Adding upstream version 1.22.1.upstream/1.22.1

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

1 files changed, 1886 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/debug.go b/src/cmd/compile/internal/ssa/debug.go
new file mode 100644
index 0000000..05a7278
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/debug.go
@@ -0,0 +1,1886 @@
+// 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/compile/internal/abi"
+	"cmd/compile/internal/abt"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/types"
+	"cmd/internal/dwarf"
+	"cmd/internal/obj"
+	"cmd/internal/src"
+	"encoding/hex"
+	"fmt"
+	"internal/buildcfg"
+	"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 []*ir.Name
+	// 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
+	// Register-resident output parameters for the function. This is filled in at
+	// SSA generation time.
+	RegOutputParams []*ir.Name
+	// Variable declarations that were removed during optimization
+	OptDcl []*ir.Name
+
+	// Filled in by the user. Translates Block and Value ID to PC.
+	//
+	// NOTE: block is only used if value is BlockStart.ID or BlockEnd.ID.
+	// Otherwise, it is ignored.
+	GetPC func(block, value ID) int64
+}
+
+type BlockDebug struct {
+	// State at the start and end of the block. These are initialized,
+	// and updated from new information that flows on back edges.
+	startState, endState abt.T
+	// Use these to avoid excess work in the merge. If none of the
+	// predecessors has changed since the last check, the old answer is
+	// still good.
+	lastCheckedTime, lastChangedTime int32
+	// Whether the block had any changes to user variables at all.
+	relevant bool
+	// false until the block has been processed at least once. This
+	// affects how the merge is done; the goal is to maximize sharing
+	// and avoid allocation.
+	everProcessed bool
+}
+
+// A liveSlot is a slot that's live in loc at entry/exit of a block.
+type liveSlot struct {
+	VarLoc
+}
+
+func (ls *liveSlot) String() string {
+	return fmt.Sprintf("0x%x.%d.%d", ls.Registers, ls.stackOffsetValue(), int32(ls.StackOffset)&1)
+}
+
+func (ls liveSlot) absent() bool {
+	return ls.Registers == 0 && !ls.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 abt.T) {
+	slots, registers := state.slots, state.registers
+	for i := range slots {
+		slots[i] = VarLoc{}
+	}
+	for i := range registers {
+		registers[i] = registers[i][:0]
+	}
+	for it := live.Iterator(); !it.Done(); {
+		k, d := it.Next()
+		live := d.(*liveSlot)
+		slots[k] = live.VarLoc
+		if live.VarLoc.Registers == 0 {
+			continue
+		}
+
+		mask := uint64(live.VarLoc.Registers)
+		for {
+			if mask == 0 {
+				break
+			}
+			reg := uint8(bits.TrailingZeros64(mask))
+			mask &^= 1 << reg
+
+			registers[reg] = append(registers[reg], SlotID(k))
+		}
+	}
+	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, fmt.Sprintf("@%+d", loc.stackOffsetValue()))
+	}
+
+	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()
+}
+
+func (loc VarLoc) intersect(other VarLoc) VarLoc {
+	if !loc.onStack() || !other.onStack() || loc.StackOffset != other.StackOffset {
+		loc.StackOffset = 0
+	}
+	loc.Registers &= other.Registers
+	return loc
+}
+
+var BlockStart = &Value{
+	ID:  -10000,
+	Op:  OpInvalid,
+	Aux: StringToAux("BlockStart"),
+}
+
+var BlockEnd = &Value{
+	ID:  -20000,
+	Op:  OpInvalid,
+	Aux: StringToAux("BlockEnd"),
+}
+
+var FuncEnd = &Value{
+	ID:  -30000,
+	Op:  OpInvalid,
+	Aux: StringToAux("FuncEnd"),
+}
+
+// RegisterSet is a bitmap of registers, indexed by Register.num.
+type RegisterSet uint64
+
+// logf prints debug-specific logging to stdout (always stdout) if the
+// current function is tagged by GOSSAFUNC (for ssa output directed
+// either to stdout or html).
+func (s *debugState) logf(msg string, args ...interface{}) {
+	if s.f.PrintOrHtmlSSA {
+		fmt.Printf(msg, args...)
+	}
+}
+
+type debugState struct {
+	// See FuncDebug.
+	slots    []LocalSlot
+	vars     []*ir.Name
+	varSlots [][]SlotID
+	lists    [][]byte
+
+	// The user variable that each slot rolls up to, indexed by SlotID.
+	slotVars []VarID
+
+	f             *Func
+	loggingLevel  int
+	convergeCount int // testing; iterate over block debug state this many times
+	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
+	changedVars  *sparseSet
+	changedSlots *sparseSet
+
+	// The pending location list entry for each user variable, indexed by VarID.
+	pendingEntries []pendingEntry
+
+	varParts         map[*ir.Name][]SlotID
+	blockDebug       []BlockDebug
+	pendingSlotLocs  []VarLoc
+	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)]
+	}
+
+	// A relatively small slice, but used many times as the return from processValue.
+	state.changedVars = newSparseSet(numVars)
+	state.changedSlots = newSparseSet(numSlots)
+
+	// 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
+		}
+	}
+}
+
+func (state *debugState) allocBlock(b *Block) *BlockDebug {
+	return &state.blockDebug[b.ID]
+}
+
+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, "")
+}
+
+// slotCanonicalizer is a table used to lookup and canonicalize
+// LocalSlot's in a type insensitive way (e.g. taking into account the
+// base name, offset, and width of the slot, but ignoring the slot
+// type).
