Adding upstream version 1.22.1.upstream/1.22.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 938 insertions, 0 deletions

diff --git a/src/runtime/mbitmap_noallocheaders.go b/src/runtime/mbitmap_noallocheaders.go
new file mode 100644
index 0000000..383993a
--- /dev/null
+++ b/src/runtime/mbitmap_noallocheaders.go
@@ -0,0 +1,938 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !goexperiment.allocheaders
+
+// Garbage collector: type and heap bitmaps.
+//
+// Stack, data, and bss bitmaps
+//
+// Stack frames and global variables in the data and bss sections are
+// described by bitmaps with 1 bit per pointer-sized word. A "1" bit
+// means the word is a live pointer to be visited by the GC (referred to
+// as "pointer"). A "0" bit means the word should be ignored by GC
+// (referred to as "scalar", though it could be a dead pointer value).
+//
+// Heap bitmap
+//
+// The heap bitmap comprises 1 bit for each pointer-sized word in the heap,
+// recording whether a pointer is stored in that word or not. This bitmap
+// is stored in the heapArena metadata backing each heap arena.
+// That is, if ha is the heapArena for the arena starting at "start",
+// then ha.bitmap[0] holds the 64 bits for the 64 words "start"
+// through start+63*ptrSize, ha.bitmap[1] holds the entries for
+// start+64*ptrSize through start+127*ptrSize, and so on.
+// Bits correspond to words in little-endian order. ha.bitmap[0]&1 represents
+// the word at "start", ha.bitmap[0]>>1&1 represents the word at start+8, etc.
+// (For 32-bit platforms, s/64/32/.)
+//
+// We also keep a noMorePtrs bitmap which allows us to stop scanning
+// the heap bitmap early in certain situations. If ha.noMorePtrs[i]>>j&1
+// is 1, then the object containing the last word described by ha.bitmap[8*i+j]
+// has no more pointers beyond those described by ha.bitmap[8*i+j].
+// If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
+// beyond must all be zero until the start of the next object.
+//
+// The bitmap for noscan spans is set to all zero at span allocation time.
+//
+// The bitmap for unallocated objects in scannable spans is not maintained
+// (can be junk).
+
+package runtime
+
+import (
+	"internal/abi"
+	"internal/goarch"
+	"runtime/internal/sys"
+	"unsafe"
+)
+
+const (
+	// For compatibility with the allocheaders GOEXPERIMENT.
+	mallocHeaderSize       = 0
+	minSizeForMallocHeader = ^uintptr(0)
+)
+
+// For compatibility with the allocheaders GOEXPERIMENT.
+//
+//go:nosplit
+func heapBitsInSpan(_ uintptr) bool {
+	return false
+}
+
+// heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC.
+type heapArenaPtrScalar struct {
+	// bitmap stores the pointer/scalar bitmap for the words in
+	// this arena. See mbitmap.go for a description.
+	// This array uses 1 bit per word of heap, or 1.6% of the heap size (for 64-bit).
+	bitmap [heapArenaBitmapWords]uintptr
+
+	// If the ith bit of noMorePtrs is true, then there are no more
+	// pointers for the object containing the word described by the
+	// high bit of bitmap[i].
+	// In that case, bitmap[i+1], ... must be zero until the start
+	// of the next object.
+	// We never operate on these entries using bit-parallel techniques,
+	// so it is ok if they are small. Also, they can't be bigger than
+	// uint16 because at that size a single noMorePtrs entry
+	// represents 8K of memory, the minimum size of a span. Any larger
+	// and we'd have to worry about concurrent updates.
+	// This array uses 1 bit per word of bitmap, or .024% of the heap size (for 64-bit).
+	noMorePtrs [heapArenaBitmapWords / 8]uint8
+}
+
+// heapBits provides access to the bitmap bits for a single heap word.
+// The methods on heapBits take value receivers so that the compiler
+// can more easily inline calls to those methods and registerize the
+// struct fields independently.
+type heapBits struct {
+	// heapBits will report on pointers in the range [addr,addr+size).
+	// The low bit of mask contains the pointerness of the word at addr
+	// (assuming valid>0).
+	addr, size uintptr
+
+	// The next few pointer bits representing words starting at addr.
+	// Those bits already returned by next() are zeroed.
+	mask uintptr
+	// Number of bits in mask that are valid. mask is always less than 1<<valid.
