cmd/compile, etc: store method tables as offsets

This CL introduces the typeOff type and a lookup method of the same
name that can turn a typeOff offset into an *rtype.

In a typical Go binary (built with buildmode=exe, pie, c-archive, or
c-shared), there is one moduledata and all typeOff values are offsets
relative to firstmoduledata.types. This makes computing the pointer
cheap in typical programs.

With buildmode=shared (and one day, buildmode=plugin) there are
multiple modules whose relative offset is determined at runtime.
We identify a type in the general case by the pair of the original
*rtype that references it and its typeOff value. We determine
the module from the original pointer, and then use the typeOff from
there to compute the final *rtype.

To ensure there is only one *rtype representing each type, the
runtime initializes a typemap for each module, using any identical
type from an earlier module when resolving that offset. This means
that types computed from an offset match the type mapped by the
pointer dynamic relocations.

A series of followup CLs will replace other *rtype values with typeOff
(and name/*string with nameOff).

For types created at runtime by reflect, type offsets are treated as
global IDs and reference into a reflect offset map kept by the runtime.

darwin/amd64:
	cmd/go:  -57KB (0.6%)
	jujud:  -557KB (0.8%)

linux/amd64 PIE:
	cmd/go: -361KB (3.0%)
	jujud:  -3.5MB (4.2%)

For #6853.

Change-Id: Icf096fd884a0a0cb9f280f46f7a26c70a9006c96
Reviewed-on: https://go-review.googlesource.com/21285
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/type.go

@@ -131,6 +131,92 @@ func (t *_type) name() string {
 	return t._string[i+1:]
 }
 
+// reflectOffs holds type offsets defined at run time by the reflect package.
+//
+// When a type is defined at run time, its *rtype data lives on the heap.
+// There are a wide range of possible addresses the heap may use, that
+// may not be representable as a 32-bit offset. Moreover the GC may
+// one day start moving heap memory, in which case there is no stable
+// offset that can be defined.
+//
+// To provide stable offsets, we add pin *rtype objects in a global map
+// and treat the offset as an identifier. We use negative offsets that
+// do not overlap with any compile-time module offsets.
+//
+// Entries are created by reflect.addReflectOff.
+var reflectOffs struct {
+	lock mutex
+	next int32
+	m    map[int32]unsafe.Pointer
+	minv map[unsafe.Pointer]int32
+}
+
+func (t *_type) typeOff(off typeOff) *_type {
+	if off == 0 {
+		return nil
+	}
+	base := uintptr(unsafe.Pointer(t))
+	var md *moduledata
+	for next := &firstmoduledata; next != nil; next = next.next {
+		if base >= next.types && base < next.etypes {
+			md = next
+			break
+		}
+	}
+	if md == nil {
+		lock(&reflectOffs.lock)
+		res := reflectOffs.m[int32(off)]
+		unlock(&reflectOffs.lock)
+		if res == nil {
+			println("runtime: typeOff", 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: type offset base pointer out of range")
+		}
+		return (*_type)(res)
+	}
+	if t := md.typemap[off]; t != nil {
+		return t
+	}
+	res := md.types + uintptr(off)
+	if res > md.etypes {
+		println("runtime: typeOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
+		throw("runtime: type offset out of range")
+	}
+	return (*_type)(unsafe.Pointer(res))
+}
+
+func (t *_type) textOff(off textOff) unsafe.Pointer {
+	base := uintptr(unsafe.Pointer(t))
+	var md *moduledata
+	for next := &firstmoduledata; next != nil; next = next.next {
+		if base >= next.types && base < next.etypes {
+			md = next
+			break
+		}
+	}
+	if md == nil {
+		lock(&reflectOffs.lock)
+		res := reflectOffs.m[int32(off)]
+		unlock(&reflectOffs.lock)
+		if res == nil {
+			println("runtime: textOff", 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: text offset base pointer out of range")
+		}
+		return res
+	}
+	res := md.text + uintptr(off)
+	if res > md.etext {
+		println("runtime: textOff", hex(off), "out of range", hex(md.text), "-", hex(md.etext))
+		throw("runtime: text offset out of range")
+	}
+	return unsafe.Pointer(res)
+}
+
 func (t *functype) in() []*_type {
 	// See funcType in reflect/type.go for details on data layout.
 	uadd := uintptr(unsafe.Sizeof(functype{}))
@@ -154,16 +240,20 @@ func (t *functype) dotdotdot() bool {
 	return t.outCount&(1<<15) != 0
 }
 
