src/runtime/symtab.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

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

// Frames may be used to get function/file/line information for a
// slice of PC values returned by Callers.
type Frames struct {
	// callers is a slice of PCs that have not yet been expanded to frames.
	callers []uintptr

	// frames is a slice of Frames that have yet to be returned.
	frames     []Frame
	frameStore [2]Frame
}

// Frame is the information returned by Frames for each call frame.
type Frame struct {
	// PC is the program counter for the location in this frame.
	// For a frame that calls another frame, this will be the
	// program counter of a call instruction. Because of inlining,
	// multiple frames may have the same PC value, but different
	// symbolic information.
	PC uintptr

	// Func is the Func value of this call frame. This may be nil
	// for non-Go code or fully inlined functions.
	Func *Func

	// Function is the package path-qualified function name of
	// this call frame. If non-empty, this string uniquely
	// identifies a single function in the program.
	// This may be the empty string if not known.
	// If Func is not nil then Function == Func.Name().
	Function string

	// File and Line are the file name and line number of the
	// location in this frame. For non-leaf frames, this will be
	// the location of a call. These may be the empty string and
	// zero, respectively, if not known.
	File string
	Line int

	// Entry point program counter for the function; may be zero
	// if not known. If Func is not nil then Entry ==
	// Func.Entry().
	Entry uintptr

	// The runtime's internal view of the function. This field
	// is set (funcInfo.valid() returns true) only for Go functions,
	// not for C functions.
	funcInfo funcInfo
}

// CallersFrames takes a slice of PC values returned by Callers and
// prepares to return function/file/line information.
// Do not change the slice until you are done with the Frames.
func CallersFrames(callers []uintptr) *Frames {
	f := &Frames{callers: callers}
	f.frames = f.frameStore[:0]
	return f
}

// Next returns frame information for the next caller.
// If more is false, there are no more callers (the Frame value is valid).
func (ci *Frames) Next() (frame Frame, more bool) {
	for len(ci.frames) < 2 {
		// Find the next frame.
		// We need to look for 2 frames so we know what
		// to return for the "more" result.
		if len(ci.callers) == 0 {
			break
		}
		pc := ci.callers[0]
		ci.callers = ci.callers[1:]
		funcInfo := findfunc(pc)
		if !funcInfo.valid() {
			if cgoSymbolizer != nil {
				// Pre-expand cgo frames. We could do this
				// incrementally, too, but there's no way to
				// avoid allocation in this case anyway.
				ci.frames = append(ci.frames, expandCgoFrames(pc)...)
			}
			continue
		}
		f := funcInfo._Func()
		entry := f.Entry()
		if pc > entry {
			// We store the pc of the start of the instruction following
			// the instruction in question (the call or the inline mark).
			// This is done for historical reasons, and to make FuncForPC
			// work correctly for entries in the result of runtime.Callers.
			pc--
		}
		name := funcname(funcInfo)
		if inldata := funcdata(funcInfo, _FUNCDATA_InlTree); inldata != nil {
			inltree := (*[1 << 20]inlinedCall)(inldata)
			ix := pcdatavalue(funcInfo, _PCDATA_InlTreeIndex, pc, nil)
			if ix >= 0 {
				// Note: entry is not modified. It always refers to a real frame, not an inlined one.
				f = nil
				name = funcnameFromNameoff(funcInfo, inltree[ix].func_)
				// File/line is already correct.
				// TODO: remove file/line from InlinedCall?
			}
		}
		ci.frames = append(ci.frames, Frame{
			PC:       pc,
			Func:     f,
			Function: name,
			Entry:    entry,
			funcInfo: funcInfo,
			// Note: File,Line set below
		})
	}

	// Pop one frame from the frame list. Keep the rest.
	// Avoid allocation in the common case, which is 1 or 2 frames.
	switch len(ci.frames) {
	case 0: // In the rare case when there are no frames at all, we return Frame{}.
		return
	case 1:
		frame = ci.frames[0]
		ci.frames = ci.frameStore[:0]
	case 2:
		frame = ci.frames[0]
		ci.frameStore[0] = ci.frames[1]
		ci.frames = ci.frameStore[:1]
	default:
		frame = ci.frames[0]
		ci.frames = ci.frames[1:]
	}
	more = len(ci.frames) > 0
	if frame.funcInfo.valid() {
		// Compute file/line just before we need to return it,
		// as it can be expensive. This avoids computing file/line
		// for the Frame we find but don't return. See issue 32093.
		file, line := funcline1(frame.funcInfo, frame.PC, false)
		frame.File, frame.Line = file, int(line)
	}
	return
}

