runtime: add the allocation headers GOEXPERIMENT and fork files

[gostls13.git] / src / runtime / mbitmap.go

// 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.

package runtime

import (
        "internal/goarch"
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

// addb returns the byte pointer p+n.
//
//go:nowritebarrier
//go:nosplit
func addb(p *byte, n uintptr) *byte {
        // Note: wrote out full expression instead of calling add(p, n)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}

// subtractb returns the byte pointer p-n.
//
//go:nowritebarrier
//go:nosplit
func subtractb(p *byte, n uintptr) *byte {
        // Note: wrote out full expression instead of calling add(p, -n)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - n))
}

// add1 returns the byte pointer p+1.
//
//go:nowritebarrier
//go:nosplit
func add1(p *byte) *byte {
        // Note: wrote out full expression instead of calling addb(p, 1)
        // to reduce the number of temporaries generated by the
        // compiler for this trivial expression during inlining.
        return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + 1))
}

// subtract1 returns the byte pointer p-1.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nowritebarrier
//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))
}

// markBits provides access to the mark bit for an object in the heap.
// bytep points to the byte holding the mark bit.
// mask is a byte with a single bit set that can be &ed with *bytep
// to see if the bit has been set.
// *m.byte&m.mask != 0 indicates the mark bit is set.
// index can be used along with span information to generate
// the address of the object in the heap.
// We maintain one set of mark bits for allocation and one for
// marking purposes.
type markBits struct {
        bytep *uint8
        mask  uint8
        index uintptr
}

//go:nosplit
func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
        bytep, mask := s.allocBits.bitp(allocBitIndex)
        return markBits{bytep, mask, allocBitIndex}
}

// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
// and negates them so that ctz (count trailing zeros) instructions
// can be used. It then places these 8 bytes into the cached 64 bit
// s.allocCache.
func (s *mspan) refillAllocCache(whichByte uint16) {
        bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(uintptr(whichByte))))
        aCache := uint64(0)
        aCache |= uint64(bytes[0])
        aCache |= uint64(bytes[1]) << (1 * 8)
        aCache |= uint64(bytes[2]) << (2 * 8)
        aCache |= uint64(bytes[3]) << (3 * 8)
        aCache |= uint64(bytes[4]) << (4 * 8)
        aCache |= uint64(bytes[5]) << (5 * 8)
        aCache |= uint64(bytes[6]) << (6 * 8)
        aCache |= uint64(bytes[7]) << (7 * 8)
        s.allocCache = ^aCache
}

// nextFreeIndex returns the index of the next free object in s at
// or after s.freeindex.
// There are hardware instructions that can be used to make this
// faster if profiling warrants it.
func (s *mspan) nextFreeIndex() uint16 {
        sfreeindex := s.freeindex
        snelems := s.nelems
        if sfreeindex == snelems {
                return sfreeindex
        }
        if sfreeindex > snelems {
                throw("s.freeindex > s.nelems")
        }

        aCache := s.allocCache

        bitIndex := sys.TrailingZeros64(aCache)
        for bitIndex == 64 {
                // Move index to start of next cached bits.
                sfreeindex = (sfreeindex + 64) &^ (64 - 1)
                if sfreeindex >= snelems {
                        s.freeindex = snelems
                        return snelems
                }
                whichByte := sfreeindex / 8
                // Refill s.allocCache with the next 64 alloc bits.
                s.refillAllocCache(whichByte)
                aCache = s.allocCache
                bitIndex = sys.TrailingZeros64(aCache)
                // nothing available in cached bits
                // grab the next 8 bytes and try again.
        }
        result := sfreeindex + uint16(bitIndex)
        if result >= snelems {
                s.freeindex = snelems
                return snelems
        }

        s.allocCache >>= uint(bitIndex + 1)
        sfreeindex = result + 1

        if sfreeindex%64 == 0 && sfreeindex != snelems {
                // We just incremented s.freeindex so it isn't 0.
                // As each 1 in s.allocCache was encountered and used for allocation
                // it was shifted away. At this point s.allocCache contains all 0s.
                // Refill s.allocCache so that it corresponds
                // to the bits at s.allocBits starting at s.freeindex.
                whichByte := sfreeindex / 8
                s.refillAllocCache(whichByte)
        }
        s.freeindex = sfreeindex
        return result
}

