[dev.link] cmd/compile, cmd/link: generate itablink at link time

Currently, at compile time, for each itab symbol, we create an
"itablink" symbol which holds solely the address of the itab
symbol. At link time, all the itablink symbols are grouped
together to form the itablinks slice.

This CL removes the itablink symbols, and directly generate the
itablinks slice in the linker. This removes a number of symbols,
which are dupOK and generally have long names. And also removes
a special handling of itablink symbols in the deadcode pass which
iterates through all symbols.

Change-Id: I475c3c8899e9fbeec9abc7647b1e4a69aa5c3c5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/245901
Reviewed-by: Jeremy Faller <jeremy@golang.org>

9 files changed, 49 insertions, 63 deletions

diff --git a/src/cmd/compile/internal/gc/reflect.go b/src/cmd/compile/internal/gc/reflect.go
index ce4838ce80..59b00168e0 100644
--- a/src/cmd/compile/internal/gc/reflect.go
+++ b/src/cmd/compile/internal/gc/reflect.go
@@ -1560,9 +1560,6 @@ func dumptabs() {
 		// Nothing writes static itabs, so they are read only.
 		ggloblsym(i.lsym, int32(o), int16(obj.DUPOK|obj.RODATA))
 		i.lsym.Set(obj.AttrContentAddressable, true)
-		ilink := itablinkpkg.Lookup(i.t.ShortString() + "," + i.itype.ShortString()).Linksym()
-		dsymptr(ilink, 0, i.lsym, 0)
-		ggloblsym(ilink, int32(Widthptr), int16(obj.DUPOK|obj.RODATA))
 	}
 
 	// process ptabs
diff --git a/src/cmd/internal/goobj2/objfile.go b/src/cmd/internal/goobj2/objfile.go
index c3d00122e7..6f0df86b06 100644
--- a/src/cmd/internal/goobj2/objfile.go
+++ b/src/cmd/internal/goobj2/objfile.go
@@ -292,6 +292,7 @@ const (
 // Sym.Flag2
 const (
 	SymFlagUsedInIface = 1 << iota
+	SymFlagItab
 )
 
 // Returns the length of the name of the symbol.
@@ -321,6 +322,7 @@ func (s *Sym) ReflectMethod() bool { return s.Flag()&SymFlagReflectMethod != 0 }
 func (s *Sym) IsGoType() bool      { return s.Flag()&SymFlagGoType != 0 }
 func (s *Sym) TopFrame() bool      { return s.Flag()&SymFlagTopFrame != 0 }
 func (s *Sym) UsedInIface() bool   { return s.Flag2()&SymFlagUsedInIface != 0 }
+func (s *Sym) IsItab() bool        { return s.Flag2()&SymFlagItab != 0 }
 
 func (s *Sym) SetName(x string, w *Writer) {
 	binary.LittleEndian.PutUint32(s[:], uint32(len(x)))
diff --git a/src/cmd/internal/obj/objfile2.go b/src/cmd/internal/obj/objfile2.go
index 74f4fc63ba..17021d9294 100644
--- a/src/cmd/internal/obj/objfile2.go
+++ b/src/cmd/internal/obj/objfile2.go
@@ -300,6 +300,9 @@ func (w *writer) Sym(s *LSym) {
 	if s.UsedInIface() {
 		flag2 |= goobj2.SymFlagUsedInIface
 	}
+	if strings.HasPrefix(s.Name, "go.itab.") && s.Type == objabi.SRODATA {
+		flag2 |= goobj2.SymFlagItab
+	}
 	name := s.Name
 	if strings.HasPrefix(name, "gofile..") {
 		name = filepath.ToSlash(name)
diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index 9bc0f021b1..39f65364b7 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -1904,14 +1904,14 @@ func (state *dodataState) allocateDataSections(ctxt *Link) {
 	sect.Length = uint64(state.datsize) - sect.Vaddr
 
