Source file src/cmd/internal/obj/dwarf.go

Documentation: cmd/internal/obj

     1  // Copyright 2019 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  // Writes dwarf information to object files.
     6  
     7  package obj
     8  
     9  import (
    10  	"cmd/internal/dwarf"
    11  	"cmd/internal/objabi"
    12  	"cmd/internal/src"
    13  	"fmt"
    14  	"sort"
    15  	"sync"
    16  )
    17  
    18  // Generate a sequence of opcodes that is as short as possible.
    19  // See section 6.2.5
    20  const (
    21  	LINE_BASE   = -4
    22  	LINE_RANGE  = 10
    23  	PC_RANGE    = (255 - OPCODE_BASE) / LINE_RANGE
    24  	OPCODE_BASE = 11
    25  )
    26  
    27  // generateDebugLinesSymbol fills the debug lines symbol of a given function.
    28  //
    29  // It's worth noting that this function doesn't generate the full debug_lines
    30  // DWARF section, saving that for the linker. This function just generates the
    31  // state machine part of debug_lines. The full table is generated by the
    32  // linker.  Also, we use the file numbers from the full package (not just the
    33  // function in question) when generating the state machine. We do this so we
    34  // don't have to do a fixup on the indices when writing the full section.
    35  func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
    36  	dctxt := dwCtxt{ctxt}
    37  
    38  	// Emit a LNE_set_address extended opcode, so as to establish the
    39  	// starting text address of this function.
    40  	dctxt.AddUint8(lines, 0)
    41  	dwarf.Uleb128put(dctxt, lines, 1+int64(ctxt.Arch.PtrSize))
    42  	dctxt.AddUint8(lines, dwarf.DW_LNE_set_address)
    43  	dctxt.AddAddress(lines, s, 0)
    44  
    45  	// Set up the debug_lines state machine to the default values
    46  	// we expect at the start of a new sequence.
    47  	stmt := true
    48  	line := int64(1)
    49  	pc := s.Func().Text.Pc
    50  	var lastpc int64 // last PC written to line table, not last PC in func
    51  	name := ""
    52  	prologue, wrotePrologue := false, false
    53  	// Walk the progs, generating the DWARF table.
    54  	for p := s.Func().Text; p != nil; p = p.Link {
    55  		prologue = prologue || (p.Pos.Xlogue() == src.PosPrologueEnd)
    56  		// If we're not at a real instruction, keep looping!
    57  		if p.Pos.Line() == 0 || (p.Link != nil && p.Link.Pc == p.Pc) {
    58  			continue
    59  		}
    60  		newStmt := p.Pos.IsStmt() != src.PosNotStmt
    61  		newName, newLine := linkgetlineFromPos(ctxt, p.Pos)
    62  
    63  		// Output debug info.
    64  		wrote := false
    65  		if name != newName {
    66  			newFile := ctxt.PosTable.FileIndex(newName) + 1 // 1 indexing for the table.
    67  			dctxt.AddUint8(lines, dwarf.DW_LNS_set_file)
    68  			dwarf.Uleb128put(dctxt, lines, int64(newFile))
    69  			name = newName
    70  			wrote = true
    71  		}
    72  		if prologue && !wrotePrologue {
    73  			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_set_prologue_end))
    74  			wrotePrologue = true
    75  			wrote = true
    76  		}
    77  		if stmt != newStmt {
    78  			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_negate_stmt))
    79  			stmt = newStmt
    80  			wrote = true
    81  		}
    82  
    83  		if line != int64(newLine) || wrote {
    84  			pcdelta := p.Pc - pc
    85  			lastpc = p.Pc
    86  			putpclcdelta(ctxt, dctxt, lines, uint64(pcdelta), int64(newLine)-line)
    87  			line, pc = int64(newLine), p.Pc
    88  		}
    89  	}
    90  
    91  	// Because these symbols will be concatenated together by the
    92  	// linker, we need to reset the state machine that controls the
    93  	// debug symbols. Do this using an end-of-sequence operator.
    94  	//
    95  	// Note: at one point in time, Delve did not support multiple end
    96  	// sequence ops within a compilation unit (bug for this:
    97  	// https://github.com/go-delve/delve/issues/1694), however the bug
    98  	// has since been fixed (Oct 2019).
