cmd/compile: do more type conversion inline

The code to do the conversion is smaller than the
call to the runtime.
The 1-result asserts need to call panic if they fail, but that
code is out of line.

The only conversions left in the runtime are those which
might allocate and those which might need to generate an itab.

Given the following types:
  type E interface{}
  type I interface { foo() }
  type I2 iterface { foo(); bar() }
  type Big [10]int
  func (b Big) foo() { ... }

This CL inlines the following conversions:

was assertE2T
  var e E = ...
  b := i.(Big)
was assertE2T2
  var e E = ...
  b, ok := i.(Big)
was assertI2T
  var i I = ...
  b := i.(Big)
was assertI2T2
  var i I = ...
  b, ok := i.(Big)
was assertI2E
  var i I = ...
  e := i.(E)
was assertI2E2
  var i I = ...
  e, ok := i.(E)

These are the remaining runtime calls:

convT2E:
  var b Big = ...
  var e E = b
convT2I:
  var b Big = ...
  var i I = b
convI2I:
  var i2 I2 = ...
  var i I = i2
assertE2I:
  var e E = ...
  i := e.(I)
assertE2I2:
  var e E = ...
  i, ok := e.(I)
assertI2I:
  var i I = ...
  i2 := i.(I2)
assertI2I2:
  var i I = ...
  i2, ok := i.(I2)

Fixes #17405
Fixes #8422

Change-Id: Ida2367bf8ce3cd2c6bb599a1814f1d275afabe21
Reviewed-on: https://go-review.googlesource.com/32313
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

13 files changed, 355 insertions, 451 deletions

diff --git a/src/cmd/compile/internal/gc/builtin.go b/src/cmd/compile/internal/gc/builtin.go
index adde1bd8c2..e02e2feb01 100644
--- a/src/cmd/compile/internal/gc/builtin.go
+++ b/src/cmd/compile/internal/gc/builtin.go
@@ -49,19 +49,12 @@ var runtimeDecls = [...]struct {
 	{"convI2I", funcTag, 53},
 	{"convT2E", funcTag, 54},
 	{"convT2I", funcTag, 54},
-	{"assertE2E", funcTag, 55},
-	{"assertE2E2", funcTag, 56},
-	{"assertE2I", funcTag, 55},
-	{"assertE2I2", funcTag, 56},
-	{"assertE2T", funcTag, 55},
-	{"assertE2T2", funcTag, 56},
-	{"assertI2E", funcTag, 55},
-	{"assertI2E2", funcTag, 56},
-	{"assertI2I", funcTag, 55},
-	{"assertI2I2", funcTag, 56},
-	{"assertI2T", funcTag, 55},
-	{"assertI2T2", funcTag, 56},
-	{"panicdottype", funcTag, 57},
+	{"assertE2I", funcTag, 53},
+	{"assertE2I2", funcTag, 55},
+	{"assertI2I", funcTag, 53},
+	{"assertI2I2", funcTag, 55},
+	{"panicdottype", funcTag, 56},
+	{"panicnildottype", funcTag, 57},
 	{"ifaceeq", funcTag, 58},
 	{"efaceeq", funcTag, 58},
 	{"makemap", funcTag, 60},
@@ -97,43 +90,43 @@ var runtimeDecls = [...]struct {
 	{"selectrecv", funcTag, 73},
 	{"selectrecv2", funcTag, 86},
 	{"selectdefault", funcTag, 87},
-	{"selectgo", funcTag, 88},
+	{"selectgo", funcTag, 57},
 	{"block", funcTag, 5},
-	{"makeslice", funcTag, 90},
-	{"makeslice64", funcTag, 91},
-	{"growslice", funcTag, 92},
-	{"memmove", funcTag, 93},
-	{"memclrNoHeapPointers", funcTag, 94},
-	{"memclrHasPointers", funcTag, 94},
-	{"memequal", funcTag, 95},
-	{"memequal8", funcTag, 96},
-	{"memequal16", funcTag, 96},
-	{"memequal32", funcTag, 96},
-	{"memequal64", funcTag, 96},
-	{"memequal128", funcTag, 96},
-	{"int64div", funcTag, 97},
-	{"uint64div", funcTag, 98},
-	{"int64mod", funcTag, 97},
-	{"uint64mod", funcTag, 98},
-	{"float64toint64", funcTag, 99},
-	{"float64touint64", funcTag, 100},
-	{"float64touint32", funcTag, 102},
-	{"int64tofloat64", funcTag, 103},
-	{"uint64tofloat64", funcTag, 104},
-	{"uint32tofloat64", funcTag, 105},
-	{"complex128div", funcTag, 106},
-	{"racefuncenter", funcTag, 107},
+	{"makeslice", funcTag, 89},
+	{"makeslice64", funcTag, 90},
+	{"growslice", funcTag, 91},
+	{"memmove", funcTag, 92},
+	{"memclrNoHeapPointers", funcTag, 93},
+	{"memclrHasPointers", funcTag, 93},
+	{"memequal", funcTag, 94},
+	{"memequal8", funcTag, 95},
+	{"memequal16", funcTag, 95},
+	{"memequal32", funcTag, 95},
+	{"memequal64", funcTag, 95},
+	{"memequal128", funcTag, 95},
+	{"int64div", funcTag, 96},
+	{"uint64div", funcTag, 97},
+	{"int64mod", funcTag, 96},
+	{"uint64mod", funcTag, 97},
+	{"float64toint64", funcTag, 98},
+	{"float64touint64", funcTag, 99},
+	{"float64touint32", funcTag, 101},
+	{"int64tofloat64", funcTag, 102},
+	{"uint64tofloat64", funcTag, 103},
+	{"uint32tofloat64", funcTag, 104},
+	{"complex128div", funcTag, 105},
+	{"racefuncenter", funcTag, 106},
 	{"racefuncexit", funcTag, 5},
-	{"raceread", funcTag, 107},
-	{"racewrite", funcTag, 107},
-	{"racereadrange", funcTag, 108},
-	{"racewriterange", funcTag, 108},
-	{"msanread", funcTag, 108},
-	{"msanwrite", funcTag, 108},
+	{"raceread", funcTag, 106},
+	{"racewrite", funcTag, 106},
+	{"racereadrange", funcTag, 107},
+	{"racewriterange", funcTag, 107},
+	{"msanread", funcTag, 107},
+	{"msanwrite", funcTag, 107},
 }
 
