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// 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 bitmaps
//
// 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 at the end of a span for small objects and is unrolled at
// runtime from type metadata for all larger objects. Objects without
// pointers have neither a bitmap nor associated type metadata.
//
// Bits in all cases correspond to words in little-endian order.
//
// For small objects, if s is the mspan for the span starting at "start",
// then s.heapBits() returns a slice containing the bitmap for the whole span.
// That is, s.heapBits()[0] holds the goarch.PtrSize*8 bits for the first
// goarch.PtrSize*8 words from "start" through "start+63*ptrSize" in the span.
// On a related note, small objects are always small enough that their bitmap
// fits in goarch.PtrSize*8 bits, so writing out bitmap data takes two bitmap
// writes at most (because object boundaries don't generally lie on
// s.heapBits()[i] boundaries).
//
// For larger objects, if t is the type for the object starting at "start",
// within some span whose mspan is s, then the bitmap at t.GCData is "tiled"
// from "start" through "start+s.elemsize".
// Specifically, the first bit of t.GCData corresponds to the word at "start",
// the second to the word after "start", and so on up to t.PtrBytes. At t.PtrBytes,
// we skip to "start+t.Size_" and begin again from there. This process is
// repeated until we hit "start+s.elemsize".
// This tiling algorithm supports array data, since the type always refers to
// the element type of the array. Single objects are considered the same as
// single-element arrays.
// The tiling algorithm may scan data past the end of the compiler-recognized
// object, but any unused data within the allocation slot (i.e. within s.elemsize)
// is zeroed, so the GC just observes nil pointers.
// Note that this "tiled" bitmap isn't stored anywhere; it is generated on-the-fly.
//
// For objects without their own span, the type metadata is stored in the first
// word before the object at the beginning of the allocation slot. For objects
// with their own span, the type metadata is stored in the mspan.
//
// The bitmap for small unallocated objects in scannable spans is not maintained
// (can be junk).
package runtime
import (
"internal/abi"
"internal/goarch"
"runtime/internal/sys"
"unsafe"
)
const (
// A malloc header is functionally a single type pointer, but
// we need to use 8 here to ensure 8-byte alignment of allocations
// on 32-bit platforms. It's wasteful, but a lot of code relies on
// 8-byte alignment for 8-byte atomics.
mallocHeaderSize = 8
// The minimum object size that has a malloc header, exclusive.
//
// The size of this value controls overheads from the malloc header.
// The minimum size is bound by writeHeapBitsSmall, which assumes that the
// pointer bitmap for objects of a size smaller than this doesn't cross
// more than one pointer-word boundary. This sets an upper-bound on this
// value at the number of bits in a uintptr, multiplied by the pointer
// size in bytes.
//
// We choose a value here that has a natural cutover point in terms of memory
// overheads. This value just happens to be the maximum possible value this
// can be.
//
// A span with heap bits in it will have 128 bytes of heap bits on 64-bit
// platforms, and 256 bytes of heap bits on 32-bit platforms. The first size
// class where malloc headers match this overhead for 64-bit platforms is
// 512 bytes (8 KiB / 512 bytes * 8 bytes-per-header = 128 bytes of overhead).
// On 32-bit platforms, this same point is the 256 byte size class
// (8 KiB / 256 bytes * 8 bytes-per-header = 256 bytes of overhead).
//
// Guaranteed to be exactly at a size class boundary. The reason this value is
// an exclusive minimum is subtle. Suppose we're allocating a 504-byte object
// and its rounded up to 512 bytes for the size class. If minSizeForMallocHeader
// is 512 and an inclusive minimum, then a comparison against minSizeForMallocHeader
// by the two values would produce different results. In other words, the comparison
// would not be invariant to size-class rounding. Eschewing this property means a
// more complex check or possibly storing additional state to determine whether a
// span has malloc headers.
minSizeForMallocHeader = goarch.PtrSize * ptrBits
)
// heapBitsInSpan returns true if the size of an object implies its ptr/scalar
// data is stored at the end of the span, and is accessible via span.heapBits.
//
// Note: this works for both rounded-up sizes (span.elemsize) and unrounded
// type sizes because minSizeForMallocHeader is guaranteed to be at a size
// class boundary.
//
//go:nosplit
func heapBitsInSpan(userSize uintptr) bool {
// N.B. minSizeForMallocHeader is an exclusive minimum so that this function is
// invariant under size-class rounding on its input.
return userSize <= minSizeForMallocHeader
}
// heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC.
type heapArenaPtrScalar struct {
// N.B. This is no longer necessary with allocation headers.
}
// typePointers is an iterator over the pointers in a heap object.
