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Diffstat (limited to 'src/runtime/mbitmap.go')
| -rw-r--r-- | src/runtime/mbitmap.go | 2026 |
1 files changed, 2026 insertions, 0 deletions
diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go new file mode 100644 index 0000000..fbfaae0 --- /dev/null +++ b/src/runtime/mbitmap.go @@ -0,0 +1,2026 @@ +// 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 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 is a pointer/scalar bit, just +// like in the stack/data bitmaps described above. The upper bit +// indicates scan/dead: a "1" value ("scan") indicates that there may +// be pointers in later words of the allocation, and a "0" value +// ("dead") indicates there are no more pointers in the allocation. If +// the upper bit is 0, the lower bit must also be 0, and this +// indicates scanning can ignore the rest of the allocation. +// +// The 2-bit entries are split when written into the byte, so that the top half +// of the byte contains 4 high (scan) 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 means scalar/dead. This property 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. + +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 +} + +// 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. +// 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 + } +} + +// 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 +} + +// 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 + + const ( + mask1 = bitPointer | bitScan // 00010001 + mask2 = bitPointer | bitScan | mask1<<heapBitsShift // 00110011 + mask3 = bitPointer | bitScan | mask2<<heapBitsShift // 01110111 + ) + + // 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) + + // 2-word objects only have 4 bitmap bits and 3-word objects only have 6 bitmap bits. + // Therefore, these objects share a heap bitmap byte with the objects next to them. + // These are called out as a special case primarily so the code below can assume all + // objects are at least 4 words long and that their bitmaps start either at the beginning + // of a bitmap byte, or half-way in (h.shift of 0 and 2 respectively). + + 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 array of pointer. + *h.bitp |= (bitPointer | bitScan | (bitPointer|bitScan)<<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 + hb |= bitScanAll & ((bitScan << (typ.ptrdata / sys.PtrSize)) - 1) + // Clear the bits for this object so we can set the + // appropriate ones. + *h.bitp &^= (bitPointer | bitScan | ((bitPointer | bitScan) << heapBitsShift)) << h.shift + *h.bitp |= uint8(hb << h.shift) + return + } else if size == 3*sys.PtrSize { + b := uint8(*ptrmask) + if doubleCheck { + if b == 0 { + println("runtime: invalid type ", typ.string()) + throw("heapBitsSetType: called with non-pointer type") + } + if sys.PtrSize != 8 { + throw("heapBitsSetType: unexpected 3 pointer wide size class on 32 bit") + } + if typ.kind&kindGCProg != 0 { + throw("heapBitsSetType: unexpected GC prog for 3 pointer wide size class") + } + if typ.size == 2*sys.PtrSize { + print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, "\n") + throw("heapBitsSetType: inconsistent object sizes") + } + } + if typ.size == sys.PtrSize { + // The type contains a pointer otherwise heapBitsSetType wouldn't have been called. + // Since the type is only 1 pointer wide and contains a pointer, its gcdata must be exactly 1. + if doubleCheck && *typ.gcdata != 1 { + print("runtime: heapBitsSetType size=", size, " typ.size=", typ.size, "but *typ.gcdata", *typ.gcdata, "\n") + throw("heapBitsSetType: unexpected gcdata for 1 pointer wide type size in 3 pointer wide size class") + } + // 3 element array of pointers. Unrolling ptrmask 3 times into p yields 00000111. + b = 7 + } + + hb := b & 7 + // Set bitScan bits for all pointers. + hb |= hb << wordsPerBitmapByte + // First bitScan bit is always set since the type contains pointers. + hb |= bitScan + // Second bitScan bit needs to also be set if the third bitScan bit is set. + hb |= hb & (bitScan << (2 * heapBitsShift)) >> 1 + + // For h.shift > 1 heap bits cross a byte boundary and need to be written part + // to h.bitp and part to the next h.bitp. + switch h.shift { + case 0: + *h.bitp &^= mask3 << 0 + *h.bitp |= hb << 0 + case 1: + *h.bitp &^= mask3 << 1 + *h.bitp |= hb << 1 + case 2: + *h.bitp &^= mask2 << 2 + *h.bitp |= (hb & mask2) << 2 + // Two words written to the first byte. + // Advance two words to get to the next byte. + h = h.next().next() + *h.bitp &^= mask1 + *h.bitp |= (hb >> 2) & mask1 + case 3: + *h.bitp &^= mask1 << 3 + *h.bitp |= (hb & mask1) << 3 + // One word written to the first byte. + // Advance one word to get to the next byte. + h = h.next() + *h.bitp &^= mask2 + *h.bitp |= (hb >> 1) & mask2 + } + 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 + } + + // 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. + // + // This is a fast path for small objects. + // + // 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. + // In all following words, we set the scan/dead + // appropriately to indicate that the object continues + // to the next 2-bit entry in the bitmap. + // + // We set four bits at a time here, but if the object + // is fewer than four words, phase 3 will clear + // unnecessary bits. + hb = b & bitPointerAll + hb |= bitScanAll + if w += 4; w >= nw { + goto Phase3 + } + *hbitp = uint8(hb) + hbitp = add1(hbitp) + b >>= 4 + nb -= 4 + + case h.shift == 2: + // Ptrmask and heap bitmap are misaligned. + // + // On 32 bit architectures only the 6-word object that corresponds + // to a 24 bytes size class can start with h.shift of 2 here since + // all other non 16 byte aligned size classes have been handled by + // special code paths at the beginning of heapBitsSetType on 32 bit. + // + // Many size classes are only 16 byte aligned. On 64 bit architectures + // this results in a heap bitmap position starting with a h.shift of 2. + // + // 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, 2-word, and 3-word objects above, + // so this is at least a 6-word object. + hb = (b & (bitPointer | bitPointer<<heapBitsShift)) << (2 * heapBitsShift) + hb |= bitScan << (2 * heapBitsShift) + if nw > 1 { + hb |= bitScan << (3 * heapBitsShift) + } + b >>= 2 + nb -= 2 + *hbitp &^= uint8((bitPointer | bitScan | ((bitPointer | bitScan) << 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 and 3-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 || h.shift == 2) { + print("x=", x, " size=", size, " cnw=", h.shift, "\n") + throw("bad start shift") + } + } + if 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 advance it the way we do above. + end := heapBitsForAddr(x + size - sys.PtrSize) + if outOfPlace { + // In out-of-place copying, we just advance + // using next. + end = end.next() + } else { + // Don't use next because that may advance to + // the next arena and the in-place logic + // doesn't do that. + end.shift += heapBitsShift + if end.shift == 4*heapBitsShift { + end.bitp, end.shift = add1(end.bitp), 0 + } + } + 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 { + if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 { + // heapBitsSetTypeGCProg always fills + // in full nibbles of bitScan. + want = bitScan + } + } else { + if j < nptr && (*addb(ptrmask, j/8)>>(j%8))&1 != 0 { + want |= bitPointer + } + want |= 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 { + // Each word of ptrdata needs one bit in the bitmap. + bitmapBytes := divRoundUp(ptrdata, 8*sys.PtrSize) + // Compute the number of pages needed for bitmapBytes. + pages := divRoundUp(bitmapBytes, pageSize) + s := mheap_.allocManual(pages, spanAllocPtrScalarBits) + runGCProg(addb(prog, 4), nil, (*byte)(unsafe.Pointer(s.startAddr)), 1) + 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. + +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 !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 +} |