Source file src/cmd/compile/internal/ssa/debug.go

Documentation: cmd/compile/internal/ssa

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ssa
     6  
     7  import (
     8  	"cmd/compile/internal/abi"
     9  	"cmd/compile/internal/abt"
    10  	"cmd/compile/internal/ir"
    11  	"cmd/compile/internal/types"
    12  	"cmd/internal/dwarf"
    13  	"cmd/internal/obj"
    14  	"cmd/internal/src"
    15  	"encoding/hex"
    16  	"fmt"
    17  	"internal/buildcfg"
    18  	"math/bits"
    19  	"sort"
    20  	"strings"
    21  )
    22  
    23  type SlotID int32
    24  type VarID int32
    25  
    26  // A FuncDebug contains all the debug information for the variables in a
    27  // function. Variables are identified by their LocalSlot, which may be
    28  // the result of decomposing a larger variable.
    29  type FuncDebug struct {
    30  	// Slots is all the slots used in the debug info, indexed by their SlotID.
    31  	Slots []LocalSlot
    32  	// The user variables, indexed by VarID.
    33  	Vars []*ir.Name
    34  	// The slots that make up each variable, indexed by VarID.
    35  	VarSlots [][]SlotID
    36  	// The location list data, indexed by VarID. Must be processed by PutLocationList.
    37  	LocationLists [][]byte
    38  	// Register-resident output parameters for the function. This is filled in at
    39  	// SSA generation time.
    40  	RegOutputParams []*ir.Name
    41  	// Variable declarations that were removed during optimization
    42  	OptDcl []*ir.Name
    43  
    44  	// Filled in by the user. Translates Block and Value ID to PC.
    45  	//
    46  	// NOTE: block is only used if value is BlockStart.ID or BlockEnd.ID.
    47  	// Otherwise, it is ignored.
    48  	GetPC func(block, value ID) int64
    49  }
    50  
    51  type BlockDebug struct {
    52  	// State at the start and end of the block. These are initialized,
    53  	// and updated from new information that flows on back edges.
    54  	startState, endState abt.T
    55  	// Use these to avoid excess work in the merge. If none of the
    56  	// predecessors has changed since the last check, the old answer is
    57  	// still good.
    58  	lastCheckedTime, lastChangedTime int32
    59  	// Whether the block had any changes to user variables at all.
    60  	relevant bool
    61  	// false until the block has been processed at least once. This
    62  	// affects how the merge is done; the goal is to maximize sharing
    63  	// and avoid allocation.
    64  	everProcessed bool
    65  }
    66  
    67  // A liveSlot is a slot that's live in loc at entry/exit of a block.
    68  type liveSlot struct {
    69  	VarLoc
    70  }
    71  
    72  func (ls *liveSlot) String() string {
    73  	return fmt.Sprintf("0x%x.%d.%d", ls.Registers, ls.stackOffsetValue(), int32(ls.StackOffset)&1)
    74  }
    75  
    76  func (ls liveSlot) absent() bool {
    77  	return ls.Registers == 0 && !ls.onStack()
    78  }
    79  
    80  // StackOffset encodes whether a value is on the stack and if so, where.
    81  // It is a 31-bit integer followed by a presence flag at the low-order
    82  // bit.
    83  type StackOffset int32
    84  
    85  func (s StackOffset) onStack() bool {
    86  	return s != 0
    87  }
    88  
    89  func (s StackOffset) stackOffsetValue() int32 {
    90  	return int32(s) >> 1
    91  }
    92  
    93  // stateAtPC is the current state of all variables at some point.
    94  type stateAtPC struct {
    95  	// The location of each known slot, indexed by SlotID.
    96  	slots []VarLoc
    97  	// The slots present in each register, indexed by register number.
    98  	registers [][]SlotID
    99  }
   100  
   101  // reset fills state with the live variables from live.
   102  func (state *stateAtPC) reset(live abt.T) {
   103  	slots, registers := state.slots, state.registers
   104  	for i := range slots {
   105  		slots[i] = VarLoc{}
   106  	}
   107  	for i := range registers {
   108  		registers[i] = registers[i][:0]
   109  	}
   110  	for it := live.Iterator(); !it.Done(); {
   111  		k, d := it.Next()
   112  		live := d.(*liveSlot)
   113  		slots[k] = live.VarLoc
   114  		if live.VarLoc.Registers == 0 {
   115  			continue
   116  		}
   117  
   118  		mask := uint64(live.VarLoc.Registers)
   119  		for {
   120  			if mask == 0 {
   121  				break
   122  			}
   123  			reg := uint8(bits.TrailingZeros64(mask))
   124  			mask &^= 1 << reg
   125  
   126  			registers[reg] = append(registers[reg], SlotID(k))
   127  		}
   128  	}
   129  	state.slots, state.registers = slots, registers
   130  }
   131  
   132  func (s *debugState) LocString(loc VarLoc) string {
   133  	if loc.absent() {
   134  		return "<nil>"
   135  	}
   136  
   137  	var storage []string
   138  	if loc.onStack() {
   139  		storage = append(storage, fmt.Sprintf("@%+d", loc.stackOffsetValue()))
   140  	}
   141  
   142  	mask := uint64(loc.Registers)
   143  	for {
   144  		if mask == 0 {
   145  			break
   146  		}
   147  		reg := uint8(bits.TrailingZeros64(mask))
   148  		mask &^= 1 << reg
   149  
   150  		storage = append(storage, s.registers[reg].String())
   151  	}
   152  	return strings.Join(storage, ",")
   153  }
   154  
   155  // A VarLoc describes the storage for part of a user variable.
   156  type VarLoc struct {
   157  	// The registers this variable is available in. There can be more than
   158  	// one in various situations, e.g. it's being moved between registers.
   159  	Registers RegisterSet
   160  
   161  	StackOffset
   162  }
   163  
   164  func (loc VarLoc) absent() bool {
   165  	return loc.Registers == 0 && !loc.onStack()
   166  }
   167  
   168  func (loc VarLoc) intersect(other VarLoc) VarLoc {
   169  	if !loc.onStack() || !other.onStack() || loc.StackOffset != other.StackOffset {
   170  		loc.StackOffset = 0
   171  	}
   172  	loc.Registers &= other.Registers
   173  	return loc
   174  }
   175  
   176  var BlockStart = &Value{
   177  	ID:  -10000,
   178  	Op:  OpInvalid,
   179  	Aux: StringToAux("BlockStart"),
   180  }
   181  
   182  var BlockEnd = &Value{
   183  	ID:  -20000,
   184  	Op:  OpInvalid,
   185  	Aux: StringToAux("BlockEnd"),
   186  }
   187  
   188  var FuncEnd = &Value{
   189  	ID:  -30000,
   190  	Op:  OpInvalid,
   191  	Aux: StringToAux("FuncEnd"),
   192  }
   193  
   194  // RegisterSet is a bitmap of registers, indexed by Register.num.
   195  type RegisterSet uint64
   196  
   197  // logf prints debug-specific logging to stdout (always stdout) if the
   198  // current function is tagged by GOSSAFUNC (for ssa output directed
   199  // either to stdout or html).
   200  func (s *debugState) logf(msg string, args ...interface{}) {
   201  	if s.f.PrintOrHtmlSSA {
   202  		fmt.Printf(msg, args...)
   203  	}
   204  }
   205  
   206  type debugState struct {
   207  	// See FuncDebug.
   208  	slots    []LocalSlot
   209  	vars     []*ir.Name
   210  	varSlots [][]SlotID
   211  	lists    [][]byte
   212  
   213  	// The user variable that each slot rolls up to, indexed by SlotID.
   214  	slotVars []VarID
   215  
   216  	f             *Func
   217  	loggingLevel  int
   218  	convergeCount int // testing; iterate over block debug state this many times
   219  	registers     []Register
   220  	stackOffset   func(LocalSlot) int32
   221  	ctxt          *obj.Link
   222  
   223  	// The names (slots) associated with each value, indexed by Value ID.
   224  	valueNames [][]SlotID
   225  
   226  	// The current state of whatever analysis is running.
   227  	currentState stateAtPC
   228  	changedVars  *sparseSet
   229  	changedSlots *sparseSet
   230  
   231  	// The pending location list entry for each user variable, indexed by VarID.
   232  	pendingEntries []pendingEntry
   233  
   234  	varParts         map[*ir.Name][]SlotID
   235  	blockDebug       []BlockDebug
   236  	pendingSlotLocs  []VarLoc
   237  	partsByVarOffset sort.Interface
   238  }
   239  
   240  func (state *debugState) initializeCache(f *Func, numVars, numSlots int) {
   241  	// One blockDebug per block. Initialized in allocBlock.
   242  	if cap(state.blockDebug) < f.NumBlocks() {
   243  		state.blockDebug = make([]BlockDebug, f.NumBlocks())
   244  	} else {
   245  		// This local variable, and the ones like it below, enable compiler
   246  		// optimizations. Don't inline them.
   247  		b := state.blockDebug[:f.NumBlocks()]
   248  		for i := range b {
   249  			b[i] = BlockDebug{}
   250  		}
   251  	}
   252  
   253  	// A list of slots per Value. Reuse the previous child slices.
   254  	if cap(state.valueNames) < f.NumValues() {
   255  		old := state.valueNames
   256  		state.valueNames = make([][]SlotID, f.NumValues())
   257  		copy(state.valueNames, old)
   258  	}
   259  	vn := state.valueNames[:f.NumValues()]
   260  	for i := range vn {
   261  		vn[i] = vn[i][:0]
   262  	}
   263  
   264  	// Slot and register contents for currentState. Cleared by reset().
