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Source file src/go/types/initorder.go

Documentation: go/types

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package types
     6  
     7  import (
     8  	"container/heap"
     9  	"fmt"
    10  )
    11  
    12  // initOrder computes the Info.InitOrder for package variables.
    13  func (check *Checker) initOrder() {
    14  	// An InitOrder may already have been computed if a package is
    15  	// built from several calls to (*Checker).Files. Clear it.
    16  	check.Info.InitOrder = check.Info.InitOrder[:0]
    17  
    18  	// Compute the object dependency graph and initialize
    19  	// a priority queue with the list of graph nodes.
    20  	pq := nodeQueue(dependencyGraph(check.objMap))
    21  	heap.Init(&pq)
    22  
    23  	const debug = false
    24  	if debug {
    25  		fmt.Printf("Computing initialization order for %s\n\n", check.pkg)
    26  		fmt.Println("Object dependency graph:")
    27  		for obj, d := range check.objMap {
    28  			// only print objects that may appear in the dependency graph
    29  			if obj, _ := obj.(dependency); obj != nil {
    30  				if len(d.deps) > 0 {
    31  					fmt.Printf("\t%s depends on\n", obj.Name())
    32  					for dep := range d.deps {
    33  						fmt.Printf("\t\t%s\n", dep.Name())
    34  					}
    35  				} else {
    36  					fmt.Printf("\t%s has no dependencies\n", obj.Name())
    37  				}
    38  			}
    39  		}
    40  		fmt.Println()
    41  
    42  		fmt.Println("Transposed object dependency graph (functions eliminated):")
    43  		for _, n := range pq {
    44  			fmt.Printf("\t%s depends on %d nodes\n", n.obj.Name(), n.ndeps)
    45  			for p := range n.pred {
    46  				fmt.Printf("\t\t%s is dependent\n", p.obj.Name())
    47  			}
    48  		}
    49  		fmt.Println()
    50  
    51  		fmt.Println("Processing nodes:")
    52  	}
    53  
    54  	// Determine initialization order by removing the highest priority node
    55  	// (the one with the fewest dependencies) and its edges from the graph,
    56  	// repeatedly, until there are no nodes left.
    57  	// In a valid Go program, those nodes always have zero dependencies (after
    58  	// removing all incoming dependencies), otherwise there are initialization
    59  	// cycles.
    60  	emitted := make(map[*declInfo]bool)
    61  	for len(pq) > 0 {
    62  		// get the next node
    63  		n := heap.Pop(&pq).(*graphNode)
    64  
    65  		if debug {
    66  			fmt.Printf("\t%s (src pos %d) depends on %d nodes now\n",
    67  				n.obj.Name(), n.obj.order(), n.ndeps)
    68  		}
    69  
    70  		// if n still depends on other nodes, we have a cycle
    71  		if n.ndeps > 0 {
    72  			cycle := findPath(check.objMap, n.obj, n.obj, make(objSet))
    73  			// If n.obj is not part of the cycle (e.g., n.obj->b->c->d->c),
    74  			// cycle will be nil. Don't report anything in that case since
    75  			// the cycle is reported when the algorithm gets to an object
    76  			// in the cycle.
    77  			// Furthermore, once an object in the cycle is encountered,
    78  			// the cycle will be broken (dependency count will be reduced
    79  			// below), and so the remaining nodes in the cycle don't trigger
    80  			// another error (unless they are part of multiple cycles).
    81  			if cycle != nil {
    82  				check.reportCycle(cycle)
    83  			}
    84  			// Ok to continue, but the variable initialization order
    85  			// will be incorrect at this point since it assumes no
    86  			// cycle errors.
