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runtime: add the allocation headers GOEXPERIMENT and fork files 

This change adds the allocation headers GOEXPERIMENT which is a no-op.
It forks two runtime files temporarily to make the GOEXPERIMENT easier
to maintain. The forked files are mbitmap.go and msize.go.

Change-Id: I60202c00e614e4517de7dd000029cf80dd0121ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/537980
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Michael Anthony Knyszek 2023-10-28 16:17:02 +00:00 committed by Michael Knyszek
parent 9f63534858
commit 25867485a7
8 changed files with 1766 additions and 842 deletions
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8
src/internal/goexperiment/exp_allocheaders_off.go Normal file
View File

@ -0,0 +1,8 @@
// Code generated by mkconsts.go. DO NOT EDIT.
//go:build !goexperiment.allocheaders
package goexperiment
const AllocHeaders = false
const AllocHeadersInt = 0

8
src/internal/goexperiment/exp_allocheaders_on.go Normal file
View File

@ -0,0 +1,8 @@
// Code generated by mkconsts.go. DO NOT EDIT.
//go:build goexperiment.allocheaders
package goexperiment
const AllocHeaders = true
const AllocHeadersInt = 1

7
src/internal/goexperiment/flags.go
View File

@ -116,4 +116,11 @@ type Flags struct {
// RangeFunc enables range over func.
RangeFunc bool
// Range enables range over int and func.
Range bool
// AllocHeaders enables a different, more efficient way for the GC to
// manage heap metadata.
AllocHeaders bool
}