   265  	if cap(state.currentState.slots) < numSlots {
   266  		state.currentState.slots = make([]VarLoc, numSlots)
   267  	} else {
   268  		state.currentState.slots = state.currentState.slots[:numSlots]
   269  	}
   270  	if cap(state.currentState.registers) < len(state.registers) {
   271  		state.currentState.registers = make([][]SlotID, len(state.registers))
   272  	} else {
   273  		state.currentState.registers = state.currentState.registers[:len(state.registers)]
   274  	}
   275  
   276  	// A relatively small slice, but used many times as the return from processValue.
   277  	state.changedVars = newSparseSet(numVars)
   278  	state.changedSlots = newSparseSet(numSlots)
   279  
   280  	// A pending entry per user variable, with space to track each of its pieces.
   281  	numPieces := 0
   282  	for i := range state.varSlots {
   283  		numPieces += len(state.varSlots[i])
   284  	}
   285  	if cap(state.pendingSlotLocs) < numPieces {
   286  		state.pendingSlotLocs = make([]VarLoc, numPieces)
   287  	} else {
   288  		psl := state.pendingSlotLocs[:numPieces]
   289  		for i := range psl {
   290  			psl[i] = VarLoc{}
   291  		}
   292  	}
   293  	if cap(state.pendingEntries) < numVars {
   294  		state.pendingEntries = make([]pendingEntry, numVars)
   295  	}
   296  	pe := state.pendingEntries[:numVars]
   297  	freePieceIdx := 0
   298  	for varID, slots := range state.varSlots {
   299  		pe[varID] = pendingEntry{
   300  			pieces: state.pendingSlotLocs[freePieceIdx : freePieceIdx+len(slots)],
   301  		}
   302  		freePieceIdx += len(slots)
   303  	}
   304  	state.pendingEntries = pe
   305  
   306  	if cap(state.lists) < numVars {
   307  		state.lists = make([][]byte, numVars)
   308  	} else {
   309  		state.lists = state.lists[:numVars]
   310  		for i := range state.lists {
   311  			state.lists[i] = nil
   312  		}
   313  	}
   314  }
   315  
   316  func (state *debugState) allocBlock(b *Block) *BlockDebug {
   317  	return &state.blockDebug[b.ID]
   318  }
   319  
   320  func (s *debugState) blockEndStateString(b *BlockDebug) string {
   321  	endState := stateAtPC{slots: make([]VarLoc, len(s.slots)), registers: make([][]SlotID, len(s.registers))}
   322  	endState.reset(b.endState)
   323  	return s.stateString(endState)
   324  }
   325  
   326  func (s *debugState) stateString(state stateAtPC) string {
   327  	var strs []string
   328  	for slotID, loc := range state.slots {
   329  		if !loc.absent() {
   330  			strs = append(strs, fmt.Sprintf("\t%v = %v\n", s.slots[slotID], s.LocString(loc)))
   331  		}
   332  	}
   333  
   334  	strs = append(strs, "\n")
   335  	for reg, slots := range state.registers {
   336  		if len(slots) != 0 {
   337  			var slotStrs []string
   338  			for _, slot := range slots {
   339  				slotStrs = append(slotStrs, s.slots[slot].String())
   340  			}
   341  			strs = append(strs, fmt.Sprintf("\t%v = %v\n", &s.registers[reg], slotStrs))
   342  		}
   343  	}
   344  
   345  	if len(strs) == 1 {
   346  		return "(no vars)\n"
   347  	}
   348  	return strings.Join(strs, "")
   349  }
   350  
   351  // slotCanonicalizer is a table used to lookup and canonicalize
   352  // LocalSlot's in a type insensitive way (e.g. taking into account the
   353  // base name, offset, and width of the slot, but ignoring the slot
   354  // type).
   355  type slotCanonicalizer struct {
   356  	slmap  map[slotKey]SlKeyIdx
   357  	slkeys []LocalSlot
   358  }
   359  
   360  func newSlotCanonicalizer() *slotCanonicalizer {
   361  	return &slotCanonicalizer{
   362  		slmap:  make(map[slotKey]SlKeyIdx),
   363  		slkeys: []LocalSlot{LocalSlot{N: nil}},
   364  	}
   365  }
   366  
   367  type SlKeyIdx uint32
   368  
   369  const noSlot = SlKeyIdx(0)
   370  
   371  // slotKey is a type-insensitive encapsulation of a LocalSlot; it
   372  // is used to key a map within slotCanonicalizer.
   373  type slotKey struct {
   374  	name        *ir.Name
   375  	offset      int64
   376  	width       int64
   377  	splitOf     SlKeyIdx // idx in slkeys slice in slotCanonicalizer
   378  	splitOffset int64
   379  }
   380  
   381  // lookup looks up a LocalSlot in the slot canonicalizer "sc", returning
   382  // a canonical index for the slot, and adding it to the table if need
   383  // be. Return value is the canonical slot index, and a boolean indicating
   384  // whether the slot was found in the table already (TRUE => found).
   385  func (sc *slotCanonicalizer) lookup(ls LocalSlot) (SlKeyIdx, bool) {
   386  	split := noSlot
   387  	if ls.SplitOf != nil {
   388  		split, _ = sc.lookup(*ls.SplitOf)
   389  	}
   390  	k := slotKey{
   391  		name: ls.N, offset: ls.Off, width: ls.Type.Size(),
   392  		splitOf: split, splitOffset: ls.SplitOffset,
   393  	}
   394  	if idx, ok := sc.slmap[k]; ok {
   395  		return idx, true
   396  	}
   397  	rv := SlKeyIdx(len(sc.slkeys))
   398  	sc.slkeys = append(sc.slkeys, ls)
   399  	sc.slmap[k] = rv
   400  	return rv, false
   401  }
   402  
   403  func (sc *slotCanonicalizer) canonSlot(idx SlKeyIdx) LocalSlot {
   404  	return sc.slkeys[idx]
   405  }
   406  
   407  // PopulateABIInRegArgOps examines the entry block of the function
   408  // and looks for incoming parameters that have missing or partial
   409  // OpArg{Int,Float}Reg values, inserting additional values in
   410  // cases where they are missing. Example:
   411  //
   412  //	func foo(s string, used int, notused int) int {
   413  //	  return len(s) + used
   414  //	}
   415  //
   416  // In the function above, the incoming parameter "used" is fully live,
   417  // "notused" is not live, and "s" is partially live (only the length
   418  // field of the string is used). At the point where debug value
   419  // analysis runs, we might expect to see an entry block with:
   420  //
   421  //	b1:
   422  //	  v4 = ArgIntReg <uintptr> {s+8} [0] : BX
   423  //	  v5 = ArgIntReg <int> {used} [0] : CX
   424  //
   425  // While this is an accurate picture of the live incoming params,
   426  // we also want to have debug locations for non-live params (or
   427  // their non-live pieces), e.g. something like
   428  //
   429  //	b1:
   430  //	  v9 = ArgIntReg <*uint8> {s+0} [0] : AX
   431  //	  v4 = ArgIntReg <uintptr> {s+8} [0] : BX
   432  //	  v5 = ArgIntReg <int> {used} [0] : CX
   433  //	  v10 = ArgIntReg <int> {unused} [0] : DI
   434  //
   435  // This function examines the live OpArg{Int,Float}Reg values and
   436  // synthesizes new (dead) values for the non-live params or the
   437  // non-live pieces of partially live params.
   438  func PopulateABIInRegArgOps(f *Func) {
   439  	pri := f.ABISelf.ABIAnalyzeFuncType(f.Type)
   440  
   441  	// When manufacturing new slots that correspond to splits of
   442  	// composite parameters, we want to avoid creating a new sub-slot
   443  	// that differs from some existing sub-slot only by type, since
   444  	// the debug location analysis will treat that slot as a separate
   445  	// entity. To achieve this, create a lookup table of existing
   446  	// slots that is type-insenstitive.
   447  	sc := newSlotCanonicalizer()
   448  	for _, sl := range f.Names {
   449  		sc.lookup(*sl)
   450  	}
   451  
   452  	// Add slot -> value entry to f.NamedValues if not already present.
   453  	addToNV := func(v *Value, sl LocalSlot) {
   454  		values, ok := f.NamedValues[sl]
   455  		if !ok {
   456  			// Haven't seen this slot yet.
   457  			sla := f.localSlotAddr(sl)
   458  			f.Names = append(f.Names, sla)
   459  		} else {
   460  			for _, ev := range values {
   461  				if v == ev {
   462  					return
   463  				}
   464  			}
   465  		}
   466  		values = append(values, v)
   467  		f.NamedValues[sl] = values
   468  	}
   469  
   470  	newValues := []*Value{}
   471  
   472  	abiRegIndexToRegister := func(reg abi.RegIndex) int8 {
   473  		i := f.ABISelf.FloatIndexFor(reg)
   474  		if i >= 0 { // float PR
   475  			return f.Config.floatParamRegs[i]
   476  		} else {
   477  			return f.Config.intParamRegs[reg]
   478  		}
   479  	}
   480  
   481  	// Helper to construct a new OpArg{Float,Int}Reg op value.
   482  	var pos src.XPos
   483  	if len(f.Entry.Values) != 0 {
   484  		pos = f.Entry.Values[0].Pos
   485  	}
   486  	synthesizeOpIntFloatArg := func(n *ir.Name, t *types.Type, reg abi.RegIndex, sl LocalSlot) *Value {
   487  		aux := &AuxNameOffset{n, sl.Off}
   488  		op, auxInt := ArgOpAndRegisterFor(reg, f.ABISelf)
   489  		v := f.newValueNoBlock(op, t, pos)
   490  		v.AuxInt = auxInt
   491  		v.Aux = aux
   492  		v.Args = nil
   493  		v.Block = f.Entry
   494  		newValues = append(newValues, v)
   495  		addToNV(v, sl)
   496  		f.setHome(v, &f.Config.registers[abiRegIndexToRegister(reg)])
   497  		return v
   498  	}
   499  
   500  	// Make a pass through the entry block looking for
   501  	// OpArg{Int,Float}Reg ops. Record the slots they use in a table
   502  	// ("sc"). We use a type-insensitive lookup for the slot table,
   503  	// since the type we get from the ABI analyzer won't always match
   504  	// what the compiler uses when creating OpArg{Int,Float}Reg ops.
