[dev.regabi] cmd/compile: merge OBLOCK and OEMPTY

OEMPTY is an empty *statement*, but it confusingly
gets handled as an expression in a few places.
More confusingly, OEMPTY often has an init list,
making it not empty at all. Replace uses and analysis
of OEMPTY with OBLOCK instead.

Passes buildall w/ toolstash -cmp.

Change-Id: I8d4fcef151e4f441fa19b1b96da5272d778131d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/274594
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -302,7 +302,7 @@ func (e *Escape) stmt(n ir.Node) {
 	default:
 		base.Fatalf("unexpected stmt: %v", n)
 
-	case ir.ODCLCONST, ir.ODCLTYPE, ir.OEMPTY, ir.OFALL, ir.OINLMARK:
+	case ir.ODCLCONST, ir.ODCLTYPE, ir.OFALL, ir.OINLMARK:
 		// nop
 
 	case ir.OBREAK, ir.OCONTINUE, ir.OGOTO:

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

@@ -1048,6 +1048,17 @@ func (w *exportWriter) stmt(n ir.Node) {
 	}
 
 	switch op := n.Op(); op {
+	case ir.OBLOCK:
+		// No OBLOCK in export data.
+		// Inline content into this statement list,
+		// like the init list above.
+		// (At the moment neither the parser nor the typechecker
+		// generate OBLOCK nodes except to denote an empty
+		// function body, although that may change.)
+		for _, n := range n.List().Slice() {
+			w.stmt(n)
+		}
+
 	case ir.ODCL:
 		w.op(ir.ODCL)
 		w.pos(n.Left().Pos())
@@ -1129,9 +1140,6 @@ func (w *exportWriter) stmt(n ir.Node) {
 		w.op(ir.OFALL)
 		w.pos(n.Pos())
 
-	case ir.OEMPTY:
-		// nothing to emit
-
 	case ir.OBREAK, ir.OCONTINUE, ir.OGOTO, ir.OLABEL:
 		w.op(op)
 		w.pos(n.Pos())

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

@@ -742,7 +742,9 @@ func (r *importReader) stmtList() []ir.Node {
 		if n == nil {
 			break
 		}
-		// OBLOCK nodes may be created when importing ODCL nodes - unpack them
+		// OBLOCK nodes are not written to the import data directly,
+		// but the handling of ODCL calls liststmt, which creates one.
+		// Inline them into the statement list.
 		if n.Op() == ir.OBLOCK {
 			list = append(list, n.List().Slice()...)
 		} else {

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

@@ -88,9 +88,11 @@ func fninit(n []ir.Node) {
 		s := lookupN("init.", i)
 		fn := ir.AsNode(s.Def).Name().Defn
 		// Skip init functions with empty bodies.
-		if fn.Body().Len() == 1 && fn.Body().First().Op() == ir.OEMPTY {
+		if fn.Body().Len() == 1 {
+			if stmt := fn.Body().First(); stmt.Op() == ir.OBLOCK && stmt.List().Len() == 0 {
 				continue
 			}
+		}
 		fns = append(fns, s.Linksym())
 	}
 

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

@@ -396,7 +396,7 @@ func (v *hairyVisitor) visit(n ir.Node) bool {
 	case ir.OAPPEND:
 		v.budget -= inlineExtraAppendCost
 
-	case ir.ODCLCONST, ir.OEMPTY, ir.OFALL:
+	case ir.ODCLCONST, ir.OFALL:
 		// These nodes don't produce code; omit from inlining budget.
 		return false
 
@@ -425,6 +425,11 @@ func (v *hairyVisitor) visit(n ir.Node) bool {
 			v.usedLocals[n] = true
 		}
 
+	case ir.OBLOCK:
+		// The only OBLOCK we should see at this point is an empty one.
+		// In any event, let the visitList(n.List()) below take care of the statements,
+		// and don't charge for the OBLOCK itself. The ++ undoes the -- below.
+		v.budget++
 	}
 
 	v.budget--

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

@@ -164,7 +164,7 @@ func (p *noder) funcBody(fn *ir.Func, block *syntax.BlockStmt) {
 	if block != nil {
 		body := p.stmts(block.List)
 		if body == nil {
-			body = []ir.Node{ir.Nod(ir.OEMPTY, nil, nil)}
+			body = []ir.Node{ir.Nod(ir.OBLOCK, nil, nil)}
 		}
 		fn.PtrBody().Set(body)
 
