr != 0 {
badPointer(s, p, refBase, refOff)
}
return
}
// 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
}
}
return
}
// next returns the heapBits describing the next pointer-sized word in memory.
// That is, if h describes address p, h.next() describes p+ptrSize.
// Note that next does not modify h. The caller must record the result.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) next() heapBits {
if h.shift < 3*heapBitsShift {
h.shift += heapBitsShift
} else if h.bitp != h.last {
h.bitp, h.shift = add1(h.bitp), 0
} else {
// Move to the next arena.
return h.nextArena()
}
return h
}
// nextArena advances h to the beginning of the next heap arena.
//
// This is a slow-path helper to next. gc's inliner knows that
// heapBits.next can be inlined even though it calls this. This is
// marked noinline so it doesn't get inlined into next and cause next
// to be too big to inline.
//
//go:nosplit
//go:noinline
func (h heapBits) nextArena() heapBits {
h.arena++
ai := arenaIdx(h.arena)
l2 := mheap_.arenas[ai.l1()]
if l2 == nil {
// We just passed the end of the object, which
// was also the end of the heap. Poison h. It
// should never be dereferenced at this point.
return heapBits{}
}
ha := l2[ai.l2()]
if ha == nil {
return heapBits{}
}
h.bitp, h.shift = &ha.bitmap[0], 0
h.last = &ha.bitmap[len(ha.bitmap)-1]
return h
}
// forward returns the heapBits describing n pointer-sized words ahead of h in memory.
// That is, if h describes address p, h.forward(n) describes p+n*ptrSize.
// h.forward(1) is equivalent to h.next(), just slower.
// Note that forward does not modify h. The caller must record the result.
// bits returns the heap bits for the current word.
//go:nosplit
func (h heapBits) forward(n uintptr) heapBits {
n += uintptr(h.shift) / heapBitsShift
nbitp := uintptr(unsafe.Pointer(h.bitp)) + n/4
h.shift = uint32(n%4) * heapBitsShift
if nbitp <= uintptr(unsafe.Pointer(h.last)) {
h.bitp = (*uint8)(unsafe.Pointer(nbitp))
return h
}
// We're in a new heap arena.
past := nbitp - (uintptr(unsafe.Pointer(h.last)) + 1)
h.arena += 1 + uint32(past/heapArenaBitmapBytes)
ai := arenaIdx(h.arena)
if l2 := mheap_.arenas[ai.l1()]; l2 != nil && l2[ai.l2()] != nil {
a := l2[ai.l2()]
h.bitp = &a.bitmap[past%heapArenaBitmapBytes]
h.last = &a.bitmap[len(a.bitmap)-1]
} else {
h.bitp, h.last = nil, nil
}
return h
}
// forwardOrBoundary is like forward, but stops at boundaries between
// contiguous sections of the bitmap. It returns the number of words
// advanced over, which will be <= n.
func (h heapBits) forwardOrBoundary(n uintptr) (heapBits, uintptr) {
maxn := 4 * ((uintptr(unsafe.Pointer(h.last)) + 1) - uintptr(unsafe.Pointer(h.bitp)))
if n > maxn {
n = maxn
}
return h.forward(n), n
}
// The caller can test morePointers and isPointer by &-ing with bitScan and bitPointer.
// The result includes in its higher bits the bits for subsequent words
// described by the same bitmap byte.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) bits() uint32 {
// The (shift & 31) eliminates a test and conditional branch
// from the generated code.
return uint32(*h.bitp) >> (h.shift & 31)
}
// morePointers reports whether this word and all remaining words in this object
// are scalars.
// h must not describe the second word of the object.
func (h heapBits) morePointers() bool {
return h.bits()&bitScan != 0
}
// isPointer reports whether the heap bits describe a pointer word.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) isPointer() bool {
return h.bits()&bitPointer != 0
}
// 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 a write barrier
// for every pointer slot in the memory range [src, src+size),
// using pointer/scalar information from [dst, dst+size).
// This executes the write barriers necessary before a memmove.
// src, dst, and size must be pointer-aligned.
// The range [dst, dst+size) must lie within a single object.
// It does not perform the actual writes.
//
// As a special case, src == 0 indicates that this is being used for a
// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
// barrier.
//
// Callers should call bulkBarrierPreWrite immediately before
// calling memmove(dst, src, size). This function is marked nosplit
// to avoid being preempted; the GC must not stop the goroutine
// between the memmove and the execution of the barriers.
// The caller is also responsible for cgo pointer checks if this
// may be writing Go pointers into non-Go memory.
//
// The pointer bitmap is not maintained for allocations containing
// no pointers at all; any caller of bulkBarrierPreWrite must first
// make sure the underlying allocation contains pointers, usually
// by checking typ.ptrdata.
//
// Callers must perform cgo checks if writeBarrier.cgo.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
if (dst|src|size)&(sys.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.needed {
return
}
if s := spanOf(dst); s == nil {
// If dst is a global, use the data or BSS bitmaps to
// execute write barriers.
for _, datap := range activeModules() {
if datap.data <= dst && dst < datap.edata {
bulkBarrierBitmap(dst, src, size, dst-datap.data, datap.gcdatamask.bytedata)
return
}
}
for _, datap := range activeModules() {
if datap.bss <= dst && dst < datap.ebss {
bulkBarrierBitmap(dst, src, size, dst-datap.bss, datap.gcbssmask.bytedata)
return
}
}
return
} else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
// dst was heap memory at some point, but isn't now.