// runtime_expandFinalInlineFrame expands the final pc in stk to include all
// "callers" if pc is inline.
//
//go:linkname runtime_expandFinalInlineFrame runtime/pprof.runtime_expandFinalInlineFrame
func runtime_expandFinalInlineFrame(stk []uintptr) []uintptr {
	if len(stk) == 0 {
		return stk
	}
	pc := stk[len(stk)-1]
	tracepc := pc - 1

	f := findfunc(tracepc)
	if !f.valid() {
		// Not a Go function.
		return stk
	}

	inldata := funcdata(f, _FUNCDATA_InlTree)
	if inldata == nil {
		// Nothing inline in f.
		return stk
	}

	// Treat the previous func as normal. We haven't actually checked, but
	// since this pc was included in the stack, we know it shouldn't be
	// elided.
	lastFuncID := funcID_normal

	// Remove pc from stk; we'll re-add it below.
	stk = stk[:len(stk)-1]

	// See inline expansion in gentraceback.
	var cache pcvalueCache
	inltree := (*[1 << 20]inlinedCall)(inldata)
	for {
		ix := pcdatavalue(f, _PCDATA_InlTreeIndex, tracepc, &cache)
		if ix < 0 {
			break
		}
		if inltree[ix].funcID == funcID_wrapper && elideWrapperCalling(lastFuncID) {
			// ignore wrappers
		} else {
			stk = append(stk, pc)
		}
		lastFuncID = inltree[ix].funcID
		// Back up to an instruction in the "caller".
		tracepc = f.entry + uintptr(inltree[ix].parentPc)
		pc = tracepc + 1
	}

	// N.B. we want to keep the last parentPC which is not inline.
	stk = append(stk, pc)

	return stk
}

// expandCgoFrames expands frame information for pc, known to be
// a non-Go function, using the cgoSymbolizer hook. expandCgoFrames
// returns nil if pc could not be expanded.
func expandCgoFrames(pc uintptr) []Frame {
	arg := cgoSymbolizerArg{pc: pc}
	callCgoSymbolizer(&arg)

	if arg.file == nil && arg.funcName == nil {
		// No useful information from symbolizer.
		return nil
	}

	var frames []Frame
	for {
		frames = append(frames, Frame{
			PC:       pc,
			Func:     nil,
			Function: gostring(arg.funcName),
			File:     gostring(arg.file),
			Line:     int(arg.lineno),
			Entry:    arg.entry,
			// funcInfo is zero, which implies !funcInfo.valid().
			// That ensures that we use the File/Line info given here.
		})
		if arg.more == 0 {
			break
		}
		callCgoSymbolizer(&arg)
	}

	// No more frames for this PC. Tell the symbolizer we are done.
	// We don't try to maintain a single cgoSymbolizerArg for the
	// whole use of Frames, because there would be no good way to tell
	// the symbolizer when we are done.
	arg.pc = 0
	callCgoSymbolizer(&arg)

	return frames
}

// NOTE: Func does not expose the actual unexported fields, because we return *Func
// values to users, and we want to keep them from being able to overwrite the data
// with (say) *f = Func{}.
// All code operating on a *Func must call raw() to get the *_func
// or funcInfo() to get the funcInfo instead.

// A Func represents a Go function in the running binary.
type Func struct {
	opaque struct{} // unexported field to disallow conversions
}

func (f *Func) raw() *_func {
	return (*_func)(unsafe.Pointer(f))
}

func (f *Func) funcInfo() funcInfo {
	fn := f.raw()
	return funcInfo{fn, findmoduledatap(fn.entry)}
}

// PCDATA and FUNCDATA table indexes.
//
// See funcdata.h and ../cmd/internal/objabi/funcdata.go.
const (
	_PCDATA_UnsafePoint   = 0
	_PCDATA_StackMapIndex = 1
	_PCDATA_InlTreeIndex  = 2

	_FUNCDATA_ArgsPointerMaps    = 0
	_FUNCDATA_LocalsPointerMaps  = 1
	_FUNCDATA_StackObjects       = 2
	_FUNCDATA_InlTree            = 3
	_FUNCDATA_OpenCodedDeferInfo = 4

	_ArgsSizeUnknown = -0x80000000
)

const (
	// PCDATA_UnsafePoint values.
	_PCDATA_UnsafePointSafe   = -1 // Safe for async preemption
	_PCDATA_UnsafePointUnsafe = -2 // Unsafe for async preemption