+	valid uintptr
+}
+
+// heapBitsForAddr returns the heapBits for the address addr.
+// The caller must ensure [addr,addr+size) is in an allocated span.
+// In particular, be careful not to point past the end of an object.
+//
+// nosplit because it is used during write barriers and must not be preempted.
+//
+//go:nosplit
+func heapBitsForAddr(addr, size uintptr) heapBits {
+	// Find arena
+	ai := arenaIndex(addr)
+	ha := mheap_.arenas[ai.l1()][ai.l2()]
+
+	// Word index in arena.
+	word := addr / goarch.PtrSize % heapArenaWords
+
+	// Word index and bit offset in bitmap array.
+	idx := word / ptrBits
+	off := word % ptrBits
+
+	// Grab relevant bits of bitmap.
+	mask := ha.bitmap[idx] >> off
+	valid := ptrBits - off
+
+	// Process depending on where the object ends.
+	nptr := size / goarch.PtrSize
+	if nptr < valid {
+		// Bits for this object end before the end of this bitmap word.
+		// Squash bits for the following objects.
+		mask &= 1<<(nptr&(ptrBits-1)) - 1
+		valid = nptr
+	} else if nptr == valid {
+		// Bits for this object end at exactly the end of this bitmap word.
+		// All good.
+	} else {
+		// Bits for this object extend into the next bitmap word. See if there
+		// may be any pointers recorded there.
+		if uintptr(ha.noMorePtrs[idx/8])>>(idx%8)&1 != 0 {
+			// No more pointers in this object after this bitmap word.
+			// Update size so we know not to look there.
+			size = valid * goarch.PtrSize
+		}
+	}
+
+	return heapBits{addr: addr, size: size, mask: mask, valid: valid}
+}
+
+// Returns the (absolute) address of the next known pointer and
+// a heapBits iterator representing any remaining pointers.
+// If there are no more pointers, returns address 0.
+// Note that next does not modify h. The caller must record the result.
+//
+// nosplit because it is used during write barriers and must not be preempted.
+//
+//go:nosplit
+func (h heapBits) next() (heapBits, uintptr) {
+	for {
+		if h.mask != 0 {
+			var i int
+			if goarch.PtrSize == 8 {
+				i = sys.TrailingZeros64(uint64(h.mask))
+			} else {
+				i = sys.TrailingZeros32(uint32(h.mask))
+			}
+			h.mask ^= uintptr(1) << (i & (ptrBits - 1))
+			return h, h.addr + uintptr(i)*goarch.PtrSize
+		}
+
+		// Skip words that we've already processed.
+		h.addr += h.valid * goarch.PtrSize
+		h.size -= h.valid * goarch.PtrSize
+		if h.size == 0 {
+			return h, 0 // no more pointers
+		}
+
+		// Grab more bits and try again.
+		h = heapBitsForAddr(h.addr, h.size)
+	}
+}
+
+// nextFast is like next, but can return 0 even when there are more pointers
+// to be found. Callers should call next if nextFast returns 0 as its second
+// return value.
+//
+//	if addr, h = h.nextFast(); addr == 0 {
+//	    if addr, h = h.next(); addr == 0 {
+//	        ... no more pointers ...
+//	    }
+//	}
+//	... process pointer at addr ...
+//
+// nextFast is designed to be inlineable.
+//
+//go:nosplit
+func (h heapBits) nextFast() (heapBits, uintptr) {
+	// TESTQ/JEQ
+	if h.mask == 0 {
+		return h, 0
+	}
+	// BSFQ
+	var i int
+	if goarch.PtrSize == 8 {
+		i = sys.TrailingZeros64(uint64(h.mask))
+	} else {
+		i = sys.TrailingZeros32(uint32(h.mask))
+	}
+	// BTCQ
+	h.mask ^= uintptr(1) << (i & (ptrBits - 1))
+	// LEAQ (XX)(XX*8)
+	return h, h.addr + uintptr(i)*goarch.PtrSize
+}
+
+// bulkBarrierPreWrite executes a write barrier
+// for every pointer slot in the memory range [src, src+size),
+// using pointer/scalar information from [dst, dst+size).
+// This executes the write barriers necessary before a memmove.
+// src, dst, and size must be pointer-aligned.
+// The range [dst, dst+size) must lie within a single object.
+// It does not perform the actual writes.