    99  	//
   100  	// Issue 38192: the DWARF standard specifies that when you issue
   101  	// an end-sequence op, the PC value should be one past the last
   102  	// text address in the translation unit, so apply a delta to the
   103  	// text address before the end sequence op. If this isn't done,
   104  	// GDB will assign a line number of zero the last row in the line
   105  	// table, which we don't want.
   106  	lastlen := uint64(s.Size - (lastpc - s.Func().Text.Pc))
   107  	dctxt.AddUint8(lines, dwarf.DW_LNS_advance_pc)
   108  	dwarf.Uleb128put(dctxt, lines, int64(lastlen))
   109  	dctxt.AddUint8(lines, 0) // start extended opcode
   110  	dwarf.Uleb128put(dctxt, lines, 1)
   111  	dctxt.AddUint8(lines, dwarf.DW_LNE_end_sequence)
   112  }
   113  
   114  func putpclcdelta(linkctxt *Link, dctxt dwCtxt, s *LSym, deltaPC uint64, deltaLC int64) {
   115  	// Choose a special opcode that minimizes the number of bytes needed to
   116  	// encode the remaining PC delta and LC delta.
   117  	var opcode int64
   118  	if deltaLC < LINE_BASE {
   119  		if deltaPC >= PC_RANGE {
   120  			opcode = OPCODE_BASE + (LINE_RANGE * PC_RANGE)
   121  		} else {
   122  			opcode = OPCODE_BASE + (LINE_RANGE * int64(deltaPC))
   123  		}
   124  	} else if deltaLC < LINE_BASE+LINE_RANGE {
   125  		if deltaPC >= PC_RANGE {
   126  			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * PC_RANGE)
   127  			if opcode > 255 {
   128  				opcode -= LINE_RANGE
   129  			}
   130  		} else {
   131  			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * int64(deltaPC))
   132  		}
   133  	} else {
   134  		if deltaPC <= PC_RANGE {
   135  			opcode = OPCODE_BASE + (LINE_RANGE - 1) + (LINE_RANGE * int64(deltaPC))
   136  			if opcode > 255 {
   137  				opcode = 255
   138  			}
   139  		} else {
   140  			// Use opcode 249 (pc+=23, lc+=5) or 255 (pc+=24, lc+=1).
   141  			//
   142  			// Let x=deltaPC-PC_RANGE.  If we use opcode 255, x will be the remaining
   143  			// deltaPC that we need to encode separately before emitting 255.  If we
   144  			// use opcode 249, we will need to encode x+1.  If x+1 takes one more
   145  			// byte to encode than x, then we use opcode 255.
   146  			//
   147  			// In all other cases x and x+1 take the same number of bytes to encode,
   148  			// so we use opcode 249, which may save us a byte in encoding deltaLC,
   149  			// for similar reasons.
   150  			switch deltaPC - PC_RANGE {
   151  			// PC_RANGE is the largest deltaPC we can encode in one byte, using
   152  			// DW_LNS_const_add_pc.
   153  			//
   154  			// (1<<16)-1 is the largest deltaPC we can encode in three bytes, using
   155  			// DW_LNS_fixed_advance_pc.
   156  			//
   157  			// (1<<(7n))-1 is the largest deltaPC we can encode in n+1 bytes for
   158  			// n=1,3,4,5,..., using DW_LNS_advance_pc.
   159  			case PC_RANGE, (1 << 7) - 1, (1 << 16) - 1, (1 << 21) - 1, (1 << 28) - 1,
   160  				(1 << 35) - 1, (1 << 42) - 1, (1 << 49) - 1, (1 << 56) - 1, (1 << 63) - 1:
   161  				opcode = 255
   162  			default:
   163  				opcode = OPCODE_BASE + LINE_RANGE*PC_RANGE - 1 // 249
   164  			}
   165  		}
   166  	}
   167  	if opcode < OPCODE_BASE || opcode > 255 {
   168  		panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
   169  	}
   170  
   171  	// Subtract from deltaPC and deltaLC the amounts that the opcode will add.
