cmd/compile: optimize non-empty-interface type conversions

When doing i.(T) for non-empty-interface i and concrete type T,
there's no need to read the type out of the itab. Just compare the
itab to the itab we expect for that interface/type pair.

Also optimize type switches by putting the type hash of the
concrete type in the itab. That way we don't need to load the
type pointer out of the itab.

Update #18492

Change-Id: I49e280a21e5687e771db5b8a56b685291ac168ce
Reviewed-on: https://go-review.googlesource.com/34810
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/builtin.go

@@ -54,7 +54,8 @@ var runtimeDecls = [...]struct {
 	{"assertE2I2", funcTag, 54},
 	{"assertI2I", funcTag, 52},
 	{"assertI2I2", funcTag, 54},
-	{"panicdottype", funcTag, 55},
+	{"panicdottypeE", funcTag, 55},
+	{"panicdottypeI", funcTag, 55},
 	{"panicnildottype", funcTag, 56},
 	{"ifaceeq", funcTag, 57},
 	{"efaceeq", funcTag, 57},

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -67,7 +67,8 @@ func assertE2I(typ *byte, iface any) (ret any)
 func assertE2I2(typ *byte, iface any) (ret any, b bool)
 func assertI2I(typ *byte, iface any) (ret any)
 func assertI2I2(typ *byte, iface any) (ret any, b bool)
-func panicdottype(have, want, iface *byte)
+func panicdottypeE(have, want, iface *byte)
+func panicdottypeI(have, want, iface *byte)
 func panicnildottype(want *byte)
 
 func ifaceeq(i1 any, i2 any) (ret bool)

src/cmd/compile/internal/gc/go.go

@@ -376,7 +376,8 @@ var (
 	panicslice,
 	panicdivide,
 	growslice,
-	panicdottype,
+	panicdottypeE,
+	panicdottypeI,
 	panicnildottype,
 	assertE2I,
 	assertE2I2,

src/cmd/compile/internal/gc/pgen.go

@@ -306,7 +306,8 @@ func compile(fn *Node) {
 		panicslice = Sysfunc("panicslice")
 		panicdivide = Sysfunc("panicdivide")
 		growslice = Sysfunc("growslice")
-		panicdottype = Sysfunc("panicdottype")
+		panicdottypeE = Sysfunc("panicdottypeE")
+		panicdottypeI = Sysfunc("panicdottypeI")
 		panicnildottype = Sysfunc("panicnildottype")
 		assertE2I = Sysfunc("assertE2I")
 		assertE2I2 = Sysfunc("assertE2I2")

src/cmd/compile/internal/gc/reflect.go

@@ -1411,13 +1411,17 @@ func dumptypestructs() {
 		//   inter  *interfacetype
 		//   _type  *_type
 		//   link   *itab
-		//   bad    int32
-		//   unused int32
+		//   hash   uint32
+		//   bad    bool
+		//   inhash bool
+		//   unused [2]byte
 		//   fun    [1]uintptr // variable sized
 		// }
 		o := dsymptr(i.sym, 0, dtypesym(i.itype), 0)
 		o = dsymptr(i.sym, o, dtypesym(i.t), 0)
-		o += Widthptr + 8                      // skip link/bad/inhash fields
+		o += Widthptr                          // skip link field
+		o = duint32(i.sym, o, typehash(i.t))   // copy of type hash
+		o += 4                                 // skip bad/inhash/unused fields
 		o += len(imethods(i.itype)) * Widthptr // skip fun method pointers
 		// at runtime the itab will contain pointers to types, other itabs and
 		// method functions. None are allocated on heap, so we can use obj.NOPTR.

src/cmd/compile/internal/gc/ssa.go

@@ -4013,48 +4013,6 @@ func (s *state) floatToUint(cvttab *f2uCvtTab, n *Node, x *ssa.Value, ft, tt *Ty
 	return s.variable(n, n.Type)
 }
 
