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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Garbage collector: type and heap bitmaps.
//
// Stack, data, and bss bitmaps
//
// Stack frames and global variables in the data and bss sections are described
// by 1-bit bitmaps in which 0 means uninteresting and 1 means live pointer
// to be visited during GC. The bits in each byte are consumed starting with
// the low bit: 1<<0, 1<<1, and so on.
//
// Heap bitmap
//
// The allocated heap comes from a subset of the memory in the range [start, used),
// where start == mheap_.arena_start and used == mheap_.arena_used.
// The heap bitmap comprises 2 bits for each pointer-sized word in that range,
// stored in bytes indexed backward in memory from start.
// That is, the byte at address start-1 holds the 2-bit entries for the four words
// start through start+3*ptrSize, the byte at start-2 holds the entries for
// start+4*ptrSize through start+7*ptrSize, and so on.
//
// In each 2-bit entry, the lower bit holds the same information as in the 1-bit
// bitmaps: 0 means uninteresting and 1 means live pointer to be visited during GC.
// The meaning of the high bit depends on the position of the word being described
// in its allocated object. In the first word, the high bit is the GC ``marked'' bit.
// In the second word, the high bit is the GC ``checkmarked'' bit (see below).
// In the third and later words, the high bit indicates that the object is still
// being described. In these words, if a bit pair with a high bit 0 is encountered,
// the low bit can also be assumed to be 0, and the object description is over.
// This 00 is called the ``dead'' encoding: it signals that the rest of the words
// in the object are uninteresting to the garbage collector.
//
// The 2-bit entries are split when written into the byte, so that the top half
// of the byte contains 4 mark bits and the bottom half contains 4 pointer bits.
// This form allows a copy from the 1-bit to the 4-bit form to keep the
// pointer bits contiguous, instead of having to space them out.
//
// The code makes use of the fact that the zero value for a heap bitmap
// has no live pointer bit set and is (depending on position), not marked,
// not checkmarked, and is the dead encoding.
// These properties must be preserved when modifying the encoding.
//
// Checkmarks
//
// In a concurrent garbage collector, one worries about failing to mark
// a live object due to mutations without write barriers or bugs in the
// collector implementation. As a sanity check, the GC has a 'checkmark'
// mode that retraverses the object graph with the world stopped, to make
// sure that everything that should be marked is marked.
// In checkmark mode, in the heap bitmap, the high bit of the 2-bit entry
// for the second word of the object holds the checkmark bit.
// When not in checkmark mode, this bit is set to 1.
//
// The smallest possible allocation is 8 bytes. On a 32-bit machine, that
// means every allocated object has two words, so there is room for the
// checkmark bit. On a 64-bit machine, however, the 8-byte allocation is
// just one word, so the second bit pair is not available for encoding the
// checkmark. However, because non-pointer allocations are combined
// into larger 16-byte (maxTinySize) allocations, a plain 8-byte allocation
// must be a pointer, so the type bit in the first word is not actually needed.
// It is still used in general, except in checkmark the type bit is repurposed
// as the checkmark bit and then reinitialized (to 1) as the type bit when
// finished.
package runtime
import (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
bitPointer = 1 << 0
bitMarked = 1 << 4
heapBitsShift = 1 // shift offset between successive bitPointer or bitMarked entries
heapBitmapScale = sys.PtrSize * (8 / 2) // number of data bytes described by one heap bitmap byte
// all mark/pointer bits in a byte
bitMarkedAll = bitMarked | bitMarked<<heapBitsShift | bitMarked<<(2*heapBitsShift) | bitMarked<<(3*heapBitsShift)
bitPointerAll = bitPointer | bitPointer<<heapBitsShift | bitPointer<<(2*heapBitsShift) | bitPointer<<(3*heapBitsShift)
)
// addb returns the byte pointer p+n.
//go:nowritebarrier
func addb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}
// subtractb returns the byte pointer p-n.
//go:nowritebarrier
func subtractb(p *byte, n uintptr) *byte {
// Note: wrote out full expression instead of calling add(p, -n)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
}
// add1 returns the byte pointer p+1.
//go:nowritebarrier
func add1(p *byte) *byte {
// Note: wrote out full expression instead of calling addb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
}
// subtract1 returns the byte pointer p-1.
//go:nowritebarrier
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func subtract1(p *byte) *byte {
// Note: wrote out full expression instead of calling subtractb(p, 1)
// to reduce the number of temporaries generated by the
// compiler for this trivial expression during inlining.
return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
}
// mHeap_MapBits is called each time arena_used is extended.
// It maps any additional bitmap memory needed for the new arena memory.
// It must be called with the expected new value of arena_used,
// *before* h.arena_used has been updated.
// Waiting to update arena_used until after the memory has been mapped
// avoids faults when other threads try access the bitmap immediately
// after observing the change to arena_used.
//
//go:nowritebarrier
func (h *mheap) mapBits(arena_used uintptr) {
// Caller has added extra mappings to the arena.
// Add extra mappings of bitmap words as needed.
// We allocate extra bitmap pieces in chunks of bitmapChunk.
const bitmapChunk = 8192
n := (arena_used - mheap_.arena_start) / heapBitmapScale
n = round(n, bitmapChunk)
n = round(n, sys.PhysPageSize)
if h.bitmap_mapped >= n {
return
}
sysMap(unsafe.Pointer(h.arena_start-n), n-h.bitmap_mapped, h.arena_reserved, &memstats.gc_sys)
h.bitmap_mapped = n
}
// heapBits provides access to the bitmap bits for a single heap word.
// The methods on heapBits take value receivers so that the compiler
// can more easily inline calls to those methods and registerize the
// struct fields independently.
type heapBits struct {
bitp *uint8
shift uint32
}
// heapBitsForAddr returns the heapBits for the address addr.
// The caller must have already checked that addr is in the range [mheap_.arena_start, mheap_.arena_used).
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func heapBitsForAddr(addr uintptr) heapBits {
// 2 bits per work, 4 pairs per byte, and a mask is hard coded.
off := (addr - mheap_.arena_start) / sys.PtrSize
return heapBits{(*uint8)(unsafe.Pointer(mheap_.arena_start - off/4 - 1)), uint32(off & 3)}
}
// heapBitsForSpan returns the heapBits for the span base address base.
func heapBitsForSpan(base uintptr) (hbits heapBits) {
if base < mheap_.arena_start || base >= mheap_.arena_used {
throw("heapBitsForSpan: base out of range")
}
hbits = heapBitsForAddr(base)
if hbits.shift != 0 {
throw("heapBitsForSpan: unaligned start")
}
return hbits
}
// heapBitsForObject returns the base address for the heap object
// containing the address p, along with the heapBits for base.
// If p does not point into a heap object,
// return base == 0
// otherwise return the base of the object.
//
// refBase and refOff optionally give the base address of the object
// in which the pointer p was found and the byte offset at which it
// was found. These are used for error reporting.
func heapBitsForObject(p, refBase, refOff uintptr) (base uintptr, hbits heapBits, s *mspan) {
arenaStart := mheap_.arena_start
if p < arenaStart || p >= mheap_.arena_used {
return
}
off := p - arenaStart
idx := off >> _PageShift
// p points into the heap, but possibly to the middle of an object.