+//
+// As a special case, src == 0 indicates that this is being used for a
+// memclr. bulkBarrierPreWrite will pass 0 for the src of each write
+// barrier.
+//
+// Callers should call bulkBarrierPreWrite immediately before
+// calling memmove(dst, src, size). This function is marked nosplit
+// to avoid being preempted; the GC must not stop the goroutine
+// between the memmove and the execution of the barriers.
+// The caller is also responsible for cgo pointer checks if this
+// may be writing Go pointers into non-Go memory.
+//
+// The pointer bitmap is not maintained for allocations containing
+// no pointers at all; any caller of bulkBarrierPreWrite must first
+// make sure the underlying allocation contains pointers, usually
+// by checking typ.PtrBytes.
+//
+// The type of the space can be provided purely as an optimization,
+// however it is not used with GOEXPERIMENT=noallocheaders.
+//
+// Callers must perform cgo checks if goexperiment.CgoCheck2.
+//
+//go:nosplit
+func bulkBarrierPreWrite(dst, src, size uintptr, _ *abi.Type) {
+	if (dst|src|size)&(goarch.PtrSize-1) != 0 {
+		throw("bulkBarrierPreWrite: unaligned arguments")
+	}
+	if !writeBarrier.enabled {
+		return
+	}
+	if s := spanOf(dst); s == nil {
+		// If dst is a global, use the data or BSS bitmaps to
+		// execute write barriers.
+		for _, datap := range activeModules() {
+			if datap.data <= dst && dst < datap.edata {
+				bulkBarrierBitmap(dst, src, size, dst-datap.data, datap.gcdatamask.bytedata)
+				return
+			}
+		}
+		for _, datap := range activeModules() {
+			if datap.bss <= dst && dst < datap.ebss {
+				bulkBarrierBitmap(dst, src, size, dst-datap.bss, datap.gcbssmask.bytedata)
+				return
+			}
+		}
+		return
+	} else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
+		// dst was heap memory at some point, but isn't now.
+		// It can't be a global. It must be either our stack,
+		// or in the case of direct channel sends, it could be
+		// another stack. Either way, no need for barriers.
+		// This will also catch if dst is in a freed span,
+		// though that should never have.
+		return
+	}
+
+	buf := &getg().m.p.ptr().wbBuf
+	h := heapBitsForAddr(dst, size)
+	if src == 0 {
+		for {
+			var addr uintptr
+			if h, addr = h.next(); addr == 0 {
+				break
+			}
+			dstx := (*uintptr)(unsafe.Pointer(addr))
+			p := buf.get1()
+			p[0] = *dstx
+		}
+	} else {
+		for {
+			var addr uintptr
+			if h, addr = h.next(); addr == 0 {
+				break
+			}
+			dstx := (*uintptr)(unsafe.Pointer(addr))
+			srcx := (*uintptr)(unsafe.Pointer(src + (addr - dst)))
+			p := buf.get2()
+			p[0] = *dstx
+			p[1] = *srcx
+		}
+	}
+}
+
+// bulkBarrierPreWriteSrcOnly is like bulkBarrierPreWrite but
+// does not execute write barriers for [dst, dst+size).
+//
+// In addition to the requirements of bulkBarrierPreWrite
+// callers need to ensure [dst, dst+size) is zeroed.
+//
+// This is used for special cases where e.g. dst was just
+// created and zeroed with malloc.
+//
+// The type of the space can be provided purely as an optimization,
+// however it is not used with GOEXPERIMENT=noallocheaders.
+//
+//go:nosplit
+func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr, _ *abi.Type) {
+	if (dst|src|size)&(goarch.PtrSize-1) != 0 {
+		throw("bulkBarrierPreWrite: unaligned arguments")
+	}
+	if !writeBarrier.enabled {
+		return
+	}
+	buf := &getg().m.p.ptr().wbBuf
+	h := heapBitsForAddr(dst, size)
+	for {
+		var addr uintptr
+		if h, addr = h.next(); addr == 0 {
+			break
+		}
+		srcx := (*uintptr)(unsafe.Pointer(addr - dst + src))
+		p := buf.get1()
+		p[0] = *srcx
+	}
+}
+
+// initHeapBits initializes the heap bitmap for a span.
+// If this is a span of single pointer allocations, it initializes all
+// words to pointer. If force is true, clears all bits.