	// _PCDATA_Restart1(2) apply on a sequence of instructions, within
	// which if an async preemption happens, we should back off the PC
	// to the start of the sequence when resume.
	// We need two so we can distinguish the start/end of the sequence
	// in case that two sequences are next to each other.
	_PCDATA_Restart1 = -3
	_PCDATA_Restart2 = -4

	// Like _PCDATA_RestartAtEntry, but back to function entry if async
	// preempted.
	_PCDATA_RestartAtEntry = -5
)

// A FuncID identifies particular functions that need to be treated
// specially by the runtime.
// Note that in some situations involving plugins, there may be multiple
// copies of a particular special runtime function.
// Note: this list must match the list in cmd/internal/objabi/funcid.go.
type funcID uint8

const (
	funcID_normal funcID = iota // not a special function
	funcID_asmcgocall
	funcID_asyncPreempt
	funcID_cgocallback
	funcID_debugCallV1
	funcID_externalthreadhandler
	funcID_gcBgMarkWorker
	funcID_goexit
	funcID_gogo
	funcID_gopanic
	funcID_handleAsyncEvent
	funcID_jmpdefer
	funcID_mcall
	funcID_morestack
	funcID_mstart
	funcID_panicwrap
	funcID_rt0_go
	funcID_runfinq
	funcID_runtime_main
	funcID_sigpanic
	funcID_systemstack
	funcID_systemstack_switch
	funcID_wrapper // any autogenerated code (hash/eq algorithms, method wrappers, etc.)
)

// A FuncFlag holds bits about a function.
// This list must match the list in cmd/internal/objabi/funcid.go.
type funcFlag uint8

const (
	// TOPFRAME indicates a function that appears at the top of its stack.
	// The traceback routine stop at such a function and consider that a
	// successful, complete traversal of the stack.
	// Examples of TOPFRAME functions include goexit, which appears
	// at the top of a user goroutine stack, and mstart, which appears
	// at the top of a system goroutine stack.
	funcFlag_TOPFRAME funcFlag = 1 << iota

	// SPWRITE indicates a function that writes an arbitrary value to SP
	// (any write other than adding or subtracting a constant amount).
	// The traceback routines cannot encode such changes into the
	// pcsp tables, so the function traceback cannot safely unwind past
	// SPWRITE functions. Stopping at an SPWRITE function is considered
	// to be an incomplete unwinding of the stack. In certain contexts
	// (in particular garbage collector stack scans) that is a fatal error.
	funcFlag_SPWRITE
)

// pcHeader holds data used by the pclntab lookups.
type pcHeader struct {
	magic          uint32  // 0xFFFFFFFA
	pad1, pad2     uint8   // 0,0
	minLC          uint8   // min instruction size
	ptrSize        uint8   // size of a ptr in bytes
	nfunc          int     // number of functions in the module
	nfiles         uint    // number of entries in the file tab.
	funcnameOffset uintptr // offset to the funcnametab variable from pcHeader
	cuOffset       uintptr // offset to the cutab variable from pcHeader
	filetabOffset  uintptr // offset to the filetab variable from pcHeader
	pctabOffset    uintptr // offset to the pctab varible from pcHeader
	pclnOffset     uintptr // offset to the pclntab variable from pcHeader
}

// moduledata records information about the layout of the executable
// image. It is written by the linker. Any changes here must be
// matched changes to the code in cmd/internal/ld/symtab.go:symtab.
// moduledata is stored in statically allocated non-pointer memory;
// none of the pointers here are visible to the garbage collector.
type moduledata struct {
	pcHeader     *pcHeader
	funcnametab  []byte
	cutab        []uint32
	filetab      []byte
	pctab        []byte
	pclntable    []byte
	ftab         []functab
	findfunctab  uintptr
	minpc, maxpc uintptr

	text, etext           uintptr
	noptrdata, enoptrdata uintptr
	data, edata           uintptr
	bss, ebss             uintptr
	noptrbss, enoptrbss   uintptr
	end, gcdata, gcbss    uintptr
	types, etypes         uintptr

	textsectmap []textsect
	typelinks   []int32 // offsets from types
	itablinks   []*itab