// isFree reports whether the index'th object in s is unallocated.
//
// The caller must ensure s.state is mSpanInUse, and there must have
// been no preemption points since ensuring this (which could allow a
// GC transition, which would allow the state to change).
func (s *mspan) isFree(index uintptr) bool {
        if index < uintptr(s.freeIndexForScan) {
                return false
        }
        bytep, mask := s.allocBits.bitp(index)
        return *bytep&mask == 0
}

// divideByElemSize returns n/s.elemsize.
// n must be within [0, s.npages*_PageSize),
// or may be exactly s.npages*_PageSize
// if s.elemsize is from sizeclasses.go.
//
// nosplit, because it is called by objIndex, which is nosplit
//
//go:nosplit
func (s *mspan) divideByElemSize(n uintptr) uintptr {
        const doubleCheck = false

        // See explanation in mksizeclasses.go's computeDivMagic.
        q := uintptr((uint64(n) * uint64(s.divMul)) >> 32)

        if doubleCheck && q != n/s.elemsize {
                println(n, "/", s.elemsize, "should be", n/s.elemsize, "but got", q)
                throw("bad magic division")
        }
        return q
}

// nosplit, because it is called by other nosplit code like findObject
//
//go:nosplit
func (s *mspan) objIndex(p uintptr) uintptr {
        return s.divideByElemSize(p - s.base())
}

func markBitsForAddr(p uintptr) markBits {
        s := spanOf(p)
        objIndex := s.objIndex(p)
        return s.markBitsForIndex(objIndex)
}

func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
        bytep, mask := s.gcmarkBits.bitp(objIndex)
        return markBits{bytep, mask, objIndex}
}

func (s *mspan) markBitsForBase() markBits {
        return markBits{&s.gcmarkBits.x, uint8(1), 0}
}

// isMarked reports whether mark bit m is set.
func (m markBits) isMarked() bool {
        return *m.bytep&m.mask != 0
}

// setMarked sets the marked bit in the markbits, atomically.
func (m markBits) setMarked() {
        // Might be racing with other updates, so use atomic update always.
        // We used to be clever here and use a non-atomic update in certain
        // cases, but it's not worth the risk.
        atomic.Or8(m.bytep, m.mask)
}

// setMarkedNonAtomic sets the marked bit in the markbits, non-atomically.
func (m markBits) setMarkedNonAtomic() {
        *m.bytep |= m.mask
}

// clearMarked clears the marked bit in the markbits, atomically.
func (m markBits) clearMarked() {
        // Might be racing with other updates, so use atomic update always.
        // We used to be clever here and use a non-atomic update in certain
        // cases, but it's not worth the risk.
        atomic.And8(m.bytep, ^m.mask)
}

// markBitsForSpan returns the markBits for the span base address base.
func markBitsForSpan(base uintptr) (mbits markBits) {
        mbits = markBitsForAddr(base)
        if mbits.mask != 1 {
                throw("markBitsForSpan: unaligned start")
        }
        return mbits
}

// advance advances the markBits to the next object in the span.
func (m *markBits) advance() {
        if m.mask == 1<<7 {
                m.bytep = (*uint8)(unsafe.Pointer(uintptr(unsafe.Pointer(m.bytep)) + 1))
                m.mask = 1
        } else {
                m.mask = m.mask << 1
        }
        m.index++
}

