Editing: mbitmap_noallocheaders.go

// Copyright 2023 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. //go:build !goexperiment.allocheaders // 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/abi" "internal/goarch" "runtime/internal/sys" "unsafe" ) const ( // For compatibility with the allocheaders GOEXPERIMENT. mallocHeaderSize = 0 minSizeForMallocHeader = ^uintptr(0) ) // For compatibility with the allocheaders GOEXPERIMENT. // //go:nosplit func heapBitsInSpan(_ uintptr) bool { return false } // heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC. type heapArenaPtrScalar struct { // bitmap stores the pointer/scalar bitmap for the words in // this arena. See mbitmap.go for a description. // This array uses 1 bit per word of heap, or 1.6% of the heap size (for 64-bit). bitmap [heapArenaBitmapWords]uintptr // If the ith bit of noMorePtrs is true, then there are no more // pointers for the object containing the word described by the // high bit of bitmap[i]. // In that case, bitmap[i+1], ... must be zero until the start // of the next object. // We never operate on these entries using bit-parallel techniques, // so it is ok if they are small. Also, they can't be bigger than // uint16 because at that size a single noMorePtrs entry // represents 8K of memory, the minimum size of a span. Any larger // and we'd have to worry about concurrent updates. // This array uses 1 bit per word of bitmap, or .024% of the heap size (for 64-bit). noMorePtrs [heapArenaBitmapWords / 8]uint8 } // 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. // // The type of the space can be provided purely as an optimization, // however it is not used with GOEXPERIMENT=noallocheaders. // // Callers must perform cgo checks if goexperiment.CgoCheck2. // //go:nosplit func bulkBarrierPreWrite(dst, src, size uintptr, _ *abi.Type) { if (dst|src|size)&(goarch.PtrSize-1) != 0 { throw("bulkBarrierPreWrite: unaligned arguments") } if !writeBarrier.enabled { 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. // // The type of the space can be provided purely as an optimization, // however it is not used with GOEXPERIMENT=noallocheaders. // //go:nosplit func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr, _ *abi.Type) { if (dst|src|size)&(goarch.PtrSize-1) != 0 { throw("bulkBarrierPreWrite: unaligned arguments") } if !writeBarrier.enabled { 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 } } // 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) } 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 } } // 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") } } } // For goexperiment.AllocHeaders func heapSetType(x, dataSize uintptr, typ *_type, header **_type, span *mspan) (scanSize uintptr) { return 0 } // Testing. // 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] } // Make sure we keep ep alive. We may have stopped referencing // ep's data pointer sometime before this point and it's possible // for that memory to get freed. KeepAlive(ep) 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(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 } // userArenaHeapBitsSetType is the equivalent of heapBitsSetType but for // non-slice-backing-store Go values allocated in a user arena chunk. It // sets up the heap bitmap for the value with type typ allocated at address ptr. // base is the base address of the arena chunk. func userArenaHeapBitsSetType(typ *_type, ptr unsafe.Pointer, s *mspan) { base := s.base() h := writeHeapBitsForAddr(uintptr(ptr)) // Our last allocation might have ended right at a noMorePtrs mark, // which we would not have erased. We need to erase that mark here, // because we're going to start adding new heap bitmap bits. // We only need to clear one mark, because below we make sure to // pad out the bits with zeroes and only write one noMorePtrs bit // for each new object. // (This is only necessary at noMorePtrs boundaries, as noMorePtrs // marks within an object allocated with newAt will be erased by // the normal writeHeapBitsForAddr mechanism.) // // Note that we skip this if this is the first allocation in the // arena because there's definitely no previous noMorePtrs mark // (in fact, we *must* do this, because we're going to try to back // up a pointer to fix this up). if uintptr(ptr)%(8*goarch.PtrSize*goarch.PtrSize) == 0 && uintptr(ptr) != base { // Back up one pointer and rewrite that pointer. That will // cause the writeHeapBits implementation to clear the // noMorePtrs bit we need to clear. r := heapBitsForAddr(uintptr(ptr)-goarch.PtrSize, goarch.PtrSize) _, p := r.next() b := uintptr(0) if p == uintptr(ptr)-goarch.PtrSize { b = 1 } h = writeHeapBitsForAddr(uintptr(ptr) - goarch.PtrSize) h = h.write(b, 1) } p := typ.GCData // start of 1-bit pointer mask (or GC program) var gcProgBits uintptr if typ.Kind_&kindGCProg != 0 { // Expand gc program, using the object itself for storage. gcProgBits = runGCProg(addb(p, 4), (*byte)(ptr)) p = (*byte)(ptr) } nb := typ.PtrBytes / goarch.PtrSize for i := uintptr(0); i < nb; i += ptrBits { k := nb - i if k > ptrBits { k = ptrBits } h = h.write(readUintptr(addb(p, i/8)), k) } // Note: we call pad here to ensure we emit explicit 0 bits // for the pointerless tail of the object. This ensures that // there's only a single noMorePtrs mark for the next object // to clear. We don't need to do this to clear stale noMorePtrs // markers from previous uses because arena chunk pointer bitmaps // are always fully cleared when reused. h = h.pad(typ.Size_ - typ.PtrBytes) h.flush(uintptr(ptr), typ.Size_) if typ.Kind_&kindGCProg != 0 { // Zero out temporary ptrmask buffer inside object. memclrNoHeapPointers(ptr, (gcProgBits+7)/8) } // Double-check that the bitmap was written out correctly. // // Derived from heapBitsSetType. const doubleCheck = false if doubleCheck { size := typ.Size_ x := uintptr(ptr) h := heapBitsForAddr(x, size) for i := uintptr(0); i < size; i += goarch.PtrSize { // Compute the pointer bit we want at offset i. want := false 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("userArenaHeapBitsSetType: pointer entry not correct") } } } if _, addr := h.next(); addr != 0 { throw("userArenaHeapBitsSetType: extra pointer") } } } // For goexperiment.AllocHeaders. type typePointers struct { addr uintptr } // For goexperiment.AllocHeaders. // //go:nosplit func (span *mspan) typePointersOf(addr, size uintptr) typePointers { panic("not implemented") } // For goexperiment.AllocHeaders. // //go:nosplit func (span *mspan) typePointersOfUnchecked(addr uintptr) typePointers { panic("not implemented") } // For goexperiment.AllocHeaders. // //go:nosplit func (tp typePointers) nextFast() (typePointers, uintptr) { panic("not implemented") } // For goexperiment.AllocHeaders. // //go:nosplit func (tp typePointers) next(limit uintptr) (typePointers, uintptr) { panic("not implemented") } // For goexperiment.AllocHeaders. // //go:nosplit func (tp typePointers) fastForward(n, limit uintptr) typePointers { panic("not implemented") } // For goexperiment.AllocHeaders, to pass TestIntendedInlining. func (s *mspan) writeUserArenaHeapBits() { panic("not implemented") } // For goexperiment.AllocHeaders, to pass TestIntendedInlining. func heapBitsSlice() { panic("not implemented") }

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