// It can't be a global. It must be either our stack,
// or in the case of direct channel sends, it could be
// another stack. Either way, no need for barriers.
// This will also catch if dst is in a freed span,
// though that should never have.
return
}
buf := &getg().m.p.ptr().wbBuf
h := heapBitsForAddr(dst)
if src == 0 {
for i := uintptr(0); i < size; i += sys.PtrSize {
if h.isPointer() {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
if !buf.putFast(*dstx, 0) {
wbBufFlush(nil, 0)
}
}
h = h.next()
}
} else {
for i := uintptr(0); i < size; i += sys.PtrSize {
if h.isPointer() {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
srcx := (*uintptr)(unsafe.Pointer(src + i))
if !buf.putFast(*dstx, *srcx) {
wbBufFlush(nil, 0)
}
}
h = h.next()
}
}
}
// bulkBarrierPreWriteSrcOnly is like bulkBarrierPreWrite but
// does not execute write barriers for [dst, dst+size).
//
// In addition to the requirements of bulkBarrierPreWrite
// callers need to ensure [dst, dst+size) is zeroed.
//
// This is used for special cases where e.g. dst was just
// created and zeroed with malloc.
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
if (dst|src|size)&(sys.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.needed {
return
}
buf := &getg().m.p.ptr().wbBuf
h := heapBitsForAddr(dst)
for i := uintptr(0); i < size; i += sys.PtrSize {
if h.isPointer() {
srcx := (*uintptr)(unsafe.Pointer(src + i))
if !buf.putFast(0, *srcx) {
wbBufFlush(nil, 0)
}
}
h = h.next()
}
}
// bulkBarrierBitmap executes write barriers for copying from [src,
// src+size) to [dst, dst+size) using a 1-bit pointer bitmap. src is
// assumed to start maskOffset bytes into the data covered by the
// bitmap in bits (which may not be a multiple of 8).
//
// This is used by bulkBarrierPreWrite for writes to data and BSS.
//
//go:nosplit
func bulkBarrierBitmap(dst, src, size, maskOffset uintptr, bits *uint8) {
word := maskOffset / sys.PtrSize
bits = addb(bits, word/8)
mask := uint8(1) << (word % 8)
buf := &getg().m.p.ptr().wbBuf
for i := uintptr(0); i < size; i += sys.PtrSize {
if mask == 0 {
bits = addb(bits, 1)
if *bits == 0 {
// Skip 8 words.
i += 7 * sys.PtrSize
continue
}
mask = 1
}
if *bits&mask != 0 {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
if src == 0 {
if !buf.putFast(*dstx, 0) {
wbBufFlush(nil, 0)
}
} else {
srcx := (*uintptr)(unsafe.Pointer(src + i))
if !buf.putFast(*dstx, *srcx) {
wbBufFlush(nil, 0)
}
}
}
mask <<= 1
}
}
// typeBitsBulkBarrier executes a write barrier for every
// pointer that would be copied from [src, src+size) to [dst,
// dst+size) by a memmove using the type bitmap to locate those
// pointer slots.
//
// The type typ must correspond exactly to [src, src+size) and [dst, dst+size).
// dst, src, and size must be pointer-aligned.
// The type typ must have a plain bitmap, not a GC program.
// The only use of this function is in channel sends, and the
// 64 kB channel element limit takes care of this for us.
//
// Must not be preempted because it typically runs right before memmove,
// and the GC must observe them as an atomic action.
//
// Callers must perform cgo checks if writeBarrier.cgo.
//
//go:nosplit
func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
if typ == nil {
throw("runtime: typeBitsBulkBarrier without type")
}
if typ.size != size {
println("runtime: typeBitsBulkBarrier with type ", typ.string(), " of size ", typ.size, " but memory size", size)
throw("runtime: invalid typeBitsBulkBarrier")
}
if typ.kind&kindGCProg != 0 {
println("runtime: typeBitsBulkBarrier with type ", typ.string(), " with GC prog")
throw("runtime: invalid typeBitsBulkBarrier")
}
if !writeBarrier.needed {
return
}
ptrmask := typ.gcdata
buf := &getg().m.p.ptr().wbBuf
var bits uint32
for i := uintptr(0); i < typ.ptrdata; i += sys.PtrSize {
if i&(sys.PtrSize*8-1) == 0 {
bits = uint32(*ptrmask)
ptrmask = addb(ptrmask, 1)
} else {
bits = bits >> 1
}
if bits&1 != 0 {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
srcx := (*uintptr)(unsafe.Pointer(src + i))
if !buf.putFast(*dstx, *srcx) {
wbBufFlush(nil, 0)
}
}
}
}
// The methods operating on spans all require that h has been returned
// by heapBitsForSpan and that size, n, total are the span layout description
// returned by the mspan's layout method.
