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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 bitmaps with 1 bit per pointer-sized word. A "1" bit
// means the word is a live pointer to be visited by the GC (referred to
// as "pointer"). A "0" bit means the word should be ignored by GC
// (referred to as "scalar", though it could be a dead pointer value).
//
// Heap bitmap
//
// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting at "start",
// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
// through start+63*ptrSize, ha.bitmap[1] holds the entries for
// start+64*ptrSize through start+127*ptrSize, and so on.
// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
// (For 32-bit platforms, s/64/32/.)
//
// We also keep a noMorePtrs bitmap which allows us to stop scanning
// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
// has no more pointers beyond those described by ha.bitmap[8*i+j].
// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
// beyond must all be zero until the start of the next object.
//
// The bitmap for noscan spans is set to all zero at span allocation time.
//
// The bitmap for unallocated objects in scannable spans is not maintained
// (can be junk).
package runtime
import (
"internal/goarch"
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
// addb returns the byte pointer p+n.
//
//go:nowritebarrier
//go:nosplit
func addb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}
// subtractb returns the byte pointer p-n.
//
//go:nowritebarrier
//go:nosplit
func subtractb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, -n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
}
// add1 returns the byte pointer p+1.
//
//go:nowritebarrier
//go:nosplit
func add1(p *byte) *byte {
// Note: wrote out full expression instead of calling addb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
}
// subtract1 returns the byte pointer p-1.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nowritebarrier
//go:nosplit
func subtract1(p *byte) *byte {
// Note: wrote out full expression instead of calling subtractb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
}
// markBits provides access to the mark bit for an object in the heap.
// bytep points to the byte holding the mark bit.
// mask is a byte with a single bit set that can be &ed with *bytep
// to see if the bit has been set.
// *m.byte&m.mask != 0 indicates the mark bit is set.
// index can be used along with span information to generate
// the address of the object in the heap.
// We maintain one set of mark bits for allocation and one for
// marking purposes.
type markBits struct {
bytep *uint8
mask uint8
index uintptr
}
//go:nosplit
func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
bytep, mask := s.allocBits.bitp(allocBitIndex)
return markBits{bytep, mask, allocBitIndex}
}
// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
// and negates them so that ctz (count trailing zeros) instructions
// can be used. It then places these 8 bytes into the cached 64 bit
// s.allocCache.
func (s *mspan) refillAllocCache(whichByte uintptr) {
bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(whichByte)))
aCache := uint64(0)
aCache |= uint64(bytes[0])
aCache |= uint64(bytes[1]) << (1 * 8)
aCache |= uint64(bytes[2]) << (2 * 8)
aCache |= uint64(bytes[3]) << (3 * 8)
aCache |= uint64(bytes[4]) << (4 * 8)
aCache |= uint64(bytes[5]) << (5 * 8)
aCache |= uint64(bytes[6]) << (6 * 8)
aCache |= uint64(bytes[7]) << (7 * 8)
s.allocCache = ^aCache
}
// nextFreeIndex returns the index of the next free object in s at
// or after s.freeindex.
// There are hardware instructions that can be used to make this
// faster if profiling warrants it.
func (s *mspan) nextFreeIndex() uintptr {
sfreeindex := s.freeindex
snelems := s.nelems
if sfreeindex == snelems {
return sfreeindex
}
if sfreeindex > snelems {
throw("s.freeindex > s.nelems")
}
aCache := s.allocCache
bitIndex := sys.TrailingZeros64(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.TrailingZeros64(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.freeIndexForScan {
return false
}
bytep, mask := s.allocBits.bitp(index)
return *bytep&mask == 0
}
// divideByElemSize returns n/s.elemsize.
// n must be within [0, s.npages*_PageSize),
// or may be exactly s.npages*_PageSize
// if s.elemsize is from sizeclasses.go.
//
// nosplit, because it is called by objIndex, which is nosplit
//
//go:nosplit
func (s *mspan) divideByElemSize(n uintptr) uintptr {
const doubleCheck = false
// See explanation in mksizeclasses.go's computeDivMagic.
q := uintptr((uint64(n) * uint64(s.divMul)) >> 32)
if doubleCheck && q != n/s.elemsize {
println(n, "/", s.elemsize, "should be", n/s.elemsize, "but got", q)
throw("bad magic division")
}
return q
}
// nosplit, because it is called by other nosplit code like findObject
//
//go:nosplit
func (s *mspan) objIndex(p uintptr) uintptr {
return s.divideByElemSize(p - s.base())
}
func markBitsForAddr(p uintptr) markBits {
s := spanOf(p)
objIndex := s.objIndex(p)
return s.markBitsForIndex(objIndex)
}
func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
bytep, mask := s.gcmarkBits.bitp(objIndex)
return markBits{bytep, mask, objIndex}
}
func (s *mspan) markBitsForBase() markBits {
return markBits{&s.gcmarkBits.x, uint8(1), 0}
}
// isMarked reports whether mark bit m is set.
func (m markBits) isMarked() bool {
return *m.bytep&m.mask != 0
}
// setMarked sets the marked bit in the markbits, atomically.
func (m markBits) setMarked() {
// Might be racing with other updates, so use atomic update always.