+type slotCanonicalizer struct {
+	slmap  map[slotKey]SlKeyIdx
+	slkeys []LocalSlot
+}
+
+func newSlotCanonicalizer() *slotCanonicalizer {
+	return &slotCanonicalizer{
+		slmap:  make(map[slotKey]SlKeyIdx),
+		slkeys: []LocalSlot{LocalSlot{N: nil}},
+	}
+}
+
+type SlKeyIdx uint32
+
+const noSlot = SlKeyIdx(0)
+
+// slotKey is a type-insensitive encapsulation of a LocalSlot; it
+// is used to key a map within slotCanonicalizer.
+type slotKey struct {
+	name        *ir.Name
+	offset      int64
+	width       int64
+	splitOf     SlKeyIdx // idx in slkeys slice in slotCanonicalizer
+	splitOffset int64
+}
+
+// lookup looks up a LocalSlot in the slot canonicalizer "sc", returning
+// a canonical index for the slot, and adding it to the table if need
+// be. Return value is the canonical slot index, and a boolean indicating
+// whether the slot was found in the table already (TRUE => found).
+func (sc *slotCanonicalizer) lookup(ls LocalSlot) (SlKeyIdx, bool) {
+	split := noSlot
+	if ls.SplitOf != nil {
+		split, _ = sc.lookup(*ls.SplitOf)
+	}
+	k := slotKey{
+		name: ls.N, offset: ls.Off, width: ls.Type.Size(),
+		splitOf: split, splitOffset: ls.SplitOffset,
+	}
+	if idx, ok := sc.slmap[k]; ok {
+		return idx, true
+	}
+	rv := SlKeyIdx(len(sc.slkeys))
+	sc.slkeys = append(sc.slkeys, ls)
+	sc.slmap[k] = rv
+	return rv, false
+}
+
+func (sc *slotCanonicalizer) canonSlot(idx SlKeyIdx) LocalSlot {
+	return sc.slkeys[idx]
+}
+
+// PopulateABIInRegArgOps examines the entry block of the function
+// and looks for incoming parameters that have missing or partial
+// OpArg{Int,Float}Reg values, inserting additional values in
+// cases where they are missing. Example:
+//
+//	func foo(s string, used int, notused int) int {
+//	  return len(s) + used
+//	}
+//
+// In the function above, the incoming parameter "used" is fully live,
+// "notused" is not live, and "s" is partially live (only the length
+// field of the string is used). At the point where debug value
+// analysis runs, we might expect to see an entry block with:
+//
+//	b1:
+//	  v4 = ArgIntReg <uintptr> {s+8} [0] : BX
+//	  v5 = ArgIntReg <int> {used} [0] : CX
+//
+// While this is an accurate picture of the live incoming params,
+// we also want to have debug locations for non-live params (or
+// their non-live pieces), e.g. something like
+//
+//	b1:
+//	  v9 = ArgIntReg <*uint8> {s+0} [0] : AX
+//	  v4 = ArgIntReg <uintptr> {s+8} [0] : BX
+//	  v5 = ArgIntReg <int> {used} [0] : CX
+//	  v10 = ArgIntReg <int> {unused} [0] : DI
+//
+// This function examines the live OpArg{Int,Float}Reg values and
+// synthesizes new (dead) values for the non-live params or the
+// non-live pieces of partially live params.
+func PopulateABIInRegArgOps(f *Func) {
+	pri := f.ABISelf.ABIAnalyzeFuncType(f.Type)
+
+	// When manufacturing new slots that correspond to splits of
+	// composite parameters, we want to avoid creating a new sub-slot
+	// that differs from some existing sub-slot only by type, since
+	// the debug location analysis will treat that slot as a separate
+	// entity. To achieve this, create a lookup table of existing
+	// slots that is type-insenstitive.
+	sc := newSlotCanonicalizer()
+	for _, sl := range f.Names {
+		sc.lookup(*sl)
+	}
+
+	// Add slot -> value entry to f.NamedValues if not already present.
+	addToNV := func(v *Value, sl LocalSlot) {
+		values, ok := f.NamedValues[sl]
+		if !ok {
+			// Haven't seen this slot yet.
+			sla := f.localSlotAddr(sl)
+			f.Names = append(f.Names, sla)
+		} else {
+			for _, ev := range values {
+				if v == ev {
+					return
+				}
+			}
+		}
+		values = append(values, v)
+		f.NamedValues[sl] = values
+	}
+
+	newValues := []*Value{}
+
+	abiRegIndexToRegister := func(reg abi.RegIndex) int8 {
+		i := f.ABISelf.FloatIndexFor(reg)
+		if i >= 0 { // float PR
+			return f.Config.floatParamRegs[i]
+		} else {
+			return f.Config.intParamRegs[reg]
+		}
+	}
+
+	// Helper to construct a new OpArg{Float,Int}Reg op value.
+	var pos src.XPos
+	if len(f.Entry.Values) != 0 {
+		pos = f.Entry.Values[0].Pos
+	}
+	synthesizeOpIntFloatArg := func(n *ir.Name, t *types.Type, reg abi.RegIndex, sl LocalSlot) *Value {
+		aux := &AuxNameOffset{n, sl.Off}
+		op, auxInt := ArgOpAndRegisterFor(reg, f.ABISelf)
+		v := f.newValueNoBlock(op, t, pos)
+		v.AuxInt = auxInt
+		v.Aux = aux
+		v.Args = nil
+		v.Block = f.Entry
+		newValues = append(newValues, v)
+		addToNV(v, sl)
+		f.setHome(v, &f.Config.registers[abiRegIndexToRegister(reg)])
+		return v
+	}
+
+	// Make a pass through the entry block looking for
+	// OpArg{Int,Float}Reg ops. Record the slots they use in a table
+	// ("sc"). We use a type-insensitive lookup for the slot table,
+	// since the type we get from the ABI analyzer won't always match
+	// what the compiler uses when creating OpArg{Int,Float}Reg ops.
