1 //===- LazyCallGraph.cpp - Analysis of a Module's call graph --------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #define DEBUG_TYPE "lcg"
11 #include "llvm/Analysis/LazyCallGraph.h"
12 #include "llvm/ADT/STLExtras.h"
13 #include "llvm/IR/CallSite.h"
14 #include "llvm/IR/InstVisitor.h"
15 #include "llvm/IR/Instructions.h"
16 #include "llvm/IR/PassManager.h"
17 #include "llvm/Support/Debug.h"
18 #include "llvm/Support/raw_ostream.h"
19 
20 using namespace llvm;
21 
22 static void findCallees(
23     SmallVectorImpl<Constant *> &Worklist, SmallPtrSetImpl<Constant *> &Visited,
24     SmallVectorImpl<PointerUnion<Function *, LazyCallGraph::Node *>> &Callees,
25     SmallPtrSetImpl<Function *> &CalleeSet) {
26   while (!Worklist.empty()) {
27     Constant *C = Worklist.pop_back_val();
28 
29     if (Function *F = dyn_cast<Function>(C)) {
30       // Note that we consider *any* function with a definition to be a viable
31       // edge. Even if the function's definition is subject to replacement by
32       // some other module (say, a weak definition) there may still be
33       // optimizations which essentially speculate based on the definition and
34       // a way to check that the specific definition is in fact the one being
35       // used. For example, this could be done by moving the weak definition to
36       // a strong (internal) definition and making the weak definition be an
37       // alias. Then a test of the address of the weak function against the new
38       // strong definition's address would be an effective way to determine the
39       // safety of optimizing a direct call edge.
40       if (!F->isDeclaration() && CalleeSet.insert(F)) {
41         DEBUG(dbgs() << "    Added callable function: " << F->getName()
42                      << "\n");
43         Callees.push_back(F);
44       }
45       continue;
46     }
47 
48     for (Value *Op : C->operand_values())
49       if (Visited.insert(cast<Constant>(Op)))
50         Worklist.push_back(cast<Constant>(Op));
51   }
52 }
53 
54 LazyCallGraph::Node::Node(LazyCallGraph &G, Function &F)
55     : G(&G), F(F), DFSNumber(0), LowLink(0) {
56   DEBUG(dbgs() << "  Adding functions called by '" << F.getName()
57                << "' to the graph.\n");
58 
59   SmallVector<Constant *, 16> Worklist;
60   SmallPtrSet<Constant *, 16> Visited;
61   // Find all the potential callees in this function. First walk the
62   // instructions and add every operand which is a constant to the worklist.
63   for (BasicBlock &BB : F)
64     for (Instruction &I : BB)
65       for (Value *Op : I.operand_values())
66         if (Constant *C = dyn_cast<Constant>(Op))
67           if (Visited.insert(C))
68             Worklist.push_back(C);
69 
70   // We've collected all the constant (and thus potentially function or
71   // function containing) operands to all of the instructions in the function.
72   // Process them (recursively) collecting every function found.
73   findCallees(Worklist, Visited, Callees, CalleeSet);
74 }
75 
76 LazyCallGraph::LazyCallGraph(Module &M) : NextDFSNumber(0) {
77   DEBUG(dbgs() << "Building CG for module: " << M.getModuleIdentifier()
78                << "\n");
79   for (Function &F : M)
80     if (!F.isDeclaration() && !F.hasLocalLinkage())
81       if (EntryNodeSet.insert(&F)) {
82         DEBUG(dbgs() << "  Adding '" << F.getName()
83                      << "' to entry set of the graph.\n");
84         EntryNodes.push_back(&F);
85       }
86 
87   // Now add entry nodes for functions reachable via initializers to globals.