   505  	for _, v := range f.Entry.Values {
   506  		if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
   507  			aux := v.Aux.(*AuxNameOffset)
   508  			sl := LocalSlot{N: aux.Name, Type: v.Type, Off: aux.Offset}
   509  			// install slot in lookup table
   510  			idx, _ := sc.lookup(sl)
   511  			// add to f.NamedValues if not already present
   512  			addToNV(v, sc.canonSlot(idx))
   513  		} else if v.Op.IsCall() {
   514  			// if we hit a call, we've gone too far.
   515  			break
   516  		}
   517  	}
   518  
   519  	// Now make a pass through the ABI in-params, looking for params
   520  	// or pieces of params that we didn't encounter in the loop above.
   521  	for _, inp := range pri.InParams() {
   522  		if !isNamedRegParam(inp) {
   523  			continue
   524  		}
   525  		n := inp.Name
   526  
   527  		// Param is spread across one or more registers. Walk through
   528  		// each piece to see whether we've seen an arg reg op for it.
   529  		types, offsets := inp.RegisterTypesAndOffsets()
   530  		for k, t := range types {
   531  			// Note: this recipe for creating a LocalSlot is designed
   532  			// to be compatible with the one used in expand_calls.go
   533  			// as opposed to decompose.go. The expand calls code just
   534  			// takes the base name and creates an offset into it,
   535  			// without using the SplitOf/SplitOffset fields. The code
   536  			// in decompose.go does the opposite -- it creates a
   537  			// LocalSlot object with "Off" set to zero, but with
   538  			// SplitOf pointing to a parent slot, and SplitOffset
   539  			// holding the offset into the parent object.
   540  			pieceSlot := LocalSlot{N: n, Type: t, Off: offsets[k]}
   541  
   542  			// Look up this piece to see if we've seen a reg op
   543  			// for it. If not, create one.
   544  			_, found := sc.lookup(pieceSlot)
   545  			if !found {
   546  				// This slot doesn't appear in the map, meaning it
   547  				// corresponds to an in-param that is not live, or
   548  				// a portion of an in-param that is not live/used.
   549  				// Add a new dummy OpArg{Int,Float}Reg for it.
   550  				synthesizeOpIntFloatArg(n, t, inp.Registers[k],
   551  					pieceSlot)
   552  			}
   553  		}
   554  	}
   555  
   556  	// Insert the new values into the head of the block.
   557  	f.Entry.Values = append(newValues, f.Entry.Values...)
   558  }
   559  
   560  // BuildFuncDebug debug information for f, placing the results
   561  // in "rval". f must be fully processed, so that each Value is where it
   562  // will be when machine code is emitted.
   563  func BuildFuncDebug(ctxt *obj.Link, f *Func, loggingLevel int, stackOffset func(LocalSlot) int32, rval *FuncDebug) {
   564  	if f.RegAlloc == nil {
   565  		f.Fatalf("BuildFuncDebug on func %v that has not been fully processed", f)
   566  	}
   567  	state := &f.Cache.debugState
   568  	state.loggingLevel = loggingLevel % 1000
   569  
   570  	// A specific number demands exactly that many iterations. Under
   571  	// particular circumstances it make require more than the total of
   572  	// 2 passes implied by a single run through liveness and a single
   573  	// run through location list generation.
   574  	state.convergeCount = loggingLevel / 1000
   575  	state.f = f
   576  	state.registers = f.Config.registers
   577  	state.stackOffset = stackOffset
   578  	state.ctxt = ctxt
   579  
   580  	if buildcfg.Experiment.RegabiArgs {
   581  		PopulateABIInRegArgOps(f)
   582  	}
   583  
   584  	if state.loggingLevel > 0 {
   585  		state.logf("Generating location lists for function %q\n", f.Name)
   586  	}
   587  
   588  	if state.varParts == nil {
   589  		state.varParts = make(map[*ir.Name][]SlotID)
   590  	} else {
   591  		for n := range state.varParts {
   592  			delete(state.varParts, n)
   593  		}
   594  	}
   595  
   596  	// Recompose any decomposed variables, and establish the canonical
   597  	// IDs for each var and slot by filling out state.vars and state.slots.
   598  
   599  	state.slots = state.slots[:0]
   600  	state.vars = state.vars[:0]
   601  	for i, slot := range f.Names {
   602  		state.slots = append(state.slots, *slot)
   603  		if ir.IsSynthetic(slot.N) {
   604  			continue
   605  		}
   606  
   607  		topSlot := slot
   608  		for topSlot.SplitOf != nil {
   609  			topSlot = topSlot.SplitOf
   610  		}
   611  		if _, ok := state.varParts[topSlot.N]; !ok {
   612  			state.vars = append(state.vars, topSlot.N)
   613  		}
   614  		state.varParts[topSlot.N] = append(state.varParts[topSlot.N], SlotID(i))
   615  	}
   616  
   617  	// Recreate the LocalSlot for each stack-only variable.
   618  	// This would probably be better as an output from stackframe.
   619  	for _, b := range f.Blocks {
   620  		for _, v := range b.Values {
   621  			if v.Op == OpVarDef {
   622  				n := v.Aux.(*ir.Name)
   623  				if ir.IsSynthetic(n) {
   624  					continue
   625  				}
   626  
   627  				if _, ok := state.varParts[n]; !ok {
   628  					slot := LocalSlot{N: n, Type: v.Type, Off: 0}
   629  					state.slots = append(state.slots, slot)
   630  					state.varParts[n] = []SlotID{SlotID(len(state.slots) - 1)}
   631  					state.vars = append(state.vars, n)
   632  				}
   633  			}
   634  		}
   635  	}
   636  
   637  	// Fill in the var<->slot mappings.
   638  	if cap(state.varSlots) < len(state.vars) {
   639  		state.varSlots = make([][]SlotID, len(state.vars))
   640  	} else {
   641  		state.varSlots = state.varSlots[:len(state.vars)]
   642  		for i := range state.varSlots {
   643  			state.varSlots[i] = state.varSlots[i][:0]
   644  		}
   645  	}
   646  	if cap(state.slotVars) < len(state.slots) {
   647  		state.slotVars = make([]VarID, len(state.slots))
   648  	} else {
   649  		state.slotVars = state.slotVars[:len(state.slots)]
   650  	}
   651  
   652  	if state.partsByVarOffset == nil {
   653  		state.partsByVarOffset = &partsByVarOffset{}
   654  	}
   655  	for varID, n := range state.vars {
   656  		parts := state.varParts[n]
   657  		state.varSlots[varID] = parts
   658  		for _, slotID := range parts {
   659  			state.slotVars[slotID] = VarID(varID)
   660  		}
   661  		*state.partsByVarOffset.(*partsByVarOffset) = partsByVarOffset{parts, state.slots}
   662  		sort.Sort(state.partsByVarOffset)
   663  	}
   664  
   665  	state.initializeCache(f, len(state.varParts), len(state.slots))
   666  
   667  	for i, slot := range f.Names {
   668  		if ir.IsSynthetic(slot.N) {
   669  			continue
   670  		}
   671  		for _, value := range f.NamedValues[*slot] {
   672  			state.valueNames[value.ID] = append(state.valueNames[value.ID], SlotID(i))
   673  		}
   674  	}
   675  
   676  	blockLocs := state.liveness()
   677  	state.buildLocationLists(blockLocs)
   678  
   679  	// Populate "rval" with what we've computed.
   680  	rval.Slots = state.slots
   681  	rval.VarSlots = state.varSlots
   682  	rval.Vars = state.vars
   683  	rval.LocationLists = state.lists
   684  }
   685  
   686  // liveness walks the function in control flow order, calculating the start
   687  // and end state of each block.
   688  func (state *debugState) liveness() []*BlockDebug {
   689  	blockLocs := make([]*BlockDebug, state.f.NumBlocks())
   690  	counterTime := int32(1)
   691  
   692  	// Reverse postorder: visit a block after as many as possible of its
   693  	// predecessors have been visited.
   694  	po := state.f.Postorder()
   695  	converged := false
   696  
   697  	// The iteration rule is that by default, run until converged, but
   698  	// if a particular iteration count is specified, run that many
   699  	// iterations, no more, no less.  A count is specified as the
   700  	// thousands digit of the location lists debug flag,
   701  	// e.g. -d=locationlists=4000
   702  	keepGoing := func(k int) bool {
   703  		if state.convergeCount == 0 {
   704  			return !converged
   705  		}
   706  		return k < state.convergeCount
   707  	}
   708  	for k := 0; keepGoing(k); k++ {
   709  		if state.loggingLevel > 0 {
   710  			state.logf("Liveness pass %d\n", k)
   711  		}
   712  		converged = true
   713  		for i := len(po) - 1; i >= 0; i-- {
   714  			b := po[i]
   715  			locs := blockLocs[b.ID]
   716  			if locs == nil {
   717  				locs = state.allocBlock(b)
   718  				blockLocs[b.ID] = locs
   719  			}
   720  
   721  			// Build the starting state for the block from the final
   722  			// state of its predecessors.
   723  			startState, blockChanged := state.mergePredecessors(b, blockLocs, nil, false)
   724  			locs.lastCheckedTime = counterTime
   725  			counterTime++
   726  			if state.loggingLevel > 1 {
   727  				state.logf("Processing %v, block changed %v, initial state:\n%v", b, blockChanged, state.stateString(state.currentState))
   728  			}
   729  
   730  			if blockChanged {
   731  				// If the start did not change, then the old endState is good
   732  				converged = false
   733  				changed := false
   734  				state.changedSlots.clear()
   735  
   736  				// Update locs/registers with the effects of each Value.
   737  				for _, v := range b.Values {
   738  					slots := state.valueNames[v.ID]
   739  
   740  					// Loads and stores inherit the names of their sources.
