dwarf.go

     1  // Copyright 2011 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 dwarfgen
     6  
     7  import (
     8  	"bytes"
     9  	"flag"
    10  	"fmt"
    11  	"internal/buildcfg"
    12  	"slices"
    13  	"sort"
    14  	"strings"
    15  
    16  	"cmd/compile/internal/base"
    17  	"cmd/compile/internal/ir"
    18  	"cmd/compile/internal/reflectdata"
    19  	"cmd/compile/internal/ssa"
    20  	"cmd/compile/internal/ssagen"
    21  	"cmd/compile/internal/typecheck"
    22  	"cmd/compile/internal/types"
    23  	"cmd/internal/dwarf"
    24  	"cmd/internal/obj"
    25  	"cmd/internal/objabi"
    26  	"cmd/internal/src"
    27  )
    28  
    29  func Info(ctxt *obj.Link, fnsym *obj.LSym, infosym *obj.LSym, curfn obj.Func) (scopes []dwarf.Scope, inlcalls dwarf.InlCalls) {
    30  	fn := curfn.(*ir.Func)
    31  
    32  	if fn.Nname != nil {
    33  		expect := fn.Linksym()
    34  		if fnsym.ABI() == obj.ABI0 {
    35  			expect = fn.LinksymABI(obj.ABI0)
    36  		}
    37  		if fnsym != expect {
    38  			base.Fatalf("unexpected fnsym: %v != %v", fnsym, expect)
    39  		}
    40  	}
    41  
    42  	// Back when there were two different *Funcs for a function, this code
    43  	// was not consistent about whether a particular *Node being processed
    44  	// was an ODCLFUNC or ONAME node. Partly this is because inlined function
    45  	// bodies have no ODCLFUNC node, which was it's own inconsistency.
    46  	// In any event, the handling of the two different nodes for DWARF purposes
    47  	// was subtly different, likely in unintended ways. CL 272253 merged the
    48  	// two nodes' Func fields, so that code sees the same *Func whether it is
    49  	// holding the ODCLFUNC or the ONAME. This resulted in changes in the
    50  	// DWARF output. To preserve the existing DWARF output and leave an
    51  	// intentional change for a future CL, this code does the following when
    52  	// fn.Op == ONAME:
    53  	//
    54  	// 1. Disallow use of createComplexVars in createDwarfVars.
    55  	//    It was not possible to reach that code for an ONAME before,
    56  	//    because the DebugInfo was set only on the ODCLFUNC Func.
    57  	//    Calling into it in the ONAME case causes an index out of bounds panic.
    58  	//
    59  	// 2. Do not populate apdecls. fn.Func.Dcl was in the ODCLFUNC Func,
    60  	//    not the ONAME Func. Populating apdecls for the ONAME case results
    61  	//    in selected being populated after createSimpleVars is called in
    62  	//    createDwarfVars, and then that causes the loop to skip all the entries
    63  	//    in dcl, meaning that the RecordAutoType calls don't happen.
    64  	//
    65  	// These two adjustments keep toolstash -cmp working for now.
    66  	// Deciding the right answer is, as they say, future work.
    67  	//
    68  	// We can tell the difference between the old ODCLFUNC and ONAME
    69  	// cases by looking at the infosym.Name. If it's empty, DebugInfo is
    70  	// being called from (*obj.Link).populateDWARF, which used to use
    71  	// the ODCLFUNC. If it's non-empty (the name will end in $abstract),
    72  	// DebugInfo is being called from (*obj.Link).DwarfAbstractFunc,
    73  	// which used to use the ONAME form.
    74  	isODCLFUNC := infosym.Name == ""
    75  
    76  	var apdecls []*ir.Name
    77  	// Populate decls for fn.
