cmd/compile, etc: use name for type pkgPath

By replacing the *string used to represent pkgPath with a
reflect.name everywhere, the embedded *string for package paths
inside the reflect.name can be replaced by an offset, nameOff.
This reduces the number of pointers in the type information.

This also moves all reflect.name types into the same section, making
it possible to use nameOff more widely in later CLs.

No significant binary size change for normal binaries, but:

linux/amd64 PIE:
	cmd/go: -440KB (3.7%)
	jujud:  -2.6MB (3.2%)

For #6853.

Change-Id: I3890b132a784a1090b1b72b32febfe0bea77eaee
Reviewed-on: https://go-review.googlesource.com/21395
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 82 insertions, 50 deletions

diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index 5df49b56d6..8411d2d0ac 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -20,7 +20,7 @@ const (
 type Pkg struct {
 	Name     string // package name, e.g. "sys"
 	Path     string // string literal used in import statement, e.g. "runtime/internal/sys"
-	Pathsym  *Sym
+	Pathsym  *obj.LSym
 	Prefix   string // escaped path for use in symbol table
 	Imported bool   // export data of this package was parsed
 	Exported bool   // import line written in export data
diff --git a/src/cmd/compile/internal/gc/reflect.go b/src/cmd/compile/internal/gc/reflect.go
index 2bd50b4665..70a75f9324 100644
--- a/src/cmd/compile/internal/gc/reflect.go
+++ b/src/cmd/compile/internal/gc/reflect.go
@@ -412,8 +412,6 @@ func imethods(t *Type) []*Sig {
 	return methods
 }
 
-var dimportpath_gopkg *Pkg
-
 func dimportpath(p *Pkg) {
 	if p.Pathsym != nil {
 		return
@@ -426,27 +424,18 @@ func dimportpath(p *Pkg) {
 		return
 	}
 
-	if dimportpath_gopkg == nil {
-		dimportpath_gopkg = mkpkg("go")
-		dimportpath_gopkg.Name = "go"
-	}
-
-	nam := "importpath." + p.Prefix + "."
-
-	n := Nod(ONAME, nil, nil)
-	n.Sym = Pkglookup(nam, dimportpath_gopkg)
-
-	n.Class = PEXTERN
-	n.Xoffset = 0
-	p.Pathsym = n.Sym
-
+	var str string
 	if p == localpkg {
 		// Note: myimportpath != "", or else dgopkgpath won't call dimportpath.
-		gdatastring(n, myimportpath)
+		str = myimportpath
 	} else {
-		gdatastring(n, p.Path)
+		str = p.Path
 	}
-	ggloblsym(n.Sym, int32(Types[TSTRING].Width), obj.DUPOK|obj.RODATA)
+
+	s := obj.Linklookup(Ctxt, "go.importpath."+p.Prefix+".", 0)
+	ot := dnameData(s, 0, str, "", nil, false)
+	ggloblLSym(s, int32(ot), obj.DUPOK|obj.RODATA)
+	p.Pathsym = s
 }
 
 func dgopkgpath(s *Sym, ot int, pkg *Pkg) int {
@@ -469,7 +458,23 @@ func dgopkgpathLSym(s *obj.LSym, ot int, pkg *Pkg) int {
 	}
 
 	dimportpath(pkg)
-	return dsymptrLSym(s, ot, Linksym(pkg.Pathsym), 0)
+	return dsymptrLSym(s, ot, pkg.Pathsym, 0)
+}
+
+// dgopkgpathOffLSym writes an offset relocation in s at offset ot to the pkg path symbol.
+func dgopkgpathOffLSym(s *obj.LSym, ot int, pkg *Pkg) int {
+	if pkg == localpkg && myimportpath == "" {
+		// If we don't know the full import path of the package being compiled
+		// (i.e. -p was not passed on the compiler command line), emit a reference to
+		// go.importpath.""., which the linker will rewrite using the correct import path.
+		// Every package that imports this one directly defines the symbol.
+		// See also https://groups.google.com/forum/#!topic/golang-dev/myb9s53HxGQ.
+		ns := obj.Linklookup(Ctxt, `go.importpath."".`, 0)
+		return dsymptrOffLSym(s, ot, ns, 0)
+	}
+
+	dimportpath(pkg)
+	return dsymptrOffLSym(s, ot, pkg.Pathsym, 0)
 }
 
 // isExportedField reports whether a struct field is exported.
@@ -495,13 +500,12 @@ func dnameField(s *Sym, ot int, ft *Field) int {
 	if ft.Note != nil {
 		tag = *ft.Note
 	}
-	return dname(s, ot, name, tag, nil, isExportedField(ft))
+	nsym := dname(name, tag, nil, isExportedField(ft))
+	return dsymptrLSym(Linksym(s), ot, nsym, 0)
 }
 
-var dnameCount int
-
-// dname dumps a reflect.name for a struct field or method.
-func dname(s *Sym, ot int, name, tag string, pkg *Pkg, exported bool) int {
+// dnameData writes the contents of a reflect.name into s at offset ot.
+func dnameData(s *obj.LSym, ot int, name, tag string, pkg *Pkg, exported bool) int {
 	if len(name) > 1<<16-1 {
 		Fatalf("name too long: %s", name)
 	}
@@ -534,31 +538,46 @@ func dname(s *Sym, ot int, name, tag string, pkg *Pkg, exported bool) int {
 		copy(tb[2:], tag)
 	}
 