// If total > size*n, it means that there is extra leftover memory in the span,
// usually due to rounding.
//
// TODO(rsc): Perhaps introduce a different heapBitsSpan type.
// initSpan initializes the heap bitmap for a span.
// 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) {
// Clear bits corresponding to objects.
nw := (s.npages << _PageShift) / sys.PtrSize
if nw%wordsPerBitmapByte != 0 {
throw("initSpan: unaligned length")
}
if h.shift != 0 {
throw("initSpan: unaligned base")
}
isPtrs := sys.PtrSize == 8 && s.elemsize == sys.PtrSize
for nw > 0 {
hNext, anw := h.forwardOrBoundary(nw)
nbyte := anw / wordsPerBitmapByte
if isPtrs {
bitp := h.bitp
for i := uintptr(0); i < nbyte; i++ {
*bitp = bitPointerAll | bitScanAll
bitp = add1(bitp)
}
} else {
memclrNoHeapPointers(unsafe.Pointer(h.bitp), nbyte)
}
h = hNext
nw -= anw
}
}
// 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.
for i := uintptr(0); i < n; i += wordsPerBitmapByte {
*h.bitp &^= bitPointerAll
h = h.forward(wordsPerBitmapByte)
}
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.
for i := uintptr(0); i < n; i += wordsPerBitmapByte {
*h.bitp |= bitPointerAll
h = h.forward(wordsPerBitmapByte)
}
}
}
// 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}
// countAlloc returns the number of objects allocated in span s by
// scanning the allocation bitmap.
// TODO:(rlh) Use popcount intrinsic.
func (s *mspan) countAlloc() int {
count := 0
maxIndex := s.nelems / 8
for i := uintptr(0); i < maxIndex; i++ {
mrkBits := *s.gcmarkBits.bytep(i)
count += int(oneBitCount[mrkBits])
}
if bitsInLastByte := s.nelems % 8; bitsInLastByte != 0 {
mrkBits := *s.gcmarkBits.bytep(maxIndex)
mask := uint8((1 << bitsInLastByte) - 1)
bits := mrkBits & mask
count += int(oneBitCount[bits])
}
return count
}
// heapBitsSetType records that the new allocation [x, x+size)
// holds in [x, x+dataSize) one or more values of type typ.
// (The number of values is given by dataSize / typ.size.)
// If dataSize < size, the fragment [x+dataSize, x+size) is
// recorded as non-pointer data.
// It is known that the type has pointers somewhere;
// malloc does not call heapBitsSetType when there are no pointers,
// because all free objects are marked as noscan during
// heapBitsSweepSpan.
//
// There can only be one allocation from a given span active at a time,
// and the bitmap for a span always falls on byte boundaries,
// so there are no write-write races for access to the heap bitmap.
// Hence, heapBitsSetType can access the bitmap without atomics.
//
// There can be read-write races between heapBitsSetType and things
// that read the heap bitmap like scanobject. However, since
// heapBitsSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// bits that belong to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
const doubleCheck = false // slow but helpful; enable to test modifications to this code
// 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.
outOfPlace := false
if arenaIndex(x+size-1) != arenaIdx(h.arena) || (doubleCheck && fastrand()%2 == 0) {
// This object spans heap arenas, so the bitmap may be
// discontiguous. Unroll it into the object instead
// and then copy it out.
//
// In doubleCheck mode, we randomly do this anyway to
// stress test the bitmap copying path.
outOfPlace = true
h.bitp = (*uint8)(unsafe.Pointer(x))
h.last = nil
}
var (
// Ptrmask input.
p *byte // last ptrmask byte read
b uintptr // ptrmask bits already loaded
nb uintptr // number of bits in b at next read
endp *byte // final ptrmask byte to read (then repeat)
endnb uintptr // number of valid bits in *endp
pbits uintptr // alternate source of bits
// Heap bitmap output.
w uintptr // words processed
nw uintptr // number of words to process
hbitp *byte // next heap bitmap byte to write
hb uintptr // bits being prepared for *hbitp
)
hbitp = h.bitp
// Handle GC program. Delayed until this part of the code
// so that we can use the same double-checking mechanism
// as the 1-bit case. Nothing above could have encountered
// GC programs: the cases were all too small.
if typ.kind&kindGCProg != 0 {
heapBitsSetTypeGCProg(h, typ.ptrdata, typ.size, dataSize, size, addb(typ.gcdata, 4))
if doubleCheck {
// Double-check the heap bits written by GC program
// by running the GC program to create a 1-bit pointer mask
// and then jumping to the double-check code below.
// This doesn't catch bugs shared between the 1-bit and 4-bit
// GC program execution, but it does catch mistakes specific
// to just one of those and bugs in heapBitsSetTypeGCProg's
// implementation of arrays.
lock(&debugPtrmask.lock)
if debugPtrmask.data == nil {
debugPtrmask.data = (*byte)(persistentalloc(1<<20, 1, &memstats.other_sys))
}
ptrmask = debugPtrmask.data
runGCProg(addb(typ.gcdata, 4), nil, ptrmask, 1)
}
goto Phase4
}
// Note about sizes:
//
// typ.size is the number of words in the object,
// and typ.ptrdata is the number of words in the prefix
// of the object that contains pointers. That is, the final
// typ.size - typ.ptrdata words contain no pointers.
