debug.go

// Copyright 2017 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 ssa

import (
	"cmd/internal/dwarf"
	"cmd/internal/obj"
	"encoding/hex"
	"fmt"
	"math/bits"
	"sort"
	"strings"
)

type SlotID int32
type VarID int32

// A FuncDebug contains all the debug information for the variables in a
// function. Variables are identified by their LocalSlot, which may be the
// result of decomposing a larger variable.
type FuncDebug struct {
	// Slots is all the slots used in the debug info, indexed by their SlotID.
	Slots []LocalSlot
	// The user variables, indexed by VarID.
	Vars []GCNode
	// The slots that make up each variable, indexed by VarID.
	VarSlots [][]SlotID
	// The location list data, indexed by VarID. Must be processed by PutLocationList.
	LocationLists [][]byte

	// Filled in by the user. Translates Block and Value ID to PC.
	GetPC func(ID, ID) int64
}

type BlockDebug struct {
	// Whether the block had any changes to user variables at all.
	relevant bool
	// State at the end of the block if it's fully processed. Immutable once initialized.
	endState []liveSlot
}

// A liveSlot is a slot that's live in loc at entry/exit of a block.
type liveSlot struct {
	// An inlined VarLoc, so it packs into 16 bytes instead of 20.
	Registers RegisterSet
	StackOffset

	slot SlotID
}

func (loc liveSlot) absent() bool {
	return loc.Registers == 0 && !loc.onStack()
}

// StackOffset encodes whether a value is on the stack and if so, where. It is
// a 31-bit integer followed by a presence flag at the low-order bit.
type StackOffset int32

func (s StackOffset) onStack() bool {
	return s != 0
}

func (s StackOffset) stackOffsetValue() int32 {
	return int32(s) >> 1
}

// stateAtPC is the current state of all variables at some point.
type stateAtPC struct {
	// The location of each known slot, indexed by SlotID.
	slots []VarLoc
	// The slots present in each register, indexed by register number.
	registers [][]SlotID
}

// reset fills state with the live variables from live.
func (state *stateAtPC) reset(live []liveSlot) {
	slots, registers := state.slots, state.registers
	for i := range slots {
		slots[i] = VarLoc{}
	}
	for i := range registers {
		registers[i] = registers[i][:0]
	}
	for _, live := range live {
		slots[live.slot] = VarLoc{live.Registers, live.StackOffset}
		if live.Registers == 0 {
			continue
		}

		mask := uint64(live.Registers)
		for {
			if mask == 0 {
				break
			}
			reg := uint8(bits.TrailingZeros64(mask))
			mask &^= 1 << reg

			registers[reg] = append(registers[reg], live.slot)
		}
	}
	state.slots, state.registers = slots, registers
}

func (s *debugState) LocString(loc VarLoc) string {
	if loc.absent() {
		return "<nil>"
	}

	var storage []string
	if loc.onStack() {
		storage = append(storage, "stack")
	}

	mask := uint64(loc.Registers)
	for {
		if mask == 0 {
			break
		}
		reg := uint8(bits.TrailingZeros64(mask))
		mask &^= 1 << reg

		storage = append(storage, s.registers[reg].String())
	}
	return strings.Join(storage, ",")
}

// A VarLoc describes the storage for part of a user variable.
type VarLoc struct {
	// The registers this variable is available in. There can be more than
	// one in various situations, e.g. it's being moved between registers.
	Registers RegisterSet

	StackOffset
}

func (loc VarLoc) absent() bool {
	return loc.Registers == 0 && !loc.onStack()
}

var BlockStart = &Value{
	ID:  -10000,
	Op:  OpInvalid,
	Aux: "BlockStart",
}

var BlockEnd = &Value{
	ID:  -20000,
	Op:  OpInvalid,
	Aux: "BlockEnd",
}

// RegisterSet is a bitmap of registers, indexed by Register.num.
type RegisterSet uint64

func (s *debugState) logf(msg string, args ...interface{}) {
	s.f.Logf(msg, args...)
}

type debugState struct {
	// See FuncDebug.
	slots    []LocalSlot
	vars     []GCNode
	varSlots [][]SlotID
	lists    [][]byte

	// The user variable that each slot rolls up to, indexed by SlotID.
	slotVars []VarID

	f              *Func
	loggingEnabled bool
	registers      []Register
	stackOffset    func(LocalSlot) int32
	ctxt           *obj.Link

