runtime: implement experiment to replace heap bitmap with alloc headers

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

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build goexperiment.allocheaders

// Garbage collector: type and heap bitmaps.
//
// Stack, data, and bss bitmaps
//
// Stack frames and global variables in the data and bss sections are
// described by bitmaps with 1 bit per pointer-sized word. A "1" bit
// means the word is a live pointer to be visited by the GC (referred to
// as "pointer"). A "0" bit means the word should be ignored by GC
// (referred to as "scalar", though it could be a dead pointer value).
//
// Heap bitmaps
//
// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
// recording whether a pointer is stored in that word or not. This bitmap
// is stored at the end of a span for small objects and is unrolled at
// runtime from type metadata for all larger objects. Objects without
// pointers have neither a bitmap nor associated type metadata.
//
// Bits in all cases correspond to words in little-endian order.
//
// For small objects, if s is the mspan for the span starting at "start",
// then s.heapBits() returns a slice containing the bitmap for the whole span.
// That is, s.heapBits()[0] holds the goarch.PtrSize*8 bits for the first
// goarch.PtrSize*8 words from "start" through "start+63*ptrSize" in the span.
// On a related note, small objects are always small enough that their bitmap
// fits in goarch.PtrSize*8 bits, so writing out bitmap data takes two bitmap
// writes at most (because object boundaries don't generally lie on
// s.heapBits()[i] boundaries).
//
// For larger objects, if t is the type for the object starting at "start",
// within some span whose mspan is s, then the bitmap at t.GCData is "tiled"
// from "start" through "start+s.elemsize".
// Specifically, the first bit of t.GCData corresponds to the word at "start",
// the second to the word after "start", and so on up to t.PtrBytes. At t.PtrBytes,
// we skip to "start+t.Size_" and begin again from there. This process is
// repeated until we hit "start+s.elemsize".
// This tiling algorithm supports array data, since the type always refers to
// the element type of the array. Single objects are considered the same as
// single-element arrays.
// The tiling algorithm may scan data past the end of the compiler-recognized
// object, but any unused data within the allocation slot (i.e. within s.elemsize)
// is zeroed, so the GC just observes nil pointers.
// Note that this "tiled" bitmap isn't stored anywhere; it is generated on-the-fly.
//
// For objects without their own span, the type metadata is stored in the first
// word before the object at the beginning of the allocation slot. For objects
// with their own span, the type metadata is stored in the mspan.
//
// The bitmap for small unallocated objects in scannable spans is not maintained
// (can be junk).

package runtime

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

const (
        // A malloc header is functionally a single type pointer, but
        // we need to use 8 here to ensure 8-byte alignment of allocations
        // on 32-bit platforms. It's wasteful, but a lot of code relies on
        // 8-byte alignment for 8-byte atomics.
        mallocHeaderSize = 8

        // The minimum object size that has a malloc header, exclusive.
        //
        // The size of this value controls overheads from the malloc header.
        // The minimum size is bound by writeHeapBitsSmall, which assumes that the
        // pointer bitmap for objects of a size smaller than this doesn't cross
        // more than one pointer-word boundary. This sets an upper-bound on this
        // value at the number of bits in a uintptr, multiplied by the pointer
        // size in bytes.
        //
        // We choose a value here that has a natural cutover point in terms of memory
        // overheads. This value just happens to be the maximum possible value this
        // can be.
        //
        // A span with heap bits in it will have 128 bytes of heap bits on 64-bit
        // platforms, and 256 bytes of heap bits on 32-bit platforms. The first size
        // class where malloc headers match this overhead for 64-bit platforms is
        // 512 bytes (8 KiB / 512 bytes * 8 bytes-per-header = 128 bytes of overhead).
        // On 32-bit platforms, this same point is the 256 byte size class
        // (8 KiB / 256 bytes * 8 bytes-per-header = 256 bytes of overhead).
        //
        // Guaranteed to be exactly at a size class boundary. The reason this value is
        // an exclusive minimum is subtle. Suppose we're allocating a 504-byte object
        // and its rounded up to 512 bytes for the size class. If minSizeForMallocHeader
        // is 512 and an inclusive minimum, then a comparison against minSizeForMallocHeader
        // by the two values would produce different results. In other words, the comparison
        // would not be invariant to size-class rounding. Eschewing this property means a
        // more complex check or possibly storing additional state to determine whether a
        // span has malloc headers.
        minSizeForMallocHeader = goarch.PtrSize * ptrBits
)

// heapBitsInSpan returns true if the size of an object implies its ptr/scalar
// data is stored at the end of the span, and is accessible via span.heapBits.
//
// Note: this works for both rounded-up sizes (span.elemsize) and unrounded
// type sizes because minSizeForMallocHeader is guaranteed to be at a size
// class boundary.
//
//go:nosplit
func heapBitsInSpan(userSize uintptr) bool {
        // N.B. minSizeForMallocHeader is an exclusive minimum so that this function is
        // invariant under size-class rounding on its input.
        return userSize <= minSizeForMallocHeader
}

// heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC.
type heapArenaPtrScalar struct {
        // N.B. This is no longer necessary with allocation headers.
}

// typePointers is an iterator over the pointers in a heap object.
//
// Iteration through this type implements the tiling algorithm described at the
// top of this file.
type typePointers struct {
        // elem is the address of the current array element of type typ being iterated over.
        // Objects that are not arrays are treated as single-element arrays, in which case
        // this value does not change.
        elem uintptr

        // addr is the address the iterator is currently working from and describes
        // the address of the first word referenced by mask.
        addr uintptr

        // mask is a bitmask where each bit corresponds to pointer-words after addr.
        // Bit 0 is the pointer-word at addr, Bit 1 is the next word, and so on.
        // If a bit is 1, then there is a pointer at that word.
        // nextFast and next mask out bits in this mask as their pointers are processed.
        mask uintptr

        // typ is a pointer to the type information for the heap object's type.
        // This may be nil if the object is in a span where heapBitsInSpan(span.elemsize) is true.
        typ *_type
}

// typePointersOf returns an iterator over all heap pointers in the range [addr, addr+size).
//
// addr and addr+size must be in the range [span.base(), span.limit).
//
// Note: addr+size must be passed as the limit argument to the iterator's next method on
// each iteration. This slightly awkward API is to allow typePointers to be destructured
// by the compiler.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (span *mspan) typePointersOf(addr, size uintptr) typePointers {
        base := span.objBase(addr)
        tp := span.typePointersOfUnchecked(base)
        if base == addr && size == span.elemsize {
                return tp
        }
        return tp.fastForward(addr-tp.addr, addr+size)
}

// typePointersOfUnchecked is like typePointersOf, but assumes addr is the base
// pointer of an object in span. It returns an iterator that generates all pointers
// in the range [addr, addr+span.elemsize).
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (span *mspan) typePointersOfUnchecked(addr uintptr) typePointers {
        const doubleCheck = false
        if doubleCheck && span.objBase(addr) != addr {
                print("runtime: addr=", addr, " base=", span.objBase(addr), "\n")
                throw("typePointersOfUnchecked consisting of non-base-address for object")
        }

        spc := span.spanclass
        if spc.noscan() {
                return typePointers{}
        }
        if heapBitsInSpan(span.elemsize) {
                // Handle header-less objects.
                return typePointers{elem: addr, addr: addr, mask: span.heapBitsSmallForAddr(addr)}
        }

        // All of these objects have a header.
        var typ *_type
        if spc.sizeclass() != 0 {
                // Pull the allocation header from the first word of the object.
                typ = *(**_type)(unsafe.Pointer(addr))
                addr += mallocHeaderSize
        } else {
                typ = span.largeType
        }
        gcdata := typ.GCData
        return typePointers{elem: addr, addr: addr, mask: readUintptr(gcdata), typ: typ}
}

// nextFast is the fast path of next. nextFast is written to be inlineable and,
// as the name implies, fast.
//
// Callers that are performance-critical should iterate using the following
// pattern:
//
//      for {
//              var addr uintptr
//              if tp, addr = tp.nextFast(); addr == 0 {
//                      if tp, addr = tp.next(limit); addr == 0 {
//                              break
//                      }
//              }
//              // Use addr.
//              ...
//      }
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) nextFast() (typePointers, uintptr) {
        // TESTQ/JEQ
        if tp.mask == 0 {
                return tp, 0
        }
        // BSFQ
        var i int
        if goarch.PtrSize == 8 {
                i = sys.TrailingZeros64(uint64(tp.mask))
        } else {
                i = sys.TrailingZeros32(uint32(tp.mask))
        }
        // BTCQ
        tp.mask ^= uintptr(1) << (i & (ptrBits - 1))
        // LEAQ (XX)(XX*8)
        return tp, tp.addr + uintptr(i)*goarch.PtrSize
}

// next advances the pointers iterator, returning the updated iterator and
// the address of the next pointer.
//
// limit must be the same each time it is passed to next.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) next(limit uintptr) (typePointers, uintptr) {
        for {
                if tp.mask != 0 {
                        return tp.nextFast()
                }

                // Stop if we don't actually have type information.
                if tp.typ == nil {
                        return typePointers{}, 0
                }

                // Advance to the next element if necessary.
                if tp.addr+goarch.PtrSize*ptrBits >= tp.elem+tp.typ.PtrBytes {
                        tp.elem += tp.typ.Size_
                        tp.addr = tp.elem
                } else {
                        tp.addr += ptrBits * goarch.PtrSize
                }