+func (s *mspan) initHeapBits(forceClear bool) {
+	if forceClear || s.spanclass.noscan() {
+		// Set all the pointer bits to zero. We do this once
+		// when the span is allocated so we don't have to do it
+		// for each object allocation.
+		base := s.base()
+		size := s.npages * pageSize
+		h := writeHeapBitsForAddr(base)
+		h.flush(base, size)
+		return
+	}
+	isPtrs := goarch.PtrSize == 8 && s.elemsize == goarch.PtrSize
+	if !isPtrs {
+		return // nothing to do
+	}
+	h := writeHeapBitsForAddr(s.base())
+	size := s.npages * pageSize
+	nptrs := size / goarch.PtrSize
+	for i := uintptr(0); i < nptrs; i += ptrBits {
+		h = h.write(^uintptr(0), ptrBits)
+	}
+	h.flush(s.base(), size)
+}
+
+type writeHeapBits struct {
+	addr  uintptr // address that the low bit of mask represents the pointer state of.
+	mask  uintptr // some pointer bits starting at the address addr.
+	valid uintptr // number of bits in buf that are valid (including low)
+	low   uintptr // number of low-order bits to not overwrite
+}
+
+func writeHeapBitsForAddr(addr uintptr) (h writeHeapBits) {
+	// We start writing bits maybe in the middle of a heap bitmap word.
+	// Remember how many bits into the word we started, so we can be sure
+	// not to overwrite the previous bits.
+	h.low = addr / goarch.PtrSize % ptrBits
+
+	// round down to heap word that starts the bitmap word.
+	h.addr = addr - h.low*goarch.PtrSize
+
+	// We don't have any bits yet.
+	h.mask = 0
+	h.valid = h.low
+
+	return
+}
+
+// write appends the pointerness of the next valid pointer slots
+// using the low valid bits of bits. 1=pointer, 0=scalar.
+func (h writeHeapBits) write(bits, valid uintptr) writeHeapBits {
+	if h.valid+valid <= ptrBits {
+		// Fast path - just accumulate the bits.
+		h.mask |= bits << h.valid
+		h.valid += valid
+		return h
+	}
+	// Too many bits to fit in this word. Write the current word
+	// out and move on to the next word.
+
+	data := h.mask | bits<<h.valid       // mask for this word
+	h.mask = bits >> (ptrBits - h.valid) // leftover for next word
+	h.valid += valid - ptrBits           // have h.valid+valid bits, writing ptrBits of them
+
+	// Flush mask to the memory bitmap.
+	// TODO: figure out how to cache arena lookup.
+	ai := arenaIndex(h.addr)
+	ha := mheap_.arenas[ai.l1()][ai.l2()]
+	idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
+	m := uintptr(1)<<h.low - 1
+	ha.bitmap[idx] = ha.bitmap[idx]&m | data
+	// Note: no synchronization required for this write because
+	// the allocator has exclusive access to the page, and the bitmap
+	// entries are all for a single page. Also, visibility of these
+	// writes is guaranteed by the publication barrier in mallocgc.
+
+	// Clear noMorePtrs bit, since we're going to be writing bits
+	// into the following word.
+	ha.noMorePtrs[idx/8] &^= uint8(1) << (idx % 8)
+	// Note: same as above
+
+	// Move to next word of bitmap.
+	h.addr += ptrBits * goarch.PtrSize
+	h.low = 0
+	return h
+}
+
+// Add padding of size bytes.
+func (h writeHeapBits) pad(size uintptr) writeHeapBits {
+	if size == 0 {
+		return h
+	}
+	words := size / goarch.PtrSize
+	for words > ptrBits {
+		h = h.write(0, ptrBits)
+		words -= ptrBits
+	}
+	return h.write(0, words)
+}
+
+// Flush the bits that have been written, and add zeros as needed
+// to cover the full object [addr, addr+size).
+func (h writeHeapBits) flush(addr, size uintptr) {
+	// zeros counts the number of bits needed to represent the object minus the
+	// number of bits we've already written. This is the number of 0 bits
+	// that need to be added.
+	zeros := (addr+size-h.addr)/goarch.PtrSize - h.valid
+
+	// Add zero bits up to the bitmap word boundary
+	if zeros > 0 {
+		z := ptrBits - h.valid
+		if z > zeros {
+			z = zeros
+		}
+		h.valid += z
+		zeros -= z
+	}
+
+	// Find word in bitmap that we're going to write.