   172  	deltaPC -= uint64((opcode - OPCODE_BASE) / LINE_RANGE)
   173  	deltaLC -= (opcode-OPCODE_BASE)%LINE_RANGE + LINE_BASE
   174  
   175  	// Encode deltaPC.
   176  	if deltaPC != 0 {
   177  		if deltaPC <= PC_RANGE {
   178  			// Adjust the opcode so that we can use the 1-byte DW_LNS_const_add_pc
   179  			// instruction.
   180  			opcode -= LINE_RANGE * int64(PC_RANGE-deltaPC)
   181  			if opcode < OPCODE_BASE {
   182  				panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
   183  			}
   184  			dctxt.AddUint8(s, dwarf.DW_LNS_const_add_pc)
   185  		} else if (1<<14) <= deltaPC && deltaPC < (1<<16) {
   186  			dctxt.AddUint8(s, dwarf.DW_LNS_fixed_advance_pc)
   187  			dctxt.AddUint16(s, uint16(deltaPC))
   188  		} else {
   189  			dctxt.AddUint8(s, dwarf.DW_LNS_advance_pc)
   190  			dwarf.Uleb128put(dctxt, s, int64(deltaPC))
   191  		}
   192  	}
   193  
   194  	// Encode deltaLC.
   195  	if deltaLC != 0 {
   196  		dctxt.AddUint8(s, dwarf.DW_LNS_advance_line)
   197  		dwarf.Sleb128put(dctxt, s, deltaLC)
   198  	}
   199  
   200  	// Output the special opcode.
   201  	dctxt.AddUint8(s, uint8(opcode))
   202  }
   203  
   204  // implement dwarf.Context
   205  type dwCtxt struct{ *Link }
   206  
   207  func (c dwCtxt) PtrSize() int {
   208  	return c.Arch.PtrSize
   209  }
   210  func (c dwCtxt) AddInt(s dwarf.Sym, size int, i int64) {
   211  	ls := s.(*LSym)
   212  	ls.WriteInt(c.Link, ls.Size, size, i)
   213  }
   214  func (c dwCtxt) AddUint16(s dwarf.Sym, i uint16) {
   215  	c.AddInt(s, 2, int64(i))
   216  }
   217  func (c dwCtxt) AddUint8(s dwarf.Sym, i uint8) {
   218  	b := []byte{byte(i)}
   219  	c.AddBytes(s, b)
   220  }
   221  func (c dwCtxt) AddBytes(s dwarf.Sym, b []byte) {
   222  	ls := s.(*LSym)
   223  	ls.WriteBytes(c.Link, ls.Size, b)
   224  }
   225  func (c dwCtxt) AddString(s dwarf.Sym, v string) {
   226  	ls := s.(*LSym)
   227  	ls.WriteString(c.Link, ls.Size, len(v), v)
   228  	ls.WriteInt(c.Link, ls.Size, 1, 0)
   229  }
   230  func (c dwCtxt) AddAddress(s dwarf.Sym, data interface{}, value int64) {
   231  	ls := s.(*LSym)
   232  	size := c.PtrSize()
   233  	if data != nil {
   234  		rsym := data.(*LSym)
   235  		ls.WriteAddr(c.Link, ls.Size, size, rsym, value)
   236  	} else {
   237  		ls.WriteInt(c.Link, ls.Size, size, value)
   238  	}
   239  }
   240  func (c dwCtxt) AddCURelativeAddress(s dwarf.Sym, data interface{}, value int64) {
   241  	ls := s.(*LSym)
   242  	rsym := data.(*LSym)
   243  	ls.WriteCURelativeAddr(c.Link, ls.Size, rsym, value)
   244  }
   245  func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64) {
   246  	panic("should be used only in the linker")
   247  }
   248  func (c dwCtxt) AddDWARFAddrSectionOffset(s dwarf.Sym, t interface{}, ofs int64) {
   249  	size := 4
   250  	if isDwarf64(c.Link) {
   251  		size = 8
   252  	}
   253  
   254  	ls := s.(*LSym)
   255  	rsym := t.(*LSym)
   256  	ls.WriteAddr(c.Link, ls.Size, size, rsym, ofs)
   257  	r := &ls.R[len(ls.R)-1]
   258  	r.Type = objabi.R_DWARFSECREF
   259  }
   260  
   261  func (c dwCtxt) AddFileRef(s dwarf.Sym, f interface{}) {
   262  	ls := s.(*LSym)
   263  	rsym := f.(*LSym)
   264  	fidx := c.Link.PosTable.FileIndex(rsym.Name)
   265  	// Note the +1 here -- the value we're writing is going to be an
   266  	// index into the DWARF line table file section, whose entries
   267  	// are numbered starting at 1, not 0.
