From 32bc0976f9c4333e4a9247d6acc908972c6c5d79 Mon Sep 17 00:00:00 2001
From: Than McIntosh <thanm@google.com>
Date: Thu, 28 Jun 2018 15:42:20 -0400
Subject: [PATCH] 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 |  4 +++-
 src/cmd/link/internal/ld/go.go       |  2 +-
 src/cmd/link/internal/ld/link.go     |  3 +++
 src/cmd/link/internal/ld/main.go     |  5 +++++
 src/cmd/link/internal/sym/symbol.go  | 21 ++++++++++-----------
 5 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index ce0fe1f7a13..540f4068cb8 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/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)
 }
 
diff --git a/src/cmd/link/internal/ld/go.go b/src/cmd/link/internal/ld/go.go
index 8d50332c7c6..eb6c2ccc831 100644
--- a/src/cmd/link/internal/ld/go.go
+++ b/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)
 				}
diff --git a/src/cmd/link/internal/ld/link.go b/src/cmd/link/internal/ld/link.go
index 2e66cf857c3..bf575443578 100644
--- a/src/cmd/link/internal/ld/link.go
+++ b/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 {
diff --git a/src/cmd/link/internal/ld/main.go b/src/cmd/link/internal/ld/main.go
index d7929d59fdd..23462f1154b 100644
--- a/src/cmd/link/internal/ld/main.go
+++ b/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
diff --git a/src/cmd/link/internal/sym/symbol.go b/src/cmd/link/internal/sym/symbol.go
index b3ff6c4e190..8893dcf0d69 100644
--- a/src/cmd/link/internal/sym/symbol.go
+++ b/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