+	ai := arenaIndex(h.addr)
+	ha := mheap_.arenas[ai.l1()][ai.l2()]
+	idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
+
+	// Write remaining bits.
+	if h.valid != h.low {
+		m := uintptr(1)<<h.low - 1      // don't clear existing bits below "low"
+		m |= ^(uintptr(1)<<h.valid - 1) // don't clear existing bits above "valid"
+		ha.bitmap[idx] = ha.bitmap[idx]&m | h.mask
+	}
+	if zeros == 0 {
+		return
+	}
+
+	// Record in the noMorePtrs map that there won't be any more 1 bits,
+	// so readers can stop early.
+	ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
+
+	// Advance to next bitmap word.
+	h.addr += ptrBits * goarch.PtrSize
+
+	// Continue on writing zeros for the rest of the object.
+	// For standard use of the ptr bits this is not required, as
+	// the bits are read from the beginning of the object. Some uses,
+	// like noscan spans, oblets, bulk write barriers, and cgocheck, might
+	// start mid-object, so these writes are still required.
+	for {
+		// Write zero bits.
+		ai := arenaIndex(h.addr)
+		ha := mheap_.arenas[ai.l1()][ai.l2()]
+		idx := h.addr / (ptrBits * goarch.PtrSize) % heapArenaBitmapWords
+		if zeros < ptrBits {
+			ha.bitmap[idx] &^= uintptr(1)<<zeros - 1
+			break
+		} else if zeros == ptrBits {
+			ha.bitmap[idx] = 0
+			break
+		} else {
+			ha.bitmap[idx] = 0
+			zeros -= ptrBits
+		}
+		ha.noMorePtrs[idx/8] |= uint8(1) << (idx % 8)
+		h.addr += ptrBits * goarch.PtrSize
+	}
+}
+
+// heapBitsSetType records that the new allocation [x, x+size)
+// holds in [x, x+dataSize) one or more values of type typ.
+// (The number of values is given by dataSize / typ.Size.)
+// If dataSize < size, the fragment [x+dataSize, x+size) is
+// recorded as non-pointer data.
+// It is known that the type has pointers somewhere;
+// malloc does not call heapBitsSetType when there are no pointers,
+// because all free objects are marked as noscan during
+// heapBitsSweepSpan.
+//
+// There can only be one allocation from a given span active at a time,
+// and the bitmap for a span always falls on word boundaries,
+// so there are no write-write races for access to the heap bitmap.
+// Hence, heapBitsSetType can access the bitmap without atomics.
+//
+// There can be read-write races between heapBitsSetType and things
+// that read the heap bitmap like scanobject. However, since
+// heapBitsSetType is only used for objects that have not yet been
+// made reachable, readers will ignore bits being modified by this
+// function. This does mean this function cannot transiently modify
+// bits that belong to neighboring objects. Also, on weakly-ordered
+// machines, callers must execute a store/store (publication) barrier
+// between calling this function and making the object reachable.
+func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
+	const doubleCheck = false // slow but helpful; enable to test modifications to this code
+
+	if doubleCheck && dataSize%typ.Size_ != 0 {
+		throw("heapBitsSetType: dataSize not a multiple of typ.Size")
+	}
+
+	if goarch.PtrSize == 8 && size == goarch.PtrSize {
+		// It's one word and it has pointers, it must be a pointer.
+		// Since all allocated one-word objects are pointers
+		// (non-pointers are aggregated into tinySize allocations),
+		// (*mspan).initHeapBits sets the pointer bits for us.
+		// Nothing to do here.
+		if doubleCheck {
+			h, addr := heapBitsForAddr(x, size).next()
+			if addr != x {
+				throw("heapBitsSetType: pointer bit missing")
+			}
+			_, addr = h.next()
+			if addr != 0 {
+				throw("heapBitsSetType: second pointer bit found")
+			}
+		}
+		return
+	}
+
+	h := writeHeapBitsForAddr(x)
+
+	// Handle GC program.
+	if typ.Kind_&kindGCProg != 0 {
+		// Expand the gc program into the storage we're going to use for the actual object.
+		obj := (*uint8)(unsafe.Pointer(x))
+		n := runGCProg(addb(typ.GCData, 4), obj)
+		// Use the expanded program to set the heap bits.
+		for i := uintptr(0); true; i += typ.Size_ {
+			// Copy expanded program to heap bitmap.
