src/cmd/compile/internal/gc/initorder.go - go - Git at Google

 // Copyright 2019 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 gc

 import (
 	"bytes"
 	"container/heap"
 	"fmt"
 )

 // Package initialization
 //
 // Here we implement the algorithm for ordering package-level variable
 // initialization. The spec is written in terms of variable
 // initialization, but multiple variables initialized by a single
 // assignment are handled together, so here we instead focus on
 // ordering initialization assignments. Conveniently, this maps well
 // to how we represent package-level initializations using the Node
 // AST.
 //
 // Assignments are in one of three phases: NotStarted, Pending, or
 // Done. For assignments in the Pending phase, we use Xoffset to
 // record the number of unique variable dependencies whose
 // initialization assignment is not yet Done. We also maintain a
 // "blocking" map that maps assignments back to all of the assignments
 // that depend on it.
 //
 // For example, for an initialization like:
 //
 //     var x = f(a, b, b)
 //     var a, b = g()
 //
 // the "x = f(a, b, b)" assignment depends on two variables (a and b),
 // so its Xoffset will be 2. Correspondingly, the "a, b = g()"
 // assignment's "blocking" entry will have two entries back to x's
 // assignment.
 //
 // Logically, initialization works by (1) taking all NotStarted
 // assignments, calculating their dependencies, and marking them
 // Pending; (2) adding all Pending assignments with Xoffset==0 to a
 // "ready" priority queue (ordered by variable declaration position);
 // and (3) iteratively processing the next Pending assignment from the
 // queue, decreasing the Xoffset of assignments it's blocking, and
 // adding them to the queue if decremented to 0.
 //
 // As an optimization, we actually apply each of these three steps for
 // each assignment. This yields the same order, but keeps queue size
 // down and thus also heap operation costs.

 // Static initialization phase.
 // These values are stored in two bits in Node.flags.
 const (
 	InitNotStarted = iota
 	InitDone
 	InitPending
 )

 type InitOrder struct {
 	// blocking maps initialization assignments to the assignments
 	// that depend on it.
 	blocking map[*Node][]*Node

 	// ready is the queue of Pending initialization assignments
 	// that are ready for initialization.
 	ready declOrder
 }

 // initOrder computes initialization order for a list l of
 // package-level declarations (in declaration order) and outputs the
 // corresponding list of statements to include in the init() function
 // body.
 func initOrder(l []*Node) []*Node {
 	s := InitSchedule{
 		initplans: make(map[*Node]*InitPlan),
 		inittemps: make(map[*Node]*Node),
 	}
 	o := InitOrder{
 		blocking: make(map[*Node][]*Node),
 	}

 	// Process all package-level assignment in declaration order.
 	for _, n := range l {
 		switch n.Op {
 		case OAS, OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
 			o.processAssign(n)
 			o.flushReady(s.staticInit)
 		case ODCLCONST, ODCLFUNC, ODCLTYPE:
 			// nop
 		default:
 			Fatalf("unexpected package-level statement: %v", n)
 		}
 	}

 	// Check that all assignments are now Done; if not, there must
 	// have been a dependency cycle.
 	for _, n := range l {
 		switch n.Op {
 		case OAS, OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
 			if n.Initorder() != InitDone {
 				// If there have already been errors
 				// printed, those errors may have
 				// confused us and there might not be
 				// a loop. Let the user fix those
 				// first.
 				if nerrors > 0 {
 					errorexit()
 				}

 				findInitLoopAndExit(firstLHS(n), new([]*Node))
 				Fatalf("initialization unfinished, but failed to identify loop")
 			}
 		}
 	}

 	// Invariant consistency check. If this is non-zero, then we
 	// should have found a cycle above.
 	if len(o.blocking) != 0 {
 		Fatalf("expected empty map: %v", o.blocking)
 	}

 	return s.out
 }

 func (o *InitOrder) processAssign(n *Node) {
 	if n.Initorder() != InitNotStarted || n.Xoffset != BADWIDTH {
 		Fatalf("unexpected state: %v, %v, %v", n, n.Initorder(), n.Xoffset)
 	}

 	n.SetInitorder(InitPending)
 	n.Xoffset = 0

 	// Compute number of variable dependencies and build the
 	// inverse dependency ("blocking") graph.
 	for dep := range collectDeps(n, true) {
 		defn := dep.Name.Defn
 		// Skip dependencies on functions (PFUNC) and
 		// variables already initialized (InitDone).
 		if dep.Class() != PEXTERN || defn.Initorder() == InitDone {
 			continue
 		}
 		n.Xoffset++
 		o.blocking[defn] = append(o.blocking[defn], n)
 	}