+type typeOff int32
+type textOff int32
+
 type method struct {
 	name name
-	mtyp *_type
-	ifn  unsafe.Pointer
-	tfn  unsafe.Pointer
+	mtyp typeOff
+	ifn  textOff
+	tfn  textOff
 }
 
 type uncommontype struct {
 	pkgpath *string
-	mhdr    []method
+	mcount  uint16 // number of methods
+	moff    uint16 // offset from this uncommontype to [mcount]method
 }
 
 type imethod struct {
@@ -270,6 +360,18 @@ func (n *name) name() (s string) {
 	return s
 }
 
+func (n *name) tag() (s string) {
+	tl := n.tagLen()
+	if tl == 0 {
+		return ""
+	}
+	nl := n.nameLen()
+	hdr := (*stringStruct)(unsafe.Pointer(&s))
+	hdr.str = unsafe.Pointer(n.data(3 + nl + 2))
+	hdr.len = tl
+	return s
+}
+
 func (n *name) pkgPath() *string {
 	if *n.data(0)&(1<<2) == 0 {
 		return nil
@@ -281,3 +383,200 @@ func (n *name) pkgPath() *string {
 	off = int(round(uintptr(off), sys.PtrSize))
 	return *(**string)(unsafe.Pointer(n.data(off)))
 }
+
+// typelinksinit scans the types from extra modules and builds the
+// moduledata typemap used to de-duplicate type pointers.
+func typelinksinit() {
+	if firstmoduledata.next == nil {
+		return
+	}
+	typehash := make(map[uint32][]*_type)
+
+	modules := []*moduledata{}
+	for md := &firstmoduledata; md != nil; md = md.next {
+		modules = append(modules, md)
+	}
+	prev, modules := modules[len(modules)-1], modules[:len(modules)-1]
+	for len(modules) > 0 {
+		// Collect types from the previous module into typehash.
+	collect:
+		for _, tl := range prev.typelinks {
+			var t *_type
+			if prev.typemap == nil {
+				t = (*_type)(unsafe.Pointer(prev.types + uintptr(tl)))
+			} else {
+				t = prev.typemap[typeOff(tl)]
+			}
+			// Add to typehash if not seen before.
+			tlist := typehash[t.hash]
+			for _, tcur := range tlist {
+				if tcur == t {
+					continue collect
+				}
+			}
+			typehash[t.hash] = append(tlist, t)
+		}
+
+		// If any of this module's typelinks match a type from a
+		// prior module, prefer that prior type by adding the offset
+		// to this module's typemap.
+		md := modules[len(modules)-1]
+		md.typemap = make(map[typeOff]*_type, len(md.typelinks))
+		for _, tl := range md.typelinks {
+			t := (*_type)(unsafe.Pointer(md.types + uintptr(tl)))
+			for _, candidate := range typehash[t.hash] {
+				if typesEqual(t, candidate) {
+					t = candidate
+					break
+				}
+			}
+			md.typemap[typeOff(tl)] = t
+		}
+
+		prev, modules = md, modules[:len(modules)-1]
+	}
+}
+
+// typesEqual reports whether two types are equal.
+//
+// Everywhere in the runtime and reflect packages, it is assumed that
+// there is exactly one *_type per Go type, so that pointer equality
+// can be used to test if types are equal. There is one place that
+// breaks this assumption: buildmode=shared. In this case a type can
+// appear as two different pieces of memory. This is hidden from the
+// runtime and reflect package by the per-module typemap built in
+// typelinksinit. It uses typesEqual to map types from later modules
+// back into earlier ones.
+//
+// Only typelinksinit needs this function.
+func typesEqual(t, v *_type) bool {
+	if t == v {
+		return true