	ptab []ptabEntry

	pluginpath string
	pkghashes  []modulehash

	modulename   string
	modulehashes []modulehash

	hasmain uint8 // 1 if module contains the main function, 0 otherwise

	gcdatamask, gcbssmask bitvector

	typemap map[typeOff]*_type // offset to *_rtype in previous module

	bad bool // module failed to load and should be ignored

	next *moduledata
}

// A modulehash is used to compare the ABI of a new module or a
// package in a new module with the loaded program.
//
// For each shared library a module links against, the linker creates an entry in the
// moduledata.modulehashes slice containing the name of the module, the abi hash seen
// at link time and a pointer to the runtime abi hash. These are checked in
// moduledataverify1 below.
//
// For each loaded plugin, the pkghashes slice has a modulehash of the
// newly loaded package that can be used to check the plugin's version of
// a package against any previously loaded version of the package.
// This is done in plugin.lastmoduleinit.
type modulehash struct {
	modulename   string
	linktimehash string
	runtimehash  *string
}

// pinnedTypemaps are the map[typeOff]*_type from the moduledata objects.
//
// These typemap objects are allocated at run time on the heap, but the
// only direct reference to them is in the moduledata, created by the
// linker and marked SNOPTRDATA so it is ignored by the GC.
//
// To make sure the map isn't collected, we keep a second reference here.
var pinnedTypemaps []map[typeOff]*_type

var firstmoduledata moduledata  // linker symbol
var lastmoduledatap *moduledata // linker symbol
var modulesSlice *[]*moduledata // see activeModules

// activeModules returns a slice of active modules.
//
// A module is active once its gcdatamask and gcbssmask have been
// assembled and it is usable by the GC.
//
// This is nosplit/nowritebarrier because it is called by the
// cgo pointer checking code.
//go:nosplit
//go:nowritebarrier
func activeModules() []*moduledata {
	p := (*[]*moduledata)(atomic.Loadp(unsafe.Pointer(&modulesSlice)))
	if p == nil {
		return nil
	}
	return *p
}

// modulesinit creates the active modules slice out of all loaded modules.
//
// When a module is first loaded by the dynamic linker, an .init_array
// function (written by cmd/link) is invoked to call addmoduledata,
// appending to the module to the linked list that starts with
// firstmoduledata.
//
// There are two times this can happen in the lifecycle of a Go
// program. First, if compiled with -linkshared, a number of modules
// built with -buildmode=shared can be loaded at program initialization.
// Second, a Go program can load a module while running that was built
// with -buildmode=plugin.
//
// After loading, this function is called which initializes the
// moduledata so it is usable by the GC and creates a new activeModules
// list.
//
// Only one goroutine may call modulesinit at a time.
func modulesinit() {
	modules := new([]*moduledata)
	for md := &firstmoduledata; md != nil; md = md.next {
		if md.bad {
			continue
		}
		*modules = append(*modules, md)
		if md.gcdatamask == (bitvector{}) {
			md.gcdatamask = progToPointerMask((*byte)(unsafe.Pointer(md.gcdata)), md.edata-md.data)
			md.gcbssmask = progToPointerMask((*byte)(unsafe.Pointer(md.gcbss)), md.ebss-md.bss)
		}
	}

	// Modules appear in the moduledata linked list in the order they are
	// loaded by the dynamic loader, with one exception: the
	// firstmoduledata itself the module that contains the runtime. This
	// is not always the first module (when using -buildmode=shared, it
	// is typically libstd.so, the second module). The order matters for
	// typelinksinit, so we swap the first module with whatever module
	// contains the main function.
	//
	// See Issue #18729.
	for i, md := range *modules {
		if md.hasmain != 0 {
			(*modules)[0] = md
			(*modules)[i] = &firstmoduledata
			break
		}
	}

	atomicstorep(unsafe.Pointer(&modulesSlice), unsafe.Pointer(modules))
}

type functab struct {
	entry   uintptr
	funcoff uintptr
}

// Mapping information for secondary text sections

type textsect struct {
	vaddr    uintptr // prelinked section vaddr
	length   uintptr // section length
	baseaddr uintptr // relocated section address
}

const minfunc = 16                 // minimum function size
const pcbucketsize = 256 * minfunc // size of bucket in the pc->func lookup table