 	if n.Xoffset == 0 {
 		heap.Push(&o.ready, n)
 	}
 }

 // flushReady repeatedly applies initialize to the earliest (in
 // declaration order) assignment ready for initialization and updates
 // the inverse dependency ("blocking") graph.
 func (o *InitOrder) flushReady(initialize func(*Node)) {
 	for o.ready.Len() != 0 {
 		n := heap.Pop(&o.ready).(*Node)
 		if n.Initorder() != InitPending || n.Xoffset != 0 {
 			Fatalf("unexpected state: %v, %v, %v", n, n.Initorder(), n.Xoffset)
 		}

 		initialize(n)
 		n.SetInitorder(InitDone)
 		n.Xoffset = BADWIDTH

 		blocked := o.blocking[n]
 		delete(o.blocking, n)

 		for _, m := range blocked {
 			m.Xoffset--
 			if m.Xoffset == 0 {
 				heap.Push(&o.ready, m)
 			}
 		}
 	}
 }

 // findInitLoopAndExit searches for an initialization loop involving variable
 // or function n. If one is found, it reports the loop as an error and exits.
 //
 // path points to a slice used for tracking the sequence of
 // variables/functions visited. Using a pointer to a slice allows the
 // slice capacity to grow and limit reallocations.
 func findInitLoopAndExit(n *Node, path *[]*Node) {
 	// We implement a simple DFS loop-finding algorithm. This
 	// could be faster, but initialization cycles are rare.

 	for i, x := range *path {
 		if x == n {
 			reportInitLoopAndExit((*path)[i:])
 			return
 		}
 	}

 	// There might be multiple loops involving n; by sorting
 	// references, we deterministically pick the one reported.
 	refers := collectDeps(n.Name.Defn, false).Sorted(func(ni, nj *Node) bool {
 		return ni.Pos.Before(nj.Pos)
 	})

 	*path = append(*path, n)
 	for _, ref := range refers {
 		// Short-circuit variables that were initialized.
 		if ref.Class() == PEXTERN && ref.Name.Defn.Initorder() == InitDone {
 			continue
 		}

 		findInitLoopAndExit(ref, path)
 	}
 	*path = (*path)[:len(*path)-1]
 }

 // reportInitLoopAndExit reports and initialization loop as an error
 // and exits. However, if l is not actually an initialization loop, it
 // simply returns instead.
 func reportInitLoopAndExit(l []*Node) {
 	// Rotate loop so that the earliest variable declaration is at
 	// the start.
 	i := -1
 	for j, n := range l {
 		if n.Class() == PEXTERN && (i == -1 || n.Pos.Before(l[i].Pos)) {
 			i = j
 		}
 	}
 	if i == -1 {
 		// False positive: loop only involves recursive
 		// functions. Return so that findInitLoop can continue
 		// searching.
 		return
 	}
 	l = append(l[i:], l[:i]...)

 	// TODO(mdempsky): Method values are printed as "T.m-fm"
 	// rather than "T.m". Figure out how to avoid that.

 	var msg bytes.Buffer
 	fmt.Fprintf(&msg, "initialization loop:\n")
 	for _, n := range l {
 		fmt.Fprintf(&msg, "\t%v: %v refers to\n", n.Line(), n)
 	}
 	fmt.Fprintf(&msg, "\t%v: %v", l[0].Line(), l[0])

 	yyerrorl(l[0].Pos, msg.String())
 	errorexit()
 }

 // collectDeps returns all of the package-level functions and
 // variables that declaration n depends on. If transitive is true,
 // then it also includes the transitive dependencies of any depended
 // upon functions (but not variables).
 func collectDeps(n *Node, transitive bool) NodeSet {
 	d := initDeps{transitive: transitive}
 	switch n.Op {
 	case OAS:
 		d.inspect(n.Right)
 	case OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
 		d.inspect(n.Right)
 	case ODCLFUNC:
 		d.inspectList(n.Nbody)
 	default:
 		Fatalf("unexpected Op: %v", n.Op)
 	}
 	return d.seen
 }

 type initDeps struct {
 	transitive bool
 	seen       NodeSet
 }

 func (d *initDeps) inspect(n *Node)     { inspect(n, d.visit) }
 func (d *initDeps) inspectList(l Nodes) { inspectList(l, d.visit) }

