cmd/compile: refactor generic visitor code

[mdempsky]

There's a bunch of implementations of ad hoc, generic IR tree walking code within the compiler frontend.

Examples:

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

Lines 789 to 814 in 351bc2f

	if n.Left() != nil && hasCall(n.Left()) {
	return true
	}
	if n.Right() != nil && hasCall(n.Right()) {
	return true
	}
	for _, x := range n.Init().Slice() {
	if hasCall(x) {
	return true
	}
	}
	for _, x := range n.Body().Slice() {
	if hasCall(x) {
	return true
	}
	}
	for _, x := range n.List().Slice() {
	if hasCall(x) {
	return true
	}
	}
	for _, x := range n.Rlist().Slice() {
	if hasCall(x) {
	return true
	}
	}

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

Lines 810 to 818 in 351bc2f

	if hascallchan(n.Left()) || hascallchan(n.Right()) {
	return true
	}
	for _, n1 := range n.List().Slice() {
	if hascallchan(n1) {
	return true
	}
	}
	for _, n2 := range n.Rlist().Slice() {

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

Lines 445 to 447 in 351bc2f

	return v.visit(n.Left()) || v.visit(n.Right()) ||
	v.visitList(n.List()) || v.visitList(n.Rlist()) ||
	v.visitList(n.Init()) || v.visitList(n.Body())

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

Lines 490 to 503 in 351bc2f

	cnt += countNodes(n.Left())
	cnt += countNodes(n.Right())
	for _, n1 := range n.Init().Slice() {
	cnt += countNodes(n1)
	}
	for _, n1 := range n.Body().Slice() {
	cnt += countNodes(n1)
	}
	for _, n1 := range n.List().Slice() {
	cnt += countNodes(n1)
	}
	for _, n1 := range n.Rlist().Slice() {
	cnt += countNodes(n1)
	}

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

Lines 828 to 845 in 351bc2f

	if a := v.visit(n.Left()); a != nil {
	return a
	}
	if a := v.visit(n.Right()); a != nil {
	return a
	}
	if a := v.visitList(n.List()); a != nil {
	return a
	}
	if a := v.visitList(n.Rlist()); a != nil {
	return a
	}
	if a := v.visitList(n.Init()); a != nil {
	return a
	}
	if a := v.visitList(n.Body()); a != nil {
	return a
	}

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

Lines 3700 to 3705 in 351bc2f

	markbreak(labels, n.Left(), implicit)
	markbreak(labels, n.Right(), implicit)
	markbreaklist(labels, n.Init(), implicit)
	markbreaklist(labels, n.Body(), implicit)
	markbreaklist(labels, n.List(), implicit)
	markbreaklist(labels, n.Rlist(), implicit)

It should be possible to rewrite a lot of this code to use ir.Inspect or ir.InspectList instead. This will eventually be helpful for getting rid of the generic Left/Right/etc methods.

For the markbreak case (and maybe others), it might be useful to provide ir analogs to ast.Walk and ast.Visitor. (See also how ast.Inspect is implemented, as that API was inspirational for ir.Inspect.)

/cc @cuonglm

[mdempsky]

/cc @rsc @zephyrtronium

[gopherbot]

Change https://golang.org/cl/275305 mentions this issue: [dev.regabi] cmd/compile: add RefersTo and DeclaredBy helpers

[rsc]

I have been working on this the last couple days. Please don't start. CLs coming soon!

[rsc]

https://go-review.googlesource.com/c/go/+/275311 is the CL introducing the API. See the rest of the stack for uses.
Best to discuss on the CLs themselves - I get too much GitHub spam to keep up with the issue tracker.

[mdempsky]

Russ's CLs have landed. This is done now.