+			p := obj
+			j := n
+			for j > 8 {
+				h = h.write(uintptr(*p), 8)
+				p = add1(p)
+				j -= 8
+			}
+			h = h.write(uintptr(*p), j)
+
+			if i+typ.Size_ == dataSize {
+				break // no padding after last element
+			}
+
+			// Pad with zeros to the start of the next element.
+			h = h.pad(typ.Size_ - n*goarch.PtrSize)
+		}
+
+		h.flush(x, size)
+
+		// Erase the expanded GC program.
+		memclrNoHeapPointers(unsafe.Pointer(obj), (n+7)/8)
+		return
+	}
+
+	// Note about sizes:
+	//
+	// typ.Size is the number of words in the object,
+	// and typ.PtrBytes is the number of words in the prefix
+	// of the object that contains pointers. That is, the final
+	// typ.Size - typ.PtrBytes words contain no pointers.
+	// This allows optimization of a common pattern where
+	// an object has a small header followed by a large scalar
+	// buffer. If we know the pointers are over, we don't have
+	// to scan the buffer's heap bitmap at all.
+	// The 1-bit ptrmasks are sized to contain only bits for
+	// the typ.PtrBytes prefix, zero padded out to a full byte
+	// of bitmap. If there is more room in the allocated object,
+	// that space is pointerless. The noMorePtrs bitmap will prevent
+	// scanning large pointerless tails of an object.
+	//
+	// Replicated copies are not as nice: if there is an array of
+	// objects with scalar tails, all but the last tail does have to
+	// be initialized, because there is no way to say "skip forward".
+
+	ptrs := typ.PtrBytes / goarch.PtrSize
+	if typ.Size_ == dataSize { // Single element
+		if ptrs <= ptrBits { // Single small element
+			m := readUintptr(typ.GCData)
+			h = h.write(m, ptrs)
+		} else { // Single large element
+			p := typ.GCData
+			for {
+				h = h.write(readUintptr(p), ptrBits)
+				p = addb(p, ptrBits/8)
+				ptrs -= ptrBits
+				if ptrs <= ptrBits {
+					break
+				}
+			}
+			m := readUintptr(p)
+			h = h.write(m, ptrs)
+		}
+	} else { // Repeated element
+		words := typ.Size_ / goarch.PtrSize // total words, including scalar tail
+		if words <= ptrBits {               // Repeated small element
+			n := dataSize / typ.Size_
+			m := readUintptr(typ.GCData)
+			// Make larger unit to repeat
+			for words <= ptrBits/2 {
+				if n&1 != 0 {
+					h = h.write(m, words)
+				}
+				n /= 2
+				m |= m << words
+				ptrs += words
+				words *= 2
+				if n == 1 {
+					break
+				}
+			}
+			for n > 1 {
+				h = h.write(m, words)
+				n--
+			}
+			h = h.write(m, ptrs)
+		} else { // Repeated large element
+			for i := uintptr(0); true; i += typ.Size_ {
+				p := typ.GCData
+				j := ptrs
+				for j > ptrBits {
+					h = h.write(readUintptr(p), ptrBits)
+					p = addb(p, ptrBits/8)
+					j -= ptrBits
+				}
+				m := readUintptr(p)
+				h = h.write(m, j)
+				if i+typ.Size_ == dataSize {
+					break // don't need the trailing nonptr bits on the last element.
+				}
+				// Pad with zeros to the start of the next element.
+				h = h.pad(typ.Size_ - typ.PtrBytes)
+			}
+		}
+	}
+	h.flush(x, size)
+
+	if doubleCheck {
+		h := heapBitsForAddr(x, size)
+		for i := uintptr(0); i < size; i += goarch.PtrSize {
+			// Compute the pointer bit we want at offset i.
+			want := false
+			if i < dataSize {
+				off := i % typ.Size_
+				if off < typ.PtrBytes {
+					j := off / goarch.PtrSize
+					want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
+				}
+			}
+			if want {
+				var addr uintptr
+				h, addr = h.next()
+				if addr != x+i {
+					throw("heapBitsSetType: pointer entry not correct")
+				}
+			}
+		}
+		if _, addr := h.next(); addr != 0 {
+			throw("heapBitsSetType: extra pointer")
+		}
+	}
+}
+
+// For goexperiment.AllocHeaders
+func heapSetType(x, dataSize uintptr, typ *_type, header **_type, span *mspan) (scanSize uintptr) {
+	return 0
+}
+
+// Testing.