	// The names (slots) associated with each value, indexed by Value ID.
	valueNames [][]SlotID

	// The current state of whatever analysis is running.
	currentState stateAtPC
	liveCount    []int
	changedVars  *sparseSet

	// The pending location list entry for each user variable, indexed by VarID.
	pendingEntries []pendingEntry

	varParts           map[GCNode][]SlotID
	blockDebug         []BlockDebug
	pendingSlotLocs    []VarLoc
	liveSlots          []liveSlot
	liveSlotSliceBegin int
	partsByVarOffset   sort.Interface
}

func (state *debugState) initializeCache(f *Func, numVars, numSlots int) {
	// One blockDebug per block. Initialized in allocBlock.
	if cap(state.blockDebug) < f.NumBlocks() {
		state.blockDebug = make([]BlockDebug, f.NumBlocks())
	} else {
		// This local variable, and the ones like it below, enable compiler
		// optimizations. Don't inline them.
		b := state.blockDebug[:f.NumBlocks()]
		for i := range b {
			b[i] = BlockDebug{}
		}
	}

	// A list of slots per Value. Reuse the previous child slices.
	if cap(state.valueNames) < f.NumValues() {
		old := state.valueNames
		state.valueNames = make([][]SlotID, f.NumValues())
		copy(state.valueNames, old)
	}
	vn := state.valueNames[:f.NumValues()]
	for i := range vn {
		vn[i] = vn[i][:0]
	}

	// Slot and register contents for currentState. Cleared by reset().
	if cap(state.currentState.slots) < numSlots {
		state.currentState.slots = make([]VarLoc, numSlots)
	} else {
		state.currentState.slots = state.currentState.slots[:numSlots]
	}
	if cap(state.currentState.registers) < len(state.registers) {
		state.currentState.registers = make([][]SlotID, len(state.registers))
	} else {
		state.currentState.registers = state.currentState.registers[:len(state.registers)]
	}

	// Used many times by mergePredecessors.
	if cap(state.liveCount) < numSlots {
		state.liveCount = make([]int, numSlots)
	} else {
		state.liveCount = state.liveCount[:numSlots]
	}

	// A relatively small slice, but used many times as the return from processValue.
	state.changedVars = newSparseSet(numVars)

	// A pending entry per user variable, with space to track each of its pieces.
	numPieces := 0
	for i := range state.varSlots {
		numPieces += len(state.varSlots[i])
	}
	if cap(state.pendingSlotLocs) < numPieces {
		state.pendingSlotLocs = make([]VarLoc, numPieces)
	} else {
		psl := state.pendingSlotLocs[:numPieces]
		for i := range psl {
			psl[i] = VarLoc{}
		}
	}
	if cap(state.pendingEntries) < numVars {
		state.pendingEntries = make([]pendingEntry, numVars)
	}
	pe := state.pendingEntries[:numVars]
	freePieceIdx := 0
	for varID, slots := range state.varSlots {
		pe[varID] = pendingEntry{
			pieces: state.pendingSlotLocs[freePieceIdx : freePieceIdx+len(slots)],
		}
		freePieceIdx += len(slots)
	}
	state.pendingEntries = pe

	if cap(state.lists) < numVars {
		state.lists = make([][]byte, numVars)
	} else {
		state.lists = state.lists[:numVars]
		for i := range state.lists {
			state.lists[i] = nil
		}
	}

	state.liveSlots = state.liveSlots[:0]
	state.liveSlotSliceBegin = 0
}

func (state *debugState) allocBlock(b *Block) *BlockDebug {
	return &state.blockDebug[b.ID]
}

func (state *debugState) appendLiveSlot(ls liveSlot) {
	state.liveSlots = append(state.liveSlots, ls)
}

func (state *debugState) getLiveSlotSlice() []liveSlot {
	s := state.liveSlots[state.liveSlotSliceBegin:]
	state.liveSlotSliceBegin = len(state.liveSlots)
	return s
}

func (s *debugState) blockEndStateString(b *BlockDebug) string {
	endState := stateAtPC{slots: make([]VarLoc, len(s.slots)), registers: make([][]SlotID, len(s.registers))}
	endState.reset(b.endState)
	return s.stateString(endState)
}

func (s *debugState) stateString(state stateAtPC) string {
	var strs []string
	for slotID, loc := range state.slots {
		if !loc.absent() {
			strs = append(strs, fmt.Sprintf("\t%v = %v\n", s.slots[slotID], s.LocString(loc)))
		}
	}