@@ -967,7 +967,9 @@ func (p *noder) stmtsFall(stmts []syntax.Stmt, fallOK bool) []ir.Node {
 	for i, stmt := range stmts {
 		s := p.stmtFall(stmt, fallOK && i+1 == len(stmts))
 		if s == nil {
-		} else if s.Op() == ir.OBLOCK && s.Init().Len() == 0 {
+		} else if s.Op() == ir.OBLOCK && s.List().Len() > 0 {
+			// Inline non-empty block.
+			// Empty blocks must be preserved for checkreturn.
 			nodes = append(nodes, s.List().Slice()...)
 		} else {
 			nodes = append(nodes, s)
@@ -991,7 +993,7 @@ func (p *noder) stmtFall(stmt syntax.Stmt, fallOK bool) ir.Node {
 		l := p.blockStmt(stmt)
 		if len(l) == 0 {
 			// TODO(mdempsky): Line number?
-			return ir.Nod(ir.OEMPTY, nil, nil)
+			return ir.Nod(ir.OBLOCK, nil, nil)
 		}
 		return liststmt(l)
 	case *syntax.ExprStmt:
@@ -1166,7 +1168,7 @@ func (p *noder) ifStmt(stmt *syntax.IfStmt) ir.Node {
 	n.PtrBody().Set(p.blockStmt(stmt.Then))
 	if stmt.Else != nil {
 		e := p.stmt(stmt.Else)
-		if e.Op() == ir.OBLOCK && e.Init().Len() == 0 {
+		if e.Op() == ir.OBLOCK {
 			n.PtrRlist().Set(e.List().Slice())
 		} else {
 			n.PtrRlist().Set1(e)
@@ -1319,7 +1321,7 @@ func (p *noder) labeledStmt(label *syntax.LabeledStmt, fallOK bool) ir.Node {
 
 	l := []ir.Node{lhs}
 	if ls != nil {
-		if ls.Op() == ir.OBLOCK && ls.Init().Len() == 0 {
+		if ls.Op() == ir.OBLOCK {
 			l = append(l, ls.List().Slice()...)
 		} else {
 			l = append(l, ls)

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

@@ -676,7 +676,7 @@ func (o *Order) stmt(n ir.Node) {
 		o.cleanTemp(t)
 
 	// Special: does not save n onto out.
-	case ir.OBLOCK, ir.OEMPTY:
+	case ir.OBLOCK:
 		o.stmtList(n.List())
 
 	// Special: n->left is not an expression; save as is.

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

@@ -1071,7 +1071,7 @@ func (s *state) stmt(n ir.Node) {
 		s.stmtList(n.List())
 
 	// No-ops
-	case ir.OEMPTY, ir.ODCLCONST, ir.ODCLTYPE, ir.OFALL:
+	case ir.ODCLCONST, ir.ODCLTYPE, ir.OFALL:
 
 	// Expression statements
 	case ir.OCALLFUNC:

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

@@ -1950,13 +1950,16 @@ func typecheck1(n ir.Node, top int) (res ir.Node) {
 	case ir.OBREAK,
 		ir.OCONTINUE,
 		ir.ODCL,
-		ir.OEMPTY,
 		ir.OGOTO,
 		ir.OFALL,
 		ir.OVARKILL,
 		ir.OVARLIVE:
 		ok |= ctxStmt
 
+	case ir.OBLOCK:
+		ok |= ctxStmt
+		typecheckslice(n.List().Slice(), ctxStmt)
+
 	case ir.OLABEL:
 		ok |= ctxStmt
 		decldepth++
@@ -1964,7 +1967,7 @@ func typecheck1(n ir.Node, top int) (res ir.Node) {
 			// Empty identifier is valid but useless.
 			// Eliminate now to simplify life later.
 			// See issues 7538, 11589, 11593.
-			n = ir.NodAt(n.Pos(), ir.OEMPTY, nil, nil)
+			n = ir.NodAt(n.Pos(), ir.OBLOCK, nil, nil)
 		}
 
 	case ir.ODEFER:
@@ -3808,7 +3811,7 @@ func deadcode(fn *ir.Func) {
 		}
 	}
 
-	fn.PtrBody().Set([]ir.Node{ir.Nod(ir.OEMPTY, nil, nil)})
+	fn.PtrBody().Set([]ir.Node{ir.Nod(ir.OBLOCK, nil, nil)})
 }
 
 func deadcodeslice(nn *ir.Nodes) {

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

@@ -142,7 +142,6 @@ func walkstmt(n ir.Node) ir.Node {
 		ir.OPRINT,
 		ir.OPRINTN,
 		ir.OPANIC,
-		ir.OEMPTY,
 		ir.ORECOVER,
 		ir.OGETG:
 		if n.Typecheck() == 0 {
@@ -155,7 +154,7 @@ func walkstmt(n ir.Node) ir.Node {
 		if wascopy && n.Op() == ir.ONAME {
 			// copy rewrote to a statement list and a temp for the length.
 			// Throw away the temp to avoid plain values as statements.
-			n = ir.NodAt(n.Pos(), ir.OEMPTY, nil, nil)
+			n = ir.NodAt(n.Pos(), ir.OBLOCK, nil, nil)
 		}
 		n = addinit(n, init.Slice())
 