    78  	if isODCLFUNC {
    79  		for _, n := range fn.Dcl {
    80  			if n.Op() != ir.ONAME { // might be OTYPE or OLITERAL
    81  				continue
    82  			}
    83  			switch n.Class {
    84  			case ir.PAUTO:
    85  				if !n.Used() {
    86  					// Text == nil -> generating abstract function
    87  					if fnsym.Func().Text != nil {
    88  						base.Fatalf("debuginfo unused node (AllocFrame should truncate fn.Func.Dcl)")
    89  					}
    90  					continue
    91  				}
    92  			case ir.PPARAM, ir.PPARAMOUT:
    93  			default:
    94  				continue
    95  			}
    96  			if !ssa.IsVarWantedForDebug(n) {
    97  				continue
    98  			}
    99  			apdecls = append(apdecls, n)
   100  			if n.Type().Kind() == types.TSSA {
   101  				// Can happen for TypeInt128 types. This only happens for
   102  				// spill locations, so not a huge deal.
   103  				continue
   104  			}
   105  			fnsym.Func().RecordAutoType(reflectdata.TypeLinksym(n.Type()))
   106  		}
   107  	}
   108  
   109  	var closureVars map[*ir.Name]int64
   110  	if fn.Needctxt() {
   111  		closureVars = make(map[*ir.Name]int64)
   112  		csiter := typecheck.NewClosureStructIter(fn.ClosureVars)
   113  		for {
   114  			n, _, offset := csiter.Next()
   115  			if n == nil {
   116  				break
   117  			}
   118  			closureVars[n] = offset
   119  			if n.Heapaddr != nil {
   120  				closureVars[n.Heapaddr] = offset
   121  			}
   122  		}
   123  	}
   124  
   125  	decls, dwarfVars := createDwarfVars(fnsym, isODCLFUNC, fn, apdecls, closureVars)
   126  
   127  	// For each type referenced by the functions auto vars but not
   128  	// already referenced by a dwarf var, attach an R_USETYPE relocation to
   129  	// the function symbol to insure that the type included in DWARF
   130  	// processing during linking.
   131  	// Do the same with R_USEIFACE relocations from the function symbol for the
   132  	// same reason.
   133  	// All these R_USETYPE relocations are only looked at if the function
   134  	// survives deadcode elimination in the linker.
   135  	typesyms := []*obj.LSym{}
   136  	for t := range fnsym.Func().Autot {
   137  		typesyms = append(typesyms, t)
   138  	}
   139  	for i := range fnsym.R {
   140  		if fnsym.R[i].Type == objabi.R_USEIFACE && !strings.HasPrefix(fnsym.R[i].Sym.Name, "go:itab.") {
   141  			// Types referenced through itab will be referenced from somewhere else
   142  			typesyms = append(typesyms, fnsym.R[i].Sym)
   143  		}
   144  	}
   145  	slices.SortFunc(typesyms, func(a, b *obj.LSym) int {
   146  		return strings.Compare(a.Name, b.Name)
   147  	})
   148  	var lastsym *obj.LSym
   149  	for _, sym := range typesyms {
   150  		if sym == lastsym {
   151  			continue
   152  		}
   153  		lastsym = sym
   154  		infosym.AddRel(ctxt, obj.Reloc{Type: objabi.R_USETYPE, Sym: sym})
   155  	}
   156  	fnsym.Func().Autot = nil
   157  
   158  	var varScopes []ir.ScopeID
   159  	for _, decl := range decls {
   160  		pos := declPos(decl)
   161  		varScopes = append(varScopes, findScope(fn.Marks, pos))
   162  	}
   163  
   164  	scopes = assembleScopes(fnsym, fn, dwarfVars, varScopes)
   165  	if base.Flag.GenDwarfInl > 0 {
   166  		inlcalls = assembleInlines(fnsym, dwarfVars)
   167  	}
   168  	return scopes, inlcalls
   169  }
   170  
   171  func declPos(decl *ir.Name) src.XPos {
   172  	return decl.Canonical().Pos()
   173  }
   174  
   175  // createDwarfVars process fn, returning a list of DWARF variables and the
   176  // Nodes they represent.
