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// Copyright 2014 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 cha computes the call graph of a Go program using the Class
// Hierarchy Analysis (CHA) algorithm.
//
// CHA was first described in "Optimization of Object-Oriented Programs
// Using Static Class Hierarchy Analysis", Jeffrey Dean, David Grove,
// and Craig Chambers, ECOOP'95.
//
// CHA is related to RTA (see go/callgraph/rta); the difference is that
// CHA conservatively computes the entire "implements" relation between
// interfaces and concrete types ahead of time, whereas RTA uses dynamic
// programming to construct it on the fly as it encounters new functions
// reachable from main. CHA may thus include spurious call edges for
// types that haven't been instantiated yet, or types that are never
// instantiated.
//
// Since CHA conservatively assumes that all functions are address-taken
// and all concrete types are put into interfaces, it is sound to run on
// partial programs, such as libraries without a main or test function.
//
package cha // import "golang.org/x/tools/go/callgraph/cha"
import (
"go/types"
"golang.org/x/tools/go/callgraph"
"golang.org/x/tools/go/ssa"
"golang.org/x/tools/go/ssa/ssautil"
"golang.org/x/tools/go/types/typeutil"
)
// CallGraph computes the call graph of the specified program using the
// Class Hierarchy Analysis algorithm.
//
func CallGraph(prog *ssa.Program) *callgraph.Graph {
cg := callgraph.New(nil) // TODO(adonovan) eliminate concept of rooted callgraph
allFuncs := ssautil.AllFunctions(prog)
// funcsBySig contains all functions, keyed by signature. It is
// the effective set of address-taken functions used to resolve
// a dynamic call of a particular signature.
var funcsBySig typeutil.Map // value is []*ssa.Function
// methodsByName contains all methods,
// grouped by name for efficient lookup.
// (methodsById would be better but not every SSA method has a go/types ID.)
methodsByName := make(map[string][]*ssa.Function)
// An imethod represents an interface method I.m.
// (There's no go/types object for it;
// a *types.Func may be shared by many interfaces due to interface embedding.)
type imethod struct {
I *types.Interface
id string
}
// methodsMemo records, for every abstract method call I.m on
// interface type I, the set of concrete methods C.m of all
// types C that satisfy interface I.
//
// Abstract methods may be shared by several interfaces,
// hence we must pass I explicitly, not guess from m.
//
// methodsMemo is just a cache, so it needn't be a typeutil.Map.
methodsMemo := make(map[imethod][]*ssa.Function)
lookupMethods := func(I *types.Interface, m *types.Func) []*ssa.Function {
id := m.Id()
methods, ok := methodsMemo[imethod{I, id}]
if !ok {
for _, f := range methodsByName[m.Name()] {
C := f.Signature.Recv().Type() // named or *named
if types.Implements(C, I) {
methods = append(methods, f)
}
}
methodsMemo[imethod{I, id}] = methods
}
return methods
}
for f := range allFuncs {
if f.Signature.Recv() == nil {
// Package initializers can never be address-taken.
if f.Name() == "init" && f.Synthetic == "package initializer" {
continue
}
funcs, _ := funcsBySig.At(f.Signature).([]*ssa.Function)
funcs = append(funcs, f)
funcsBySig.Set(f.Signature, funcs)
} else {
methodsByName[f.Name()] = append(methodsByName[f.Name()], f)
}
}
addEdge := func(fnode *callgraph.Node, site ssa.CallInstruction, g *ssa.Function) {
gnode := cg.CreateNode(g)
callgraph.AddEdge(fnode, site, gnode)
}
addEdges := func(fnode *callgraph.Node, site ssa.CallInstruction, callees []*ssa.Function) {
// Because every call to a highly polymorphic and
// frequently used abstract method such as
// (io.Writer).Write is assumed to call every concrete
// Write method in the program, the call graph can
// contain a lot of duplication.
//
// TODO(adonovan): opt: consider factoring the callgraph
// API so that the Callers component of each edge is a
// slice of nodes, not a singleton.
for _, g := range callees {
addEdge(fnode, site, g)
}
}
for f := range allFuncs {
fnode := cg.CreateNode(f)
for _, b := range f.Blocks {
for _, instr := range b.Instrs {
if site, ok := instr.(ssa.CallInstruction); ok {
call := site.Common()
if call.IsInvoke() {
tiface := call.Value.Type().Underlying().(*types.Interface)
addEdges(fnode, site, lookupMethods(tiface, call.Method))
} else if g := call.StaticCallee(); g != nil {
addEdge(fnode, site, g)
} else if _, ok := call.Value.(*ssa.Builtin); !ok {
callees, _ := funcsBySig.At(call.Signature()).([]*ssa.Function)
addEdges(fnode, site, callees)
}
}
}
}
}
return cg
}
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