-// ifaceType returns the value for the word containing the type.
-// t is the type of the interface expression.
-// v is the corresponding value.
-func (s *state) ifaceType(t *Type, v *ssa.Value) *ssa.Value {
-	byteptr := ptrto(Types[TUINT8]) // type used in runtime prototypes for runtime type (*byte)
-
-	if t.IsEmptyInterface() {
-		// Have eface. The type is the first word in the struct.
-		return s.newValue1(ssa.OpITab, byteptr, v)
-	}
-
-	// Have iface.
-	// The first word in the struct is the itab.
-	// If the itab is nil, return 0.
-	// Otherwise, the second word in the itab is the type.
-
-	tab := s.newValue1(ssa.OpITab, byteptr, v)
-	s.vars[&typVar] = tab
-	isnonnil := s.newValue2(ssa.OpNeqPtr, Types[TBOOL], tab, s.constNil(byteptr))
-	b := s.endBlock()
-	b.Kind = ssa.BlockIf
-	b.SetControl(isnonnil)
-	b.Likely = ssa.BranchLikely
-
-	bLoad := s.f.NewBlock(ssa.BlockPlain)
-	bEnd := s.f.NewBlock(ssa.BlockPlain)
-
-	b.AddEdgeTo(bLoad)
-	b.AddEdgeTo(bEnd)
-	bLoad.AddEdgeTo(bEnd)
-
-	s.startBlock(bLoad)
-	off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(Widthptr), tab)
-	s.vars[&typVar] = s.newValue2(ssa.OpLoad, byteptr, off, s.mem())
-	s.endBlock()
-
-	s.startBlock(bEnd)
-	typ := s.variable(&typVar, byteptr)
-	delete(s.vars, &typVar)
-	return typ
-}
-
 // dottype generates SSA for a type assertion node.
 // commaok indicates whether to panic or return a bool.
 // If commaok is false, resok will be nil.
@@ -4157,11 +4115,18 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 
 	// Converting to a concrete type.
 	direct := isdirectiface(n.Type)
-	typ := s.ifaceType(n.Left.Type, iface) // actual concrete type of input interface
-
+	itab := s.newValue1(ssa.OpITab, byteptr, iface) // type word of interface
 	if Debug_typeassert > 0 {
 		Warnl(n.Pos, "type assertion inlined")
 	}
+	var targetITab *ssa.Value
+	if n.Left.Type.IsEmptyInterface() {
+		// Looking for pointer to target type.
+		targetITab = target
+	} else {
+		// Looking for pointer to itab for target type and source interface.
+		targetITab = s.expr(itabname(n.Type, n.Left.Type))
+	}
 
 	var tmp *Node       // temporary for use with large types
 	var addr *ssa.Value // address of tmp
@@ -4173,9 +4138,7 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 		s.vars[&memVar] = s.newValue1A(ssa.OpVarDef, ssa.TypeMem, tmp, s.mem())
 	}
 
-	// TODO:  If we have a nonempty interface and its itab field is nil,
-	// then this test is redundant and ifaceType should just branch directly to bFail.
-	cond := s.newValue2(ssa.OpEqPtr, Types[TBOOL], typ, target)
+	cond := s.newValue2(ssa.OpEqPtr, Types[TBOOL], itab, targetITab)
 	b := s.endBlock()
 	b.Kind = ssa.BlockIf
 	b.SetControl(cond)
@@ -4190,7 +4153,11 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 		// on failure, panic by calling panicdottype
 		s.startBlock(bFail)
 		taddr := s.newValue1A(ssa.OpAddr, byteptr, &ssa.ExternSymbol{Typ: byteptr, Sym: Linksym(typenamesym(n.Left.Type))}, s.sb)
-		s.rtcall(panicdottype, false, nil, typ, target, taddr)
+		if n.Left.Type.IsEmptyInterface() {
+			s.rtcall(panicdottypeE, false, nil, itab, target, taddr)
+		} else {
+			s.rtcall(panicdottypeI, false, nil, itab, target, taddr)
+		}
 
 		// on success, return data from interface
 		s.startBlock(bOk)

src/cmd/compile/internal/gc/swt.go

@@ -729,11 +729,13 @@ func (s *typeSwitch) walk(sw *Node) {
 	// Use a similar strategy for non-empty interfaces.
 
 	// Get interface descriptor word.
-	typ := nod(OITAB, s.facename, nil)
+	// For empty interfaces this will be the type.
+	// For non-empty interfaces this will be the itab.
+	itab := nod(OITAB, s.facename, nil)
 
 	// Check for nil first.
 	i := nod(OIF, nil, nil)
-	i.Left = nod(OEQ, typ, nodnil())
+	i.Left = nod(OEQ, itab, nodnil())
 	if clauses.niljmp != nil {
 		// Do explicit nil case right here.
 		i.Nbody.Set1(clauses.niljmp)
@@ -749,16 +751,16 @@ func (s *typeSwitch) walk(sw *Node) {
 	i.Left = typecheck(i.Left, Erv)
 	cas = append(cas, i)
 