	strs = append(strs, "\n")
	for reg, slots := range state.registers {
		if len(slots) != 0 {
			var slotStrs []string
			for _, slot := range slots {
				slotStrs = append(slotStrs, s.slots[slot].String())
			}
			strs = append(strs, fmt.Sprintf("\t%v = %v\n", &s.registers[reg], slotStrs))
		}
	}

	if len(strs) == 1 {
		return "(no vars)\n"
	}
	return strings.Join(strs, "")
}

// BuildFuncDebug returns debug information for f.
// f must be fully processed, so that each Value is where it will be when
// machine code is emitted.
func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset func(LocalSlot) int32) *FuncDebug {
	if f.RegAlloc == nil {
		f.Fatalf("BuildFuncDebug on func %v that has not been fully processed", f)
	}
	state := &f.Cache.debugState
	state.loggingEnabled = loggingEnabled
	state.f = f
	state.registers = f.Config.registers
	state.stackOffset = stackOffset
	state.ctxt = ctxt

	if state.loggingEnabled {
		state.logf("Generating location lists for function %q\n", f.Name)
	}

	if state.varParts == nil {
		state.varParts = make(map[GCNode][]SlotID)
	} else {
		for n := range state.varParts {
			delete(state.varParts, n)
		}
	}

	// Recompose any decomposed variables, and establish the canonical
	// IDs for each var and slot by filling out state.vars and state.slots.

	state.slots = state.slots[:0]
	state.vars = state.vars[:0]
	for i, slot := range f.Names {
		state.slots = append(state.slots, slot)
		if slot.N.IsSynthetic() {
			continue
		}

		topSlot := &slot
		for topSlot.SplitOf != nil {
			topSlot = topSlot.SplitOf
		}
		if _, ok := state.varParts[topSlot.N]; !ok {
			state.vars = append(state.vars, topSlot.N)
		}
		state.varParts[topSlot.N] = append(state.varParts[topSlot.N], SlotID(i))
	}

	// Recreate the LocalSlot for each stack-only variable.
	// This would probably be better as an output from stackframe.
	for _, b := range f.Blocks {
		for _, v := range b.Values {
			if v.Op == OpVarDef || v.Op == OpVarKill {
				n := v.Aux.(GCNode)
				if n.IsSynthetic() {
					continue
				}

				if _, ok := state.varParts[n]; !ok {
					slot := LocalSlot{N: n, Type: v.Type, Off: 0}
					state.slots = append(state.slots, slot)
					state.varParts[n] = []SlotID{SlotID(len(state.slots) - 1)}
					state.vars = append(state.vars, n)
				}
			}
		}
	}

	// Fill in the var<->slot mappings.
	if cap(state.varSlots) < len(state.vars) {
		state.varSlots = make([][]SlotID, len(state.vars))
	} else {
		state.varSlots = state.varSlots[:len(state.vars)]
		for i := range state.varSlots {
			state.varSlots[i] = state.varSlots[i][:0]
		}
	}
	if cap(state.slotVars) < len(state.slots) {
		state.slotVars = make([]VarID, len(state.slots))
	} else {
		state.slotVars = state.slotVars[:len(state.slots)]
	}

	if state.partsByVarOffset == nil {
		state.partsByVarOffset = &partsByVarOffset{}
	}
	for varID, n := range state.vars {
		parts := state.varParts[n]
		state.varSlots[varID] = parts
		for _, slotID := range parts {
			state.slotVars[slotID] = VarID(varID)
		}
		*state.partsByVarOffset.(*partsByVarOffset) = partsByVarOffset{parts, state.slots}
		sort.Sort(state.partsByVarOffset)
	}

	state.initializeCache(f, len(state.varParts), len(state.slots))

	for i, slot := range f.Names {
		if slot.N.IsSynthetic() {
			continue
		}
		for _, value := range f.NamedValues[slot] {
			state.valueNames[value.ID] = append(state.valueNames[value.ID], SlotID(i))
		}
	}

	blockLocs := state.liveness()
	state.buildLocationLists(blockLocs)

	return &FuncDebug{
		Slots:         state.slots,
		VarSlots:      state.varSlots,
		Vars:          state.vars,
		LocationLists: state.lists,
	}
}