+	for _, v := range f.Entry.Values {
+		if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
+			aux := v.Aux.(*AuxNameOffset)
+			sl := LocalSlot{N: aux.Name, Type: v.Type, Off: aux.Offset}
+			// install slot in lookup table
+			idx, _ := sc.lookup(sl)
+			// add to f.NamedValues if not already present
+			addToNV(v, sc.canonSlot(idx))
+		} else if v.Op.IsCall() {
+			// if we hit a call, we've gone too far.
+			break
+		}
+	}
+
+	// Now make a pass through the ABI in-params, looking for params
+	// or pieces of params that we didn't encounter in the loop above.
+	for _, inp := range pri.InParams() {
+		if !isNamedRegParam(inp) {
+			continue
+		}
+		n := inp.Name
+
+		// Param is spread across one or more registers. Walk through
+		// each piece to see whether we've seen an arg reg op for it.
+		types, offsets := inp.RegisterTypesAndOffsets()
+		for k, t := range types {
+			// Note: this recipe for creating a LocalSlot is designed
+			// to be compatible with the one used in expand_calls.go
+			// as opposed to decompose.go. The expand calls code just
+			// takes the base name and creates an offset into it,
+			// without using the SplitOf/SplitOffset fields. The code
+			// in decompose.go does the opposite -- it creates a
+			// LocalSlot object with "Off" set to zero, but with
+			// SplitOf pointing to a parent slot, and SplitOffset
+			// holding the offset into the parent object.
+			pieceSlot := LocalSlot{N: n, Type: t, Off: offsets[k]}
+
+			// Look up this piece to see if we've seen a reg op
+			// for it. If not, create one.
+			_, found := sc.lookup(pieceSlot)
+			if !found {
+				// This slot doesn't appear in the map, meaning it
+				// corresponds to an in-param that is not live, or
+				// a portion of an in-param that is not live/used.
+				// Add a new dummy OpArg{Int,Float}Reg for it.
+				synthesizeOpIntFloatArg(n, t, inp.Registers[k],
+					pieceSlot)
+			}
+		}
+	}
+
+	// Insert the new values into the head of the block.
+	f.Entry.Values = append(newValues, f.Entry.Values...)
+}
+
+// BuildFuncDebug debug information for f, placing the results
+// in "rval". 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, loggingLevel int, stackOffset func(LocalSlot) int32, rval *FuncDebug) {
+	if f.RegAlloc == nil {
+		f.Fatalf("BuildFuncDebug on func %v that has not been fully processed", f)
+	}
+	state := &f.Cache.debugState
+	state.loggingLevel = loggingLevel % 1000
+
+	// A specific number demands exactly that many iterations. Under
+	// particular circumstances it make require more than the total of
+	// 2 passes implied by a single run through liveness and a single
+	// run through location list generation.
+	state.convergeCount = loggingLevel / 1000
+	state.f = f
+	state.registers = f.Config.registers
+	state.stackOffset = stackOffset
+	state.ctxt = ctxt
+
+	if buildcfg.Experiment.RegabiArgs {
+		PopulateABIInRegArgOps(f)
+	}
+
+	if state.loggingLevel > 0 {
+		state.logf("Generating location lists for function %q\n", f.Name)
+	}
+
+	if state.varParts == nil {
+		state.varParts = make(map[*ir.Name][]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 ir.IsSynthetic(slot.N) {
+			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 {
+				n := v.Aux.(*ir.Name)
+				if ir.IsSynthetic(n) {
+					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 ir.IsSynthetic(slot.N) {
+			continue
+		}
+		for _, value := range f.NamedValues[*slot] {
+			state.valueNames[value.ID] = append(state.valueNames[value.ID], SlotID(i))
+		}
+	}
+
+	blockLocs := state.liveness()
+	state.buildLocationLists(blockLocs)
+
+	// Populate "rval" with what we've computed.
+	rval.Slots = state.slots
+	rval.VarSlots = state.varSlots
+	rval.Vars = state.vars
+	rval.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())
+	counterTime := int32(1)
+
+	// Reverse postorder: visit a block after as many as possible of its
+	// predecessors have been visited.
+	po := state.f.Postorder()
+	converged := false
+
+	// The iteration rule is that by default, run until converged, but
+	// if a particular iteration count is specified, run that many
+	// iterations, no more, no less.  A count is specified as the
+	// thousands digit of the location lists debug flag,
+	// e.g. -d=locationlists=4000
+	keepGoing := func(k int) bool {
+		if state.convergeCount == 0 {
+			return !converged
+		}
+		return k < state.convergeCount
+	}
+	for k := 0; keepGoing(k); k++ {
+		if state.loggingLevel > 0 {
+			state.logf("Liveness pass %d\n", k)
+		}
+		converged = true
+		for i := len(po) - 1; i >= 0; i-- {
+			b := po[i]
+			locs := blockLocs[b.ID]
+			if locs == nil {
+				locs = state.allocBlock(b)
+				blockLocs[b.ID] = locs
+			}
+
+			// Build the starting state for the block from the final
+			// state of its predecessors.
