1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
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 // This family of functions performs analyses on basic blocks, and instructions
11 // contained within basic blocks.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Analysis/CFG.h"
16 #include "llvm/ADT/SmallSet.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/IR/Dominators.h"
19
20 using namespace llvm;
21
22 /// FindFunctionBackedges - Analyze the specified function to find all of the
23 /// loop backedges in the function and return them. This is a relatively cheap
24 /// (compared to computing dominators and loop info) analysis.
25 ///
26 /// The output is added to Result, as pairs of <from,to> edge info.
FindFunctionBackedges(const Function & F,SmallVectorImpl<std::pair<const BasicBlock *,const BasicBlock * >> & Result)27 void llvm::FindFunctionBackedges(const Function &F,
28 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
29 const BasicBlock *BB = &F.getEntryBlock();
30 if (succ_empty(BB))
31 return;
32
33 SmallPtrSet<const BasicBlock*, 8> Visited;
34 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
35 SmallPtrSet<const BasicBlock*, 8> InStack;
36
37 Visited.insert(BB);
38 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
39 InStack.insert(BB);
40 do {
41 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
42 const BasicBlock *ParentBB = Top.first;
43 succ_const_iterator &I = Top.second;
44
45 bool FoundNew = false;
46 while (I != succ_end(ParentBB)) {
47 BB = *I++;
48 if (Visited.insert(BB).second) {
49 FoundNew = true;
50 break;
51 }
52 // Successor is in VisitStack, it's a back edge.
53 if (InStack.count(BB))
54 Result.push_back(std::make_pair(ParentBB, BB));
55 }
56
57 if (FoundNew) {
58 // Go down one level if there is a unvisited successor.
59 InStack.insert(BB);
60 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
61 } else {
62 // Go up one level.
63 InStack.erase(VisitStack.pop_back_val().first);
64 }
65 } while (!VisitStack.empty());
66 }
67
68 /// GetSuccessorNumber - Search for the specified successor of basic block BB
69 /// and return its position in the terminator instruction's list of
70 /// successors. It is an error to call this with a block that is not a
71 /// successor.
GetSuccessorNumber(const BasicBlock * BB,const BasicBlock * Succ)72 unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
73 const BasicBlock *Succ) {
74 const TerminatorInst *Term = BB->getTerminator();
75 #ifndef NDEBUG
76 unsigned e = Term->getNumSuccessors();
77 #endif
78 for (unsigned i = 0; ; ++i) {
79 assert(i != e && "Didn't find edge?");
80 if (Term->getSuccessor(i) == Succ)
81 return i;
82 }
83 }
84
85 /// isCriticalEdge - Return true if the specified edge is a critical edge.
86 /// Critical edges are edges from a block with multiple successors to a block
87 /// with multiple predecessors.
isCriticalEdge(const TerminatorInst * TI,unsigned SuccNum,bool AllowIdenticalEdges)88 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
89 bool AllowIdenticalEdges) {
90 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
91 if (TI->getNumSuccessors() == 1) return false;
92
93 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
94 const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
95
96 // If there is more than one predecessor, this is a critical edge...
97 assert(I != E && "No preds, but we have an edge to the block?");
98 const BasicBlock *FirstPred = *I;
99 ++I; // Skip one edge due to the incoming arc from TI.
100 if (!AllowIdenticalEdges)
101 return I != E;
102
103 // If AllowIdenticalEdges is true, then we allow this edge to be considered
104 // non-critical iff all preds come from TI's block.
105 for (; I != E; ++I)
106 if (*I != FirstPred)
107 return true;
108 return false;
109 }
110
111 // LoopInfo contains a mapping from basic block to the innermost loop. Find
112 // the outermost loop in the loop nest that contains BB.
getOutermostLoop(const LoopInfo * LI,const BasicBlock * BB)113 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
114 const Loop *L = LI->getLoopFor(BB);
115 if (L) {
116 while (const Loop *Parent = L->getParentLoop())
117 L = Parent;
118 }
119 return L;
120 }
121
122 // True if there is a loop which contains both BB1 and BB2.
loopContainsBoth(const LoopInfo * LI,const BasicBlock * BB1,const BasicBlock * BB2)123 static bool loopContainsBoth(const LoopInfo *LI,
124 const BasicBlock *BB1, const BasicBlock *BB2) {
125 const Loop *L1 = getOutermostLoop(LI, BB1);
126 const Loop *L2 = getOutermostLoop(LI, BB2);
127 return L1 != nullptr && L1 == L2;
128 }
129
isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock * > & Worklist,BasicBlock * StopBB,const DominatorTree * DT,const LoopInfo * LI)130 bool llvm::isPotentiallyReachableFromMany(
131 SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
132 const DominatorTree *DT, const LoopInfo *LI) {
133 // When the stop block is unreachable, it's dominated from everywhere,
134 // regardless of whether there's a path between the two blocks.