   268  	ls.WriteInt(c.Link, ls.Size, 4, int64(fidx+1))
   269  }
   270  
   271  func (c dwCtxt) CurrentOffset(s dwarf.Sym) int64 {
   272  	ls := s.(*LSym)
   273  	return ls.Size
   274  }
   275  
   276  // Here "from" is a symbol corresponding to an inlined or concrete
   277  // function, "to" is the symbol for the corresponding abstract
   278  // function, and "dclIdx" is the index of the symbol of interest with
   279  // respect to the Dcl slice of the original pre-optimization version
   280  // of the inlined function.
   281  func (c dwCtxt) RecordDclReference(from dwarf.Sym, to dwarf.Sym, dclIdx int, inlIndex int) {
   282  	ls := from.(*LSym)
   283  	tls := to.(*LSym)
   284  	ridx := len(ls.R) - 1
   285  	c.Link.DwFixups.ReferenceChildDIE(ls, ridx, tls, dclIdx, inlIndex)
   286  }
   287  
   288  func (c dwCtxt) RecordChildDieOffsets(s dwarf.Sym, vars []*dwarf.Var, offsets []int32) {
   289  	ls := s.(*LSym)
   290  	c.Link.DwFixups.RegisterChildDIEOffsets(ls, vars, offsets)
   291  }
   292  
   293  func (c dwCtxt) Logf(format string, args ...interface{}) {
   294  	c.Link.Logf(format, args...)
   295  }
   296  
   297  func isDwarf64(ctxt *Link) bool {
   298  	return ctxt.Headtype == objabi.Haix
   299  }
   300  
   301  func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym, dwarfAbsFnSym, dwarfDebugLines *LSym) {
   302  	if s.Type != objabi.STEXT {
   303  		ctxt.Diag("dwarfSym of non-TEXT %v", s)
   304  	}
   305  	fn := s.Func()
   306  	if fn.dwarfInfoSym == nil {
   307  		fn.dwarfInfoSym = &LSym{
   308  			Type: objabi.SDWARFFCN,
   309  		}
   310  		if ctxt.Flag_locationlists {
   311  			fn.dwarfLocSym = &LSym{
   312  				Type: objabi.SDWARFLOC,
   313  			}
   314  		}
   315  		fn.dwarfRangesSym = &LSym{
   316  			Type: objabi.SDWARFRANGE,
   317  		}
   318  		fn.dwarfDebugLinesSym = &LSym{
   319  			Type: objabi.SDWARFLINES,
   320  		}
   321  		if s.WasInlined() {
   322  			fn.dwarfAbsFnSym = ctxt.DwFixups.AbsFuncDwarfSym(s)
   323  		}
   324  	}
   325  	return fn.dwarfInfoSym, fn.dwarfLocSym, fn.dwarfRangesSym, fn.dwarfAbsFnSym, fn.dwarfDebugLinesSym
   326  }
   327  
   328  func (s *LSym) Length(dwarfContext interface{}) int64 {
   329  	return s.Size
   330  }
   331  
   332  // fileSymbol returns a symbol corresponding to the source file of the
   333  // first instruction (prog) of the specified function. This will
   334  // presumably be the file in which the function is defined.
   335  func (ctxt *Link) fileSymbol(fn *LSym) *LSym {
   336  	p := fn.Func().Text
   337  	if p != nil {
   338  		f, _ := linkgetlineFromPos(ctxt, p.Pos)
   339  		fsym := ctxt.Lookup(f)
   340  		return fsym
   341  	}
   342  	return nil
   343  }
   344  
   345  // populateDWARF fills in the DWARF Debugging Information Entries for
   346  // TEXT symbol 's'. The various DWARF symbols must already have been
   347  // initialized in InitTextSym.
