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>

1 files changed, 47 insertions, 0 deletions

diff --git a/test/codegen/typeswitch.go b/test/codegen/typeswitch.go
new file mode 100644
index 0000000000..495853ed3c
--- /dev/null
+++ b/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))
+}