//
// Iteration through this type implements the tiling algorithm described at the
// top of this file.
type typePointers struct {
// elem is the address of the current array element of type typ being iterated over.
// Objects that are not arrays are treated as single-element arrays, in which case
// this value does not change.
elem uintptr
// addr is the address the iterator is currently working from and describes
// the address of the first word referenced by mask.
addr uintptr
// mask is a bitmask where each bit corresponds to pointer-words after addr.
// Bit 0 is the pointer-word at addr, Bit 1 is the next word, and so on.
// If a bit is 1, then there is a pointer at that word.
// nextFast and next mask out bits in this mask as their pointers are processed.
mask uintptr
// typ is a pointer to the type information for the heap object's type.
// This may be nil if the object is in a span where heapBitsInSpan(span.elemsize) is true.
typ *_type
}
// typePointersOf returns an iterator over all heap pointers in the range [addr, addr+size).
//
// addr and addr+size must be in the range [span.base(), span.limit).
//
// Note: addr+size must be passed as the limit argument to the iterator's next method on
// each iteration. This slightly awkward API is to allow typePointers to be destructured
// by the compiler.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (span *mspan) typePointersOf(addr, size uintptr) typePointers {
base := span.objBase(addr)
tp := span.typePointersOfUnchecked(base)
if base == addr && size == span.elemsize {
return tp
}
return tp.fastForward(addr-tp.addr, addr+size)
}
// typePointersOfUnchecked is like typePointersOf, but assumes addr is the base
// of an allocation slot in a span (the start of the object if no header, the
// header otherwise). It returns an iterator that generates all pointers
// in the range [addr, addr+span.elemsize).
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (span *mspan) typePointersOfUnchecked(addr uintptr) typePointers {
const doubleCheck = false
if doubleCheck && span.objBase(addr) != addr {
print("runtime: addr=", addr, " base=", span.objBase(addr), "\n")
throw("typePointersOfUnchecked consisting of non-base-address for object")
}
spc := span.spanclass
if spc.noscan() {
return typePointers{}
}
if heapBitsInSpan(span.elemsize) {
// Handle header-less objects.
return typePointers{elem: addr, addr: addr, mask: span.heapBitsSmallForAddr(addr)}
}
// All of these objects have a header.
var typ *_type
if spc.sizeclass() != 0 {
// Pull the allocation header from the first word of the object.
typ = *(**_type)(unsafe.Pointer(addr))
addr += mallocHeaderSize
} else {
typ = span.largeType
}
gcdata := typ.GCData
return typePointers{elem: addr, addr: addr, mask: readUintptr(gcdata), typ: typ}
}
// typePointersOfType is like typePointersOf, but assumes addr points to one or more
// contiguous instances of the provided type. The provided type must not be nil and
// it must not have its type metadata encoded as a gcprog.
//
// It returns an iterator that tiles typ.GCData starting from addr. It's the caller's
// responsibility to limit iteration.
//
// nosplit because its callers are nosplit and require all their callees to be nosplit.
//
//go:nosplit
func (span *mspan) typePointersOfType(typ *abi.Type, addr uintptr) typePointers {
const doubleCheck = false
if doubleCheck && (typ == nil || typ.Kind_&kindGCProg != 0) {
throw("bad type passed to typePointersOfType")
}
if span.spanclass.noscan() {
return typePointers{}
}
// Since we have the type, pretend we have a header.
gcdata := typ.GCData
return typePointers{elem: addr, addr: addr, mask: readUintptr(gcdata), typ: typ}
}
// nextFast is the fast path of next. nextFast is written to be inlineable and,
// as the name implies, fast.
//
// Callers that are performance-critical should iterate using the following
// pattern:
//
// for {
// var addr uintptr
// if tp, addr = tp.nextFast(); addr == 0 {
// if tp, addr = tp.next(limit); addr == 0 {
// break
// }
// }
// // Use addr.
// ...
// }
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) nextFast() (typePointers, uintptr) {
// TESTQ/JEQ
if tp.mask == 0 {
return tp, 0
}
// BSFQ
var i int
if goarch.PtrSize == 8 {
i = sys.TrailingZeros64(uint64(tp.mask))
} else {
i = sys.TrailingZeros32(uint32(tp.mask))
}
// BTCQ
tp.mask ^= uintptr(1) << (i & (ptrBits - 1))
// LEAQ (XX)(XX*8)
return tp, tp.addr + uintptr(i)*goarch.PtrSize
}
// next advances the pointers iterator, returning the updated iterator and
// the address of the next pointer.