842
src/runtime/mbitmap.go
View File

@ -2,41 +2,6 @@
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Garbage collector: type and heap bitmaps.
//
// Stack, data, and bss bitmaps
//
// Stack frames and global variables in the data and bss sections are
// described by bitmaps with 1 bit per pointer-sized word. A "1" bit
// means the word is a live pointer to be visited by the GC (referred to
// as "pointer"). A "0" bit means the word should be ignored by GC
// (referred to as "scalar", though it could be a dead pointer value).
//
// Heap bitmap
//
// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting at "start",
// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
// through start+63*ptrSize, ha.bitmap[1] holds the entries for
// start+64*ptrSize through start+127*ptrSize, and so on.
// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
// (For 32-bit platforms, s/64/32/.)
//
// We also keep a noMorePtrs bitmap which allows us to stop scanning
// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
// has no more pointers beyond those described by ha.bitmap[8*i+j].
// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
// beyond must all be zero until the start of the next object.
//
// The bitmap for noscan spans is set to all zero at span allocation time.
//
// The bitmap for unallocated objects in scannable spans is not maintained
// (can be junk).
package runtime
import (
@ -46,27 +11,6 @@ import (
"unsafe"
)
// 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
}
// addb returns the byte pointer p+n.
//
//go:nowritebarrier
@ -404,251 +348,6 @@ func reflect_verifyNotInHeapPtr(p uintptr) bool {
const ptrBits = 8 * goarch.PtrSize
// heapBits provides access to the bitmap bits for a single heap word.
// The methods on heapBits take value receivers so that the compiler
// can more easily inline calls to those methods and registerize the
// struct fields independently.
type heapBits struct {
// heapBits will report on pointers in the range [addr,addr+size).
// The low bit of mask contains the pointerness of the word at addr
// (assuming valid>0).
addr, size uintptr
// The next few pointer bits representing words starting at addr.
// Those bits already returned by next() are zeroed.
mask uintptr
// Number of bits in mask that are valid. mask is always less than 1<<valid.
valid uintptr
}
// heapBitsForAddr returns the heapBits for the address addr.
// The caller must ensure [addr,addr+size) is in an allocated span.
// In particular, be careful not to point past the end of an object.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func heapBitsForAddr(addr, size uintptr) heapBits {
// Find arena
ai := arenaIndex(addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
// Word index in arena.
word := addr / goarch.PtrSize % heapArenaWords
// Word index and bit offset in bitmap array.
idx := word / ptrBits
off := word % ptrBits
// Grab relevant bits of bitmap.
mask := ha.bitmap[idx] >> off
valid := ptrBits - off
// Process depending on where the object ends.
nptr := size / goarch.PtrSize
if nptr < valid {
// Bits for this object end before the end of this bitmap word.
// Squash bits for the following objects.
mask &= 1<<(nptr&(ptrBits-1)) - 1
valid = nptr
} else if nptr == valid {
// Bits for this object end at exactly the end of this bitmap word.
// All good.
} else {
// Bits for this object extend into the next bitmap word. See if there
// may be any pointers recorded there.
if uintptr(ha.noMorePtrs[idx/8])>>(idx%8)&1 != 0 {
// No more pointers in this object after this bitmap word.
// Update size so we know not to look there.
size = valid * goarch.PtrSize
}
}
return heapBits{addr: addr, size: size, mask: mask, valid: valid}
}
// Returns the (absolute) address of the next known pointer and
// a heapBits iterator representing any remaining pointers.
// If there are no more pointers, returns address 0.
// Note that next does not modify h. The caller must record the result.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (h heapBits) next() (heapBits, uintptr) {
for {
if h.mask != 0 {
var i int
if goarch.PtrSize == 8 {
i = sys.TrailingZeros64(uint64(h.mask))
} else {
i = sys.TrailingZeros32(uint32(h.mask))
}
h.mask ^= uintptr(1) << (i & (ptrBits - 1))
return h, h.addr + uintptr(i)*goarch.PtrSize
}
// Skip words that we've already processed.
h.addr += h.valid * goarch.PtrSize
h.size -= h.valid * goarch.PtrSize
if h.size == 0 {
return h, 0 // no more pointers
}
// Grab more bits and try again.
h = heapBitsForAddr(h.addr, h.size)
}
}
// nextFast is like next, but can return 0 even when there are more pointers
// to be found. Callers should call next if nextFast returns 0 as its second
// return value.
//
// if addr, h = h.nextFast(); addr == 0 {
// if addr, h = h.next(); addr == 0 {
// ... no more pointers ...
// }
// }
// ... process pointer at addr ...