                // Check if we've exceeded the limit with the last update.
                if tp.addr >= limit {
                        return typePointers{}, 0
                }

                // Grab more bits and try again.
                tp.mask = readUintptr(addb(tp.typ.GCData, (tp.addr-tp.elem)/goarch.PtrSize/8))
                if tp.addr+goarch.PtrSize*ptrBits > limit {
                        bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
                        tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
                }
        }
}

// fastForward moves the iterator forward by n bytes. n must be a multiple
// of goarch.PtrSize. limit must be the same limit passed to next for this
// iterator.
//
// nosplit because it is used during write barriers and must not be preempted.
//
//go:nosplit
func (tp typePointers) fastForward(n, limit uintptr) typePointers {
        // Basic bounds check.
        target := tp.addr + n
        if target >= limit {
                return typePointers{}
        }
        if tp.typ == nil {
                // Handle small objects.
                // Clear any bits before the target address.
                tp.mask &^= (1 << ((target - tp.addr) / goarch.PtrSize)) - 1
                // Clear any bits past the limit.
                if tp.addr+goarch.PtrSize*ptrBits > limit {
                        bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
                        tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
                }
                return tp
        }

        // Move up elem and addr.
        // Offsets within an element are always at a ptrBits*goarch.PtrSize boundary.
        if n >= tp.typ.Size_ {
                // elem needs to be moved to the element containing
                // tp.addr + n.
                oldelem := tp.elem
                tp.elem += (tp.addr - tp.elem + n) / tp.typ.Size_ * tp.typ.Size_
                tp.addr = tp.elem + alignDown(n-(tp.elem-oldelem), ptrBits*goarch.PtrSize)
        } else {
                tp.addr += alignDown(n, ptrBits*goarch.PtrSize)
        }

        if tp.addr-tp.elem >= tp.typ.PtrBytes {
                // We're starting in the non-pointer area of an array.
                // Move up to the next element.
                tp.elem += tp.typ.Size_
                tp.addr = tp.elem
                tp.mask = readUintptr(tp.typ.GCData)

                // We may have exceeded the limit after this. Bail just like next does.
                if tp.addr >= limit {
                        return typePointers{}
                }
        } else {
                // Grab the mask, but then clear any bits before the target address and any
                // bits over the limit.
                tp.mask = readUintptr(addb(tp.typ.GCData, (tp.addr-tp.elem)/goarch.PtrSize/8))
                tp.mask &^= (1 << ((target - tp.addr) / goarch.PtrSize)) - 1
        }
        if tp.addr+goarch.PtrSize*ptrBits > limit {
                bits := (tp.addr + goarch.PtrSize*ptrBits - limit) / goarch.PtrSize
                tp.mask &^= ((1 << (bits)) - 1) << (ptrBits - bits)
        }
        return tp
}

// objBase returns the base pointer for the object containing addr in span.
//
// Assumes that addr points into a valid part of span (span.base() <= addr < span.limit).
//
//go:nosplit
func (span *mspan) objBase(addr uintptr) uintptr {
        return span.base() + span.objIndex(addr)*span.elemsize
}

// bulkBarrierPreWrite executes a write barrier
// for every pointer slot in the memory range [src, src+size),
// using pointer/scalar information from [dst, dst+size).
// This executes the write barriers necessary before a memmove.
// src, dst, and size must be pointer-aligned.
// The range [dst, dst+size) must lie within a single object.
// It does not perform the actual writes.
//
// As a special case, src == 0 indicates that this is being used for a
// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
// barrier.
//
// Callers should call bulkBarrierPreWrite immediately before
// calling memmove(dst, src, size). This function is marked nosplit
// to avoid being preempted; the GC must not stop the goroutine
// between the memmove and the execution of the barriers.
// The caller is also responsible for cgo pointer checks if this
// may be writing Go pointers into non-Go memory.
//
// 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
        }
        s := spanOf(dst)
        if s == nil {
                // If dst is a global, use the data or BSS bitmaps to
                // execute write barriers.
                for _, datap := range activeModules() {
                        if datap.data <= dst && dst < datap.edata {
                                bulkBarrierBitmap(dst, src, size, dst-datap.data, datap.gcdatamask.bytedata)
                                return
                        }
                }
                for _, datap := range activeModules() {
                        if datap.bss <= dst && dst < datap.ebss {
                                bulkBarrierBitmap(dst, src, size, dst-datap.bss, datap.gcbssmask.bytedata)
                                return
                        }
                }
                return
        } else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
                // dst was heap memory at some point, but isn't now.
                // It can't be a global. It must be either our stack,
                // or in the case of direct channel sends, it could be
                // another stack. Either way, no need for barriers.
                // This will also catch if dst is in a freed span,
                // though that should never have.
                return
        }
        buf := &getg().m.p.ptr().wbBuf

