cmd/compile: don’t generate pointless gotos during inlining

Their only purpose in life was to suppress an error.
Suppress that error explicitly instead by reusing
an existing, aptly named Node field.

This generates fewer blocks during ssa construction.

name       old alloc/op     new alloc/op     delta
Template       47.5MB ± 0%      47.2MB ± 0%  -0.72%        (p=0.000 n=15+15)
Unicode        36.8MB ± 0%      36.8MB ± 0%    ~           (p=0.775 n=15+15)
GoTypes         143MB ± 0%       142MB ± 0%  -1.03%        (p=0.000 n=15+14)
Compiler        686MB ± 0%       674MB ± 0%  -1.75%        (p=0.000 n=15+15)

name       old allocs/op    new allocs/op    delta
Template         446k ± 0%        445k ± 0%  -0.20%        (p=0.000 n=15+15)
Unicode          355k ± 0%        355k ± 0%    ~           (p=0.235 n=13+15)
GoTypes         1.36M ± 0%       1.36M ± 0%  -0.41%        (p=0.000 n=13+15)
Compiler        5.77M ± 0%       5.70M ± 0%  -1.16%        (p=0.000 n=15+15)


Change-Id: I5f14afb833c9d355688d9a229eb820e95c7657bf
Reviewed-on: https://go-review.googlesource.com/27461
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: josharian

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

@@ -217,6 +217,9 @@ func newlab(n *Node) *Label {
 		lab = new(Label)
 		lab.Sym = s
 		s.Label = lab
+		if n.Used {
+			lab.Used = true
+		}
 		labellist = append(labellist, lab)
 	}
 

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

@@ -778,8 +778,9 @@ func mkinlcall1(n *Node, fn *Node, isddd bool) *Node {
 
 	body := subst.list(fn.Func.Inl)
 
-	body = append(body, Nod(OGOTO, retlabel, nil)) // avoid 'not used' when function doesn't have return
-	body = append(body, Nod(OLABEL, retlabel, nil))
+	lab := Nod(OLABEL, retlabel, nil)
+	lab.Used = true // avoid 'not used' when function doesn't have return
+	body = append(body, lab)
 
 	typecheckslice(body, Etop)
 

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

@@ -193,7 +193,7 @@ func buildssa(fn *Node) *ssa.Func {
 
 	// Check that we used all labels
 	for name, lab := range s.labels {
-		if !lab.used() && !lab.reported {
+		if !lab.used() && !lab.reported && !lab.defNode.Used {
 			yyerrorl(lab.defNode.Lineno, "label %v defined and not used", name)
 			lab.reported = true
 		}