cmd/link: split off 'Dynimp' string fields to reduce sym.Symbol size

The linker's sym.Symbol struct contains two string fields, "Dynimplib"
and "Dynimpvers" that are used only in very specific circumstances
(for many symbols, such as DWARF syms, they are wasted space). Split
these two off into a separate struct, then point to an instance of
that struct when needed. This reduces the size of sym.Symbol so as to
save space in the common case.

Updates #26186

Change-Id: Id9c74824e78423a215c8cbc105b72665525a1eff
Reviewed-on: https://go-review.googlesource.com/121916Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/elf.go

@@ -1030,8 +1030,8 @@ func elfdynhash(ctxt *Link) {
 			continue
 		}
 
-		if sy.Dynimpvers != "" {
-			need[sy.Dynid] = addelflib(&needlib, sy.Dynimplib, sy.Dynimpvers)
+		if sy.Dynimpvers() != "" {
+			need[sy.Dynid] = addelflib(&needlib, sy.Dynimplib(), sy.Dynimpvers())
 		}
 
 		name := sy.Extname
@@ -2287,8 +2287,8 @@ func elfadddynsym(ctxt *Link, s *sym.Symbol) {
 		/* size of object */
 		d.AddUint64(ctxt.Arch, uint64(s.Size))
 
-		if ctxt.Arch.Family == sys.AMD64 && !s.Attr.CgoExportDynamic() && s.Dynimplib != "" && !seenlib[s.Dynimplib] {
-			Elfwritedynent(ctxt, ctxt.Syms.Lookup(".dynamic", 0), DT_NEEDED, uint64(Addstring(ctxt.Syms.Lookup(".dynstr", 0), s.Dynimplib)))
+		if ctxt.Arch.Family == sys.AMD64 && !s.Attr.CgoExportDynamic() && s.Dynimplib() != "" && !seenlib[s.Dynimplib()] {
+			Elfwritedynent(ctxt, ctxt.Syms.Lookup(".dynamic", 0), DT_NEEDED, uint64(Addstring(ctxt.Syms.Lookup(".dynstr", 0), s.Dynimplib())))
 		}
 	} else {
 		s.Dynid = int32(Nelfsym)

src/cmd/link/internal/ld/go.go

@@ -155,9 +155,9 @@ func loadcgo(ctxt *Link, file string, pkg string, p string) {
 			}
 			s := ctxt.Syms.Lookup(local, 0)
 			if s.Type == 0 || s.Type == sym.SXREF || s.Type == sym.SHOSTOBJ {
-				s.Dynimplib = lib
+				s.SetDynimplib(lib)
 				s.Extname = remote
-				s.Dynimpvers = q
+				s.SetDynimpvers(q)
 				if s.Type != sym.SHOSTOBJ {
 					s.Type = sym.SDYNIMPORT
 				}
@@ -198,10 +198,9 @@ func loadcgo(ctxt *Link, file string, pkg string, p string) {
 
 			// export overrides import, for openbsd/cgo.
 			// see issue 4878.
-			if s.Dynimplib != "" {
-				s.Dynimplib = ""
+			if s.Dynimplib() != "" {
+				s.ResetDyninfo()
 				s.Extname = ""
-				s.Dynimpvers = ""
 				s.Type = 0
 			}
 

src/cmd/link/internal/ld/lib.go

@@ -416,7 +416,7 @@ func (ctxt *Link) loadlib() {
 				// cgo_import_static and cgo_import_dynamic,
 				// then we want to make it cgo_import_dynamic
 				// now.
-				if s.Extname != "" && s.Dynimplib != "" && !s.Attr.CgoExport() {
+				if s.Extname != "" && s.Dynimplib() != "" && !s.Attr.CgoExport() {
 					s.Type = sym.SDYNIMPORT
 				} else {
 					s.Type = 0