    87  		}
    88  
    89  		// reduce dependency count of all dependent nodes
    90  		// and update priority queue
    91  		for p := range n.pred {
    92  			p.ndeps--
    93  			heap.Fix(&pq, p.index)
    94  		}
    95  
    96  		// record the init order for variables with initializers only
    97  		v, _ := n.obj.(*Var)
    98  		info := check.objMap[v]
    99  		if v == nil || !info.hasInitializer() {
   100  			continue
   101  		}
   102  
   103  		// n:1 variable declarations such as: a, b = f()
   104  		// introduce a node for each lhs variable (here: a, b);
   105  		// but they all have the same initializer - emit only
   106  		// one, for the first variable seen
   107  		if emitted[info] {
   108  			continue // initializer already emitted, if any
   109  		}
   110  		emitted[info] = true
   111  
   112  		infoLhs := info.lhs // possibly nil (see declInfo.lhs field comment)
   113  		if infoLhs == nil {
   114  			infoLhs = []*Var{v}
   115  		}
   116  		init := &Initializer{infoLhs, info.init}
   117  		check.Info.InitOrder = append(check.Info.InitOrder, init)
   118  	}
   119  
   120  	if debug {
   121  		fmt.Println()
   122  		fmt.Println("Initialization order:")
   123  		for _, init := range check.Info.InitOrder {
   124  			fmt.Printf("\t%s\n", init)
   125  		}
   126  		fmt.Println()
   127  	}
   128  }
   129  
   130  // findPath returns the (reversed) list of objects []Object{to, ... from}
   131  // such that there is a path of object dependencies from 'from' to 'to'.
   132  // If there is no such path, the result is nil.
   133  func findPath(objMap map[Object]*declInfo, from, to Object, visited objSet) []Object {
   134  	if visited[from] {
   135  		return nil // node already seen
   136  	}
   137  	visited[from] = true
   138  
   139  	for d := range objMap[from].deps {
   140  		if d == to {
   141  			return []Object{d}
   142  		}
   143  		if P := findPath(objMap, d, to, visited); P != nil {
   144  			return append(P, d)
   145  		}
   146  	}
   147  
   148  	return nil
   149  }
   150  
   151  // reportCycle reports an error for the given cycle.
   152  func (check *Checker) reportCycle(cycle []Object) {
   153  	obj := cycle[0]
   154  	check.errorf(obj.Pos(), "initialization cycle for %s", obj.Name())
   155  	// subtle loop: print cycle[i] for i = 0, n-1, n-2, ... 1 for len(cycle) = n
   156  	for i := len(cycle) - 1; i >= 0; i-- {
   157  		check.errorf(obj.Pos(), "\t%s refers to", obj.Name()) // secondary error, \t indented
   158  		obj = cycle[i]
   159  	}
   160  	// print cycle[0] again to close the cycle
   161  	check.errorf(obj.Pos(), "\t%s", obj.Name())
   162  }
   163  
   164  // ----------------------------------------------------------------------------
   165  // Object dependency graph
   166  
   167  // A dependency is an object that may be a dependency in an initialization
   168  // expression. Only constants, variables, and functions can be dependencies.
   169  // Constants are here because constant expression cycles are reported during
   170  // initialization order computation.
   171  type dependency interface {
   172  	Object
   173  	isDependency()
   174  }
   175  
   176  // A graphNode represents a node in the object dependency graph.
   177  // Each node p in n.pred represents an edge p->n, and each node
   178  // s in n.succ represents an edge n->s; with a->b indicating that
   179  // a depends on b.
   180  type graphNode struct {
   181  	obj        dependency // object represented by this node
   182  	pred, succ nodeSet    // consumers and dependencies of this node (lazily initialized)
   183  	index      int        // node index in graph slice/priority queue
   184  	ndeps      int        // number of outstanding dependencies before this object can be initialized
   185  }
   186  
   187  type nodeSet map[*graphNode]bool
   188  
   189  func (s *nodeSet) add(p *graphNode) {
   190  	if *s == nil {
   191  		*s = make(nodeSet)
   192  	}
   193  	(*s)[p] = true
   194  }
   195  
   196  // dependencyGraph computes the object dependency graph from the given objMap,
   197  // with any function nodes removed. The resulting graph contains only constants
   198  // and variables.