// liveness walks the function in control flow order, calculating the start
// and end state of each block.
func (state *debugState) liveness() []*BlockDebug {
	blockLocs := make([]*BlockDebug, state.f.NumBlocks())

	// Reverse postorder: visit a block after as many as possible of its
	// predecessors have been visited.
	po := state.f.Postorder()
	for i := len(po) - 1; i >= 0; i-- {
		b := po[i]

		// Build the starting state for the block from the final
		// state of its predecessors.
		startState, startValid := state.mergePredecessors(b, blockLocs)
		changed := false
		if state.loggingEnabled {
			state.logf("Processing %v, initial state:\n%v", b, state.stateString(state.currentState))
		}

		// Update locs/registers with the effects of each Value.
		for _, v := range b.Values {
			slots := state.valueNames[v.ID]

			// Loads and stores inherit the names of their sources.
			var source *Value
			switch v.Op {
			case OpStoreReg:
				source = v.Args[0]
			case OpLoadReg:
				switch a := v.Args[0]; a.Op {
				case OpArg, OpPhi:
					source = a
				case OpStoreReg:
					source = a.Args[0]
				default:
					if state.loggingEnabled {
						state.logf("at %v: load with unexpected source op: %v (%v)\n", v, a.Op, a)
					}
				}
			}
			// Update valueNames with the source so that later steps
			// don't need special handling.
			if source != nil {
				slots = append(slots, state.valueNames[source.ID]...)
				state.valueNames[v.ID] = slots
			}

			reg, _ := state.f.getHome(v.ID).(*Register)
			c := state.processValue(v, slots, reg)
			changed = changed || c
		}

		if state.loggingEnabled {
			state.f.Logf("Block %v done, locs:\n%v", b, state.stateString(state.currentState))
		}

		locs := state.allocBlock(b)
		locs.relevant = changed
		if !changed && startValid {
			locs.endState = startState
		} else {
			for slotID, slotLoc := range state.currentState.slots {
				if slotLoc.absent() {
					continue
				}
				state.appendLiveSlot(liveSlot{slot: SlotID(slotID), Registers: slotLoc.Registers, StackOffset: slotLoc.StackOffset})
			}
			locs.endState = state.getLiveSlotSlice()
		}
		blockLocs[b.ID] = locs
	}
	return blockLocs
}

// mergePredecessors takes the end state of each of b's predecessors and
// intersects them to form the starting state for b. It returns that state in
// the BlockDebug, and fills state.currentState with it.
func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug) ([]liveSlot, bool) {
	// Filter out back branches.
	var predsBuf [10]*Block
	preds := predsBuf[:0]
	for _, pred := range b.Preds {
		if blockLocs[pred.b.ID] != nil {
			preds = append(preds, pred.b)
		}
	}

	if state.loggingEnabled {
		// The logf below would cause preds to be heap-allocated if
		// it were passed directly.
		preds2 := make([]*Block, len(preds))
		copy(preds2, preds)
		state.logf("Merging %v into %v\n", preds2, b)
	}

	if len(preds) == 0 {
		if state.loggingEnabled {
		}
		state.currentState.reset(nil)
		return nil, true
	}

	p0 := blockLocs[preds[0].ID].endState
	if len(preds) == 1 {
		state.currentState.reset(p0)
		return p0, true
	}

	if state.loggingEnabled {
		state.logf("Starting %v with state from %v:\n%v", b, preds[0], state.blockEndStateString(blockLocs[preds[0].ID]))
	}

	slotLocs := state.currentState.slots
	for _, predSlot := range p0 {
		slotLocs[predSlot.slot] = VarLoc{predSlot.Registers, predSlot.StackOffset}
		state.liveCount[predSlot.slot] = 1
	}
	for i := 1; i < len(preds); i++ {
		if state.loggingEnabled {
			state.logf("Merging in state from %v:\n%v", preds[i], state.blockEndStateString(blockLocs[preds[i].ID]))
		}
		for _, predSlot := range blockLocs[preds[i].ID].endState {
			state.liveCount[predSlot.slot]++
			liveLoc := slotLocs[predSlot.slot]
			if !liveLoc.onStack() || !predSlot.onStack() || liveLoc.StackOffset != predSlot.StackOffset {
				liveLoc.StackOffset = 0
			}
			liveLoc.Registers &= predSlot.Registers
			slotLocs[predSlot.slot] = liveLoc
		}
	}