// This allows optimization of a common pattern where
// an object has a small header followed by a large scalar
// buffer. If we know the pointers are over, we don't have
// to scan the buffer's heap bitmap at all.
// The 1-bit ptrmasks are sized to contain only bits for
// the typ.ptrdata prefix, zero padded out to a full byte
// of bitmap. This code sets nw (below) so that heap bitmap
// bits are only written for the typ.ptrdata prefix; if there is
// more room in the allocated object, the next heap bitmap
// entry is a 00, indicating that there are no more pointers
// to scan. So only the ptrmask for the ptrdata bytes is needed.
//
// Replicated copies are not as nice: if there is an array of
// objects with scalar tails, all but the last tail does have to
// be initialized, because there is no way to say "skip forward".
// However, because of the possibility of a repeated type with
// size not a multiple of 4 pointers (one heap bitmap byte),
// the code already must handle the last ptrmask byte specially
// by treating it as containing only the bits for endnb pointers,
// where endnb <= 4. We represent large scalar tails that must
// be expanded in the replication by setting endnb larger than 4.
// This will have the effect of reading many bits out of b,
// but once the real bits are shifted out, b will supply as many
// zero bits as we try to read, which is exactly what we need.
p = ptrmask
if typ.size < dataSize {
// Filling in bits for an array of typ.
// Set up for repetition of ptrmask during main loop.
// Note that ptrmask describes only a prefix of
const maxBits = sys.PtrSize*8 - 7
if typ.ptrdata/sys.PtrSize <= maxBits {
// Entire ptrmask fits in uintptr with room for a byte fragment.
// Load into pbits and never read from ptrmask again.
// This is especially important when the ptrmask has
// fewer than 8 bits in it; otherwise the reload in the middle
// of the Phase 2 loop would itself need to loop to gather
// at least 8 bits.
// Accumulate ptrmask into b.
// ptrmask is sized to describe only typ.ptrdata, but we record
// it as describing typ.size bytes, since all the high bits are zero.
nb = typ.ptrdata / sys.PtrSize
for i := uintptr(0); i < nb; i += 8 {
b |= uintptr(*p) << i
p = add1(p)
}
nb = typ.size / sys.PtrSize
// Replicate ptrmask to fill entire pbits uintptr.
// Doubling and truncating is fewer steps than
// iterating by nb each time. (nb could be 1.)
// Since we loaded typ.ptrdata/sys.PtrSize bits
// but are pretending to have typ.size/sys.PtrSize,
// there might be no replication necessary/possible.
pbits = b
endnb = nb
if nb+nb <= maxBits {
for endnb <= sys.PtrSize*8 {
pbits |= pbits << endnb
endnb += endnb
}
// Truncate to a multiple of original ptrmask.
// Because nb+nb <= maxBits, nb fits in a byte.
// Byte division is cheaper than uintptr division.
endnb = uintptr(maxBits/byte(nb)) * nb
pbits &= 1<<endnb - 1
b = pbits
nb = endnb
}
// Clear p and endp as sentinel for using pbits.
// Checked during Phase 2 loop.
p = nil
endp = nil
} else {
// Ptrmask is larger. Read it multiple times.
n := (typ.ptrdata/sys.PtrSize+7)/8 - 1
endp = addb(ptrmask, n)
endnb = typ.size/sys.PtrSize - n*8
}
}
if p != nil {
b = uintptr(*p)
p = add1(p)
nb = 8
}
if typ.size == dataSize {
// Single entry: can stop once we reach the non-pointer data.
nw = typ.ptrdata / sys.PtrSize
} else {
// Repeated instances of typ in an array.
// Have to process first N-1 entries in full, but can stop
// once we reach the non-pointer data in the final entry.
nw = ((dataSize/typ.size-1)*typ.size + typ.ptrdata) / sys.PtrSize
}
if nw == 0 {
// No pointers! Caller was supposed to check.
println("runtime: invalid type ", typ.string())
throw("heapBitsSetType: called with non-pointer type")
return
}
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==2).
// The leading byte is special because it contains the bits for word 1,
// which does not have the scan bit set.
// The leading half-byte is special because it's a half a byte,
// so we have to be careful with the bits already there.
switch {
default:
throw("heapBitsSetType: unexpected shift")
case h.shift == 0:
// Ptrmask and heap bitmap are aligned.
// 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 = add1(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 = add1(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 = add1(hbitp)
b >>= 4
// Load more bits. b has nb right now.
if p != endp {
// Fast path: keep reading from ptrmask.
// nb unmodified: we just loaded 8 bits,
// and the next iteration will consume 8 bits,
// leaving us with the same nb the next time we're here.
if nb < 8 {
b |= uintptr(*p) << nb
p = add1(p)
} else {
// Reduce the number of bits in b.
// This is important if we skipped
// over a scalar tail, since nb could
// be larger than the bit width of b.
nb -= 8
}
} else if p == nil {
// Almost as fast path: track bit count and refill from pbits.