// findfunctab is an array of these structures.
// Each bucket represents 4096 bytes of the text segment.
// Each subbucket represents 256 bytes of the text segment.
// To find a function given a pc, locate the bucket and subbucket for
// that pc. Add together the idx and subbucket value to obtain a
// function index. Then scan the functab array starting at that
// index to find the target function.
// This table uses 20 bytes for every 4096 bytes of code, or ~0.5% overhead.
type findfuncbucket struct {
	idx        uint32
	subbuckets [16]byte
}

func moduledataverify() {
	for datap := &firstmoduledata; datap != nil; datap = datap.next {
		moduledataverify1(datap)
	}
}

const debugPcln = false

func moduledataverify1(datap *moduledata) {
	// Check that the pclntab's format is valid.
	hdr := datap.pcHeader
	if hdr.magic != 0xfffffffa || hdr.pad1 != 0 || hdr.pad2 != 0 || hdr.minLC != sys.PCQuantum || hdr.ptrSize != sys.PtrSize {
		println("runtime: function symbol table header:", hex(hdr.magic), hex(hdr.pad1), hex(hdr.pad2), hex(hdr.minLC), hex(hdr.ptrSize))
		throw("invalid function symbol table\n")
	}

	// ftab is lookup table for function by program counter.
	nftab := len(datap.ftab) - 1
	for i := 0; i < nftab; i++ {
		// NOTE: ftab[nftab].entry is legal; it is the address beyond the final function.
		if datap.ftab[i].entry > datap.ftab[i+1].entry {
			f1 := funcInfo{(*_func)(unsafe.Pointer(&datap.pclntable[datap.ftab[i].funcoff])), datap}
			f2 := funcInfo{(*_func)(unsafe.Pointer(&datap.pclntable[datap.ftab[i+1].funcoff])), datap}
			f2name := "end"
			if i+1 < nftab {
				f2name = funcname(f2)
			}
			println("function symbol table not sorted by program counter:", hex(datap.ftab[i].entry), funcname(f1), ">", hex(datap.ftab[i+1].entry), f2name)
			for j := 0; j <= i; j++ {
				print("\t", hex(datap.ftab[j].entry), " ", funcname(funcInfo{(*_func)(unsafe.Pointer(&datap.pclntable[datap.ftab[j].funcoff])), datap}), "\n")
			}
			if GOOS == "aix" && isarchive {
				println("-Wl,-bnoobjreorder is mandatory on aix/ppc64 with c-archive")
			}
			throw("invalid runtime symbol table")
		}
	}

	if datap.minpc != datap.ftab[0].entry ||
		datap.maxpc != datap.ftab[nftab].entry {
		throw("minpc or maxpc invalid")
	}

	for _, modulehash := range datap.modulehashes {
		if modulehash.linktimehash != *modulehash.runtimehash {
			println("abi mismatch detected between", datap.modulename, "and", modulehash.modulename)
			throw("abi mismatch")
		}
	}
}

// FuncForPC returns a *Func describing the function that contains the
// given program counter address, or else nil.
//
// If pc represents multiple functions because of inlining, it returns
// the *Func describing the innermost function, but with an entry of
// the outermost function.
func FuncForPC(pc uintptr) *Func {
	f := findfunc(pc)
	if !f.valid() {
		return nil
	}
	if inldata := funcdata(f, _FUNCDATA_InlTree); inldata != nil {
		// Note: strict=false so bad PCs (those between functions) don't crash the runtime.
		// We just report the preceding function in that situation. See issue 29735.
		// TODO: Perhaps we should report no function at all in that case.
		// The runtime currently doesn't have function end info, alas.
		if ix := pcdatavalue1(f, _PCDATA_InlTreeIndex, pc, nil, false); ix >= 0 {
			inltree := (*[1 << 20]inlinedCall)(inldata)
			name := funcnameFromNameoff(f, inltree[ix].func_)
			file, line := funcline(f, pc)
			fi := &funcinl{
				entry: f.entry, // entry of the real (the outermost) function.
				name:  name,
				file:  file,
				line:  int(line),
			}
			return (*Func)(unsafe.Pointer(fi))
		}
	}
	return f._Func()
}

// Name returns the name of the function.
func (f *Func) Name() string {
	if f == nil {
		return ""
	}
	fn := f.raw()
	if fn.entry == 0 { // inlined version
		fi := (*funcinl)(unsafe.Pointer(fn))
		return fi.name
	}
	return funcname(f.funcInfo())
}

// Entry returns the entry address of the function.
func (f *Func) Entry() uintptr {
	fn := f.raw()
	if fn.entry == 0 { // inlined version
		fi := (*funcinl)(unsafe.Pointer(fn))
		return fi.entry
	}
	return fn.entry
}