// We used to be clever here and use a non-atomic update in certain
// cases, but it's not worth the risk.
atomic.Or8(m.bytep, m.mask)
}
// setMarkedNonAtomic sets the marked bit in the markbits, non-atomically.
func (m markBits) setMarkedNonAtomic() {
*m.bytep |= m.mask
}
// clearMarked clears the marked bit in the markbits, atomically.
func (m markBits) clearMarked() {
// Might be racing with other updates, so use atomic update always.
// We used to be clever here and use a non-atomic update in certain
// cases, but it's not worth the risk.
atomic.And8(m.bytep, ^m.mask)
}
// markBitsForSpan returns the markBits for the span base address base.
func markBitsForSpan(base uintptr) (mbits markBits) {
mbits = markBitsForAddr(base)
if mbits.mask != 1 {
throw("markBitsForSpan: unaligned start")
}
return mbits
}
// advance advances the markBits to the next object in the span.
func (m *markBits) advance() {
if m.mask == 1<<7 {
m.bytep = (*uint8)(unsafe.Pointer(uintptr(unsafe.Pointer(m.bytep)) + 1))
m.mask = 1
} else {
m.mask = m.mask << 1
}
m.index++
}
// clobberdeadPtr is a special value that is used by the compiler to
// clobber dead stack slots, when -clobberdead flag is set.
const clobberdeadPtr = uintptr(0xdeaddead | 0xdeaddead<<((^uintptr(0)>>63)*32))
// badPointer throws bad pointer in heap panic.
func badPointer(s *mspan, p, refBase, refOff uintptr) {
// Typically this indicates an incorrect use
// of unsafe or cgo to store a bad pointer in
// the Go heap. It may also indicate a runtime
// bug.
//
// TODO(austin): We could be more aggressive
// and detect pointers to unallocated objects
// in allocated spans.
printlock()
print("runtime: pointer ", hex(p))
if s != nil {
state := s.state.get()
if state != mSpanInUse {
print(" to unallocated span")
} else {
print(" to unused region of span")
}
print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", state)
}
print("\n")
if refBase != 0 {
print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
gcDumpObject("object", refBase, refOff)
}
getg().m.traceback = 2
throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
}
// findObject returns the base address for the heap object containing
// the address p, the object's span, and the index of the object in s.
// If p does not point into a heap object, it returns base == 0.
//
// If p points is an invalid heap pointer and debug.invalidptr != 0,
// findObject panics.
//
// refBase and refOff optionally give the base address of the object
// in which the pointer p was found and the byte offset at which it
// was found. These are used for error reporting.
//
// It is nosplit so it is safe for p to be a pointer to the current goroutine's stack.
// Since p is a uintptr, it would not be adjusted if the stack were to move.
//
//go:nosplit
func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex uintptr) {
s = spanOf(p)
// If s is nil, the virtual address has never been part of the heap.
// This pointer may be to some mmap'd region, so we allow it.
if s == nil {
if (GOARCH == "amd64" || GOARCH == "arm64") && p == clobberdeadPtr && debug.invalidptr != 0 {
// Crash if clobberdeadPtr is seen. Only on AMD64 and ARM64 for now,
// as they are the only platform where compiler's clobberdead mode is
// implemented. On these platforms clobberdeadPtr cannot be a valid address.
badPointer(s, p, refBase, refOff)
}
return
}
// If p is a bad pointer, it may not be in s's bounds.
//
// Check s.state to synchronize with span initialization
// before checking other fields. See also spanOfHeap.
if state := s.state.get(); state != mSpanInUse || p < s.base() || p >= s.limit {
// Pointers into stacks are also ok, the runtime manages these explicitly.
if state == mSpanManual {
return
}
// The following ensures that we are rigorous about what data
// structures hold valid pointers.
if debug.invalidptr != 0 {
badPointer(s, p, refBase, refOff)
}
return
}
objIndex = s.objIndex(p)
base = s.base() + objIndex*s.elemsize
return
}
// reflect_verifyNotInHeapPtr reports whether converting the not-in-heap pointer into a unsafe.Pointer is ok.
//
//go:linkname reflect_verifyNotInHeapPtr reflect.verifyNotInHeapPtr
func reflect_verifyNotInHeapPtr(p uintptr) bool {
// Conversion to a pointer is ok as long as findObject above does not call badPointer.
// Since we're already promised that p doesn't point into the heap, just disallow heap
// pointers and the special clobbered pointer.
return spanOf(p) == nil && p != clobberdeadPtr
}
const ptrBits = 8 * goarch.PtrSize
// heapBits provides access to the bitmap bits for a single heap word.
// The methods on heapBits take value receivers so that the compiler
// can more easily inline calls to those methods and registerize the
// struct fields independently.
type heapBits struct {
// heapBits will report on pointers in the range [addr,addr+size).