        tp := s.typePointersOf(dst, size)
        if src == 0 {
                for {
                        var addr uintptr
                        if tp, addr = tp.next(dst + size); addr == 0 {
                                break
                        }
                        dstx := (*uintptr)(unsafe.Pointer(addr))
                        p := buf.get1()
                        p[0] = *dstx
                }
        } else {
                for {
                        var addr uintptr
                        if tp, addr = tp.next(dst + size); addr == 0 {
                                break
                        }
                        dstx := (*uintptr)(unsafe.Pointer(addr))
                        srcx := (*uintptr)(unsafe.Pointer(src + (addr - dst)))
                        p := buf.get2()
                        p[0] = *dstx
                        p[1] = *srcx
                }
        }
}

// bulkBarrierPreWriteSrcOnly is like bulkBarrierPreWrite but
// does not execute write barriers for [dst, dst+size).
//
// In addition to the requirements of bulkBarrierPreWrite
// callers need to ensure [dst, dst+size) is zeroed.
//
// This is used for special cases where e.g. dst was just
// created and zeroed with malloc.
//
//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
        tp := spanOf(dst).typePointersOf(dst, size)
        for {
                var addr uintptr
                if tp, addr = tp.next(dst + size); addr == 0 {
                        break
                }
                srcx := (*uintptr)(unsafe.Pointer(addr - dst + src))
                p := buf.get1()
                p[0] = *srcx
        }
}

// initHeapBits initializes the heap bitmap for a span.
//
// TODO(mknyszek): This should set the heap bits for single pointer
// allocations eagerly to avoid calling heapSetType at allocation time,
// just to write one bit.
func (s *mspan) initHeapBits(forceClear bool) {
        if (!s.spanclass.noscan() && heapBitsInSpan(s.elemsize)) || s.isUserArenaChunk {
                b := s.heapBits()
                for i := range b {
                        b[i] = 0
                }
        }
}

type writeHeapBits struct {
        offset uintptr // offset in span that the low bit of mask represents the pointer state of.
        mask   uintptr // some pointer bits starting at the address addr.
        valid  uintptr // number of bits in buf that are valid (including low)
        low    uintptr // number of low-order bits to not overwrite
}

func (s *mspan) writeHeapBits(addr uintptr) (h writeHeapBits) {
        offset := addr - s.base()

        // We start writing bits maybe in the middle of a heap bitmap word.
        // Remember how many bits into the word we started, so we can be sure
        // not to overwrite the previous bits.
        h.low = offset / goarch.PtrSize % ptrBits

        // round down to heap word that starts the bitmap word.
        h.offset = offset - h.low*goarch.PtrSize

        // We don't have any bits yet.
        h.mask = 0
        h.valid = h.low

        return
}

// write appends the pointerness of the next valid pointer slots
// using the low valid bits of bits. 1=pointer, 0=scalar.
func (h writeHeapBits) write(s *mspan, 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.
        idx := h.offset / (ptrBits * goarch.PtrSize)
        m := uintptr(1)<<h.low - 1
        bitmap := s.heapBits()
        bitmap[idx] = bitmap[idx]&m | data
        // Note: no synchronization required for this write because
        // the allocator has exclusive access to the page, and the bitmap
        // entries are all for a single page. Also, visibility of these
        // writes is guaranteed by the publication barrier in mallocgc.

        // Move to next word of bitmap.
        h.offset += ptrBits * goarch.PtrSize
        h.low = 0
        return h
}

// Add padding of size bytes.
func (h writeHeapBits) pad(s *mspan, size uintptr) writeHeapBits {
        if size == 0 {
                return h
        }
        words := size / goarch.PtrSize
        for words > ptrBits {
                h = h.write(s, 0, ptrBits)
                words -= ptrBits
        }
        return h.write(s, 0, words)
}

// Flush the bits that have been written, and add zeros as needed
// to cover the full object [addr, addr+size).
func (h writeHeapBits) flush(s *mspan, addr, size uintptr) {
        offset := addr - s.base()

        // zeros counts the number of bits needed to represent the object minus the
        // number of bits we've already written. This is the number of 0 bits
        // that need to be added.
        zeros := (offset+size-h.offset)/goarch.PtrSize - h.valid

        // Add zero bits up to the bitmap word boundary
        if zeros > 0 {
                z := ptrBits - h.valid
                if z > zeros {
                        z = zeros
                }
                h.valid += z
                zeros -= z
        }

        // Find word in bitmap that we're going to write.
        bitmap := s.heapBits()
        idx := h.offset / (ptrBits * goarch.PtrSize)

        // Write remaining bits.
        if h.valid != h.low {
                m := uintptr(1)<<h.low - 1      // don't clear existing bits below "low"
                m |= ^(uintptr(1)<<h.valid - 1) // don't clear existing bits above "valid"
                bitmap[idx] = bitmap[idx]&m | h.mask
        }
        if zeros == 0 {
                return
        }