 func runtimeTypes() []*Type {
-	var typs [109]*Type
+	var typs [108]*Type
 	typs[0] = bytetype
 	typs[1] = typPtr(typs[0])
 	typs[2] = Types[TANY]
@@ -189,9 +182,9 @@ func runtimeTypes() []*Type {
 	typs[52] = functype(nil, []*Node{anonfield(typs[2]), anonfield(typs[2])}, []*Node{anonfield(typs[33])})
 	typs[53] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2])}, []*Node{anonfield(typs[2])})
 	typs[54] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3])}, []*Node{anonfield(typs[2])})
-	typs[55] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2]), anonfield(typs[3])}, nil)
-	typs[56] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2]), anonfield(typs[3])}, []*Node{anonfield(typs[13])})
-	typs[57] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[1])}, nil)
+	typs[55] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2])}, []*Node{anonfield(typs[2]), anonfield(typs[13])})
+	typs[56] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[1])}, nil)
+	typs[57] = functype(nil, []*Node{anonfield(typs[1])}, nil)
 	typs[58] = functype(nil, []*Node{anonfield(typs[2]), anonfield(typs[2])}, []*Node{anonfield(typs[13])})
 	typs[59] = typMap(typs[2], typs[2])
 	typs[60] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[17]), anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[59])})
@@ -222,26 +215,25 @@ func runtimeTypes() []*Type {
 	typs[85] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[17]), anonfield(typs[10])}, nil)
 	typs[86] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[71]), anonfield(typs[3]), anonfield(typs[83])}, []*Node{anonfield(typs[13])})
 	typs[87] = functype(nil, []*Node{anonfield(typs[1])}, []*Node{anonfield(typs[13])})
-	typs[88] = functype(nil, []*Node{anonfield(typs[1])}, nil)
-	typs[89] = typSlice(typs[2])
-	typs[90] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[33]), anonfield(typs[33])}, []*Node{anonfield(typs[89])})
-	typs[91] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[89])})
-	typs[92] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[89]), anonfield(typs[33])}, []*Node{anonfield(typs[89])})
-	typs[93] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[50])}, nil)
-	typs[94] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[50])}, nil)
-	typs[95] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[50])}, []*Node{anonfield(typs[13])})
-	typs[96] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[13])})
-	typs[97] = functype(nil, []*Node{anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[17])})
-	typs[98] = functype(nil, []*Node{anonfield(typs[19]), anonfield(typs[19])}, []*Node{anonfield(typs[19])})
-	typs[99] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[17])})
-	typs[100] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[19])})
-	typs[101] = Types[TUINT32]
-	typs[102] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[101])})
-	typs[103] = functype(nil, []*Node{anonfield(typs[17])}, []*Node{anonfield(typs[15])})
-	typs[104] = functype(nil, []*Node{anonfield(typs[19])}, []*Node{anonfield(typs[15])})
-	typs[105] = functype(nil, []*Node{anonfield(typs[101])}, []*Node{anonfield(typs[15])})
-	typs[106] = functype(nil, []*Node{anonfield(typs[21]), anonfield(typs[21])}, []*Node{anonfield(typs[21])})
-	typs[107] = functype(nil, []*Node{anonfield(typs[50])}, nil)
-	typs[108] = functype(nil, []*Node{anonfield(typs[50]), anonfield(typs[50])}, nil)
+	typs[88] = typSlice(typs[2])
+	typs[89] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[33]), anonfield(typs[33])}, []*Node{anonfield(typs[88])})
+	typs[90] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[88])})
+	typs[91] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[88]), anonfield(typs[33])}, []*Node{anonfield(typs[88])})
+	typs[92] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[50])}, nil)
+	typs[93] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[50])}, nil)
+	typs[94] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[50])}, []*Node{anonfield(typs[13])})
+	typs[95] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[13])})
+	typs[96] = functype(nil, []*Node{anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[17])})
+	typs[97] = functype(nil, []*Node{anonfield(typs[19]), anonfield(typs[19])}, []*Node{anonfield(typs[19])})
+	typs[98] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[17])})
+	typs[99] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[19])})
+	typs[100] = Types[TUINT32]
+	typs[101] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[100])})
+	typs[102] = functype(nil, []*Node{anonfield(typs[17])}, []*Node{anonfield(typs[15])})
+	typs[103] = functype(nil, []*Node{anonfield(typs[19])}, []*Node{anonfield(typs[15])})
+	typs[104] = functype(nil, []*Node{anonfield(typs[100])}, []*Node{anonfield(typs[15])})
+	typs[105] = functype(nil, []*Node{anonfield(typs[21]), anonfield(typs[21])}, []*Node{anonfield(typs[21])})
+	typs[106] = functype(nil, []*Node{anonfield(typs[50])}, nil)
+	typs[107] = functype(nil, []*Node{anonfield(typs[50]), anonfield(typs[50])}, nil)
 	return typs[:]
 }
diff --git a/src/cmd/compile/internal/gc/builtin/runtime.go b/src/cmd/compile/internal/gc/builtin/runtime.go
index ff2da79e81..98e25fefb8 100644
--- a/src/cmd/compile/internal/gc/builtin/runtime.go
+++ b/src/cmd/compile/internal/gc/builtin/runtime.go
@@ -62,19 +62,12 @@ func convT2E(typ *byte, elem *any) (ret any)
 func convT2I(tab *byte, elem *any) (ret any)
 