   348  func (ctxt *Link) populateDWARF(curfn interface{}, s *LSym, myimportpath string) {
   349  	info, loc, ranges, absfunc, lines := ctxt.dwarfSym(s)
   350  	if info.Size != 0 {
   351  		ctxt.Diag("makeFuncDebugEntry double process %v", s)
   352  	}
   353  	var scopes []dwarf.Scope
   354  	var inlcalls dwarf.InlCalls
   355  	if ctxt.DebugInfo != nil {
   356  		scopes, inlcalls = ctxt.DebugInfo(s, info, curfn)
   357  	}
   358  	var err error
   359  	dwctxt := dwCtxt{ctxt}
   360  	filesym := ctxt.fileSymbol(s)
   361  	fnstate := &dwarf.FnState{
   362  		Name:          s.Name,
   363  		Importpath:    myimportpath,
   364  		Info:          info,
   365  		Filesym:       filesym,
   366  		Loc:           loc,
   367  		Ranges:        ranges,
   368  		Absfn:         absfunc,
   369  		StartPC:       s,
   370  		Size:          s.Size,
   371  		External:      !s.Static(),
   372  		Scopes:        scopes,
   373  		InlCalls:      inlcalls,
   374  		UseBASEntries: ctxt.UseBASEntries,
   375  	}
   376  	if absfunc != nil {
   377  		err = dwarf.PutAbstractFunc(dwctxt, fnstate)
   378  		if err != nil {
   379  			ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
   380  		}
   381  		err = dwarf.PutConcreteFunc(dwctxt, fnstate)
   382  	} else {
   383  		err = dwarf.PutDefaultFunc(dwctxt, fnstate)
   384  	}
   385  	if err != nil {
   386  		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
   387  	}
   388  	// Fill in the debug lines symbol.
   389  	ctxt.generateDebugLinesSymbol(s, lines)
   390  }
   391  
   392  // DwarfIntConst creates a link symbol for an integer constant with the
   393  // given name, type and value.
   394  func (ctxt *Link) DwarfIntConst(myimportpath, name, typename string, val int64) {
   395  	if myimportpath == "" {
   396  		return
   397  	}
   398  	s := ctxt.LookupInit(dwarf.ConstInfoPrefix+myimportpath, func(s *LSym) {
   399  		s.Type = objabi.SDWARFCONST
   400  		ctxt.Data = append(ctxt.Data, s)
   401  	})
   402  	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), myimportpath+"."+name, val)
   403  }
   404  
   405  // DwarfGlobal creates a link symbol containing a DWARF entry for
   406  // a global variable.
   407  func (ctxt *Link) DwarfGlobal(myimportpath, typename string, varSym *LSym) {
   408  	if myimportpath == "" || varSym.Local() {
   409  		return
   410  	}
   411  	var varname string
   412  	if varSym.Pkg == "_" {
   413  		// The frontend uses package "_" to mark symbols that should not
   414  		// be referenced by index, e.g. linkname'd symbols.
   415  		varname = varSym.Name
   416  	} else {
   417  		// Convert "".<name> into a fully qualified package.sym name.
   418  		varname = objabi.PathToPrefix(myimportpath) + varSym.Name[len(`""`):]
   419  	}
   420  	dieSymName := dwarf.InfoPrefix + varname
   421  	dieSym := ctxt.LookupInit(dieSymName, func(s *LSym) {
   422  		s.Type = objabi.SDWARFVAR
   423  		s.Set(AttrDuplicateOK, true) // needed for shared linkage
   424  		ctxt.Data = append(ctxt.Data, s)
   425  	})
   426  	typeSym := ctxt.Lookup(dwarf.InfoPrefix + typename)
   427  	dwarf.PutGlobal(dwCtxt{ctxt}, dieSym, typeSym, varSym, varname)
   428  }
   429  
   430  func (ctxt *Link) DwarfAbstractFunc(curfn interface{}, s *LSym, myimportpath string) {
   431  	absfn := ctxt.DwFixups.AbsFuncDwarfSym(s)
   432  	if absfn.Size != 0 {
   433  		ctxt.Diag("internal error: DwarfAbstractFunc double process %v", s)
   434  	}
   435  	if s.Func() == nil {
   436  		s.NewFuncInfo()
   437  	}
   438  	scopes, _ := ctxt.DebugInfo(s, absfn, curfn)
   439  	dwctxt := dwCtxt{ctxt}
   440  	filesym := ctxt.fileSymbol(s)
   441  	fnstate := dwarf.FnState{
   442  		Name:          s.Name,
   443  		Importpath:    myimportpath,
   444  		Info:          absfn,
   445  		Filesym:       filesym,
   446  		Absfn:         absfn,
   447  		External:      !s.Static(),
   448  		Scopes:        scopes,
   449  		UseBASEntries: ctxt.UseBASEntries,
   450  	}
   451  	if err := dwarf.PutAbstractFunc(dwctxt, &fnstate); err != nil {
   452  		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
   453  	}
   454  }
   455  
   456  // This table is designed to aid in the creation of references between
   457  // DWARF subprogram DIEs.