        // Advance to next bitmap word.
        h.offset += ptrBits * goarch.PtrSize

        // Continue on writing zeros for the rest of the object.
        // For standard use of the ptr bits this is not required, as
        // the bits are read from the beginning of the object. Some uses,
        // like noscan spans, oblets, bulk write barriers, and cgocheck, might
        // start mid-object, so these writes are still required.
        for {
                // Write zero bits.
                idx := h.offset / (ptrBits * goarch.PtrSize)
                if zeros < ptrBits {
                        bitmap[idx] &^= uintptr(1)<<zeros - 1
                        break
                } else if zeros == ptrBits {
                        bitmap[idx] = 0
                        break
                } else {
                        bitmap[idx] = 0
                        zeros -= ptrBits
                }
                h.offset += ptrBits * goarch.PtrSize
        }
}

// heapBits returns the heap ptr/scalar bits stored at the end of the span for
// small object spans.
//
// heapBitsInSpan(span.elemsize) or span.isUserArenaChunk must be true.
//
//go:nosplit
func (span *mspan) heapBits() []uintptr {
        const doubleCheck = false

        if doubleCheck && !span.isUserArenaChunk {
                if span.spanclass.noscan() {
                        throw("heapBits called for noscan")
                }
                if span.elemsize > minSizeForMallocHeader {
                        throw("heapBits called for span class that should have a malloc header")
                }
        }
        // Find the bitmap at the end of the span.
        //
        // Nearly every span with heap bits is exactly one page in size. Arenas are the only exception.
        if span.npages == 1 {
                // This will be inlined and constant-folded down.
                return heapBitsSlice(span.base(), pageSize)
        }
        return heapBitsSlice(span.base(), span.npages*pageSize)
}

// Helper for constructing a slice for the span's heap bits.
//
//go:nosplit
func heapBitsSlice(spanBase, spanSize uintptr) []uintptr {
        bitmapSize := spanSize / goarch.PtrSize / 8
        elems := int(bitmapSize / goarch.PtrSize)
        var sl notInHeapSlice
        sl = notInHeapSlice{(*notInHeap)(unsafe.Pointer(spanBase + spanSize - bitmapSize)), elems, elems}
        return *(*[]uintptr)(unsafe.Pointer(&sl))
}

// heapBitsSmallForAddr loads the heap bits for the object stored at addr from span.heapBits.
//
// addr must be the base pointer of an object in the span. heapBitsInSpan(span.elemsize)
// must be true.
//
//go:nosplit
func (span *mspan) heapBitsSmallForAddr(addr uintptr) uintptr {
        spanSize := span.npages * pageSize
        bitmapSize := spanSize / goarch.PtrSize / 8
        hbits := (*byte)(unsafe.Pointer(span.base() + spanSize - bitmapSize))

        // These objects are always small enough that their bitmaps
        // fit in a single word, so just load the word or two we need.
        //
        // Mirrors mspan.writeHeapBitsSmall.
        //
        // We should be using heapBits(), but unfortunately it introduces
        // both bounds checks panics and throw which causes us to exceed
        // the nosplit limit in quite a few cases.
        i := (addr - span.base()) / goarch.PtrSize / ptrBits
        j := (addr - span.base()) / goarch.PtrSize % ptrBits
        bits := span.elemsize / goarch.PtrSize
        word0 := (*uintptr)(unsafe.Pointer(addb(hbits, goarch.PtrSize*(i+0))))
        word1 := (*uintptr)(unsafe.Pointer(addb(hbits, goarch.PtrSize*(i+1))))

        var read uintptr
        if j+bits > ptrBits {
                // Two reads.
                bits0 := ptrBits - j
                bits1 := bits - bits0
                read = *word0 >> j
                read |= (*word1 & ((1 << bits1) - 1)) << bits0
        } else {
                // One read.
                read = (*word0 >> j) & ((1 << bits) - 1)
        }
        return read
}

// writeHeapBitsSmall writes the heap bits for small objects whose ptr/scalar data is
// stored as a bitmap at the end of the span.
//
// Assumes dataSize is <= ptrBits*goarch.PtrSize. x must be a pointer into the span.
// heapBitsInSpan(dataSize) must be true. dataSize must be >= typ.Size_.
//
//go:nosplit
func (span *mspan) writeHeapBitsSmall(x, dataSize uintptr, typ *_type) (scanSize uintptr) {
        // The objects here are always really small, so a single load is sufficient.
        src0 := readUintptr(typ.GCData)