   741  					var source *Value
   742  					switch v.Op {
   743  					case OpStoreReg:
   744  						source = v.Args[0]
   745  					case OpLoadReg:
   746  						switch a := v.Args[0]; a.Op {
   747  						case OpArg, OpPhi:
   748  							source = a
   749  						case OpStoreReg:
   750  							source = a.Args[0]
   751  						default:
   752  							if state.loggingLevel > 1 {
   753  								state.logf("at %v: load with unexpected source op: %v (%v)\n", v, a.Op, a)
   754  							}
   755  						}
   756  					}
   757  					// Update valueNames with the source so that later steps
   758  					// don't need special handling.
   759  					if source != nil && k == 0 {
   760  						// limit to k == 0 otherwise there are duplicates.
   761  						slots = append(slots, state.valueNames[source.ID]...)
   762  						state.valueNames[v.ID] = slots
   763  					}
   764  
   765  					reg, _ := state.f.getHome(v.ID).(*Register)
   766  					c := state.processValue(v, slots, reg)
   767  					changed = changed || c
   768  				}
   769  
   770  				if state.loggingLevel > 1 {
   771  					state.logf("Block %v done, locs:\n%v", b, state.stateString(state.currentState))
   772  				}
   773  
   774  				locs.relevant = locs.relevant || changed
   775  				if !changed {
   776  					locs.endState = startState
   777  				} else {
   778  					for _, id := range state.changedSlots.contents() {
   779  						slotID := SlotID(id)
   780  						slotLoc := state.currentState.slots[slotID]
   781  						if slotLoc.absent() {
   782  							startState.Delete(int32(slotID))
   783  							continue
   784  						}
   785  						old := startState.Find(int32(slotID)) // do NOT replace existing values
   786  						if oldLS, ok := old.(*liveSlot); !ok || oldLS.VarLoc != slotLoc {
   787  							startState.Insert(int32(slotID),
   788  								&liveSlot{VarLoc: slotLoc})
   789  						}
   790  					}
   791  					locs.endState = startState
   792  				}
   793  				locs.lastChangedTime = counterTime
   794  			}
   795  			counterTime++
   796  		}
   797  	}
   798  	return blockLocs
   799  }
   800  
   801  // mergePredecessors takes the end state of each of b's predecessors and
   802  // intersects them to form the starting state for b. It puts that state
   803  // in blockLocs[b.ID].startState, and fills state.currentState with it.
   804  // It returns the start state and whether this is changed from the
   805  // previously approximated value of startState for this block.  After
   806  // the first call, subsequent calls can only shrink startState.
   807  //
   808  // Passing forLocationLists=true enables additional side-effects that
   809  // are necessary for building location lists but superfluous while still
   810  // iterating to an answer.
   811  //
   812  // If previousBlock is non-nil, it registers changes vs. that block's
   813  // end state in state.changedVars. Note that previousBlock will often
   814  // not be a predecessor.
   815  //
   816  // Note that mergePredecessors behaves slightly differently between
   817  // first and subsequent calls for a block.  For the first call, the
   818  // starting state is approximated by taking the state from the
   819  // predecessor whose state is smallest, and removing any elements not
   820  // in all the other predecessors; this makes the smallest number of
   821  // changes and shares the most state.  On subsequent calls the old
   822  // value of startState is adjusted with new information; this is judged
   823  // to do the least amount of extra work.
   824  //
   825  // To improve performance, each block's state information is marked with
   826  // lastChanged and lastChecked "times" so unchanged predecessors can be
   827  // skipped on after-the-first iterations.  Doing this allows extra
   828  // iterations by the caller to be almost free.
   829  //
   830  // It is important to know that the set representation used for
   831  // startState, endState, and merges can share data for two sets where
   832  // one is a small delta from the other.  Doing this does require a
   833  // little care in how sets are updated, both in mergePredecessors, and
   834  // using its result.
   835  func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug, previousBlock *Block, forLocationLists bool) (abt.T, bool) {
   836  	// Filter out back branches.
   837  	var predsBuf [10]*Block
   838  
   839  	preds := predsBuf[:0]
   840  	locs := blockLocs[b.ID]
   841  
   842  	blockChanged := !locs.everProcessed // the first time it always changes.
   843  	updating := locs.everProcessed
   844  
   845  	// For the first merge, exclude predecessors that have not been seen yet.
   846  	// I.e., backedges.
   847  	for _, pred := range b.Preds {
   848  		if bl := blockLocs[pred.b.ID]; bl != nil && bl.everProcessed {
   849  			// crucially, a self-edge has bl != nil, but bl.everProcessed is false the first time.
   850  			preds = append(preds, pred.b)
   851  		}
   852  	}
   853  
   854  	locs.everProcessed = true
   855  
   856  	if state.loggingLevel > 1 {
   857  		// The logf below would cause preds to be heap-allocated if
   858  		// it were passed directly.
   859  		preds2 := make([]*Block, len(preds))
   860  		copy(preds2, preds)
   861  		state.logf("Merging %v into %v (changed=%d, checked=%d)\n", preds2, b, locs.lastChangedTime, locs.lastCheckedTime)
   862  	}
   863  
   864  	state.changedVars.clear()
   865  
   866  	markChangedVars := func(slots, merged abt.T) {
   867  		if !forLocationLists {
   868  			return
   869  		}
   870  		// Fill changedVars with those that differ between the previous
   871  		// block (in the emit order, not necessarily a flow predecessor)
   872  		// and the start state for this block.
   873  		for it := slots.Iterator(); !it.Done(); {
   874  			k, v := it.Next()
   875  			m := merged.Find(k)
   876  			if m == nil || v.(*liveSlot).VarLoc != m.(*liveSlot).VarLoc {
   877  				state.changedVars.add(ID(state.slotVars[k]))
   878  			}
   879  		}
   880  	}
   881  
   882  	reset := func(ourStartState abt.T) {
   883  		if !(forLocationLists || blockChanged) {
   884  			// there is no change and this is not for location lists, do
   885  			// not bother to reset currentState because it will not be
   886  			// examined.
   887  			return
   888  		}
   889  		state.currentState.reset(ourStartState)
   890  	}
   891  
   892  	// Zero predecessors
   893  	if len(preds) == 0 {
   894  		if previousBlock != nil {
   895  			state.f.Fatalf("Function %v, block %s with no predecessors is not first block, has previous %s", state.f, b.String(), previousBlock.String())
   896  		}
   897  		// startState is empty
   898  		reset(abt.T{})
   899  		return abt.T{}, blockChanged
   900  	}
   901  
   902  	// One predecessor
   903  	l0 := blockLocs[preds[0].ID]
   904  	p0 := l0.endState
   905  	if len(preds) == 1 {
   906  		if previousBlock != nil && preds[0].ID != previousBlock.ID {
   907  			// Change from previous block is its endState minus the predecessor's endState
   908  			markChangedVars(blockLocs[previousBlock.ID].endState, p0)
   909  		}
   910  		locs.startState = p0
   911  		blockChanged = blockChanged || l0.lastChangedTime > locs.lastCheckedTime
   912  		reset(p0)
   913  		return p0, blockChanged
   914  	}
   915  
   916  	// More than one predecessor
   917  
   918  	if updating {
   919  		// After the first approximation, i.e., when updating, results
   920  		// can only get smaller, because initially backedge
   921  		// predecessors do not participate in the intersection.  This
   922  		// means that for the update, given the prior approximation of
   923  		// startState, there is no need to re-intersect with unchanged
   924  		// blocks.  Therefore remove unchanged blocks from the
   925  		// predecessor list.
   926  		for i := len(preds) - 1; i >= 0; i-- {
   927  			pred := preds[i]
   928  			if blockLocs[pred.ID].lastChangedTime > locs.lastCheckedTime {
   929  				continue // keep this predecessor
   930  			}
   931  			preds[i] = preds[len(preds)-1]
   932  			preds = preds[:len(preds)-1]
   933  			if state.loggingLevel > 2 {
   934  				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)
   935  			}
   936  		}
   937  		// Check for an early out; this should always hit for the update
   938  		// if there are no cycles.
   939  		if len(preds) == 0 {
   940  			blockChanged = false
   941  
   942  			reset(locs.startState)
   943  			if state.loggingLevel > 2 {
   944  				state.logf("Early out, no predecessors changed since last check\n")
   945  			}
   946  			if previousBlock != nil {
   947  				markChangedVars(blockLocs[previousBlock.ID].endState, locs.startState)
   948  			}
   949  			return locs.startState, blockChanged
   950  		}
   951  	}
   952  
   953  	baseID := preds[0].ID
   954  	baseState := p0
   955  
   956  	// Choose the predecessor with the smallest endState for intersection work
   957  	for _, pred := range preds[1:] {
   958  		if blockLocs[pred.ID].endState.Size() < baseState.Size() {
   959  			baseState = blockLocs[pred.ID].endState
   960  			baseID = pred.ID
   961  		}
   962  	}
   963  
   964  	if state.loggingLevel > 2 {
   965  		state.logf("Starting %v with state from b%v:\n%v", b, baseID, state.blockEndStateString(blockLocs[baseID]))
   966  		for _, pred := range preds {
   967  			if pred.ID == baseID {
   968  				continue
   969  			}
   970  			state.logf("Merging in state from %v:\n%v", pred, state.blockEndStateString(blockLocs[pred.ID]))
   971  		}
   972  	}
   973  
   974  	state.currentState.reset(abt.T{})
   975  	// The normal logic of "reset" is included in the intersection loop below.
   976  
   977  	slotLocs := state.currentState.slots
   978  
   979  	// If this is the first call, do updates on the "baseState"; if this
   980  	// is a subsequent call, tweak the startState instead. Note that
   981  	// these "set" values are values; there are no side effects to
   982  	// other values as these are modified.