// Consult the span table to find the block beginning.
k := p >> _PageShift
s = h_spans[idx]
if s == nil || pageID(k) < s.start || p >= s.limit || s.state != mSpanInUse {
if s == nil || s.state == _MSpanStack {
// If s is nil, the virtual address has never been part of the heap.
// This pointer may be to some mmap'd region, so we allow it.
// Pointers into stacks are also ok, the runtime manages these explicitly.
return
}
// The following ensures that we are rigorous about what data
// structures hold valid pointers.
if debug.invalidptr != 0 {
// Typically this indicates an incorrect use
// of unsafe or cgo to store a bad pointer in
// the Go heap. It may also indicate a runtime
// bug.
//
// TODO(austin): We could be more aggressive
// and detect pointers to unallocated objects
// in allocated spans.
printlock()
print("runtime: pointer ", hex(p))
if s.state != mSpanInUse {
print(" to unallocated span")
} else {
print(" to unused region of span")
}
print("idx=", hex(idx), " span.start=", hex(s.start<<_PageShift), " span.limit=", hex(s.limit), " span.state=", s.state, "\n")
if refBase != 0 {
print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
gcDumpObject("object", refBase, refOff)
}
throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
}
return
}
// If this span holds object of a power of 2 size, just mask off the bits to
// the interior of the object. Otherwise use the size to get the base.
if s.baseMask != 0 {
// optimize for power of 2 sized objects.
base = s.base()
base = base + (p-base)&s.baseMask
// base = p & s.baseMask is faster for small spans,
// but doesn't work for large spans.
// Overall, it's faster to use the more general computation above.
} else {
base = s.base()
if p-base >= s.elemsize {
// n := (p - base) / s.elemsize, using division by multiplication
n := uintptr(uint64(p-base) >> s.divShift * uint64(s.divMul) >> s.divShift2)
base += n * s.elemsize
}
}
// Now that we know the actual base, compute heapBits to return to caller.
hbits = heapBitsForAddr(base)
return
}
// prefetch the bits.
func (h heapBits) prefetch() {
prefetchnta(uintptr(unsafe.Pointer((h.bitp))))
}
// next returns the heapBits describing the next pointer-sized word in memory.
// That is, if h describes address p, h.next() describes p+ptrSize.
// Note that next does not modify h. The caller must record the result.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) next() heapBits {
if h.shift < 3*heapBitsShift {
return heapBits{h.bitp, h.shift + heapBitsShift}
}
return heapBits{subtract1(h.bitp), 0}
}
// forward returns the heapBits describing n pointer-sized words ahead of h in memory.
// That is, if h describes address p, h.forward(n) describes p+n*ptrSize.
// h.forward(1) is equivalent to h.next(), just slower.
// Note that forward does not modify h. The caller must record the result.
// bits returns the heap bits for the current word.
func (h heapBits) forward(n uintptr) heapBits {
n += uintptr(h.shift) / heapBitsShift
return heapBits{subtractb(h.bitp, n/4), uint32(n%4) * heapBitsShift}
}
// The caller can test isMarked and isPointer by &-ing with bitMarked and bitPointer.
// The result includes in its higher bits the bits for subsequent words
// described by the same bitmap byte.
func (h heapBits) bits() uint32 {
return uint32(*h.bitp) >> h.shift
}
// isMarked reports whether the heap bits have the marked bit set.
// h must describe the initial word of the object.
func (h heapBits) isMarked() bool {
return *h.bitp&(bitMarked<<h.shift) != 0
}
// setMarked sets the marked bit in the heap bits, atomically.
// h must describe the initial word of the object.
func (h heapBits) setMarked() {
// Each byte of GC bitmap holds info for four words.
// Might be racing with other updates, so use atomic update always.
// We used to be clever here and use a non-atomic update in certain
// cases, but it's not worth the risk.
atomic.Or8(h.bitp, bitMarked<<h.shift)
}
// setMarkedNonAtomic sets the marked bit in the heap bits, non-atomically.
// h must describe the initial word of the object.
func (h heapBits) setMarkedNonAtomic() {
*h.bitp |= bitMarked << h.shift
}
// isPointer reports whether the heap bits describe a pointer word.
// h must describe the initial word of the object.
//
// nosplit because it is used during write barriers and must not be preempted.
//go:nosplit
func (h heapBits) isPointer() bool {
return (*h.bitp>>h.shift)&bitPointer != 0
}
// hasPointers reports whether the given object has any pointers.
// It must be told how large the object at h is, so that it does not read too
// far into the bitmap.
// h must describe the initial word of the object.
func (h heapBits) hasPointers(size uintptr) bool {
if size == sys.PtrSize { // 1-word objects are always pointers
return true
}
// Otherwise, at least a 2-word object, and at least 2-word aligned,
// so h.shift is either 0 or 4, so we know we can get the bits for the
// first two words out of *h.bitp.
// If either of the first two words is a pointer, not pointer free.
b := uint32(*h.bitp >> h.shift)
if b&(bitPointer|bitPointer<<heapBitsShift) != 0 {
return true
}
if size == 2*sys.PtrSize {
return false
}
// At least a 4-word object. Check scan bit (aka marked bit) in third word.
if h.shift == 0 {
return b&(bitMarked<<(2*heapBitsShift)) != 0
}
return uint32(*subtract1(h.bitp))&bitMarked != 0
}
// isCheckmarked reports whether the heap bits have the checkmarked bit set.
// It must be told how large the object at h is, because the encoding of the
// checkmark bit varies by size.
// h must describe the initial word of the object.
func (h heapBits) isCheckmarked(size uintptr) bool {
if size == sys.PtrSize {
return (*h.bitp>>h.shift)&bitPointer != 0
}
// All multiword objects are 2-word aligned,
// so we know that the initial word's 2-bit pair
// and the second word's 2-bit pair are in the
// same heap bitmap byte, *h.bitp.
return (*h.bitp>>(heapBitsShift+h.shift))&bitMarked != 0
}
// setCheckmarked sets the checkmarked bit.
// It must be told how large the object at h is, because the encoding of the
// checkmark bit varies by size.
// h must describe the initial word of the object.
func (h heapBits) setCheckmarked(size uintptr) {
if size == sys.PtrSize {
atomic.Or8(h.bitp, bitPointer<<h.shift)
return
}
atomic.Or8(h.bitp, bitMarked<<(heapBitsShift+h.shift))
}
// heapBitsBulkBarrier executes writebarrierptr_nostore
// for every pointer slot in the memory range [p, p+size),
// using the heap bitmap to locate those pointer slots.
// This executes the write barriers necessary after a memmove.
// Both p and size must be pointer-aligned.
// The range [p, p+size) must lie within a single allocation.
//
// Callers should call heapBitsBulkBarrier immediately after
// calling memmove(p, 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 heap bitmap is not maintained for allocations containing
// no pointers at all; any caller of heapBitsBulkBarrier must first
// make sure the underlying allocation contains pointers, usually
// by checking typ.kind&kindNoPointers.