135 if (DT && !DT->isReachableFromEntry(StopBB))
136 DT = nullptr;
137
138 // Limit the number of blocks we visit. The goal is to avoid run-away compile
139 // times on large CFGs without hampering sensible code. Arbitrarily chosen.
140 unsigned Limit = 32;
141 SmallPtrSet<const BasicBlock*, 32> Visited;
142 do {
143 BasicBlock *BB = Worklist.pop_back_val();
144 if (!Visited.insert(BB).second)
145 continue;
146 if (BB == StopBB)
147 return true;
148 if (DT && DT->dominates(BB, StopBB))
149 return true;
150 if (LI && loopContainsBoth(LI, BB, StopBB))
151 return true;
152
153 if (!--Limit) {
154 // We haven't been able to prove it one way or the other. Conservatively
155 // answer true -- that there is potentially a path.
156 return true;
157 }
158
159 if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : nullptr) {
160 // All blocks in a single loop are reachable from all other blocks. From
161 // any of these blocks, we can skip directly to the exits of the loop,
162 // ignoring any other blocks inside the loop body.
163 Outer->getExitBlocks(Worklist);
164 } else {
165 Worklist.append(succ_begin(BB), succ_end(BB));
166 }
167 } while (!Worklist.empty());
168
169 // We have exhausted all possible paths and are certain that 'To' can not be
170 // reached from 'From'.
171 return false;
172 }
173
isPotentiallyReachable(const BasicBlock * A,const BasicBlock * B,const DominatorTree * DT,const LoopInfo * LI)174 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
175 const DominatorTree *DT, const LoopInfo *LI) {
176 assert(A->getParent() == B->getParent() &&
177 "This analysis is function-local!");
178
179 SmallVector<BasicBlock*, 32> Worklist;
180 Worklist.push_back(const_cast<BasicBlock*>(A));
181
182 return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
183 DT, LI);
184 }
185
isPotentiallyReachable(const Instruction * A,const Instruction * B,const DominatorTree * DT,const LoopInfo * LI)186 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
187 const DominatorTree *DT, const LoopInfo *LI) {
188 assert(A->getParent()->getParent() == B->getParent()->getParent() &&
189 "This analysis is function-local!");
190
191 SmallVector<BasicBlock*, 32> Worklist;
192
193 if (A->getParent() == B->getParent()) {
194 // The same block case is special because it's the only time we're looking
195 // within a single block to see which instruction comes first. Once we
196 // start looking at multiple blocks, the first instruction of the block is
197 // reachable, so we only need to determine reachability between whole
198 // blocks.
199 BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
200
201 // If the block is in a loop then we can reach any instruction in the block
202 // from any other instruction in the block by going around a backedge.
203 if (LI && LI->getLoopFor(BB) != nullptr)
204 return true;
205
206 // Linear scan, start at 'A', see whether we hit 'B' or the end first.
207 for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
208 ++I) {
209 if (&*I == B)
210 return true;
211 }
212
213 // Can't be in a loop if it's the entry block -- the entry block may not
214 // have predecessors.
215 if (BB == &BB->getParent()->getEntryBlock())
216 return false;
217
218 // Otherwise, continue doing the normal per-BB CFG walk.
219 Worklist.append(succ_begin(BB), succ_end(BB));
220
221 if (Worklist.empty()) {
222 // We've proven that there's no path!
223 return false;
224 }
225 } else {
226 Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
227 }
228
229 if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
230 return true;
231 if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
232 return false;
233
234 return isPotentiallyReachableFromMany(
235 Worklist, const_cast<BasicBlock *>(B->getParent()), DT, LI);
236 }
237