+			startState, blockChanged := state.mergePredecessors(b, blockLocs, nil, false)
+			locs.lastCheckedTime = counterTime
+			counterTime++
+			if state.loggingLevel > 1 {
+				state.logf("Processing %v, block changed %v, initial state:\n%v", b, blockChanged, state.stateString(state.currentState))
+			}
+
+			if blockChanged {
+				// If the start did not change, then the old endState is good
+				converged = false
+				changed := false
+				state.changedSlots.clear()
+
+				// 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.loggingLevel > 1 {
+								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 && k == 0 {
+						// limit to k == 0 otherwise there are duplicates.
+						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.loggingLevel > 1 {
+					state.logf("Block %v done, locs:\n%v", b, state.stateString(state.currentState))
+				}
+
+				locs.relevant = locs.relevant || changed
+				if !changed {
+					locs.endState = startState
+				} else {
+					for _, id := range state.changedSlots.contents() {
+						slotID := SlotID(id)
+						slotLoc := state.currentState.slots[slotID]
+						if slotLoc.absent() {
+							startState.Delete(int32(slotID))
+							continue
+						}
+						old := startState.Find(int32(slotID)) // do NOT replace existing values
+						if oldLS, ok := old.(*liveSlot); !ok || oldLS.VarLoc != slotLoc {
+							startState.Insert(int32(slotID),
+								&liveSlot{VarLoc: slotLoc})
+						}
+					}
+					locs.endState = startState
+				}
+				locs.lastChangedTime = counterTime
+			}
+			counterTime++
+		}
+	}
+	return blockLocs
+}
+
+// mergePredecessors takes the end state of each of b's predecessors and
+// intersects them to form the starting state for b. It puts that state
+// in blockLocs[b.ID].startState, and fills state.currentState with it.
+// It returns the start state and whether this is changed from the
+// previously approximated value of startState for this block.  After
+// the first call, subsequent calls can only shrink startState.
+//
+// Passing forLocationLists=true enables additional side-effects that
+// are necessary for building location lists but superfluous while still
+// iterating to an answer.
+//
+// If previousBlock is non-nil, it registers changes vs. that block's
+// end state in state.changedVars. Note that previousBlock will often
+// not be a predecessor.
+//
+// Note that mergePredecessors behaves slightly differently between
+// first and subsequent calls for a block.  For the first call, the
+// starting state is approximated by taking the state from the
+// predecessor whose state is smallest, and removing any elements not
+// in all the other predecessors; this makes the smallest number of
+// changes and shares the most state.  On subsequent calls the old
+// value of startState is adjusted with new information; this is judged
+// to do the least amount of extra work.
+//
+// To improve performance, each block's state information is marked with
+// lastChanged and lastChecked "times" so unchanged predecessors can be
+// skipped on after-the-first iterations.  Doing this allows extra
+// iterations by the caller to be almost free.
+//
+// It is important to know that the set representation used for
+// startState, endState, and merges can share data for two sets where
+// one is a small delta from the other.  Doing this does require a
+// little care in how sets are updated, both in mergePredecessors, and
+// using its result.
+func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug, previousBlock *Block, forLocationLists bool) (abt.T, bool) {
+	// Filter out back branches.
+	var predsBuf [10]*Block
+
+	preds := predsBuf[:0]
+	locs := blockLocs[b.ID]
+
+	blockChanged := !locs.everProcessed // the first time it always changes.
+	updating := locs.everProcessed
+
+	// For the first merge, exclude predecessors that have not been seen yet.
+	// I.e., backedges.
+	for _, pred := range b.Preds {
+		if bl := blockLocs[pred.b.ID]; bl != nil && bl.everProcessed {
+			// crucially, a self-edge has bl != nil, but bl.everProcessed is false the first time.
+			preds = append(preds, pred.b)
+		}
+	}
+
+	locs.everProcessed = true
+
+	if state.loggingLevel > 1 {
+		// 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 (changed=%d, checked=%d)\n", preds2, b, locs.lastChangedTime, locs.lastCheckedTime)
+	}
+
+	state.changedVars.clear()
+
+	markChangedVars := func(slots, merged abt.T) {
+		if !forLocationLists {
+			return
+		}
+		// Fill changedVars with those that differ between the previous
+		// block (in the emit order, not necessarily a flow predecessor)
+		// and the start state for this block.
+		for it := slots.Iterator(); !it.Done(); {
+			k, v := it.Next()
+			m := merged.Find(k)
+			if m == nil || v.(*liveSlot).VarLoc != m.(*liveSlot).VarLoc {
+				state.changedVars.add(ID(state.slotVars[k]))
+			}
+		}
+	}
+
+	reset := func(ourStartState abt.T) {
+		if !(forLocationLists || blockChanged) {
+			// there is no change and this is not for location lists, do
+			// not bother to reset currentState because it will not be
+			// examined.
+			return
+		}
+		state.currentState.reset(ourStartState)
+	}
+
+	// Zero predecessors
+	if len(preds) == 0 {
+		if previousBlock != nil {
+			state.f.Fatalf("Function %v, block %s with no predecessors is not first block, has previous %s", state.f, b.String(), previousBlock.String())
+		}
+		// startState is empty
+		reset(abt.T{})
+		return abt.T{}, blockChanged
+	}
+
+	// One predecessor
+	l0 := blockLocs[preds[0].ID]
+	p0 := l0.endState
+	if len(preds) == 1 {
+		if previousBlock != nil && preds[0].ID != previousBlock.ID {
+			// Change from previous block is its endState minus the predecessor's endState
+			markChangedVars(blockLocs[previousBlock.ID].endState, p0)
+		}
+		locs.startState = p0
+		blockChanged = blockChanged || l0.lastChangedTime > locs.lastCheckedTime
+		reset(p0)
+		return p0, blockChanged
+	}
+
+	// More than one predecessor
+
+	if updating {
+		// After the first approximation, i.e., when updating, results
+		// can only get smaller, because initially backedge
+		// predecessors do not participate in the intersection.  This
+		// means that for the update, given the prior approximation of
+		// startState, there is no need to re-intersect with unchanged
+		// blocks.  Therefore remove unchanged blocks from the
+		// predecessor list.