   177  func createDwarfVars(fnsym *obj.LSym, complexOK bool, fn *ir.Func, apDecls []*ir.Name, closureVars map[*ir.Name]int64) ([]*ir.Name, []*dwarf.Var) {
   178  	// Collect a raw list of DWARF vars.
   179  	var vars []*dwarf.Var
   180  	var decls []*ir.Name
   181  	var selected ir.NameSet
   182  
   183  	if base.Ctxt.Flag_locationlists && base.Ctxt.Flag_optimize && fn.DebugInfo != nil && complexOK {
   184  		decls, vars, selected = createComplexVars(fnsym, fn, closureVars)
   185  	} else if fn.ABI == obj.ABIInternal && base.Flag.N != 0 && complexOK {
   186  		decls, vars, selected = createABIVars(fnsym, fn, apDecls, closureVars)
   187  	} else {
   188  		decls, vars, selected = createSimpleVars(fnsym, apDecls, closureVars)
   189  	}
   190  	if fn.DebugInfo != nil {
   191  		// Recover zero sized variables eliminated by the stackframe pass
   192  		for _, n := range fn.DebugInfo.(*ssa.FuncDebug).OptDcl {
   193  			if n.Class != ir.PAUTO {
   194  				continue
   195  			}
   196  			types.CalcSize(n.Type())
   197  			if n.Type().Size() == 0 {
   198  				decls = append(decls, n)
   199  				vars = append(vars, createSimpleVar(fnsym, n, closureVars))
   200  				vars[len(vars)-1].StackOffset = 0
   201  				fnsym.Func().RecordAutoType(reflectdata.TypeLinksym(n.Type()))
   202  			}
   203  		}
   204  	}
   205  
   206  	dcl := apDecls
   207  	if fnsym.WasInlined() {
   208  		dcl = preInliningDcls(fnsym)
   209  	} else {
   210  		// The backend's stackframe pass prunes away entries from the
   211  		// fn's Dcl list, including PARAMOUT nodes that correspond to
   212  		// output params passed in registers. Add back in these
   213  		// entries here so that we can process them properly during
   214  		// DWARF-gen. See issue 48573 for more details.
   215  		debugInfo := fn.DebugInfo.(*ssa.FuncDebug)
   216  		for _, n := range debugInfo.RegOutputParams {
   217  			if !ssa.IsVarWantedForDebug(n) {
   218  				continue
   219  			}
   220  			if n.Class != ir.PPARAMOUT || !n.IsOutputParamInRegisters() {
   221  				base.Fatalf("invalid ir.Name on debugInfo.RegOutputParams list")
   222  			}
   223  			dcl = append(dcl, n)
   224  		}
   225  	}
   226  
   227  	// If optimization is enabled, the list above will typically be
   228  	// missing some of the original pre-optimization variables in the
   229  	// function (they may have been promoted to registers, folded into
   230  	// constants, dead-coded away, etc).  Input arguments not eligible
   231  	// for SSA optimization are also missing.  Here we add back in entries
   232  	// for selected missing vars. Note that the recipe below creates a
   233  	// conservative location. The idea here is that we want to
   234  	// communicate to the user that "yes, there is a variable named X
   235  	// in this function, but no, I don't have enough information to
   236  	// reliably report its contents."
   237  	// For non-SSA-able arguments, however, the correct information
   238  	// is known -- they have a single home on the stack.
   239  	for _, n := range dcl {
   240  		if selected.Has(n) {
   241  			continue
   242  		}
   243  		c := n.Sym().Name[0]
   244  		if c == '.' || n.Type().IsUntyped() {
   245  			continue
   246  		}
   247  		if n.Class == ir.PPARAM && !ssa.CanSSA(n.Type()) {
   248  			// SSA-able args get location lists, and may move in and
   249  			// out of registers, so those are handled elsewhere.
   250  			// Autos and named output params seem to get handled
   251  			// with VARDEF, which creates location lists.