   983  	newState := baseState
   984  	if updating {
   985  		newState = blockLocs[b.ID].startState
   986  	}
   987  
   988  	for it := newState.Iterator(); !it.Done(); {
   989  		k, d := it.Next()
   990  		thisSlot := d.(*liveSlot)
   991  		x := thisSlot.VarLoc
   992  		x0 := x // initial value in newState
   993  
   994  		// Intersect this slot with the slot in all the predecessors
   995  		for _, other := range preds {
   996  			if !updating && other.ID == baseID {
   997  				continue
   998  			}
   999  			otherSlot := blockLocs[other.ID].endState.Find(k)
  1000  			if otherSlot == nil {
  1001  				x = VarLoc{}
  1002  				break
  1003  			}
  1004  			y := otherSlot.(*liveSlot).VarLoc
  1005  			x = x.intersect(y)
  1006  			if x.absent() {
  1007  				x = VarLoc{}
  1008  				break
  1009  			}
  1010  		}
  1011  
  1012  		// Delete if necessary, but not otherwise (in order to maximize sharing).
  1013  		if x.absent() {
  1014  			if !x0.absent() {
  1015  				blockChanged = true
  1016  				newState.Delete(k)
  1017  			}
  1018  			slotLocs[k] = VarLoc{}
  1019  			continue
  1020  		}
  1021  		if x != x0 {
  1022  			blockChanged = true
  1023  			newState.Insert(k, &liveSlot{VarLoc: x})
  1024  		}
  1025  
  1026  		slotLocs[k] = x
  1027  		mask := uint64(x.Registers)
  1028  		for {
  1029  			if mask == 0 {
  1030  				break
  1031  			}
  1032  			reg := uint8(bits.TrailingZeros64(mask))
  1033  			mask &^= 1 << reg
  1034  			state.currentState.registers[reg] = append(state.currentState.registers[reg], SlotID(k))
  1035  		}
  1036  	}
  1037  
  1038  	if previousBlock != nil {
  1039  		markChangedVars(blockLocs[previousBlock.ID].endState, newState)
  1040  	}
  1041  	locs.startState = newState
  1042  	return newState, blockChanged
  1043  }
  1044  
  1045  // processValue updates locs and state.registerContents to reflect v, a
  1046  // value with the names in vSlots and homed in vReg.  "v" becomes
  1047  // visible after execution of the instructions evaluating it. It
  1048  // returns which VarIDs were modified by the Value's execution.
  1049  func (state *debugState) processValue(v *Value, vSlots []SlotID, vReg *Register) bool {
  1050  	locs := state.currentState
  1051  	changed := false
  1052  	setSlot := func(slot SlotID, loc VarLoc) {
  1053  		changed = true
  1054  		state.changedVars.add(ID(state.slotVars[slot]))
  1055  		state.changedSlots.add(ID(slot))
  1056  		state.currentState.slots[slot] = loc
  1057  	}
  1058  
  1059  	// Handle any register clobbering. Call operations, for example,
  1060  	// clobber all registers even though they don't explicitly write to
  1061  	// them.
  1062  	clobbers := uint64(opcodeTable[v.Op].reg.clobbers)
  1063  	for {
  1064  		if clobbers == 0 {
  1065  			break
  1066  		}
  1067  		reg := uint8(bits.TrailingZeros64(clobbers))
  1068  		clobbers &^= 1 << reg
  1069  
  1070  		for _, slot := range locs.registers[reg] {
  1071  			if state.loggingLevel > 1 {
  1072  				state.logf("at %v: %v clobbered out of %v\n", v, state.slots[slot], &state.registers[reg])
  1073  			}
  1074  
  1075  			last := locs.slots[slot]
  1076  			if last.absent() {
  1077  				state.f.Fatalf("at %v: slot %v in register %v with no location entry", v, state.slots[slot], &state.registers[reg])
  1078  				continue
  1079  			}
  1080  			regs := last.Registers &^ (1 << reg)
  1081  			setSlot(slot, VarLoc{regs, last.StackOffset})
  1082  		}
  1083  
  1084  		locs.registers[reg] = locs.registers[reg][:0]
  1085  	}
  1086  
  1087  	switch {
  1088  	case v.Op == OpVarDef:
  1089  		n := v.Aux.(*ir.Name)
  1090  		if ir.IsSynthetic(n) {
  1091  			break
  1092  		}
  1093  
  1094  		slotID := state.varParts[n][0]
  1095  		var stackOffset StackOffset
  1096  		if v.Op == OpVarDef {
  1097  			stackOffset = StackOffset(state.stackOffset(state.slots[slotID])<<1 | 1)
  1098  		}
  1099  		setSlot(slotID, VarLoc{0, stackOffset})
  1100  		if state.loggingLevel > 1 {
  1101  			if v.Op == OpVarDef {
  1102  				state.logf("at %v: stack-only var %v now live\n", v, state.slots[slotID])
  1103  			} else {
  1104  				state.logf("at %v: stack-only var %v now dead\n", v, state.slots[slotID])
  1105  			}
  1106  		}
  1107  
  1108  	case v.Op == OpArg:
  1109  		home := state.f.getHome(v.ID).(LocalSlot)
  1110  		stackOffset := state.stackOffset(home)<<1 | 1
  1111  		for _, slot := range vSlots {
  1112  			if state.loggingLevel > 1 {
  1113  				state.logf("at %v: arg %v now on stack in location %v\n", v, state.slots[slot], home)
  1114  				if last := locs.slots[slot]; !last.absent() {
  1115  					state.logf("at %v: unexpected arg op on already-live slot %v\n", v, state.slots[slot])
  1116  				}
  1117  			}
  1118  
  1119  			setSlot(slot, VarLoc{0, StackOffset(stackOffset)})
  1120  		}
  1121  
  1122  	case v.Op == OpStoreReg:
  1123  		home := state.f.getHome(v.ID).(LocalSlot)
  1124  		stackOffset := state.stackOffset(home)<<1 | 1
  1125  		for _, slot := range vSlots {
  1126  			last := locs.slots[slot]
  1127  			if last.absent() {
  1128  				if state.loggingLevel > 1 {
  1129  					state.logf("at %v: unexpected spill of unnamed register %s\n", v, vReg)
  1130  				}
  1131  				break
  1132  			}
  1133  
  1134  			setSlot(slot, VarLoc{last.Registers, StackOffset(stackOffset)})
  1135  			if state.loggingLevel > 1 {
  1136  				state.logf("at %v: %v spilled to stack location %v@%d\n", v, state.slots[slot], home, state.stackOffset(home))
  1137  			}
  1138  		}
  1139  
  1140  	case vReg != nil:
  1141  		if state.loggingLevel > 1 {
  1142  			newSlots := make([]bool, len(state.slots))
  1143  			for _, slot := range vSlots {
  1144  				newSlots[slot] = true
  1145  			}
  1146  
  1147  			for _, slot := range locs.registers[vReg.num] {
  1148  				if !newSlots[slot] {
  1149  					state.logf("at %v: overwrote %v in register %v\n", v, state.slots[slot], vReg)
  1150  				}
  1151  			}
  1152  		}
  1153  
  1154  		for _, slot := range locs.registers[vReg.num] {
  1155  			last := locs.slots[slot]
  1156  			setSlot(slot, VarLoc{last.Registers &^ (1 << uint8(vReg.num)), last.StackOffset})
  1157  		}
  1158  		locs.registers[vReg.num] = locs.registers[vReg.num][:0]
  1159  		locs.registers[vReg.num] = append(locs.registers[vReg.num], vSlots...)
  1160  		for _, slot := range vSlots {
  1161  			if state.loggingLevel > 1 {
  1162  				state.logf("at %v: %v now in %s\n", v, state.slots[slot], vReg)
  1163  			}
  1164  
  1165  			last := locs.slots[slot]
  1166  			setSlot(slot, VarLoc{1<<uint8(vReg.num) | last.Registers, last.StackOffset})
  1167  		}
  1168  	}
  1169  	return changed
  1170  }
  1171  
  1172  // varOffset returns the offset of slot within the user variable it was
  1173  // decomposed from. This has nothing to do with its stack offset.
  1174  func varOffset(slot LocalSlot) int64 {
  1175  	offset := slot.Off
  1176  	s := &slot
  1177  	for ; s.SplitOf != nil; s = s.SplitOf {
  1178  		offset += s.SplitOffset
  1179  	}
  1180  	return offset
  1181  }
  1182  
  1183  type partsByVarOffset struct {
  1184  	slotIDs []SlotID
  1185  	slots   []LocalSlot
  1186  }
  1187  
  1188  func (a partsByVarOffset) Len() int { return len(a.slotIDs) }
  1189  func (a partsByVarOffset) Less(i, j int) bool {
  1190  	return varOffset(a.slots[a.slotIDs[i]]) < varOffset(a.slots[a.slotIDs[j]])
  1191  }
  1192  func (a partsByVarOffset) Swap(i, j int) { a.slotIDs[i], a.slotIDs[j] = a.slotIDs[j], a.slotIDs[i] }
  1193  
  1194  // A pendingEntry represents the beginning of a location list entry, missing
  1195  // only its end coordinate.
  1196  type pendingEntry struct {
  1197  	present                bool
  1198  	startBlock, startValue ID
  1199  	// The location of each piece of the variable, in the same order as the
  1200  	// SlotIDs in varParts.
  1201  	pieces []VarLoc
  1202  }
  1203  
  1204  func (e *pendingEntry) clear() {
  1205  	e.present = false
  1206  	e.startBlock = 0
  1207  	e.startValue = 0
  1208  	for i := range e.pieces {
  1209  		e.pieces[i] = VarLoc{}
  1210  	}
  1211  }
  1212  
  1213  // canMerge reports whether a new location description is a superset
  1214  // of the (non-empty) pending location description, if so, the two
  1215  // can be merged (i.e., pending is still a valid and useful location
  1216  // description).