// clobberdeadPtr is a special value that is used by the compiler to
// clobber dead stack slots, when -clobberdead flag is set.
const clobberdeadPtr = uintptr(0xdeaddead | 0xdeaddead<<((^uintptr(0)>>63)*32))

// badPointer throws bad pointer in heap panic.
func badPointer(s *mspan, p, refBase, refOff uintptr) {
        // Typically this indicates an incorrect use
        // of unsafe or cgo to store a bad pointer in
        // the Go heap. It may also indicate a runtime
        // bug.
        //
        // TODO(austin): We could be more aggressive
        // and detect pointers to unallocated objects
        // in allocated spans.
        printlock()
        print("runtime: pointer ", hex(p))
        if s != nil {
                state := s.state.get()
                if state != mSpanInUse {
                        print(" to unallocated span")
                } else {
                        print(" to unused region of span")
                }
                print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", state)
        }
        print("\n")
        if refBase != 0 {
                print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
                gcDumpObject("object", refBase, refOff)
        }
        getg().m.traceback = 2
        throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
}

// findObject returns the base address for the heap object containing
// the address p, the object's span, and the index of the object in s.
// If p does not point into a heap object, it returns base == 0.
//
// If p points is an invalid heap pointer and debug.invalidptr != 0,
// findObject panics.
//
// refBase and refOff optionally give the base address of the object
// in which the pointer p was found and the byte offset at which it
// was found. These are used for error reporting.
//
// It is nosplit so it is safe for p to be a pointer to the current goroutine's stack.
// Since p is a uintptr, it would not be adjusted if the stack were to move.
//
//go:nosplit
func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex uintptr) {
        s = spanOf(p)
        // If s is nil, the virtual address has never been part of the heap.
        // This pointer may be to some mmap'd region, so we allow it.
        if s == nil {
                if (GOARCH == "amd64" || GOARCH == "arm64") && p == clobberdeadPtr && debug.invalidptr != 0 {
                        // Crash if clobberdeadPtr is seen. Only on AMD64 and ARM64 for now,
                        // as they are the only platform where compiler's clobberdead mode is
                        // implemented. On these platforms clobberdeadPtr cannot be a valid address.
                        badPointer(s, p, refBase, refOff)
                }
                return
        }
        // If p is a bad pointer, it may not be in s's bounds.
        //
        // Check s.state to synchronize with span initialization
        // before checking other fields. See also spanOfHeap.
        if state := s.state.get(); state != mSpanInUse || p < s.base() || p >= s.limit {
                // Pointers into stacks are also ok, the runtime manages these explicitly.
                if state == mSpanManual {
                        return
                }
                // The following ensures that we are rigorous about what data
                // structures hold valid pointers.
                if debug.invalidptr != 0 {
                        badPointer(s, p, refBase, refOff)
                }
                return
        }

        objIndex = s.objIndex(p)
        base = s.base() + objIndex*s.elemsize
        return
}

// reflect_verifyNotInHeapPtr reports whether converting the not-in-heap pointer into a unsafe.Pointer is ok.
//
//go:linkname reflect_verifyNotInHeapPtr reflect.verifyNotInHeapPtr
func reflect_verifyNotInHeapPtr(p uintptr) bool {
        // Conversion to a pointer is ok as long as findObject above does not call badPointer.
        // Since we're already promised that p doesn't point into the heap, just disallow heap
        // pointers and the special clobbered pointer.
        return spanOf(p) == nil && p != clobberdeadPtr
}

const ptrBits = 8 * goarch.PtrSize

// bulkBarrierBitmap executes write barriers for copying from [src,
// src+size) to [dst, dst+size) using a 1-bit pointer bitmap. src is
// assumed to start maskOffset bytes into the data covered by the
// bitmap in bits (which may not be a multiple of 8).
//
// This is used by bulkBarrierPreWrite for writes to data and BSS.
//
//go:nosplit
func bulkBarrierBitmap(dst, src, size, maskOffset uintptr, bits *uint8) {
        word := maskOffset / goarch.PtrSize
        bits = addb(bits, word/8)
        mask := uint8(1) << (word % 8)