   252  			// Args not of SSA-able type are treated here; they
   253  			// are homed on the stack in a single place for the
   254  			// entire call.
   255  			vars = append(vars, createSimpleVar(fnsym, n, closureVars))
   256  			decls = append(decls, n)
   257  			continue
   258  		}
   259  		typename := dwarf.InfoPrefix + types.TypeSymName(n.Type())
   260  		decls = append(decls, n)
   261  		tag := dwarf.DW_TAG_variable
   262  		isReturnValue := (n.Class == ir.PPARAMOUT)
   263  		if n.Class == ir.PPARAM || n.Class == ir.PPARAMOUT {
   264  			tag = dwarf.DW_TAG_formal_parameter
   265  		}
   266  		inlIndex := 0
   267  		if base.Flag.GenDwarfInl > 1 {
   268  			if n.InlFormal() || n.InlLocal() {
   269  				inlIndex = posInlIndex(n.Pos()) + 1
   270  				if n.InlFormal() {
   271  					tag = dwarf.DW_TAG_formal_parameter
   272  				}
   273  			}
   274  		}
   275  		declpos := base.Ctxt.InnermostPos(n.Pos())
   276  		dvar := &dwarf.Var{
   277  			Name:          n.Sym().Name,
   278  			IsReturnValue: isReturnValue,
   279  			Tag:           tag,
   280  			WithLoclist:   true,
   281  			StackOffset:   int32(n.FrameOffset()),
   282  			Type:          base.Ctxt.Lookup(typename),
   283  			DeclFile:      declpos.RelFilename(),
   284  			DeclLine:      declpos.RelLine(),
   285  			DeclCol:       declpos.RelCol(),
   286  			InlIndex:      int32(inlIndex),
   287  			ChildIndex:    -1,
   288  			DictIndex:     n.DictIndex,
   289  			ClosureOffset: closureOffset(n, closureVars),
   290  		}
   291  		if n.Esc() == ir.EscHeap {
   292  			if n.Heapaddr == nil {
   293  				base.Fatalf("invalid heap allocated var without Heapaddr")
   294  			}
   295  			debug := fn.DebugInfo.(*ssa.FuncDebug)
   296  			list := createHeapDerefLocationList(n, debug.EntryID)
   297  			dvar.PutLocationList = func(listSym, startPC dwarf.Sym) {
   298  				debug.PutLocationList(list, base.Ctxt, listSym.(*obj.LSym), startPC.(*obj.LSym))
   299  			}
   300  		}
   301  		vars = append(vars, dvar)
   302  		// Record go type to ensure that it gets emitted by the linker.
   303  		fnsym.Func().RecordAutoType(reflectdata.TypeLinksym(n.Type()))
   304  	}
   305  
   306  	// Sort decls and vars.
   307  	sortDeclsAndVars(fn, decls, vars)
   308  
   309  	return decls, vars
   310  }
   311  
   312  // sortDeclsAndVars sorts the decl and dwarf var lists according to
   313  // parameter declaration order, so as to insure that when a subprogram
   314  // DIE is emitted, its parameter children appear in declaration order.
   315  // Prior to the advent of the register ABI, sorting by frame offset
   316  // would achieve this; with the register we now need to go back to the
   317  // original function signature.