  1217  func canMerge(pending, new VarLoc) bool {
  1218  	if pending.absent() && new.absent() {
  1219  		return true
  1220  	}
  1221  	if pending.absent() || new.absent() {
  1222  		return false
  1223  	}
  1224  	// pending is not absent, therefore it has either a stack mapping,
  1225  	// or registers, or both.
  1226  	if pending.onStack() && pending.StackOffset != new.StackOffset {
  1227  		// if pending has a stack offset, then new must also, and it
  1228  		// must be the same (StackOffset encodes onStack).
  1229  		return false
  1230  	}
  1231  	if pending.Registers&new.Registers != pending.Registers {
  1232  		// There is at least one register in pending not mentioned in new.
  1233  		return false
  1234  	}
  1235  	return true
  1236  }
  1237  
  1238  // firstReg returns the first register in set that is present.
  1239  func firstReg(set RegisterSet) uint8 {
  1240  	if set == 0 {
  1241  		// This is wrong, but there seem to be some situations where we
  1242  		// produce locations with no storage.
  1243  		return 0
  1244  	}
  1245  	return uint8(bits.TrailingZeros64(uint64(set)))
  1246  }
  1247  
  1248  // buildLocationLists builds location lists for all the user variables
  1249  // in state.f, using the information about block state in blockLocs.
  1250  // The returned location lists are not fully complete. They are in
  1251  // terms of SSA values rather than PCs, and have no base address/end
  1252  // entries. They will be finished by PutLocationList.
  1253  func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
  1254  	// Run through the function in program text order, building up location
  1255  	// lists as we go. The heavy lifting has mostly already been done.
  1256  
  1257  	var prevBlock *Block
  1258  	for _, b := range state.f.Blocks {
  1259  		state.mergePredecessors(b, blockLocs, prevBlock, true)
  1260  
  1261  		// Handle any differences among predecessor blocks and previous block (perhaps not a predecessor)
  1262  		for _, varID := range state.changedVars.contents() {
  1263  			state.updateVar(VarID(varID), b, BlockStart)
  1264  		}
  1265  		state.changedVars.clear()
  1266  
  1267  		if !blockLocs[b.ID].relevant {
  1268  			continue
  1269  		}
  1270  
  1271  		mustBeFirst := func(v *Value) bool {
  1272  			return v.Op == OpPhi || v.Op.isLoweredGetClosurePtr() ||
  1273  				v.Op == OpArgIntReg || v.Op == OpArgFloatReg
  1274  		}
  1275  
  1276  		blockPrologComplete := func(v *Value) bool {
  1277  			if b.ID != state.f.Entry.ID {
  1278  				return !opcodeTable[v.Op].zeroWidth
  1279  			} else {
  1280  				return v.Op == OpInitMem
  1281  			}
  1282  		}
  1283  
  1284  		// Examine the prolog portion of the block to process special
  1285  		// zero-width ops such as Arg, Phi, LoweredGetClosurePtr (etc)
  1286  		// whose lifetimes begin at the block starting point. In an
  1287  		// entry block, allow for the possibility that we may see Arg
  1288  		// ops that appear _after_ other non-zero-width operations.
  1289  		// Example:
  1290  		//
  1291  		//   v33 = ArgIntReg <uintptr> {foo+0} [0] : AX (foo)
  1292  		//   v34 = ArgIntReg <uintptr> {bar+0} [0] : BX (bar)
  1293  		//   ...
  1294  		//   v77 = StoreReg <unsafe.Pointer> v67 : ctx+8[unsafe.Pointer]
  1295  		//   v78 = StoreReg <unsafe.Pointer> v68 : ctx[unsafe.Pointer]
  1296  		//   v79 = Arg <*uint8> {args} : args[*uint8] (args[*uint8])
  1297  		//   v80 = Arg <int> {args} [8] : args+8[int] (args+8[int])
  1298  		//   ...
  1299  		//   v1 = InitMem <mem>
  1300  		//
  1301  		// We can stop scanning the initial portion of the block when
  1302  		// we either see the InitMem op (for entry blocks) or the
  1303  		// first non-zero-width op (for other blocks).
  1304  		for idx := 0; idx < len(b.Values); idx++ {
  1305  			v := b.Values[idx]
  1306  			if blockPrologComplete(v) {
  1307  				break
  1308  			}
  1309  			// Consider only "lifetime begins at block start" ops.
  1310  			if !mustBeFirst(v) && v.Op != OpArg {
  1311  				continue
  1312  			}
  1313  			slots := state.valueNames[v.ID]
  1314  			reg, _ := state.f.getHome(v.ID).(*Register)
  1315  			changed := state.processValue(v, slots, reg) // changed == added to state.changedVars
  1316  			if changed {
  1317  				for _, varID := range state.changedVars.contents() {
  1318  					state.updateVar(VarID(varID), v.Block, BlockStart)
  1319  				}
  1320  				state.changedVars.clear()
  1321  			}
  1322  		}
  1323  
  1324  		// Now examine the block again, handling things other than the
  1325  		// "begins at block start" lifetimes.
  1326  		zeroWidthPending := false
  1327  		prologComplete := false
  1328  		// expect to see values in pattern (apc)* (zerowidth|real)*
  1329  		for _, v := range b.Values {
  1330  			if blockPrologComplete(v) {
  1331  				prologComplete = true
  1332  			}
  1333  			slots := state.valueNames[v.ID]
  1334  			reg, _ := state.f.getHome(v.ID).(*Register)
  1335  			changed := state.processValue(v, slots, reg) // changed == added to state.changedVars
  1336  
  1337  			if opcodeTable[v.Op].zeroWidth {
  1338  				if prologComplete && mustBeFirst(v) {
  1339  					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))
  1340  				}
  1341  				if changed {
  1342  					if mustBeFirst(v) || v.Op == OpArg {
  1343  						// already taken care of above
  1344  						continue
  1345  					}
  1346  					zeroWidthPending = true
  1347  				}
  1348  				continue
  1349  			}
  1350  			if !changed && !zeroWidthPending {
  1351  				continue
  1352  			}
  1353  
  1354  			// Not zero-width; i.e., a "real" instruction.
  1355  			zeroWidthPending = false
  1356  			for _, varID := range state.changedVars.contents() {
  1357  				state.updateVar(VarID(varID), v.Block, v)
  1358  			}
  1359  			state.changedVars.clear()
  1360  		}
  1361  		for _, varID := range state.changedVars.contents() {
  1362  			state.updateVar(VarID(varID), b, BlockEnd)
  1363  		}
  1364  
  1365  		prevBlock = b
  1366  	}
  1367  
  1368  	if state.loggingLevel > 0 {
  1369  		state.logf("location lists:\n")
  1370  	}
  1371  
  1372  	// Flush any leftover entries live at the end of the last block.
  1373  	for varID := range state.lists {
  1374  		state.writePendingEntry(VarID(varID), -1, FuncEnd.ID)
  1375  		list := state.lists[varID]
  1376  		if state.loggingLevel > 0 {
  1377  			if len(list) == 0 {
  1378  				state.logf("\t%v : empty list\n", state.vars[varID])
  1379  			} else {
  1380  				state.logf("\t%v : %q\n", state.vars[varID], hex.EncodeToString(state.lists[varID]))
  1381  			}
  1382  		}
  1383  	}
  1384  }
  1385  
  1386  // updateVar updates the pending location list entry for varID to
  1387  // reflect the new locations in curLoc, beginning at v in block b.
  1388  // v may be one of the special values indicating block start or end.
  1389  func (state *debugState) updateVar(varID VarID, b *Block, v *Value) {
  1390  	curLoc := state.currentState.slots
  1391  	// Assemble the location list entry with whatever's live.
  1392  	empty := true
  1393  	for _, slotID := range state.varSlots[varID] {
  1394  		if !curLoc[slotID].absent() {
  1395  			empty = false
  1396  			break
  1397  		}
  1398  	}
  1399  	pending := &state.pendingEntries[varID]
  1400  	if empty {
  1401  		state.writePendingEntry(varID, b.ID, v.ID)
  1402  		pending.clear()
  1403  		return
  1404  	}
  1405  
  1406  	// Extend the previous entry if possible.
  1407  	if pending.present {
  1408  		merge := true
  1409  		for i, slotID := range state.varSlots[varID] {
  1410  			if !canMerge(pending.pieces[i], curLoc[slotID]) {
  1411  				merge = false
  1412  				break
  1413  			}
  1414  		}
  1415  		if merge {
  1416  			return
  1417  		}
  1418  	}
  1419  
  1420  	state.writePendingEntry(varID, b.ID, v.ID)
  1421  	pending.present = true
  1422  	pending.startBlock = b.ID
  1423  	pending.startValue = v.ID
  1424  	for i, slot := range state.varSlots[varID] {
  1425  		pending.pieces[i] = curLoc[slot]
  1426  	}
  1427  }
  1428  
  1429  // writePendingEntry writes out the pending entry for varID, if any,
  1430  // terminated at endBlock/Value.
  1431  func (state *debugState) writePendingEntry(varID VarID, endBlock, endValue ID) {
  1432  	pending := state.pendingEntries[varID]
  1433  	if !pending.present {
  1434  		return
  1435  	}
  1436  
  1437  	// Pack the start/end coordinates into the start/end addresses
  1438  	// of the entry, for decoding by PutLocationList.
  1439  	start, startOK := encodeValue(state.ctxt, pending.startBlock, pending.startValue)
  1440  	end, endOK := encodeValue(state.ctxt, endBlock, endValue)
  1441  	if !startOK || !endOK {
  1442  		// If someone writes a function that uses >65K values,
  1443  		// they get incomplete debug info on 32-bit platforms.
  1444  		return
  1445  	}
  1446  	if start == end {
  1447  		if state.loggingLevel > 1 {
  1448  			// Printf not logf so not gated by GOSSAFUNC; this should fire very rarely.
  1449  			// TODO this fires a lot, need to figure out why.