+		for i := len(preds) - 1; i >= 0; i-- {
+			pred := preds[i]
+			if blockLocs[pred.ID].lastChangedTime > locs.lastCheckedTime {
+				continue // keep this predecessor
+			}
+			preds[i] = preds[len(preds)-1]
+			preds = preds[:len(preds)-1]
+			if state.loggingLevel > 2 {
+				state.logf("Pruned b%d, lastChanged was %d but b%d lastChecked is %d\n", pred.ID, blockLocs[pred.ID].lastChangedTime, b.ID, locs.lastCheckedTime)
+			}
+		}
+		// Check for an early out; this should always hit for the update
+		// if there are no cycles.
+		if len(preds) == 0 {
+			blockChanged = false
+
+			reset(locs.startState)
+			if state.loggingLevel > 2 {
+				state.logf("Early out, no predecessors changed since last check\n")
+			}
+			if previousBlock != nil {
+				markChangedVars(blockLocs[previousBlock.ID].endState, locs.startState)
+			}
+			return locs.startState, blockChanged
+		}
+	}
+
+	baseID := preds[0].ID
+	baseState := p0
+
+	// Choose the predecessor with the smallest endState for intersection work
+	for _, pred := range preds[1:] {
+		if blockLocs[pred.ID].endState.Size() < baseState.Size() {
+			baseState = blockLocs[pred.ID].endState
+			baseID = pred.ID
+		}
+	}
+
+	if state.loggingLevel > 2 {
+		state.logf("Starting %v with state from b%v:\n%v", b, baseID, state.blockEndStateString(blockLocs[baseID]))
+		for _, pred := range preds {
+			if pred.ID == baseID {
+				continue
+			}
+			state.logf("Merging in state from %v:\n%v", pred, state.blockEndStateString(blockLocs[pred.ID]))
+		}
+	}
+
+	state.currentState.reset(abt.T{})
+	// The normal logic of "reset" is included in the intersection loop below.
+
+	slotLocs := state.currentState.slots
+
+	// If this is the first call, do updates on the "baseState"; if this
+	// is a subsequent call, tweak the startState instead. Note that
+	// these "set" values are values; there are no side effects to
+	// other values as these are modified.
+	newState := baseState
+	if updating {
+		newState = blockLocs[b.ID].startState
+	}
+
+	for it := newState.Iterator(); !it.Done(); {
+		k, d := it.Next()
+		thisSlot := d.(*liveSlot)
+		x := thisSlot.VarLoc
+		x0 := x // initial value in newState
+
+		// Intersect this slot with the slot in all the predecessors
+		for _, other := range preds {
+			if !updating && other.ID == baseID {
+				continue
+			}
+			otherSlot := blockLocs[other.ID].endState.Find(k)
+			if otherSlot == nil {
+				x = VarLoc{}
+				break
+			}
+			y := otherSlot.(*liveSlot).VarLoc
+			x = x.intersect(y)
+			if x.absent() {
+				x = VarLoc{}
+				break
+			}
+		}
+
+		// Delete if necessary, but not otherwise (in order to maximize sharing).
+		if x.absent() {
+			if !x0.absent() {
+				blockChanged = true
+				newState.Delete(k)
+			}
+			slotLocs[k] = VarLoc{}
+			continue
+		}
+		if x != x0 {
+			blockChanged = true
+			newState.Insert(k, &liveSlot{VarLoc: x})
+		}
+
+		slotLocs[k] = x
+		mask := uint64(x.Registers)
+		for {
+			if mask == 0 {
+				break
+			}
+			reg := uint8(bits.TrailingZeros64(mask))
+			mask &^= 1 << reg
+			state.currentState.registers[reg] = append(state.currentState.registers[reg], SlotID(k))
+		}
+	}
+
+	if previousBlock != nil {
+		markChangedVars(blockLocs[previousBlock.ID].endState, newState)
+	}
+	locs.startState = newState
+	return newState, blockChanged
+}
+
+// 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.changedSlots.add(ID(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.loggingLevel > 1 {
+				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:
+		n := v.Aux.(*ir.Name)
+		if ir.IsSynthetic(n) {
+			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.loggingLevel > 1 {
+			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.loggingLevel > 1 {
+				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.loggingLevel > 1 {
+					state.logf("at %v: unexpected spill of unnamed register %s\n", v, vReg)
+				}
+				break
+			}
+
+			setSlot(slot, VarLoc{last.Registers, StackOffset(stackOffset)})
+			if state.loggingLevel > 1 {
+				state.logf("at %v: %v spilled to stack location %v@%d\n", v, state.slots[slot], home, state.stackOffset(home))
+			}
+		}
+
+	case vReg != nil:
+		if state.loggingLevel > 1 {
+			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.loggingLevel > 1 {
+				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 reports whether a new location description is a superset
+// of the (non-empty) pending location description, if so, the two
+// can be merged (i.e., pending is still a valid and useful location
+// description).
+func canMerge(pending, new VarLoc) bool {
+	if pending.absent() && new.absent() {
+		return true
+	}
+	if pending.absent() || new.absent() {
+		return false
+	}
+	// pending is not absent, therefore it has either a stack mapping,
+	// or registers, or both.