// FileLine returns the file name and line number of the
// source code corresponding to the program counter pc.
// The result will not be accurate if pc is not a program
// counter within f.
func (f *Func) FileLine(pc uintptr) (file string, line int) {
	fn := f.raw()
	if fn.entry == 0 { // inlined version
		fi := (*funcinl)(unsafe.Pointer(fn))
		return fi.file, fi.line
	}
	// Pass strict=false here, because anyone can call this function,
	// and they might just be wrong about targetpc belonging to f.
	file, line32 := funcline1(f.funcInfo(), pc, false)
	return file, int(line32)
}

func findmoduledatap(pc uintptr) *moduledata {
	for datap := &firstmoduledata; datap != nil; datap = datap.next {
		if datap.minpc <= pc && pc < datap.maxpc {
			return datap
		}
	}
	return nil
}

type funcInfo struct {
	*_func
	datap *moduledata
}

func (f funcInfo) valid() bool {
	return f._func != nil
}

func (f funcInfo) _Func() *Func {
	return (*Func)(unsafe.Pointer(f._func))
}

func findfunc(pc uintptr) funcInfo {
	datap := findmoduledatap(pc)
	if datap == nil {
		return funcInfo{}
	}
	const nsub = uintptr(len(findfuncbucket{}.subbuckets))

	x := pc - datap.minpc
	b := x / pcbucketsize
	i := x % pcbucketsize / (pcbucketsize / nsub)

	ffb := (*findfuncbucket)(add(unsafe.Pointer(datap.findfunctab), b*unsafe.Sizeof(findfuncbucket{})))
	idx := ffb.idx + uint32(ffb.subbuckets[i])

	// If the idx is beyond the end of the ftab, set it to the end of the table and search backward.
	// This situation can occur if multiple text sections are generated to handle large text sections
	// and the linker has inserted jump tables between them.

	if idx >= uint32(len(datap.ftab)) {
		idx = uint32(len(datap.ftab) - 1)
	}
	if pc < datap.ftab[idx].entry {
		// With multiple text sections, the idx might reference a function address that
		// is higher than the pc being searched, so search backward until the matching address is found.

		for datap.ftab[idx].entry > pc && idx > 0 {
			idx--
		}
		if idx == 0 {
			throw("findfunc: bad findfunctab entry idx")
		}
	} else {
		// linear search to find func with pc >= entry.
		for datap.ftab[idx+1].entry <= pc {
			idx++
		}
	}
	funcoff := datap.ftab[idx].funcoff
	if funcoff == ^uintptr(0) {
		// With multiple text sections, there may be functions inserted by the external
		// linker that are not known by Go. This means there may be holes in the PC
		// range covered by the func table. The invalid funcoff value indicates a hole.
		// See also cmd/link/internal/ld/pcln.go:pclntab
		return funcInfo{}
	}
	return funcInfo{(*_func)(unsafe.Pointer(&datap.pclntable[funcoff])), datap}
}

type pcvalueCache struct {
	entries [2][8]pcvalueCacheEnt
}

type pcvalueCacheEnt struct {
	// targetpc and off together are the key of this cache entry.
	targetpc uintptr
	off      uint32
	// val is the value of this cached pcvalue entry.
	val int32
}

// pcvalueCacheKey returns the outermost index in a pcvalueCache to use for targetpc.
// It must be very cheap to calculate.
// For now, align to sys.PtrSize and reduce mod the number of entries.
// In practice, this appears to be fairly randomly and evenly distributed.
func pcvalueCacheKey(targetpc uintptr) uintptr {
	return (targetpc / sys.PtrSize) % uintptr(len(pcvalueCache{}.entries))
}

// Returns the PCData value, and the PC where this value starts.
// TODO: the start PC is returned only when cache is nil.
func pcvalue(f funcInfo, off uint32, targetpc uintptr, cache *pcvalueCache, strict bool) (int32, uintptr) {
	if off == 0 {
		return -1, 0
	}