+
+// Returns GC type info for the pointer stored in ep for testing.
+// If ep points to the stack, only static live information will be returned
+// (i.e. not for objects which are only dynamically live stack objects).
+func getgcmask(ep any) (mask []byte) {
+	e := *efaceOf(&ep)
+	p := e.data
+	t := e._type
+	// data or bss
+	for _, datap := range activeModules() {
+		// data
+		if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
+			bitmap := datap.gcdatamask.bytedata
+			n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
+			mask = make([]byte, n/goarch.PtrSize)
+			for i := uintptr(0); i < n; i += goarch.PtrSize {
+				off := (uintptr(p) + i - datap.data) / goarch.PtrSize
+				mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
+			}
+			return
+		}
+
+		// bss
+		if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
+			bitmap := datap.gcbssmask.bytedata
+			n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
+			mask = make([]byte, n/goarch.PtrSize)
+			for i := uintptr(0); i < n; i += goarch.PtrSize {
+				off := (uintptr(p) + i - datap.bss) / goarch.PtrSize
+				mask[i/goarch.PtrSize] = (*addb(bitmap, off/8) >> (off % 8)) & 1
+			}
+			return
+		}
+	}
+
+	// heap
+	if base, s, _ := findObject(uintptr(p), 0, 0); base != 0 {
+		if s.spanclass.noscan() {
+			return nil
+		}
+		n := s.elemsize
+		hbits := heapBitsForAddr(base, n)
+		mask = make([]byte, n/goarch.PtrSize)
+		for {
+			var addr uintptr
+			if hbits, addr = hbits.next(); addr == 0 {
+				break
+			}
+			mask[(addr-base)/goarch.PtrSize] = 1
+		}
+		// Callers expect this mask to end at the last pointer.
+		for len(mask) > 0 && mask[len(mask)-1] == 0 {
+			mask = mask[:len(mask)-1]
+		}
+
+		// Make sure we keep ep alive. We may have stopped referencing
+		// ep's data pointer sometime before this point and it's possible
+		// for that memory to get freed.
+		KeepAlive(ep)
+		return
+	}
+
+	// stack
+	if gp := getg(); gp.m.curg.stack.lo <= uintptr(p) && uintptr(p) < gp.m.curg.stack.hi {
+		found := false
+		var u unwinder
+		for u.initAt(gp.m.curg.sched.pc, gp.m.curg.sched.sp, 0, gp.m.curg, 0); u.valid(); u.next() {
+			if u.frame.sp <= uintptr(p) && uintptr(p) < u.frame.varp {
+				found = true
+				break
+			}
+		}
+		if found {
+			locals, _, _ := u.frame.getStackMap(false)
+			if locals.n == 0 {
+				return
+			}
+			size := uintptr(locals.n) * goarch.PtrSize
+			n := (*ptrtype)(unsafe.Pointer(t)).Elem.Size_
+			mask = make([]byte, n/goarch.PtrSize)
+			for i := uintptr(0); i < n; i += goarch.PtrSize {
+				off := (uintptr(p) + i - u.frame.varp + size) / goarch.PtrSize
+				mask[i/goarch.PtrSize] = locals.ptrbit(off)
+			}
+		}
+		return
+	}
+
+	// otherwise, not something the GC knows about.
+	// possibly read-only data, like malloc(0).
+	// must not have pointers
+	return
+}
+
+// userArenaHeapBitsSetType is the equivalent of heapBitsSetType but for
+// non-slice-backing-store Go values allocated in a user arena chunk. It
+// sets up the heap bitmap for the value with type typ allocated at address ptr.
+// base is the base address of the arena chunk.
+func userArenaHeapBitsSetType(typ *_type, ptr unsafe.Pointer, s *mspan) {
+	base := s.base()
+	h := writeHeapBitsForAddr(uintptr(ptr))
+
+	// Our last allocation might have ended right at a noMorePtrs mark,
+	// which we would not have erased. We need to erase that mark here,
+	// because we're going to start adding new heap bitmap bits.
+	// We only need to clear one mark, because below we make sure to
+	// pad out the bits with zeroes and only write one noMorePtrs bit
+	// for each new object.
+	// (This is only necessary at noMorePtrs boundaries, as noMorePtrs
+	// marks within an object allocated with newAt will be erased by
+	// the normal writeHeapBitsForAddr mechanism.)