//
//go:nosplit
func heapBitsBulkBarrier(p, size uintptr) {
if (p|size)&(sys.PtrSize-1) != 0 {
throw("heapBitsBulkBarrier: unaligned arguments")
}
if !writeBarrier.needed {
return
}
if !inheap(p) {
// If p is on the stack and in a higher frame than the
// caller, we either need to execute write barriers on
// it (which is what happens for normal stack writes
// through pointers to higher frames), or we need to
// force the mark termination stack scan to scan the
// frame containing p.
//
// Executing write barriers on p is complicated in the
// general case because we either need to unwind the
// stack to get the stack map, or we need the type's
// bitmap, which may be a GC program.
//
// Hence, we opt for forcing the re-scan to scan the
// frame containing p, which we can do by simply
// unwinding the stack barriers between the current SP
// and p's frame.
gp := getg().m.curg
if gp != nil && gp.stack.lo <= p && p < gp.stack.hi {
// Run on the system stack to give it more
// stack space.
systemstack(func() {
gcUnwindBarriers(gp, p)
})
}
return
}
h := heapBitsForAddr(p)
for i := uintptr(0); i < size; i += sys.PtrSize {
if h.isPointer() {
x := (*uintptr)(unsafe.Pointer(p + i))
writebarrierptr_nostore(x, *x)
}
h = h.next()
}
}
// typeBitsBulkBarrier executes writebarrierptr_nostore
// for every pointer slot in the memory range [p, p+size),
// using the type bitmap to locate those pointer slots.
// The type typ must correspond exactly to [p, p+size).
// This executes the write barriers necessary after a copy.
// Both p and size must be pointer-aligned.
// The type typ must have a plain bitmap, not a GC program.
// The only use of this function is in channel sends, and the
// 64 kB channel element limit takes care of this for us.
//
// Must not be preempted because it typically runs right after memmove,
// and the GC must not complete between those two.
//
//go:nosplit
func typeBitsBulkBarrier(typ *_type, p, size uintptr) {
if typ == nil {
throw("runtime: typeBitsBulkBarrier without type")
}
if typ.size != size {
println("runtime: typeBitsBulkBarrier with type ", *typ._string, " of size ", typ.size, " but memory size", size)
throw("runtime: invalid typeBitsBulkBarrier")
}
if typ.kind&kindGCProg != 0 {
println("runtime: typeBitsBulkBarrier with type ", *typ._string, " with GC prog")
throw("runtime: invalid typeBitsBulkBarrier")
}
if !writeBarrier.needed {
return
}
ptrmask := typ.gcdata
var bits uint32
for i := uintptr(0); i < typ.ptrdata; i += sys.PtrSize {
if i&(sys.PtrSize*8-1) == 0 {
bits = uint32(*ptrmask)
ptrmask = addb(ptrmask, 1)
} else {
bits = bits >> 1
}
if bits&1 != 0 {
x := (*uintptr)(unsafe.Pointer(p + i))
writebarrierptr_nostore(x, *x)
}
}
}
// The methods operating on spans all require that h has been returned
// by heapBitsForSpan and that size, n, total are the span layout description
// returned by the mspan's layout method.
// If total > size*n, it means that there is extra leftover memory in the span,
// usually due to rounding.
//
// TODO(rsc): Perhaps introduce a different heapBitsSpan type.
// initSpan initializes the heap bitmap for a span.
func (h heapBits) initSpan(size, n, total uintptr) {
if total%heapBitmapScale != 0 {
throw("initSpan: unaligned length")
}
nbyte := total / heapBitmapScale
if sys.PtrSize == 8 && size == sys.PtrSize {
end := h.bitp
bitp := subtractb(end, nbyte-1)
for {
*bitp = bitPointerAll
if bitp == end {
break
}
bitp = add1(bitp)
}
return
}
memclr(unsafe.Pointer(subtractb(h.bitp, nbyte-1)), nbyte)
}
// initCheckmarkSpan initializes a span for being checkmarked.
// It clears the checkmark bits, which are set to 1 in normal operation.
func (h heapBits) initCheckmarkSpan(size, n, total uintptr) {
// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
if sys.PtrSize == 8 && size == sys.PtrSize {
// Checkmark bit is type bit, bottom bit of every 2-bit entry.
// Only possible on 64-bit system, since minimum size is 8.
// Must clear type bit (checkmark bit) of every word.
// The type bit is the lower of every two-bit pair.
bitp := h.bitp
for i := uintptr(0); i < n; i += 4 {
*bitp &^= bitPointerAll
bitp = subtract1(bitp)
}
return
}
for i := uintptr(0); i < n; i++ {
*h.bitp &^= bitMarked << (heapBitsShift + h.shift)
h = h.forward(size / sys.PtrSize)
}
}
// clearCheckmarkSpan undoes all the checkmarking in a span.
// The actual checkmark bits are ignored, so the only work to do
// is to fix the pointer bits. (Pointer bits are ignored by scanobject
// but consulted by typedmemmove.)
func (h heapBits) clearCheckmarkSpan(size, n, total uintptr) {
// The ptrSize == 8 is a compile-time constant false on 32-bit and eliminates this code entirely.
if sys.PtrSize == 8 && size == sys.PtrSize {
// Checkmark bit is type bit, bottom bit of every 2-bit entry.
// Only possible on 64-bit system, since minimum size is 8.
// Must clear type bit (checkmark bit) of every word.
// The type bit is the lower of every two-bit pair.
bitp := h.bitp
for i := uintptr(0); i < n; i += 4 {
*bitp |= bitPointerAll
bitp = subtract1(bitp)
}
}
}
// heapBitsSweepSpan coordinates the sweeping of a span by reading
// and updating the corresponding heap bitmap entries.
// For each free object in the span, heapBitsSweepSpan sets the type
// bits for the first two words (or one for single-word objects) to typeDead
// and then calls f(p), where p is the object's base address.
// f is expected to add the object to a free list.
// For non-free objects, heapBitsSweepSpan turns off the marked bit.
func heapBitsSweepSpan(base, size, n uintptr, f func(uintptr)) {
h := heapBitsForSpan(base)
switch {
default:
throw("heapBitsSweepSpan")
case sys.PtrSize == 8 && size == sys.PtrSize:
// Consider mark bits in all four 2-bit entries of each bitmap byte.
bitp := h.bitp
for i := uintptr(0); i < n; i += 4 {
x := uint32(*bitp)
// Note that unlike the other size cases, we leave the pointer bits set here.
// These are initialized during initSpan when the span is created and left
// in place the whole time the span is used for pointer-sized objects.
// That lets heapBitsSetType avoid an atomic update to set the pointer bit
// during allocation.
if x&bitMarked != 0 {
x &^= bitMarked
} else {
f(base + i*sys.PtrSize)
}
if x&(bitMarked<<heapBitsShift) != 0 {
x &^= bitMarked << heapBitsShift
} else {
f(base + (i+1)*sys.PtrSize)
}
if x&(bitMarked<<(2*heapBitsShift)) != 0 {
x &^= bitMarked << (2 * heapBitsShift)
} else {
f(base + (i+2)*sys.PtrSize)
}
if x&(bitMarked<<(3*heapBitsShift)) != 0 {
x &^= bitMarked << (3 * heapBitsShift)
} else {
f(base + (i+3)*sys.PtrSize)
}
*bitp = uint8(x)
bitp = subtract1(bitp)
}
case size%(4*sys.PtrSize) == 0:
// Mark bit is in first word of each object.