 	/* itablink */
-	sect = state.allocateNamedSectionAndAssignSyms(seg, genrelrosecname(".itablink"), sym.SITABLINK, sym.Sxxx, relroSecPerm)
-	ldr.SetSymSect(ldr.LookupOrCreateSym("runtime.itablink", 0), sect)
-	ldr.SetSymSect(ldr.LookupOrCreateSym("runtime.eitablink", 0), sect)
-	if ctxt.HeadType == objabi.Haix {
-		// Store .itablink size because its symbols are wrapped
-		// under an outer symbol: runtime.itablink.
-		xcoffUpdateOuterSize(ctxt, int64(sect.Length), sym.SITABLINK)
-	}
+	sect = state.allocateNamedDataSection(seg, genrelrosecname(".itablink"), []sym.SymKind{sym.SITABLINK}, relroSecPerm)
+
+	itablink := ldr.CreateSymForUpdate("runtime.itablink", 0)
+	ldr.SetSymSect(itablink.Sym(), sect)
+	itablink.SetType(sym.SRODATA)
+	state.datsize += itablink.Size()
+	state.checkdatsize(sym.SITABLINK)
+	sect.Length = uint64(state.datsize) - sect.Vaddr
 
 	/* gosymtab */
 	sect = state.allocateNamedSectionAndAssignSyms(seg, genrelrosecname(".gosymtab"), sym.SSYMTAB, sym.SRODATA, relroSecPerm)
@@ -2414,11 +2414,10 @@ func (ctxt *Link) address() []*sym.Segment {
 
 	ldr := ctxt.loader
 	var (
-		rodata   = ldr.SymSect(ldr.LookupOrCreateSym("runtime.rodata", 0))
-		itablink = ldr.SymSect(ldr.LookupOrCreateSym("runtime.itablink", 0))
-		symtab   = ldr.SymSect(ldr.LookupOrCreateSym("runtime.symtab", 0))
-		pclntab  = ldr.SymSect(ldr.LookupOrCreateSym("runtime.pclntab", 0))
-		types    = ldr.SymSect(ldr.LookupOrCreateSym("runtime.types", 0))
+		rodata  = ldr.SymSect(ldr.LookupOrCreateSym("runtime.rodata", 0))
+		symtab  = ldr.SymSect(ldr.LookupOrCreateSym("runtime.symtab", 0))
+		pclntab = ldr.SymSect(ldr.LookupOrCreateSym("runtime.pclntab", 0))
+		types   = ldr.SymSect(ldr.LookupOrCreateSym("runtime.types", 0))
 	)
 
 	for _, s := range ctxt.datap {
@@ -2474,8 +2473,6 @@ func (ctxt *Link) address() []*sym.Segment {
 	ctxt.xdefine("runtime.erodata", sym.SRODATA, int64(rodata.Vaddr+rodata.Length))
 	ctxt.xdefine("runtime.types", sym.SRODATA, int64(types.Vaddr))
 	ctxt.xdefine("runtime.etypes", sym.SRODATA, int64(types.Vaddr+types.Length))
-	ctxt.xdefine("runtime.itablink", sym.SRODATA, int64(itablink.Vaddr))
-	ctxt.xdefine("runtime.eitablink", sym.SRODATA, int64(itablink.Vaddr+itablink.Length))
 
 	s := ldr.Lookup("runtime.gcdata", 0)
 	ldr.SetAttrLocal(s, true)
diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index 53ab0db9d0..a169772dde 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/src/cmd/link/internal/ld/deadcode.go
@@ -306,22 +306,6 @@ func deadcode(ctxt *Link) {
 		}
 		d.flood()
 	}
-
-	n := ldr.NSym()
-
-	if ctxt.BuildMode != BuildModeShared {
-		// Keep a itablink if the symbol it points at is being kept.
-		// (When BuildModeShared, always keep itablinks.)
-		for i := 1; i < n; i++ {
-			s := loader.Sym(i)
-			if ldr.IsItabLink(s) {
-				relocs := ldr.Relocs(s)
-				if relocs.Count() > 0 && ldr.AttrReachable(relocs.At(0).Sym()) {
-					ldr.SetAttrReachable(s, true)
-				}
-			}
-		}
-	}
 }
 