//
// limit must be the same each time it is passed to next.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) next(limit uintptr) (typePointers, uintptr) {
for {
if tp.mask != 0 {
return tp.nextFast()
}
// Stop if we don't actually have type information.
if tp.typ == nil {
return typePointers{}, 0
}
// Advance to the next element if necessary.
if tp.addr+goarch.PtrSize*ptrBits >= tp.elem+tp.typ.PtrBytes {
tp.elem += tp.typ.Size_
tp.addr = tp.elem
} else {
tp.addr += ptrBits * goarch.PtrSize
}
// Check if we've exceeded the limit with the last update.
if tp.addr >= limit {
return typePointers{}, 0
}
// Grab more bits and try again.
tp.mask = readUintptr(addb(tp.typ.GCData, (tp.addr-tp.elem)/goarch.PtrSize/8))
if tp.addr+goarch.PtrSize*ptrBits > limit {
bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
}
}
}
// fastForward moves the iterator forward by n bytes. n must be a multiple
// of goarch.PtrSize. limit must be the same limit passed to next for this
// iterator.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) fastForward(n, limit uintptr) typePointers {
// Basic bounds check.
target := tp.addr + n
if target >= limit {
return typePointers{}
}
if tp.typ == nil {
// Handle small objects.
// Clear any bits before the target address.
tp.mask &^= (1 << ((target - tp.addr) / goarch.PtrSize)) - 1
// Clear any bits past the limit.
if tp.addr+goarch.PtrSize*ptrBits > limit {
bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
}
return tp
}
// Move up elem and addr.
// Offsets within an element are always at a ptrBits*goarch.PtrSize boundary.
if n >= tp.typ.Size_ {
// elem needs to be moved to the element containing
// tp.addr + n.
oldelem := tp.elem
tp.elem += (tp.addr - tp.elem + n) / tp.typ.Size_ * tp.typ.Size_
tp.addr = tp.elem + alignDown(n-(tp.elem-oldelem), ptrBits*goarch.PtrSize)
} else {
tp.addr += alignDown(n, ptrBits*goarch.PtrSize)
}
if tp.addr-tp.elem >= tp.typ.PtrBytes {
// We're starting in the non-pointer area of an array.
// Move up to the next element.
tp.elem += tp.typ.Size_
tp.addr = tp.elem
tp.mask = readUintptr(tp.typ.GCData)
// We may have exceeded the limit after this. Bail just like next does.
if tp.addr >= limit {
return typePointers{}
}
} else {
// Grab the mask, but then clear any bits before the target address and any
// bits over the limit.
tp.mask = readUintptr(addb(tp.typ.GCData, (tp.addr-tp.elem)/goarch.PtrSize/8))
tp.mask &^= (1 << ((target - tp.addr) / goarch.PtrSize)) - 1
}
if tp.addr+goarch.PtrSize*ptrBits > limit {
bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
}
return tp
}
// objBase returns the base pointer for the object containing addr in span.
//
// Assumes that addr points into a valid part of span (span.base() <= addr < span.limit).
//
//go:nosplit
func (span *mspan) objBase(addr uintptr) uintptr {
return span.base() + span.objIndex(addr)*span.elemsize
}
// 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.
//
// Pointer data 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 typ argument is the type of the space at src and dst (and the
// element type if src and dst refer to arrays) and it is optional.
// If typ is nil, the barrier will still behave as expected and typ
// is used purely as an optimization. However, it must be used with
// care.
//
// If typ is not nil, then src and dst must point to one or more values
// of type typ. The caller must ensure that the ranges [src, src+size)
// and [dst, dst+size) refer to one or more whole values of type src and
// dst (leaving off the pointerless tail of the space is OK). If this
// precondition is not followed, this function will fail to scan the
// right pointers.
//
// When in doubt, pass nil for typ. That is safe and will always work.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr, typ *abi.Type) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.enabled {
return
}
s := spanOf(dst)
if 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
// Double-check that the bitmaps generated in the two possible paths match.
const doubleCheck = false
if doubleCheck {
doubleCheckTypePointersOfType(s, typ, dst, size)
}
var tp typePointers
if typ != nil && typ.Kind_&kindGCProg == 0 {
tp = s.typePointersOfType(typ, dst)
} else {
tp = s.typePointersOf(dst, size)
}
if src == 0 {
for {
var addr uintptr
if tp, addr = tp.next(dst + size); addr == 0 {
break
}
dstx := (*uintptr)(unsafe.Pointer(addr))
p := buf.get1()
p[0] = *dstx
}
} else {
for {
var addr uintptr
if tp, addr = tp.next(dst + size); 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.