	// Check if the final state is the same as the first predecessor's
	// final state, and reuse it if so. In principle it could match any,
	// but it's probably not worth checking more than the first.
	unchanged := true
	for _, predSlot := range p0 {
		if state.liveCount[predSlot.slot] != len(preds) ||
			slotLocs[predSlot.slot].Registers != predSlot.Registers ||
			slotLocs[predSlot.slot].StackOffset != predSlot.StackOffset {
			unchanged = false
			break
		}
	}
	if unchanged {
		if state.loggingEnabled {
			state.logf("After merge, %v matches %v exactly.\n", b, preds[0])
		}
		state.currentState.reset(p0)
		return p0, true
	}

	for reg := range state.currentState.registers {
		state.currentState.registers[reg] = state.currentState.registers[reg][:0]
	}

	// A slot is live if it was seen in all predecessors, and they all had
	// some storage in common.
	for _, predSlot := range p0 {
		slotLoc := slotLocs[predSlot.slot]

		if state.liveCount[predSlot.slot] != len(preds) {
			// Seen in only some predecessors. Clear it out.
			slotLocs[predSlot.slot] = VarLoc{}
			continue
		}

		// Present in all predecessors.
		mask := uint64(slotLoc.Registers)
		for {
			if mask == 0 {
				break
			}
			reg := uint8(bits.TrailingZeros64(mask))
			mask &^= 1 << reg

			state.currentState.registers[reg] = append(state.currentState.registers[reg], predSlot.slot)
		}
	}
	return nil, false
}

// processValue updates locs and state.registerContents to reflect v, a value with
// the names in vSlots and homed in vReg.  "v" becomes visible after execution of
// the instructions evaluating it. It returns which VarIDs were modified by the
// Value's execution.
func (state *debugState) processValue(v *Value, vSlots []SlotID, vReg *Register) bool {
	locs := state.currentState
	changed := false
	setSlot := func(slot SlotID, loc VarLoc) {
		changed = true
		state.changedVars.add(ID(state.slotVars[slot]))
		state.currentState.slots[slot] = loc
	}

	// Handle any register clobbering. Call operations, for example,
	// clobber all registers even though they don't explicitly write to
	// them.
	clobbers := uint64(opcodeTable[v.Op].reg.clobbers)
	for {
		if clobbers == 0 {
			break
		}
		reg := uint8(bits.TrailingZeros64(clobbers))
		clobbers &^= 1 << reg

		for _, slot := range locs.registers[reg] {
			if state.loggingEnabled {
				state.logf("at %v: %v clobbered out of %v\n", v, state.slots[slot], &state.registers[reg])
			}

			last := locs.slots[slot]
			if last.absent() {
				state.f.Fatalf("at %v: slot %v in register %v with no location entry", v, state.slots[slot], &state.registers[reg])
				continue
			}
			regs := last.Registers &^ (1 << reg)
			setSlot(slot, VarLoc{regs, last.StackOffset})
		}

		locs.registers[reg] = locs.registers[reg][:0]
	}

	switch {
	case v.Op == OpVarDef, v.Op == OpVarKill:
		n := v.Aux.(GCNode)
		if n.IsSynthetic() {
			break
		}

		slotID := state.varParts[n][0]
		var stackOffset StackOffset
		if v.Op == OpVarDef {
			stackOffset = StackOffset(state.stackOffset(state.slots[slotID])<<1 | 1)
		}
		setSlot(slotID, VarLoc{0, stackOffset})
		if state.loggingEnabled {
			if v.Op == OpVarDef {
				state.logf("at %v: stack-only var %v now live\n", v, state.slots[slotID])
			} else {
				state.logf("at %v: stack-only var %v now dead\n", v, state.slots[slotID])
			}
		}

	case v.Op == OpArg:
		home := state.f.getHome(v.ID).(LocalSlot)
		stackOffset := state.stackOffset(home)<<1 | 1
		for _, slot := range vSlots {
			if state.loggingEnabled {
				state.logf("at %v: arg %v now on stack in location %v\n", v, state.slots[slot], home)
				if last := locs.slots[slot]; !last.absent() {
					state.logf("at %v: unexpected arg op on already-live slot %v\n", v, state.slots[slot])
				}
			}

