libgo/go/runtime/debuglog.go - gofrontend - Git at Google

 // Copyright 2019 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.

 // This file provides an internal debug logging facility. The debug
 // log is a lightweight, in-memory, per-M ring buffer. By default, the
 // runtime prints the debug log on panic.
 //
 // To print something to the debug log, call dlog to obtain a dlogger
 // and use the methods on that to add values. The values will be
 // space-separated in the output (much like println).
 //
 // This facility can be enabled by passing -tags debuglog when
 // building. Without this tag, dlog calls compile to nothing.

 package runtime

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

 // debugLogBytes is the size of each per-M ring buffer. This is
 // allocated off-heap to avoid blowing up the M and hence the GC'd
 // heap size.
 const debugLogBytes = 16 << 10

 // debugLogStringLimit is the maximum number of bytes in a string.
 // Above this, the string will be truncated with "..(n more bytes).."
 const debugLogStringLimit = debugLogBytes / 8

 // dlog returns a debug logger. The caller can use methods on the
 // returned logger to add values, which will be space-separated in the
 // final output, much like println. The caller must call end() to
 // finish the message.
 //
 // dlog can be used from highly-constrained corners of the runtime: it
 // is safe to use in the signal handler, from within the write
 // barrier, from within the stack implementation, and in places that
 // must be recursively nosplit.
 //
 // This will be compiled away if built without the debuglog build tag.
 // However, argument construction may not be. If any of the arguments
 // are not literals or trivial expressions, consider protecting the
 // call with "if dlogEnabled".
 //
 //go:nosplit
 //go:nowritebarrierrec
 func dlog() *dlogger {
 	if !dlogEnabled {
 		return nil
 	}

 	// Get the time.
 	tick, nano := uint64(cputicks()), uint64(nanotime())

 	// Try to get a cached logger.
 	l := getCachedDlogger()

 	// If we couldn't get a cached logger, try to get one from the
 	// global pool.
 	if l == nil {
 		allp := (*uintptr)(unsafe.Pointer(&allDloggers))
 		all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
 		for l1 := all; l1 != nil; l1 = l1.allLink {
 			if atomic.Load(&l1.owned) == 0 && atomic.Cas(&l1.owned, 0, 1) {
 				l = l1
 				break
 			}
 		}
 	}

 	// If that failed, allocate a new logger.
 	if l == nil {
 		l = (*dlogger)(sysAlloc(unsafe.Sizeof(dlogger{}), nil))
 		if l == nil {
 			throw("failed to allocate debug log")
 		}
 		l.w.r.data = &l.w.data
 		l.owned = 1

 		// Prepend to allDloggers list.
 		headp := (*uintptr)(unsafe.Pointer(&allDloggers))
 		for {
 			head := atomic.Loaduintptr(headp)
 			l.allLink = (*dlogger)(unsafe.Pointer(head))
 			if atomic.Casuintptr(headp, head, uintptr(unsafe.Pointer(l))) {
 				break
 			}
 		}
 	}

 	// If the time delta is getting too high, write a new sync
 	// packet. We set the limit so we don't write more than 6
 	// bytes of delta in the record header.
 	const deltaLimit = 1<<(3*7) - 1 // ~2ms between sync packets
 	if tick-l.w.tick > deltaLimit || nano-l.w.nano > deltaLimit {
 		l.w.writeSync(tick, nano)
 	}

 	// Reserve space for framing header.
 	l.w.ensure(debugLogHeaderSize)
 	l.w.write += debugLogHeaderSize

 	// Write record header.
 	l.w.uvarint(tick - l.w.tick)
 	l.w.uvarint(nano - l.w.nano)
 	gp := getg()
 	if gp != nil && gp.m != nil && gp.m.p != 0 {
 		l.w.varint(int64(gp.m.p.ptr().id))
 	} else {
 		l.w.varint(-1)
 	}

 	return l
 }

 // A dlogger writes to the debug log.
 //
 // To obtain a dlogger, call dlog(). When done with the dlogger, call
 // end().
 //
 //go:notinheap
 type dlogger struct {
 	w debugLogWriter

 	// allLink is the next dlogger in the allDloggers list.
 	allLink *dlogger

 	// owned indicates that this dlogger is owned by an M. This is
 	// accessed atomically.
 	owned uint32
 }

 // allDloggers is a list of all dloggers, linked through
 // dlogger.allLink. This is accessed atomically. This is prepend only,
 // so it doesn't need to protect against ABA races.
 var allDloggers *dlogger

 //go:nosplit
 func (l *dlogger) end() {
 	if !dlogEnabled {
 		return
 	}

 	// Fill in framing header.
 	size := l.w.write - l.w.r.end
 	if !l.w.writeFrameAt(l.w.r.end, size) {
 		throw("record too large")
 	}

 	// Commit the record.
 	l.w.r.end = l.w.write

 	// Attempt to return this logger to the cache.
 	if putCachedDlogger(l) {
 		return
 	}

 	// Return the logger to the global pool.
 	atomic.Store(&l.owned, 0)
 }

 const (
 	debugLogUnknown = 1 + iota
 	debugLogBoolTrue
 	debugLogBoolFalse
 	debugLogInt
 	debugLogUint
 	debugLogHex
 	debugLogPtr
 	debugLogString
 	debugLogConstString
 	debugLogStringOverflow

 	debugLogPC
 	debugLogTraceback
 )

 //go:nosplit
 func (l *dlogger) b(x bool) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	if x {
 		l.w.byte(debugLogBoolTrue)
 	} else {
 		l.w.byte(debugLogBoolFalse)
 	}
 	return l
 }