// The low bit of mask contains the pointerness of the word at addr
// (assuming valid>0).
addr, size uintptr
// The next few pointer bits representing words starting at addr.
// Those bits already returned by next() are zeroed.
mask uintptr
// Number of bits in mask that are valid. mask is always less than 1<<valid.
valid uintptr
}
// heapBitsForAddr returns the heapBits for the address addr.
// The caller must ensure [addr,addr+size) is in an allocated span.
// In particular, be careful not to point past the end of an object.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func heapBitsForAddr(addr, size uintptr) heapBits {
// Find arena
ai := arenaIndex(addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
// Word index in arena.
word := addr / goarch.PtrSize % heapArenaWords
// Word index and bit offset in bitmap array.
idx := word / ptrBits
off := word % ptrBits
// Grab relevant bits of bitmap.
mask := ha.bitmap[idx] >> off
valid := ptrBits - off
// Process depending on where the object ends.
nptr := size / goarch.PtrSize
if nptr < valid {
// Bits for this object end before the end of this bitmap word.
// Squash bits for the following objects.
mask &= 1<<(nptr&(ptrBits-1)) - 1
valid = nptr
} else if nptr == valid {
// Bits for this object end at exactly the end of this bitmap word.
// All good.
} else {
// Bits for this object extend into the next bitmap word. See if there
// may be any pointers recorded there.
if uintptr(ha.noMorePtrs[idx/8])>>(idx%8)&1 != 0 {
// No more pointers in this object after this bitmap word.
// Update size so we know not to look there.
size = valid * goarch.PtrSize
}
}
return heapBits{addr: addr, size: size, mask: mask, valid: valid}
}
// Returns the (absolute) address of the next known pointer and
// a heapBits iterator representing any remaining pointers.
// If there are no more pointers, returns address 0.
// Note that next does not modify h. The caller must record the result.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (h heapBits) next() (heapBits, uintptr) {
for {
if h.mask != 0 {
var i int
if goarch.PtrSize == 8 {
i = sys.TrailingZeros64(uint64(h.mask))
} else {
i = sys.TrailingZeros32(uint32(h.mask))
}
h.mask ^= uintptr(1) << (i & (ptrBits - 1))
return h, h.addr + uintptr(i)*goarch.PtrSize
}
// Skip words that we've already processed.
h.addr += h.valid * goarch.PtrSize
h.size -= h.valid * goarch.PtrSize
if h.size == 0 {
return h, 0 // no more pointers
}
// Grab more bits and try again.
h = heapBitsForAddr(h.addr, h.size)
}
}
// nextFast is like next, but can return 0 even when there are more pointers
// to be found. Callers should call next if nextFast returns 0 as its second
// return value.
//
// if addr, h = h.nextFast(); addr == 0 {
// if addr, h = h.next(); addr == 0 {
// ... no more pointers ...
// }
// }
// ... process pointer at addr ...
//
// nextFast is designed to be inlineable.
//
//go:nosplit
func (h heapBits) nextFast() (heapBits, uintptr) {
// TESTQ/JEQ
if h.mask == 0 {
return h, 0
}
// BSFQ
var i int
if goarch.PtrSize == 8 {
i = sys.TrailingZeros64(uint64(h.mask))
} else {
i = sys.TrailingZeros32(uint32(h.mask))
}
// BTCQ
h.mask ^= uintptr(1) << (i & (ptrBits - 1))
// LEAQ (XX)(XX*8)
return h, h.addr + uintptr(i)*goarch.PtrSize
}
// bulkBarrierPreWrite executes a write barrier
// for every pointer slot in the memory range [src, src+size),
// using pointer/scalar information from [dst, dst+size).
// This executes the write barriers necessary before a memmove.
// src, dst, and size must be pointer-aligned.
// The range [dst, dst+size) must lie within a single object.
// It does not perform the actual writes.
//
// As a special case, src == 0 indicates that this is being used for a
// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
// barrier.
//
// Callers should call bulkBarrierPreWrite immediately before
// calling memmove(dst, src, size). This function is marked nosplit
// to avoid being preempted; the GC must not stop the goroutine
// between the memmove and the execution of the barriers.
// The caller is also responsible for cgo pointer checks if this
// may be writing Go pointers into non-Go memory.
//
// The pointer bitmap is not maintained for allocations containing
// no pointers at all; any caller of bulkBarrierPreWrite must first
// make sure the underlying allocation contains pointers, usually
// by checking typ.PtrBytes.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.needed {
return
}
if s := spanOf(dst); s == nil {
// If dst is a global, use the data or BSS bitmaps to
// execute write barriers.
for _, datap := range activeModules() {
if datap.data <= dst && dst < datap.edata {
bulkBarrierBitmap(dst, src, size, dst-datap.data, datap.gcdatamask.bytedata)
return
}
}
for _, datap := range activeModules() {
if datap.bss <= dst && dst < datap.ebss {
bulkBarrierBitmap(dst, src, size, dst-datap.bss, datap.gcbssmask.bytedata)
return
}
}
return
} else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
// dst was heap memory at some point, but isn't now.
