cmd/compile: load properly constant values from itabs

While looking at the SSA of following code, i noticed
that these rules do not work properly, and the types
are loaded indirectly through an itab, instead of statically.

type M interface{ M() }
type A interface{ A() }

type Impl struct{}
func (*Impl) M() {}
func (*Impl) A() {}

func main() {
        var a M = &Impl{}
        a.(A).A()
}

Change-Id: Ia275993f81a2e7302102d4ff87ac28586023d13c
GitHub-Last-Rev: 4bfc9019172929d0b0f1c8a1b7eb28cdbc9b87ef
GitHub-Pull-Request: golang/go#71784
Reviewed-on: https://go-review.googlesource.com/c/go/+/649500
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/test/codegen/load_type_from_itab.go b/test/codegen/load_type_from_itab.go
new file mode 100644
index 0000000000..b47044fcbd
--- /dev/null
+++ b/test/codegen/load_type_from_itab.go
@@ -0,0 +1,24 @@
+// asmcheck
+
+// Copyright 2025 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.
+
+// This test makes sure that we statically load a type from an itab, instead
+// of doing a indirect load from thet itab.
+
+package codegen
+
+type M interface{ M() }
+type A interface{ A() }
+
+type Impl struct{}
+
+func (*Impl) M() {}
+func (*Impl) A() {}
+
+func main() {
+	var a M = &Impl{}
+	// amd64:`LEAQ\ttype:.*Impl`
+	a.(A).A()
+}