cmd/compile: allow inlining of "for" loops

We already allow inlining "if" and "goto" statements, so we might as
well allow "for" loops too. The majority of frontend support is
already there too.

The critical missing feature at the moment is that inlining doesn't
properly reassociate OLABEL nodes with their control statement (e.g.,
OFOR) after inlining. This eventually causes SSA construction to fail.

As a workaround, this CL only enables inlining for unlabeled "for"
loops. It's left to a (yet unplanned) future CL to add support for
labeled "for" loops.

The increased opportunity for inlining leads to a small growth in
binary size. For example:

$ size go.old go.new
   text	   data	    bss	    dec	    hex	filename
9740163	 320064	 230656	10290883	 9d06c3	go.old
9793399	 320064	 230656	10344119	 9dd6b7	go.new

Updates #14768.
Fixes #41474.

Change-Id: I827db0b2b9d9fa2934db05caf6baa463f0cd032a
Reviewed-on: https://go-review.googlesource.com/c/go/+/256459
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

Author: mdempsky

src/cmd/compile/internal/gc/inl.go

@@ -385,25 +385,35 @@ func (v *hairyVisitor) visit(n *Node) bool {
 	case OCLOSURE,
 		OCALLPART,
 		ORANGE,
-		OFOR,
-		OFORUNTIL,
 		OSELECT,
 		OTYPESW,
 		OGO,
 		ODEFER,
 		ODCLTYPE, // can't print yet
-		OBREAK,
 		ORETJMP:
 		v.reason = "unhandled op " + n.Op.String()
 		return true
 
 	case OAPPEND:
 		v.budget -= inlineExtraAppendCost
 
-	case ODCLCONST, OEMPTY, OFALL, OLABEL:
+	case ODCLCONST, OEMPTY, OFALL:
 		// These nodes don't produce code; omit from inlining budget.
 		return false
 
+	case OLABEL:
+		// TODO(mdempsky): Add support for inlining labeled control statements.
+		if n.labeledControl() != nil {
+			v.reason = "labeled control"
+			return true
+		}
+
+	case OBREAK, OCONTINUE:
+		if n.Sym != nil {
+			// Should have short-circuited due to labeledControl above.
+			Fatalf("unexpected labeled break/continue: %v", n)
+		}
+
 	case OIF:
 		if Isconst(n.Left, CTBOOL) {
 			// This if and the condition cost nothing.

test/closure3.dir/main.go

@@ -238,8 +238,7 @@ func main() {
 				if c != 4 {
 					ppanic("c != 4")
 				}
-				for i := 0; i < 10; i++ { // prevent inlining
-				}
+				recover() // prevent inlining
 			}()
 		}()
 		if c != 4 {

test/inline.go

@@ -197,3 +197,26 @@ func gg(x int) { // ERROR "can inline gg"
 func hh(x int) { // ERROR "can inline hh"
 	ff(x - 1) // ERROR "inlining call to ff"  // ERROR "inlining call to gg"
 }
+
+// Issue #14768 - make sure we can inline for loops.
+func for1(fn func() bool) { // ERROR "can inline for1" "fn does not escape"
+	for {
+		if fn() {
+			break
+		} else {
+			continue
+		}
+	}
+}
+
+// BAD: for2 should be inlineable too.
+func for2(fn func() bool) { // ERROR "fn does not escape"
+Loop:
+	for {
+		if fn() {
+			break Loop
+		} else {
+			continue Loop
+		}
+	}
+}