cmd/link: use side table instead of sym.Symbol 'Reachparent' field

The sym.Symbol 'Reachparent' field is used only when field tracking
is enabled. So as to use less memory for the common case where
field tracking is not enabled, remove this field and use a side
table stored in the context to achieve the same functionality.

Updates #26186

Change-Id: Idc5f8b0aa323689d4d51dddb5d1b0341a37bb7d2
Reviewed-on: https://go-review.googlesource.com/121915
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -190,7 +190,9 @@ func (d *deadcodepass) mark(s, parent *sym.Symbol) {
 		fmt.Printf("%s -> %s\n", p, s.Name)
 	}
 	s.Attr |= sym.AttrReachable
-	s.Reachparent = parent
+	if d.ctxt.Reachparent != nil {
+		d.ctxt.Reachparent[s] = parent
+	}
 	d.markQueue = append(d.markQueue, s)
 }
 

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

@@ -294,7 +294,7 @@ func fieldtrack(ctxt *Link) {
 			s.Attr |= sym.AttrNotInSymbolTable
 			if s.Attr.Reachable() {
 				buf.WriteString(s.Name[9:])
-				for p := s.Reachparent; p != nil; p = p.Reachparent {
+				for p := ctxt.Reachparent[s]; p != nil; p = ctxt.Reachparent[p] {
 					buf.WriteString("\t")
 					buf.WriteString(p.Name)
 				}

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

@@ -86,6 +86,9 @@ type Link struct {
 	// unresolvedSymSet is a set of erroneous unresolved references.
 	// Used to avoid duplicated error messages.
 	unresolvedSymSet map[unresolvedSymKey]bool
+
+	// Used to implement field tracking.
+	Reachparent map[*sym.Symbol]*sym.Symbol
 }
 
 type unresolvedSymKey struct {

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

@@ -34,6 +34,7 @@ import (
 	"bufio"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
+	"cmd/link/internal/sym"
 	"flag"
 	"log"
 	"os"
@@ -144,6 +145,10 @@ func Main(arch *sys.Arch, theArch Arch) {
 		}
 	}
 
+	if objabi.Fieldtrack_enabled != 0 {
+		ctxt.Reachparent = make(map[*sym.Symbol]*sym.Symbol)
+	}
+
 	startProfile()
 	if ctxt.BuildMode == BuildModeUnset {
 		ctxt.BuildMode = BuildModeExe

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

@@ -31,17 +31,16 @@ 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
-	Reachparent *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
+	Dynimplib  string
+	Dynimpvers string
+	Sect       *Section
+	FuncInfo   *FuncInfo
+	Lib        *Library // Package defining this symbol
 	// P contains the raw symbol data.
 	P []byte
 	R []Reloc