// See bulkBarrierPreWrite's comment for more details -- use this
// optimization with great care.
//
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr, typ *abi.Type) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.enabled {
return
}
buf := &getg().m.p.ptr().wbBuf
s := spanOf(dst)
// Double-check that the bitmaps generated in the two possible paths match.
const doubleCheck = false
if doubleCheck {
doubleCheckTypePointersOfType(s, typ, dst, size)
}
var tp typePointers
if typ != nil && typ.Kind_&kindGCProg == 0 {
tp = s.typePointersOfType(typ, dst)
} else {
tp = s.typePointersOf(dst, size)
}
for {
var addr uintptr
if tp, addr = tp.next(dst + size); addr == 0 {
break
}
srcx := (*uintptr)(unsafe.Pointer(addr - dst + src))
p := buf.get1()
p[0] = *srcx
}
}
// initHeapBits initializes the heap bitmap for a span.
//
// TODO(mknyszek): This should set the heap bits for single pointer
// allocations eagerly to avoid calling heapSetType at allocation time,
// just to write one bit.
func (s *mspan) initHeapBits(forceClear bool) {
if (!s.spanclass.noscan() && heapBitsInSpan(s.elemsize)) || s.isUserArenaChunk {
b := s.heapBits()
clear(b)
}
}
// bswapIfBigEndian swaps the byte order of the uintptr on goarch.BigEndian platforms,
// and leaves it alone elsewhere.
func bswapIfBigEndian(x uintptr) uintptr {
if goarch.BigEndian {
if goarch.PtrSize == 8 {
return uintptr(sys.Bswap64(uint64(x)))
}
return uintptr(sys.Bswap32(uint32(x)))
}
return x
}
type writeUserArenaHeapBits struct {
offset uintptr // offset in span 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 (s *mspan) writeUserArenaHeapBits(addr uintptr) (h writeUserArenaHeapBits) {
offset := addr - s.base()
// 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 = offset / goarch.PtrSize % ptrBits
// round down to heap word that starts the bitmap word.
h.offset = offset - 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 writeUserArenaHeapBits) write(s *mspan, bits, valid uintptr) writeUserArenaHeapBits {
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.
idx := h.offset / (ptrBits * goarch.PtrSize)
m := uintptr(1)<<h.low - 1
bitmap := s.heapBits()
bitmap[idx] = bswapIfBigEndian(bswapIfBigEndian(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.
// Move to next word of bitmap.
h.offset += ptrBits * goarch.PtrSize
h.low = 0
return h
}
// Add padding of size bytes.
func (h writeUserArenaHeapBits) pad(s *mspan, size uintptr) writeUserArenaHeapBits {
if size == 0 {
return h
}
words := size / goarch.PtrSize
for words > ptrBits {
h = h.write(s, 0, ptrBits)
words -= ptrBits
}
return h.write(s, 0, words)
}
// Flush the bits that have been written, and add zeros as needed
// to cover the full object [addr, addr+size).
func (h writeUserArenaHeapBits) flush(s *mspan, addr, size uintptr) {
offset := addr - s.base()
// 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 := (offset+size-h.offset)/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.
bitmap := s.heapBits()
idx := h.offset / (ptrBits * goarch.PtrSize)
// 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"
bitmap[idx] = bswapIfBigEndian(bswapIfBigEndian(bitmap[idx])&m | h.mask)
}
if zeros == 0 {
return
}
// Advance to next bitmap word.
h.offset += 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.
idx := h.offset / (ptrBits * goarch.PtrSize)
if zeros < ptrBits {
bitmap[idx] = bswapIfBigEndian(bswapIfBigEndian(bitmap[idx]) &^ (uintptr(1)<<zeros - 1))
break
} else if zeros == ptrBits {
bitmap[idx] = 0
break
} else {
bitmap[idx] = 0
zeros -= ptrBits
}
h.offset += ptrBits * goarch.PtrSize
}
}
// heapBits returns the heap ptr/scalar bits stored at the end of the span for
// small object spans and heap arena spans.
//
// Note that the uintptr of each element means something different for small object
// spans and for heap arena spans. Small object spans are easy: they're never interpreted
// as anything but uintptr, so they're immune to differences in endianness. However, the
// heapBits for user arena spans is exposed through a dummy type descriptor, so the byte
// ordering needs to match the same byte ordering the compiler would emit. The compiler always
// emits the bitmap data in little endian byte ordering, so on big endian platforms these
// uintptrs will have their byte orders swapped from what they normally would be.
//
// heapBitsInSpan(span.elemsize) or span.isUserArenaChunk must be true.