   199  func dependencyGraph(objMap map[Object]*declInfo) []*graphNode {
   200  	// M is the dependency (Object) -> graphNode mapping
   201  	M := make(map[dependency]*graphNode)
   202  	for obj := range objMap {
   203  		// only consider nodes that may be an initialization dependency
   204  		if obj, _ := obj.(dependency); obj != nil {
   205  			M[obj] = &graphNode{obj: obj}
   206  		}
   207  	}
   208  
   209  	// compute edges for graph M
   210  	// (We need to include all nodes, even isolated ones, because they still need
   211  	// to be scheduled for initialization in correct order relative to other nodes.)
   212  	for obj, n := range M {
   213  		// for each dependency obj -> d (= deps[i]), create graph edges n->s and s->n
   214  		for d := range objMap[obj].deps {
   215  			// only consider nodes that may be an initialization dependency
   216  			if d, _ := d.(dependency); d != nil {
   217  				d := M[d]
   218  				n.succ.add(d)
   219  				d.pred.add(n)
   220  			}
   221  		}
   222  	}
   223  
   224  	// remove function nodes and collect remaining graph nodes in G
   225  	// (Mutually recursive functions may introduce cycles among themselves
   226  	// which are permitted. Yet such cycles may incorrectly inflate the dependency
   227  	// count for variables which in turn may not get scheduled for initialization
   228  	// in correct order.)
   229  	var G []*graphNode
   230  	for obj, n := range M {
   231  		if _, ok := obj.(*Func); ok {
   232  			// connect each predecessor p of n with each successor s
   233  			// and drop the function node (don't collect it in G)
   234  			for p := range n.pred {
   235  				// ignore self-cycles
   236  				if p != n {
   237  					// Each successor s of n becomes a successor of p, and
   238  					// each predecessor p of n becomes a predecessor of s.
   239  					for s := range n.succ {
   240  						// ignore self-cycles
   241  						if s != n {
   242  							p.succ.add(s)
   243  							s.pred.add(p)
   244  							delete(s.pred, n) // remove edge to n
   245  						}
   246  					}
   247  					delete(p.succ, n) // remove edge to n
   248  				}
   249  			}
   250  		} else {
   251  			// collect non-function nodes
   252  			G = append(G, n)
   253  		}
   254  	}
   255  
   256  	// fill in index and ndeps fields
   257  	for i, n := range G {
   258  		n.index = i
   259  		n.ndeps = len(n.succ)
   260  	}
   261  
   262  	return G
   263  }
   264  
   265  // ----------------------------------------------------------------------------
   266  // Priority queue
   267  
   268  // nodeQueue implements the container/heap interface;
   269  // a nodeQueue may be used as a priority queue.
   270  type nodeQueue []*graphNode
   271  
   272  func (a nodeQueue) Len() int { return len(a) }
   273  
   274  func (a nodeQueue) Swap(i, j int) {
   275  	x, y := a[i], a[j]
   276  	a[i], a[j] = y, x
   277  	x.index, y.index = j, i
   278  }
   279  
   280  func (a nodeQueue) Less(i, j int) bool {
   281  	x, y := a[i], a[j]
   282  	// nodes are prioritized by number of incoming dependencies (1st key)
   283  	// and source order (2nd key)
   284  	return x.ndeps < y.ndeps || x.ndeps == y.ndeps && x.obj.order() < y.obj.order()
   285  }
   286  
   287  func (a *nodeQueue) Push(x interface{}) {
   288  	panic("unreachable")
   289  }
   290  
   291  func (a *nodeQueue) Pop() interface{} {
   292  	n := len(*a)
   293  	x := (*a)[n-1]
   294  	x.index = -1 // for safety
   295  	*a = (*a)[:n-1]
   296  	return x
   297  }
   298  

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