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>

src/runtime/type.go

@@ -6,10 +6,7 @@
 
 package runtime
 
-import (
-	"runtime/internal/sys"
-	"unsafe"
-)
+import "unsafe"
 
 // tflag is documented in ../reflect/type.go.
 type tflag uint8
@@ -151,6 +148,33 @@ var reflectOffs struct {
 	minv map[unsafe.Pointer]int32
 }
 
+func resolveNameOff(ptrInModule unsafe.Pointer, off nameOff) name {
+	if off == 0 {
+		return name{}
+	}
+	base := uintptr(ptrInModule)
+	var md *moduledata
+	for next := &firstmoduledata; next != nil; next = next.next {
+		if base >= next.types && base < next.etypes {
+			md = next
+			break
+		}
+	}
+	if md == nil {
+		println("runtime: nameOff", hex(off), "base", hex(base), "not in ranges:")
+		for next := &firstmoduledata; next != nil; next = next.next {
+			println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
+		}
+		throw("runtime: name offset base pointer out of range")
+	}
+	res := md.types + uintptr(off)
+	if res > md.etypes {
+		println("runtime: nameOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
+		throw("runtime: name offset out of range")
+	}
+	return name{(*byte)(unsafe.Pointer(res))}
+}
+
 func (t *_type) typeOff(off typeOff) *_type {
 	if off == 0 {
 		return nil
@@ -240,6 +264,7 @@ func (t *functype) dotdotdot() bool {
 	return t.outCount&(1<<15) != 0
 }
 
+type nameOff int32
 type typeOff int32
 type textOff int32
 
@@ -251,7 +276,7 @@ type method struct {
 }
 
 type uncommontype struct {
-	pkgpath *string
+	pkgpath name
 	mcount  uint16 // number of methods
 	moff    uint16 // offset from this uncommontype to [mcount]method
 }
@@ -263,7 +288,7 @@ type imethod struct {
 
 type interfacetype struct {
 	typ     _type
-	pkgpath *string
+	pkgpath name
 	mhdr    []imethod
 }
 
@@ -319,7 +344,7 @@ type structfield struct {
 
 type structtype struct {
 	typ     _type
-	pkgPath *string
+	pkgPath name
 	fields  []structfield
 }
 
@@ -350,6 +375,9 @@ func (n *name) tagLen() int {
 }
 
 func (n *name) name() (s string) {
+	if n.bytes == nil {
+		return ""
+	}
 	nl := n.nameLen()
 	if nl == 0 {
 		return ""
@@ -372,16 +400,18 @@ func (n *name) tag() (s string) {
 	return s
 }
 
-func (n *name) pkgPath() *string {
-	if *n.data(0)&(1<<2) == 0 {
-		return nil
+func (n *name) pkgPath() string {
+	if n.bytes == nil || *n.data(0)&(1<<2) == 0 {
+		return ""
 	}
 	off := 3 + n.nameLen()
 	if tl := n.tagLen(); tl > 0 {
 		off += 2 + tl
 	}
-	off = int(round(uintptr(off), sys.PtrSize))
-	return *(**string)(unsafe.Pointer(n.data(off)))
+	var nameOff nameOff
+	copy((*[4]byte)(unsafe.Pointer(&nameOff))[:], (*[4]byte)(unsafe.Pointer(n.data(off)))[:])
+	pkgPathName := resolveNameOff(unsafe.Pointer(n.bytes), nameOff)
+	return pkgPathName.name()
 }
 
 // typelinksinit scans the types from extra modules and builds the
@@ -466,7 +496,7 @@ func typesEqual(t, v *_type) bool {
 		if ut == nil || uv == nil {
 			return false
 		}
-		if !pkgPathEqual(ut.pkgpath, uv.pkgpath) {
+		if ut.pkgpath.name() != uv.pkgpath.name() {
 			return false
 		}
 	}
@@ -506,7 +536,7 @@ func typesEqual(t, v *_type) bool {
 	case kindInterface:
 		it := (*interfacetype)(unsafe.Pointer(t))
 		iv := (*interfacetype)(unsafe.Pointer(v))
-		if !pkgPathEqual(it.pkgpath, iv.pkgpath) {
+		if it.pkgpath.name() != iv.pkgpath.name() {
 			return false
 		}
 		if len(it.mhdr) != len(iv.mhdr) {
@@ -518,7 +548,7 @@ func typesEqual(t, v *_type) bool {
 			if tm.name.name() != vm.name.name() {
 				return false
 			}
-			if !pkgPathEqual(tm.name.pkgPath(), vm.name.pkgPath()) {
+			if tm.name.pkgPath() != vm.name.pkgPath() {
 				return false
 			}
 			if !typesEqual(tm._type, vm._type) {
@@ -550,7 +580,7 @@ func typesEqual(t, v *_type) bool {
 			if tf.name.name() != vf.name.name() {
 				return false
 			}
-			if !pkgPathEqual(tf.name.pkgPath(), vf.name.pkgPath()) {
+			if tf.name.pkgPath() != vf.name.pkgPath() {
 				return false
 			}
 			if !typesEqual(tf.typ, vf.typ) {
@@ -570,13 +600,3 @@ func typesEqual(t, v *_type) bool {
 		return false
 	}
 }
-
-func pkgPathEqual(p, q *string) bool {
-	if p == q {
-		return true
-	}
-	if p == nil || q == nil {
-		return false
-	}
-	return *p == *q
-}