			setSlot(slot, VarLoc{0, StackOffset(stackOffset)})
		}

	case v.Op == OpStoreReg:
		home := state.f.getHome(v.ID).(LocalSlot)
		stackOffset := state.stackOffset(home)<<1 | 1
		for _, slot := range vSlots {
			last := locs.slots[slot]
			if last.absent() {
				if state.loggingEnabled {
					state.logf("at %v: unexpected spill of unnamed register %s\n", v, vReg)
				}
				break
			}

			setSlot(slot, VarLoc{last.Registers, StackOffset(stackOffset)})
			if state.loggingEnabled {
				state.logf("at %v: %v spilled to stack location %v\n", v, state.slots[slot], home)
			}
		}

	case vReg != nil:
		if state.loggingEnabled {
			newSlots := make([]bool, len(state.slots))
			for _, slot := range vSlots {
				newSlots[slot] = true
			}

			for _, slot := range locs.registers[vReg.num] {
				if !newSlots[slot] {
					state.logf("at %v: overwrote %v in register %v\n", v, state.slots[slot], vReg)
				}
			}
		}

		for _, slot := range locs.registers[vReg.num] {
			last := locs.slots[slot]
			setSlot(slot, VarLoc{last.Registers &^ (1 << uint8(vReg.num)), last.StackOffset})
		}
		locs.registers[vReg.num] = locs.registers[vReg.num][:0]
		locs.registers[vReg.num] = append(locs.registers[vReg.num], vSlots...)
		for _, slot := range vSlots {
			if state.loggingEnabled {
				state.logf("at %v: %v now in %s\n", v, state.slots[slot], vReg)
			}

			last := locs.slots[slot]
			setSlot(slot, VarLoc{1<<uint8(vReg.num) | last.Registers, last.StackOffset})
		}
	}
	return changed
}

// varOffset returns the offset of slot within the user variable it was
// decomposed from. This has nothing to do with its stack offset.
func varOffset(slot LocalSlot) int64 {
	offset := slot.Off
	s := &slot
	for ; s.SplitOf != nil; s = s.SplitOf {
		offset += s.SplitOffset
	}
	return offset
}

type partsByVarOffset struct {
	slotIDs []SlotID
	slots   []LocalSlot
}

func (a partsByVarOffset) Len() int { return len(a.slotIDs) }
func (a partsByVarOffset) Less(i, j int) bool {
	return varOffset(a.slots[a.slotIDs[i]]) < varOffset(a.slots[a.slotIDs[j]])
}
func (a partsByVarOffset) Swap(i, j int) { a.slotIDs[i], a.slotIDs[j] = a.slotIDs[j], a.slotIDs[i] }

// A pendingEntry represents the beginning of a location list entry, missing
// only its end coordinate.
type pendingEntry struct {
	present                bool
	startBlock, startValue ID
	// The location of each piece of the variable, in the same order as the
	// SlotIDs in varParts.
	pieces []VarLoc
}

func (e *pendingEntry) clear() {
	e.present = false
	e.startBlock = 0
	e.startValue = 0
	for i := range e.pieces {
		e.pieces[i] = VarLoc{}
	}
}

// canMerge returns true if the location description for new is the same as
// pending.
func canMerge(pending, new VarLoc) bool {
	if pending.absent() && new.absent() {
		return true
	}
	if pending.absent() || new.absent() {
		return false
	}
	if pending.onStack() {
		return pending.StackOffset == new.StackOffset
	}
	if pending.Registers != 0 && new.Registers != 0 {
		return firstReg(pending.Registers) == firstReg(new.Registers)
	}
	return false
}

// firstReg returns the first register in set that is present.
func firstReg(set RegisterSet) uint8 {
	if set == 0 {
		// This is wrong, but there seem to be some situations where we
		// produce locations with no storage.
		return 0
	}
	return uint8(bits.TrailingZeros64(uint64(set)))
}

// buildLocationLists builds location lists for all the user variables in
// state.f, using the information about block state in blockLocs.
// The returned location lists are not fully complete. They are in terms of
// SSA values rather than PCs, and have no base address/end entries. They will
// be finished by PutLocationList.
func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
	// Run through the function in program text order, building up location
	// lists as we go. The heavy lifting has mostly already been done.
	for _, b := range state.f.Blocks {
		if !blockLocs[b.ID].relevant {
			continue
		}

		state.mergePredecessors(b, blockLocs)

		zeroWidthPending := false
		for _, v := range b.Values {
			slots := state.valueNames[v.ID]
			reg, _ := state.f.getHome(v.ID).(*Register)
			changed := state.processValue(v, slots, reg)

			if opcodeTable[v.Op].zeroWidth {
				if changed {
					zeroWidthPending = true
				}
				continue
			}

			if !changed && !zeroWidthPending {
				continue
			}

			zeroWidthPending = false
			for _, varID := range state.changedVars.contents() {
				state.updateVar(VarID(varID), v, state.currentState.slots)
			}
			state.changedVars.clear()
		}