 // interface type assertions  x.(T)
-func assertE2E(typ *byte, iface any, ret *any)
-func assertE2E2(typ *byte, iface any, ret *any) bool
-func assertE2I(typ *byte, iface any, ret *any)
-func assertE2I2(typ *byte, iface any, ret *any) bool
-func assertE2T(typ *byte, iface any, ret *any)
-func assertE2T2(typ *byte, iface any, ret *any) bool
-func assertI2E(typ *byte, iface any, ret *any)
-func assertI2E2(typ *byte, iface any, ret *any) bool
-func assertI2I(typ *byte, iface any, ret *any)
-func assertI2I2(typ *byte, iface any, ret *any) bool
-func assertI2T(typ *byte, iface any, ret *any)
-func assertI2T2(typ *byte, iface any, ret *any) bool
+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 panicnildottype(want *byte)
 
 func ifaceeq(i1 any, i2 any) (ret bool)
 func efaceeq(i1 any, i2 any) (ret bool)
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index 089b6668b9..d427755386 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -363,18 +363,18 @@ var pcloc int32
 
 var Thearch Arch
 
-var Newproc *Node
-
-var Deferproc *Node
-
-var Deferreturn *Node
-
-var panicindex *Node
-
-var panicslice *Node
-
-var panicdivide *Node
-
-var growslice *Node
-
-var panicdottype *Node
+var (
+	Newproc,
+	Deferproc,
+	Deferreturn,
+	panicindex,
+	panicslice,
+	panicdivide,
+	growslice,
+	panicdottype,
+	panicnildottype,
+	assertE2I,
+	assertE2I2,
+	assertI2I,
+	assertI2I2 *Node
+)
diff --git a/src/cmd/compile/internal/gc/pgen.go b/src/cmd/compile/internal/gc/pgen.go
index 25611e9e60..5ae493bc82 100644
--- a/src/cmd/compile/internal/gc/pgen.go
+++ b/src/cmd/compile/internal/gc/pgen.go
@@ -304,6 +304,11 @@ func compile(fn *Node) {
 		panicdivide = Sysfunc("panicdivide")
 		growslice = Sysfunc("growslice")
 		panicdottype = Sysfunc("panicdottype")
+		panicnildottype = Sysfunc("panicnildottype")
+		assertE2I = Sysfunc("assertE2I")
+		assertE2I2 = Sysfunc("assertE2I2")
+		assertI2I = Sysfunc("assertI2I")
+		assertI2I2 = Sysfunc("assertI2I2")
 	}
 
 	defer func(lno int32) {
diff --git a/src/cmd/compile/internal/gc/racewalk.go b/src/cmd/compile/internal/gc/racewalk.go
index 29ccbd18cc..bfb82b91b2 100644
--- a/src/cmd/compile/internal/gc/racewalk.go
+++ b/src/cmd/compile/internal/gc/racewalk.go
@@ -318,6 +318,15 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 		instrumentnode(&n.Left, init, 0, 0)
 		goto ret
 
+	case OAS2DOTTYPE:
+		instrumentnode(&n.Left, init, 1, 0)
+		instrumentnode(&n.Right, init, 0, 0)
+		goto ret
+
+	case ODOTTYPE, ODOTTYPE2:
+		instrumentnode(&n.Left, init, 0, 0)
+		goto ret
+
 		// should not appear in AST by now
 	case OSEND,
 		ORECV,
@@ -345,9 +354,6 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 		// lowered to call
 		OCMPSTR,
 		OADDSTR,
-		ODOTTYPE,
-		ODOTTYPE2,
-		OAS2DOTTYPE,
 		OCALLPART,
 		// lowered to PTRLIT
 		OCLOSURE,  // lowered to PTRLIT
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index b77dafd345..fce08a11af 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -292,7 +292,6 @@ var (
 	newlenVar = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "newlen"}}
 	capVar    = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "cap"}}
 	typVar    = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "typ"}}
-	idataVar  = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "idata"}}
 	okVar     = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "ok"}}
 )
 
@@ -539,7 +538,22 @@ func (s *state) stmt(n *Node) {
 
 	case OAS2DOTTYPE:
 		res, resok := s.dottype(n.Rlist.First(), true)
-		s.assign(n.List.First(), res, needwritebarrier(n.List.First(), n.Rlist.First()), false, n.Lineno, 0, false)
+		deref := false
+		if !canSSAType(n.Rlist.First().Type) {
+			if res.Op != ssa.OpLoad {
+				s.Fatalf("dottype of non-load")
+			}
+			mem := s.mem()
+			if mem.Op == ssa.OpVarKill {
+				mem = mem.Args[0]
+			}
+			if res.Args[1] != mem {
+				s.Fatalf("memory no longer live from 2-result dottype load")
+			}
+			deref = true
+			res = res.Args[0]
+		}
+		s.assign(n.List.First(), res, needwritebarrier(n.List.First(), n.Rlist.First()), deref, n.Lineno, 0, false)
 		s.assign(n.List.Second(), resok, false, false, n.Lineno, 0, false)
 		return
 