//
//go:nosplit
func (span *mspan) heapBits() []uintptr {
const doubleCheck = false
if doubleCheck && !span.isUserArenaChunk {
if span.spanclass.noscan() {
throw("heapBits called for noscan")
}
if span.elemsize > minSizeForMallocHeader {
throw("heapBits called for span class that should have a malloc header")
}
}
// Find the bitmap at the end of the span.
//
// Nearly every span with heap bits is exactly one page in size. Arenas are the only exception.
if span.npages == 1 {
// This will be inlined and constant-folded down.
return heapBitsSlice(span.base(), pageSize)
}
return heapBitsSlice(span.base(), span.npages*pageSize)
}
// Helper for constructing a slice for the span's heap bits.
//
//go:nosplit
func heapBitsSlice(spanBase, spanSize uintptr) []uintptr {
bitmapSize := spanSize / goarch.PtrSize / 8
elems := int(bitmapSize / goarch.PtrSize)
var sl notInHeapSlice
sl = notInHeapSlice{(*notInHeap)(unsafe.Pointer(spanBase + spanSize - bitmapSize)), elems, elems}
return *(*[]uintptr)(unsafe.Pointer(&sl))
}
// heapBitsSmallForAddr loads the heap bits for the object stored at addr from span.heapBits.
//
// addr must be the base pointer of an object in the span. heapBitsInSpan(span.elemsize)
// must be true.
//
//go:nosplit
func (span *mspan) heapBitsSmallForAddr(addr uintptr) uintptr {
spanSize := span.npages * pageSize
bitmapSize := spanSize / goarch.PtrSize / 8
hbits := (*byte)(unsafe.Pointer(span.base() + spanSize - bitmapSize))
// These objects are always small enough that their bitmaps
// fit in a single word, so just load the word or two we need.
//
// Mirrors mspan.writeHeapBitsSmall.
//
// We should be using heapBits(), but unfortunately it introduces
// both bounds checks panics and throw which causes us to exceed
// the nosplit limit in quite a few cases.
i := (addr - span.base()) / goarch.PtrSize / ptrBits
j := (addr - span.base()) / goarch.PtrSize % ptrBits
bits := span.elemsize / goarch.PtrSize
word0 := (*uintptr)(unsafe.Pointer(addb(hbits, goarch.PtrSize*(i+0))))
word1 := (*uintptr)(unsafe.Pointer(addb(hbits, goarch.PtrSize*(i+1))))
var read uintptr
if j+bits > ptrBits {
// Two reads.
bits0 := ptrBits - j
bits1 := bits - bits0
read = *word0 >> j
read |= (*word1 & ((1 << bits1) - 1)) << bits0
} else {
// One read.
read = (*word0 >> j) & ((1 << bits) - 1)
}
return read
}
// writeHeapBitsSmall writes the heap bits for small objects whose ptr/scalar data is
// stored as a bitmap at the end of the span.
//
// Assumes dataSize is <= ptrBits*goarch.PtrSize. x must be a pointer into the span.
// heapBitsInSpan(dataSize) must be true. dataSize must be >= typ.Size_.
//
//go:nosplit
func (span *mspan) writeHeapBitsSmall(x, dataSize uintptr, typ *_type) (scanSize uintptr) {
// The objects here are always really small, so a single load is sufficient.
src0 := readUintptr(typ.GCData)
// Create repetitions of the bitmap if we have a small array.
bits := span.elemsize / goarch.PtrSize
scanSize = typ.PtrBytes
src := src0
switch typ.Size_ {
case goarch.PtrSize:
src = (1 << (dataSize / goarch.PtrSize)) - 1
default:
for i := typ.Size_; i < dataSize; i += typ.Size_ {
src |= src0 << (i / goarch.PtrSize)
scanSize += typ.Size_
}
}
// Since we're never writing more than one uintptr's worth of bits, we're either going
// to do one or two writes.
dst := span.heapBits()
o := (x - span.base()) / goarch.PtrSize
i := o / ptrBits
j := o % ptrBits
if j+bits > ptrBits {
// Two writes.
bits0 := ptrBits - j
bits1 := bits - bits0
dst[i+0] = dst[i+0]&(^uintptr(0)>>bits0) | (src << j)
dst[i+1] = dst[i+1]&^((1<<bits1)-1) | (src >> bits0)
} else {
// One write.
dst[i] = (dst[i] &^ (((1 << bits) - 1) << j)) | (src << j)
}
const doubleCheck = false
if doubleCheck {
srcRead := span.heapBitsSmallForAddr(x)
if srcRead != src {
print("runtime: x=", hex(x), " i=", i, " j=", j, " bits=", bits, "\n")
print("runtime: dataSize=", dataSize, " typ.Size_=", typ.Size_, " typ.PtrBytes=", typ.PtrBytes, "\n")
print("runtime: src0=", hex(src0), " src=", hex(src), " srcRead=", hex(srcRead), "\n")
throw("bad pointer bits written for small object")
}
}
return
}
// For !goexperiment.AllocHeaders.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
}
// heapSetType 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 heapSetType when there are no pointers.