 // methodsig is a typed method signature (name + type).
diff --git a/src/cmd/link/internal/ld/symtab.go b/src/cmd/link/internal/ld/symtab.go
index 8d2cbd8133..97d7a22537 100644
--- a/src/cmd/link/internal/ld/symtab.go
+++ b/src/cmd/link/internal/ld/symtab.go
@@ -416,8 +416,6 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 
 	// Define these so that they'll get put into the symbol table.
 	// data.c:/^address will provide the actual values.
-	ctxt.xdefine("runtime.itablink", sym.SRODATA, 0)
-	ctxt.xdefine("runtime.eitablink", sym.SRODATA, 0)
 	ctxt.xdefine("runtime.rodata", sym.SRODATA, 0)
 	ctxt.xdefine("runtime.erodata", sym.SRODATA, 0)
 	ctxt.xdefine("runtime.types", sym.SRODATA, 0)
@@ -489,16 +487,11 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 		}
 	}
 
-	symitablink := ldr.CreateSymForUpdate("runtime.itablink", 0)
-	symitablink.SetType(sym.SITABLINK)
-
 	symt := ldr.CreateSymForUpdate("runtime.symtab", 0)
 	symt.SetType(sym.SSYMTAB)
 	symt.SetSize(0)
 	symt.SetLocal(true)
 
-	nitablinks := 0
-
 	// assign specific types so that they sort together.
 	// within a type they sort by size, so the .* symbols
 	// just defined above will be first.
@@ -536,12 +529,6 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 			// names, as they can be referred to by a section offset.
 			symGroupType[s] = sym.STYPERELRO
 
-		case strings.HasPrefix(name, "go.itablink."):
-			nitablinks++
-			symGroupType[s] = sym.SITABLINK
-			ldr.SetAttrNotInSymbolTable(s, true)
-			ldr.SetCarrierSym(s, symitablink.Sym())
-
 		case strings.HasPrefix(name, "go.string."):
 			symGroupType[s] = sym.SGOSTRING
 			ldr.SetAttrNotInSymbolTable(s, true)
@@ -672,7 +659,9 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 	moduledata.AddUint(ctxt.Arch, ntypelinks)
 	moduledata.AddUint(ctxt.Arch, ntypelinks)
 	// The itablinks slice
-	moduledata.AddAddr(ctxt.Arch, symitablink.Sym())
+	itablinkSym := ldr.Lookup("runtime.itablink", 0)
+	nitablinks := uint64(ldr.SymSize(itablinkSym)) / uint64(ctxt.Arch.PtrSize)
+	moduledata.AddAddr(ctxt.Arch, itablinkSym)
 	moduledata.AddUint(ctxt.Arch, uint64(nitablinks))
 	moduledata.AddUint(ctxt.Arch, uint64(nitablinks))
 	// The ptab slice
diff --git a/src/cmd/link/internal/ld/typelink.go b/src/cmd/link/internal/ld/typelink.go
index 2476efe75c..5eca6e0181 100644
--- a/src/cmd/link/internal/ld/typelink.go
+++ b/src/cmd/link/internal/ld/typelink.go
@@ -28,9 +28,15 @@ func (s byTypeStr) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 func (ctxt *Link) typelink() {
 	ldr := ctxt.loader
 	typelinks := byTypeStr{}
+	var itabs []loader.Sym
 	for s := loader.Sym(1); s < loader.Sym(ldr.NSym()); s++ {
-		if ldr.AttrReachable(s) && ldr.IsTypelink(s) {
+		if !ldr.AttrReachable(s) {
+			continue
+		}
+		if ldr.IsTypelink(s) {
 			typelinks = append(typelinks, typelinkSortKey{decodetypeStr(ldr, ctxt.Arch, s), s})
+		} else if ldr.IsItab(s) {
+			itabs = append(itabs, s)
 		}
 	}
 	sort.Sort(typelinks)
@@ -48,4 +54,19 @@ func (ctxt *Link) typelink() {
 		r.SetSiz(4)
 		r.SetType(objabi.R_ADDROFF)
 	}
+
+	ptrsize := ctxt.Arch.PtrSize
+	il := ldr.CreateSymForUpdate("runtime.itablink", 0)
+	il.SetType(sym.SITABLINK)
+	ldr.SetAttrLocal(il.Sym(), true)
+	il.SetSize(int64(ptrsize * len(itabs)))
+	il.Grow(il.Size())
+	relocs = il.AddRelocs(len(itabs))
+	for i, s := range itabs {
+		r := relocs.At(i)
+		r.SetSym(s)
+		r.SetOff(int32(i * ptrsize))
+		r.SetSiz(uint8(ptrsize))
+		r.SetType(objabi.R_ADDR)
+	}
 }
diff --git a/src/cmd/link/internal/ld/xcoff.go b/src/cmd/link/internal/ld/xcoff.go
index cc299cfb6b..7eb7f94ca4 100644
--- a/src/cmd/link/internal/ld/xcoff.go
+++ b/src/cmd/link/internal/ld/xcoff.go
@@ -605,9 +605,6 @@ func xcoffUpdateOuterSize(ctxt *Link, size int64, stype sym.SymKind) {
 		outerSymSize["go.funcrel.*"] = size
 	case sym.SGCBITS:
 		outerSymSize["runtime.gcbits.*"] = size
-	case sym.SITABLINK:
-		outerSymSize["runtime.itablink"] = size
-
 	}
 }
 