 // visit calls foundDep on any package-level functions or variables
 // referenced by n, if any.
 func (d *initDeps) visit(n *Node) bool {
 	switch n.Op {
 	case ONAME:
 		if n.isMethodExpression() {
 			d.foundDep(asNode(n.Type.FuncType().Nname))
 			return false
 		}

 		switch n.Class() {
 		case PEXTERN, PFUNC:
 			d.foundDep(n)
 		}

 	case OCLOSURE:
 		d.inspectList(n.Func.Closure.Nbody)

 	case ODOTMETH, OCALLPART:
 		d.foundDep(asNode(n.Type.FuncType().Nname))
 	}

 	return true
 }

 // foundDep records that we've found a dependency on n by adding it to
 // seen.
 func (d *initDeps) foundDep(n *Node) {
 	// Can happen with method expressions involving interface
 	// types; e.g., fixedbugs/issue4495.go.
 	if n == nil {
 		return
 	}

 	// Names without definitions aren't interesting as far as
 	// initialization ordering goes.
 	if n.Name.Defn == nil {
 		return
 	}

 	if d.seen.Has(n) {
 		return
 	}
 	d.seen.Add(n)
 	if d.transitive && n.Class() == PFUNC {
 		d.inspectList(n.Name.Defn.Nbody)
 	}
 }

 // declOrder implements heap.Interface, ordering assignment statements
 // by the position of their first LHS expression.
 //
 // N.B., the Pos of the first LHS expression is used because because
 // an OAS node's Pos may not be unique. For example, given the
 // declaration "var a, b = f(), g()", "a" must be ordered before "b",
 // but both OAS nodes use the "=" token's position as their Pos.
 type declOrder []*Node

 func (s declOrder) Len() int           { return len(s) }
 func (s declOrder) Less(i, j int) bool { return firstLHS(s[i]).Pos.Before(firstLHS(s[j]).Pos) }
 func (s declOrder) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

 func (s *declOrder) Push(x interface{}) { *s = append(*s, x.(*Node)) }
 func (s *declOrder) Pop() interface{} {
 	n := (*s)[len(*s)-1]
 	*s = (*s)[:len(*s)-1]
 	return n
 }

 // firstLHS returns the first expression on the left-hand side of
 // assignment n.
 func firstLHS(n *Node) *Node {
 	switch n.Op {
 	case OAS:
 		return n.Left
 	case OAS2DOTTYPE, OAS2FUNC, OAS2RECV, OAS2MAPR:
 		return n.List.First()
 	}

 	Fatalf("unexpected Op: %v", n.Op)
 	return nil
 }
	// Copyright 2019 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 gc

	import (
	"bytes"
	"container/heap"
	"fmt"
	)

	// Package initialization
	//
	// Here we implement the algorithm for ordering package-level variable
	// initialization. The spec is written in terms of variable
	// initialization, but multiple variables initialized by a single
	// assignment are handled together, so here we instead focus on
	// ordering initialization assignments. Conveniently, this maps well
	// to how we represent package-level initializations using the Node
	// AST.
	//
	// Assignments are in one of three phases: NotStarted, Pending, or
	// Done. For assignments in the Pending phase, we use Xoffset to
	// record the number of unique variable dependencies whose
	// initialization assignment is not yet Done. We also maintain a
	// "blocking" map that maps assignments back to all of the assignments
	// that depend on it.
	//
	// For example, for an initialization like:
	//
	// var x = f(a, b, b)
	// var a, b = g()
	//
	// the "x = f(a, b, b)" assignment depends on two variables (a and b),
	// so its Xoffset will be 2. Correspondingly, the "a, b = g()"
	// assignment's "blocking" entry will have two entries back to x's
	// assignment.
	//
	// Logically, initialization works by (1) taking all NotStarted
	// assignments, calculating their dependencies, and marking them
	// Pending; (2) adding all Pending assignments with Xoffset==0 to a
	// "ready" priority queue (ordered by variable declaration position);
	// and (3) iteratively processing the next Pending assignment from the
	// queue, decreasing the Xoffset of assignments it's blocking, and
	// adding them to the queue if decremented to 0.
	//
	// As an optimization, we actually apply each of these three steps for
	// each assignment. This yields the same order, but keeps queue size
	// down and thus also heap operation costs.