@@ -3078,7 +3092,15 @@ func (s *state) addr(n *Node, bounded bool) (*ssa.Value, bool) {
 		return s.newValue1(ssa.OpCopy, t, addr), isVolatile // ensure that addr has the right type
 	case OCALLFUNC, OCALLINTER, OCALLMETH:
 		return s.call(n, callNormal), true
-
+	case ODOTTYPE:
+		v, _ := s.dottype(n, false)
+		if v.Op != ssa.OpLoad {
+			s.Fatalf("dottype of non-load")
+		}
+		if v.Args[1] != s.mem() {
+			s.Fatalf("memory no longer live from dottype load")
+		}
+		return v.Args[0], false
 	default:
 		s.Fatalf("unhandled addr %v", n.Op)
 		return nil, false
@@ -3822,20 +3844,20 @@ func (s *state) floatToUint(cvttab *f2uCvtTab, n *Node, x *ssa.Value, ft, tt *Ty
 }
 
 // ifaceType returns the value for the word containing the type.
-// n is the node for the interface expression.
+// t is the type of the interface expression.
 // v is the corresponding value.
-func (s *state) ifaceType(n *Node, v *ssa.Value) *ssa.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 n.Type.IsEmptyInterface() {
-		// Have *eface. The type is the first word in the struct.
+	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.
+	// 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
@@ -3867,18 +3889,120 @@ func (s *state) ifaceType(n *Node, v *ssa.Value) *ssa.Value {
 // commaok indicates whether to panic or return a bool.
 // If commaok is false, resok will be nil.
 func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
-	iface := s.expr(n.Left)
-	typ := s.ifaceType(n.Left, iface)  // actual concrete type
+	iface := s.expr(n.Left)            // input interface
 	target := s.expr(typename(n.Type)) // target type
-	if !isdirectiface(n.Type) {
-		// walk rewrites ODOTTYPE/OAS2DOTTYPE into runtime calls except for this case.
-		Fatalf("dottype needs a direct iface type %v", n.Type)
+	byteptr := ptrto(Types[TUINT8])
+
+	if n.Type.IsInterface() {
+		if n.Type.IsEmptyInterface() {
+			// Converting to an empty interface.
+			// Input could be an empty or nonempty interface.
+			if Debug_typeassert > 0 {
+				Warnl(n.Lineno, "type assertion inlined")
+			}
+
+			// Get itab/type field from input.
+			itab := s.newValue1(ssa.OpITab, byteptr, iface)
+			// Conversion succeeds iff that field is not nil.
+			cond := s.newValue2(ssa.OpNeqPtr, Types[TBOOL], itab, s.constNil(byteptr))
+
+			if n.Left.Type.IsEmptyInterface() && commaok {
+				// Converting empty interface to empty interface with ,ok is just a nil check.
+				return iface, cond
+			}
+
+			// Branch on nilness.
+			b := s.endBlock()
+			b.Kind = ssa.BlockIf
+			b.SetControl(cond)
+			b.Likely = ssa.BranchLikely
+			bOk := s.f.NewBlock(ssa.BlockPlain)
+			bFail := s.f.NewBlock(ssa.BlockPlain)
+			b.AddEdgeTo(bOk)
+			b.AddEdgeTo(bFail)
+
+			if !commaok {
+				// On failure, panic by calling panicnildottype.
+				s.startBlock(bFail)
+				s.rtcall(panicnildottype, false, nil, target)
+
+				// On success, return (perhaps modified) input interface.
+				s.startBlock(bOk)
+				if n.Left.Type.IsEmptyInterface() {
+					res = iface // Use input interface unchanged.
+					return
+				}
+				// Load type out of itab, build interface with existing idata.
+				off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(Widthptr), itab)
+				typ := s.newValue2(ssa.OpLoad, byteptr, off, s.mem())
+				idata := s.newValue1(ssa.OpIData, n.Type, iface)
+				res = s.newValue2(ssa.OpIMake, n.Type, typ, idata)
+				return
+			}
+
+			s.startBlock(bOk)
+			// nonempty -> empty
+			// Need to load type from itab
+			off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(Widthptr), itab)
+			s.vars[&typVar] = s.newValue2(ssa.OpLoad, byteptr, off, s.mem())
+			s.endBlock()
+
+			// itab is nil, might as well use that as the nil result.
+			s.startBlock(bFail)
+			s.vars[&typVar] = itab
+			s.endBlock()
+
+			// Merge point.
+			bEnd := s.f.NewBlock(ssa.BlockPlain)
+			bOk.AddEdgeTo(bEnd)
+			bFail.AddEdgeTo(bEnd)
+			s.startBlock(bEnd)
+			idata := s.newValue1(ssa.OpIData, n.Type, iface)
+			res = s.newValue2(ssa.OpIMake, n.Type, s.variable(&typVar, byteptr), idata)
+			resok = cond
+			delete(s.vars, &typVar)
+			return
+		}
+		// converting to a nonempty interface needs a runtime call.
+		if Debug_typeassert > 0 {
+			Warnl(n.Lineno, "type assertion not inlined")
+		}
+		if n.Left.Type.IsEmptyInterface() {
+			if commaok {
+				call := s.rtcall(assertE2I2, true, []*Type{n.Type, Types[TBOOL]}, target, iface)
+				return call[0], call[1]
+			}
+			return s.rtcall(assertE2I, true, []*Type{n.Type}, target, iface)[0], nil
+		}
+		if commaok {
+			call := s.rtcall(assertI2I2, true, []*Type{n.Type, Types[TBOOL]}, target, iface)
+			return call[0], call[1]
+		}
+		return s.rtcall(assertI2I, true, []*Type{n.Type}, target, iface)[0], nil
 	}
 
 	if Debug_typeassert > 0 {
 		Warnl(n.Lineno, "type assertion inlined")
 	}
 
+	// Converting to a concrete type.
+	direct := isdirectiface(n.Type)
+	typ := s.ifaceType(n.Left.Type, iface) // actual concrete type of input interface
+
+	if Debug_typeassert > 0 {
+		Warnl(n.Lineno, "type assertion inlined")
+	}
+
+	var tmp *Node       // temporary for use with large types
+	var addr *ssa.Value // address of tmp
+	if commaok && !canSSAType(n.Type) {
+		// unSSAable type, use temporary.
+		// TODO: get rid of some of these temporaries.
+		tmp = temp(n.Type)
+		addr, _ = s.addr(tmp, false)
+		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)
@@ -3887,8 +4011,6 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 	b.SetControl(cond)
 	b.Likely = ssa.BranchLikely
 