-	if !cond.Right.Type.IsEmptyInterface() {
-		// Load type from itab.
-		typ = itabType(typ)
-	}
-	// Load hash from type.
-	h := nodSym(ODOTPTR, typ, nil)
+	// Load hash from type or itab.
+	h := nodSym(ODOTPTR, itab, nil)
 	h.Type = Types[TUINT32]
 	h.Typecheck = 1
-	h.Xoffset = int64(2 * Widthptr) // offset of hash in runtime._type
-	h.Bounded = true                // guaranteed not to fault
+	if cond.Right.Type.IsEmptyInterface() {
+		h.Xoffset = int64(2 * Widthptr) // offset of hash in runtime._type
+	} else {
+		h.Xoffset = int64(3 * Widthptr) // offset of hash in runtime.itab
+	}
+	h.Bounded = true // guaranteed not to fault
 	a = nod(OAS, s.hashname, h)
 	a = typecheck(a, Etop)
 	cas = append(cas, a)

src/runtime/iface.go

@@ -53,7 +53,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 		}
 		for m = (*itab)(atomic.Loadp(unsafe.Pointer(&hash[h]))); m != nil; m = m.link {
 			if m.inter == inter && m._type == typ {
-				if m.bad != 0 {
+				if m.bad {
 					if !canfail {
 						// this can only happen if the conversion
 						// was already done once using the , ok form
@@ -78,7 +78,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 	m._type = typ
 	additab(m, true, canfail)
 	unlock(&ifaceLock)
-	if m.bad != 0 {
+	if m.bad {
 		return nil
 	}
 	return m
@@ -130,7 +130,7 @@ func additab(m *itab, locked, canfail bool) {
 			}
 			panic(&TypeAssertionError{"", typ.string(), inter.typ.string(), iname})
 		}
-		m.bad = 1
+		m.bad = true
 		break
 	nextimethod:
 	}
@@ -139,7 +139,7 @@ func additab(m *itab, locked, canfail bool) {
 	}
 	h := itabhash(inter, typ)
 	m.link = hash[h]
-	m.inhash = 1
+	m.inhash = true
 	atomicstorep(unsafe.Pointer(&hash[h]), unsafe.Pointer(m))
 }
 
@@ -152,7 +152,7 @@ func itabsinit() {
 			// and thanks to the way global symbol resolution works, the
 			// pointed-to itab may already have been inserted into the
 			// global 'hash'.
-			if i.inhash == 0 {
+			if !i.inhash {
 				additab(i, true, false)
 			}
 		}
@@ -160,11 +160,11 @@ func itabsinit() {
 	unlock(&ifaceLock)
 }
 
-// panicdottype is called when doing an i.(T) conversion and the conversion fails.
+// panicdottypeE is called when doing an e.(T) conversion and the conversion fails.
 // have = the dynamic type we have.
 // want = the static type we're trying to convert to.
 // iface = the static type we're converting from.
-func panicdottype(have, want, iface *_type) {
+func panicdottypeE(have, want, iface *_type) {
 	haveString := ""
 	if have != nil {
 		haveString = have.string()
@@ -172,6 +172,16 @@ func panicdottype(have, want, iface *_type) {
 	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
 }
 
+// panicdottypeI is called when doing an i.(T) conversion and the conversion fails.
+// Same args as panicdottypeE, but "have" is the dynamic itab we have.
+func panicdottypeI(have *itab, want, iface *_type) {
+	var t *_type
+	if have != nil {
+		t = have._type
+	}
+	panicdottypeE(t, want, iface)
+}
+
 // panicnildottype is called when doing a i.(T) conversion and the interface i is nil.
 // want = the static type we're trying to convert to.
 func panicnildottype(want *_type) {

src/runtime/plugin.go

@@ -56,7 +56,7 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 
 	lock(&ifaceLock)
 	for _, i := range md.itablinks {
-		if i.inhash == 0 {
+		if !i.inhash {
 			additab(i, true, false)
 		}
 	}

src/runtime/runtime2.go

@@ -644,8 +644,10 @@ type itab struct {
 	inter  *interfacetype
 	_type  *_type
 	link   *itab
-	bad    int32
-	inhash int32      // has this itab been added to hash?
+	hash   uint32 // copy of _type.hash. Used for type switches.
+	bad    bool   // type does not implement interface
+	inhash bool   // has this itab been added to hash?
+	unused [2]byte
 	fun    [1]uintptr // variable sized
 }