//
// nextFast is designed to be inlineable.
//
//go:nosplit
func (h heapBits) nextFast() (heapBits, uintptr) {
// TESTQ/JEQ
if h.mask == 0 {
return h, 0
}
// BSFQ
var i int
if goarch.PtrSize == 8 {
i = sys.TrailingZeros64(uint64(h.mask))
} else {
i = sys.TrailingZeros32(uint32(h.mask))
}
// BTCQ
h.mask ^= uintptr(1) << (i & (ptrBits - 1))
// LEAQ (XX)(XX*8)
return h, h.addr + uintptr(i)*goarch.PtrSize
}
// bulkBarrierPreWrite executes a write barrier
// for every pointer slot in the memory range [src, src+size),
// using pointer/scalar information from [dst, dst+size).
// This executes the write barriers necessary before a memmove.
// src, dst, and size must be pointer-aligned.
// The range [dst, dst+size) must lie within a single object.
// It does not perform the actual writes.
//
// As a special case, src == 0 indicates that this is being used for a
// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
// barrier.
//
// Callers should call bulkBarrierPreWrite immediately before
// calling memmove(dst, src, size). This function is marked nosplit
// to avoid being preempted; the GC must not stop the goroutine
// between the memmove and the execution of the barriers.
// The caller is also responsible for cgo pointer checks if this
// may be writing Go pointers into non-Go memory.
//
// The pointer bitmap is not maintained for allocations containing
// no pointers at all; any caller of bulkBarrierPreWrite must first
// make sure the underlying allocation contains pointers, usually
// by checking typ.PtrBytes.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.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.
//
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
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
}
}
// 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
@ -741,33 +440,6 @@ func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
}
}
// initHeapBits initializes the heap bitmap for a span.
// If this is a span of single pointer allocations, it initializes all
// words to pointer. If force is true, clears all bits.
func (s *mspan) initHeapBits(forceClear bool) {
if forceClear || s.spanclass.noscan() {
// Set all the pointer bits to zero. We do this once
// when the span is allocated so we don't have to do it
// for each object allocation.
base := s.base()
size := s.npages * pageSize
h := writeHeapBitsForAddr(base)
h.flush(base, size)
return
}
isPtrs := goarch.PtrSize == 8 && s.elemsize == goarch.PtrSize
if !isPtrs {
return // nothing to do
}
h := writeHeapBitsForAddr(s.base())
size := s.npages * pageSize
nptrs := size / goarch.PtrSize
for i := uintptr(0); i < nptrs; i += ptrBits {
h = h.write(^uintptr(0), ptrBits)
}
h.flush(s.base(), size)
}
// countAlloc returns the number of objects allocated in span s by
// scanning the allocation bitmap.
func (s *mspan) countAlloc() int {
@ -788,146 +460,6 @@ func (s *mspan) countAlloc() int {
return count
}
type writeHeapBits struct {
addr uintptr // address that the low bit of mask represents the pointer state of.
mask uintptr // some pointer bits starting at the address addr.
valid uintptr // number of bits in buf that are valid (including low)
low uintptr // number of low-order bits to not overwrite
}
func writeHeapBitsForAddr(addr uintptr) (h writeHeapBits) {
// We start writing bits maybe in the middle of a heap bitmap word.
// Remember how many bits into the word we started, so we can be sure
// not to overwrite the previous bits.
h.low = addr / goarch.PtrSize % ptrBits
// round down to heap word that starts the bitmap word.
h.addr = addr - h.low*goarch.PtrSize
// We don't have any bits yet.
h.mask = 0
h.valid = h.low
return
}
// write appends the pointerness of the next valid pointer slots
// using the low valid bits of bits. 1=pointer, 0=scalar.
func (h writeHeapBits) write(bits, valid uintptr) writeHeapBits {
if h.valid+valid <= ptrBits {
// Fast path - just accumulate the bits.
h.mask |= bits << h.valid
h.valid += valid
return h
}
// Too many bits to fit in this word. Write the current word
// out and move on to the next word.
data := h.mask | bits<<h.valid // mask for this word
h.mask = bits >> (ptrBits - h.valid) // leftover for next word
h.valid += valid - ptrBits // have h.valid+valid bits, writing ptrBits of them
// Flush mask to the memory bitmap.
// TODO: figure out how to cache arena lookup.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
m := uintptr(1)<<h.low - 1
ha.bitmap[idx] = ha.bitmap[idx]&m | data
// Note: no synchronization required for this write because
// the allocator has exclusive access to the page, and the bitmap
// entries are all for a single page. Also, visibility of these
// writes is guaranteed by the publication barrier in mallocgc.
// Clear noMorePtrs bit, since we're going to be writing bits
// into the following word.
ha.noMorePtrs[idx/8] &^= uint8(1) << (idx % 8)