	// Check the cache. This speeds up walks of deep stacks, which
	// tend to have the same recursive functions over and over.
	//
	// This cache is small enough that full associativity is
	// cheaper than doing the hashing for a less associative
	// cache.
	if cache != nil {
		x := pcvalueCacheKey(targetpc)
		for i := range cache.entries[x] {
			// We check off first because we're more
			// likely to have multiple entries with
			// different offsets for the same targetpc
			// than the other way around, so we'll usually
			// fail in the first clause.
			ent := &cache.entries[x][i]
			if ent.off == off && ent.targetpc == targetpc {
				return ent.val, 0
			}
		}
	}

	if !f.valid() {
		if strict && panicking == 0 {
			print("runtime: no module data for ", hex(f.entry), "\n")
			throw("no module data")
		}
		return -1, 0
	}
	datap := f.datap
	p := datap.pctab[off:]
	pc := f.entry
	prevpc := pc
	val := int32(-1)
	for {
		var ok bool
		p, ok = step(p, &pc, &val, pc == f.entry)
		if !ok {
			break
		}
		if targetpc < pc {
			// Replace a random entry in the cache. Random
			// replacement prevents a performance cliff if
			// a recursive stack's cycle is slightly
			// larger than the cache.
			// Put the new element at the beginning,
			// since it is the most likely to be newly used.
			if cache != nil {
				x := pcvalueCacheKey(targetpc)
				e := &cache.entries[x]
				ci := fastrand() % uint32(len(cache.entries[x]))
				e[ci] = e[0]
				e[0] = pcvalueCacheEnt{
					targetpc: targetpc,
					off:      off,
					val:      val,
				}
			}

			return val, prevpc
		}
		prevpc = pc
	}

	// If there was a table, it should have covered all program counters.
	// If not, something is wrong.
	if panicking != 0 || !strict {
		return -1, 0
	}

	print("runtime: invalid pc-encoded table f=", funcname(f), " pc=", hex(pc), " targetpc=", hex(targetpc), " tab=", p, "\n")

	p = datap.pctab[off:]
	pc = f.entry
	val = -1
	for {
		var ok bool
		p, ok = step(p, &pc, &val, pc == f.entry)
		if !ok {
			break
		}
		print("\tvalue=", val, " until pc=", hex(pc), "\n")
	}

	throw("invalid runtime symbol table")
	return -1, 0
}

func cfuncname(f funcInfo) *byte {
	if !f.valid() || f.nameoff == 0 {
		return nil
	}
	return &f.datap.funcnametab[f.nameoff]
}

func funcname(f funcInfo) string {
	return gostringnocopy(cfuncname(f))
}

func funcpkgpath(f funcInfo) string {
	name := funcname(f)
	i := len(name) - 1
	for ; i > 0; i-- {
		if name[i] == '/' {
			break
		}
	}
	for ; i < len(name); i++ {
		if name[i] == '.' {
			break
		}
	}
	return name[:i]
}

func cfuncnameFromNameoff(f funcInfo, nameoff int32) *byte {
	if !f.valid() {
		return nil
	}
	return &f.datap.funcnametab[nameoff]
}

func funcnameFromNameoff(f funcInfo, nameoff int32) string {
	return gostringnocopy(cfuncnameFromNameoff(f, nameoff))
}

func funcfile(f funcInfo, fileno int32) string {
	datap := f.datap
	if !f.valid() {
		return "?"
	}
	// Make sure the cu index and file offset are valid
	if fileoff := datap.cutab[f.cuOffset+uint32(fileno)]; fileoff != ^uint32(0) {
		return gostringnocopy(&datap.filetab[fileoff])
	}
	// pcln section is corrupt.
	return "?"
}

func funcline1(f funcInfo, targetpc uintptr, strict bool) (file string, line int32) {
	datap := f.datap
	if !f.valid() {
		return "?", 0
	}
	fileno, _ := pcvalue(f, f.pcfile, targetpc, nil, strict)
	line, _ = pcvalue(f, f.pcln, targetpc, nil, strict)
	if fileno == -1 || line == -1 || int(fileno) >= len(datap.filetab) {
		// print("looking for ", hex(targetpc), " in ", funcname(f), " got file=", fileno, " line=", lineno, "\n")
		return "?", 0
	}
	file = funcfile(f, fileno)
	return
}

func funcline(f funcInfo, targetpc uintptr) (file string, line int32) {
	return funcline1(f, targetpc, true)
}

func funcspdelta(f funcInfo, targetpc uintptr, cache *pcvalueCache) int32 {
	x, _ := pcvalue(f, f.pcsp, targetpc, cache, true)
	if x&(sys.PtrSize-1) != 0 {
		print("invalid spdelta ", funcname(f), " ", hex(f.entry), " ", hex(targetpc), " ", hex(f.pcsp), " ", x, "\n")
	}
	return x
}