 //go:nosplit
 func (l *dlogger) i(x int) *dlogger {
 	return l.i64(int64(x))
 }

 //go:nosplit
 func (l *dlogger) i8(x int8) *dlogger {
 	return l.i64(int64(x))
 }

 //go:nosplit
 func (l *dlogger) i16(x int16) *dlogger {
 	return l.i64(int64(x))
 }

 //go:nosplit
 func (l *dlogger) i32(x int32) *dlogger {
 	return l.i64(int64(x))
 }

 //go:nosplit
 func (l *dlogger) i64(x int64) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogInt)
 	l.w.varint(x)
 	return l
 }

 //go:nosplit
 func (l *dlogger) u(x uint) *dlogger {
 	return l.u64(uint64(x))
 }

 //go:nosplit
 func (l *dlogger) uptr(x uintptr) *dlogger {
 	return l.u64(uint64(x))
 }

 //go:nosplit
 func (l *dlogger) u8(x uint8) *dlogger {
 	return l.u64(uint64(x))
 }

 //go:nosplit
 func (l *dlogger) u16(x uint16) *dlogger {
 	return l.u64(uint64(x))
 }

 //go:nosplit
 func (l *dlogger) u32(x uint32) *dlogger {
 	return l.u64(uint64(x))
 }

 //go:nosplit
 func (l *dlogger) u64(x uint64) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogUint)
 	l.w.uvarint(x)
 	return l
 }

 //go:nosplit
 func (l *dlogger) hex(x uint64) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogHex)
 	l.w.uvarint(x)
 	return l
 }

 //go:nosplit
 func (l *dlogger) p(x any) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogPtr)
 	if x == nil {
 		l.w.uvarint(0)
 	} else {
 		v := efaceOf(&x)
 		switch v._type.kind & kindMask {
 		case kindChan, kindFunc, kindMap, kindPtr, kindUnsafePointer:
 			l.w.uvarint(uint64(uintptr(v.data)))
 		default:
 			throw("not a pointer type")
 		}
 	}
 	return l
 }

 // C function to get an address before the read-only data segment.
 func getText() uintptr

 // C function to get an address after the read-only data segment.
 func getEtext() uintptr

 //go:nosplit
 func (l *dlogger) s(x string) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	str := stringStructOf(&x)
 	text := getText()
 	etext := getEtext()
 	if len(x) > 4 && text != 0 && etext != 0 && text <= uintptr(str.str) && uintptr(str.str) < etext {
 		// String constants are in the rodata section, which
 		// isn't recorded in moduledata. But it has to be
 		// somewhere between etext and end.
 		l.w.byte(debugLogConstString)
 		l.w.uvarint(uint64(str.len))
 		l.w.uvarint(uint64(uintptr(str.str) - text))
 	} else {
 		l.w.byte(debugLogString)
 		var b []byte
 		bb := (*slice)(unsafe.Pointer(&b))
 		bb.array = str.str
 		bb.len, bb.cap = str.len, str.len
 		if len(b) > debugLogStringLimit {
 			b = b[:debugLogStringLimit]
 		}
 		l.w.uvarint(uint64(len(b)))
 		l.w.bytes(b)
 		if len(b) != len(x) {
 			l.w.byte(debugLogStringOverflow)
 			l.w.uvarint(uint64(len(x) - len(b)))
 		}
 	}
 	return l
 }

 //go:nosplit
 func (l *dlogger) pc(x uintptr) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogPC)
 	l.w.uvarint(uint64(x))
 	return l
 }

 //go:nosplit
 func (l *dlogger) traceback(x []uintptr) *dlogger {
 	if !dlogEnabled {
 		return l
 	}
 	l.w.byte(debugLogTraceback)
 	l.w.uvarint(uint64(len(x)))
 	for _, pc := range x {
 		l.w.uvarint(uint64(pc))
 	}
 	return l
 }

 // A debugLogWriter is a ring buffer of binary debug log records.
 //
 // A log record consists of a 2-byte framing header and a sequence of
 // fields. The framing header gives the size of the record as a little
 // endian 16-bit value. Each field starts with a byte indicating its
 // type, followed by type-specific data. If the size in the framing
 // header is 0, it's a sync record consisting of two little endian
 // 64-bit values giving a new time base.
 //
 // Because this is a ring buffer, new records will eventually
 // overwrite old records. Hence, it maintains a reader that consumes
 // the log as it gets overwritten. That reader state is where an
 // actual log reader would start.
 //
 //go:notinheap
 type debugLogWriter struct {
 	write uint64
 	data  debugLogBuf

 	// tick and nano are the time bases from the most recently
 	// written sync record.
 	tick, nano uint64

 	// r is a reader that consumes records as they get overwritten
 	// by the writer. It also acts as the initial reader state
 	// when printing the log.
 	r debugLogReader

 	// buf is a scratch buffer for encoding. This is here to
 	// reduce stack usage.
 	buf [10]byte
 }

 //go:notinheap
 type debugLogBuf [debugLogBytes]byte

 const (
 	// debugLogHeaderSize is the number of bytes in the framing
 	// header of every dlog record.
 	debugLogHeaderSize = 2

 	// debugLogSyncSize is the number of bytes in a sync record.
 	debugLogSyncSize = debugLogHeaderSize + 2*8
 )

 //go:nosplit
 func (l *debugLogWriter) ensure(n uint64) {
 	for l.write+n >= l.r.begin+uint64(len(l.data)) {
 		// Consume record at begin.
 		if l.r.skip() == ^uint64(0) {
 			// Wrapped around within a record.
 			//
 			// TODO(austin): It would be better to just
 			// eat the whole buffer at this point, but we
 			// have to communicate that to the reader
 			// somehow.
 			throw("record wrapped around")
 		}
 	}
 }