// Note: same as above
// Move to next word of bitmap.
h.addr += ptrBits * goarch.PtrSize
h.low = 0
return h
}
// Add padding of size bytes.
func (h writeHeapBits) pad(size uintptr) writeHeapBits {
if size == 0 {
return h
}
words := size / goarch.PtrSize
for words > ptrBits {
h = h.write(0, ptrBits)
words -= ptrBits
}
return h.write(0, words)
}
// Flush the bits that have been written, and add zeros as needed
// to cover the full object [addr, addr+size).
func (h writeHeapBits) flush(addr, size uintptr) {
// zeros counts the number of bits needed to represent the object minus the
// number of bits we've already written. This is the number of 0 bits
// that need to be added.
zeros := (addr+size-h.addr)/goarch.PtrSize - h.valid
// Add zero bits up to the bitmap word boundary
if zeros > 0 {
z := ptrBits - h.valid
if z > zeros {
z = zeros
}
h.valid += z
zeros -= z
}
// Find word in bitmap that we're going to write.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
// Write remaining bits.
if h.valid != h.low {
m := uintptr(1)<<h.low - 1 // don't clear existing bits below "low"
m |= ^(uintptr(1)<<h.valid - 1) // don't clear existing bits above "valid"
ha.bitmap[idx] = ha.bitmap[idx]&m | h.mask
}
if zeros == 0 {
return
}
// Record in the noMorePtrs map that there won't be any more 1 bits,
// so readers can stop early.
ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
// Advance to next bitmap word.
h.addr += ptrBits * goarch.PtrSize
// Continue on writing zeros for the rest of the object.
// For standard use of the ptr bits this is not required, as
// the bits are read from the beginning of the object. Some uses,
// like noscan spans, oblets, bulk write barriers, and cgocheck, might
// start mid-object, so these writes are still required.
for {
// Write zero bits.
ai := arenaIndex(h.addr)
ha := mheap_.arenas[ai.l1()][ai.l2()]
idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
if zeros < ptrBits {
ha.bitmap[idx] &^= uintptr(1)<<zeros - 1
break
} else if zeros == ptrBits {
ha.bitmap[idx] = 0
break
} else {
ha.bitmap[idx] = 0
zeros -= ptrBits
}
ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
h.addr += ptrBits * goarch.PtrSize
}
}
// Read the bytes starting at the aligned pointer p into a uintptr.
// Read is little-endian.
func readUintptr(p *byte) uintptr {
@ -941,197 +473,6 @@ func readUintptr(p *byte) uintptr {
return x
}
// heapBitsSetType records that the new allocation [x, x+size)
// holds in [x, x+dataSize) one or more values of type typ.
// (The number of values is given by dataSize / typ.Size.)
// If dataSize < size, the fragment [x+dataSize, x+size) is
// recorded as non-pointer data.
// It is known that the type has pointers somewhere;
// malloc does not call heapBitsSetType when there are no pointers,
// because all free objects are marked as noscan during
// heapBitsSweepSpan.
//
// There can only be one allocation from a given span active at a time,
// and the bitmap for a span always falls on word boundaries,
// so there are no write-write races for access to the heap bitmap.
// Hence, heapBitsSetType can access the bitmap without atomics.
//
// There can be read-write races between heapBitsSetType and things
// that read the heap bitmap like scanobject. However, since
// heapBitsSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// bits that belong to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
const doubleCheck = false // slow but helpful; enable to test modifications to this code
if doubleCheck && dataSize%typ.Size_ != 0 {
throw("heapBitsSetType: dataSize not a multiple of typ.Size")
}
if goarch.PtrSize == 8 && size == goarch.PtrSize {
// It's one word and it has pointers, it must be a pointer.
// Since all allocated one-word objects are pointers
// (non-pointers are aggregated into tinySize allocations),
// (*mspan).initHeapBits sets the pointer bits for us.
// Nothing to do here.
if doubleCheck {
h, addr := heapBitsForAddr(x, size).next()
if addr != x {
throw("heapBitsSetType: pointer bit missing")
}
_, addr = h.next()
if addr != 0 {
throw("heapBitsSetType: second pointer bit found")
}
}
return
}
h := writeHeapBitsForAddr(x)
// Handle GC program.
if typ.Kind_&kindGCProg != 0 {
// Expand the gc program into the storage we're going to use for the actual object.
obj := (*uint8)(unsafe.Pointer(x))
n := runGCProg(addb(typ.GCData, 4), obj)
// Use the expanded program to set the heap bits.
for i := uintptr(0); true; i += typ.Size_ {
// Copy expanded program to heap bitmap.
p := obj
j := n
for j > 8 {
h = h.write(uintptr(*p), 8)
p = add1(p)
j -= 8
}
h = h.write(uintptr(*p), j)
if i+typ.Size_ == dataSize {
break // no padding after last element
}
// Pad with zeros to the start of the next element.
h = h.pad(typ.Size_ - n*goarch.PtrSize)
}
h.flush(x, size)
// Erase the expanded GC program.
memclrNoHeapPointers(unsafe.Pointer(obj), (n+7)/8)