//
// There can be read-write races between heapSetType and things
// that read the heap metadata like scanobject. However, since
// heapSetType 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
// shared memory that belongs 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 heapSetType(x, dataSize uintptr, typ *_type, header **_type, span *mspan) (scanSize uintptr) {
const doubleCheck = false
gctyp := typ
if header == nil {
if doubleCheck && (!heapBitsInSpan(dataSize) || !heapBitsInSpan(span.elemsize)) {
throw("tried to write heap bits, but no heap bits in span")
}
// Handle the case where we have no malloc header.
scanSize = span.writeHeapBitsSmall(x, dataSize, typ)
} else {
if typ.Kind_&kindGCProg != 0 {
// Allocate space to unroll the gcprog. This space will consist of
// a dummy _type value and the unrolled gcprog. The dummy _type will
// refer to the bitmap, and the mspan will refer to the dummy _type.
if span.spanclass.sizeclass() != 0 {
throw("GCProg for type that isn't large")
}
spaceNeeded := alignUp(unsafe.Sizeof(_type{}), goarch.PtrSize)
heapBitsOff := spaceNeeded
spaceNeeded += alignUp(typ.PtrBytes/goarch.PtrSize/8, goarch.PtrSize)
npages := alignUp(spaceNeeded, pageSize) / pageSize
var progSpan *mspan
systemstack(func() {
progSpan = mheap_.allocManual(npages, spanAllocPtrScalarBits)
memclrNoHeapPointers(unsafe.Pointer(progSpan.base()), progSpan.npages*pageSize)
})
// Write a dummy _type in the new space.
//
// We only need to write size, PtrBytes, and GCData, since that's all
// the GC cares about.
gctyp = (*_type)(unsafe.Pointer(progSpan.base()))
gctyp.Size_ = typ.Size_
gctyp.PtrBytes = typ.PtrBytes
gctyp.GCData = (*byte)(add(unsafe.Pointer(progSpan.base()), heapBitsOff))
gctyp.TFlag = abi.TFlagUnrolledBitmap
// Expand the GC program into space reserved at the end of the new span.
runGCProg(addb(typ.GCData, 4), gctyp.GCData)
}
// Write out the header.
*header = gctyp
scanSize = span.elemsize
}
if doubleCheck {
doubleCheckHeapPointers(x, dataSize, gctyp, header, span)
// To exercise the less common path more often, generate
// a random interior pointer and make sure iterating from
// that point works correctly too.
maxIterBytes := span.elemsize
if header == nil {
maxIterBytes = dataSize
}
off := alignUp(uintptr(cheaprand())%dataSize, goarch.PtrSize)
size := dataSize - off
if size == 0 {
off -= goarch.PtrSize
size += goarch.PtrSize
}
interior := x + off
size -= alignDown(uintptr(cheaprand())%size, goarch.PtrSize)
if size == 0 {
size = goarch.PtrSize
}
// Round up the type to the size of the type.
size = (size + gctyp.Size_ - 1) / gctyp.Size_ * gctyp.Size_
if interior+size > x+maxIterBytes {
size = x + maxIterBytes - interior
}
doubleCheckHeapPointersInterior(x, interior, size, dataSize, gctyp, header, span)
}
return
}
func doubleCheckHeapPointers(x, dataSize uintptr, typ *_type, header **_type, span *mspan) {
// Check that scanning the full object works.
tp := span.typePointersOfUnchecked(span.objBase(x))
maxIterBytes := span.elemsize
if header == nil {
maxIterBytes = dataSize
}
bad := false
for i := uintptr(0); i < maxIterBytes; i += goarch.PtrSize {
// Compute the pointer bit we want at offset i.