   458  //
   459  // In most cases when one DWARF DIE has to refer to another DWARF DIE,
   460  // the target of the reference has an LSym, which makes it easy to use
   461  // the existing relocation mechanism. For DWARF inlined routine DIEs,
   462  // however, the subprogram DIE has to refer to a child
   463  // parameter/variable DIE of the abstract subprogram. This child DIE
   464  // doesn't have an LSym, and also of interest is the fact that when
   465  // DWARF generation is happening for inlined function F within caller
   466  // G, it's possible that DWARF generation hasn't happened yet for F,
   467  // so there is no way to know the offset of a child DIE within F's
   468  // abstract function. Making matters more complex, each inlined
   469  // instance of F may refer to a subset of the original F's variables
   470  // (depending on what happens with optimization, some vars may be
   471  // eliminated).
   472  //
   473  // The fixup table below helps overcome this hurdle. At the point
   474  // where a parameter/variable reference is made (via a call to
   475  // "ReferenceChildDIE"), a fixup record is generate that records
   476  // the relocation that is targeting that child variable. At a later
   477  // point when the abstract function DIE is emitted, there will be
   478  // a call to "RegisterChildDIEOffsets", at which point the offsets
   479  // needed to apply fixups are captured. Finally, once the parallel
   480  // portion of the compilation is done, fixups can actually be applied
   481  // during the "Finalize" method (this can't be done during the
   482  // parallel portion of the compile due to the possibility of data
   483  // races).
   484  //
   485  // This table is also used to record the "precursor" function node for
   486  // each function that is the target of an inline -- child DIE references
   487  // have to be made with respect to the original pre-optimization
   488  // version of the function (to allow for the fact that each inlined
   489  // body may be optimized differently).
   490  type DwarfFixupTable struct {
   491  	ctxt      *Link
   492  	mu        sync.Mutex
   493  	symtab    map[*LSym]int // maps abstract fn LSYM to index in svec
   494  	svec      []symFixups
   495  	precursor map[*LSym]fnState // maps fn Lsym to precursor Node, absfn sym
   496  }
   497  
   498  type symFixups struct {
   499  	fixups   []relFixup
   500  	doffsets []declOffset
   501  	inlIndex int32
   502  	defseen  bool
   503  }
   504  
   505  type declOffset struct {
   506  	// Index of variable within DCL list of pre-optimization function
   507  	dclIdx int32
   508  	// Offset of var's child DIE with respect to containing subprogram DIE
   509  	offset int32
   510  }
   511  
   512  type relFixup struct {
   513  	refsym *LSym
   514  	relidx int32
   515  	dclidx int32
   516  }
   517  
   518  type fnState struct {
   519  	// precursor function (really *gc.Node)
   520  	precursor interface{}
   521  	// abstract function symbol
   522  	absfn *LSym
   523  }
   524  
   525  func NewDwarfFixupTable(ctxt *Link) *DwarfFixupTable {
   526  	return &DwarfFixupTable{
   527  		ctxt:      ctxt,
   528  		symtab:    make(map[*LSym]int),
   529  		precursor: make(map[*LSym]fnState),
   530  	}
   531  }
   532  
   533  func (ft *DwarfFixupTable) GetPrecursorFunc(s *LSym) interface{} {
   534  	if fnstate, found := ft.precursor[s]; found {
   535  		return fnstate.precursor
   536  	}
   537  	return nil
   538  }
   539  
   540  func (ft *DwarfFixupTable) SetPrecursorFunc(s *LSym, fn interface{}) {
   541  	if _, found := ft.precursor[s]; found {
   542  		ft.ctxt.Diag("internal error: DwarfFixupTable.SetPrecursorFunc double call on %v", s)
   543  	}
   544  
   545  	// initialize abstract function symbol now. This is done here so
   546  	// as to avoid data races later on during the parallel portion of
   547  	// the back end.