+	if pending.onStack() && pending.StackOffset != new.StackOffset {
+		// if pending has a stack offset, then new must also, and it
+		// must be the same (StackOffset encodes onStack).
+		return false
+	}
+	if pending.Registers&new.Registers != pending.Registers {
+		// There is at least one register in pending not mentioned in new.
+		return false
+	}
+	return true
+}
+
+// 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.
+
+	var prevBlock *Block
+	for _, b := range state.f.Blocks {
+		state.mergePredecessors(b, blockLocs, prevBlock, true)
+
+		// Handle any differences among predecessor blocks and previous block (perhaps not a predecessor)
+		for _, varID := range state.changedVars.contents() {
+			state.updateVar(VarID(varID), b, BlockStart)
+		}
+		state.changedVars.clear()
+
+		if !blockLocs[b.ID].relevant {
+			continue
+		}
+
+		mustBeFirst := func(v *Value) bool {
+			return v.Op == OpPhi || v.Op.isLoweredGetClosurePtr() ||
+				v.Op == OpArgIntReg || v.Op == OpArgFloatReg
+		}
+
+		blockPrologComplete := func(v *Value) bool {
+			if b.ID != state.f.Entry.ID {
+				return !opcodeTable[v.Op].zeroWidth
+			} else {
+				return v.Op == OpInitMem
+			}
+		}
+
+		// Examine the prolog portion of the block to process special
+		// zero-width ops such as Arg, Phi, LoweredGetClosurePtr (etc)
+		// whose lifetimes begin at the block starting point. In an
+		// entry block, allow for the possibility that we may see Arg
+		// ops that appear _after_ other non-zero-width operations.
+		// Example:
+		//
+		//   v33 = ArgIntReg <uintptr> {foo+0} [0] : AX (foo)
+		//   v34 = ArgIntReg <uintptr> {bar+0} [0] : BX (bar)
+		//   ...
+		//   v77 = StoreReg <unsafe.Pointer> v67 : ctx+8[unsafe.Pointer]
+		//   v78 = StoreReg <unsafe.Pointer> v68 : ctx[unsafe.Pointer]
+		//   v79 = Arg <*uint8> {args} : args[*uint8] (args[*uint8])
+		//   v80 = Arg <int> {args} [8] : args+8[int] (args+8[int])
+		//   ...
+		//   v1 = InitMem <mem>
+		//
+		// We can stop scanning the initial portion of the block when
+		// we either see the InitMem op (for entry blocks) or the
+		// first non-zero-width op (for other blocks).
+		for idx := 0; idx < len(b.Values); idx++ {
+			v := b.Values[idx]
+			if blockPrologComplete(v) {
+				break
+			}
+			// Consider only "lifetime begins at block start" ops.
+			if !mustBeFirst(v) && v.Op != OpArg {
+				continue
+			}
+			slots := state.valueNames[v.ID]
+			reg, _ := state.f.getHome(v.ID).(*Register)
+			changed := state.processValue(v, slots, reg) // changed == added to state.changedVars
+			if changed {
+				for _, varID := range state.changedVars.contents() {
+					state.updateVar(VarID(varID), v.Block, BlockStart)
+				}
+				state.changedVars.clear()
+			}
+		}
+
+		// Now examine the block again, handling things other than the
+		// "begins at block start" lifetimes.
+		zeroWidthPending := false
+		prologComplete := false
+		// expect to see values in pattern (apc)* (zerowidth|real)*
+		for _, v := range b.Values {
+			if blockPrologComplete(v) {
+				prologComplete = true
+			}
+			slots := state.valueNames[v.ID]
+			reg, _ := state.f.getHome(v.ID).(*Register)
+			changed := state.processValue(v, slots, reg) // changed == added to state.changedVars
+
+			if opcodeTable[v.Op].zeroWidth {
+				if prologComplete && mustBeFirst(v) {
+					panic(fmt.Errorf("Unexpected placement of op '%s' appearing after non-pseudo-op at beginning of block %s in %s\n%s", v.LongString(), b, b.Func.Name, b.Func))
+				}
+				if changed {
+					if mustBeFirst(v) || v.Op == OpArg {
+						// already taken care of above
+						continue
+					}
+					zeroWidthPending = true
+				}
+				continue
+			}
+			if !changed && !zeroWidthPending {
+				continue
+			}
+
+			// Not zero-width; i.e., a "real" instruction.
+			zeroWidthPending = false
+			for _, varID := range state.changedVars.contents() {
+				state.updateVar(VarID(varID), v.Block, v)
+			}
+			state.changedVars.clear()
+		}
+		for _, varID := range state.changedVars.contents() {
+			state.updateVar(VarID(varID), b, BlockEnd)
+		}
+
+		prevBlock = b
+	}
+
+	if state.loggingLevel > 0 {
+		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), -1, FuncEnd.ID)
+		list := state.lists[varID]
+		if state.loggingLevel > 0 {
+			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, beginning at v in block b.
+// v may be one of the special values indicating block start or end.
+func (state *debugState) updateVar(varID VarID, b *Block, v *Value) {
+	curLoc := state.currentState.slots
+	// 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, b.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, b.ID, v.ID)
+	pending.present = true
+	pending.startBlock = b.ID
+	pending.startValue = v.ID
+	for i, slot := range state.varSlots[varID] {
+		pending.pieces[i] = curLoc[slot]
+	}
+}
+
+// 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
+	}
+	if start == end {
+		if state.loggingLevel > 1 {
+			// Printf not logf so not gated by GOSSAFUNC; this should fire very rarely.