   318  func sortDeclsAndVars(fn *ir.Func, decls []*ir.Name, vars []*dwarf.Var) {
   319  	paramOrder := make(map[*ir.Name]int)
   320  	idx := 1
   321  	for _, f := range fn.Type().RecvParamsResults() {
   322  		if n, ok := f.Nname.(*ir.Name); ok {
   323  			paramOrder[n] = idx
   324  			idx++
   325  		}
   326  	}
   327  	sort.Stable(varsAndDecls{decls, vars, paramOrder})
   328  }
   329  
   330  type varsAndDecls struct {
   331  	decls      []*ir.Name
   332  	vars       []*dwarf.Var
   333  	paramOrder map[*ir.Name]int
   334  }
   335  
   336  func (v varsAndDecls) Len() int {
   337  	return len(v.decls)
   338  }
   339  
   340  func (v varsAndDecls) Less(i, j int) bool {
   341  	nameLT := func(ni, nj *ir.Name) bool {
   342  		oi, foundi := v.paramOrder[ni]
   343  		oj, foundj := v.paramOrder[nj]
   344  		if foundi {
   345  			if foundj {
   346  				return oi < oj
   347  			} else {
   348  				return true
   349  			}
   350  		}
   351  		return false
   352  	}
   353  	return nameLT(v.decls[i], v.decls[j])
   354  }
   355  
   356  func (v varsAndDecls) Swap(i, j int) {
   357  	v.vars[i], v.vars[j] = v.vars[j], v.vars[i]
   358  	v.decls[i], v.decls[j] = v.decls[j], v.decls[i]
   359  }
   360  
   361  // Given a function that was inlined at some point during the
   362  // compilation, return a sorted list of nodes corresponding to the
   363  // autos/locals in that function prior to inlining. If this is a
   364  // function that is not local to the package being compiled, then the
   365  // names of the variables may have been "versioned" to avoid conflicts
   366  // with local vars; disregard this versioning when sorting.
   367  func preInliningDcls(fnsym *obj.LSym) []*ir.Name {
   368  	fn := base.Ctxt.DwFixups.GetPrecursorFunc(fnsym).(*ir.Func)
   369  	var rdcl []*ir.Name
   370  	for _, n := range fn.Inl.Dcl {
   371  		c := n.Sym().Name[0]
   372  		// Avoid reporting "_" parameters, since if there are more than
   373  		// one, it can result in a collision later on, as in #23179.
   374  		if n.Sym().Name == "_" || c == '.' || n.Type().IsUntyped() {
   375  			continue
   376  		}
   377  		rdcl = append(rdcl, n)
   378  	}
   379  	return rdcl
   380  }
   381  
   382  // createSimpleVars creates a DWARF entry for every variable declared in the
   383  // function, claiming that they are permanently on the stack.
   384  func createSimpleVars(fnsym *obj.LSym, apDecls []*ir.Name, closureVars map[*ir.Name]int64) ([]*ir.Name, []*dwarf.Var, ir.NameSet) {
   385  	var vars []*dwarf.Var
   386  	var decls []*ir.Name
   387  	var selected ir.NameSet
   388  	for _, n := range apDecls {
   389  		if ir.IsAutoTmp(n) {
   390  			continue
   391  		}
   392  
   393  		decls = append(decls, n)
   394  		vars = append(vars, createSimpleVar(fnsym, n, closureVars))
   395  		selected.Add(n)
   396  	}
   397  	return decls, vars, selected
   398  }
   399  
   400  func createSimpleVar(fnsym *obj.LSym, n *ir.Name, closureVars map[*ir.Name]int64) *dwarf.Var {
   401  	var tag int
   402  	var offs int64
   403  
   404  	localAutoOffset := func() int64 {
   405  		offs = n.FrameOffset()
   406  		if base.Ctxt.Arch.FixedFrameSize == 0 {
   407  			offs -= int64(types.PtrSize)
   408  		}
   409  		if buildcfg.FramePointerEnabled {
   410  			offs -= int64(types.PtrSize)
   411  		}
   412  		return offs
   413  	}
   414  
   415  	switch n.Class {
   416  	case ir.PAUTO:
   417  		offs = localAutoOffset()
   418  		tag = dwarf.DW_TAG_variable
   419  	case ir.PPARAM, ir.PPARAMOUT:
   420  		tag = dwarf.DW_TAG_formal_parameter
   421  		if n.IsOutputParamInRegisters() {