 //go:nosplit
 func (l *debugLogWriter) writeFrameAt(pos, size uint64) bool {
 	l.data[pos%uint64(len(l.data))] = uint8(size)
 	l.data[(pos+1)%uint64(len(l.data))] = uint8(size >> 8)
 	return size <= 0xFFFF
 }

 //go:nosplit
 func (l *debugLogWriter) writeSync(tick, nano uint64) {
 	l.tick, l.nano = tick, nano
 	l.ensure(debugLogHeaderSize)
 	l.writeFrameAt(l.write, 0)
 	l.write += debugLogHeaderSize
 	l.writeUint64LE(tick)
 	l.writeUint64LE(nano)
 	l.r.end = l.write
 }

 //go:nosplit
 func (l *debugLogWriter) writeUint64LE(x uint64) {
 	var b [8]byte
 	b[0] = byte(x)
 	b[1] = byte(x >> 8)
 	b[2] = byte(x >> 16)
 	b[3] = byte(x >> 24)
 	b[4] = byte(x >> 32)
 	b[5] = byte(x >> 40)
 	b[6] = byte(x >> 48)
 	b[7] = byte(x >> 56)
 	l.bytes(b[:])
 }

 //go:nosplit
 func (l *debugLogWriter) byte(x byte) {
 	l.ensure(1)
 	pos := l.write
 	l.write++
 	l.data[pos%uint64(len(l.data))] = x
 }

 //go:nosplit
 func (l *debugLogWriter) bytes(x []byte) {
 	l.ensure(uint64(len(x)))
 	pos := l.write
 	l.write += uint64(len(x))
 	for len(x) > 0 {
 		n := copy(l.data[pos%uint64(len(l.data)):], x)
 		pos += uint64(n)
 		x = x[n:]
 	}
 }

 //go:nosplit
 func (l *debugLogWriter) varint(x int64) {
 	var u uint64
 	if x < 0 {
 		u = (^uint64(x) << 1) | 1 // complement i, bit 0 is 1
 	} else {
 		u = (uint64(x) << 1) // do not complement i, bit 0 is 0
 	}
 	l.uvarint(u)
 }

 //go:nosplit
 func (l *debugLogWriter) uvarint(u uint64) {
 	i := 0
 	for u >= 0x80 {
 		l.buf[i] = byte(u) | 0x80
 		u >>= 7
 		i++
 	}
 	l.buf[i] = byte(u)
 	i++
 	l.bytes(l.buf[:i])
 }

 type debugLogReader struct {
 	data *debugLogBuf

 	// begin and end are the positions in the log of the beginning
 	// and end of the log data, modulo len(data).
 	begin, end uint64

 	// tick and nano are the current time base at begin.
 	tick, nano uint64
 }

 //go:nosplit
 func (r *debugLogReader) skip() uint64 {
 	// Read size at pos.
 	if r.begin+debugLogHeaderSize > r.end {
 		return ^uint64(0)
 	}
 	size := uint64(r.readUint16LEAt(r.begin))
 	if size == 0 {
 		// Sync packet.
 		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
 		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
 		size = debugLogSyncSize
 	}
 	if r.begin+size > r.end {
 		return ^uint64(0)
 	}
 	r.begin += size
 	return size
 }

 //go:nosplit
 func (r *debugLogReader) readUint16LEAt(pos uint64) uint16 {
 	return uint16(r.data[pos%uint64(len(r.data))]) |
 		uint16(r.data[(pos+1)%uint64(len(r.data))])<<8
 }

 //go:nosplit
 func (r *debugLogReader) readUint64LEAt(pos uint64) uint64 {
 	var b [8]byte
 	for i := range b {
 		b[i] = r.data[pos%uint64(len(r.data))]
 		pos++
 	}
 	return uint64(b[0]) | uint64(b[1])<<8 |
 		uint64(b[2])<<16 | uint64(b[3])<<24 |
 		uint64(b[4])<<32 | uint64(b[5])<<40 |
 		uint64(b[6])<<48 | uint64(b[7])<<56
 }

 func (r *debugLogReader) peek() (tick uint64) {
 	// Consume any sync records.
 	size := uint64(0)
 	for size == 0 {
 		if r.begin+debugLogHeaderSize > r.end {
 			return ^uint64(0)
 		}
 		size = uint64(r.readUint16LEAt(r.begin))
 		if size != 0 {
 			break
 		}
 		if r.begin+debugLogSyncSize > r.end {
 			return ^uint64(0)
 		}
 		// Sync packet.
 		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
 		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
 		r.begin += debugLogSyncSize
 	}

 	// Peek tick delta.
 	if r.begin+size > r.end {
 		return ^uint64(0)
 	}
 	pos := r.begin + debugLogHeaderSize
 	var u uint64
 	for i := uint(0); ; i += 7 {
 		b := r.data[pos%uint64(len(r.data))]
 		pos++
 		u |= uint64(b&^0x80) << i
 		if b&0x80 == 0 {
 			break
 		}
 	}
 	if pos > r.begin+size {
 		return ^uint64(0)
 	}
 	return r.tick + u
 }

 func (r *debugLogReader) header() (end, tick, nano uint64, p int) {
 	// Read size. We've already skipped sync packets and checked
 	// bounds in peek.
 	size := uint64(r.readUint16LEAt(r.begin))
 	end = r.begin + size
 	r.begin += debugLogHeaderSize