	// Static initialization phase.
	// These values are stored in two bits in Node.flags.
	const (
	InitNotStarted = iota
	InitDone
	InitPending
	)

	type InitOrder struct {
	// blocking maps initialization assignments to the assignments
	// that depend on it.
	blocking map[Node][]Node

	// ready is the queue of Pending initialization assignments
	// that are ready for initialization.
	ready declOrder
	}

	// initOrder computes initialization order for a list l of
	// package-level declarations (in declaration order) and outputs the
	// corresponding list of statements to include in the init() function
	// body.
	func initOrder(l []Node) []Node {
	s := InitSchedule{
	initplans: make(map[Node]InitPlan),
	inittemps: make(map[Node]Node),
	}
	o := InitOrder{
	blocking: make(map[Node][]Node),
	}

	// Process all package-level assignment in declaration order.
	for _, n := range l {
	switch n.Op {
	case OAS, OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
	o.processAssign(n)
	o.flushReady(s.staticInit)
	case ODCLCONST, ODCLFUNC, ODCLTYPE:
	// nop
	default:
	Fatalf("unexpected package-level statement: %v", n)
	}
	}

	// Check that all assignments are now Done; if not, there must
	// have been a dependency cycle.
	for _, n := range l {
	switch n.Op {
	case OAS, OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
	if n.Initorder() != InitDone {
	// If there have already been errors
	// printed, those errors may have
	// confused us and there might not be
	// a loop. Let the user fix those
	// first.
	if nerrors > 0 {
	errorexit()
	}

	findInitLoopAndExit(firstLHS(n), new([]*Node))
	Fatalf("initialization unfinished, but failed to identify loop")
	}
	}
	}

	// Invariant consistency check. If this is non-zero, then we
	// should have found a cycle above.
	if len(o.blocking) != 0 {
	Fatalf("expected empty map: %v", o.blocking)
	}

	return s.out
	}

	func (o InitOrder) processAssign(n Node) {
	if n.Initorder() != InitNotStarted \|\| n.Xoffset != BADWIDTH {
	Fatalf("unexpected state: %v, %v, %v", n, n.Initorder(), n.Xoffset)
	}

	n.SetInitorder(InitPending)
	n.Xoffset = 0

	// Compute number of variable dependencies and build the
	// inverse dependency ("blocking") graph.
	for dep := range collectDeps(n, true) {
	defn := dep.Name.Defn
	// Skip dependencies on functions (PFUNC) and
	// variables already initialized (InitDone).
	if dep.Class() != PEXTERN \|\| defn.Initorder() == InitDone {
	continue
	}
	n.Xoffset++
	o.blocking[defn] = append(o.blocking[defn], n)
	}

	if n.Xoffset == 0 {
	heap.Push(&o.ready, n)
	}
	}

	// flushReady repeatedly applies initialize to the earliest (in
	// declaration order) assignment ready for initialization and updates
	// the inverse dependency ("blocking") graph.
	func (o InitOrder) flushReady(initialize func(Node)) {
	for o.ready.Len() != 0 {
	n := heap.Pop(&o.ready).(*Node)
	if n.Initorder() != InitPending \|\| n.Xoffset != 0 {
	Fatalf("unexpected state: %v, %v, %v", n, n.Initorder(), n.Xoffset)
	}

	initialize(n)
	n.SetInitorder(InitDone)
	n.Xoffset = BADWIDTH

	blocked := o.blocking[n]
	delete(o.blocking, n)

	for _, m := range blocked {
	m.Xoffset--
	if m.Xoffset == 0 {
	heap.Push(&o.ready, m)
	}
	}
	}
	}

	// findInitLoopAndExit searches for an initialization loop involving variable
	// or function n. If one is found, it reports the loop as an error and exits.
	//
	// path points to a slice used for tracking the sequence of
	// variables/functions visited. Using a pointer to a slice allows the
	// slice capacity to grow and limit reallocations.
	func findInitLoopAndExit(n Node, path []*Node) {
	// We implement a simple DFS loop-finding algorithm. This
	// could be faster, but initialization cycles are rare.

	for i, x := range *path {
	if x == n {
	reportInitLoopAndExit((*path)[i:])
	return
	}
	}

	// There might be multiple loops involving n; by sorting
	// references, we deterministically pick the one reported.
	refers := collectDeps(n.Name.Defn, false).Sorted(func(ni, nj *Node) bool {
	return ni.Pos.Before(nj.Pos)
	})

	path = append(path, n)
	for _, ref := range refers {
	// Short-circuit variables that were initialized.
	if ref.Class() == PEXTERN && ref.Name.Defn.Initorder() == InitDone {
	continue
	}

	findInitLoopAndExit(ref, path)
	}
	path = (path)[:len(*path)-1]
	}

	// reportInitLoopAndExit reports and initialization loop as an error
	// and exits. However, if l is not actually an initialization loop, it
	// simply returns instead.
	func reportInitLoopAndExit(l []*Node) {
	// Rotate loop so that the earliest variable declaration is at
	// the start.
	i := -1
	for j, n := range l {
	if n.Class() == PEXTERN && (i == -1 \|\| n.Pos.Before(l[i].Pos)) {
	i = j
	}
	}
	if i == -1 {
	// False positive: loop only involves recursive
	// functions. Return so that findInitLoop can continue
	// searching.
	return
	}
	l = append(l[i:], l[:i]...)