return
}
// Note about sizes:
//
// typ.Size is the number of words in the object,
// and typ.PtrBytes is the number of words in the prefix
// of the object that contains pointers. That is, the final
// typ.Size - typ.PtrBytes words contain no pointers.
// This allows optimization of a common pattern where
// an object has a small header followed by a large scalar
// buffer. If we know the pointers are over, we don't have
// to scan the buffer's heap bitmap at all.
// The 1-bit ptrmasks are sized to contain only bits for
// the typ.PtrBytes prefix, zero padded out to a full byte
// of bitmap. If there is more room in the allocated object,
// that space is pointerless. The noMorePtrs bitmap will prevent
// scanning large pointerless tails of an object.
//
// Replicated copies are not as nice: if there is an array of
// objects with scalar tails, all but the last tail does have to
// be initialized, because there is no way to say "skip forward".
ptrs := typ.PtrBytes / goarch.PtrSize
if typ.Size_ == dataSize { // Single element
if ptrs <= ptrBits { // Single small element
m := readUintptr(typ.GCData)
h = h.write(m, ptrs)
} else { // Single large element
p := typ.GCData
for {
h = h.write(readUintptr(p), ptrBits)
p = addb(p, ptrBits/8)
ptrs -= ptrBits
if ptrs <= ptrBits {
break
}
}
m := readUintptr(p)
h = h.write(m, ptrs)
}
} else { // Repeated element
words := typ.Size_ / goarch.PtrSize // total words, including scalar tail
if words <= ptrBits { // Repeated small element
n := dataSize / typ.Size_
m := readUintptr(typ.GCData)
// Make larger unit to repeat
for words <= ptrBits/2 {
if n&1 != 0 {
h = h.write(m, words)
}
n /= 2
m |= m << words
ptrs += words
words *= 2
if n == 1 {
break
}
}
for n > 1 {
h = h.write(m, words)
n--
}
h = h.write(m, ptrs)
} else { // Repeated large element
for i := uintptr(0); true; i += typ.Size_ {
p := typ.GCData
j := ptrs
for j > ptrBits {
h = h.write(readUintptr(p), ptrBits)
p = addb(p, ptrBits/8)
j -= ptrBits
}
m := readUintptr(p)
h = h.write(m, j)
if i+typ.Size_ == dataSize {
break // don't need the trailing nonptr bits on the last element.
}
// Pad with zeros to the start of the next element.
h = h.pad(typ.Size_ - typ.PtrBytes)
}
}
}
h.flush(x, size)
if doubleCheck {
h := heapBitsForAddr(x, size)
for i := uintptr(0); i < size; i += goarch.PtrSize {
// Compute the pointer bit we want at offset i.
want := false
if i < dataSize {
off := i % typ.Size_
if off < typ.PtrBytes {
j := off / goarch.PtrSize
want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
}
}
if want {
var addr uintptr
h, addr = h.next()
if addr != x+i {
throw("heapBitsSetType: pointer entry not correct")
}
}
}
if _, addr := h.next(); addr != 0 {
throw("heapBitsSetType: extra pointer")
}
}
}
var debugPtrmask struct {
lock mutex
data *byte
@ -1432,186 +773,3 @@ func dumpGCProg(p *byte) {
func reflect_gcbits(x any) []byte {
return getgcmask(x)
}
// Returns GC type info for the pointer stored in ep for testing.
// If ep points to the stack, only static live information will be returned
// (i.e. not for objects which are only dynamically live stack objects).
func getgcmask(ep any) (mask []byte) {
e := *efaceOf(&ep)
p := e.data
t := e._type
// data or bss
for _, datap := range activeModules() {
// data
if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
bitmap := datap.gcdatamask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
mask = make([]byte, n/goarch.PtrSize)
for i := uintptr(0); i < n; i += goarch.PtrSize {
off := (uintptr(p) + i - datap.data) / goarch.PtrSize
mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
// bss
if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
bitmap := datap.gcbssmask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
mask = make([]byte, n/goarch.PtrSize)
for i := uintptr(0); i < n; i += goarch.PtrSize {
off := (uintptr(p) + i - datap.bss) / goarch.PtrSize
mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
}
// heap
if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
if s.spanclass.noscan() {
return nil
}
n := s.elemsize
hbits := heapBitsForAddr(base, n)
mask = make([]byte, n/goarch.PtrSize)
for {
var addr uintptr
if hbits, addr = hbits.next(); addr == 0 {
break
}
mask[(addr-base)/goarch.PtrSize] = 1
}
// Callers expect this mask to end at the last pointer.
for len(mask) > 0 && mask[len(mask)-1] == 0 {
mask = mask[:len(mask)-1]
}
return
}
// stack
if gp := getg(); gp.m.curg.stack.lo <= uintptr(p) && uintptr(p) < gp.m.curg.stack.hi {
found := false
var u unwinder
for u.initAt(gp.m.curg.sched.pc, gp.m.curg.sched.sp, 0, gp.m.curg, 0); u.valid(); u.next() {
if u.frame.sp <= uintptr(p) && uintptr(p) < u.frame.varp {
found = true
break
}
}
if found {
locals, _, _ := u.frame.getStackMap(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, base uintptr) {
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")
}
}
}