// It can't be a global. It must be either our stack,
// or in the case of direct channel sends, it could be
// another stack. Either way, no need for barriers.
// This will also catch if dst is in a freed span,
// though that should never have.
return
}
buf := &getg().m.p.ptr().wbBuf
h := heapBitsForAddr(dst, size)
if src == 0 {
for {
var addr uintptr
if h, addr = h.next(); addr == 0 {
break
}
dstx := (*uintptr)(unsafe.Pointer(addr))
p := buf.get1()
p[0] = *dstx
}
} else {
for {
var addr uintptr
if h, addr = h.next(); addr == 0 {
break
}
dstx := (*uintptr)(unsafe.Pointer(addr))
srcx := (*uintptr)(unsafe.Pointer(src + (addr - dst)))
p := buf.get2()
p[0] = *dstx
p[1] = *srcx
}
}
}
// bulkBarrierPreWriteSrcOnly is like bulkBarrierPreWrite but
// does not execute write barriers for [dst, dst+size).
//
// In addition to the requirements of bulkBarrierPreWrite
// callers need to ensure [dst, dst+size) is zeroed.
//
// This is used for special cases where e.g. dst was just
// created and zeroed with malloc.
//
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.needed {
return
}
buf := &getg().m.p.ptr().wbBuf
h := heapBitsForAddr(dst, size)
for {
var addr uintptr
if h, addr = h.next(); addr == 0 {
break
}
srcx := (*uintptr)(unsafe.Pointer(addr - dst + src))
p := buf.get1()
p[0] = *srcx
}
}
// bulkBarrierBitmap executes write barriers for copying from [src,
// src+size) to [dst, dst+size) using a 1-bit pointer bitmap. src is
// assumed to start maskOffset bytes into the data covered by the
// bitmap in bits (which may not be a multiple of 8).
//
// This is used by bulkBarrierPreWrite for writes to data and BSS.
//
//go:nosplit
func bulkBarrierBitmap(dst, src, size, maskOffset uintptr, bits *uint8) {
word := maskOffset / goarch.PtrSize
bits = addb(bits, word/8)
mask := uint8(1) << (word % 8)
buf := &getg().m.p.ptr().wbBuf
for i := uintptr(0); i < size; i += goarch.PtrSize {
if mask == 0 {
bits = addb(bits, 1)
if *bits == 0 {
// Skip 8 words.
i += 7 * goarch.PtrSize
continue
}
mask = 1
}
if *bits&mask != 0 {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
if src == 0 {
p := buf.get1()
p[0] = *dstx
} else {
srcx := (*uintptr)(unsafe.Pointer(src + i))
p := buf.get2()
p[0] = *dstx
p[1] = *srcx
}
}
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 goexperiment.CgoCheck2.
//
//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 ", toRType(typ).string(), " of size ", typ.Size_, " but memory size", size)
throw("runtime: invalid typeBitsBulkBarrier")
}
if typ.Kind_&kindGCProg != 0 {
println("runtime: typeBitsBulkBarrier with type ", toRType(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.PtrBytes; i += goarch.PtrSize {
if i&(goarch.PtrSize*8-1) == 0 {
bits = uint32(*ptrmask)
ptrmask = addb(ptrmask, 1)
} else {
bits = bits >> 1
}
if bits&1 != 0 {
dstx := (*uintptr)(unsafe.Pointer(dst + i))
srcx := (*uintptr)(unsafe.Pointer(src + i))
p := buf.get2()
p[0] = *dstx
p[1] = *srcx
}
}
}
// initHeapBits initializes the heap bitmap for a span.
// If this is a span of single pointer allocations, it initializes all
// words to pointer. If force is true, clears all bits.
func (s *mspan) initHeapBits(forceClear bool) {
if forceClear || s.spanclass.noscan() {
// Set all the pointer bits to zero. We do this once
// when the span is allocated so we don't have to do it
// for each object allocation.
base := s.base()
size := s.npages * pageSize
h := writeHeapBitsForAddr(base)
h.flush(base, size)
return
}
isPtrs := goarch.PtrSize == 8 && s.elemsize == goarch.PtrSize
if !isPtrs {
return // nothing to do
}
h := writeHeapBitsForAddr(s.base())
size := s.npages * pageSize
nptrs := size / goarch.PtrSize
for i := uintptr(0); i < nptrs; i += ptrBits {
h = h.write(^uintptr(0), ptrBits)
}
h.flush(s.base(), size)
}
// countAlloc returns the number of objects allocated in span s by
// scanning the allocation bitmap.
func (s *mspan) countAlloc() int {
count := 0
bytes := divRoundUp(s.nelems, 8)
// Iterate over each 8-byte chunk and count allocations
// with an intrinsic. Note that newMarkBits guarantees that
// gcmarkBits will be 8-byte aligned, so we don't have to
// worry about edge cases, irrelevant bits will simply be zero.
for i := uintptr(0); i < bytes; i += 8 {
// Extract 64 bits from the byte pointer and get a OnesCount.