+	}
+	kind := t.kind & kindMask
+	if kind != v.kind&kindMask {
+		return false
+	}
+	if t._string != v._string {
+		return false
+	}
+	ut := t.uncommon()
+	uv := v.uncommon()
+	if ut != nil || uv != nil {
+		if ut == nil || uv == nil {
+			return false
+		}
+		if !pkgPathEqual(ut.pkgpath, uv.pkgpath) {
+			return false
+		}
+	}
+	if kindBool <= kind && kind <= kindComplex128 {
+		return true
+	}
+	switch kind {
+	case kindString, kindUnsafePointer:
+		return true
+	case kindArray:
+		at := (*arraytype)(unsafe.Pointer(t))
+		av := (*arraytype)(unsafe.Pointer(v))
+		return typesEqual(at.elem, av.elem) && at.len == av.len
+	case kindChan:
+		ct := (*chantype)(unsafe.Pointer(t))
+		cv := (*chantype)(unsafe.Pointer(v))
+		return ct.dir == cv.dir && typesEqual(ct.elem, cv.elem)
+	case kindFunc:
+		ft := (*functype)(unsafe.Pointer(t))
+		fv := (*functype)(unsafe.Pointer(v))
+		if ft.outCount != fv.outCount || ft.inCount != fv.inCount {
+			return false
+		}
+		tin, vin := ft.in(), fv.in()
+		for i := 0; i < len(tin); i++ {
+			if !typesEqual(tin[i], vin[i]) {
+				return false
+			}
+		}
+		tout, vout := ft.out(), fv.out()
+		for i := 0; i < len(tout); i++ {
+			if !typesEqual(tout[i], vout[i]) {
+				return false
+			}
+		}
+		return true
+	case kindInterface:
+		it := (*interfacetype)(unsafe.Pointer(t))
+		iv := (*interfacetype)(unsafe.Pointer(v))
+		if !pkgPathEqual(it.pkgpath, iv.pkgpath) {
+			return false
+		}
+		if len(it.mhdr) != len(iv.mhdr) {
+			return false
+		}
+		for i := range it.mhdr {
+			tm := &it.mhdr[i]
+			vm := &iv.mhdr[i]
+			if tm.name.name() != vm.name.name() {
+				return false
+			}
+			if !pkgPathEqual(tm.name.pkgPath(), vm.name.pkgPath()) {
+				return false
+			}
+			if !typesEqual(tm._type, vm._type) {
+				return false
+			}
+		}
+		return true
+	case kindMap:
+		mt := (*maptype)(unsafe.Pointer(t))
+		mv := (*maptype)(unsafe.Pointer(v))
+		return typesEqual(mt.key, mv.key) && typesEqual(mt.elem, mv.elem)
+	case kindPtr:
+		pt := (*ptrtype)(unsafe.Pointer(t))
+		pv := (*ptrtype)(unsafe.Pointer(v))
+		return typesEqual(pt.elem, pv.elem)
+	case kindSlice:
+		st := (*slicetype)(unsafe.Pointer(t))
+		sv := (*slicetype)(unsafe.Pointer(v))
+		return typesEqual(st.elem, sv.elem)
+	case kindStruct:
+		st := (*structtype)(unsafe.Pointer(t))
+		sv := (*structtype)(unsafe.Pointer(v))
+		if len(st.fields) != len(sv.fields) {
+			return false
+		}
+		for i := range st.fields {
+			tf := &st.fields[i]
+			vf := &sv.fields[i]
+			if tf.name.name() != vf.name.name() {
+				return false
+			}
+			if !pkgPathEqual(tf.name.pkgPath(), vf.name.pkgPath()) {
+				return false
+			}
+			if !typesEqual(tf.typ, vf.typ) {
+				return false
+			}
+			if tf.name.tag() != vf.name.tag() {
+				return false
+			}
+			if tf.offset != vf.offset {
+				return false
+			}
+		}
+		return true
+	default:
+		println("runtime: impossible type kind", kind)
+		throw("runtime: impossible type kind")
+		return false
+	}
+}
+
+func pkgPathEqual(p, q *string) bool {
+	if p == q {
+		return true
+	}
+	if p == nil || q == nil {
+		return false
+	}
+	return *p == *q
+}