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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 bitmap
//
// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting at "start",
// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
// through start+63*ptrSize, ha.bitmap[1] holds the entries for
// start+64*ptrSize through start+127*ptrSize, and so on.
// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
// (For 32-bit platforms, s/64/32/.)
//
// We also keep a noMorePtrs bitmap which allows us to stop scanning
// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
// has no more pointers beyond those described by ha.bitmap[8*i+j].
// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
// beyond must all be zero until the start of the next object.
//
// The bitmap for noscan spans is set to all zero at span allocation time.
//
// The bitmap for unallocated objects in scannable spans is not maintained
// (can be junk).
package runtime
import (
"internal/goarch"
"runtime/internal/sys"
"unsafe"
)
// 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.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.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.
//
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
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")
}
}
}
// 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]
}
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, base uintptr) {
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")
}
}
}

857
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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 bitmap
//
// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored in the heapArena metadata backing each heap arena.
// That is, if ha is the heapArena for the arena starting at "start",
// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
// through start+63*ptrSize, ha.bitmap[1] holds the entries for
// start+64*ptrSize through start+127*ptrSize, and so on.
// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
// (For 32-bit platforms, s/64/32/.)
//
// We also keep a noMorePtrs bitmap which allows us to stop scanning
// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
// has no more pointers beyond those described by ha.bitmap[8*i+j].
// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
// beyond must all be zero until the start of the next object.
//
// The bitmap for noscan spans is set to all zero at span allocation time.
//
// The bitmap for unallocated objects in scannable spans is not maintained
// (can be junk).
package runtime
import (
"internal/goarch"
"runtime/internal/sys"
"unsafe"
)
// 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.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func bulkBarrierPreWrite(dst, src, size uintptr) {
if (dst|src|size)&(goarch.PtrSize-1) != 0 {
throw("bulkBarrierPreWrite: unaligned arguments")
}
if !writeBarrier.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.
//
//go:nosplit
func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
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")
}
}
}
// 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]
}
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, base uintptr) {
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")
}
}
}

2
src/runtime/msize.go → src/runtime/msize_allocheaders.go
View File

@ -2,6 +2,8 @@
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build goexperiment.allocheaders
// Malloc small size classes.
//
// See malloc.go for overview.

27
src/runtime/msize_noallocheaders.go Normal file
View File

@ -0,0 +1,27 @@
// 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.
//go:build !goexperiment.allocheaders
// Malloc small size classes.
//
// See malloc.go for overview.
// See also mksizeclasses.go for how we decide what size classes to use.
package runtime
// Returns size of the memory block that mallocgc will allocate if you ask for the size.
func roundupsize(size uintptr) uintptr {
if size < _MaxSmallSize {
if size <= smallSizeMax-8 {
return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
} else {
return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
}
}
if size+_PageSize < size {
return size
}
return alignUp(size, _PageSize)
}