want := false
if i < span.elemsize {
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
tp, addr = tp.next(x + span.elemsize)
if addr == 0 {
println("runtime: found bad iterator")
}
if addr != x+i {
print("runtime: addr=", hex(addr), " x+i=", hex(x+i), "\n")
bad = true
}
}
}
if !bad {
var addr uintptr
tp, addr = tp.next(x + span.elemsize)
if addr == 0 {
return
}
println("runtime: extra pointer:", hex(addr))
}
print("runtime: hasHeader=", header != nil, " typ.Size_=", typ.Size_, " hasGCProg=", typ.Kind_&kindGCProg != 0, "\n")
print("runtime: x=", hex(x), " dataSize=", dataSize, " elemsize=", span.elemsize, "\n")
print("runtime: typ=", unsafe.Pointer(typ), " typ.PtrBytes=", typ.PtrBytes, "\n")
print("runtime: limit=", hex(x+span.elemsize), "\n")
tp = span.typePointersOfUnchecked(x)
dumpTypePointers(tp)
for {
var addr uintptr
if tp, addr = tp.next(x + span.elemsize); addr == 0 {
println("runtime: would've stopped here")
dumpTypePointers(tp)
break
}
print("runtime: addr=", hex(addr), "\n")
dumpTypePointers(tp)
}
throw("heapSetType: pointer entry not correct")
}
func doubleCheckHeapPointersInterior(x, interior, size, dataSize uintptr, typ *_type, header **_type, span *mspan) {
bad := false
if interior < x {
print("runtime: interior=", hex(interior), " x=", hex(x), "\n")
throw("found bad interior pointer")
}
off := interior - x
tp := span.typePointersOf(interior, size)
for i := off; i < off+size; i += goarch.PtrSize {
// Compute the pointer bit we want at offset i.
want := false
if i < span.elemsize {
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
tp, addr = tp.next(interior + size)
if addr == 0 {
println("runtime: found bad iterator")
bad = true
}
if addr != x+i {
print("runtime: addr=", hex(addr), " x+i=", hex(x+i), "\n")
bad = true
}
}
}
if !bad {
var addr uintptr
tp, addr = tp.next(interior + size)
if addr == 0 {
return
}
println("runtime: extra pointer:", hex(addr))
}
print("runtime: hasHeader=", header != nil, " typ.Size_=", typ.Size_, "\n")
print("runtime: x=", hex(x), " dataSize=", dataSize, " elemsize=", span.elemsize, " interior=", hex(interior), " size=", size, "\n")
print("runtime: limit=", hex(interior+size), "\n")
tp = span.typePointersOf(interior, size)
dumpTypePointers(tp)
for {
var addr uintptr
if tp, addr = tp.next(interior + size); addr == 0 {
println("runtime: would've stopped here")
dumpTypePointers(tp)
break
}
print("runtime: addr=", hex(addr), "\n")
dumpTypePointers(tp)
}
print("runtime: want: ")
for i := off; i < off+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 {
print("1")
} else {
print("0")
}
}
println()
throw("heapSetType: pointer entry not correct")
}
//go:nosplit
func doubleCheckTypePointersOfType(s *mspan, typ *_type, addr, size uintptr) {
if typ == nil || typ.Kind_&kindGCProg != 0 {
return
}
if typ.Kind_&kindMask == kindInterface {
// Interfaces are unfortunately inconsistently handled
// when it comes to the type pointer, so it's easy to
// produce a lot of false positives here.
return
}
tp0 := s.typePointersOfType(typ, addr)
tp1 := s.typePointersOf(addr, size)
failed := false
for {
var addr0, addr1 uintptr
tp0, addr0 = tp0.next(addr + size)
tp1, addr1 = tp1.next(addr + size)
if addr0 != addr1 {
failed = true
break
}
if addr0 == 0 {
break
}
}
if failed {
tp0 := s.typePointersOfType(typ, addr)
tp1 := s.typePointersOf(addr, size)
print("runtime: addr=", hex(addr), " size=", size, "\n")
print("runtime: type=", toRType(typ).string(), "\n")
dumpTypePointers(tp0)
dumpTypePointers(tp1)
for {
var addr0, addr1 uintptr
tp0, addr0 = tp0.next(addr + size)
tp1, addr1 = tp1.next(addr + size)
print("runtime: ", hex(addr0), " ", hex(addr1), "\n")
if addr0 == 0 && addr1 == 0 {
break
}
}
throw("mismatch between typePointersOfType and typePointersOf")
}
}
func dumpTypePointers(tp typePointers) {
print("runtime: tp.elem=", hex(tp.elem), " tp.typ=", unsafe.Pointer(tp.typ), "\n")
print("runtime: tp.addr=", hex(tp.addr), " tp.mask=")
for i := uintptr(0); i < ptrBits; i++ {
if tp.mask&(uintptr(1)<<i) != 0 {
print("1")
} else {
print("0")
}
}
println()
}
// 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
var et *_type
if t.Kind_&kindMask != kindPtr {
throw("bad argument to getgcmask: expected type to be a pointer to the value type whose mask is being queried")
}
et = (*ptrtype)(unsafe.Pointer(t)).Elem
// data or bss
for _, datap := range activeModules() {
// data
if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
bitmap := datap.gcdatamask.bytedata
n := et.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 := et.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
}
limit := base + s.elemsize
// Move the base up to the iterator's start, because
// we want to hide evidence of a malloc header from the
// caller.
