cmd/compile: support inlining of type switches

This CL adds support for inlining type switches, including exporting
and importing them.

Type switches are represented mostly the same as expression switches.
However, if the type switch guard includes a short variable
declaration, then there are two differences: (1) there's an ONONAME
(in the OTYPESW's Left) to represent the overall pseudo declaration;
and (2) there's an ONAME (in each OCASE's Rlist) to represent the
per-case variables.

For simplicity, this CL simply writes out each variable separately
using iimport/iiexport's normal Vargen mechanism for disambiguating
identically named variables within a function. This could be improved
somewhat, but inlinable type switches are probably too uncommon to
merit the complexity.

While here, remove "case OCASE" from typecheck1. We only type check
"case" clauses as part of a "select" or "switch" statement, never as
standalone statements.

Fixes #37837

Change-Id: I8f42f6c9afdd821d6202af4a6bf1dbcbba0ef424
Reviewed-on: https://go-review.googlesource.com/c/go/+/266203
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

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

@@ -1138,13 +1138,10 @@ func (w *exportWriter) stmt(n *Node) {
 		w.pos(n.Pos)
 		w.stmtList(n.Ninit)
 		w.exprsOrNil(n.Left, nil)
-		w.stmtList(n.List)
+		w.caseList(n)
 
-	case OCASE:
-		w.op(OCASE)
-		w.pos(n.Pos)
-		w.stmtList(n.List)
-		w.stmtList(n.Nbody)
+	// case OCASE:
+	//	handled by caseList
 
 	case OFALL:
 		w.op(OFALL)
@@ -1168,6 +1165,24 @@ func (w *exportWriter) stmt(n *Node) {
 	}
 }
 
+func (w *exportWriter) caseList(sw *Node) {
+	namedTypeSwitch := sw.Op == OSWITCH && sw.Left != nil && sw.Left.Op == OTYPESW && sw.Left.Left != nil
+
+	cases := sw.List.Slice()
+	w.uint64(uint64(len(cases)))
+	for _, cas := range cases {
+		if cas.Op != OCASE {
+			Fatalf("expected OCASE, got %v", cas)
+		}
+		w.pos(cas.Pos)
+		w.stmtList(cas.List)
+		if namedTypeSwitch {
+			w.localName(cas.Rlist.First())
+		}
+		w.stmtList(cas.Nbody)
+	}
+}
+
 func (w *exportWriter) exprList(list Nodes) {
 	for _, n := range list.Slice() {
 		w.expr(n)
@@ -1232,6 +1247,19 @@ func (w *exportWriter) expr(n *Node) {
 		w.op(OTYPE)
 		w.typ(n.Type)
 
+	case OTYPESW:
+		w.op(OTYPESW)
+		w.pos(n.Pos)
+		var s *types.Sym
+		if n.Left != nil {
+			if n.Left.Op != ONONAME {
+				Fatalf("expected ONONAME, got %v", n.Left)
+			}
+			s = n.Left.Sym
+		}
+		w.localIdent(s, 0) // declared pseudo-variable, if any
+		w.exprsOrNil(n.Right, nil)
+
 	// case OTARRAY, OTMAP, OTCHAN, OTSTRUCT, OTINTER, OTFUNC:
 	// 	should have been resolved by typechecking - handled by default case
 

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

@@ -784,6 +784,28 @@ func (r *importReader) stmtList() []*Node {
 	return list
 }
 
+func (r *importReader) caseList(sw *Node) []*Node {
+	namedTypeSwitch := sw.Op == OSWITCH && sw.Left != nil && sw.Left.Op == OTYPESW && sw.Left.Left != nil
+
+	cases := make([]*Node, r.uint64())
+	for i := range cases {
+		cas := nodl(r.pos(), OCASE, nil, nil)
+		cas.List.Set(r.stmtList())
+		if namedTypeSwitch {
+			// Note: per-case variables will have distinct, dotted
+			// names after import. That's okay: swt.go only needs
+			// Sym for diagnostics anyway.
+			caseVar := newnamel(cas.Pos, r.ident())
+			declare(caseVar, dclcontext)
+			cas.Rlist.Set1(caseVar)
+			caseVar.Name.Defn = sw.Left
+		}
+		cas.Nbody.Set(r.stmtList())
+		cases[i] = cas
+	}
+	return cases
+}
+
 func (r *importReader) exprList() []*Node {
 	var list []*Node
 	for {
@@ -831,6 +853,14 @@ func (r *importReader) node() *Node {
 	case OTYPE:
 		return typenod(r.typ())
 
+	case OTYPESW:
+		n := nodl(r.pos(), OTYPESW, nil, nil)
+		if s := r.ident(); s != nil {
+			n.Left = npos(n.Pos, newnoname(s))
+		}
+		n.Right, _ = r.exprsOrNil()
+		return n
+
 	// case OTARRAY, OTMAP, OTCHAN, OTSTRUCT, OTINTER, OTFUNC:
 	//      unreachable - should have been resolved by typechecking
 