// For short repetitions.
if nb < 8 {
b |= pbits << nb
nb += endnb
}
nb -= 8 // for next iteration
} else {
// Slow path: reached end of ptrmask.
// Process final partial byte and rewind to start.
b |= uintptr(*p) << nb
nb += endnb
if nb < 8 {
b |= uintptr(*ptrmask) << nb
p = add1(ptrmask)
} else {
nb -= 8
p = ptrmask
}
}
// Emit bitmap byte.
hb = b & bitPointerAll
hb |= bitScanAll
if w += 4; w >= nw {
break
}
*hbitp = uint8(hb)
hbitp = add1(hbitp)
b >>= 4
}
Phase3:
// Phase 3: Write last byte or partial byte and zero the rest of the bitmap entries.
if w > nw {
// Counting the 4 entries in hb not yet written to memory,
// there are more entries than possible pointer slots.
// Discard the excess entries (can't be more than 3).
mask := uintptr(1)<<(4-(w-nw)) - 1
hb &= mask | mask<<4 // apply mask to both pointer bits and scan bits
}
// Change nw from counting possibly-pointer words to total words in allocation.
nw = size / sys.PtrSize
// Write whole bitmap bytes.
// The first is hb, the rest are zero.
if w <= nw {
*hbitp = uint8(hb)
hbitp = add1(hbitp)
hb = 0 // for possible final half-byte below
for w += 4; w <= nw; w += 4 {
*hbitp = 0
hbitp = add1(hbitp)
}
}
// Write final partial bitmap byte if any.
// We know w > nw, or else we'd still be in the loop above.
// It can be bigger only due to the 4 entries in hb that it counts.
// If w == nw+4 then there's nothing left to do: we wrote all nw entries
// and can discard the 4 sitting in hb.
// But if w == nw+2, we need to write first two in hb.
// The byte is shared with the next object, so be careful with
// existing bits.
if w == nw+2 {
*hbitp = *hbitp&^(bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift) | uint8(hb)
}
Phase4:
// Phase 4: Copy unrolled bitmap to per-arena bitmaps, if necessary.
if outOfPlace {
// TODO: We could probably make this faster by
// handling [x+dataSize, x+size) specially.
h := heapBitsForAddr(x)
// cnw is the number of heap words, or bit pairs
// remaining (like nw above).
cnw := size / sys.PtrSize
src := (*uint8)(unsafe.Pointer(x))
// We know the first and last byte of the bitmap are
// not the same, but it's still possible for small
// objects span arenas, so it may share bitmap bytes
// with neighboring objects.
//
// Handle the first byte specially if it's shared. See
// Phase 1 for why this is the only special case we need.
if doubleCheck {
if !(h.shift == 0 || (sys.PtrSize == 8 && h.shift == 2)) {
print("x=", x, " size=", size, " cnw=", h.shift, "\n")
throw("bad start shift")
}
}
if sys.PtrSize == 8 && h.shift == 2 {
*h.bitp = *h.bitp&^((bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift)<<(2*heapBitsShift)) | *src
h = h.next().next()
cnw -= 2
src = addb(src, 1)
}
// We're now byte aligned. Copy out to per-arena
// bitmaps until the last byte (which may again be
// partial).
for cnw >= 4 {
// This loop processes four words at a time,
// so round cnw down accordingly.
hNext, words := h.forwardOrBoundary(cnw / 4 * 4)
// n is the number of bitmap bytes to copy.
n := words / 4
memmove(unsafe.Pointer(h.bitp), unsafe.Pointer(src), n)
cnw -= words
h = hNext
src = addb(src, n)
}
if doubleCheck && h.shift != 0 {
print("cnw=", cnw, " h.shift=", h.shift, "\n")
throw("bad shift after block copy")
}
// Handle the last byte if it's shared.
if cnw == 2 {
*h.bitp = *h.bitp&^(bitPointer|bitScan|(bitPointer|bitScan)<<heapBitsShift) | *src
src = addb(src, 1)
h = h.next().next()
}
if doubleCheck {
if uintptr(unsafe.Pointer(src)) > x+size {
throw("copy exceeded object size")
}
if !(cnw == 0 || cnw == 2) {
print("x=", x, " size=", size, " cnw=", cnw, "\n")
throw("bad number of remaining words")
}
// Set up hbitp so doubleCheck code below can check it.
hbitp = h.bitp
}
// Zero the object where we wrote the bitmap.
memclrNoHeapPointers(unsafe.Pointer(x), uintptr(unsafe.Pointer(src))-x)
}
// Double check the whole bitmap.
if doubleCheck {
// x+size may not point to the heap, so back up one
// word and then call next().
end := heapBitsForAddr(x + size - sys.PtrSize).next()
endAI := arenaIdx(end.arena)
if !outOfPlace && (end.bitp == nil || (end.shift == 0 && end.bitp == &mheap_.arenas[endAI.l1()][endAI.l2()].bitmap[0])) {
// The unrolling code above walks hbitp just
// past the bitmap without moving to the next
// arena. Synthesize this for end.bitp.