-	byteptr := ptrto(Types[TUINT8])
-
 	bOk := s.f.NewBlock(ssa.BlockPlain)
 	bFail := s.f.NewBlock(ssa.BlockPlain)
 	b.AddEdgeTo(bOk)
@@ -3900,34 +4022,60 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 		taddr := s.newValue1A(ssa.OpAddr, byteptr, &ssa.ExternSymbol{Typ: byteptr, Sym: typenamesym(n.Left.Type)}, s.sb)
 		s.rtcall(panicdottype, false, nil, typ, target, taddr)
 
-		// on success, return idata field
+		// on success, return data from interface
 		s.startBlock(bOk)
-		return s.newValue1(ssa.OpIData, n.Type, iface), nil
+		if direct {
+			return s.newValue1(ssa.OpIData, n.Type, iface), nil
+		}
+		p := s.newValue1(ssa.OpIData, ptrto(n.Type), iface)
+		return s.newValue2(ssa.OpLoad, n.Type, p, s.mem()), nil
 	}
 
 	// commaok is the more complicated case because we have
 	// a control flow merge point.
 	bEnd := s.f.NewBlock(ssa.BlockPlain)
+	// Note that we need a new valVar each time (unlike okVar where we can
+	// reuse the variable) because it might have a different type every time.
+	valVar := &Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "val"}}
 
 	// type assertion succeeded
 	s.startBlock(bOk)
-	s.vars[&idataVar] = s.newValue1(ssa.OpIData, n.Type, iface)
+	if tmp == nil {
+		if direct {
+			s.vars[valVar] = s.newValue1(ssa.OpIData, n.Type, iface)
+		} else {
+			p := s.newValue1(ssa.OpIData, ptrto(n.Type), iface)
+			s.vars[valVar] = s.newValue2(ssa.OpLoad, n.Type, p, s.mem())
+		}
+	} else {
+		p := s.newValue1(ssa.OpIData, ptrto(n.Type), iface)
+		s.vars[&memVar] = s.newValue3I(ssa.OpMove, ssa.TypeMem, sizeAlignAuxInt(n.Type), addr, p, s.mem())
+	}
 	s.vars[&okVar] = s.constBool(true)
 	s.endBlock()
 	bOk.AddEdgeTo(bEnd)
 
 	// type assertion failed
 	s.startBlock(bFail)
-	s.vars[&idataVar] = s.constNil(byteptr)
+	if tmp == nil {
+		s.vars[valVar] = s.zeroVal(n.Type)
+	} else {
+		s.vars[&memVar] = s.newValue2I(ssa.OpZero, ssa.TypeMem, sizeAlignAuxInt(n.Type), addr, s.mem())
+	}
 	s.vars[&okVar] = s.constBool(false)
 	s.endBlock()
 	bFail.AddEdgeTo(bEnd)
 
 	// merge point
 	s.startBlock(bEnd)
-	res = s.variable(&idataVar, byteptr)
+	if tmp == nil {
+		res = s.variable(valVar, n.Type)
+		delete(s.vars, valVar)
+	} else {
+		res = s.newValue2(ssa.OpLoad, n.Type, addr, s.mem())
+		s.vars[&memVar] = s.newValue1A(ssa.OpVarKill, ssa.TypeMem, tmp, s.mem())
+	}
 	resok = s.variable(&okVar, Types[TBOOL])
-	delete(s.vars, &idataVar)
 	delete(s.vars, &okVar)
 	return res, resok
 }
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index a3d55916e5..cfb65ebb40 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -730,34 +730,6 @@ opswitch:
 		default:
 			n.Right = walkexpr(n.Right, init)
 
-		case ODOTTYPE:
-			// TODO(rsc): The isfat is for consistency with componentgen and orderexpr.
-			// It needs to be removed in all three places.
-			// That would allow inlining x.(struct{*int}) the same as x.(*int).
-			if isdirectiface(n.Right.Type) && !isfat(n.Right.Type) && !instrumenting {
-				// handled directly during cgen
-				n.Right = walkexpr(n.Right, init)
-				break
-			}
-
-			// x = i.(T); n.Left is x, n.Right.Left is i.
-			// orderstmt made sure x is addressable.
-			n.Right.Left = walkexpr(n.Right.Left, init)
-
-			n1 := nod(OADDR, n.Left, nil)
-			r := n.Right // i.(T)
-
-			if Debug_typeassert > 0 {
-				Warn("type assertion not inlined")
-			}
-
-			fn := syslook(assertFuncName(r.Left.Type, r.Type, false))
-			fn = substArgTypes(fn, r.Left.Type, r.Type)
-
-			n = mkcall1(fn, nil, init, typename(r.Type), r.Left, n1)
-			n = walkexpr(n, init)
-			break opswitch
-
 		case ORECV:
 			// x = <-c; n.Left is x, n.Right.Left is c.
 			// orderstmt made sure x is addressable.
@@ -935,112 +907,11 @@ opswitch:
 		n = mkcall1(mapfndel("mapdelete", t), nil, init, typename(t), map_, key)
 