   422  			offs = localAutoOffset()
   423  		} else {
   424  			offs = n.FrameOffset() + base.Ctxt.Arch.FixedFrameSize
   425  		}
   426  
   427  	default:
   428  		base.Fatalf("createSimpleVar unexpected class %v for node %v", n.Class, n)
   429  	}
   430  
   431  	typename := dwarf.InfoPrefix + types.TypeSymName(n.Type())
   432  	delete(fnsym.Func().Autot, reflectdata.TypeLinksym(n.Type()))
   433  	inlIndex := 0
   434  	if base.Flag.GenDwarfInl > 1 {
   435  		if n.InlFormal() || n.InlLocal() {
   436  			inlIndex = posInlIndex(n.Pos()) + 1
   437  			if n.InlFormal() {
   438  				tag = dwarf.DW_TAG_formal_parameter
   439  			}
   440  		}
   441  	}
   442  	declpos := base.Ctxt.InnermostPos(declPos(n))
   443  	return &dwarf.Var{
   444  		Name:          n.Sym().Name,
   445  		IsReturnValue: n.Class == ir.PPARAMOUT,
   446  		IsInlFormal:   n.InlFormal(),
   447  		Tag:           tag,
   448  		StackOffset:   int32(offs),
   449  		Type:          base.Ctxt.Lookup(typename),
   450  		DeclFile:      declpos.RelFilename(),
   451  		DeclLine:      declpos.RelLine(),
   452  		DeclCol:       declpos.RelCol(),
   453  		InlIndex:      int32(inlIndex),
   454  		ChildIndex:    -1,
   455  		DictIndex:     n.DictIndex,
   456  		ClosureOffset: closureOffset(n, closureVars),
   457  	}
   458  }
   459  
   460  // createABIVars creates DWARF variables for functions in which the
   461  // register ABI is enabled but optimization is turned off. It uses a
   462  // hybrid approach in which register-resident input params are
   463  // captured with location lists, and all other vars use the "simple"
   464  // strategy.
   465  func createABIVars(fnsym *obj.LSym, fn *ir.Func, apDecls []*ir.Name, closureVars map[*ir.Name]int64) ([]*ir.Name, []*dwarf.Var, ir.NameSet) {
   466  
   467  	// Invoke createComplexVars to generate dwarf vars for input parameters
   468  	// that are register-allocated according to the ABI rules.
   469  	decls, vars, selected := createComplexVars(fnsym, fn, closureVars)
   470  
   471  	// Now fill in the remainder of the variables: input parameters
   472  	// that are not register-resident, output parameters, and local
   473  	// variables.
   474  	for _, n := range apDecls {
   475  		if ir.IsAutoTmp(n) {
   476  			continue
   477  		}
   478  		if _, ok := selected[n]; ok {
   479  			// already handled
   480  			continue
   481  		}
   482  
   483  		decls = append(decls, n)
   484  		vars = append(vars, createSimpleVar(fnsym, n, closureVars))
   485  		selected.Add(n)
   486  	}
   487  
   488  	return decls, vars, selected
   489  }
   490  
   491  // createComplexVars creates recomposed DWARF vars with location lists,
   492  // suitable for describing optimized code.
   493  func createComplexVars(fnsym *obj.LSym, fn *ir.Func, closureVars map[*ir.Name]int64) ([]*ir.Name, []*dwarf.Var, ir.NameSet) {
   494  	debugInfo := fn.DebugInfo.(*ssa.FuncDebug)
   495  
   496  	// Produce a DWARF variable entry for each user variable.
   497  	var decls []*ir.Name
   498  	var vars []*dwarf.Var
   499  	var ssaVars ir.NameSet
   500  
   501  	for varID, dvar := range debugInfo.Vars {
   502  		n := dvar
   503  		ssaVars.Add(n)
   504  		for _, slot := range debugInfo.VarSlots[varID] {
   505  			ssaVars.Add(debugInfo.Slots[slot].N)
   506  		}
   507  
   508  		if dvar := createComplexVar(fnsym, fn, ssa.VarID(varID), closureVars); dvar != nil {
   509  			decls = append(decls, n)
   510  			vars = append(vars, dvar)
   511  		}
   512  	}
   513  
   514  	return decls, vars, ssaVars
   515  }
   516  
   517  // createComplexVar builds a single DWARF variable entry and location list.