// Each object starts at bit 0 of a heap bitmap byte.
bitp := h.bitp
step := size / heapBitmapScale
for i := uintptr(0); i < n; i++ {
x := uint32(*bitp)
if x&bitMarked != 0 {
x &^= bitMarked
} else {
x = 0
f(base + i*size)
}
*bitp = uint8(x)
bitp = subtractb(bitp, step)
}
case size%(4*sys.PtrSize) == 2*sys.PtrSize:
// Mark bit is in first word of each object,
// but every other object starts halfway through a heap bitmap byte.
// Unroll loop 2x to handle alternating shift count and step size.
bitp := h.bitp
step := size / heapBitmapScale
var i uintptr
for i = uintptr(0); i < n; i += 2 {
x := uint32(*bitp)
if x&bitMarked != 0 {
x &^= bitMarked
} else {
x &^= bitMarked | bitPointer | (bitMarked|bitPointer)<<heapBitsShift
f(base + i*size)
if size > 2*sys.PtrSize {
x = 0
}
}
*bitp = uint8(x)
if i+1 >= n {
break
}
bitp = subtractb(bitp, step)
x = uint32(*bitp)
if x&(bitMarked<<(2*heapBitsShift)) != 0 {
x &^= bitMarked << (2 * heapBitsShift)
} else {
x &^= (bitMarked|bitPointer)<<(2*heapBitsShift) | (bitMarked|bitPointer)<<(3*heapBitsShift)
f(base + (i+1)*size)
if size > 2*sys.PtrSize {
*subtract1(bitp) = 0
}
}
*bitp = uint8(x)
bitp = subtractb(bitp, step+1)
}
}
}
// 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,
// so this code is not racing with other instances of itself,
// and we don't allocate from a span until it has been swept,
// so this code is not racing with heapBitsSweepSpan.
// It is, however, racing with the concurrent GC mark phase,
// which can be setting the mark bit in the leading 2-bit entry
// of an allocated block. The block we are modifying is not quite
// allocated yet, so the GC marker is not racing with updates to x's bits,
// but if the start or end of x shares a bitmap byte with an adjacent
// object, the GC marker is racing with updates to those object's mark bits.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
const doubleCheck = false // slow but helpful; enable to test modifications to this code
// dataSize is always size rounded up to the next malloc size class,
// except in the case of allocating a defer block, in which case
// size is sizeof(_defer{}) (at least 6 words) and dataSize may be
// arbitrarily larger.
//
// The checks for size == ptrSize and size == 2*ptrSize can therefore
// assume that dataSize == size without checking it explicitly.
if sys.PtrSize == 8 && size == sys.PtrSize {
// It's one word and it has pointers, it must be a pointer.
// In general we'd need an atomic update here if the
// concurrent GC were marking objects in this span,
// because each bitmap byte describes 3 other objects
// in addition to the one being allocated.
// However, since all allocated one-word objects are pointers
// (non-pointers are aggregated into tinySize allocations),
// initSpan sets the pointer bits for us. Nothing to do here.
if doubleCheck {
h := heapBitsForAddr(x)
if !h.isPointer() {
throw("heapBitsSetType: pointer bit missing")
}
}
return
}
h := heapBitsForAddr(x)
ptrmask := typ.gcdata // start of 1-bit pointer mask (or GC program, handled below)
// Heap bitmap bits for 2-word object are only 4 bits,
// so also shared with objects next to it; use atomic updates.
// This is called out as a special case primarily for 32-bit systems,
// so that on 32-bit systems the code below can assume all objects
// are 4-word aligned (because they're all 16-byte aligned).
if size == 2*sys.PtrSize {
if typ.size == sys.PtrSize {
// We're allocating a block big enough to hold two pointers.
// On 64-bit, that means the actual object must be two pointers,
// or else we'd have used the one-pointer-sized block.
// On 32-bit, however, this is the 8-byte block, the smallest one.
// So it could be that we're allocating one pointer and this was
// just the smallest block available. Distinguish by checking dataSize.
// (In general the number of instances of typ being allocated is
// dataSize/typ.size.)
if sys.PtrSize == 4 && dataSize == sys.PtrSize {
// 1 pointer.
if gcphase == _GCoff {
*h.bitp |= bitPointer << h.shift
} else {
atomic.Or8(h.bitp, bitPointer<<h.shift)
}
} else {
// 2-element slice of pointer.
if gcphase == _GCoff {
*h.bitp |= (bitPointer | bitPointer<<heapBitsShift) << h.shift
} else {
atomic.Or8(h.bitp, (bitPointer|bitPointer<<heapBitsShift)<<h.shift)
}
}
return
}
// Otherwise typ.size must be 2*ptrSize, and typ.kind&kindGCProg == 0.
if doubleCheck {
if typ.size != 2*sys.PtrSize || typ.kind&kindGCProg != 0 {
print("runtime: heapBitsSetType size=", size, " but typ.size=", typ.size, " gcprog=", typ.kind&kindGCProg != 0, "\n")
throw("heapBitsSetType")
}
}
b := uint32(*ptrmask)
hb := b & 3
if gcphase == _GCoff {
*h.bitp |= uint8(hb << h.shift)
} else {
atomic.Or8(h.bitp, uint8(hb<<h.shift))
}
return
}
// Copy from 1-bit ptrmask into 2-bit bitmap.
// The basic approach is to use a single uintptr as a bit buffer,
// alternating between reloading the buffer and writing bitmap bytes.
// In general, one load can supply two bitmap byte writes.
// This is a lot of lines of code, but it compiles into relatively few
// machine instructions.
var (
// Ptrmask input.
p *byte // last ptrmask byte read
b uintptr // ptrmask bits already loaded
nb uintptr // number of bits in b at next read
endp *byte // final ptrmask byte to read (then repeat)
endnb uintptr // number of valid bits in *endp
pbits uintptr // alternate source of bits
// Heap bitmap output.
w uintptr // words processed
nw uintptr // number of words to process
hbitp *byte // next heap bitmap byte to write
hb uintptr // bits being prepared for *hbitp
)
hbitp = h.bitp
// Handle GC program. Delayed until this part of the code
// so that we can use the same double-checking mechanism
// as the 1-bit case. Nothing above could have encountered
// GC programs: the cases were all too small.
if typ.kind&kindGCProg != 0 {
heapBitsSetTypeGCProg(h, typ.ptrdata, typ.size, dataSize, size, addb(typ.gcdata, 4))
if doubleCheck {
// Double-check the heap bits written by GC program
// by running the GC program to create a 1-bit pointer mask
// and then jumping to the double-check code below.