end.arena--
endAI = arenaIdx(end.arena)
end.bitp = addb(&mheap_.arenas[endAI.l1()][endAI.l2()].bitmap[0], heapArenaBitmapBytes)
end.last = nil
}
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, " outOfPlace=", outOfPlace, "\n")
print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
h0 := heapBitsForAddr(x)
print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
print("current bits h.bitp=", h.bitp, " h.shift=", h.shift, " *h.bitp=", hex(*h.bitp), "\n")
print("ptrmask=", ptrmask, " p=", p, " endp=", endp, " endnb=", endnb, " pbits=", hex(pbits), " b=", hex(b), " nb=", nb, "\n")
println("at word", i, "offset", i*sys.PtrSize, "have", hex(have), "want", hex(want))
if typ.kind&kindGCProg != 0 {
println("GC program:")
dumpGCProg(addb(typ.gcdata, 4))
}
throw("bad heapBitsSetType")
}
h = h.next()
}
if ptrmask == debugPtrmask.data {
unlock(&debugPtrmask.lock)
}
}
}
var debugPtrmask struct {
lock mutex
data *byte
}
// heapBitsSetTypeGCProg implements heapBitsSetType using a GC program.
// progSize is the size of the memory described by the program.
// elemSize is the size of the element that the GC program describes (a prefix of).
// dataSize is the total size of the intended data, a multiple of elemSize.
// allocSize is the total size of the allocated memory.
//
// GC programs are only used for large allocations.
// heapBitsSetType requires that allocSize is a multiple of 4 words,
// so that the relevant bitmap bytes are not shared with surrounding
// objects.
func heapBitsSetTypeGCProg(h heapBits, progSize, elemSize, dataSize, allocSize uintptr, prog *byte) {
if sys.PtrSize == 8 && allocSize%(4*sys.PtrSize) != 0 {
// Alignment will be wrong.
throw("heapBitsSetTypeGCProg: small allocation")
}
var totalBits uintptr
if elemSize == dataSize {
totalBits = runGCProg(prog, nil, h.bitp, 2)
if totalBits*sys.PtrSize != progSize {
println("runtime: heapBitsSetTypeGCProg: total bits", totalBits, "but progSize", progSize)
throw("heapBitsSetTypeGCProg: unexpected bit count")
}
} else {
count := dataSize / elemSize
// Piece together program trailer to run after prog that does:
// literal(0)
// repeat(1, elemSize-progSize-1) // zeros to fill element size
// repeat(elemSize, count-1) // repeat that element for count
// This zero-pads the data remaining in the first element and then
// repeats that first element to fill the array.
var trailer [40]byte // 3 varints (max 10 each) + some bytes
i := 0
if n := elemSize/sys.PtrSize - progSize/sys.PtrSize; n > 0 {
// literal(0)
trailer[i] = 0x01
i++
trailer[i] = 0
i++
if n > 1 {
// repeat(1, n-1)
trailer[i] = 0x81
i++
n--
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
}
}
// repeat(elemSize/ptrSize, count-1)
trailer[i] = 0x80
i++
n := elemSize / sys.PtrSize
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
n = count - 1
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
trailer[i] = 0
i++
runGCProg(prog, &trailer[0], h.bitp, 2)
// Even though we filled in the full array just now,
// record that we only filled in up to the ptrdata of the
// last element. This will cause the code below to
// memclr the dead section of the final array element,
// so that scanobject can stop early in the final element.
totalBits = (elemSize*(count-1) + progSize) / sys.PtrSize
}
endProg := unsafe.Pointer(addb(h.bitp, (totalBits+3)/4))
endAlloc := unsafe.Pointer(addb(h.bitp, allocSize/sys.PtrSize/wordsPerBitmapByte))
memclrNoHeapPointers(endProg, uintptr(endAlloc)-uintptr(endProg))
}
// progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
// size the size of the region described by prog, in bytes.
// The resulting bitvector will have no more than size/sys.PtrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
n := (size/sys.PtrSize + 7) / 8
x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
x[len(x)-1] = 0xa1 // overflow check sentinel
n = runGCProg(prog, nil, &x[0], 1)
if x[len(x)-1] != 0xa1 {
throw("progToPointerMask: overflow")
}
return bitvector{int32(n), &x[0]}
}
// Packed GC pointer bitmaps, aka GC programs.
//
// For large types containing arrays, the type information has a
// natural repetition that can be encoded to save space in the
// binary and in the memory representation of the type information.
//
// The encoding is a simple Lempel-Ziv style bytecode machine
// with the following instructions:
//
// 00000000: stop
// 0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
// 10000000 n c: repeat the previous n bits c times; n, c are varints
// 1nnnnnnn c: repeat the previous n bits c times; c is a varint
// runGCProg executes the GC program prog, and then trailer if non-nil,
// writing to dst with entries of the given size.
// If size == 1, dst is a 1-bit pointer mask laid out moving forward from dst.
// If size == 2, dst is the 2-bit heap bitmap, and writes move backward
// starting at dst (because the heap bitmap does). In this case, the caller guarantees
// that only whole bytes in dst need to be written.