tp := s.typePointersOfUnchecked(base)
base = tp.addr
// Unroll the full bitmap the GC would actually observe.
maskFromHeap := make([]byte, (limit-base)/goarch.PtrSize)
for {
var addr uintptr
if tp, addr = tp.next(limit); addr == 0 {
break
}
maskFromHeap[(addr-base)/goarch.PtrSize] = 1
}
// Double-check that every part of the ptr/scalar we're not
// showing the caller is zeroed. This keeps us honest that
// that information is actually irrelevant.
for i := limit; i < s.elemsize; i++ {
if *(*byte)(unsafe.Pointer(i)) != 0 {
throw("found non-zeroed tail of allocation")
}
}
// Callers (and a check we're about to run) expects this mask
// to end at the last pointer.
for len(maskFromHeap) > 0 && maskFromHeap[len(maskFromHeap)-1] == 0 {
maskFromHeap = maskFromHeap[:len(maskFromHeap)-1]
}
if et.Kind_&kindGCProg == 0 {
// Unroll again, but this time from the type information.
maskFromType := make([]byte, (limit-base)/goarch.PtrSize)
tp = s.typePointersOfType(et, base)
for {
var addr uintptr
if tp, addr = tp.next(limit); addr == 0 {
break
}
maskFromType[(addr-base)/goarch.PtrSize] = 1
}
// Validate that the prefix of maskFromType is equal to
// maskFromHeap. maskFromType may contain more pointers than
// maskFromHeap produces because maskFromHeap may be able to
// get exact type information for certain classes of objects.
// With maskFromType, we're always just tiling the type bitmap
// through to the elemsize.
//
// It's OK if maskFromType has pointers in elemsize that extend
// past the actual populated space; we checked above that all
// that space is zeroed, so just the GC will just see nil pointers.
differs := false
for i := range maskFromHeap {
if maskFromHeap[i] != maskFromType[i] {
differs = true
break
}
}
if differs {
print("runtime: heap mask=")
for _, b := range maskFromHeap {
print(b)
}
println()
print("runtime: type mask=")
for _, b := range maskFromType {
print(b)
}
println()
print("runtime: type=", toRType(et).string(), "\n")
throw("found two different masks from two different methods")
}
}
// Select the heap mask to return. We may not have a type mask.
mask = maskFromHeap
// 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 heapSetType but for
// non-slice-backing-store Go values allocated in a user arena chunk. It
// sets up the type metadata 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 := s.writeUserArenaHeapBits(uintptr(ptr))
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
}
// N.B. On big endian platforms we byte swap the data that we
// read from GCData, which is always stored in little-endian order
// by the compiler. writeUserArenaHeapBits handles data in
// a platform-ordered way for efficiency, but stores back the
// data in little endian order, since we expose the bitmap through
// a dummy type.
h = h.write(s, 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(s, typ.Size_-typ.PtrBytes)
h.flush(s, uintptr(ptr), typ.Size_)
if typ.Kind_&kindGCProg != 0 {
// Zero out temporary ptrmask buffer inside object.
memclrNoHeapPointers(ptr, (gcProgBits+7)/8)
}
// Update the PtrBytes value in the type information. After this
// point, the GC will observe the new bitmap.
s.largeType.PtrBytes = uintptr(ptr) - base + typ.PtrBytes
// Double-check that the bitmap was written out correctly.
const doubleCheck = false
if doubleCheck {
doubleCheckHeapPointersInterior(uintptr(ptr), uintptr(ptr), typ.Size_, typ.Size_, typ, &s.largeType, s)
}
}
// For !goexperiment.AllocHeaders, to pass TestIntendedInlining.
func writeHeapBitsForAddr() {
panic("not implemented")
}
// For !goexperiment.AllocHeaders.
type heapBits struct {
}
// For !goexperiment.AllocHeaders.
//
//go:nosplit
func heapBitsForAddr(addr, size uintptr) heapBits {
panic("not implemented")
}
// For !goexperiment.AllocHeaders.
//
//go:nosplit
func (h heapBits) next() (heapBits, uintptr) {
panic("not implemented")
}
// For !goexperiment.AllocHeaders.
//
//go:nosplit
func (h heapBits) nextFast() (heapBits, uintptr) {
panic("not implemented")
}