   548  	absfn := ft.ctxt.LookupDerived(s, dwarf.InfoPrefix+s.Name+dwarf.AbstractFuncSuffix)
   549  	absfn.Set(AttrDuplicateOK, true)
   550  	absfn.Type = objabi.SDWARFABSFCN
   551  	ft.ctxt.Data = append(ft.ctxt.Data, absfn)
   552  
   553  	// In the case of "late" inlining (inlines that happen during
   554  	// wrapper generation as opposed to the main inlining phase) it's
   555  	// possible that we didn't cache the abstract function sym for the
   556  	// text symbol -- do so now if needed. See issue 38068.
   557  	if fn := s.Func(); fn != nil && fn.dwarfAbsFnSym == nil {
   558  		fn.dwarfAbsFnSym = absfn
   559  	}
   560  
   561  	ft.precursor[s] = fnState{precursor: fn, absfn: absfn}
   562  }
   563  
   564  // Make a note of a child DIE reference: relocation 'ridx' within symbol 's'
   565  // is targeting child 'c' of DIE with symbol 'tgt'.
   566  func (ft *DwarfFixupTable) ReferenceChildDIE(s *LSym, ridx int, tgt *LSym, dclidx int, inlIndex int) {
   567  	// Protect against concurrent access if multiple backend workers
   568  	ft.mu.Lock()
   569  	defer ft.mu.Unlock()
   570  
   571  	// Create entry for symbol if not already present.
   572  	idx, found := ft.symtab[tgt]
   573  	if !found {
   574  		ft.svec = append(ft.svec, symFixups{inlIndex: int32(inlIndex)})
   575  		idx = len(ft.svec) - 1
   576  		ft.symtab[tgt] = idx
   577  	}
   578  
   579  	// Do we have child DIE offsets available? If so, then apply them,
   580  	// otherwise create a fixup record.
   581  	sf := &ft.svec[idx]
   582  	if len(sf.doffsets) > 0 {
   583  		found := false
   584  		for _, do := range sf.doffsets {
   585  			if do.dclIdx == int32(dclidx) {
   586  				off := do.offset
   587  				s.R[ridx].Add += int64(off)
   588  				found = true
   589  				break
   590  			}
   591  		}
   592  		if !found {
   593  			ft.ctxt.Diag("internal error: DwarfFixupTable.ReferenceChildDIE unable to locate child DIE offset for dclIdx=%d src=%v tgt=%v", dclidx, s, tgt)
   594  		}
   595  	} else {
   596  		sf.fixups = append(sf.fixups, relFixup{s, int32(ridx), int32(dclidx)})
   597  	}
   598  }
   599  
   600  // Called once DWARF generation is complete for a given abstract function,
   601  // whose children might have been referenced via a call above. Stores
   602  // the offsets for any child DIEs (vars, params) so that they can be
   603  // consumed later in on DwarfFixupTable.Finalize, which applies any
   604  // outstanding fixups.
   605  func (ft *DwarfFixupTable) RegisterChildDIEOffsets(s *LSym, vars []*dwarf.Var, coffsets []int32) {
   606  	// Length of these two slices should agree
   607  	if len(vars) != len(coffsets) {
   608  		ft.ctxt.Diag("internal error: RegisterChildDIEOffsets vars/offsets length mismatch")
   609  		return
   610  	}
   611  
   612  	// Generate the slice of declOffset's based in vars/coffsets
   613  	doffsets := make([]declOffset, len(coffsets))
   614  	for i := range coffsets {
   615  		doffsets[i].dclIdx = vars[i].ChildIndex
   616  		doffsets[i].offset = coffsets[i]
   617  	}
   618  
   619  	ft.mu.Lock()
   620  	defer ft.mu.Unlock()
   621  
   622  	// Store offsets for this symbol.