  1450  			state.logf("Skipping empty location list for %v in %s\n", state.vars[varID], state.f.Name)
  1451  		}
  1452  		return
  1453  	}
  1454  
  1455  	list := state.lists[varID]
  1456  	list = appendPtr(state.ctxt, list, start)
  1457  	list = appendPtr(state.ctxt, list, end)
  1458  	// Where to write the length of the location description once
  1459  	// we know how big it is.
  1460  	sizeIdx := len(list)
  1461  	list = list[:len(list)+2]
  1462  
  1463  	if state.loggingLevel > 1 {
  1464  		var partStrs []string
  1465  		for i, slot := range state.varSlots[varID] {
  1466  			partStrs = append(partStrs, fmt.Sprintf("%v@%v", state.slots[slot], state.LocString(pending.pieces[i])))
  1467  		}
  1468  		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, " "))
  1469  	}
  1470  
  1471  	for i, slotID := range state.varSlots[varID] {
  1472  		loc := pending.pieces[i]
  1473  		slot := state.slots[slotID]
  1474  
  1475  		if !loc.absent() {
  1476  			if loc.onStack() {
  1477  				if loc.stackOffsetValue() == 0 {
  1478  					list = append(list, dwarf.DW_OP_call_frame_cfa)
  1479  				} else {
  1480  					list = append(list, dwarf.DW_OP_fbreg)
  1481  					list = dwarf.AppendSleb128(list, int64(loc.stackOffsetValue()))
  1482  				}
  1483  			} else {
  1484  				regnum := state.ctxt.Arch.DWARFRegisters[state.registers[firstReg(loc.Registers)].ObjNum()]
  1485  				if regnum < 32 {
  1486  					list = append(list, dwarf.DW_OP_reg0+byte(regnum))
  1487  				} else {
  1488  					list = append(list, dwarf.DW_OP_regx)
  1489  					list = dwarf.AppendUleb128(list, uint64(regnum))
  1490  				}
  1491  			}
  1492  		}
  1493  
  1494  		if len(state.varSlots[varID]) > 1 {
  1495  			list = append(list, dwarf.DW_OP_piece)
  1496  			list = dwarf.AppendUleb128(list, uint64(slot.Type.Size()))
  1497  		}
  1498  	}
  1499  	state.ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
  1500  	state.lists[varID] = list
  1501  }
  1502  
  1503  // PutLocationList adds list (a location list in its intermediate representation) to listSym.
  1504  func (debugInfo *FuncDebug) PutLocationList(list []byte, ctxt *obj.Link, listSym, startPC *obj.LSym) {
  1505  	getPC := debugInfo.GetPC
  1506  
  1507  	if ctxt.UseBASEntries {
  1508  		listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, ^0)
  1509  		listSym.WriteAddr(ctxt, listSym.Size, ctxt.Arch.PtrSize, startPC, 0)
  1510  	}
  1511  
  1512  	// Re-read list, translating its address from block/value ID to PC.
  1513  	for i := 0; i < len(list); {
  1514  		begin := getPC(decodeValue(ctxt, readPtr(ctxt, list[i:])))
  1515  		end := getPC(decodeValue(ctxt, readPtr(ctxt, list[i+ctxt.Arch.PtrSize:])))
  1516  
  1517  		// Horrible hack. If a range contains only zero-width
  1518  		// instructions, e.g. an Arg, and it's at the beginning of the
  1519  		// function, this would be indistinguishable from an
  1520  		// end entry. Fudge it.
  1521  		if begin == 0 && end == 0 {
  1522  			end = 1
  1523  		}
  1524  
  1525  		if ctxt.UseBASEntries {
  1526  			listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, int64(begin))
  1527  			listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, int64(end))
  1528  		} else {
  1529  			listSym.WriteCURelativeAddr(ctxt, listSym.Size, startPC, int64(begin))
  1530  			listSym.WriteCURelativeAddr(ctxt, listSym.Size, startPC, int64(end))
  1531  		}
  1532  
  1533  		i += 2 * ctxt.Arch.PtrSize
  1534  		datalen := 2 + int(ctxt.Arch.ByteOrder.Uint16(list[i:]))
  1535  		listSym.WriteBytes(ctxt, listSym.Size, list[i:i+datalen]) // copy datalen and location encoding
  1536  		i += datalen
  1537  	}
  1538  
  1539  	// Location list contents, now with real PCs.
  1540  	// End entry.
  1541  	listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, 0)
  1542  	listSym.WriteInt(ctxt, listSym.Size, ctxt.Arch.PtrSize, 0)
  1543  }
  1544  
  1545  // Pack a value and block ID into an address-sized uint, returning
  1546  // encoded value and boolean indicating whether the encoding succeeded.
  1547  // For 32-bit architectures the process may fail for very large
  1548  // procedures(the theory being that it's ok to have degraded debug
  1549  // quality in this case).
  1550  func encodeValue(ctxt *obj.Link, b, v ID) (uint64, bool) {
  1551  	if ctxt.Arch.PtrSize == 8 {
  1552  		result := uint64(b)<<32 | uint64(uint32(v))
  1553  		//ctxt.Logf("b %#x (%d) v %#x (%d) -> %#x\n", b, b, v, v, result)
  1554  		return result, true
  1555  	}
  1556  	if ctxt.Arch.PtrSize != 4 {
  1557  		panic("unexpected pointer size")
  1558  	}
  1559  	if ID(int16(b)) != b || ID(int16(v)) != v {
  1560  		return 0, false
  1561  	}
  1562  	return uint64(b)<<16 | uint64(uint16(v)), true
  1563  }
  1564  
  1565  // Unpack a value and block ID encoded by encodeValue.
  1566  func decodeValue(ctxt *obj.Link, word uint64) (ID, ID) {
  1567  	if ctxt.Arch.PtrSize == 8 {
  1568  		b, v := ID(word>>32), ID(word)
  1569  		//ctxt.Logf("%#x -> b %#x (%d) v %#x (%d)\n", word, b, b, v, v)
  1570  		return b, v
  1571  	}
  1572  	if ctxt.Arch.PtrSize != 4 {
  1573  		panic("unexpected pointer size")
  1574  	}
  1575  	return ID(word >> 16), ID(int16(word))
  1576  }
  1577  
  1578  // Append a pointer-sized uint to buf.
  1579  func appendPtr(ctxt *obj.Link, buf []byte, word uint64) []byte {
  1580  	if cap(buf) < len(buf)+20 {
  1581  		b := make([]byte, len(buf), 20+cap(buf)*2)
  1582  		copy(b, buf)
  1583  		buf = b
  1584  	}
  1585  	writeAt := len(buf)
  1586  	buf = buf[0 : len(buf)+ctxt.Arch.PtrSize]
  1587  	writePtr(ctxt, buf[writeAt:], word)
  1588  	return buf
  1589  }
  1590  
  1591  // Write a pointer-sized uint to the beginning of buf.
  1592  func writePtr(ctxt *obj.Link, buf []byte, word uint64) {
  1593  	switch ctxt.Arch.PtrSize {
  1594  	case 4:
  1595  		ctxt.Arch.ByteOrder.PutUint32(buf, uint32(word))
  1596  	case 8:
  1597  		ctxt.Arch.ByteOrder.PutUint64(buf, word)
  1598  	default:
  1599  		panic("unexpected pointer size")
  1600  	}
  1601  
  1602  }
  1603  
  1604  // Read a pointer-sized uint from the beginning of buf.
  1605  func readPtr(ctxt *obj.Link, buf []byte) uint64 {
  1606  	switch ctxt.Arch.PtrSize {
  1607  	case 4:
  1608  		return uint64(ctxt.Arch.ByteOrder.Uint32(buf))
  1609  	case 8:
  1610  		return ctxt.Arch.ByteOrder.Uint64(buf)
  1611  	default:
  1612  		panic("unexpected pointer size")
  1613  	}
  1614  
  1615  }
  1616  
  1617  // setupLocList creates the initial portion of a location list for a
  1618  // user variable. It emits the encoded start/end of the range and a
  1619  // placeholder for the size. Return value is the new list plus the
  1620  // slot in the list holding the size (to be updated later).
  1621  func setupLocList(ctxt *obj.Link, f *Func, list []byte, st, en ID) ([]byte, int) {
  1622  	start, startOK := encodeValue(ctxt, f.Entry.ID, st)
  1623  	end, endOK := encodeValue(ctxt, f.Entry.ID, en)
  1624  	if !startOK || !endOK {
  1625  		// This could happen if someone writes a function that uses
  1626  		// >65K values on a 32-bit platform. Hopefully a degraded debugging
  1627  		// experience is ok in that case.
  1628  		return nil, 0
  1629  	}
  1630  	list = appendPtr(ctxt, list, start)
  1631  	list = appendPtr(ctxt, list, end)
  1632  
  1633  	// Where to write the length of the location description once
  1634  	// we know how big it is.
  1635  	sizeIdx := len(list)
  1636  	list = list[:len(list)+2]
  1637  	return list, sizeIdx
  1638  }
  1639  
  1640  // locatePrologEnd walks the entry block of a function with incoming
  1641  // register arguments and locates the last instruction in the prolog
  1642  // that spills a register arg. It returns the ID of that instruction
  1643  // Example:
  1644  //
  1645  //	b1:
  1646  //	    v3 = ArgIntReg <int> {p1+0} [0] : AX
  1647  //	    ... more arg regs ..
  1648  //	    v4 = ArgFloatReg <float32> {f1+0} [0] : X0
  1649  //	    v52 = MOVQstore <mem> {p1} v2 v3 v1
  1650  //	    ... more stores ...
  1651  //	    v68 = MOVSSstore <mem> {f4} v2 v67 v66
  1652  //	    v38 = MOVQstoreconst <mem> {blob} [val=0,off=0] v2 v32
  1653  //
  1654  // Important: locatePrologEnd is expected to work properly only with
  1655  // optimization turned off (e.g. "-N"). If optimization is enabled
  1656  // we can't be assured of finding all input arguments spilled in the
  1657  // entry block prolog.