88   SmallVector<Constant *, 16> Worklist;
89   SmallPtrSet<Constant *, 16> Visited;
90   for (GlobalVariable &GV : M.globals())
91     if (GV.hasInitializer())
92       if (Visited.insert(GV.getInitializer()))
93         Worklist.push_back(GV.getInitializer());
94 
95   DEBUG(dbgs() << "  Adding functions referenced by global initializers to the "
96                   "entry set.\n");
97   findCallees(Worklist, Visited, EntryNodes, EntryNodeSet);
98 
99   for (auto &Entry : EntryNodes)
100     if (Function *F = Entry.dyn_cast<Function *>())
101       SCCEntryNodes.insert(F);
102     else
103       SCCEntryNodes.insert(&Entry.get<Node *>()->getFunction());
104 }
105 
106 LazyCallGraph::LazyCallGraph(LazyCallGraph &&G)
107     : BPA(std::move(G.BPA)), NodeMap(std::move(G.NodeMap)),
108       EntryNodes(std::move(G.EntryNodes)),
109       EntryNodeSet(std::move(G.EntryNodeSet)), SCCBPA(std::move(G.SCCBPA)),
110       SCCMap(std::move(G.SCCMap)), LeafSCCs(std::move(G.LeafSCCs)),
111       DFSStack(std::move(G.DFSStack)),
112       SCCEntryNodes(std::move(G.SCCEntryNodes)),
113       NextDFSNumber(G.NextDFSNumber) {
114   updateGraphPtrs();
115 }
116 
117 LazyCallGraph &LazyCallGraph::operator=(LazyCallGraph &&G) {
118   BPA = std::move(G.BPA);
119   NodeMap = std::move(G.NodeMap);
120   EntryNodes = std::move(G.EntryNodes);
121   EntryNodeSet = std::move(G.EntryNodeSet);
122   SCCBPA = std::move(G.SCCBPA);
123   SCCMap = std::move(G.SCCMap);
124   LeafSCCs = std::move(G.LeafSCCs);
125   DFSStack = std::move(G.DFSStack);
126   SCCEntryNodes = std::move(G.SCCEntryNodes);
127   NextDFSNumber = G.NextDFSNumber;
128   updateGraphPtrs();
129   return *this;
130 }
131 
132 LazyCallGraph::Node *LazyCallGraph::insertInto(Function &F, Node *&MappedN) {
133   return new (MappedN = BPA.Allocate()) Node(*this, F);
134 }
135 
136 void LazyCallGraph::updateGraphPtrs() {
137   // Process all nodes updating the graph pointers.
138   SmallVector<Node *, 16> Worklist;
139   for (auto &Entry : EntryNodes)
140     if (Node *EntryN = Entry.dyn_cast<Node *>())
141       Worklist.push_back(EntryN);
142 
143   while (!Worklist.empty()) {
144     Node *N = Worklist.pop_back_val();
145     N->G = this;
146     for (auto &Callee : N->Callees)
147       if (Node *CalleeN = Callee.dyn_cast<Node *>())
148         Worklist.push_back(CalleeN);
149   }
150 }
151 
152 LazyCallGraph::SCC *LazyCallGraph::getNextSCCInPostOrder() {
153   // When the stack is empty, there are no more SCCs to walk in this graph.
154   if (DFSStack.empty()) {
155     // If we've handled all candidate entry nodes to the SCC forest, we're done.
156     if (SCCEntryNodes.empty())
157       return nullptr;
158 
159     Node *N = get(*SCCEntryNodes.pop_back_val());
160     DFSStack.push_back(std::make_pair(N, N->begin()));
161   }
162 
163   Node *N = DFSStack.back().first;
164   if (N->DFSNumber == 0) {
165     // This node hasn't been visited before, assign it a DFS number and remove
166     // it from the entry set.
167     N->LowLink = N->DFSNumber = NextDFSNumber++;
168     SCCEntryNodes.remove(&N->getFunction());
169   }
170 
171   for (auto I = DFSStack.back().second, E = N->end(); I != E; ++I) {
172     Node *ChildN = *I;
173     if (ChildN->DFSNumber == 0) {
174       // Mark that we should start at this child when next this node is the
175       // top of the stack. We don't start at the next child to ensure this
176       // child's lowlink is reflected.
177       // FIXME: I don't actually think this is required, and we could start
178       // at the next child.
179       DFSStack.back().second = I;
180 
181       // Recurse onto this node via a tail call.
182       DFSStack.push_back(std::make_pair(ChildN, ChildN->begin()));
183       return LazyCallGraph::getNextSCCInPostOrder();
184     }
185 
186     // Track the lowest link of the childen, if any are still in the stack.
