cmd/compile: return (and receive) medium-large results

includes three tests

Change-Id: I33ac0cfe35085d4b6ad2775abcaa3d7d6527b49f
Reviewed-on: https://go-review.googlesource.com/c/go/+/297031
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/abi/double_nested_struct.go

@@ -0,0 +1,55 @@
+// run
+
+//go:build !wasm
+// +build !wasm
+
+// Copyright 2021 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.
+
+// wasm is excluded because the compiler chatter about register abi pragma ends up
+// on stdout, and causes the expected output to not match. 
+
+package main
+
+import (
+	"fmt"
+)
+
+var sink *string
+
+type stringPair struct {
+	a, b string
+}
+
+type stringPairPair struct {
+	x, y stringPair
+}
+
+// The goal of this test is to be sure that the call arg/result expander works correctly
+// for a corner case of passing a 2-nested struct that fits in registers to/from calls.
+
+//go:registerparams
+//go:noinline
+func H(spp stringPairPair) string {
+	return spp.x.a + " " + spp.x.b + " " + spp.y.a + " " + spp.y.b
+}
+
+//go:registerparams
+//go:noinline
+func G(a,b,c,d string) stringPairPair {
+	return stringPairPair{stringPair{a,b},stringPair{c,d}}
+}
+
+
+func main() {
+	spp := G("this","is","a","test")
+	s := H(spp)
+	gotVsWant(s, "this is a test")
+}
+
+func gotVsWant(got, want string) {
+	if got != want {
+		fmt.Printf("FAIL, got %s, wanted %s\n", got, want)
+	}
+}