//
// runGCProg returns the number of 1- or 2-bit entries written to memory.
func runGCProg(prog, trailer, dst *byte, size int) uintptr {
dstStart := dst
// Bits waiting to be written to memory.
var bits uintptr
var nbits uintptr
p := prog
Run:
for {
// Flush accumulated full bytes.
// The rest of the loop assumes that nbits <= 7.
for ; nbits >= 8; nbits -= 8 {
if size == 1 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
} else {
v := bits&bitPointerAll | bitScanAll
*dst = uint8(v)
dst = add1(dst)
bits >>= 4
v = bits&bitPointerAll | bitScanAll
*dst = uint8(v)
dst = add1(dst)
bits >>= 4
}
}
// Process one instruction.
inst := uintptr(*p)
p = add1(p)
n := inst & 0x7F
if inst&0x80 == 0 {
// Literal bits; n == 0 means end of program.
if n == 0 {
// Program is over; continue in trailer if present.
if trailer != nil {
p = trailer
trailer = nil
continue
}
break Run
}
nbyte := n / 8
for i := uintptr(0); i < nbyte; i++ {
bits |= uintptr(*p) << nbits
p = add1(p)
if size == 1 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
} else {
v := bits&0xf | bitScanAll
*dst = uint8(v)
dst = add1(dst)
bits >>= 4
v = bits&0xf | bitScanAll
*dst = uint8(v)
dst = add1(dst)
bits >>= 4
}
}
if n %= 8; n > 0 {
bits |= uintptr(*p) << nbits
p = add1(p)
nbits += n
}
continue Run
}
// Repeat. If n == 0, it is encoded in a varint in the next bytes.
if n == 0 {
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
n |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
}
// Count is encoded in a varint in the next bytes.
c := uintptr(0)
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
c |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
c *= n // now total number of bits to copy
// If the number of bits being repeated is small, load them
// into a register and use that register for the entire loop
// instead of repeatedly reading from memory.
// Handling fewer than 8 bits here makes the general loop simpler.
// The cutoff is sys.PtrSize*8 - 7 to guarantee that when we add
// the pattern to a bit buffer holding at most 7 bits (a partial byte)
// it will not overflow.
src := dst
const maxBits = sys.PtrSize*8 - 7
if n <= maxBits {
// Start with bits in output buffer.
pattern := bits
npattern := nbits
// If we need more bits, fetch them from memory.
if size == 1 {
src = subtract1(src)
for npattern < n {
pattern <<= 8
pattern |= uintptr(*src)
src = subtract1(src)
npattern += 8
}
} else {
src = subtract1(src)
for npattern < n {
pattern <<= 4
pattern |= uintptr(*src) & 0xf
src = subtract1(src)
npattern += 4
}
}
// We started with the whole bit output buffer,
// and then we loaded bits from whole bytes.
// Either way, we might now have too many instead of too few.
// Discard the extra.
if npattern > n {
pattern >>= npattern - n
npattern = n
}
// Replicate pattern to at most maxBits.
if npattern == 1 {
// One bit being repeated.
// If the bit is 1, make the pattern all 1s.
// If the bit is 0, the pattern is already all 0s,
// but we can claim that the number of bits
// in the word is equal to the number we need (c),
// because right shift of bits will zero fill.
if pattern == 1 {
pattern = 1<<maxBits - 1
npattern = maxBits
} else {
npattern = c
}
} else {
b := pattern
nb := npattern
if nb+nb <= maxBits {
// Double pattern until the whole uintptr is filled.
for nb <= sys.PtrSize*8 {
b |= b << nb
nb += nb
}
// Trim away incomplete copy of original pattern in high bits.
// TODO(rsc): Replace with table lookup or loop on systems without divide?
nb = maxBits / npattern * npattern
b &= 1<<nb - 1
pattern = b
npattern = nb
}
}
// Add pattern to bit buffer and flush bit buffer, c/npattern times.
// Since pattern contains >8 bits, there will be full bytes to flush
// on each iteration.
for ; c >= npattern; c -= npattern {
bits |= pattern << nbits
nbits += npattern
if size == 1 {
for nbits >= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
nbits -= 8
}
} else {
for nbits >= 4 {
*dst = uint8(bits&0xf | bitScanAll)
dst = add1(dst)
bits >>= 4
nbits -= 4
}
}
}
// Add final fragment to bit buffer.
if c > 0 {
pattern &= 1<<c - 1
bits |= pattern << nbits
nbits += c
}
continue Run
}
// Repeat; n too large to fit in a register.
// Since nbits <= 7, we know the first few bytes of repeated data
// are already written to memory.
off := n - nbits // n > nbits because n > maxBits and nbits <= 7
if size == 1 {
// Leading src fragment.
src = subtractb(src, (off+7)/8)
if frag := off & 7; frag != 0 {
bits |= uintptr(*src) >> (8 - frag) << nbits
src = add1(src)
nbits += frag
c -= frag
}
// Main loop: load one byte, write another.