	}

	if state.loggingEnabled {
		state.logf("location lists:\n")
	}

	// Flush any leftover entries live at the end of the last block.
	for varID := range state.lists {
		state.writePendingEntry(VarID(varID), state.f.Blocks[len(state.f.Blocks)-1].ID, BlockEnd.ID)
		list := state.lists[varID]
		if state.loggingEnabled {
			if len(list) == 0 {
				state.logf("\t%v : empty list\n", state.vars[varID])
			} else {
				state.logf("\t%v : %q\n", state.vars[varID], hex.EncodeToString(state.lists[varID]))
			}
		}
	}
}

// updateVar updates the pending location list entry for varID to
// reflect the new locations in curLoc, caused by v.
func (state *debugState) updateVar(varID VarID, v *Value, curLoc []VarLoc) {
	// Assemble the location list entry with whatever's live.
	empty := true
	for _, slotID := range state.varSlots[varID] {
		if !curLoc[slotID].absent() {
			empty = false
			break
		}
	}
	pending := &state.pendingEntries[varID]
	if empty {
		state.writePendingEntry(varID, v.Block.ID, v.ID)
		pending.clear()
		return
	}

	// Extend the previous entry if possible.
	if pending.present {
		merge := true
		for i, slotID := range state.varSlots[varID] {
			if !canMerge(pending.pieces[i], curLoc[slotID]) {
				merge = false
				break
			}
		}
		if merge {
			return
		}
	}

	state.writePendingEntry(varID, v.Block.ID, v.ID)
	pending.present = true
	pending.startBlock = v.Block.ID
	pending.startValue = v.ID
	for i, slot := range state.varSlots[varID] {
		pending.pieces[i] = curLoc[slot]
	}
	return

}

// writePendingEntry writes out the pending entry for varID, if any,
// terminated at endBlock/Value.
func (state *debugState) writePendingEntry(varID VarID, endBlock, endValue ID) {
	pending := state.pendingEntries[varID]
	if !pending.present {
		return
	}

	// Pack the start/end coordinates into the start/end addresses
	// of the entry, for decoding by PutLocationList.
	start, startOK := encodeValue(state.ctxt, pending.startBlock, pending.startValue)
	end, endOK := encodeValue(state.ctxt, endBlock, endValue)
	if !startOK || !endOK {
		// If someone writes a function that uses >65K values,
		// they get incomplete debug info on 32-bit platforms.
		return
	}
	list := state.lists[varID]
	list = appendPtr(state.ctxt, list, start)
	list = appendPtr(state.ctxt, list, end)
	// Where to write the length of the location description once
	// we know how big it is.
	sizeIdx := len(list)
	list = list[:len(list)+2]

	if state.loggingEnabled {
		var partStrs []string
		for i, slot := range state.varSlots[varID] {
			partStrs = append(partStrs, fmt.Sprintf("%v@%v", state.slots[slot], state.LocString(pending.pieces[i])))
		}
		state.logf("Add entry for %v: \tb%vv%v-b%vv%v = \t%v\n", state.vars[varID], pending.startBlock, pending.startValue, endBlock, endValue, strings.Join(partStrs, " "))
	}

	for i, slotID := range state.varSlots[varID] {
		loc := pending.pieces[i]
		slot := state.slots[slotID]

		if !loc.absent() {
			if loc.onStack() {
				if loc.stackOffsetValue() == 0 {
					list = append(list, dwarf.DW_OP_call_frame_cfa)
				} else {
					list = append(list, dwarf.DW_OP_fbreg)
					list = dwarf.AppendSleb128(list, int64(loc.stackOffsetValue()))
				}
			} else {
				regnum := state.ctxt.Arch.DWARFRegisters[state.registers[firstReg(loc.Registers)].ObjNum()]
				if regnum < 32 {
					list = append(list, dwarf.DW_OP_reg0+byte(regnum))
				} else {
					list = append(list, dwarf.DW_OP_regx)
					list = dwarf.AppendUleb128(list, uint64(regnum))
				}
			}
		}

		if len(state.varSlots[varID]) > 1 {
			list = append(list, dwarf.DW_OP_piece)
			list = dwarf.AppendUleb128(list, uint64(slot.Type.Size()))
		}
	}
	state.ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
	state.lists[varID] = list
}