// This doesn't catch bugs shared between the 1-bit and 4-bit
// GC program execution, but it does catch mistakes specific
// to just one of those and bugs in heapBitsSetTypeGCProg's
// implementation of arrays.
lock(&debugPtrmask.lock)
if debugPtrmask.data == nil {
debugPtrmask.data = (*byte)(persistentalloc(1<<20, 1, &memstats.other_sys))
}
ptrmask = debugPtrmask.data
runGCProg(addb(typ.gcdata, 4), nil, ptrmask, 1)
goto Phase4
}
return
}
// Note about sizes:
//
// typ.size is the number of words in the object,
// and typ.ptrdata is the number of words in the prefix
// of the object that contains pointers. That is, the final
// typ.size - typ.ptrdata words contain no pointers.
// This allows optimization of a common pattern where
// an object has a small header followed by a large scalar
// buffer. If we know the pointers are over, we don't have
// to scan the buffer's heap bitmap at all.
// The 1-bit ptrmasks are sized to contain only bits for
// the typ.ptrdata prefix, zero padded out to a full byte
// of bitmap. This code sets nw (below) so that heap bitmap
// bits are only written for the typ.ptrdata prefix; if there is
// more room in the allocated object, the next heap bitmap
// entry is a 00, indicating that there are no more pointers
// to scan. So only the ptrmask for the ptrdata bytes is needed.
//
// Replicated copies are not as nice: if there is an array of
// objects with scalar tails, all but the last tail does have to
// be initialized, because there is no way to say "skip forward".
// However, because of the possibility of a repeated type with
// size not a multiple of 4 pointers (one heap bitmap byte),
// the code already must handle the last ptrmask byte specially
// by treating it as containing only the bits for endnb pointers,
// where endnb <= 4. We represent large scalar tails that must
// be expanded in the replication by setting endnb larger than 4.
// This will have the effect of reading many bits out of b,
// but once the real bits are shifted out, b will supply as many
// zero bits as we try to read, which is exactly what we need.
p = ptrmask
if typ.size < dataSize {
// Filling in bits for an array of typ.
// Set up for repetition of ptrmask during main loop.
// Note that ptrmask describes only a prefix of
const maxBits = sys.PtrSize*8 - 7
if typ.ptrdata/sys.PtrSize <= maxBits {
// Entire ptrmask fits in uintptr with room for a byte fragment.
// Load into pbits and never read from ptrmask again.
// This is especially important when the ptrmask has
// fewer than 8 bits in it; otherwise the reload in the middle
// of the Phase 2 loop would itself need to loop to gather
// at least 8 bits.
// Accumulate ptrmask into b.
// ptrmask is sized to describe only typ.ptrdata, but we record
// it as describing typ.size bytes, since all the high bits are zero.
nb = typ.ptrdata / sys.PtrSize
for i := uintptr(0); i < nb; i += 8 {
b |= uintptr(*p) << i
p = add1(p)
}
nb = typ.size / sys.PtrSize
// Replicate ptrmask to fill entire pbits uintptr.
// Doubling and truncating is fewer steps than
// iterating by nb each time. (nb could be 1.)
// Since we loaded typ.ptrdata/ptrSize bits
// but are pretending to have typ.size/ptrSize,
// there might be no replication necessary/possible.
pbits = b
endnb = nb
if nb+nb <= maxBits {
for endnb <= sys.PtrSize*8 {
pbits |= pbits << endnb
endnb += endnb
}
// Truncate to a multiple of original ptrmask.
endnb = maxBits / nb * nb
pbits &= 1<<endnb - 1
b = pbits
nb = endnb
}
// Clear p and endp as sentinel for using pbits.
// Checked during Phase 2 loop.
p = nil
endp = nil
} else {
// Ptrmask is larger. Read it multiple times.
n := (typ.ptrdata/sys.PtrSize+7)/8 - 1
endp = addb(ptrmask, n)
endnb = typ.size/sys.PtrSize - n*8
}
}
if p != nil {
b = uintptr(*p)
p = add1(p)
nb = 8
}
if typ.size == dataSize {
// Single entry: can stop once we reach the non-pointer data.
nw = typ.ptrdata / sys.PtrSize
} else {
// Repeated instances of typ in an array.
// Have to process first N-1 entries in full, but can stop
// once we reach the non-pointer data in the final entry.
nw = ((dataSize/typ.size-1)*typ.size + typ.ptrdata) / sys.PtrSize
}
if nw == 0 {
// No pointers! Caller was supposed to check.
println("runtime: invalid type ", *typ._string)
throw("heapBitsSetType: called with non-pointer type")
return
}
if nw < 2 {
// Must write at least 2 words, because the "no scan"
// encoding doesn't take effect until the third word.
nw = 2
}
// Phase 1: Special case for leading byte (shift==0) or half-byte (shift==4).
// The leading byte is special because it contains the bits for words 0 and 1,
// which do not have the marked bits set.
// The leading half-byte is special because it's a half a byte and must be
// manipulated atomically.
switch {
default:
throw("heapBitsSetType: unexpected shift")
case h.shift == 0:
// Ptrmask and heap bitmap are aligned.
// Handle first byte of bitmap specially.
// The first byte we write out contains the first two words of the object.
// In those words, the mark bits are mark and checkmark, respectively,
// and must not be set. In all following words, we want to set the mark bit
// as a signal that the object continues to the next 2-bit entry in the bitmap.
hb = b & bitPointerAll
hb |= bitMarked<<(2*heapBitsShift) | bitMarked<<(3*heapBitsShift)
if w += 4; w >= nw {
goto Phase3
}
*hbitp = uint8(hb)
hbitp = subtract1(hbitp)
b >>= 4
nb -= 4
case sys.PtrSize == 8 && h.shift == 2:
// Ptrmask and heap bitmap are misaligned.
// The bits for the first two words are in a byte shared with another object
// and must be updated atomically.
// NOTE(rsc): The atomic here may not be necessary.
// We took care of 1-word and 2-word objects above,
// so this is at least a 6-word object, so our start bits
// are shared only with the type bits of another object,
// not with its mark bit. Since there is only one allocation
// from a given span at a time, we should be able to set
// these bits non-atomically. Not worth the risk right now.
hb = (b & 3) << (2 * heapBitsShift)
b >>= 2
nb -= 2
// Note: no bitMarker in hb because the first two words don't get markers from us.
if gcphase == _GCoff {
*hbitp |= uint8(hb)
} else {
atomic.Or8(hbitp, uint8(hb))
}
hbitp = subtract1(hbitp)
if w += 2; w >= nw {
// We know that there is more data, because we handled 2-word objects above.
// This must be at least a 6-word object. If we're out of pointer words,
// mark no scan in next bitmap byte and finish.
hb = 0
w += 4
goto Phase3
}
}
// Phase 2: Full bytes in bitmap, up to but not including write to last byte (full or partial) in bitmap.
// The loop computes the bits for that last write but does not execute the write;
// it leaves the bits in hb for processing by phase 3.
// To avoid repeated adjustment of nb, we subtract out the 4 bits we're going to
// use in the first half of the loop right now, and then we only adjust nb explicitly
// if the 8 bits used by each iteration isn't balanced by 8 bits loaded mid-loop.
nb -= 4
for {
// Emit bitmap byte.