   518  func createComplexVar(fnsym *obj.LSym, fn *ir.Func, varID ssa.VarID, closureVars map[*ir.Name]int64) *dwarf.Var {
   519  	debug := fn.DebugInfo.(*ssa.FuncDebug)
   520  	n := debug.Vars[varID]
   521  
   522  	var tag int
   523  	switch n.Class {
   524  	case ir.PAUTO:
   525  		tag = dwarf.DW_TAG_variable
   526  	case ir.PPARAM, ir.PPARAMOUT:
   527  		tag = dwarf.DW_TAG_formal_parameter
   528  	default:
   529  		return nil
   530  	}
   531  
   532  	gotype := reflectdata.TypeLinksym(n.Type())
   533  	delete(fnsym.Func().Autot, gotype)
   534  	typename := dwarf.InfoPrefix + gotype.Name[len("type:"):]
   535  	inlIndex := 0
   536  	if base.Flag.GenDwarfInl > 1 {
   537  		if n.InlFormal() || n.InlLocal() {
   538  			inlIndex = posInlIndex(n.Pos()) + 1
   539  			if n.InlFormal() {
   540  				tag = dwarf.DW_TAG_formal_parameter
   541  			}
   542  		}
   543  	}
   544  	declpos := base.Ctxt.InnermostPos(n.Pos())
   545  	dvar := &dwarf.Var{
   546  		Name:          n.Sym().Name,
   547  		IsReturnValue: n.Class == ir.PPARAMOUT,
   548  		IsInlFormal:   n.InlFormal(),
   549  		Tag:           tag,
   550  		WithLoclist:   true,
   551  		Type:          base.Ctxt.Lookup(typename),
   552  		// The stack offset is used as a sorting key, so for decomposed
   553  		// variables just give it the first one. It's not used otherwise.
   554  		// This won't work well if the first slot hasn't been assigned a stack
   555  		// location, but it's not obvious how to do better.
   556  		StackOffset:   ssagen.StackOffset(debug.Slots[debug.VarSlots[varID][0]]),
   557  		DeclFile:      declpos.RelFilename(),
   558  		DeclLine:      declpos.RelLine(),
   559  		DeclCol:       declpos.RelCol(),
   560  		InlIndex:      int32(inlIndex),
   561  		ChildIndex:    -1,
   562  		DictIndex:     n.DictIndex,
   563  		ClosureOffset: closureOffset(n, closureVars),
   564  	}
   565  	list := debug.LocationLists[varID]
   566  	if len(list) != 0 {
   567  		dvar.PutLocationList = func(listSym, startPC dwarf.Sym) {
   568  			debug.PutLocationList(list, base.Ctxt, listSym.(*obj.LSym), startPC.(*obj.LSym))
   569  		}
   570  	}
   571  	return dvar
   572  }
   573  
   574  // createHeapDerefLocationList creates a location list for a heap-escaped variable
   575  // that describes "dereference pointer at stack offset"
   576  func createHeapDerefLocationList(n *ir.Name, entryID ssa.ID) []byte {
   577  	// Get the stack offset where the heap pointer is stored
   578  	heapPtrOffset := n.Heapaddr.FrameOffset()
   579  	if base.Ctxt.Arch.FixedFrameSize == 0 {
   580  		heapPtrOffset -= int64(types.PtrSize)
   581  	}
   582  	if buildcfg.FramePointerEnabled {
   583  		heapPtrOffset -= int64(types.PtrSize)
   584  	}
   585  
   586  	// Create a location expression: DW_OP_fbreg <offset> DW_OP_deref
   587  	var locExpr []byte
   588  	var sizeIdx int
   589  	locExpr, sizeIdx = ssa.SetupLocList(base.Ctxt, entryID, locExpr, ssa.BlockStart.ID, ssa.FuncEnd.ID)
   590  	locExpr = append(locExpr, dwarf.DW_OP_fbreg)
   591  	locExpr = dwarf.AppendSleb128(locExpr, heapPtrOffset)
   592  	locExpr = append(locExpr, dwarf.DW_OP_deref)
   593  	base.Ctxt.Arch.ByteOrder.PutUint16(locExpr[sizeIdx:], uint16(len(locExpr)-sizeIdx-2))
   594  	return locExpr
   595  }
   596  
   597  // RecordFlags records the specified command-line flags to be placed
   598  // in the DWARF info.