 	// Read tick, nano, and p.
 	tick = r.uvarint() + r.tick
 	nano = r.uvarint() + r.nano
 	p = int(r.varint())

 	return
 }

 func (r *debugLogReader) uvarint() uint64 {
 	var u uint64
 	for i := uint(0); ; i += 7 {
 		b := r.data[r.begin%uint64(len(r.data))]
 		r.begin++
 		u |= uint64(b&^0x80) << i
 		if b&0x80 == 0 {
 			break
 		}
 	}
 	return u
 }

 func (r *debugLogReader) varint() int64 {
 	u := r.uvarint()
 	var v int64
 	if u&1 == 0 {
 		v = int64(u >> 1)
 	} else {
 		v = ^int64(u >> 1)
 	}
 	return v
 }

 func (r *debugLogReader) printVal() bool {
 	typ := r.data[r.begin%uint64(len(r.data))]
 	r.begin++

 	switch typ {
 	default:
 		print("<unknown field type ", hex(typ), " pos ", r.begin-1, " end ", r.end, ">\n")
 		return false

 	case debugLogUnknown:
 		print("<unknown kind>")

 	case debugLogBoolTrue:
 		print(true)

 	case debugLogBoolFalse:
 		print(false)

 	case debugLogInt:
 		print(r.varint())

 	case debugLogUint:
 		print(r.uvarint())

 	case debugLogHex, debugLogPtr:
 		print(hex(r.uvarint()))

 	case debugLogString:
 		sl := r.uvarint()
 		if r.begin+sl > r.end {
 			r.begin = r.end
 			print("<string length corrupted>")
 			break
 		}
 		for sl > 0 {
 			b := r.data[r.begin%uint64(len(r.data)):]
 			if uint64(len(b)) > sl {
 				b = b[:sl]
 			}
 			r.begin += uint64(len(b))
 			sl -= uint64(len(b))
 			gwrite(b)
 		}

 	case debugLogConstString:
 		len, ptr := int(r.uvarint()), uintptr(r.uvarint())
 		ptr += getText()
 		str := stringStruct{
 			str: unsafe.Pointer(ptr),
 			len: len,
 		}
 		s := *(*string)(unsafe.Pointer(&str))
 		print(s)

 	case debugLogStringOverflow:
 		print("..(", r.uvarint(), " more bytes)..")

 	case debugLogPC:
 		printDebugLogPC(uintptr(r.uvarint()), false)

 	case debugLogTraceback:
 		n := int(r.uvarint())
 		for i := 0; i < n; i++ {
 			print("\n\t")
 			// gentraceback PCs are always return PCs.
 			// Convert them to call PCs.
 			//
 			// TODO(austin): Expand inlined frames.
 			printDebugLogPC(uintptr(r.uvarint()), true)
 		}
 	}

 	return true
 }

 // printDebugLog prints the debug log.
 func printDebugLog() {
 	if !dlogEnabled {
 		return
 	}

 	// This function should not panic or throw since it is used in
 	// the fatal panic path and this may deadlock.

 	printlock()

 	// Get the list of all debug logs.
 	allp := (*uintptr)(unsafe.Pointer(&allDloggers))
 	all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))

 	// Count the logs.
 	n := 0
 	for l := all; l != nil; l = l.allLink {
 		n++
 	}
 	if n == 0 {
 		printunlock()
 		return
 	}

 	// Prepare read state for all logs.
 	type readState struct {
 		debugLogReader
 		first    bool
 		lost     uint64
 		nextTick uint64
 	}
 	state1 := sysAlloc(unsafe.Sizeof(readState{})*uintptr(n), nil)
 	if state1 == nil {
 		println("failed to allocate read state for", n, "logs")
 		printunlock()
 		return
 	}
 	state := (*[1 << 20]readState)(state1)[:n]
 	{
 		l := all
 		for i := range state {
 			s := &state[i]
 			s.debugLogReader = l.w.r
 			s.first = true
 			s.lost = l.w.r.begin
 			s.nextTick = s.peek()
 			l = l.allLink
 		}
 	}

 	// Print records.
 	for {
 		// Find the next record.
 		var best struct {
 			tick uint64
 			i    int
 		}
 		best.tick = ^uint64(0)
 		for i := range state {
 			if state[i].nextTick < best.tick {
 				best.tick = state[i].nextTick
 				best.i = i
 			}
 		}
 		if best.tick == ^uint64(0) {
 			break
 		}

 		// Print record.
 		s := &state[best.i]
 		if s.first {
 			print(">> begin log ", best.i)
 			if s.lost != 0 {
 				print("; lost first ", s.lost>>10, "KB")
 			}
 			print(" <<\n")
 			s.first = false
 		}

 		end, _, nano, p := s.header()
 		oldEnd := s.end
 		s.end = end

 		print("[")
 		var tmpbuf [21]byte
 		pnano := int64(nano) - runtimeInitTime
 		if pnano < 0 {
 			// Logged before runtimeInitTime was set.
 			pnano = 0
 		}
 		print(string(itoaDiv(tmpbuf[:], uint64(pnano), 9)))
 		print(" P ", p, "] ")

 		for i := 0; s.begin < s.end; i++ {
 			if i > 0 {
 				print(" ")
 			}
 			if !s.printVal() {
 				// Abort this P log.
 				print("<aborting P log>")
 				end = oldEnd
 				break
 			}
 		}
 		println()

 		// Move on to the next record.
 		s.begin = end
 		s.end = oldEnd
 		s.nextTick = s.peek()
 	}

 	printunlock()
 }

 // printDebugLogPC prints a single symbolized PC. If returnPC is true,
 // pc is a return PC that must first be converted to a call PC.
 func printDebugLogPC(pc uintptr, returnPC bool) {
 	name, file, line, _ := funcfileline(pc, -1, false)
 	entry := funcentry(pc)
 	if returnPC && (name == "" || (entry != 0 && pc > funcentry(pc))) {
 		// TODO(austin): Don't back up if the previous frame
 		// was a sigpanic.
 		pc--
 	}

 	print(hex(pc))
 	if name == "" {
 		print(" [unknown PC]")
 	} else {
 		print(" [", name, "+", hex(pc-funcentry(pc)),
 			" ", file, ":", line, "]")
 	}
 }
	// Copyright 2019 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.