// b has at least nb+4 bits, with one exception:
// if w+4 >= nw, then b has only nw-w bits,
// but we'll stop at the break and then truncate
// appropriately in Phase 3.
hb = b & bitPointerAll
hb |= bitMarkedAll
if w += 4; w >= nw {
break
}
*hbitp = uint8(hb)
hbitp = subtract1(hbitp)
b >>= 4
// Load more bits. b has nb right now.
if p != endp {
// Fast path: keep reading from ptrmask.
// nb unmodified: we just loaded 8 bits,
// and the next iteration will consume 8 bits,
// leaving us with the same nb the next time we're here.
if nb < 8 {
b |= uintptr(*p) << nb
p = add1(p)
} else {
// Reduce the number of bits in b.
// This is important if we skipped
// over a scalar tail, since nb could
// be larger than the bit width of b.
nb -= 8
}
} else if p == nil {
// Almost as fast path: track bit count and refill from pbits.
// For short repetitions.
if nb < 8 {
b |= pbits << nb
nb += endnb
}
nb -= 8 // for next iteration
} else {
// Slow path: reached end of ptrmask.
// Process final partial byte and rewind to start.
b |= uintptr(*p) << nb
nb += endnb
if nb < 8 {
b |= uintptr(*ptrmask) << nb
p = add1(ptrmask)
} else {
nb -= 8
p = ptrmask
}
}
// Emit bitmap byte.
hb = b & bitPointerAll
hb |= bitMarkedAll
if w += 4; w >= nw {
break
}
*hbitp = uint8(hb)
hbitp = subtract1(hbitp)
b >>= 4
}
Phase3:
// Phase 3: Write last byte or partial byte and zero the rest of the bitmap entries.
if w > nw {
// Counting the 4 entries in hb not yet written to memory,
// there are more entries than possible pointer slots.
// Discard the excess entries (can't be more than 3).
mask := uintptr(1)<<(4-(w-nw)) - 1
hb &= mask | mask<<4 // apply mask to both pointer bits and mark bits
}
// Change nw from counting possibly-pointer words to total words in allocation.
nw = size / sys.PtrSize
// Write whole bitmap bytes.
// The first is hb, the rest are zero.
if w <= nw {
*hbitp = uint8(hb)
hbitp = subtract1(hbitp)
hb = 0 // for possible final half-byte below
for w += 4; w <= nw; w += 4 {
*hbitp = 0
hbitp = subtract1(hbitp)
}
}
// Write final partial bitmap byte if any.
// We know w > nw, or else we'd still be in the loop above.
// It can be bigger only due to the 4 entries in hb that it counts.
// If w == nw+4 then there's nothing left to do: we wrote all nw entries
// and can discard the 4 sitting in hb.
// But if w == nw+2, we need to write first two in hb.
// The byte is shared with the next object so we may need an atomic.
if w == nw+2 {
if gcphase == _GCoff {
*hbitp = *hbitp&^(bitPointer|bitMarked|(bitPointer|bitMarked)<<heapBitsShift) | uint8(hb)
} else {
atomic.And8(hbitp, ^uint8(bitPointer|bitMarked|(bitPointer|bitMarked)<<heapBitsShift))
atomic.Or8(hbitp, uint8(hb))
}
}
Phase4:
// Phase 4: all done, but perhaps double check.
if doubleCheck {
end := heapBitsForAddr(x + size)
if typ.kind&kindGCProg == 0 && (hbitp != end.bitp || (w == nw+2) != (end.shift == 2)) {
println("ended at wrong bitmap byte for", *typ._string, "x", dataSize/typ.size)
print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
h0 := heapBitsForAddr(x)
print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
print("ended at hbitp=", hbitp, " but next starts at bitp=", end.bitp, " shift=", end.shift, "\n")
throw("bad heapBitsSetType")
}
// Double-check that bits to be written were written correctly.
// Does not check that other bits were not written, unfortunately.
h := heapBitsForAddr(x)
nptr := typ.ptrdata / sys.PtrSize
ndata := typ.size / sys.PtrSize
count := dataSize / typ.size
totalptr := ((count-1)*typ.size + typ.ptrdata) / sys.PtrSize
for i := uintptr(0); i < size/sys.PtrSize; i++ {
j := i % ndata
var have, want uint8
have = (*h.bitp >> h.shift) & (bitPointer | bitMarked)
if i >= totalptr {
want = 0 // deadmarker
if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 {
want = bitMarked
}
} else {
if j < nptr && (*addb(ptrmask, j/8)>>(j%8))&1 != 0 {
want |= bitPointer
}
if i >= 2 {
want |= bitMarked
} else {
have &^= bitMarked
}
}
if have != want {
println("mismatch writing bits for", *typ._string, "x", dataSize/typ.size)
print("typ.size=", typ.size, " typ.ptrdata=", typ.ptrdata, " dataSize=", dataSize, " size=", size, "\n")
print("kindGCProg=", typ.kind&kindGCProg != 0, "\n")
print("w=", w, " nw=", nw, " b=", hex(b), " nb=", nb, " hb=", hex(hb), "\n")
h0 := heapBitsForAddr(x)
print("initial bits h0.bitp=", h0.bitp, " h0.shift=", h0.shift, "\n")
print("current bits h.bitp=", h.bitp, " h.shift=", h.shift, " *h.bitp=", hex(*h.bitp), "\n")
print("ptrmask=", ptrmask, " p=", p, " endp=", endp, " endnb=", endnb, " pbits=", hex(pbits), " b=", hex(b), " nb=", nb, "\n")
println("at word", i, "offset", i*sys.PtrSize, "have", have, "want", want)
if typ.kind&kindGCProg != 0 {
println("GC program:")
dumpGCProg(addb(typ.gcdata, 4))
}
throw("bad heapBitsSetType")
}
h = h.next()
}
if ptrmask == debugPtrmask.data {
unlock(&debugPtrmask.lock)
}
}
}
var debugPtrmask struct {
lock mutex
data *byte
}
// heapBitsSetTypeGCProg implements heapBitsSetType using a GC program.
// progSize is the size of the memory described by the program.
// elemSize is the size of the element that the GC program describes (a prefix of).
// dataSize is the total size of the intended data, a multiple of elemSize.
// allocSize is the total size of the allocated memory.
//
// GC programs are only used for large allocations.