+			// TODO this fires a lot, need to figure out why.
+			state.logf("Skipping empty location list for %v in %s\n", state.vars[varID], state.f.Name)
+		}
+		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.loggingLevel > 1 {
+		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
+
+	if ctxt.UseBASEntries {
+		listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, ^0)
+		listSym.WriteAddr(ctxt, listSym.Size, ctxt.Arch.PtrSize, startPC, 0)
+	}
+
+	// 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
+		}
+
+		if ctxt.UseBASEntries {
+			listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, int64(begin))
+			listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, int64(end))
+		} else {
+			listSym.WriteCURelativeAddr(ctxt, listSym.Size, startPC, int64(begin))
+			listSym.WriteCURelativeAddr(ctxt, listSym.Size, startPC, int64(end))
+		}
+
+		i += 2 * ctxt.Arch.PtrSize
+		datalen := 2 + int(ctxt.Arch.ByteOrder.Uint16(list[i:]))
+		listSym.WriteBytes(ctxt, listSym.Size, list[i:i+datalen]) // copy datalen and location encoding
+		i += datalen
+	}
+
+	// Location list contents, now with real PCs.
+	// 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
+// encoded value and boolean indicating whether the encoding succeeded.
+// For 32-bit architectures the process may fail for very large
+// procedures(the theory being that it's ok to have degraded debug
+// quality in this case).
+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")
+	}
+
+}
+
+// setupLocList creates the initial portion of a location list for a
+// user variable. It emits the encoded start/end of the range and a
+// placeholder for the size. Return value is the new list plus the
+// slot in the list holding the size (to be updated later).
+func setupLocList(ctxt *obj.Link, f *Func, list []byte, st, en ID) ([]byte, int) {
+	start, startOK := encodeValue(ctxt, f.Entry.ID, st)
+	end, endOK := encodeValue(ctxt, f.Entry.ID, en)
+	if !startOK || !endOK {
+		// This could happen if someone writes a function that uses
+		// >65K values on a 32-bit platform. Hopefully a degraded debugging
+		// experience is ok in that case.
+		return nil, 0
+	}
+	list = appendPtr(ctxt, list, start)
+	list = appendPtr(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]
+	return list, sizeIdx
+}
+
+// locatePrologEnd walks the entry block of a function with incoming
+// register arguments and locates the last instruction in the prolog
+// that spills a register arg. It returns the ID of that instruction
+// Example:
+//
+//	b1:
+//	    v3 = ArgIntReg <int> {p1+0} [0] : AX
+//	    ... more arg regs ..
+//	    v4 = ArgFloatReg <float32> {f1+0} [0] : X0
+//	    v52 = MOVQstore <mem> {p1} v2 v3 v1
+//	    ... more stores ...
+//	    v68 = MOVSSstore <mem> {f4} v2 v67 v66
+//	    v38 = MOVQstoreconst <mem> {blob} [val=0,off=0] v2 v32
+//
+// Important: locatePrologEnd is expected to work properly only with
+// optimization turned off (e.g. "-N"). If optimization is enabled
+// we can't be assured of finding all input arguments spilled in the
+// entry block prolog.
+func locatePrologEnd(f *Func) ID {
+
+	// returns true if this instruction looks like it moves an ABI
+	// register to the stack, along with the value being stored.
+	isRegMoveLike := func(v *Value) (bool, ID) {
+		n, ok := v.Aux.(*ir.Name)
+		var r ID
+		if !ok || n.Class != ir.PPARAM {
+			return false, r
+		}
+		regInputs, memInputs, spInputs := 0, 0, 0
+		for _, a := range v.Args {
+			if a.Op == OpArgIntReg || a.Op == OpArgFloatReg {
+				regInputs++
+				r = a.ID
+			} else if a.Type.IsMemory() {
+				memInputs++
+			} else if a.Op == OpSP {
+				spInputs++
+			} else {
+				return false, r
+			}
+		}
+		return v.Type.IsMemory() && memInputs == 1 &&
+			regInputs == 1 && spInputs == 1, r
+	}
+
+	// OpArg*Reg values we've seen so far on our forward walk,
+	// for which we have not yet seen a corresponding spill.
+	regArgs := make([]ID, 0, 32)
+
+	// removeReg tries to remove a value from regArgs, returning true
+	// if found and removed, or false otherwise.
+	removeReg := func(r ID) bool {
+		for i := 0; i < len(regArgs); i++ {
+			if regArgs[i] == r {
+				regArgs = append(regArgs[:i], regArgs[i+1:]...)
+				return true
+			}
+		}
+		return false
+	}
+
+	// Walk forwards through the block. When we see OpArg*Reg, record
+	// the value it produces in the regArgs list. When see a store that uses
+	// the value, remove the entry. When we hit the last store (use)
+	// then we've arrived at the end of the prolog.
+	for k, v := range f.Entry.Values {
+		if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
+			regArgs = append(regArgs, v.ID)
+			continue
+		}
+		if ok, r := isRegMoveLike(v); ok {
+			if removed := removeReg(r); removed {
+				if len(regArgs) == 0 {
+					// Found our last spill; return the value after
+					// it. Note that it is possible that this spill is
+					// the last instruction in the block. If so, then
+					// return the "end of block" sentinel.
+					if k < len(f.Entry.Values)-1 {
+						return f.Entry.Values[k+1].ID
+					}
+					return BlockEnd.ID
+				}
+			}
+		}
+		if v.Op.IsCall() {
+			// if we hit a call, we've gone too far.