diff --git a/src/cmd/link/internal/loader/loader.go b/src/cmd/link/internal/loader/loader.go
index 4da77c6d32..d56c748f9c 100644
--- a/src/cmd/link/internal/loader/loader.go
+++ b/src/cmd/link/internal/loader/loader.go
@@ -217,8 +217,7 @@ type Loader struct {
 
 	align []uint8 // symbol 2^N alignment, indexed by global index
 
-	itablink         map[Sym]struct{} // itablink[j] defined if j is go.itablink.*
-	deferReturnTramp map[Sym]bool     // whether the symbol is a trampoline of a deferreturn call
+	deferReturnTramp map[Sym]bool // whether the symbol is a trampoline of a deferreturn call
 
 	objByPkg map[string]*oReader // map package path to its Go object reader
 
@@ -352,7 +351,6 @@ func NewLoader(flags uint32, elfsetstring elfsetstringFunc, reporter *ErrorRepor
 		attrCgoExportDynamic: make(map[Sym]struct{}),
 		attrCgoExportStatic:  make(map[Sym]struct{}),
 		generatedSyms:        make(map[Sym]struct{}),
-		itablink:             make(map[Sym]struct{}),
 		deferReturnTramp:     make(map[Sym]bool),
 		extStaticSyms:        make(map[nameVer]Sym),
 		builtinSyms:          make([]Sym, nbuiltin),
@@ -1163,12 +1161,13 @@ func (l *Loader) IsTypelink(i Sym) bool {
 	return l.SymAttr(i)&goobj2.SymFlagTypelink != 0
 }
 
-// Returns whether this is a "go.itablink.*" symbol.
-func (l *Loader) IsItabLink(i Sym) bool {
-	if _, ok := l.itablink[i]; ok {
-		return true
+// Returns whether this symbol is an itab symbol.
+func (l *Loader) IsItab(i Sym) bool {
+	if l.IsExternal(i) {
+		return false
 	}
-	return false
+	r, li := l.toLocal(i)
+	return r.Sym(li).IsItab()
 }
 
 // Return whether this is a trampoline of a deferreturn call.
@@ -2139,9 +2138,6 @@ func (st *loadState) preloadSyms(r *oReader, kind int) {
 		if osym.UsedInIface() {
 			l.SetAttrUsedInIface(gi, true)
 		}
-		if strings.HasPrefix(name, "go.itablink.") {
-			l.itablink[gi] = struct{}{}
-		}
 		if strings.HasPrefix(name, "runtime.") ||
 			(loadingRuntimePkg && strings.HasPrefix(name, "type.")) {
 			if bi := goobj2.BuiltinIdx(name, v); bi != -1 {