// The bits are rotating through the bit buffer.
for i := c / 8; i > 0; i-- {
bits |= uintptr(*src) << nbits
src = add1(src)
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
// Final src fragment.
if c %= 8; c > 0 {
bits |= (uintptr(*src) & (1<<c - 1)) << nbits
nbits += c
}
} else {
// Leading src fragment.
src = subtractb(src, (off+3)/4)
if frag := off & 3; frag != 0 {
bits |= (uintptr(*src) & 0xf) >> (4 - frag) << nbits
src = add1(src)
nbits += frag
c -= frag
}
// Main loop: load one byte, write another.
// The bits are rotating through the bit buffer.
for i := c / 4; i > 0; i-- {
bits |= (uintptr(*src) & 0xf) << nbits
src = add1(src)
*dst = uint8(bits&0xf | bitScanAll)
dst = add1(dst)
bits >>= 4
}
// Final src fragment.
if c %= 4; c > 0 {
bits |= (uintptr(*src) & (1<<c - 1)) << nbits
nbits += c
}
}
}
// Write any final bits out, using full-byte writes, even for the final byte.
var totalBits uintptr
if size == 1 {
totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
nbits += -nbits & 7
for ; nbits > 0; nbits -= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
} else {
totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*4 + nbits
nbits += -nbits & 3
for ; nbits > 0; nbits -= 4 {
v := bits&0xf | bitScanAll
*dst = uint8(v)
dst = add1(dst)
bits >>= 4
}
}
return totalBits
}
// materializeGCProg allocates space for the (1-bit) pointer bitmask
// for an object of size ptrdata. Then it fills that space with the
// pointer bitmask specified by the program prog.
// The bitmask starts at s.startAddr.
// The result must be deallocated with dematerializeGCProg.
func materializeGCProg(ptrdata uintptr, prog *byte) *mspan {
s := mheap_.allocManual((ptrdata/(8*sys.PtrSize)+pageSize-1)/pageSize, &memstats.gc_sys)
runGCProg(addb(prog, 4), nil, (*byte)(unsafe.Pointer(s.startAddr)), 1)
return s
}
func dematerializeGCProg(s *mspan) {
mheap_.freeManual(s, &memstats.gc_sys)
}
func dumpGCProg(p *byte) {
nptr := 0
for {
x := *p
p = add1(p)
if x == 0 {
print("\t", nptr, " end\n")
break
}
if x&0x80 == 0 {
print("\t", nptr, " lit ", x, ":")
n := int(x+7) / 8
for i := 0; i < n; i++ {
print(" ", hex(*p))
p = add1(p)
}
print("\n")
nptr += int(x)
} else {
nbit := int(x &^ 0x80)
if nbit == 0 {
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
nbit |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
}
count := 0
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
count |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
nptr += nbit * count
}
}
}
// Testing.
func getgcmaskcb(frame *stkframe, ctxt unsafe.Pointer) bool {
target := (*stkframe)(ctxt)
if frame.sp <= target.sp && target.sp < frame.varp {
*target = *frame
return false
}
return true
}
// gcbits returns the GC type info for x, for testing.
// The result is the bitmap entries (0 or 1), one entry per byte.
//go:linkname reflect_gcbits reflect.gcbits
func reflect_gcbits(x interface{}) []byte {
ret := getgcmask(x)
typ := (*ptrtype)(unsafe.Pointer(efaceOf(&x)._type)).elem
nptr := typ.ptrdata / sys.PtrSize
for uintptr(len(ret)) > nptr && ret[len(ret)-1] == 0 {
ret = ret[:len(ret)-1]
}
return ret
}
// Returns GC type info for the pointer stored in ep for testing.
// If ep points to the stack, only static live information will be returned
// (i.e. not for objects which are only dynamically live stack objects).
func getgcmask(ep interface{}) (mask []byte) {
e := *efaceOf(&ep)
p := e.data
t := e._type
// data or bss
for _, datap := range activeModules() {
// data
if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
bitmap := datap.gcdatamask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
off := (uintptr(p) + i - datap.data) / sys.PtrSize
mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
// bss
if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
bitmap := datap.gcbssmask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
off := (uintptr(p) + i - datap.bss) / sys.PtrSize
mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
}
// heap
if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
hbits := heapBitsForAddr(base)
n := s.elemsize
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
if hbits.isPointer() {
mask[i/sys.PtrSize] = 1
}
if i != 1*sys.PtrSize && !hbits.morePointers() {
mask = mask[:i/sys.PtrSize]
break
}
hbits = hbits.next()
}
return
}
// stack
if _g_ := getg(); _g_.m.curg.stack.lo <= uintptr(p) && uintptr(p) < _g_.m.curg.stack.hi {
var frame stkframe
frame.sp = uintptr(p)
_g_ := getg()
gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
if frame.fn.valid() {
locals, _, _ := getStackMap(&frame, nil, false)
if locals.n == 0 {
return
}
size := uintptr(locals.n) * sys.PtrSize
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
off := (uintptr(p) + i - frame.varp + size) / sys.PtrSize
mask[i/sys.PtrSize] = locals.ptrbit(off)
}
}
return
}
// otherwise, not something the GC knows about.
// possibly read-only data, like malloc(0).
// must not have pointers
return
}