-	// Very few names require a pkgPath *string (only those
-	// defined in a different package than their type). So if
-	// there is no pkgPath, we treat the name contents as string
-	// data that duplicates across packages.
-	var bsym *obj.LSym
+	ot = int(s.WriteBytes(Ctxt, int64(ot), b))
+
+	if pkg != nil {
+		ot = dgopkgpathOffLSym(s, ot, pkg)
+	}
+
+	return ot
+}
+
+var dnameCount int
+
+// dname creates a reflect.name for a struct field or method.
+func dname(name, tag string, pkg *Pkg, exported bool) *obj.LSym {
+	// Write out data as "type.." to signal two things to the
+	// linker, first that when dynamically linking, the symbol
+	// should be moved to a relro section, and second that the
+	// contents should not be decoded as a type.
+	sname := "type..namedata."
 	if pkg == nil {
-		_, bsym = stringsym(string(b))
+		// In the common case, share data with other packages.
+		if name == "" {
+			if exported {
+				sname += "-noname-exported." + tag
+			} else {
+				sname += "-noname-unexported." + tag
+			}
+		} else {
+			sname += name + "." + tag
+		}
 	} else {
-		// Write out data as "type.." to signal two things to the
-		// linker, first that when dynamically linking, the symbol
-		// should be moved to a relro section, and second that the
-		// contents should not be decoded as a type.
-		bsymname := fmt.Sprintf(`type..methodname."".%d`, dnameCount)
+		sname = fmt.Sprintf(`%s"".%d`, sname, dnameCount)
 		dnameCount++
-		bsym = obj.Linklookup(Ctxt, bsymname, 0)
-		bsym.P = b
-		boff := len(b)
-		boff = int(Rnd(int64(boff), int64(Widthptr)))
-		boff = dgopkgpathLSym(bsym, boff, pkg)
-		ggloblLSym(bsym, int32(boff), obj.RODATA|obj.LOCAL)
 	}
-
-	ot = dsymptrLSym(Linksym(s), ot, bsym, 0)
-
-	return ot
+	s := obj.Linklookup(Ctxt, sname, 0)
+	if len(s.P) > 0 {
+		return s
+	}
+	ot := dnameData(s, 0, name, tag, pkg, exported)
+	ggloblLSym(s, int32(ot), obj.DUPOK|obj.RODATA)
+	return s
 }
 
 // dextratype dumps the fields of a runtime.uncommontype.
@@ -627,7 +646,8 @@ func dextratypeData(s *Sym, ot int, t *Type) int {
 		if !exported && a.pkg != typePkg(t) {
 			pkg = a.pkg
 		}
-		ot = dname(s, ot, a.name, "", pkg, exported)
+		nsym := dname(a.name, "", pkg, exported)
+		ot = dsymptrLSym(lsym, ot, nsym, 0)
 		ot = dmethodptrOffLSym(lsym, ot, Linksym(dtypesym(a.mtype)))
 		ot = dmethodptrOffLSym(lsym, ot, Linksym(a.isym))
 		ot = dmethodptrOffLSym(lsym, ot, Linksym(a.tsym))
@@ -1213,7 +1233,8 @@ ok:
 			if !exported && a.pkg != tpkg {
 				pkg = a.pkg
 			}
-			ot = dname(s, ot, a.name, "", pkg, exported)
+			nsym := dname(a.name, "", pkg, exported)
+			ot = dsymptrLSym(Linksym(s), ot, nsym, 0)
 			ot = dsymptr(s, ot, dtypesym(a.type_), 0)
 		}
 
diff --git a/src/cmd/internal/obj/data.go b/src/cmd/internal/obj/data.go
index 546ff37269..d7f0840bc1 100644
--- a/src/cmd/internal/obj/data.go
+++ b/src/cmd/internal/obj/data.go
@@ -75,7 +75,11 @@ func (s *LSym) prepwrite(ctxt *Link, off int64, siz int) {
 	if s.Type == SBSS || s.Type == STLSBSS {
 		ctxt.Diag("cannot supply data for BSS var")
 	}
-	s.Grow(off + int64(siz))
+	l := off + int64(siz)
+	s.Grow(l)
+	if l > s.Size {
+		s.Size = l
+	}
 }
 
 // WriteFloat32 writes f into s at offset off.
@@ -150,6 +154,13 @@ func (s *LSym) WriteString(ctxt *Link, off int64, siz int, str string) {
 	copy(s.P[off:off+int64(siz)], str)
 }
 
+// WriteBytes writes a slice of bytes into s at offset off.
+func (s *LSym) WriteBytes(ctxt *Link, off int64, b []byte) int64 {
+	s.prepwrite(ctxt, off, len(b))
+	copy(s.P[off:], b)
+	return off + int64(len(b))
+}
+
 func Addrel(s *LSym) *Reloc {
 	s.R = append(s.R, Reloc{})
 	return &s.R[len(s.R)-1]