// Note that the unsafe cast here doesn't preserve endianness,
// but that's OK. We only care about how many bits are 1, not
// about the order we discover them in.
mrkBits := *(*uint64)(unsafe.Pointer(s.gcmarkBits.bytep(i)))
count += sys.OnesCount64(mrkBits)
}
return count
}
type writeHeapBits struct {
addr uintptr // address that the low bit of mask represents the pointer state of.
mask uintptr // some pointer bits starting at the address addr.
valid uintptr // number of bits in buf that are valid (including low)
low uintptr // number of low-order bits to not overwrite
}
func writeHeapBitsForAddr(addr uintptr) (h writeHeapBits) {
// We start writing bits maybe in the middle of a heap bitmap word.
// Remember how many bits into the word we started, so we can be sure
// not to overwrite the previous bits.
h.low = addr / goarch.PtrSize % ptrBits
// round down to heap word that starts the bitmap word.
h.addr = addr - h.low*goarch.PtrSize
// We don't have any bits yet.
h.mask = 0
h.valid = h.low
return
}
// write appends the pointerness of the next valid pointer slots
// using the low valid bits of bits. 1=pointer, 0=scalar.
func (h writeHeapBits) write(bits, valid uintptr) writeHeapBits {
if h.valid+valid <= ptrBits {
// Fast path - just accumulate the bits.
h.mask |= bits << h.valid
h.valid += valid
return h
}
// Too many bits to fit in this word. Write the current word
// out and move on to the next word.
data := h.mask | bits<<h.valid // mask for this word
h.mask = bits >> (ptrBits - h.valid) // leftover for next word
h.valid += valid - ptrBits // have h.valid+valid bits, writing ptrBits of them
// Flush mask to the memory bitmap.
// TODO: figure out how to cache arena lookup.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
m := uintptr(1)<<h.low - 1
ha.bitmap[idx] = ha.bitmap[idx]&m | data
// Note: no synchronization required for this write because
// the allocator has exclusive access to the page, and the bitmap
// entries are all for a single page. Also, visibility of these
// writes is guaranteed by the publication barrier in mallocgc.
// Clear noMorePtrs bit, since we're going to be writing bits
// into the following word.
ha.noMorePtrs[idx/8] &^= uint8(1) << (idx % 8)
// Note: same as above
// Move to next word of bitmap.
h.addr += ptrBits * goarch.PtrSize
h.low = 0
return h
}
// Add padding of size bytes.
func (h writeHeapBits) pad(size uintptr) writeHeapBits {
if size == 0 {
return h
}
words := size / goarch.PtrSize
for words > ptrBits {
h = h.write(0, ptrBits)
words -= ptrBits
}
return h.write(0, words)
}
// Flush the bits that have been written, and add zeros as needed
// to cover the full object [addr, addr+size).
func (h writeHeapBits) flush(addr, size uintptr) {
// zeros counts the number of bits needed to represent the object minus the
// number of bits we've already written. This is the number of 0 bits
// that need to be added.
zeros := (addr+size-h.addr)/goarch.PtrSize - h.valid
// Add zero bits up to the bitmap word boundary
if zeros > 0 {
z := ptrBits - h.valid
if z > zeros {
z = zeros
}
h.valid += z
zeros -= z
}
// Find word in bitmap that we're going to write.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
// Write remaining bits.
if h.valid != h.low {
m := uintptr(1)<<h.low - 1 // don't clear existing bits below "low"
m |= ^(uintptr(1)<<h.valid - 1) // don't clear existing bits above "valid"
ha.bitmap[idx] = ha.bitmap[idx]&m | h.mask
}
if zeros == 0 {
return
}
// Record in the noMorePtrs map that there won't be any more 1 bits,
// so readers can stop early.
ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
// Advance to next bitmap word.
h.addr += ptrBits * goarch.PtrSize
// Continue on writing zeros for the rest of the object.
// For standard use of the ptr bits this is not required, as
// the bits are read from the beginning of the object. Some uses,
// like noscan spans, oblets, bulk write barriers, and cgocheck, might
// start mid-object, so these writes are still required.
for {
// Write zero bits.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
if zeros < ptrBits {
ha.bitmap[idx] &^= uintptr(1)<<zeros - 1
break
} else if zeros == ptrBits {
ha.bitmap[idx] = 0
break
} else {
ha.bitmap[idx] = 0
zeros -= ptrBits
}
ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
h.addr += ptrBits * goarch.PtrSize
}
}
// Read the bytes starting at the aligned pointer p into a uintptr.
// Read is little-endian.
func readUintptr(p *byte) uintptr {
x := *(*uintptr)(unsafe.Pointer(p))
if goarch.BigEndian {
if goarch.PtrSize == 8 {
return uintptr(sys.Bswap64(uint64(x)))
}
return uintptr(sys.Bswap32(uint32(x)))
}
return x
}
// heapBitsSetType records that the new allocation [x, x+size)
// holds in [x, x+dataSize) one or more values of type typ.