	// TODO(mdempsky): Method values are printed as "T.m-fm"
	// rather than "T.m". Figure out how to avoid that.

	var msg bytes.Buffer
	fmt.Fprintf(&msg, "initialization loop:\n")
	for _, n := range l {
	fmt.Fprintf(&msg, "\t%v: %v refers to\n", n.Line(), n)
	}
	fmt.Fprintf(&msg, "\t%v: %v", l[0].Line(), l[0])

	yyerrorl(l[0].Pos, msg.String())
	errorexit()
	}

	// collectDeps returns all of the package-level functions and
	// variables that declaration n depends on. If transitive is true,
	// then it also includes the transitive dependencies of any depended
	// upon functions (but not variables).
	func collectDeps(n *Node, transitive bool) NodeSet {
	d := initDeps{transitive: transitive}
	switch n.Op {
	case OAS:
	d.inspect(n.Right)
	case OAS2DOTTYPE, OAS2FUNC, OAS2MAPR, OAS2RECV:
	d.inspect(n.Right)
	case ODCLFUNC:
	d.inspectList(n.Nbody)
	default:
	Fatalf("unexpected Op: %v", n.Op)
	}
	return d.seen
	}

	type initDeps struct {
	transitive bool
	seen NodeSet
	}

	func (d initDeps) inspect(n Node) { inspect(n, d.visit) }
	func (d *initDeps) inspectList(l Nodes) { inspectList(l, d.visit) }

	// visit calls foundDep on any package-level functions or variables
	// referenced by n, if any.
	func (d initDeps) visit(n Node) bool {
	switch n.Op {
	case ONAME:
	if n.isMethodExpression() {
	d.foundDep(asNode(n.Type.FuncType().Nname))
	return false
	}

	switch n.Class() {
	case PEXTERN, PFUNC:
	d.foundDep(n)
	}

	case OCLOSURE:
	d.inspectList(n.Func.Closure.Nbody)

	case ODOTMETH, OCALLPART:
	d.foundDep(asNode(n.Type.FuncType().Nname))
	}

	return true
	}

	// foundDep records that we've found a dependency on n by adding it to
	// seen.
	func (d initDeps) foundDep(n Node) {
	// Can happen with method expressions involving interface
	// types; e.g., fixedbugs/issue4495.go.
	if n == nil {
	return
	}

	// Names without definitions aren't interesting as far as
	// initialization ordering goes.
	if n.Name.Defn == nil {
	return
	}

	if d.seen.Has(n) {
	return
	}
	d.seen.Add(n)
	if d.transitive && n.Class() == PFUNC {
	d.inspectList(n.Name.Defn.Nbody)
	}
	}

	// declOrder implements heap.Interface, ordering assignment statements
	// by the position of their first LHS expression.
	//
	// N.B., the Pos of the first LHS expression is used because because
	// an OAS node's Pos may not be unique. For example, given the
	// declaration "var a, b = f(), g()", "a" must be ordered before "b",
	// but both OAS nodes use the "=" token's position as their Pos.
	type declOrder []*Node

	func (s declOrder) Len() int { return len(s) }
	func (s declOrder) Less(i, j int) bool { return firstLHS(s[i]).Pos.Before(firstLHS(s[j]).Pos) }
	func (s declOrder) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

	func (s declOrder) Push(x interface{}) { s = append(s, x.(Node)) }
	func (s *declOrder) Pop() interface{} {
	n := (s)[len(s)-1]
	s = (s)[:len(*s)-1]
	return n
	}

	// firstLHS returns the first expression on the left-hand side of
	// assignment n.
	func firstLHS(n Node) Node {
	switch n.Op {
	case OAS:
	return n.Left
	case OAS2DOTTYPE, OAS2FUNC, OAS2RECV, OAS2MAPR:
	return n.List.First()
	}

	Fatalf("unexpected Op: %v", n.Op)
	return nil
	}