// PutLocationList adds list (a location list in its intermediate representation) to listSym.
func (debugInfo *FuncDebug) PutLocationList(list []byte, ctxt *obj.Link, listSym, startPC *obj.LSym) {
	getPC := debugInfo.GetPC
	// Re-read list, translating its address from block/value ID to PC.
	for i := 0; i < len(list); {
		begin := getPC(decodeValue(ctxt, readPtr(ctxt, list[i:])))
		end := getPC(decodeValue(ctxt, readPtr(ctxt, list[i+ctxt.Arch.PtrSize:])))

		// Horrible hack. If a range contains only zero-width
		// instructions, e.g. an Arg, and it's at the beginning of the
		// function, this would be indistinguishable from an
		// end entry. Fudge it.
		if begin == 0 && end == 0 {
			end = 1
		}

		writePtr(ctxt, list[i:], uint64(begin))
		writePtr(ctxt, list[i+ctxt.Arch.PtrSize:], uint64(end))
		i += 2 * ctxt.Arch.PtrSize
		i += 2 + int(ctxt.Arch.ByteOrder.Uint16(list[i:]))
	}

	// Base address entry.
	listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, ^0)
	listSym.WriteAddr(ctxt, listSym.Size, ctxt.Arch.PtrSize, startPC, 0)
	// Location list contents, now with real PCs.
	listSym.WriteBytes(ctxt, listSym.Size, list)
	// End entry.
	listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, 0)
	listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, 0)
}

// Pack a value and block ID into an address-sized uint, returning ~0 if they
// don't fit.
func encodeValue(ctxt *obj.Link, b, v ID) (uint64, bool) {
	if ctxt.Arch.PtrSize == 8 {
		result := uint64(b)<<32 | uint64(uint32(v))
		//ctxt.Logf("b %#x (%d) v %#x (%d) -> %#x\n", b, b, v, v, result)
		return result, true
	}
	if ctxt.Arch.PtrSize != 4 {
		panic("unexpected pointer size")
	}
	if ID(int16(b)) != b || ID(int16(v)) != v {
		return 0, false
	}
	return uint64(b)<<16 | uint64(uint16(v)), true
}

// Unpack a value and block ID encoded by encodeValue.
func decodeValue(ctxt *obj.Link, word uint64) (ID, ID) {
	if ctxt.Arch.PtrSize == 8 {
		b, v := ID(word>>32), ID(word)
		//ctxt.Logf("%#x -> b %#x (%d) v %#x (%d)\n", word, b, b, v, v)
		return b, v
	}
	if ctxt.Arch.PtrSize != 4 {
		panic("unexpected pointer size")
	}
	return ID(word >> 16), ID(int16(word))
}

// Append a pointer-sized uint to buf.
func appendPtr(ctxt *obj.Link, buf []byte, word uint64) []byte {
	if cap(buf) < len(buf)+20 {
		b := make([]byte, len(buf), 20+cap(buf)*2)
		copy(b, buf)
		buf = b
	}
	writeAt := len(buf)
	buf = buf[0 : len(buf)+ctxt.Arch.PtrSize]
	writePtr(ctxt, buf[writeAt:], word)
	return buf
}

// Write a pointer-sized uint to the beginning of buf.
func writePtr(ctxt *obj.Link, buf []byte, word uint64) {
	switch ctxt.Arch.PtrSize {
	case 4:
		ctxt.Arch.ByteOrder.PutUint32(buf, uint32(word))
	case 8:
		ctxt.Arch.ByteOrder.PutUint64(buf, word)
	default:
		panic("unexpected pointer size")
	}

}

// Read a pointer-sized uint from the beginning of buf.
func readPtr(ctxt *obj.Link, buf []byte) uint64 {
	switch ctxt.Arch.PtrSize {
	case 4:
		return uint64(ctxt.Arch.ByteOrder.Uint32(buf))
	case 8:
		return ctxt.Arch.ByteOrder.Uint64(buf)
	default:
		panic("unexpected pointer size")
	}

}