// heapBitsSetType requires that allocSize is a multiple of 4 words,
// so that the relevant bitmap bytes are not shared with surrounding
// objects and need not be accessed with atomic instructions.
func heapBitsSetTypeGCProg(h heapBits, progSize, elemSize, dataSize, allocSize uintptr, prog *byte) {
if sys.PtrSize == 8 && allocSize%(4*sys.PtrSize) != 0 {
// Alignment will be wrong.
throw("heapBitsSetTypeGCProg: small allocation")
}
var totalBits uintptr
if elemSize == dataSize {
totalBits = runGCProg(prog, nil, h.bitp, 2)
if totalBits*sys.PtrSize != progSize {
println("runtime: heapBitsSetTypeGCProg: total bits", totalBits, "but progSize", progSize)
throw("heapBitsSetTypeGCProg: unexpected bit count")
}
} else {
count := dataSize / elemSize
// Piece together program trailer to run after prog that does:
// literal(0)
// repeat(1, elemSize-progSize-1) // zeros to fill element size
// repeat(elemSize, count-1) // repeat that element for count
// This zero-pads the data remaining in the first element and then
// repeats that first element to fill the array.
var trailer [40]byte // 3 varints (max 10 each) + some bytes
i := 0
if n := elemSize/sys.PtrSize - progSize/sys.PtrSize; n > 0 {
// literal(0)
trailer[i] = 0x01
i++
trailer[i] = 0
i++
if n > 1 {
// repeat(1, n-1)
trailer[i] = 0x81
i++
n--
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
}
}
// repeat(elemSize/ptrSize, count-1)
trailer[i] = 0x80
i++
n := elemSize / sys.PtrSize
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
n = count - 1
for ; n >= 0x80; n >>= 7 {
trailer[i] = byte(n | 0x80)
i++
}
trailer[i] = byte(n)
i++
trailer[i] = 0
i++
runGCProg(prog, &trailer[0], h.bitp, 2)
// Even though we filled in the full array just now,
// record that we only filled in up to the ptrdata of the
// last element. This will cause the code below to
// memclr the dead section of the final array element,
// so that scanobject can stop early in the final element.
totalBits = (elemSize*(count-1) + progSize) / sys.PtrSize
}
endProg := unsafe.Pointer(subtractb(h.bitp, (totalBits+3)/4))
endAlloc := unsafe.Pointer(subtractb(h.bitp, allocSize/heapBitmapScale))
memclr(add(endAlloc, 1), uintptr(endProg)-uintptr(endAlloc))
}
// progToPointerMask returns the 1-bit pointer mask output by the GC program prog.
// size the size of the region described by prog, in bytes.
// The resulting bitvector will have no more than size/ptrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
n := (size/sys.PtrSize + 7) / 8
x := (*[1 << 30]byte)(persistentalloc(n+1, 1, &memstats.buckhash_sys))[:n+1]
x[len(x)-1] = 0xa1 // overflow check sentinel
n = runGCProg(prog, nil, &x[0], 1)
if x[len(x)-1] != 0xa1 {
throw("progToPointerMask: overflow")
}
return bitvector{int32(n), &x[0]}
}
// Packed GC pointer bitmaps, aka GC programs.
//
// For large types containing arrays, the type information has a
// natural repetition that can be encoded to save space in the
// binary and in the memory representation of the type information.
//
// The encoding is a simple Lempel-Ziv style bytecode machine
// with the following instructions:
//
// 00000000: stop
// 0nnnnnnn: emit n bits copied from the next (n+7)/8 bytes
// 10000000 n c: repeat the previous n bits c times; n, c are varints
// 1nnnnnnn c: repeat the previous n bits c times; c is a varint
// runGCProg executes the GC program prog, and then trailer if non-nil,
// writing to dst with entries of the given size.
// If size == 1, dst is a 1-bit pointer mask laid out moving forward from dst.
// If size == 2, dst is the 2-bit heap bitmap, and writes move backward
// starting at dst (because the heap bitmap does). In this case, the caller guarantees
// that only whole bytes in dst need to be written.
//
// runGCProg returns the number of 1- or 2-bit entries written to memory.
func runGCProg(prog, trailer, dst *byte, size int) uintptr {
dstStart := dst
// Bits waiting to be written to memory.
var bits uintptr
var nbits uintptr
p := prog
Run:
for {
// Flush accumulated full bytes.
// The rest of the loop assumes that nbits <= 7.
for ; nbits >= 8; nbits -= 8 {
if size == 1 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
} else {
v := bits&bitPointerAll | bitMarkedAll
*dst = uint8(v)
dst = subtract1(dst)
bits >>= 4
v = bits&bitPointerAll | bitMarkedAll
*dst = uint8(v)
dst = subtract1(dst)
bits >>= 4
}
}
// Process one instruction.
inst := uintptr(*p)
p = add1(p)
n := inst & 0x7F
if inst&0x80 == 0 {
// Literal bits; n == 0 means end of program.
if n == 0 {
// Program is over; continue in trailer if present.
if trailer != nil {
//println("trailer")
p = trailer
trailer = nil
continue
}
//println("done")
break Run
}
//println("lit", n, dst)
nbyte := n / 8
for i := uintptr(0); i < nbyte; i++ {
bits |= uintptr(*p) << nbits
p = add1(p)
if size == 1 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
} else {
v := bits&0xf | bitMarkedAll
*dst = uint8(v)
dst = subtract1(dst)
bits >>= 4
v = bits&0xf | bitMarkedAll
*dst = uint8(v)
dst = subtract1(dst)
bits >>= 4
}
}
if n %= 8; n > 0 {
bits |= uintptr(*p) << nbits
p = add1(p)
nbits += n
}
continue Run
}
// Repeat. If n == 0, it is encoded in a varint in the next bytes.
if n == 0 {
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
n |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
}
// Count is encoded in a varint in the next bytes.
c := uintptr(0)
for off := uint(0); ; off += 7 {
x := uintptr(*p)
p = add1(p)
c |= (x & 0x7F) << off
if x&0x80 == 0 {
break
}
}
c *= n // now total number of bits to copy
// If the number of bits being repeated is small, load them
// into a register and use that register for the entire loop
// instead of repeatedly reading from memory.
// Handling fewer than 8 bits here makes the general loop simpler.
// The cutoff is ptrSize*8 - 7 to guarantee that when we add
// the pattern to a bit buffer holding at most 7 bits (a partial byte)
// it will not overflow.
src := dst
const maxBits = sys.PtrSize*8 - 7
if n <= maxBits {
// Start with bits in output buffer.
pattern := bits
npattern := nbits
// If we need more bits, fetch them from memory.
if size == 1 {
src = subtract1(src)
for npattern < n {
pattern <<= 8
pattern |= uintptr(*src)
src = subtract1(src)
npattern += 8
}
} else {
src = add1(src)
for npattern < n {
pattern <<= 4
pattern |= uintptr(*src) & 0xf
src = add1(src)
npattern += 4
}
}
// We started with the whole bit output buffer,
// and then we loaded bits from whole bytes.
// Either way, we might now have too many instead of too few.
// Discard the extra.
if npattern > n {
pattern >>= npattern - n
npattern = n
}
// Replicate pattern to at most maxBits.
if npattern == 1 {
// One bit being repeated.
// If the bit is 1, make the pattern all 1s.
// If the bit is 0, the pattern is already all 0s,
// but we can claim that the number of bits
// in the word is equal to the number we need (c),
// because right shift of bits will zero fill.
if pattern == 1 {
pattern = 1<<maxBits - 1
npattern = maxBits
} else {
npattern = c
}
} else {
b := pattern
nb := npattern
if nb+nb <= maxBits {
// Double pattern until the whole uintptr is filled.
for nb <= sys.PtrSize*8 {
b |= b << nb
nb += nb
}
// Trim away incomplete copy of original pattern in high bits.
// TODO(rsc): Replace with table lookup or loop on systems without divide?
nb = maxBits / npattern * npattern
b &= 1<<nb - 1
pattern = b
npattern = nb
}
}
// Add pattern to bit buffer and flush bit buffer, c/npattern times.