// funcMaxSPDelta returns the maximum spdelta at any point in f.
func funcMaxSPDelta(f funcInfo) int32 {
	datap := f.datap
	p := datap.pctab[f.pcsp:]
	pc := f.entry
	val := int32(-1)
	max := int32(0)
	for {
		var ok bool
		p, ok = step(p, &pc, &val, pc == f.entry)
		if !ok {
			return max
		}
		if val > max {
			max = val
		}
	}
}

func pcdatastart(f funcInfo, table uint32) uint32 {
	return *(*uint32)(add(unsafe.Pointer(&f.nfuncdata), unsafe.Sizeof(f.nfuncdata)+uintptr(table)*4))
}

func pcdatavalue(f funcInfo, table uint32, targetpc uintptr, cache *pcvalueCache) int32 {
	if table >= f.npcdata {
		return -1
	}
	r, _ := pcvalue(f, pcdatastart(f, table), targetpc, cache, true)
	return r
}

func pcdatavalue1(f funcInfo, table uint32, targetpc uintptr, cache *pcvalueCache, strict bool) int32 {
	if table >= f.npcdata {
		return -1
	}
	r, _ := pcvalue(f, pcdatastart(f, table), targetpc, cache, strict)
	return r
}

// Like pcdatavalue, but also return the start PC of this PCData value.
// It doesn't take a cache.
func pcdatavalue2(f funcInfo, table uint32, targetpc uintptr) (int32, uintptr) {
	if table >= f.npcdata {
		return -1, 0
	}
	return pcvalue(f, pcdatastart(f, table), targetpc, nil, true)
}

func funcdata(f funcInfo, i uint8) unsafe.Pointer {
	if i < 0 || i >= f.nfuncdata {
		return nil
	}
	p := add(unsafe.Pointer(&f.nfuncdata), unsafe.Sizeof(f.nfuncdata)+uintptr(f.npcdata)*4)
	if sys.PtrSize == 8 && uintptr(p)&4 != 0 {
		if uintptr(unsafe.Pointer(f._func))&4 != 0 {
			println("runtime: misaligned func", f._func)
		}
		p = add(p, 4)
	}
	return *(*unsafe.Pointer)(add(p, uintptr(i)*sys.PtrSize))
}

// step advances to the next pc, value pair in the encoded table.
func step(p []byte, pc *uintptr, val *int32, first bool) (newp []byte, ok bool) {
	// For both uvdelta and pcdelta, the common case (~70%)
	// is that they are a single byte. If so, avoid calling readvarint.
	uvdelta := uint32(p[0])
	if uvdelta == 0 && !first {
		return nil, false
	}
	n := uint32(1)
	if uvdelta&0x80 != 0 {
		n, uvdelta = readvarint(p)
	}
	*val += int32(-(uvdelta & 1) ^ (uvdelta >> 1))
	p = p[n:]

	pcdelta := uint32(p[0])
	n = 1
	if pcdelta&0x80 != 0 {
		n, pcdelta = readvarint(p)
	}
	p = p[n:]
	*pc += uintptr(pcdelta * sys.PCQuantum)
	return p, true
}

// readvarint reads a varint from p.
func readvarint(p []byte) (read uint32, val uint32) {
	var v, shift, n uint32
	for {
		b := p[n]
		n++
		v |= uint32(b&0x7F) << (shift & 31)
		if b&0x80 == 0 {
			break
		}
		shift += 7
	}
	return n, v
}

type stackmap struct {
	n        int32   // number of bitmaps
	nbit     int32   // number of bits in each bitmap
	bytedata [1]byte // bitmaps, each starting on a byte boundary
}

//go:nowritebarrier
func stackmapdata(stkmap *stackmap, n int32) bitvector {
	// Check this invariant only when stackDebug is on at all.
	// The invariant is already checked by many of stackmapdata's callers,
	// and disabling it by default allows stackmapdata to be inlined.
	if stackDebug > 0 && (n < 0 || n >= stkmap.n) {
		throw("stackmapdata: index out of range")
	}
	return bitvector{stkmap.nbit, addb(&stkmap.bytedata[0], uintptr(n*((stkmap.nbit+7)>>3)))}
}

// inlinedCall is the encoding of entries in the FUNCDATA_InlTree table.
type inlinedCall struct {
	parent   int16  // index of parent in the inltree, or < 0
	funcID   funcID // type of the called function
	_        byte
	file     int32 // perCU file index for inlined call. See cmd/link:pcln.go
	line     int32 // line number of the call site
	func_    int32 // offset into pclntab for name of called function
	parentPc int32 // position of an instruction whose source position is the call site (offset from entry)
}