187     if (ChildN->LowLink < N->LowLink && !SCCMap.count(&ChildN->getFunction()))
188       N->LowLink = ChildN->LowLink;
189   }
190 
191   // The tail of the stack is the new SCC. Allocate the SCC and pop the stack
192   // into it.
193   SCC *NewSCC = new (SCCBPA.Allocate()) SCC();
194 
195   // Because we don't follow the strict Tarjan recursive formulation, walk
196   // from the top of the stack down, propagating the lowest link and stopping
197   // when the DFS number is the lowest link.
198   int LowestLink = N->LowLink;
199   do {
200     Node *SCCN = DFSStack.pop_back_val().first;
201     SCCMap.insert(std::make_pair(&SCCN->getFunction(), NewSCC));
202     NewSCC->Nodes.push_back(SCCN);
203     LowestLink = std::min(LowestLink, SCCN->LowLink);
204     bool Inserted =
205         NewSCC->NodeSet.insert(&SCCN->getFunction());
206     (void)Inserted;
207     assert(Inserted && "Cannot have duplicates in the DFSStack!");
208   } while (!DFSStack.empty() && LowestLink <= DFSStack.back().first->DFSNumber);
209   assert(LowestLink == NewSCC->Nodes.back()->DFSNumber &&
210          "Cannot stop with a DFS number greater than the lowest link!");
211 
212   // A final pass over all edges in the SCC (this remains linear as we only
213   // do this once when we build the SCC) to connect it to the parent sets of
214   // its children.
215   bool IsLeafSCC = true;
216   for (Node *SCCN : NewSCC->Nodes)
217     for (Node *SCCChildN : *SCCN) {
218       if (NewSCC->NodeSet.count(&SCCChildN->getFunction()))
219         continue;
220       SCC *ChildSCC = SCCMap.lookup(&SCCChildN->getFunction());
221       assert(ChildSCC &&
222              "Must have all child SCCs processed when building a new SCC!");
223       ChildSCC->ParentSCCs.insert(NewSCC);
224       IsLeafSCC = false;
225     }
226 
227   // For the SCCs where we fine no child SCCs, add them to the leaf list.
228   if (IsLeafSCC)
229     LeafSCCs.push_back(NewSCC);
230 
231   return NewSCC;
232 }
233 
234 char LazyCallGraphAnalysis::PassID;
235 
236 LazyCallGraphPrinterPass::LazyCallGraphPrinterPass(raw_ostream &OS) : OS(OS) {}
237 
238 static void printNodes(raw_ostream &OS, LazyCallGraph::Node &N,
239                        SmallPtrSetImpl<LazyCallGraph::Node *> &Printed) {
240   // Recurse depth first through the nodes.
241   for (LazyCallGraph::Node *ChildN : N)
242     if (Printed.insert(ChildN))
243       printNodes(OS, *ChildN, Printed);
244 
245   OS << "  Call edges in function: " << N.getFunction().getName() << "\n";
246   for (LazyCallGraph::iterator I = N.begin(), E = N.end(); I != E; ++I)
247     OS << "    -> " << I->getFunction().getName() << "\n";
248 
249   OS << "\n";
250 }
251 
252 static void printSCC(raw_ostream &OS, LazyCallGraph::SCC &SCC) {
253   ptrdiff_t SCCSize = std::distance(SCC.begin(), SCC.end());
254   OS << "  SCC with " << SCCSize << " functions:\n";
255 
256   for (LazyCallGraph::Node *N : SCC)
257     OS << "    " << N->getFunction().getName() << "\n";
258 
259   OS << "\n";
260 }
261 
262 PreservedAnalyses LazyCallGraphPrinterPass::run(Module *M,
263                                                 ModuleAnalysisManager *AM) {
264   LazyCallGraph &G = AM->getResult<LazyCallGraphAnalysis>(M);
265 
266   OS << "Printing the call graph for module: " << M->getModuleIdentifier()
267      << "\n\n";
268 
269   SmallPtrSet<LazyCallGraph::Node *, 16> Printed;
270   for (LazyCallGraph::Node *N : G)
271     if (Printed.insert(N))
272       printNodes(OS, *N, Printed);
273 
274   for (LazyCallGraph::SCC *SCC : G.postorder_sccs())
275     printSCC(OS, *SCC);
276 
277   return PreservedAnalyses::all();
278 
279 }
280