	// This file provides an internal debug logging facility. The debug
	// log is a lightweight, in-memory, per-M ring buffer. By default, the
	// runtime prints the debug log on panic.
	//
	// To print something to the debug log, call dlog to obtain a dlogger
	// and use the methods on that to add values. The values will be
	// space-separated in the output (much like println).
	//
	// This facility can be enabled by passing -tags debuglog when
	// building. Without this tag, dlog calls compile to nothing.

	package runtime

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

	// debugLogBytes is the size of each per-M ring buffer. This is
	// allocated off-heap to avoid blowing up the M and hence the GC'd
	// heap size.
	const debugLogBytes = 16 << 10

	// debugLogStringLimit is the maximum number of bytes in a string.
	// Above this, the string will be truncated with "..(n more bytes).."
	const debugLogStringLimit = debugLogBytes / 8

	// dlog returns a debug logger. The caller can use methods on the
	// returned logger to add values, which will be space-separated in the
	// final output, much like println. The caller must call end() to
	// finish the message.
	//
	// dlog can be used from highly-constrained corners of the runtime: it
	// is safe to use in the signal handler, from within the write
	// barrier, from within the stack implementation, and in places that
	// must be recursively nosplit.
	//
	// This will be compiled away if built without the debuglog build tag.
	// However, argument construction may not be. If any of the arguments
	// are not literals or trivial expressions, consider protecting the
	// call with "if dlogEnabled".
	//
	//go:nosplit
	//go:nowritebarrierrec
	func dlog() *dlogger {
	if !dlogEnabled {
	return nil
	}

	// Get the time.
	tick, nano := uint64(cputicks()), uint64(nanotime())

	// Try to get a cached logger.
	l := getCachedDlogger()

	// If we couldn't get a cached logger, try to get one from the
	// global pool.
	if l == nil {
	allp := (*uintptr)(unsafe.Pointer(&allDloggers))
	all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
	for l1 := all; l1 != nil; l1 = l1.allLink {
	if atomic.Load(&l1.owned) == 0 && atomic.Cas(&l1.owned, 0, 1) {
	l = l1
	break
	}
	}
	}

	// If that failed, allocate a new logger.
	if l == nil {
	l = (*dlogger)(sysAlloc(unsafe.Sizeof(dlogger{}), nil))
	if l == nil {
	throw("failed to allocate debug log")
	}
	l.w.r.data = &l.w.data
	l.owned = 1

	// Prepend to allDloggers list.
	headp := (*uintptr)(unsafe.Pointer(&allDloggers))
	for {
	head := atomic.Loaduintptr(headp)
	l.allLink = (*dlogger)(unsafe.Pointer(head))
	if atomic.Casuintptr(headp, head, uintptr(unsafe.Pointer(l))) {
	break
	}
	}
	}

	// If the time delta is getting too high, write a new sync
	// packet. We set the limit so we don't write more than 6
	// bytes of delta in the record header.
	const deltaLimit = 1<<(3*7) - 1 // ~2ms between sync packets
	if tick-l.w.tick > deltaLimit \|\| nano-l.w.nano > deltaLimit {
	l.w.writeSync(tick, nano)
	}

	// Reserve space for framing header.
	l.w.ensure(debugLogHeaderSize)
	l.w.write += debugLogHeaderSize

	// Write record header.
	l.w.uvarint(tick - l.w.tick)
	l.w.uvarint(nano - l.w.nano)
	gp := getg()
	if gp != nil && gp.m != nil && gp.m.p != 0 {
	l.w.varint(int64(gp.m.p.ptr().id))
	} else {
	l.w.varint(-1)
	}

	return l
	}

	// A dlogger writes to the debug log.
	//
	// To obtain a dlogger, call dlog(). When done with the dlogger, call
	// end().
	//
	//go:notinheap
	type dlogger struct {
	w debugLogWriter

	// allLink is the next dlogger in the allDloggers list.
	allLink *dlogger

	// owned indicates that this dlogger is owned by an M. This is
	// accessed atomically.
	owned uint32
	}

	// allDloggers is a list of all dloggers, linked through
	// dlogger.allLink. This is accessed atomically. This is prepend only,
	// so it doesn't need to protect against ABA races.
	var allDloggers *dlogger

	//go:nosplit
	func (l *dlogger) end() {
	if !dlogEnabled {
	return
	}

	// Fill in framing header.
	size := l.w.write - l.w.r.end
	if !l.w.writeFrameAt(l.w.r.end, size) {
	throw("record too large")
	}

	// Commit the record.
	l.w.r.end = l.w.write

	// Attempt to return this logger to the cache.
	if putCachedDlogger(l) {
	return
	}

	// Return the logger to the global pool.
	atomic.Store(&l.owned, 0)
	}

	const (
	debugLogUnknown = 1 + iota
	debugLogBoolTrue
	debugLogBoolFalse
	debugLogInt
	debugLogUint
	debugLogHex
	debugLogPtr
	debugLogString
	debugLogConstString
	debugLogStringOverflow

	debugLogPC
	debugLogTraceback
	)