@@ -470,7 +469,7 @@ opswitch:
 		ir.Dump("walk", n)
 		base.Fatalf("walkexpr: switch 1 unknown op %+S", n)
 
-	case ir.ONONAME, ir.OEMPTY, ir.OGETG, ir.ONEWOBJ, ir.OMETHEXPR:
+	case ir.ONONAME, ir.OGETG, ir.ONEWOBJ, ir.OMETHEXPR:
 
 	case ir.OTYPE, ir.ONAME, ir.OLITERAL, ir.ONIL:
 		// TODO(mdempsky): Just return n; see discussion on CL 38655.
@@ -609,7 +608,7 @@ opswitch:
 		}
 
 		if oaslit(n, init) {
-			n = ir.NodAt(n.Pos(), ir.OEMPTY, nil, nil)
+			n = ir.NodAt(n.Pos(), ir.OBLOCK, nil, nil)
 			break
 		}
 
@@ -2032,10 +2031,10 @@ func walkprint(nn ir.Node, init *ir.Nodes) ir.Node {
 	typecheckslice(calls, ctxStmt)
 	walkexprlist(calls, init)
 
-	r := ir.Nod(ir.OEMPTY, nil, nil)
+	r := ir.Nod(ir.OBLOCK, nil, nil)
 	r = typecheck(r, ctxStmt)
-	r = walkexpr(r, init)
+	r = walkstmt(r)
-	r.PtrInit().Set(calls)
+	r.PtrList().Set(calls)
 	return r
 }
 

src/cmd/compile/internal/ir/fmt.go

@@ -942,6 +942,11 @@ func stmtFmt(n Node, s fmt.State, mode FmtMode) {
 			mode.Fprintf(s, "%.v = %.v", n.List(), n.Rlist())
 		}
 
+	case OBLOCK:
+		if n.List().Len() != 0 {
+			mode.Fprintf(s, "%v", n.List())
+		}
+
 	case ORETURN:
 		mode.Fprintf(s, "return %.v", n.List())
 
@@ -1044,9 +1049,6 @@ func stmtFmt(n Node, s fmt.State, mode FmtMode) {
 			mode.Fprintf(s, "%#v", n.Op())
 		}
 
-	case OEMPTY:
-		break
-
 	case OLABEL:
 		mode.Fprintf(s, "%v: ", n.Sym())
 	}
@@ -1155,12 +1157,12 @@ var OpPrec = []int{
 	OAS2MAPR:    -1,
 	OAS2RECV:    -1,
 	OASOP:       -1,
+	OBLOCK:      -1,
 	OBREAK:      -1,
 	OCASE:       -1,
 	OCONTINUE:   -1,
 	ODCL:        -1,
 	ODEFER:      -1,
-	OEMPTY:      -1,
 	OFALL:       -1,
 	OFOR:        -1,
 	OFORUNTIL:   -1,

src/cmd/compile/internal/ir/node.go

@@ -301,7 +301,7 @@ const (
 	OCASE
 	OCONTINUE // continue [Sym]
 	ODEFER    // defer Left (Left must be call)
-	OEMPTY    // no-op (empty statement)
+	OEMPTY    // TODO(rsc): Delete. (Use OBLOCK instead.)
 	OFALL     // fallthrough
 	OFOR      // for Ninit; Left; Right { Nbody }
 	// OFORUNTIL is like OFOR, but the test (Left) is applied after the body:
@@ -781,8 +781,6 @@ func NodAt(pos src.XPos, op Op, nleft, nright Node) Node {
 		n := NewTypeAssertExpr(pos, nleft, typ)
 		n.SetOp(op)
 		return n
-	case OEMPTY:
-		return NewEmptyStmt(pos)
 	case OFOR:
 		return NewForStmt(pos, nil, nleft, nright, nil)
 	case OGO:

src/cmd/compile/internal/ir/stmt.go

@@ -266,22 +266,6 @@ func (n *DeferStmt) rawCopy() Node                 { c := *n; return &c }
 func (n *DeferStmt) Left() Node     { return n.Call }
 func (n *DeferStmt) SetLeft(x Node) { n.Call = x }
 
-// An EmptyStmt is an empty statement
-type EmptyStmt struct {
-	miniStmt
-}
-
-func NewEmptyStmt(pos src.XPos) *EmptyStmt {
-	n := &EmptyStmt{}
-	n.pos = pos
-	n.op = OEMPTY
-	return n
-}
-
-func (n *EmptyStmt) String() string                { return fmt.Sprint(n) }
-func (n *EmptyStmt) Format(s fmt.State, verb rune) { FmtNode(n, s, verb) }
-func (n *EmptyStmt) rawCopy() Node                 { c := *n; return &c }
-
 // A ForStmt is a non-range for loop: for Init; Cond; Post { Body }
 // Op can be OFOR or OFORUNTIL (!Cond).
 type ForStmt struct {