+	//
+	// Note that we skip this if this is the first allocation in the
+	// arena because there's definitely no previous noMorePtrs mark
+	// (in fact, we *must* do this, because we're going to try to back
+	// up a pointer to fix this up).
+	if uintptr(ptr)%(8*goarch.PtrSize*goarch.PtrSize) == 0 && uintptr(ptr) != base {
+		// Back up one pointer and rewrite that pointer. That will
+		// cause the writeHeapBits implementation to clear the
+		// noMorePtrs bit we need to clear.
+		r := heapBitsForAddr(uintptr(ptr)-goarch.PtrSize, goarch.PtrSize)
+		_, p := r.next()
+		b := uintptr(0)
+		if p == uintptr(ptr)-goarch.PtrSize {
+			b = 1
+		}
+		h = writeHeapBitsForAddr(uintptr(ptr) - goarch.PtrSize)
+		h = h.write(b, 1)
+	}
+
+	p := typ.GCData // start of 1-bit pointer mask (or GC program)
+	var gcProgBits uintptr
+	if typ.Kind_&kindGCProg != 0 {
+		// Expand gc program, using the object itself for storage.
+		gcProgBits = runGCProg(addb(p, 4), (*byte)(ptr))
+		p = (*byte)(ptr)
+	}
+	nb := typ.PtrBytes / goarch.PtrSize
+
+	for i := uintptr(0); i < nb; i += ptrBits {
+		k := nb - i
+		if k > ptrBits {
+			k = ptrBits
+		}
+		h = h.write(readUintptr(addb(p, i/8)), k)
+	}
+	// Note: we call pad here to ensure we emit explicit 0 bits
+	// for the pointerless tail of the object. This ensures that
+	// there's only a single noMorePtrs mark for the next object
+	// to clear. We don't need to do this to clear stale noMorePtrs
+	// markers from previous uses because arena chunk pointer bitmaps
+	// are always fully cleared when reused.
+	h = h.pad(typ.Size_ - typ.PtrBytes)
+	h.flush(uintptr(ptr), typ.Size_)
+
+	if typ.Kind_&kindGCProg != 0 {
+		// Zero out temporary ptrmask buffer inside object.
+		memclrNoHeapPointers(ptr, (gcProgBits+7)/8)
+	}
+
+	// Double-check that the bitmap was written out correctly.
+	//
+	// Derived from heapBitsSetType.
+	const doubleCheck = false
+	if doubleCheck {
+		size := typ.Size_
+		x := uintptr(ptr)
+		h := heapBitsForAddr(x, size)
+		for i := uintptr(0); i < size; i += goarch.PtrSize {
+			// Compute the pointer bit we want at offset i.
+			want := false
+			off := i % typ.Size_
+			if off < typ.PtrBytes {
+				j := off / goarch.PtrSize
+				want = *addb(typ.GCData, j/8)>>(j%8)&1 != 0
+			}
+			if want {
+				var addr uintptr
+				h, addr = h.next()
+				if addr != x+i {
+					throw("userArenaHeapBitsSetType: pointer entry not correct")
+				}
+			}
+		}
+		if _, addr := h.next(); addr != 0 {
+			throw("userArenaHeapBitsSetType: extra pointer")
+		}
+	}
+}
+
+// For goexperiment.AllocHeaders.
+type typePointers struct {
+	addr uintptr
+}
+
+// For goexperiment.AllocHeaders.
+//
+//go:nosplit
+func (span *mspan) typePointersOf(addr, size uintptr) typePointers {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders.
+//
+//go:nosplit
+func (span *mspan) typePointersOfUnchecked(addr uintptr) typePointers {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders.
+//
+//go:nosplit
+func (tp typePointers) nextFast() (typePointers, uintptr) {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders.
+//
+//go:nosplit
+func (tp typePointers) next(limit uintptr) (typePointers, uintptr) {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders.
+//
+//go:nosplit
+func (tp typePointers) fastForward(n, limit uintptr) typePointers {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders, to pass TestIntendedInlining.
+func (s *mspan) writeUserArenaHeapBits() {
+	panic("not implemented")
+}
+
+// For goexperiment.AllocHeaders, to pass TestIntendedInlining.
+func heapBitsSlice() {
+	panic("not implemented")
+}