   599  func RecordFlags(flags ...string) {
   600  	if base.Ctxt.Pkgpath == "" {
   601  		base.Fatalf("missing pkgpath")
   602  	}
   603  
   604  	type BoolFlag interface {
   605  		IsBoolFlag() bool
   606  	}
   607  	type CountFlag interface {
   608  		IsCountFlag() bool
   609  	}
   610  	var cmd bytes.Buffer
   611  	for _, name := range flags {
   612  		f := flag.Lookup(name)
   613  		if f == nil {
   614  			continue
   615  		}
   616  		getter := f.Value.(flag.Getter)
   617  		if getter.String() == f.DefValue {
   618  			// Flag has default value, so omit it.
   619  			continue
   620  		}
   621  		if bf, ok := f.Value.(BoolFlag); ok && bf.IsBoolFlag() {
   622  			val, ok := getter.Get().(bool)
   623  			if ok && val {
   624  				fmt.Fprintf(&cmd, " -%s", f.Name)
   625  				continue
   626  			}
   627  		}
   628  		if cf, ok := f.Value.(CountFlag); ok && cf.IsCountFlag() {
   629  			val, ok := getter.Get().(int)
   630  			if ok && val == 1 {
   631  				fmt.Fprintf(&cmd, " -%s", f.Name)
   632  				continue
   633  			}
   634  		}
   635  		fmt.Fprintf(&cmd, " -%s=%v", f.Name, getter.Get())
   636  	}
   637  
   638  	// Adds flag to producer string signaling whether regabi is turned on or
   639  	// off.
   640  	// Once regabi is turned on across the board and the relative GOEXPERIMENT
   641  	// knobs no longer exist this code should be removed.
   642  	if buildcfg.Experiment.RegabiArgs {
   643  		cmd.Write([]byte(" regabi"))
   644  	}
   645  
   646  	if cmd.Len() == 0 {
   647  		return
   648  	}
   649  	s := base.Ctxt.Lookup(dwarf.CUInfoPrefix + "producer." + base.Ctxt.Pkgpath)
   650  	s.Type = objabi.SDWARFCUINFO
   651  	// Sometimes (for example when building tests) we can link
   652  	// together two package main archives. So allow dups.
   653  	s.Set(obj.AttrDuplicateOK, true)
   654  	base.Ctxt.Data = append(base.Ctxt.Data, s)
   655  	s.P = cmd.Bytes()[1:]
   656  }
   657  
   658  // RecordPackageName records the name of the package being
   659  // compiled, so that the linker can save it in the compile unit's DIE.
   660  func RecordPackageName() {
   661  	s := base.Ctxt.Lookup(dwarf.CUInfoPrefix + "packagename." + base.Ctxt.Pkgpath)
   662  	s.Type = objabi.SDWARFCUINFO
   663  	// Sometimes (for example when building tests) we can link
   664  	// together two package main archives. So allow dups.
   665  	s.Set(obj.AttrDuplicateOK, true)
   666  	base.Ctxt.Data = append(base.Ctxt.Data, s)
   667  	s.P = []byte(types.LocalPkg.Name)
   668  }
   669  
   670  func closureOffset(n *ir.Name, closureVars map[*ir.Name]int64) int64 {
   671  	return closureVars[n]
   672  }
   673
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