// (The number of values is given by dataSize / typ.Size.)
// If dataSize < size, the fragment [x+dataSize, x+size) is
// recorded as non-pointer data.
// It is known that the type has pointers somewhere;
// malloc does not call heapBitsSetType when there are no pointers,
// because all free objects are marked as noscan during
// heapBitsSweepSpan.
//
// There can only be one allocation from a given span active at a time,
// and the bitmap for a span always falls on word boundaries,
// so there are no write-write races for access to the heap bitmap.
// Hence, heapBitsSetType can access the bitmap without atomics.
//
// There can be read-write races between heapBitsSetType and things
// that read the heap bitmap like scanobject. However, since
// heapBitsSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// bits that belong to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
const doubleCheck = false // slow but helpful; enable to test modifications to this code
if doubleCheck && dataSize%typ.Size_ != 0 {
throw("heapBitsSetType: dataSize not a multiple of typ.Size")
}
if goarch.PtrSize == 8 && size == goarch.PtrSize {
// It's one word and it has pointers, it must be a pointer.
// Since all allocated one-word objects are pointers
// (non-pointers are aggregated into tinySize allocations),
// (*mspan).initHeapBits sets the pointer bits for us.
// Nothing to do here.
if doubleCheck {
h, addr := heapBitsForAddr(x, size).next()
if addr != x {
throw("heapBitsSetType: pointer bit missing")
}
_, addr = h.next()
if addr != 0 {
throw("heapBitsSetType: second pointer bit found")
}
}
return
}
h := writeHeapBitsForAddr(x)
// Handle GC program.
if typ.Kind_&kindGCProg != 0 {
// Expand the gc program into the storage we're going to use for the actual object.
obj := (*uint8)(unsafe.Pointer(x))
n := runGCProg(addb(typ.GCData, 4), obj)
// Use the expanded program to set the heap bits.
for i := uintptr(0); true; i += typ.Size_ {
// Copy expanded program to heap bitmap.
p := obj
j := n
for j > 8 {
h = h.write(uintptr(*p), 8)
p = add1(p)
j -= 8
}
h = h.write(uintptr(*p), j)
if i+typ.Size_ == dataSize {
break // no padding after last element
}
// Pad with zeros to the start of the next element.
h = h.pad(typ.Size_ - n*goarch.PtrSize)
}
h.flush(x, size)
// Erase the expanded GC program.
memclrNoHeapPointers(unsafe.Pointer(obj), (n+7)/8)
return
}
// Note about sizes:
//
// typ.Size is the number of words in the object,
// and typ.PtrBytes is the number of words in the prefix
// of the object that contains pointers. That is, the final
// typ.Size - typ.PtrBytes words contain no pointers.
// This allows optimization of a common pattern where
// an object has a small header followed by a large scalar
// buffer. If we know the pointers are over, we don't have
// to scan the buffer's heap bitmap at all.
// The 1-bit ptrmasks are sized to contain only bits for
// the typ.PtrBytes prefix, zero padded out to a full byte
// of bitmap. If there is more room in the allocated object,
// that space is pointerless. The noMorePtrs bitmap will prevent
// scanning large pointerless tails of an object.
//
// Replicated copies are not as nice: if there is an array of
// objects with scalar tails, all but the last tail does have to
// be initialized, because there is no way to say "skip forward".
ptrs := typ.PtrBytes / goarch.PtrSize
if typ.Size_ == dataSize { // Single element
if ptrs <= ptrBits { // Single small element
m := readUintptr(typ.GCData)
h = h.write(m, ptrs)
} else { // Single large element
p := typ.GCData
for {
h = h.write(readUintptr(p), ptrBits)
p = addb(p, ptrBits/8)
ptrs -= ptrBits
if ptrs <= ptrBits {
break
}
}
m := readUintptr(p)
h = h.write(m, ptrs)
}
} else { // Repeated element
words := typ.Size_ / goarch.PtrSize // total words, including scalar tail
if words <= ptrBits { // Repeated small element
n := dataSize / typ.Size_
m := readUintptr(typ.GCData)
// Make larger unit to repeat
for words <= ptrBits/2 {
if n&1 != 0 {
h = h.write(m, words)
}
n /= 2
m |= m << words
ptrs += words
words *= 2
if n == 1 {
break
}
}
for n > 1 {
h = h.write(m, words)
n--
}
h = h.write(m, ptrs)
} else { // Repeated large element
for i := uintptr(0); true; i += typ.Size_ {
p := typ.GCData
j := ptrs
for j > ptrBits {
h = h.write(readUintptr(p), ptrBits)
p = addb(p, ptrBits/8)
j -= ptrBits
}
m := readUintptr(p)
h = h.write(m, j)
if i+typ.Size_ == dataSize {
break // don't need the trailing nonptr bits on the last element.