        // Create repetitions of the bitmap if we have a small array.
        bits := span.elemsize / goarch.PtrSize
        scanSize = typ.PtrBytes
        src := src0
        switch typ.Size_ {
        case goarch.PtrSize:
                src = (1 << (dataSize / goarch.PtrSize)) - 1
        default:
                for i := typ.Size_; i < dataSize; i += typ.Size_ {
                        src |= src0 << (i / goarch.PtrSize)
                        scanSize += typ.Size_
                }
        }

        // Since we're never writing more than one uintptr's worth of bits, we're either going
        // to do one or two writes.
        dst := span.heapBits()
        o := (x - span.base()) / goarch.PtrSize
        i := o / ptrBits
        j := o % ptrBits
        if j+bits > ptrBits {
                // Two writes.
                bits0 := ptrBits - j
                bits1 := bits - bits0
                dst[i+0] = dst[i+0]&(^uintptr(0)>>bits0) | (src << j)
                dst[i+1] = dst[i+1]&^((1<<bits1)-1) | (src >> bits0)
        } else {
                // One write.
                dst[i] = (dst[i] &^ (((1 << bits) - 1) << j)) | (src << j)
        }

        const doubleCheck = false
        if doubleCheck {
                srcRead := span.heapBitsSmallForAddr(x)
                if srcRead != src {
                        print("runtime: x=", hex(x), " i=", i, " j=", j, " bits=", bits, "\n")
                        print("runtime: dataSize=", dataSize, " typ.Size_=", typ.Size_, " typ.PtrBytes=", typ.PtrBytes, "\n")
                        print("runtime: src0=", hex(src0), " src=", hex(src), " srcRead=", hex(srcRead), "\n")
                        throw("bad pointer bits written for small object")
                }
        }
        return
}

// For !goexperiment.AllocHeaders.
func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
}

// heapSetType records that the new allocation [x, x+size)
// holds in [x, x+dataSize) one or more values of type typ.
// (The number of values is given by dataSize / typ.Size.)
// If dataSize < size, the fragment [x+dataSize, x+size) is
// recorded as non-pointer data.
// It is known that the type has pointers somewhere;
// malloc does not call heapSetType when there are no pointers.
//
// There can be read-write races between heapSetType and things
// that read the heap metadata like scanobject. However, since
// heapSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// shared memory that belongs to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
func heapSetType(x, dataSize uintptr, typ *_type, header **_type, span *mspan) (scanSize uintptr) {
        const doubleCheck = false

        gctyp := typ
        if header == nil {
                if doubleCheck && (!heapBitsInSpan(dataSize) || !heapBitsInSpan(span.elemsize)) {
                        throw("tried to write heap bits, but no heap bits in span")
                }
                // Handle the case where we have no malloc header.
                scanSize = span.writeHeapBitsSmall(x, dataSize, typ)
        } else {
                if typ.Kind_&kindGCProg != 0 {
                        // Allocate space to unroll the gcprog. This space will consist of
                        // a dummy _type value and the unrolled gcprog. The dummy _type will
                        // refer to the bitmap, and the mspan will refer to the dummy _type.
                        if span.spanclass.sizeclass() != 0 {
                                throw("GCProg for type that isn't large")
                        }
                        spaceNeeded := alignUp(unsafe.Sizeof(_type{}), goarch.PtrSize)
                        heapBitsOff := spaceNeeded
                        spaceNeeded += alignUp(typ.PtrBytes/goarch.PtrSize/8, goarch.PtrSize)
                        npages := alignUp(spaceNeeded, pageSize) / pageSize
                        var progSpan *mspan
                        systemstack(func() {
                                progSpan = mheap_.allocManual(npages, spanAllocPtrScalarBits)
                                memclrNoHeapPointers(unsafe.Pointer(progSpan.base()), progSpan.npages*pageSize)
                        })
                        // Write a dummy _type in the new space.
                        //
                        // We only need to write size, PtrBytes, and GCData, since that's all
                        // the GC cares about.
                        gctyp = (*_type)(unsafe.Pointer(progSpan.base()))
                        gctyp.Kind_ |= kindGCProg
                        gctyp.Size_ = typ.Size_
                        gctyp.PtrBytes = typ.PtrBytes
                        gctyp.GCData = (*byte)(add(unsafe.Pointer(progSpan.base()), heapBitsOff))

                        // Expand the GC program into space reserved at the end of the object.
                        runGCProg(addb(typ.GCData, 4), gctyp.GCData)
                }

                // Write out the header.
                *header = gctyp
                scanSize = span.elemsize
        }

        if doubleCheck {
                doubleCheckHeapPointers(x, dataSize, gctyp, header, span)

