scc.go

     1  // Copyright 2011 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 ir
     6  
     7  // Strongly connected components.
     8  //
     9  // Run analysis on minimal sets of mutually recursive functions
    10  // or single non-recursive functions, bottom up.
    11  //
    12  // Finding these sets is finding strongly connected components
    13  // by reverse topological order in the static call graph.
    14  // The algorithm (known as Tarjan's algorithm) for doing that is taken from
    15  // Sedgewick, Algorithms, Second Edition, p. 482, with two adaptations.
    16  //
    17  // First, a closure function (fn.IsClosure()) cannot be
    18  // the root of a connected component. Refusing to use it as a root forces
    19  // it into the component of the function in which it appears.  This is
    20  // more convenient for escape analysis.
    21  //
    22  // Second, each function becomes two virtual nodes in the graph,
    23  // with numbers n and n+1. We record the function's node number as n
    24  // but search from node n+1. If the search tells us that the component
    25  // number (min) is n+1, we know that this is a trivial component: one function
    26  // plus its closures. If the search tells us that the component number is
    27  // n, then there was a path from node n+1 back to node n, meaning that
    28  // the function set is mutually recursive. The escape analysis can be
    29  // more precise when analyzing a single non-recursive function than
    30  // when analyzing a set of mutually recursive functions.
    31  
    32  type bottomUpVisitor struct {
    33  	analyze  func([]*Func, bool)
    34  	visitgen uint32
    35  	nodeID   map[*Func]uint32
    36  	stack    []*Func
    37  }
    38  
    39  // VisitFuncsBottomUp invokes analyze on the ODCLFUNC nodes listed in list.
    40  // It calls analyze with successive groups of functions, working from
    41  // the bottom of the call graph upward. Each time analyze is called with
    42  // a list of functions, every function on that list only calls other functions
    43  // on the list or functions that have been passed in previous invocations of
    44  // analyze. Closures appear in the same list as their outer functions.
    45  // The lists are as short as possible while preserving those requirements.
    46  // (In a typical program, many invocations of analyze will be passed just
    47  // a single function.) The boolean argument 'recursive' passed to analyze
    48  // specifies whether the functions on the list are mutually recursive.
    49  // If recursive is false, the list consists of only a single function and its closures.
    50  // If recursive is true, the list may still contain only a single function,
    51  // if that function is itself recursive.
    52  func VisitFuncsBottomUp(list []*Func, analyze func(list []*Func, recursive bool)) {
    53  	var v bottomUpVisitor
    54  	v.analyze = analyze
    55  	v.nodeID = make(map[*Func]uint32)
    56  	for _, n := range list {
    57  		if !n.IsClosure() {
    58  			v.visit(n)
    59  		}
    60  	}
    61  }
    62  
    63  func (v *bottomUpVisitor) visit(n *Func) uint32 {
    64  	if id := v.nodeID[n]; id > 0 {
    65  		// already visited
    66  		return id
    67  	}
    68  
    69  	v.visitgen++
    70  	id := v.visitgen
    71  	v.nodeID[n] = id
    72  	v.visitgen++
    73  	min := v.visitgen
    74  	v.stack = append(v.stack, n)
    75  
    76  	do := func(defn Node) {
    77  		if defn != nil {
    78  			if m := v.visit(defn.(*Func)); m < min {
    79  				min = m
    80  			}
    81  		}
    82  	}
    83  
    84  	Visit(n, func(n Node) {
    85  		switch n.Op() {
    86  		case ONAME:
    87  			if n := n.(*Name); n.Class == PFUNC {
    88  				do(n.Defn)
    89  			}
    90  		case ODOTMETH, OMETHVALUE, OMETHEXPR:
    91  			if fn := MethodExprName(n); fn != nil {
    92  				do(fn.Defn)
    93  			}
    94  		case OCLOSURE:
    95  			n := n.(*ClosureExpr)
    96  			do(n.Func)
    97  		}
    98  	})
    99  
   100  	if (min == id || min == id+1) && !n.IsClosure() {
   101  		// This node is the root of a strongly connected component.
   102  
   103  		// The original min was id+1. If the bottomUpVisitor found its way
   104  		// back to id, then this block is a set of mutually recursive functions.
   105  		// Otherwise, it's just a lone function that does not recurse.
   106  		recursive := min == id
   107  
   108  		// Remove connected component from stack and mark v.nodeID so that future
   109  		// visits return a large number, which will not affect the caller's min.
   110  		var i int
   111  		for i = len(v.stack) - 1; i >= 0; i-- {
   112  			x := v.stack[i]
   113  			v.nodeID[x] = ^uint32(0)
   114  			if x == n {
   115  				break
   116  			}
   117  		}
   118  		block := v.stack[i:]
   119  		// Call analyze on this set of functions.
   120  		v.stack = v.stack[:i]
   121  		v.analyze(block, recursive)
   122  	}
   123  
   124  	return min
   125  }
   126
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