	//go:nosplit
	func (l dlogger) b(x bool) dlogger {
	if !dlogEnabled {
	return l
	}
	if x {
	l.w.byte(debugLogBoolTrue)
	} else {
	l.w.byte(debugLogBoolFalse)
	}
	return l
	}

	//go:nosplit
	func (l dlogger) i(x int) dlogger {
	return l.i64(int64(x))
	}

	//go:nosplit
	func (l dlogger) i8(x int8) dlogger {
	return l.i64(int64(x))
	}

	//go:nosplit
	func (l dlogger) i16(x int16) dlogger {
	return l.i64(int64(x))
	}

	//go:nosplit
	func (l dlogger) i32(x int32) dlogger {
	return l.i64(int64(x))
	}

	//go:nosplit
	func (l dlogger) i64(x int64) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogInt)
	l.w.varint(x)
	return l
	}

	//go:nosplit
	func (l dlogger) u(x uint) dlogger {
	return l.u64(uint64(x))
	}

	//go:nosplit
	func (l dlogger) uptr(x uintptr) dlogger {
	return l.u64(uint64(x))
	}

	//go:nosplit
	func (l dlogger) u8(x uint8) dlogger {
	return l.u64(uint64(x))
	}

	//go:nosplit
	func (l dlogger) u16(x uint16) dlogger {
	return l.u64(uint64(x))
	}

	//go:nosplit
	func (l dlogger) u32(x uint32) dlogger {
	return l.u64(uint64(x))
	}

	//go:nosplit
	func (l dlogger) u64(x uint64) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogUint)
	l.w.uvarint(x)
	return l
	}

	//go:nosplit
	func (l dlogger) hex(x uint64) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogHex)
	l.w.uvarint(x)
	return l
	}

	//go:nosplit
	func (l dlogger) p(x any) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogPtr)
	if x == nil {
	l.w.uvarint(0)
	} else {
	v := efaceOf(&x)
	switch v._type.kind & kindMask {
	case kindChan, kindFunc, kindMap, kindPtr, kindUnsafePointer:
	l.w.uvarint(uint64(uintptr(v.data)))
	default:
	throw("not a pointer type")
	}
	}
	return l
	}

	// C function to get an address before the read-only data segment.
	func getText() uintptr

	// C function to get an address after the read-only data segment.
	func getEtext() uintptr

	//go:nosplit
	func (l dlogger) s(x string) dlogger {
	if !dlogEnabled {
	return l
	}
	str := stringStructOf(&x)
	text := getText()
	etext := getEtext()
	if len(x) > 4 && text != 0 && etext != 0 && text <= uintptr(str.str) && uintptr(str.str) < etext {
	// String constants are in the rodata section, which
	// isn't recorded in moduledata. But it has to be
	// somewhere between etext and end.
	l.w.byte(debugLogConstString)
	l.w.uvarint(uint64(str.len))
	l.w.uvarint(uint64(uintptr(str.str) - text))
	} else {
	l.w.byte(debugLogString)
	var b []byte
	bb := (*slice)(unsafe.Pointer(&b))
	bb.array = str.str
	bb.len, bb.cap = str.len, str.len
	if len(b) > debugLogStringLimit {
	b = b[:debugLogStringLimit]
	}
	l.w.uvarint(uint64(len(b)))
	l.w.bytes(b)
	if len(b) != len(x) {
	l.w.byte(debugLogStringOverflow)
	l.w.uvarint(uint64(len(x) - len(b)))
	}
	}
	return l
	}

	//go:nosplit
	func (l dlogger) pc(x uintptr) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogPC)
	l.w.uvarint(uint64(x))
	return l
	}

	//go:nosplit
	func (l dlogger) traceback(x []uintptr) dlogger {
	if !dlogEnabled {
	return l
	}
	l.w.byte(debugLogTraceback)
	l.w.uvarint(uint64(len(x)))
	for _, pc := range x {
	l.w.uvarint(uint64(pc))
	}
	return l
	}

	// A debugLogWriter is a ring buffer of binary debug log records.
	//
	// A log record consists of a 2-byte framing header and a sequence of
	// fields. The framing header gives the size of the record as a little
	// endian 16-bit value. Each field starts with a byte indicating its
	// type, followed by type-specific data. If the size in the framing
	// header is 0, it's a sync record consisting of two little endian
	// 64-bit values giving a new time base.
	//
	// Because this is a ring buffer, new records will eventually
	// overwrite old records. Hence, it maintains a reader that consumes
	// the log as it gets overwritten. That reader state is where an
	// actual log reader would start.
	//
	//go:notinheap
	type debugLogWriter struct {
	write uint64
	data debugLogBuf

	// tick and nano are the time bases from the most recently
	// written sync record.
	tick, nano uint64

	// r is a reader that consumes records as they get overwritten
	// by the writer. It also acts as the initial reader state
	// when printing the log.
	r debugLogReader

	// buf is a scratch buffer for encoding. This is here to
	// reduce stack usage.
	buf [10]byte
	}

	//go:notinheap
	type debugLogBuf [debugLogBytes]byte

	const (
	// debugLogHeaderSize is the number of bytes in the framing
	// header of every dlog record.
	debugLogHeaderSize = 2

	// debugLogSyncSize is the number of bytes in a sync record.
	debugLogSyncSize = debugLogHeaderSize + 2*8
	)