        buf := &getg().m.p.ptr().wbBuf
        for i := uintptr(0); i < size; i += goarch.PtrSize {
                if mask == 0 {
                        bits = addb(bits, 1)
                        if *bits == 0 {
                                // Skip 8 words.
                                i += 7 * goarch.PtrSize
                                continue
                        }
                        mask = 1
                }
                if *bits&mask != 0 {
                        dstx := (*uintptr)(unsafe.Pointer(dst + i))
                        if src == 0 {
                                p := buf.get1()
                                p[0] = *dstx
                        } else {
                                srcx := (*uintptr)(unsafe.Pointer(src + i))
                                p := buf.get2()
                                p[0] = *dstx
                                p[1] = *srcx
                        }
                }
                mask <<= 1
        }
}

// typeBitsBulkBarrier executes a write barrier for every
// pointer that would be copied from [src, src+size) to [dst,
// dst+size) by a memmove using the type bitmap to locate those
// pointer slots.
//
// The type typ must correspond exactly to [src, src+size) and [dst, dst+size).
// dst, src, and size must be pointer-aligned.
// The type typ must have a plain bitmap, not a GC program.
// The only use of this function is in channel sends, and the
// 64 kB channel element limit takes care of this for us.
//
// Must not be preempted because it typically runs right before memmove,
// and the GC must observe them as an atomic action.
//
// Callers must perform cgo checks if goexperiment.CgoCheck2.
//
//go:nosplit
func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
        if typ == nil {
                throw("runtime: typeBitsBulkBarrier without type")
        }
        if typ.Size_ != size {
                println("runtime: typeBitsBulkBarrier with type ", toRType(typ).string(), " of size ", typ.Size_, " but memory size", size)
                throw("runtime: invalid typeBitsBulkBarrier")
        }
        if typ.Kind_&kindGCProg != 0 {
                println("runtime: typeBitsBulkBarrier with type ", toRType(typ).string(), " with GC prog")
                throw("runtime: invalid typeBitsBulkBarrier")
        }
        if !writeBarrier.enabled {
                return
        }
        ptrmask := typ.GCData
        buf := &getg().m.p.ptr().wbBuf
        var bits uint32
        for i := uintptr(0); i < typ.PtrBytes; i += goarch.PtrSize {
                if i&(goarch.PtrSize*8-1) == 0 {
                        bits = uint32(*ptrmask)
                        ptrmask = addb(ptrmask, 1)
                } else {
                        bits = bits >> 1
                }
                if bits&1 != 0 {
                        dstx := (*uintptr)(unsafe.Pointer(dst + i))
                        srcx := (*uintptr)(unsafe.Pointer(src + i))
                        p := buf.get2()
                        p[0] = *dstx
                        p[1] = *srcx
                }
        }
}

// countAlloc returns the number of objects allocated in span s by
// scanning the allocation bitmap.
func (s *mspan) countAlloc() int {
        count := 0
        bytes := divRoundUp(uintptr(s.nelems), 8)
        // Iterate over each 8-byte chunk and count allocations
        // with an intrinsic. Note that newMarkBits guarantees that
        // gcmarkBits will be 8-byte aligned, so we don't have to
        // worry about edge cases, irrelevant bits will simply be zero.
        for i := uintptr(0); i < bytes; i += 8 {
                // Extract 64 bits from the byte pointer and get a OnesCount.
                // Note that the unsafe cast here doesn't preserve endianness,
                // but that's OK. We only care about how many bits are 1, not
                // about the order we discover them in.
                mrkBits := *(*uint64)(unsafe.Pointer(s.gcmarkBits.bytep(i)))
                count += sys.OnesCount64(mrkBits)
        }
        return count
}