 	case OAS2DOTTYPE:
-		e := n.Rlist.First() // i.(T)
-
-		// TODO(rsc): The isfat is for consistency with componentgen and orderexpr.
-		// It needs to be removed in all three places.
-		// That would allow inlining x.(struct{*int}) the same as x.(*int).
-		if isdirectiface(e.Type) && !isfat(e.Type) && !instrumenting {
-			// handled directly during gen.
-			walkexprlistsafe(n.List.Slice(), init)
-			e.Left = walkexpr(e.Left, init)
-			break
-		}
-
-		// res, ok = i.(T)
-		// orderstmt made sure a is addressable.
-		init.AppendNodes(&n.Ninit)
-
 		walkexprlistsafe(n.List.Slice(), init)
+		e := n.Rlist.First() // i.(T)
 		e.Left = walkexpr(e.Left, init)
-		t := e.Type    // T
-		from := e.Left // i
-
-		oktype := Types[TBOOL]
-		ok := n.List.Second()
-		if !isblank(ok) {
-			oktype = ok.Type
-		}
-		if !oktype.IsBoolean() {
-			Fatalf("orderstmt broken: got %L, want boolean", oktype)
-		}
-
-		fromKind := from.Type.iet()
-		toKind := t.iet()
-
-		res := n.List.First()
-		scalar := !haspointers(res.Type)
-
-		// Avoid runtime calls in a few cases of the form _, ok := i.(T).
-		// This is faster and shorter and allows the corresponding assertX2X2
-		// routines to skip nil checks on their last argument.
-		// Also avoid runtime calls for converting interfaces to scalar concrete types.
-		if isblank(res) || (scalar && toKind == 'T') {
-			var fast *Node
-			switch toKind {
-			case 'T':
-				tab := nod(OITAB, from, nil)
-				if fromKind == 'E' {
-					typ := nod(OCONVNOP, typename(t), nil)
-					typ.Type = ptrto(Types[TUINTPTR])
-					fast = nod(OEQ, tab, typ)
-					break
-				}
-				fast = nod(OANDAND,
-					nod(ONE, nodnil(), tab),
-					nod(OEQ, itabType(tab), typename(t)),
-				)
-			case 'E':
-				tab := nod(OITAB, from, nil)
-				fast = nod(ONE, nodnil(), tab)
-			}
-			if fast != nil {
-				if isblank(res) {
-					if Debug_typeassert > 0 {
-						Warn("type assertion (ok only) inlined")
-					}
-					n = nod(OAS, ok, fast)
-					n = typecheck(n, Etop)
-				} else {
-					if Debug_typeassert > 0 {
-						Warn("type assertion (scalar result) inlined")
-					}
-					n = nod(OIF, ok, nil)
-					n.Likely = 1
-					if isblank(ok) {
-						n.Left = fast
-					} else {
-						n.Ninit.Set1(nod(OAS, ok, fast))
-					}
-					n.Nbody.Set1(nod(OAS, res, ifaceData(from, res.Type)))
-					n.Rlist.Set1(nod(OAS, res, nil))
-					n = typecheck(n, Etop)
-				}
-				break
-			}
-		}
-
-		var resptr *Node // &res
-		if isblank(res) {
-			resptr = nodnil()
-		} else {
-			resptr = nod(OADDR, res, nil)
-		}
-		resptr.Etype = 1 // addr does not escape
-
-		if Debug_typeassert > 0 {
-			Warn("type assertion not inlined")
-		}
-		fn := syslook(assertFuncName(from.Type, t, true))
-		fn = substArgTypes(fn, from.Type, t)
-		call := mkcall1(fn, oktype, init, typename(t), from, resptr)
-		n = nod(OAS, ok, call)
-		n = typecheck(n, Etop)
 
 	case ODOTTYPE, ODOTTYPE2:
-		if !isdirectiface(n.Type) || isfat(n.Type) {
-			Fatalf("walkexpr ODOTTYPE") // should see inside OAS only
-		}
 		n.Left = walkexpr(n.Left, init)
 
 	case OCONVIFACE:
diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
index ca491c33d8..0137b109bd 100644
--- a/src/cmd/compile/internal/ssa/gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/gen/generic.rules
@@ -735,7 +735,9 @@
         f1
         (Store [t.FieldType(0).Size()] dst f0 mem))))
 
+// Putting struct{*byte} and similar into direct interfaces.
 (IMake typ (StructMake1 val)) -> (IMake typ val)
+(StructSelect [0] x:(IData _)) -> x
 
 // un-SSAable values use mem->mem copies
 (Store [size] dst (Load <t> src mem) mem) && !config.fe.CanSSA(t) ->
@@ -757,7 +759,9 @@
 
 (ArraySelect [0] (Load ptr mem)) -> (Load ptr mem)
 
+// Putting [1]{*byte} and similar into direct interfaces.
 (IMake typ (ArrayMake1 val)) -> (IMake typ val)
+(ArraySelect [0] x:(IData _)) -> x
 
 // string ops
 // Decomposing StringMake and lowering of StringPtr and StringLen
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index 818e08b7e0..1a2eacccf8 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -1674,6 +1674,22 @@ func rewriteValuegeneric_OpArraySelect(v *Value, config *Config) bool {
 		v.AddArg(mem)
 		return true
 	}
+	// match: (ArraySelect [0] x:(IData _))
+	// cond:
+	// result: x
+	for {
+		if v.AuxInt != 0 {
+			break
+		}
+		x := v.Args[0]
+		if x.Op != OpIData {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpCom16(v *Value, config *Config) bool {
@@ -10673,6 +10689,22 @@ func rewriteValuegeneric_OpStructSelect(v *Value, config *Config) bool {
 		v0.AddArg(mem)
 		return true
 	}
+	// match: (StructSelect [0] x:(IData _))
+	// cond:
+	// result: x
+	for {
+		if v.AuxInt != 0 {
+			break
+		}
+		x := v.Args[0]
+		if x.Op != OpIData {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpSub16(v *Value, config *Config) bool {
diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index f4a65fec18..9b765550ca 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -217,9 +217,6 @@ func BenchSetType(n int, x interface{}) {
 
 const PtrSize = sys.PtrSize
 
-var TestingAssertE2I2GC = &testingAssertE2I2GC
-var TestingAssertE2T2GC = &testingAssertE2T2GC
-
 var ForceGCPeriod = &forcegcperiod
 