	//go:nosplit
	func (l *debugLogWriter) ensure(n uint64) {
	for l.write+n >= l.r.begin+uint64(len(l.data)) {
	// Consume record at begin.
	if l.r.skip() == ^uint64(0) {
	// Wrapped around within a record.
	//
	// TODO(austin): It would be better to just
	// eat the whole buffer at this point, but we
	// have to communicate that to the reader
	// somehow.
	throw("record wrapped around")
	}
	}
	}

	//go:nosplit
	func (l *debugLogWriter) writeFrameAt(pos, size uint64) bool {
	l.data[pos%uint64(len(l.data))] = uint8(size)
	l.data[(pos+1)%uint64(len(l.data))] = uint8(size >> 8)
	return size <= 0xFFFF
	}

	//go:nosplit
	func (l *debugLogWriter) writeSync(tick, nano uint64) {
	l.tick, l.nano = tick, nano
	l.ensure(debugLogHeaderSize)
	l.writeFrameAt(l.write, 0)
	l.write += debugLogHeaderSize
	l.writeUint64LE(tick)
	l.writeUint64LE(nano)
	l.r.end = l.write
	}

	//go:nosplit
	func (l *debugLogWriter) writeUint64LE(x uint64) {
	var b [8]byte
	b[0] = byte(x)
	b[1] = byte(x >> 8)
	b[2] = byte(x >> 16)
	b[3] = byte(x >> 24)
	b[4] = byte(x >> 32)
	b[5] = byte(x >> 40)
	b[6] = byte(x >> 48)
	b[7] = byte(x >> 56)
	l.bytes(b[:])
	}

	//go:nosplit
	func (l *debugLogWriter) byte(x byte) {
	l.ensure(1)
	pos := l.write
	l.write++
	l.data[pos%uint64(len(l.data))] = x
	}

	//go:nosplit
	func (l *debugLogWriter) bytes(x []byte) {
	l.ensure(uint64(len(x)))
	pos := l.write
	l.write += uint64(len(x))
	for len(x) > 0 {
	n := copy(l.data[pos%uint64(len(l.data)):], x)
	pos += uint64(n)
	x = x[n:]
	}
	}

	//go:nosplit
	func (l *debugLogWriter) varint(x int64) {
	var u uint64
	if x < 0 {
	u = (^uint64(x) << 1) \| 1 // complement i, bit 0 is 1
	} else {
	u = (uint64(x) << 1) // do not complement i, bit 0 is 0
	}
	l.uvarint(u)
	}

	//go:nosplit
	func (l *debugLogWriter) uvarint(u uint64) {
	i := 0
	for u >= 0x80 {
	l.buf[i] = byte(u) \| 0x80
	u >>= 7
	i++
	}
	l.buf[i] = byte(u)
	i++
	l.bytes(l.buf[:i])
	}

	type debugLogReader struct {
	data *debugLogBuf

	// begin and end are the positions in the log of the beginning
	// and end of the log data, modulo len(data).
	begin, end uint64

	// tick and nano are the current time base at begin.
	tick, nano uint64
	}

	//go:nosplit
	func (r *debugLogReader) skip() uint64 {
	// Read size at pos.
	if r.begin+debugLogHeaderSize > r.end {
	return ^uint64(0)
	}
	size := uint64(r.readUint16LEAt(r.begin))
	if size == 0 {
	// Sync packet.
	r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
	r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
	size = debugLogSyncSize
	}
	if r.begin+size > r.end {
	return ^uint64(0)
	}
	r.begin += size
	return size
	}

	//go:nosplit
	func (r *debugLogReader) readUint16LEAt(pos uint64) uint16 {
	return uint16(r.data[pos%uint64(len(r.data))]) \|
	uint16(r.data[(pos+1)%uint64(len(r.data))])<<8
	}

	//go:nosplit
	func (r *debugLogReader) readUint64LEAt(pos uint64) uint64 {
	var b [8]byte
	for i := range b {
	b[i] = r.data[pos%uint64(len(r.data))]
	pos++
	}
	return uint64(b[0]) \| uint64(b[1])<<8 \|
	uint64(b[2])<<16 \| uint64(b[3])<<24 \|
	uint64(b[4])<<32 \| uint64(b[5])<<40 \|
	uint64(b[6])<<48 \| uint64(b[7])<<56
	}

	func (r *debugLogReader) peek() (tick uint64) {
	// Consume any sync records.
	size := uint64(0)
	for size == 0 {
	if r.begin+debugLogHeaderSize > r.end {
	return ^uint64(0)
	}
	size = uint64(r.readUint16LEAt(r.begin))
	if size != 0 {
	break
	}
	if r.begin+debugLogSyncSize > r.end {
	return ^uint64(0)
	}
	// Sync packet.
	r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
	r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
	r.begin += debugLogSyncSize
	}

	// Peek tick delta.
	if r.begin+size > r.end {
	return ^uint64(0)
	}
	pos := r.begin + debugLogHeaderSize
	var u uint64
	for i := uint(0); ; i += 7 {
	b := r.data[pos%uint64(len(r.data))]
	pos++
	u \|= uint64(b&^0x80) << i
	if b&0x80 == 0 {
	break
	}
	}
	if pos > r.begin+size {
	return ^uint64(0)
	}
	return r.tick + u
	}

	func (r *debugLogReader) header() (end, tick, nano uint64, p int) {
	// Read size. We've already skipped sync packets and checked
	// bounds in peek.
	size := uint64(r.readUint16LEAt(r.begin))
	end = r.begin + size
	r.begin += debugLogHeaderSize