// Read the bytes starting at the aligned pointer p into a uintptr.
// Read is little-endian.
func readUintptr(p *byte) uintptr {
        x := *(*uintptr)(unsafe.Pointer(p))
        if goarch.BigEndian {
                if goarch.PtrSize == 8 {
                        return uintptr(sys.Bswap64(uint64(x)))
                }
                return uintptr(sys.Bswap32(uint32(x)))
        }
        return x
}

var debugPtrmask struct {
        lock mutex
        data *byte
}

// 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/goarch.PtrSize bits.
func progToPointerMask(prog *byte, size uintptr) bitvector {
        n := (size/goarch.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, &x[0])
        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 returns the number of 1-bit entries written to memory.
func runGCProg(prog, dst *byte) 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 {
                        *dst = uint8(bits)
                        dst = add1(dst)
                        bits >>= 8
                }

                // 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.
                                break Run
                        }
                        nbyte := n / 8
                        for i := uintptr(0); i < nbyte; i++ {
                                bits |= uintptr(*p) << nbits
                                p = add1(p)
                                *dst = uint8(bits)
                                dst = add1(dst)
                                bits >>= 8
                        }
                        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 goarch.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 = goarch.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.
                        src = subtract1(src)
                        for npattern < n {
                                pattern <<= 8
                                pattern |= uintptr(*src)
                                src = subtract1(src)
                                npattern += 8
                        }

                        // 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 <= goarch.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
                                for nbits >= 8 {
                                        *dst = uint8(bits)
                                        dst = add1(dst)
                                        bits >>= 8
                                        nbits -= 8
                                }
                        }

                        // 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
                // 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
                }
        }

        // Write any final bits out, using full-byte writes, even for the final byte.
        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
        }
        return totalBits
}

// materializeGCProg allocates space for the (1-bit) pointer bitmask
// for an object of size ptrdata.  Then it fills that space with the
// pointer bitmask specified by the program prog.
// The bitmask starts at s.startAddr.
// The result must be deallocated with dematerializeGCProg.
func materializeGCProg(ptrdata uintptr, prog *byte) *mspan {
        // Each word of ptrdata needs one bit in the bitmap.
        bitmapBytes := divRoundUp(ptrdata, 8*goarch.PtrSize)
        // Compute the number of pages needed for bitmapBytes.
        pages := divRoundUp(bitmapBytes, pageSize)
        s := mheap_.allocManual(pages, spanAllocPtrScalarBits)
        runGCProg(addb(prog, 4), (*byte)(unsafe.Pointer(s.startAddr)))
        return s
}
func dematerializeGCProg(s *mspan) {
        mheap_.freeManual(s, spanAllocPtrScalarBits)
}

func dumpGCProg(p *byte) {
        nptr := 0
        for {
                x := *p
                p = add1(p)
                if x == 0 {
                        print("\t", nptr, " end\n")
                        break
                }
                if x&0x80 == 0 {
                        print("\t", nptr, " lit ", x, ":")
                        n := int(x+7) / 8
                        for i := 0; i < n; i++ {
                                print(" ", hex(*p))
                                p = add1(p)
                        }
                        print("\n")
                        nptr += int(x)
                } else {
                        nbit := int(x &^ 0x80)
                        if nbit == 0 {
                                for nb := uint(0); ; nb += 7 {
                                        x := *p
                                        p = add1(p)
                                        nbit |= int(x&0x7f) << nb
                                        if x&0x80 == 0 {
                                                break
                                        }
                                }
                        }
                        count := 0
                        for nb := uint(0); ; nb += 7 {
                                x := *p
                                p = add1(p)
                                count |= int(x&0x7f) << nb
                                if x&0x80 == 0 {
                                        break
                                }
                        }
                        print("\t", nptr, " repeat ", nbit, " × ", count, "\n")
                        nptr += nbit * count
                }
        }
}

// Testing.

// reflect_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 any) []byte {
        return getgcmask(x)
}