 // SetTracebackEnv is like runtime/debug.SetTraceback, but it raises
diff --git a/src/runtime/gc_test.go b/src/runtime/gc_test.go
index d53d3ee000..4a32f15167 100644
--- a/src/runtime/gc_test.go
+++ b/src/runtime/gc_test.go
@@ -5,7 +5,6 @@
 package runtime_test
 
 import (
-	"io"
 	"os"
 	"reflect"
 	"runtime"
@@ -399,37 +398,6 @@ func TestPrintGC(t *testing.T) {
 	close(done)
 }
 
-// The implicit y, ok := x.(error) for the case error
-// in testTypeSwitch used to not initialize the result y
-// before passing &y to assertE2I2GC.
-// Catch this by making assertE2I2 call runtime.GC,
-// which will force a stack scan and failure if there are
-// bad pointers, and then fill the stack with bad pointers
-// and run the type switch.
-func TestAssertE2I2Liveness(t *testing.T) {
-	// Note that this flag is defined in export_test.go
-	// and is not available to ordinary imports of runtime.
-	*runtime.TestingAssertE2I2GC = true
-	defer func() {
-		*runtime.TestingAssertE2I2GC = false
-	}()
-
-	poisonStack()
-	testTypeSwitch(io.EOF)
-	poisonStack()
-	testAssert(io.EOF)
-	poisonStack()
-	testAssertVar(io.EOF)
-}
-
-func poisonStack() uintptr {
-	var x [1000]uintptr
-	for i := range x {
-		x[i] = 0xff
-	}
-	return x[123]
-}
-
 func testTypeSwitch(x interface{}) error {
 	switch y := x.(type) {
 	case nil:
@@ -455,16 +423,6 @@ func testAssertVar(x interface{}) error {
 	return nil
 }
 
-func TestAssertE2T2Liveness(t *testing.T) {
-	*runtime.TestingAssertE2T2GC = true
-	defer func() {
-		*runtime.TestingAssertE2T2GC = false
-	}()
-
-	poisonStack()
-	testIfaceEqual(io.EOF)
-}
-
 var a bool
 
 //go:noinline
diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index 26e2956eea..c932e149dd 100644
--- a/src/runtime/iface.go
+++ b/src/runtime/iface.go
@@ -156,6 +156,34 @@ func itabsinit() {
 	unlock(&ifaceLock)
 }
 
+// panicdottype is called when doing an i.(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) {
+	haveString := ""
+	if have != nil {
+		haveString = have.string()
+	}
+	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
+}
+
+// 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) {
+	panic(&TypeAssertionError{"", "", want.string(), ""})
+	// TODO: Add the static type we're converting from as well.
+	// It might generate a better error message.
+	// Just to match other nil conversion errors, we don't for now.
+}
+
+// The conv and assert functions below do very similar things.
+// The convXXX functions are guaranteed by the compiler to succeed.
+// The assertXXX functions may fail (either panicing or returning false,
+// depending on whether they are 1-result or 2-result).
+// The convXXX functions succeed on a nil input, whereas the assertXXX
+// functions fail on a nil input.
+
 func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(unsafe.Pointer(&t)), funcPC(convT2E))
@@ -164,6 +192,7 @@ func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
 		msanread(elem, t.size)
 	}
 	if isDirectIface(t) {
+		// This case is implemented directly by the compiler.
 		throw("direct convT2E")
 	}
 	x := newobject(t)
@@ -184,6 +213,7 @@ func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
 		msanread(elem, t.size)
 	}
 	if isDirectIface(t) {
+		// This case is implemented directly by the compiler.
 		throw("direct convT2I")
 	}
 	x := newobject(t)
@@ -193,103 +223,6 @@ func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
 	return
 }
 