+			return v.ID
+		}
+	}
+	// nothing found
+	return ID(-1)
+}
+
+// isNamedRegParam returns true if the param corresponding to "p"
+// is a named, non-blank input parameter assigned to one or more
+// registers.
+func isNamedRegParam(p abi.ABIParamAssignment) bool {
+	if p.Name == nil {
+		return false
+	}
+	n := p.Name
+	if n.Sym() == nil || n.Sym().IsBlank() {
+		return false
+	}
+	if len(p.Registers) == 0 {
+		return false
+	}
+	return true
+}
+
+// BuildFuncDebugNoOptimized populates a FuncDebug object "rval" with
+// entries corresponding to the register-resident input parameters for
+// the function "f"; it is used when we are compiling without
+// optimization but the register ABI is enabled. For each reg param,
+// it constructs a 2-element location list: the first element holds
+// the input register, and the second element holds the stack location
+// of the param (the assumption being that when optimization is off,
+// each input param reg will be spilled in the prolog).
+func BuildFuncDebugNoOptimized(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset func(LocalSlot) int32, rval *FuncDebug) {
+
+	pri := f.ABISelf.ABIAnalyzeFuncType(f.Type)
+
+	// Look to see if we have any named register-promoted parameters.
+	// If there are none, bail early and let the caller sort things
+	// out for the remainder of the params/locals.
+	numRegParams := 0
+	for _, inp := range pri.InParams() {
+		if isNamedRegParam(inp) {
+			numRegParams++
+		}
+	}
+	if numRegParams == 0 {
+		return
+	}
+
+	state := debugState{f: f}
+
+	if loggingEnabled {
+		state.logf("generating -N reg param loc lists for func %q\n", f.Name)
+	}
+
+	// Allocate location lists.
+	rval.LocationLists = make([][]byte, numRegParams)
+
+	// Locate the value corresponding to the last spill of
+	// an input register.
+	afterPrologVal := locatePrologEnd(f)
+
+	// Walk the input params again and process the register-resident elements.
+	pidx := 0
+	for _, inp := range pri.InParams() {
+		if !isNamedRegParam(inp) {
+			// will be sorted out elsewhere
+			continue
+		}
+
+		n := inp.Name
+		sl := LocalSlot{N: n, Type: inp.Type, Off: 0}
+		rval.Vars = append(rval.Vars, n)
+		rval.Slots = append(rval.Slots, sl)
+		slid := len(rval.VarSlots)
+		rval.VarSlots = append(rval.VarSlots, []SlotID{SlotID(slid)})
+
+		if afterPrologVal == ID(-1) {
+			// This can happen for degenerate functions with infinite
+			// loops such as that in issue 45948. In such cases, leave
+			// the var/slot set up for the param, but don't try to
+			// emit a location list.
+			if loggingEnabled {
+				state.logf("locatePrologEnd failed, skipping %v\n", n)
+			}
+			pidx++
+			continue
+		}
+
+		// Param is arriving in one or more registers. We need a 2-element
+		// location expression for it. First entry in location list
+		// will correspond to lifetime in input registers.
+		list, sizeIdx := setupLocList(ctxt, f, rval.LocationLists[pidx],
+			BlockStart.ID, afterPrologVal)
+		if list == nil {
+			pidx++
+			continue
+		}
+		if loggingEnabled {
+			state.logf("param %v:\n  [<entry>, %d]:\n", n, afterPrologVal)
+		}
+		rtypes, _ := inp.RegisterTypesAndOffsets()
+		padding := make([]uint64, 0, 32)
+		padding = inp.ComputePadding(padding)
+		for k, r := range inp.Registers {
+			reg := ObjRegForAbiReg(r, f.Config)
+			dwreg := ctxt.Arch.DWARFRegisters[reg]
+			if dwreg < 32 {
+				list = append(list, dwarf.DW_OP_reg0+byte(dwreg))
+			} else {
+				list = append(list, dwarf.DW_OP_regx)
+				list = dwarf.AppendUleb128(list, uint64(dwreg))
+			}
+			if loggingEnabled {
+				state.logf("    piece %d -> dwreg %d", k, dwreg)
+			}
+			if len(inp.Registers) > 1 {
+				list = append(list, dwarf.DW_OP_piece)
+				ts := rtypes[k].Size()
+				list = dwarf.AppendUleb128(list, uint64(ts))
+				if padding[k] > 0 {
+					if loggingEnabled {
+						state.logf(" [pad %d bytes]", padding[k])
+					}
+					list = append(list, dwarf.DW_OP_piece)
+					list = dwarf.AppendUleb128(list, padding[k])
+				}
+			}
+			if loggingEnabled {
+				state.logf("\n")
+			}
+		}
+		// fill in length of location expression element
+		ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
+
+		// Second entry in the location list will be the stack home
+		// of the param, once it has been spilled.  Emit that now.
+		list, sizeIdx = setupLocList(ctxt, f, list,
+			afterPrologVal, FuncEnd.ID)
+		if list == nil {
+			pidx++
+			continue
+		}
+		soff := stackOffset(sl)
+		if soff == 0 {
+			list = append(list, dwarf.DW_OP_call_frame_cfa)
+		} else {
+			list = append(list, dwarf.DW_OP_fbreg)
+			list = dwarf.AppendSleb128(list, int64(soff))
+		}
+		if loggingEnabled {
+			state.logf("  [%d, <end>): stackOffset=%d\n", afterPrologVal, soff)
+		}
+
+		// fill in size
+		ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
+
+		rval.LocationLists[pidx] = list
+		pidx++
+	}
+}