                // To exercise the less common path more often, generate
                // a random interior pointer and make sure iterating from
                // that point works correctly too.
                maxIterBytes := span.elemsize
                if header == nil {
                        maxIterBytes = dataSize
                }
                off := alignUp(uintptr(fastrand())%dataSize, goarch.PtrSize)
                size := dataSize - off
                if size == 0 {
                        off -= goarch.PtrSize
                        size += goarch.PtrSize
                }
                interior := x + off
                size -= alignDown(uintptr(fastrand())%size, goarch.PtrSize)
                if size == 0 {
                        size = goarch.PtrSize
                }
                // Round up the type to the size of the type.
                size = (size + gctyp.Size_ - 1) / gctyp.Size_ * gctyp.Size_
                if interior+size > x+maxIterBytes {
                        size = x + maxIterBytes - interior
                }
                doubleCheckHeapPointersInterior(x, interior, size, dataSize, gctyp, header, span)
        }
        return
}

func doubleCheckHeapPointers(x, dataSize uintptr, typ *_type, header **_type, span *mspan) {
        // Check that scanning the full object works.
        tp := span.typePointersOfUnchecked(span.objBase(x))
        maxIterBytes := span.elemsize
        if header == nil {
                maxIterBytes = dataSize
        }
        bad := false
        for i := uintptr(0); i < maxIterBytes; i += goarch.PtrSize {
                // Compute the pointer bit we want at offset i.
                want := false
                if i < span.elemsize {
                        off := i % typ.Size_
                        if off < typ.PtrBytes {
                                j := off / goarch.PtrSize
                                want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
                        }
                }
                if want {
                        var addr uintptr
                        tp, addr = tp.next(x + span.elemsize)
                        if addr == 0 {
                                println("runtime: found bad iterator")
                        }
                        if addr != x+i {
                                print("runtime: addr=", hex(addr), " x+i=", hex(x+i), "\n")
                                bad = true
                        }
                }
        }
        if !bad {
                var addr uintptr
                tp, addr = tp.next(x + span.elemsize)
                if addr == 0 {
                        return
                }
                println("runtime: extra pointer:", hex(addr))
        }
        print("runtime: hasHeader=", header != nil, " typ.Size_=", typ.Size_, " hasGCProg=", typ.Kind_&kindGCProg != 0, "\n")
        print("runtime: x=", hex(x), " dataSize=", dataSize, " elemsize=", span.elemsize, "\n")
        print("runtime: typ=", unsafe.Pointer(typ), " typ.PtrBytes=", typ.PtrBytes, "\n")
        print("runtime: limit=", hex(x+span.elemsize), "\n")
        tp = span.typePointersOfUnchecked(x)
        dumpTypePointers(tp)
        for {
                var addr uintptr
                if tp, addr = tp.next(x + span.elemsize); addr == 0 {
                        println("runtime: would've stopped here")
                        dumpTypePointers(tp)
                        break
                }
                print("runtime: addr=", hex(addr), "\n")
                dumpTypePointers(tp)
        }
        throw("heapSetType: pointer entry not correct")
}

func doubleCheckHeapPointersInterior(x, interior, size, dataSize uintptr, typ *_type, header **_type, span *mspan) {
        bad := false
        if interior < x {
                print("runtime: interior=", hex(interior), " x=", hex(x), "\n")
                throw("found bad interior pointer")
        }
        off := interior - x
        tp := span.typePointersOf(interior, size)
        for i := off; i < off+size; i += goarch.PtrSize {
                // Compute the pointer bit we want at offset i.
                want := false
                if i < span.elemsize {
                        off := i % typ.Size_
                        if off < typ.PtrBytes {
                                j := off / goarch.PtrSize
                                want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
                        }
                }
                if want {
                        var addr uintptr
                        tp, addr = tp.next(interior + size)
                        if addr == 0 {
                                println("runtime: found bad iterator")
                                bad = true
                        }
                        if addr != x+i {
                                print("runtime: addr=", hex(addr), " x+i=", hex(x+i), "\n")
                                bad = true
                        }
                }
        }
        if !bad {
                var addr uintptr
                tp, addr = tp.next(interior + size)
                if addr == 0 {
                        return
                }
                println("runtime: extra pointer:", hex(addr))
        }
        print("runtime: hasHeader=", header != nil, " typ.Size_=", typ.Size_, "\n")
        print("runtime: x=", hex(x), " dataSize=", dataSize, " elemsize=", span.elemsize, " interior=", hex(interior), " size=", size, "\n")
        print("runtime: limit=", hex(interior+size), "\n")
        tp = span.typePointersOf(interior, size)
        dumpTypePointers(tp)
        for {
                var addr uintptr
                if tp, addr = tp.next(interior + size); addr == 0 {
                        println("runtime: would've stopped here")
                        dumpTypePointers(tp)
                        break
                }
                print("runtime: addr=", hex(addr), "\n")
                dumpTypePointers(tp)
        }