-func panicdottype(have, want, iface *_type) {
-	haveString := ""
-	if have != nil {
-		haveString = have.string()
-	}
-	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
-}
-
-func assertI2T(t *_type, i iface, r unsafe.Pointer) {
-	tab := i.tab
-	if tab == nil {
-		panic(&TypeAssertionError{"", "", t.string(), ""})
-	}
-	if tab._type != t {
-		panic(&TypeAssertionError{tab.inter.typ.string(), tab._type.string(), t.string(), ""})
-	}
-	if r != nil {
-		if isDirectIface(t) {
-			writebarrierptr((*uintptr)(r), uintptr(i.data))
-		} else {
-			typedmemmove(t, r, i.data)
-		}
-	}
-}
-
-// The compiler ensures that r is non-nil.
-func assertI2T2(t *_type, i iface, r unsafe.Pointer) bool {
-	tab := i.tab
-	if tab == nil || tab._type != t {
-		typedmemclr(t, r)
-		return false
-	}
-	if isDirectIface(t) {
-		writebarrierptr((*uintptr)(r), uintptr(i.data))
-	} else {
-		typedmemmove(t, r, i.data)
-	}
-	return true
-}
-
-func assertE2T(t *_type, e eface, r unsafe.Pointer) {
-	if e._type == nil {
-		panic(&TypeAssertionError{"", "", t.string(), ""})
-	}
-	if e._type != t {
-		panic(&TypeAssertionError{"", e._type.string(), t.string(), ""})
-	}
-	if r != nil {
-		if isDirectIface(t) {
-			writebarrierptr((*uintptr)(r), uintptr(e.data))
-		} else {
-			typedmemmove(t, r, e.data)
-		}
-	}
-}
-
-var testingAssertE2T2GC bool
-
-// The compiler ensures that r is non-nil.
-func assertE2T2(t *_type, e eface, r unsafe.Pointer) bool {
-	if testingAssertE2T2GC {
-		GC()
-	}
-	if e._type != t {
-		typedmemclr(t, r)
-		return false
-	}
-	if isDirectIface(t) {
-		writebarrierptr((*uintptr)(r), uintptr(e.data))
-	} else {
-		typedmemmove(t, r, e.data)
-	}
-	return true
-}
-
-func assertI2E(inter *interfacetype, i iface, r *eface) {
-	tab := i.tab
-	if tab == nil {
-		// explicit conversions require non-nil interface value.
-		panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
-	}
-	r._type = tab._type
-	r.data = i.data
-	return
-}
-
-// The compiler ensures that r is non-nil.
-func assertI2E2(inter *interfacetype, i iface, r *eface) bool {
-	tab := i.tab
-	if tab == nil {
-		return false
-	}
-	r._type = tab._type
-	r.data = i.data
-	return true
-}
-
 func convI2I(inter *interfacetype, i iface) (r iface) {
 	tab := i.tab
 	if tab == nil {
@@ -305,7 +238,7 @@ func convI2I(inter *interfacetype, i iface) (r iface) {
 	return
 }
 
-func assertI2I(inter *interfacetype, i iface, r *iface) {
+func assertI2I(inter *interfacetype, i iface) (r iface) {
 	tab := i.tab
 	if tab == nil {
 		// explicit conversions require non-nil interface value.
@@ -318,33 +251,27 @@ func assertI2I(inter *interfacetype, i iface, r *iface) {
 	}
 	r.tab = getitab(inter, tab._type, false)
 	r.data = i.data
+	return
 }
 
-func assertI2I2(inter *interfacetype, i iface, r *iface) bool {
+func assertI2I2(inter *interfacetype, i iface) (r iface, b bool) {
 	tab := i.tab
 	if tab == nil {
-		if r != nil {
-			*r = iface{}
-		}
-		return false
+		return
 	}
 	if tab.inter != inter {
 		tab = getitab(inter, tab._type, true)
 		if tab == nil {
-			if r != nil {
-				*r = iface{}
-			}
-			return false
+			return
 		}
 	}
-	if r != nil {
-		r.tab = tab
-		r.data = i.data
-	}
-	return true
+	r.tab = tab
+	r.data = i.data
+	b = true
+	return
 }
 
-func assertE2I(inter *interfacetype, e eface, r *iface) {
+func assertE2I(inter *interfacetype, e eface) (r iface) {
 	t := e._type
 	if t == nil {
 		// explicit conversions require non-nil interface value.
@@ -352,56 +279,27 @@ func assertE2I(inter *interfacetype, e eface, r *iface) {
 	}
 	r.tab = getitab(inter, t, false)
 	r.data = e.data
+	return
 }
 
-var testingAssertE2I2GC bool
-
-func assertE2I2(inter *interfacetype, e eface, r *iface) bool {
-	if testingAssertE2I2GC {
-		GC()
-	}
+func assertE2I2(inter *interfacetype, e eface) (r iface, b bool) {
 	t := e._type
 	if t == nil {
-		if r != nil {
-			*r = iface{}
-		}
-		return false
+		return
 	}
 	tab := getitab(inter, t, true)
 	if tab == nil {
-		if r != nil {
-			*r = iface{}
-		}
-		return false
-	}
-	if r != nil {
-		r.tab = tab
-		r.data = e.data
+		return
 	}
-	return true
+	r.tab = tab
+	r.data = e.data
+	b = true
+	return
 }
 
 //go:linkname reflect_ifaceE2I reflect.ifaceE2I
 func reflect_ifaceE2I(inter *interfacetype, e eface, dst *iface) {
-	assertE2I(inter, e, dst)
-}
-
-func assertE2E(inter *interfacetype, e eface, r *eface) {
-	if e._type == nil {
-		// explicit conversions require non-nil interface value.
-		panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
-	}
-	*r = e
-}
-
-// The compiler ensures that r is non-nil.
-func assertE2E2(inter *interfacetype, e eface, r *eface) bool {
-	if e._type == nil {
-		*r = eface{}
-		return false
-	}
-	*r = e
-	return true
+	*dst = assertE2I(inter, e)
 }
 
 func iterate_itabs(fn func(*itab)) {
diff --git a/src/runtime/mfinal.go b/src/runtime/mfinal.go
index e0da4a3ac0..7e191d4e7b 100644
--- a/src/runtime/mfinal.go
+++ b/src/runtime/mfinal.go
@@ -199,7 +199,7 @@ func runfinq() {
 					if len(ityp.mhdr) != 0 {
 						// convert to interface with methods
 						// this conversion is guaranteed to succeed - we checked in SetFinalizer
-						assertE2I(ityp, *(*eface)(frame), (*iface)(frame))
+						*(*iface)(frame) = assertE2I(ityp, *(*eface)(frame))
 					}
 				default:
 					throw("bad kind in runfinq")
@@ -384,7 +384,7 @@ func SetFinalizer(obj interface{}, finalizer interface{}) {
 			// ok - satisfies empty interface
 			goto okarg
 		}
-		if assertE2I2(ityp, *efaceOf(&obj), nil) {
+		if _, ok := assertE2I2(ityp, *efaceOf(&obj)); ok {
 			goto okarg
 		}
 	}