src/cmd/link/internal/ld/pe.go

@@ -989,7 +989,7 @@ func initdynimport(ctxt *Link) *Dll {
 			continue
 		}
 		for d = dr; d != nil; d = d.next {
-			if d.name == s.Dynimplib {
+			if d.name == s.Dynimplib() {
 				m = new(Imp)
 				break
 			}
@@ -997,7 +997,7 @@ func initdynimport(ctxt *Link) *Dll {
 
 		if d == nil {
 			d = new(Dll)
-			d.name = s.Dynimplib
+			d.name = s.Dynimplib()
 			d.next = dr
 			dr = d
 			m = new(Imp)

src/cmd/link/internal/loadelf/ldelf.go

@@ -805,7 +805,7 @@ func Load(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length i
 		s.Type = sect.sym.Type
 		s.Attr |= sym.AttrSubSymbol
 		if !s.Attr.CgoExportDynamic() {
-			s.Dynimplib = "" // satisfy dynimport
+			s.SetDynimplib("") // satisfy dynimport
 		}
 		s.Value = int64(elfsym.value)
 		s.Size = int64(elfsym.size)

src/cmd/link/internal/loadmacho/ldmacho.go

@@ -644,7 +644,7 @@ func Load(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length i
 		s.Outer = outer
 		s.Value = int64(machsym.value - sect.addr)
 		if !s.Attr.CgoExportDynamic() {
-			s.Dynimplib = "" // satisfy dynimport
+			s.SetDynimplib("") // satisfy dynimport
 		}
 		if outer.Type == sym.STEXT {
 			if s.Attr.External() && !s.Attr.DuplicateOK() {

src/cmd/link/internal/sym/symbol.go

@@ -31,21 +31,25 @@ type Symbol struct {
 	// ElfType is set for symbols read from shared libraries by ldshlibsyms. It
 	// is not set for symbols defined by the packages being linked or by symbols
 	// read by ldelf (and so is left as elf.STT_NOTYPE).
-	ElfType    elf.SymType
-	Sub        *Symbol
-	Outer      *Symbol
-	Gotype     *Symbol
-	File       string
-	Dynimplib  string
-	Dynimpvers string
-	Sect       *Section
-	FuncInfo   *FuncInfo
-	Lib        *Library // Package defining this symbol
+	ElfType  elf.SymType
+	Sub      *Symbol
+	Outer    *Symbol
+	Gotype   *Symbol
+	File     string
+	dyninfo  *dynimp
+	Sect     *Section
+	FuncInfo *FuncInfo
+	Lib      *Library // Package defining this symbol
 	// P contains the raw symbol data.
 	P []byte
 	R []Reloc
 }
 
+type dynimp struct {
+	dynimplib  string
+	dynimpvers string
+}
+
 func (s *Symbol) String() string {
 	if s.Version == 0 {
 		return s.Name
@@ -264,6 +268,40 @@ func (s *Symbol) setUintXX(arch *sys.Arch, off int64, v uint64, wid int64) int64
 	return off + wid
 }
 
+func (s *Symbol) Dynimplib() string {
+	if s.dyninfo == nil {
+		return ""
+	}
+	return s.dyninfo.dynimplib
+}
+
+func (s *Symbol) Dynimpvers() string {
+	if s.dyninfo == nil {
+		return ""
+	}
+	return s.dyninfo.dynimpvers
+}
+
+func (s *Symbol) SetDynimplib(lib string) {
+	if s.dyninfo == nil {
+		s.dyninfo = &dynimp{dynimplib: lib}
+	} else {
+		s.dyninfo.dynimplib = lib
+	}
+}
+
+func (s *Symbol) SetDynimpvers(vers string) {
+	if s.dyninfo == nil {
+		s.dyninfo = &dynimp{dynimpvers: vers}
+	} else {
+		s.dyninfo.dynimpvers = vers
+	}
+}
+
+func (s *Symbol) ResetDyninfo() {
+	s.dyninfo = nil
+}
+
 // SortSub sorts a linked-list (by Sub) of *Symbol by Value.
 // Used for sub-symbols when loading host objects (see e.g. ldelf.go).
 func SortSub(l *Symbol) *Symbol {