        print("runtime: want: ")
        for i := off; i < off+size; i += goarch.PtrSize {
                // Compute the pointer bit we want at offset i.
                want := false
                if i < dataSize {
                        off := i % typ.Size_
                        if off < typ.PtrBytes {
                                j := off / goarch.PtrSize
                                want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
                        }
                }
                if want {
                        print("1")
                } else {
                        print("0")
                }
        }
        println()

        throw("heapSetType: pointer entry not correct")
}

func dumpTypePointers(tp typePointers) {
        print("runtime: tp.elem=", hex(tp.elem), " tp.typ=", unsafe.Pointer(tp.typ), "\n")
        print("runtime: tp.addr=", hex(tp.addr), " tp.mask=")
        for i := uintptr(0); i < ptrBits; i++ {
                if tp.mask&(uintptr(1)<<i) != 0 {
                        print("1")
                } else {
                        print("0")
                }
        }
        println()
}

// Testing.

// Returns GC type info for the pointer stored in ep for testing.
// If ep points to the stack, only static live information will be returned
// (i.e. not for objects which are only dynamically live stack objects).
func getgcmask(ep any) (mask []byte) {
        e := *efaceOf(&ep)
        p := e.data
        t := e._type
        // 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
                }
                limit := base + s.elemsize

                // Move the base up to the iterator's start, because
                // we want to hide evidence of a malloc header from the
                // caller.
                tp := s.typePointersOfUnchecked(base)
                base = tp.addr

                // Unroll the full bitmap the GC would actually observe.
                mask = make([]byte, (limit-base)/goarch.PtrSize)
                for {
                        var addr uintptr
                        if tp, addr = tp.next(limit); addr == 0 {
                                break
                        }
                        mask[(addr-base)/goarch.PtrSize] = 1
                }

                // Double-check that every part of the ptr/scalar we're not
                // showing the caller is zeroed. This keeps us honest that
                // that information is actually irrelevant.
                for i := limit; i < s.elemsize; i++ {
                        if *(*byte)(unsafe.Pointer(i)) != 0 {
                                throw("found non-zeroed tail of allocation")
                        }
                }

                // Callers 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 heapSetType but for
// non-slice-backing-store Go values allocated in a user arena chunk. It
// sets up the type metadata for the value with type typ allocated at address ptr.
// base is the base address of the arena chunk.
func userArenaHeapBitsSetType(typ *_type, ptr unsafe.Pointer, s *mspan) {
        base := s.base()
        h := s.writeHeapBits(uintptr(ptr))

        p := typ.GCData // start of 1-bit pointer mask (or GC program)
        var gcProgBits uintptr
        if typ.Kind_&kindGCProg != 0 {
                // Expand gc program, using the object itself for storage.
                gcProgBits = runGCProg(addb(p, 4), (*byte)(ptr))
                p = (*byte)(ptr)
        }
        nb := typ.PtrBytes / goarch.PtrSize

        for i := uintptr(0); i < nb; i += ptrBits {
                k := nb - i
                if k > ptrBits {
                        k = ptrBits
                }
                h = h.write(s, readUintptr(addb(p, i/8)), k)
        }
        // Note: we call pad here to ensure we emit explicit 0 bits
        // for the pointerless tail of the object. This ensures that
        // there's only a single noMorePtrs mark for the next object
        // to clear. We don't need to do this to clear stale noMorePtrs
        // markers from previous uses because arena chunk pointer bitmaps
        // are always fully cleared when reused.
        h = h.pad(s, typ.Size_-typ.PtrBytes)
        h.flush(s, uintptr(ptr), typ.Size_)

        if typ.Kind_&kindGCProg != 0 {
                // Zero out temporary ptrmask buffer inside object.
                memclrNoHeapPointers(ptr, (gcProgBits+7)/8)
        }

        // Update the PtrBytes value in the type information. After this
        // point, the GC will observe the new bitmap.
        s.largeType.PtrBytes = uintptr(ptr) - base + typ.PtrBytes

        // Double-check that the bitmap was written out correctly.
        const doubleCheck = false
        if doubleCheck {
                doubleCheckHeapPointersInterior(uintptr(ptr), uintptr(ptr), typ.Size_, typ.Size_, typ, &s.largeType, s)
        }
}

// For !goexperiment.AllocHeaders, to pass TestIntendedInlining.
func writeHeapBitsForAddr() {
        panic("not implemented")
}

// For !goexperiment.AllocHeaders.
type heapBits struct {
}

// For !goexperiment.AllocHeaders.
//
//go:nosplit
func heapBitsForAddr(addr, size uintptr) heapBits {
        panic("not implemented")
}

// For !goexperiment.AllocHeaders.
//
//go:nosplit
func (h heapBits) next() (heapBits, uintptr) {
        panic("not implemented")
}

// For !goexperiment.AllocHeaders.
//
//go:nosplit
func (h heapBits) nextFast() (heapBits, uintptr) {
        panic("not implemented")
}