  1658  func locatePrologEnd(f *Func) ID {
  1659  
  1660  	// returns true if this instruction looks like it moves an ABI
  1661  	// register to the stack, along with the value being stored.
  1662  	isRegMoveLike := func(v *Value) (bool, ID) {
  1663  		n, ok := v.Aux.(*ir.Name)
  1664  		var r ID
  1665  		if !ok || n.Class != ir.PPARAM {
  1666  			return false, r
  1667  		}
  1668  		regInputs, memInputs, spInputs := 0, 0, 0
  1669  		for _, a := range v.Args {
  1670  			if a.Op == OpArgIntReg || a.Op == OpArgFloatReg {
  1671  				regInputs++
  1672  				r = a.ID
  1673  			} else if a.Type.IsMemory() {
  1674  				memInputs++
  1675  			} else if a.Op == OpSP {
  1676  				spInputs++
  1677  			} else {
  1678  				return false, r
  1679  			}
  1680  		}
  1681  		return v.Type.IsMemory() && memInputs == 1 &&
  1682  			regInputs == 1 && spInputs == 1, r
  1683  	}
  1684  
  1685  	// OpArg*Reg values we've seen so far on our forward walk,
  1686  	// for which we have not yet seen a corresponding spill.
  1687  	regArgs := make([]ID, 0, 32)
  1688  
  1689  	// removeReg tries to remove a value from regArgs, returning true
  1690  	// if found and removed, or false otherwise.
  1691  	removeReg := func(r ID) bool {
  1692  		for i := 0; i < len(regArgs); i++ {
  1693  			if regArgs[i] == r {
  1694  				regArgs = append(regArgs[:i], regArgs[i+1:]...)
  1695  				return true
  1696  			}
  1697  		}
  1698  		return false
  1699  	}
  1700  
  1701  	// Walk forwards through the block. When we see OpArg*Reg, record
  1702  	// the value it produces in the regArgs list. When see a store that uses
  1703  	// the value, remove the entry. When we hit the last store (use)
  1704  	// then we've arrived at the end of the prolog.
  1705  	for k, v := range f.Entry.Values {
  1706  		if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
  1707  			regArgs = append(regArgs, v.ID)
  1708  			continue
  1709  		}
  1710  		if ok, r := isRegMoveLike(v); ok {
  1711  			if removed := removeReg(r); removed {
  1712  				if len(regArgs) == 0 {
  1713  					// Found our last spill; return the value after
  1714  					// it. Note that it is possible that this spill is
  1715  					// the last instruction in the block. If so, then
  1716  					// return the "end of block" sentinel.
  1717  					if k < len(f.Entry.Values)-1 {
  1718  						return f.Entry.Values[k+1].ID
  1719  					}
  1720  					return BlockEnd.ID
  1721  				}
  1722  			}
  1723  		}
  1724  		if v.Op.IsCall() {
  1725  			// if we hit a call, we've gone too far.
  1726  			return v.ID
  1727  		}
  1728  	}
  1729  	// nothing found
  1730  	return ID(-1)
  1731  }
  1732  
  1733  // isNamedRegParam returns true if the param corresponding to "p"
  1734  // is a named, non-blank input parameter assigned to one or more
  1735  // registers.
  1736  func isNamedRegParam(p abi.ABIParamAssignment) bool {
  1737  	if p.Name == nil {
  1738  		return false
  1739  	}
  1740  	n := p.Name
  1741  	if n.Sym() == nil || n.Sym().IsBlank() {
  1742  		return false
  1743  	}
  1744  	if len(p.Registers) == 0 {
  1745  		return false
  1746  	}
  1747  	return true
  1748  }
  1749  
  1750  // BuildFuncDebugNoOptimized populates a FuncDebug object "rval" with
  1751  // entries corresponding to the register-resident input parameters for
  1752  // the function "f"; it is used when we are compiling without
  1753  // optimization but the register ABI is enabled. For each reg param,
  1754  // it constructs a 2-element location list: the first element holds
  1755  // the input register, and the second element holds the stack location
  1756  // of the param (the assumption being that when optimization is off,
  1757  // each input param reg will be spilled in the prolog).
  1758  func BuildFuncDebugNoOptimized(ctxt *obj.Link, f *Func, loggingEnabled bool, stackOffset func(LocalSlot) int32, rval *FuncDebug) {
  1759  
  1760  	pri := f.ABISelf.ABIAnalyzeFuncType(f.Type)
  1761  
  1762  	// Look to see if we have any named register-promoted parameters.
  1763  	// If there are none, bail early and let the caller sort things
  1764  	// out for the remainder of the params/locals.
  1765  	numRegParams := 0
  1766  	for _, inp := range pri.InParams() {
  1767  		if isNamedRegParam(inp) {
  1768  			numRegParams++
  1769  		}
  1770  	}
  1771  	if numRegParams == 0 {
  1772  		return
  1773  	}
  1774  
  1775  	state := debugState{f: f}
  1776  
  1777  	if loggingEnabled {
  1778  		state.logf("generating -N reg param loc lists for func %q\n", f.Name)
  1779  	}
  1780  
  1781  	// Allocate location lists.
  1782  	rval.LocationLists = make([][]byte, numRegParams)
  1783  
  1784  	// Locate the value corresponding to the last spill of
  1785  	// an input register.
  1786  	afterPrologVal := locatePrologEnd(f)
  1787  
  1788  	// Walk the input params again and process the register-resident elements.
  1789  	pidx := 0
  1790  	for _, inp := range pri.InParams() {
  1791  		if !isNamedRegParam(inp) {
  1792  			// will be sorted out elsewhere
  1793  			continue
  1794  		}
  1795  
  1796  		n := inp.Name
  1797  		sl := LocalSlot{N: n, Type: inp.Type, Off: 0}
  1798  		rval.Vars = append(rval.Vars, n)
  1799  		rval.Slots = append(rval.Slots, sl)
  1800  		slid := len(rval.VarSlots)
  1801  		rval.VarSlots = append(rval.VarSlots, []SlotID{SlotID(slid)})
  1802  
  1803  		if afterPrologVal == ID(-1) {
  1804  			// This can happen for degenerate functions with infinite
  1805  			// loops such as that in issue 45948. In such cases, leave
  1806  			// the var/slot set up for the param, but don't try to
  1807  			// emit a location list.
  1808  			if loggingEnabled {
  1809  				state.logf("locatePrologEnd failed, skipping %v\n", n)
  1810  			}
  1811  			pidx++
  1812  			continue
  1813  		}
  1814  
  1815  		// Param is arriving in one or more registers. We need a 2-element
  1816  		// location expression for it. First entry in location list
  1817  		// will correspond to lifetime in input registers.
  1818  		list, sizeIdx := setupLocList(ctxt, f, rval.LocationLists[pidx],
  1819  			BlockStart.ID, afterPrologVal)
  1820  		if list == nil {
  1821  			pidx++
  1822  			continue
  1823  		}
  1824  		if loggingEnabled {
  1825  			state.logf("param %v:\n  [<entry>, %d]:\n", n, afterPrologVal)
  1826  		}
  1827  		rtypes, _ := inp.RegisterTypesAndOffsets()
  1828  		padding := make([]uint64, 0, 32)
  1829  		padding = inp.ComputePadding(padding)
  1830  		for k, r := range inp.Registers {
  1831  			reg := ObjRegForAbiReg(r, f.Config)
  1832  			dwreg := ctxt.Arch.DWARFRegisters[reg]
  1833  			if dwreg < 32 {
  1834  				list = append(list, dwarf.DW_OP_reg0+byte(dwreg))
  1835  			} else {
  1836  				list = append(list, dwarf.DW_OP_regx)
  1837  				list = dwarf.AppendUleb128(list, uint64(dwreg))
  1838  			}
  1839  			if loggingEnabled {
  1840  				state.logf("    piece %d -> dwreg %d", k, dwreg)
  1841  			}
  1842  			if len(inp.Registers) > 1 {
  1843  				list = append(list, dwarf.DW_OP_piece)
  1844  				ts := rtypes[k].Size()
  1845  				list = dwarf.AppendUleb128(list, uint64(ts))
  1846  				if padding[k] > 0 {
  1847  					if loggingEnabled {
  1848  						state.logf(" [pad %d bytes]", padding[k])
  1849  					}
  1850  					list = append(list, dwarf.DW_OP_piece)
  1851  					list = dwarf.AppendUleb128(list, padding[k])
  1852  				}
  1853  			}
  1854  			if loggingEnabled {
  1855  				state.logf("\n")
  1856  			}
  1857  		}
  1858  		// fill in length of location expression element
  1859  		ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
  1860  
  1861  		// Second entry in the location list will be the stack home
  1862  		// of the param, once it has been spilled.  Emit that now.
  1863  		list, sizeIdx = setupLocList(ctxt, f, list,
  1864  			afterPrologVal, FuncEnd.ID)
  1865  		if list == nil {
  1866  			pidx++
  1867  			continue
  1868  		}
  1869  		soff := stackOffset(sl)
  1870  		if soff == 0 {
  1871  			list = append(list, dwarf.DW_OP_call_frame_cfa)
  1872  		} else {
  1873  			list = append(list, dwarf.DW_OP_fbreg)
  1874  			list = dwarf.AppendSleb128(list, int64(soff))
  1875  		}
  1876  		if loggingEnabled {
  1877  			state.logf("  [%d, <end>): stackOffset=%d\n", afterPrologVal, soff)
  1878  		}
  1879  
  1880  		// fill in size
  1881  		ctxt.Arch.ByteOrder.PutUint16(list[sizeIdx:], uint16(len(list)-sizeIdx-2))
  1882  
  1883  		rval.LocationLists[pidx] = list
  1884  		pidx++
  1885  	}
  1886  }
  1887  

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