}
// Pad with zeros to the start of the next element.
h = h.pad(typ.Size_ - typ.PtrBytes)
}
}
}
h.flush(x, size)
if doubleCheck {
h := heapBitsForAddr(x, size)
for i := uintptr(0); i < size; i += goarch.PtrSize {
// Compute the pointer bit we want at offset i.
want := false
if i < dataSize {
off := i % typ.Size_
if off < typ.PtrBytes {
j := off / goarch.PtrSize
want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
}
}
if want {
var addr uintptr
h, addr = h.next()
if addr != x+i {
throw("heapBitsSetType: pointer entry not correct")
}
}
}
if _, addr := h.next(); addr != 0 {
throw("heapBitsSetType: extra pointer")
}
}
}
var debugPtrmask struct {
lock mutex
data *byte
}
// progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
// size the size of the region described by prog, in bytes.
// The resulting bitvector will have no more than size/goarch.PtrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
n := (size/goarch.PtrSize + 7) / 8
x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
x[len(x)-1] = 0xa1 // overflow check sentinel
n = runGCProg(prog, &x[0])
if x[len(x)-1] != 0xa1 {
throw("progToPointerMask: overflow")
}
return bitvector{int32(n), &x[0]}
}
// Packed GC pointer bitmaps, aka GC programs.
//
// For large types containing arrays, the type information has a
// natural repetition that can be encoded to save space in the
// binary and in the memory representation of the type information.
//
// The encoding is a simple Lempel-Ziv style bytecode machine
// with the following instructions:
//
// 00000000: stop
// 0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
// 10000000 n c: repeat the previous n bits c times; n, c are varints
// 1nnnnnnn c: repeat the previous n bits c times; c is a varint
// runGCProg returns the number of 1-bit entries written to memory.
func runGCProg(prog, dst *byte) uintptr {
dstStart := dst
// Bits waiting to be written to memory.
var bits uintptr
var nbits uintptr
p := prog
Run:
for {
// Flush accumulated full bytes.
// The rest of the loop assumes that nbits <= 7.
for ; nbits >= 8; nbits -= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
// Process one instruction.
inst := uintptr(*p)
p = add1(p)
n := inst & 0x7F
if inst&0x80 == 0 {
// Literal bits; n == 0 means end of program.
if n == 0 {
// Program is over.
break Run
}
nbyte := n / 8
for i := uintptr(0); i < nbyte; i++ {
bits |= uintptr(*p) << nbits
p = add1(p)
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
if n %= 8; n > 0 {
bits |= uintptr(*p) << nbits
p = add1(p)
nbits += n
}
continue Run
}
// Repeat. If n == 0, it is encoded in a varint in the next bytes.
if n == 0 {
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
n |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
}
// Count is encoded in a varint in the next bytes.
c := uintptr(0)
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
c |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
c *= n // now total number of bits to copy
// If the number of bits being repeated is small, load them
// into a register and use that register for the entire loop
// instead of repeatedly reading from memory.
// Handling fewer than 8 bits here makes the general loop simpler.
// The cutoff is goarch.PtrSize*8 - 7 to guarantee that when we add
// the pattern to a bit buffer holding at most 7 bits (a partial byte)
// it will not overflow.
src := dst
const maxBits = goarch.PtrSize*8 - 7
if n <= maxBits {
// Start with bits in output buffer.
pattern := bits
npattern := nbits
// If we need more bits, fetch them from memory.
src = subtract1(src)
for npattern < n {
pattern <<= 8
pattern |= uintptr(*src)
src = subtract1(src)
npattern += 8
}
// We started with the whole bit output buffer,
// and then we loaded bits from whole bytes.
// Either way, we might now have too many instead of too few.
// Discard the extra.
if npattern > n {
pattern >>= npattern - n
npattern = n
}
// Replicate pattern to at most maxBits.
if npattern == 1 {
// One bit being repeated.
// If the bit is 1, make the pattern all 1s.
// If the bit is 0, the pattern is already all 0s,
// but we can claim that the number of bits
// in the word is equal to the number we need (c),
// because right shift of bits will zero fill.
if pattern == 1 {
pattern = 1<<maxBits - 1
npattern = maxBits
} else {
npattern = c
}
} else {
b := pattern
nb := npattern
if nb+nb <= maxBits {
// Double pattern until the whole uintptr is filled.
for nb <= goarch.PtrSize*8 {
b |= b << nb
nb += nb
}
// Trim away incomplete copy of original pattern in high bits.
// TODO(rsc): Replace with table lookup or loop on systems without divide?
nb = maxBits / npattern * npattern
b &= 1<<nb - 1
pattern = b
npattern = nb
}
}
// Add pattern to bit buffer and flush bit buffer, c/npattern times.
// Since pattern contains >8 bits, there will be full bytes to flush
// on each iteration.
for ; c >= npattern; c -= npattern {
bits |= pattern << nbits
nbits += npattern
for nbits >= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
nbits -= 8
}
}
// Add final fragment to bit buffer.
if c > 0 {
pattern &= 1<<c - 1
bits |= pattern << nbits
nbits += c
}
continue Run
}
// Repeat; n too large to fit in a register.
