cmd/compile: avoid dynamic type when possible

If the expression type is a single compile-time known type, use that
type instead of the dynamic one, so the later passes of the compiler
could skip un-necessary runtime calls.

Thanks Youlin Feng for writing the original test case.

Change-Id: I3f65ab90f041474a9731338a82136c1d394c1773
Reviewed-on: https://go-review.googlesource.com/c/go/+/616975
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/ir/type.go

@@ -67,3 +67,23 @@ func NewDynamicType(pos src.XPos, rtype Node) *DynamicType {
 	n.op = ODYNAMICTYPE
 	return n
 }
+
+// ToStatic returns static type of dt if it is actually static.
+func (dt *DynamicType) ToStatic() Node {
+	if dt.Typecheck() == 0 {
+		base.Fatalf("missing typecheck: %v", dt)
+	}
+	if dt.RType != nil && dt.RType.Op() == OADDR {
+		addr := dt.RType.(*AddrExpr)
+		if addr.X.Op() == OLINKSYMOFFSET {
+			return TypeNode(dt.Type())
+		}
+	}
+	if dt.ITab != nil && dt.ITab.Op() == OADDR {
+		addr := dt.ITab.(*AddrExpr)
+		if addr.X.Op() == OLINKSYMOFFSET {
+			return TypeNode(dt.Type())
+		}
+	}
+	return nil
+}

src/cmd/compile/internal/noder/reader.go

@@ -3242,7 +3242,11 @@ func (r *reader) exprType() ir.Node {
 
 	dt := ir.NewDynamicType(pos, rtype)
 	dt.ITab = itab
-	return typed(typ, dt)
+	dt = typed(typ, dt).(*ir.DynamicType)
+	if st := dt.ToStatic(); st != nil {
+		return st
+	}
+	return dt
 }
 
 func (r *reader) op() ir.Op {

src/cmd/compile/internal/walk/switch.go

@@ -459,23 +459,6 @@ func walkSwitchType(sw *ir.SwitchStmt) {
 				nilGoto = jmp
 				continue
 			}
-			if n1.Op() == ir.ODYNAMICTYPE {
-				// Convert dynamic to static, if the dynamic is actually static.
-				// TODO: why isn't this OTYPE to begin with?
-				dt := n1.(*ir.DynamicType)
-				if dt.RType != nil && dt.RType.Op() == ir.OADDR {
-					addr := dt.RType.(*ir.AddrExpr)
-					if addr.X.Op() == ir.OLINKSYMOFFSET {
-						n1 = ir.TypeNode(n1.Type())
-					}
-				}
-				if dt.ITab != nil && dt.ITab.Op() == ir.OADDR {
-					addr := dt.ITab.(*ir.AddrExpr)
-					if addr.X.Op() == ir.OLINKSYMOFFSET {
-						n1 = ir.TypeNode(n1.Type())
-					}
-				}
-			}
 			cases = append(cases, oneCase{
 				pos: ncase.Pos(),
 				typ: n1,

test/codegen/typeswitch.go

@@ -0,0 +1,47 @@
+// asmcheck
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codegen
+
+type Ix interface {
+	X()
+}
+
+type Iy interface {
+	Y()
+}
+
+type Iz interface {
+	Z()
+}
+
+func swXYZ(a Ix) {
+	switch t := a.(type) {
+	case Iy: // amd64:-".*typeAssert"
+		t.Y()
+	case Iz: // amd64:-".*typeAssert"
+		t.Z()
+	}
+}
+
+type Ig[T any] interface {
+	G() T
+}
+
+func swGYZ[T any](a Ig[T]) {
+	switch t := a.(type) {
+	case Iy: // amd64:-".*typeAssert"
+		t.Y()
+	case Iz: // amd64:-".*typeAssert"
+		t.Z()
+	case interface{ G() T }: // amd64:-".*typeAssert",".*assertE2I"
+		t.G()
+	}
+}
+
+func swCaller() {
+	swGYZ[int]((Ig[int])(nil))
+}