@@ -1025,16 +1055,11 @@ func (r *importReader) node() *Node {
 		n := nodl(r.pos(), op, nil, nil)
 		n.Ninit.Set(r.stmtList())
 		n.Left, _ = r.exprsOrNil()
-		n.List.Set(r.stmtList())
+		n.List.Set(r.caseList(n))
 		return n
 
-	case OCASE:
-		n := nodl(r.pos(), OCASE, nil, nil)
-		n.List.Set(r.exprList())
-		// TODO(gri) eventually we must declare variables for type switch
-		// statements (type switch statements are not yet exported)
-		n.Nbody.Set(r.stmtList())
-		return n
+	// case OCASE:
+	//	handled by caseList
 
 	case OFALL:
 		n := nodl(r.pos(), OFALL, nil, nil)

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

@@ -392,13 +392,9 @@ func (v *hairyVisitor) visit(n *Node) bool {
 		v.reason = "call to recover"
 		return true
 
-	case OCALLPART:
-		// OCALLPART is inlineable, but no extra cost to the budget
-
 	case OCLOSURE,
 		ORANGE,
 		OSELECT,
-		OTYPESW,
 		OGO,
 		ODEFER,
 		ODCLTYPE, // can't print yet

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

@@ -2065,11 +2065,6 @@ func typecheck1(n *Node, top int) (res *Node) {
 		n.Type = nil
 		return n
 
-	case OCASE:
-		ok |= ctxStmt
-		typecheckslice(n.List.Slice(), ctxExpr)
-		typecheckslice(n.Nbody.Slice(), ctxStmt)
-
 	case ODCLFUNC:
 		ok |= ctxStmt
 		typecheckfunc(n)

test/fixedbugs/issue37837.dir/a.go

@@ -0,0 +1,33 @@
+// Copyright 2020 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 a
+
+func F(i interface{}) int { // ERROR "can inline F" "i does not escape"
+	switch i.(type) {
+	case nil:
+		return 0
+	case int:
+		return 1
+	case float64:
+		return 2
+	default:
+		return 3
+	}
+}
+
+func G(i interface{}) interface{} { // ERROR "can inline G" "leaking param: i"
+	switch i := i.(type) {
+	case nil: // ERROR "moved to heap: i"
+		return &i
+	case int: // ERROR "moved to heap: i"
+		return &i
+	case float64: // ERROR "moved to heap: i"
+		return &i
+	case string, []byte: // ERROR "moved to heap: i"
+		return &i
+	default: // ERROR "moved to heap: i"
+		return &i
+	}
+}

test/fixedbugs/issue37837.dir/b.go

@@ -0,0 +1,32 @@
+// Copyright 2020 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 main
+
+import "./a"
+
+func main() {
+	// Test that inlined type switches without short variable
+	// declarations work correctly.
+	check(0, a.F(nil)) // ERROR "inlining call to a.F"
+	check(1, a.F(0))   // ERROR "inlining call to a.F" "does not escape"
+	check(2, a.F(0.0)) // ERROR "inlining call to a.F" "does not escape"
+	check(3, a.F(""))  // ERROR "inlining call to a.F" "does not escape"
+
+	// Test that inlined type switches with short variable
+	// declarations work correctly.
+	_ = a.G(nil).(*interface{})                       // ERROR "inlining call to a.G"
+	_ = a.G(1).(*int)                                 // ERROR "inlining call to a.G" "does not escape"
+	_ = a.G(2.0).(*float64)                           // ERROR "inlining call to a.G" "does not escape"
+	_ = (*a.G("").(*interface{})).(string)            // ERROR "inlining call to a.G" "does not escape"
+	_ = (*a.G(([]byte)(nil)).(*interface{})).([]byte) // ERROR "inlining call to a.G" "does not escape"
+	_ = (*a.G(true).(*interface{})).(bool)            // ERROR "inlining call to a.G" "does not escape"
+}
+
+//go:noinline
+func check(want, got int) {
+	if want != got {
+		println("want", want, "but got", got)
+	}
+}

test/fixedbugs/issue37837.go

@@ -0,0 +1,7 @@
+// errorcheckandrundir -0 -m
+
+// Copyright 2020 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 ignored

test/inline.go

@@ -152,8 +152,7 @@ func switchBreak(x, y int) int {
 	return n
 }
 
-// can't currently inline functions with a type switch
-func switchType(x interface{}) int { // ERROR "x does not escape"
+func switchType(x interface{}) int { // ERROR "can inline switchType" "x does not escape"
 	switch x.(type) {
 	case int:
 		return x.(int)