// Since nbits <= 7, we know the first few bytes of repeated data
// are already written to memory.
off := n - nbits // n > nbits because n > maxBits and nbits <= 7
// Leading src fragment.
src = subtractb(src, (off+7)/8)
if frag := off & 7; frag != 0 {
bits |= uintptr(*src) >> (8 - frag) << nbits
src = add1(src)
nbits += frag
c -= frag
}
// Main loop: load one byte, write another.
// The bits are rotating through the bit buffer.
for i := c / 8; i > 0; i-- {
bits |= uintptr(*src) << nbits
src = add1(src)
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
// Final src fragment.
if c %= 8; c > 0 {
bits |= (uintptr(*src) & (1<<c - 1)) << nbits
nbits += c
}
}
// Write any final bits out, using full-byte writes, even for the final byte.
totalBits := (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
nbits += -nbits & 7
for ; nbits > 0; nbits -= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
return totalBits
}
// materializeGCProg allocates space for the (1-bit) pointer bitmask
// for an object of size ptrdata. Then it fills that space with the
// pointer bitmask specified by the program prog.
// The bitmask starts at s.startAddr.
// The result must be deallocated with dematerializeGCProg.
func materializeGCProg(ptrdata uintptr, prog *byte) *mspan {
// Each word of ptrdata needs one bit in the bitmap.
bitmapBytes := divRoundUp(ptrdata, 8*goarch.PtrSize)
// Compute the number of pages needed for bitmapBytes.
pages := divRoundUp(bitmapBytes, pageSize)
s := mheap_.allocManual(pages, spanAllocPtrScalarBits)
runGCProg(addb(prog, 4), (*byte)(unsafe.Pointer(s.startAddr)))
return s
}
func dematerializeGCProg(s *mspan) {
mheap_.freeManual(s, spanAllocPtrScalarBits)
}
func dumpGCProg(p *byte) {
nptr := 0
for {
x := *p
p = add1(p)
if x == 0 {
print("\t", nptr, " end\n")
break
}
if x&0x80 == 0 {
print("\t", nptr, " lit ", x, ":")
n := int(x+7) / 8
for i := 0; i < n; i++ {
print(" ", hex(*p))
p = add1(p)
}
print("\n")
nptr += int(x)
} else {
nbit := int(x &^ 0x80)
if nbit == 0 {
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
nbit |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
}
count := 0
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
count |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
nptr += nbit * count
}
}
}
// Testing.
// reflect_gcbits returns the GC type info for x, for testing.
// The result is the bitmap entries (0 or 1), one entry per byte.
//
//go:linkname reflect_gcbits reflect.gcbits
func reflect_gcbits(x any) []byte {
return getgcmask(x)
}
// Returns GC type info for the pointer stored in ep for testing.
// If ep points to the stack, only static live information will be returned
// (i.e. not for objects which are only dynamically live stack objects).
func getgcmask(ep any) (mask []byte) {
e := *efaceOf(&ep)
p := e.data
t := e._type
// data or bss
for _, datap := range activeModules() {
// data
if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
bitmap := datap.gcdatamask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
mask = make([]byte, n/goarch.PtrSize)
for i := uintptr(0); i < n; i += goarch.PtrSize {
off := (uintptr(p) + i - datap.data) / goarch.PtrSize
mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
// bss
if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
bitmap := datap.gcbssmask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
mask = make([]byte, n/goarch.PtrSize)
for i := uintptr(0); i < n; i += goarch.PtrSize {
off := (uintptr(p) + i - datap.bss) / goarch.PtrSize
mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
}
// heap
if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
if s.spanclass.noscan() {
return nil
}
n := s.elemsize
hbits := heapBitsForAddr(base, n)
mask = make([]byte, n/goarch.PtrSize)
for {
var addr uintptr
if hbits, addr = hbits.next(); addr == 0 {
break
}
mask[(addr-base)/goarch.PtrSize] = 1
}
// Callers expect this mask to end at the last pointer.
for len(mask) > 0 && mask[len(mask)-1] == 0 {
mask = mask[:len(mask)-1]
}
return
}
// stack
if gp := getg(); gp.m.curg.stack.lo <= uintptr(p) && uintptr(p) < gp.m.curg.stack.hi {
found := false
var u unwinder
for u.initAt(gp.m.curg.sched.pc, gp.m.curg.sched.sp, 0, gp.m.curg, 0); u.valid(); u.next() {
if u.frame.sp <= uintptr(p) && uintptr(p) < u.frame.varp {
found = true
break
}
}
if found {
locals, _, _ := u.frame.getStackMap(nil, false)
if locals.n == 0 {
return
}
size := uintptr(locals.n) * goarch.PtrSize
n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
mask = make([]byte, n/goarch.PtrSize)
for i := uintptr(0); i < n; i += goarch.PtrSize {
off := (uintptr(p) + i - u.frame.varp + size) / goarch.PtrSize
mask[i/goarch.PtrSize] = locals.ptrbit(off)
}
}
return
}
// otherwise, not something the GC knows about.
// possibly read-only data, like malloc(0).
// must not have pointers
return
}