   623  	idx, found := ft.symtab[s]
   624  	if !found {
   625  		sf := symFixups{inlIndex: -1, defseen: true, doffsets: doffsets}
   626  		ft.svec = append(ft.svec, sf)
   627  		ft.symtab[s] = len(ft.svec) - 1
   628  	} else {
   629  		sf := &ft.svec[idx]
   630  		sf.doffsets = doffsets
   631  		sf.defseen = true
   632  	}
   633  }
   634  
   635  func (ft *DwarfFixupTable) processFixups(slot int, s *LSym) {
   636  	sf := &ft.svec[slot]
   637  	for _, f := range sf.fixups {
   638  		dfound := false
   639  		for _, doffset := range sf.doffsets {
   640  			if doffset.dclIdx == f.dclidx {
   641  				f.refsym.R[f.relidx].Add += int64(doffset.offset)
   642  				dfound = true
   643  				break
   644  			}
   645  		}
   646  		if !dfound {
   647  			ft.ctxt.Diag("internal error: DwarfFixupTable has orphaned fixup on %v targeting %v relidx=%d dclidx=%d", f.refsym, s, f.relidx, f.dclidx)
   648  		}
   649  	}
   650  }
   651  
   652  // return the LSym corresponding to the 'abstract subprogram' DWARF
   653  // info entry for a function.
   654  func (ft *DwarfFixupTable) AbsFuncDwarfSym(fnsym *LSym) *LSym {
   655  	// Protect against concurrent access if multiple backend workers
   656  	ft.mu.Lock()
   657  	defer ft.mu.Unlock()
   658  
   659  	if fnstate, found := ft.precursor[fnsym]; found {
   660  		return fnstate.absfn
   661  	}
   662  	ft.ctxt.Diag("internal error: AbsFuncDwarfSym requested for %v, not seen during inlining", fnsym)
   663  	return nil
   664  }
   665  
   666  // Called after all functions have been compiled; the main job of this
   667  // function is to identify cases where there are outstanding fixups.
   668  // This scenario crops up when we have references to variables of an
   669  // inlined routine, but that routine is defined in some other package.
   670  // This helper walks through and locate these fixups, then invokes a
   671  // helper to create an abstract subprogram DIE for each one.
   672  func (ft *DwarfFixupTable) Finalize(myimportpath string, trace bool) {
   673  	if trace {
   674  		ft.ctxt.Logf("DwarfFixupTable.Finalize invoked for %s\n", myimportpath)
   675  	}
   676  
   677  	// Collect up the keys from the precursor map, then sort the
   678  	// resulting list (don't want to rely on map ordering here).
   679  	fns := make([]*LSym, len(ft.precursor))
   680  	idx := 0
   681  	for fn := range ft.precursor {
   682  		fns[idx] = fn
   683  		idx++
   684  	}
   685  	sort.Sort(BySymName(fns))
   686  
   687  	// Should not be called during parallel portion of compilation.
   688  	if ft.ctxt.InParallel {
   689  		ft.ctxt.Diag("internal error: DwarfFixupTable.Finalize call during parallel backend")
   690  	}
   691  
   692  	// Generate any missing abstract functions.
   693  	for _, s := range fns {
   694  		absfn := ft.AbsFuncDwarfSym(s)
   695  		slot, found := ft.symtab[absfn]
   696  		if !found || !ft.svec[slot].defseen {
   697  			ft.ctxt.GenAbstractFunc(s)
   698  		}
   699  	}
   700  
   701  	// Apply fixups.
   702  	for _, s := range fns {
   703  		absfn := ft.AbsFuncDwarfSym(s)
   704  		slot, found := ft.symtab[absfn]
   705  		if !found {
   706  			ft.ctxt.Diag("internal error: DwarfFixupTable.Finalize orphan abstract function for %v", s)
   707  		} else {
   708  			ft.processFixups(slot, s)
   709  		}
   710  	}
   711  }
   712  
   713  type BySymName []*LSym
   714  
   715  func (s BySymName) Len() int           { return len(s) }
   716  func (s BySymName) Less(i, j int) bool { return s[i].Name < s[j].Name }
   717  func (s BySymName) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
   718  

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