// Since pattern contains >8 bits, there will be full bytes to flush
// on each iteration.
for ; c >= npattern; c -= npattern {
bits |= pattern << nbits
nbits += npattern
if size == 1 {
for nbits >= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
nbits -= 8
}
} else {
for nbits >= 4 {
*dst = uint8(bits&0xf | bitMarkedAll)
dst = subtract1(dst)
bits >>= 4
nbits -= 4
}
}
}
// Add final fragment to bit buffer.
if c > 0 {
pattern &= 1<<c - 1
bits |= pattern << nbits
nbits += c
}
continue Run
}
// Repeat; n too large to fit in a register.
// Since nbits <= 7, we know the first few bytes of repeated data
// are already written to memory.
off := n - nbits // n > nbits because n > maxBits and nbits <= 7
if size == 1 {
// Leading src fragment.
src = subtractb(src, (off+7)/8)
if frag := off & 7; frag != 0 {
bits |= uintptr(*src) >> (8 - frag) << nbits
src = add1(src)
nbits += frag
c -= frag
}
// Main loop: load one byte, write another.
// The bits are rotating through the bit buffer.
for i := c / 8; i > 0; i-- {
bits |= uintptr(*src) << nbits
src = add1(src)
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
// Final src fragment.
if c %= 8; c > 0 {
bits |= (uintptr(*src) & (1<<c - 1)) << nbits
nbits += c
}
} else {
// Leading src fragment.
src = addb(src, (off+3)/4)
if frag := off & 3; frag != 0 {
bits |= (uintptr(*src) & 0xf) >> (4 - frag) << nbits
src = subtract1(src)
nbits += frag
c -= frag
}
// Main loop: load one byte, write another.
// The bits are rotating through the bit buffer.
for i := c / 4; i > 0; i-- {
bits |= (uintptr(*src) & 0xf) << nbits
src = subtract1(src)
*dst = uint8(bits&0xf | bitMarkedAll)
dst = subtract1(dst)
bits >>= 4
}
// Final src fragment.
if c %= 4; c > 0 {
bits |= (uintptr(*src) & (1<<c - 1)) << nbits
nbits += c
}
}
}
// Write any final bits out, using full-byte writes, even for the final byte.
var totalBits uintptr
if size == 1 {
totalBits = (uintptr(unsafe.Pointer(dst))-uintptr(unsafe.Pointer(dstStart)))*8 + nbits
nbits += -nbits & 7
for ; nbits > 0; nbits -= 8 {
*dst = uint8(bits)
dst = add1(dst)
bits >>= 8
}
} else {
totalBits = (uintptr(unsafe.Pointer(dstStart))-uintptr(unsafe.Pointer(dst)))*4 + nbits
nbits += -nbits & 3
for ; nbits > 0; nbits -= 4 {
v := bits&0xf | bitMarkedAll
*dst = uint8(v)
dst = subtract1(dst)
bits >>= 4
}
// Clear the mark bits in the first two entries.
// They are the actual mark and checkmark bits,
// not non-dead markers. It simplified the code
// above to set the marker in every bit written and
// then clear these two as a special case at the end.
*dstStart &^= bitMarked | bitMarked<<heapBitsShift
}
return totalBits
}
func dumpGCProg(p *byte) {
nptr := 0
for {
x := *p
p = add1(p)
if x == 0 {
print("\t", nptr, " end\n")
break
}
if x&0x80 == 0 {
print("\t", nptr, " lit ", x, ":")
n := int(x+7) / 8
for i := 0; i < n; i++ {
print(" ", hex(*p))
p = add1(p)
}
print("\n")
nptr += int(x)
} else {
nbit := int(x &^ 0x80)
if nbit == 0 {
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
nbit |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
}
count := 0
for nb := uint(0); ; nb += 7 {
x := *p
p = add1(p)
count |= int(x&0x7f) << nb
if x&0x80 == 0 {
break
}
}
print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
nptr += nbit * count
}
}
}
// Testing.
func getgcmaskcb(frame *stkframe, ctxt unsafe.Pointer) bool {
target := (*stkframe)(ctxt)
if frame.sp <= target.sp && target.sp < frame.varp {
*target = *frame
return false
}
return true
}
// gcbits returns the GC type info for x, for testing.
// The result is the bitmap entries (0 or 1), one entry per byte.
//go:linkname reflect_gcbits reflect.gcbits
func reflect_gcbits(x interface{}) []byte {
ret := getgcmask(x)
typ := (*ptrtype)(unsafe.Pointer(efaceOf(&x)._type)).elem
nptr := typ.ptrdata / sys.PtrSize
for uintptr(len(ret)) > nptr && ret[len(ret)-1] == 0 {
ret = ret[:len(ret)-1]
}
return ret
}
// Returns GC type info for object p for testing.
func getgcmask(ep interface{}) (mask []byte) {
e := *efaceOf(&ep)
p := e.data
t := e._type
// data or bss
for datap := &firstmoduledata; datap != nil; datap = datap.next {
// data
if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
bitmap := datap.gcdatamask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
off := (uintptr(p) + i - datap.data) / sys.PtrSize
mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
// bss
if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
bitmap := datap.gcbssmask.bytedata
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
off := (uintptr(p) + i - datap.bss) / sys.PtrSize
mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
return
}
}
// heap
var n uintptr
var base uintptr
if mlookup(uintptr(p), &base, &n, nil) != 0 {
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
hbits := heapBitsForAddr(base + i)
if hbits.isPointer() {
mask[i/sys.PtrSize] = 1
}
if i >= 2*sys.PtrSize && !hbits.isMarked() {
mask = mask[:i/sys.PtrSize]
break
}
}
return
}
// stack
if _g_ := getg(); _g_.m.curg.stack.lo <= uintptr(p) && uintptr(p) < _g_.m.curg.stack.hi {
var frame stkframe
frame.sp = uintptr(p)
_g_ := getg()
gentraceback(_g_.m.curg.sched.pc, _g_.m.curg.sched.sp, 0, _g_.m.curg, 0, nil, 1000, getgcmaskcb, noescape(unsafe.Pointer(&frame)), 0)
if frame.fn != nil {
f := frame.fn
targetpc := frame.continpc
if targetpc == 0 {
return
}
if targetpc != f.entry {
targetpc--
}
pcdata := pcdatavalue(f, _PCDATA_StackMapIndex, targetpc, nil)
if pcdata == -1 {
return
}
stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
if stkmap == nil || stkmap.n <= 0 {
return
}
bv := stackmapdata(stkmap, pcdata)
size := uintptr(bv.n) * sys.PtrSize
n := (*ptrtype)(unsafe.Pointer(t)).elem.size
mask = make([]byte, n/sys.PtrSize)
for i := uintptr(0); i < n; i += sys.PtrSize {
bitmap := bv.bytedata
off := (uintptr(p) + i - frame.varp + size) / sys.PtrSize
mask[i/sys.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
}
}
return
}
// otherwise, not something the GC knows about.
// possibly read-only data, like malloc(0).
// must not have pointers
return
}