	// Read tick, nano, and p.
	tick = r.uvarint() + r.tick
	nano = r.uvarint() + r.nano
	p = int(r.varint())

	return
	}

	func (r *debugLogReader) uvarint() uint64 {
	var u uint64
	for i := uint(0); ; i += 7 {
	b := r.data[r.begin%uint64(len(r.data))]
	r.begin++
	u \|= uint64(b&^0x80) << i
	if b&0x80 == 0 {
	break
	}
	}
	return u
	}

	func (r *debugLogReader) varint() int64 {
	u := r.uvarint()
	var v int64
	if u&1 == 0 {
	v = int64(u >> 1)
	} else {
	v = ^int64(u >> 1)
	}
	return v
	}

	func (r *debugLogReader) printVal() bool {
	typ := r.data[r.begin%uint64(len(r.data))]
	r.begin++

	switch typ {
	default:
	print("<unknown field type ", hex(typ), " pos ", r.begin-1, " end ", r.end, ">\n")
	return false

	case debugLogUnknown:
	print("<unknown kind>")

	case debugLogBoolTrue:
	print(true)

	case debugLogBoolFalse:
	print(false)

	case debugLogInt:
	print(r.varint())

	case debugLogUint:
	print(r.uvarint())

	case debugLogHex, debugLogPtr:
	print(hex(r.uvarint()))

	case debugLogString:
	sl := r.uvarint()
	if r.begin+sl > r.end {
	r.begin = r.end
	print("<string length corrupted>")
	break
	}
	for sl > 0 {
	b := r.data[r.begin%uint64(len(r.data)):]
	if uint64(len(b)) > sl {
	b = b[:sl]
	}
	r.begin += uint64(len(b))
	sl -= uint64(len(b))
	gwrite(b)
	}

	case debugLogConstString:
	len, ptr := int(r.uvarint()), uintptr(r.uvarint())
	ptr += getText()
	str := stringStruct{
	str: unsafe.Pointer(ptr),
	len: len,
	}
	s := (string)(unsafe.Pointer(&str))
	print(s)

	case debugLogStringOverflow:
	print("..(", r.uvarint(), " more bytes)..")

	case debugLogPC:
	printDebugLogPC(uintptr(r.uvarint()), false)

	case debugLogTraceback:
	n := int(r.uvarint())
	for i := 0; i < n; i++ {
	print("\n\t")
	// gentraceback PCs are always return PCs.
	// Convert them to call PCs.
	//
	// TODO(austin): Expand inlined frames.
	printDebugLogPC(uintptr(r.uvarint()), true)
	}
	}

	return true
	}

	// printDebugLog prints the debug log.
	func printDebugLog() {
	if !dlogEnabled {
	return
	}

	// This function should not panic or throw since it is used in
	// the fatal panic path and this may deadlock.

	printlock()

	// Get the list of all debug logs.
	allp := (*uintptr)(unsafe.Pointer(&allDloggers))
	all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))

	// Count the logs.
	n := 0
	for l := all; l != nil; l = l.allLink {
	n++
	}
	if n == 0 {
	printunlock()
	return
	}

	// Prepare read state for all logs.
	type readState struct {
	debugLogReader
	first bool
	lost uint64
	nextTick uint64
	}
	state1 := sysAlloc(unsafe.Sizeof(readState{})*uintptr(n), nil)
	if state1 == nil {
	println("failed to allocate read state for", n, "logs")
	printunlock()
	return
	}
	state := (*[1 << 20]readState)(state1)[:n]
	{
	l := all
	for i := range state {
	s := &state[i]
	s.debugLogReader = l.w.r
	s.first = true
	s.lost = l.w.r.begin
	s.nextTick = s.peek()
	l = l.allLink
	}
	}

	// Print records.
	for {
	// Find the next record.
	var best struct {
	tick uint64
	i int
	}
	best.tick = ^uint64(0)
	for i := range state {
	if state[i].nextTick < best.tick {
	best.tick = state[i].nextTick
	best.i = i
	}
	}
	if best.tick == ^uint64(0) {
	break
	}

	// Print record.
	s := &state[best.i]
	if s.first {
	print(">> begin log ", best.i)
	if s.lost != 0 {
	print("; lost first ", s.lost>>10, "KB")
	}
	print(" <<\n")
	s.first = false
	}

	end, _, nano, p := s.header()
	oldEnd := s.end
	s.end = end

	print("[")
	var tmpbuf [21]byte
	pnano := int64(nano) - runtimeInitTime
	if pnano < 0 {
	// Logged before runtimeInitTime was set.
	pnano = 0
	}
	print(string(itoaDiv(tmpbuf[:], uint64(pnano), 9)))
	print(" P ", p, "] ")

	for i := 0; s.begin < s.end; i++ {
	if i > 0 {
	print(" ")
	}
	if !s.printVal() {
	// Abort this P log.
	print("<aborting P log>")
	end = oldEnd
	break
	}
	}
	println()

	// Move on to the next record.
	s.begin = end
	s.end = oldEnd
	s.nextTick = s.peek()
	}

	printunlock()
	}

	// printDebugLogPC prints a single symbolized PC. If returnPC is true,
	// pc is a return PC that must first be converted to a call PC.
	func printDebugLogPC(pc uintptr, returnPC bool) {
	name, file, line, _ := funcfileline(pc, -1, false)
	entry := funcentry(pc)
	if returnPC && (name == "" \|\| (entry != 0 && pc > funcentry(pc))) {
	// TODO(austin): Don't back up if the previous frame
	// was a sigpanic.
	pc--
	}

	print(hex(pc))
	if name == "" {
	print(" [unknown PC]")
	} else {
	print(" [", name, "+", hex(pc-funcentry(pc)),
	" ", file, ":", line, "]")
	}
	}