cmd/internal/gc: inline x := y.(*T) and x, ok := y.(*T)

These can be implemented with just a compare and a move instruction.
Do so, avoiding the overhead of a call into the runtime.

These assertions are a significant cost in Go code that uses interface{}
as a safe alternative to C's void* (or unsafe.Pointer), such as the
current version of the Go compiler.

*T here includes pointer to T but also any Go type represented as
a single pointer (chan, func, map). It does not include [1]*T or struct{*int}.
That requires more work in other parts of the compiler; there is a TODO.

Change-Id: I7ff681c20d2c3eb6ad11dd7b3a37b1f3dda23965
Reviewed-on: https://go-review.googlesource.com/7862
Reviewed-by: Rob Pike <r@golang.org>

test/interface/assertinline.go

diff --git a/test/interface/assertinline.go b/test/interface/assertinline.go
new file mode 100644
index 0000000..faa848a
--- /dev/null
+++ b/test/interface/assertinline.go
@@ -0,0 +1,53 @@
+// errorcheck -0 -d=typeassert
+
+// Copyright 2015 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 p
+
+func assertptr(x interface{}) *int {
+	return x.(*int) // ERROR "type assertion inlined"
+}
+
+func assertptr2(x interface{}) (*int, bool) {
+	z, ok := x.(*int) // ERROR "type assertion inlined"
+	return z, ok
+}
+
+func assertfunc(x interface{}) func() {
+	return x.(func()) // ERROR "type assertion inlined"
+}
+
+func assertfunc2(x interface{}) (func(), bool) {
+	z, ok := x.(func()) // ERROR "type assertion inlined"
+	return z, ok
+}
+
+// TODO(rsc): struct{*int} is stored directly in the interface
+// and should be possible to fetch back out of the interface,
+// but more of the general data movement code needs to
+// realize that before we can inline the assertion.
+
+func assertstruct(x interface{}) struct{ *int } {
+	return x.(struct{ *int }) // ERROR "type assertion not inlined"
+}
+
+func assertstruct2(x interface{}) (struct{ *int }, bool) {
+	z, ok := x.(struct{ *int }) // ERROR "type assertion not inlined"
+	return z, ok
+}
+
+func assertbig(x interface{}) complex128 {
+	return x.(complex128) // ERROR "type assertion not inlined"
+}
+
+func assertbig2(x interface{}) (complex128, bool) {
+	z, ok := x.(complex128) // ERROR "type assertion not inlined"
+	return z, ok
+}
+
+func assertbig2ok(x interface{}) (complex128, bool) {
+	_, ok := x.(complex128) // ERROR "type assertion [(]ok only[)] inlined"
+	return 0, ok
+}