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.
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.
72 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
73   TerminatorInst *Term = BB->getTerminator();
74 #ifndef NDEBUG
75   unsigned e = Term->getNumSuccessors();
76 #endif
77   for (unsigned i = 0; ; ++i) {
78     assert(i != e && "Didn't find edge?");
79     if (Term->getSuccessor(i) == Succ)
80       return i;
81   }
82 }
83 
84 /// isCriticalEdge - Return true if the specified edge is a critical edge.
85 /// Critical edges are edges from a block with multiple successors to a block
86 /// with multiple predecessors.
87 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
88                           bool AllowIdenticalEdges) {
89   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
90   if (TI->getNumSuccessors() == 1) return false;
91 
92   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
93   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
94 
95   // If there is more than one predecessor, this is a critical edge...
96   assert(I != E && "No preds, but we have an edge to the block?");
97   const BasicBlock *FirstPred = *I;
98   ++I;        // Skip one edge due to the incoming arc from TI.
99   if (!AllowIdenticalEdges)
100     return I != E;
101 
102   // If AllowIdenticalEdges is true, then we allow this edge to be considered
103   // non-critical iff all preds come from TI's block.
104   for (; I != E; ++I)
105     if (*I != FirstPred)
106       return true;
107   return false;
108 }
109 
110 // LoopInfo contains a mapping from basic block to the innermost loop. Find
111 // the outermost loop in the loop nest that contains BB.
112 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
113   const Loop *L = LI->getLoopFor(BB);
114   if (L) {
115     while (const Loop *Parent = L->getParentLoop())
116       L = Parent;
117   }
118   return L;
119 }
120 
121 // True if there is a loop which contains both BB1 and BB2.
122 static bool loopContainsBoth(const LoopInfo *LI,
123                              const BasicBlock *BB1, const BasicBlock *BB2) {
124   const Loop *L1 = getOutermostLoop(LI, BB1);
125   const Loop *L2 = getOutermostLoop(LI, BB2);
126   return L1 != nullptr && L1 == L2;
127 }
128 
129 bool llvm::isPotentiallyReachableFromMany(
130     SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
131     const DominatorTree *DT, const LoopInfo *LI) {
132   // When the stop block is unreachable, it's dominated from everywhere,
133   // regardless of whether there's a path between the two blocks.
134   if (DT && !DT->isReachableFromEntry(StopBB))
135     DT = nullptr;
136 
137   // Limit the number of blocks we visit. The goal is to avoid run-away compile
138   // times on large CFGs without hampering sensible code. Arbitrarily chosen.
139   unsigned Limit = 32;
140   SmallSet<const BasicBlock*, 64> Visited;
141   do {
142     BasicBlock *BB = Worklist.pop_back_val();
143     if (!Visited.insert(BB).second)
144       continue;
145     if (BB == StopBB)
146       return true;
147     if (DT && DT->dominates(BB, StopBB))
148       return true;
149     if (LI && loopContainsBoth(LI, BB, StopBB))
150       return true;
151 
152     if (!--Limit) {
153       // We haven't been able to prove it one way or the other. Conservatively
154       // answer true -- that there is potentially a path.
155       return true;
156     }
157 
158     if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : nullptr) {
159       // All blocks in a single loop are reachable from all other blocks. From
160       // any of these blocks, we can skip directly to the exits of the loop,
161       // ignoring any other blocks inside the loop body.
162       Outer->getExitBlocks(Worklist);
163     } else {
164       Worklist.append(succ_begin(BB), succ_end(BB));
165     }
166   } while (!Worklist.empty());
167 
168   // We have exhausted all possible paths and are certain that 'To' can not be
169   // reached from 'From'.
170   return false;
171 }
172 
173 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
174                                   const DominatorTree *DT, const LoopInfo *LI) {
175   assert(A->getParent() == B->getParent() &&
176          "This analysis is function-local!");
177 
178   SmallVector<BasicBlock*, 32> Worklist;
179   Worklist.push_back(const_cast<BasicBlock*>(A));
180 
181   return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
182                                         DT, LI);
183 }
184 
185 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
186                                   const DominatorTree *DT, const LoopInfo *LI) {
187   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
188          "This analysis is function-local!");
189 
190   SmallVector<BasicBlock*, 32> Worklist;
191 
192   if (A->getParent() == B->getParent()) {
193     // The same block case is special because it's the only time we're looking
194     // within a single block to see which instruction comes first. Once we
195     // start looking at multiple blocks, the first instruction of the block is
196     // reachable, so we only need to determine reachability between whole
197     // blocks.
198     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
199 
200     // If the block is in a loop then we can reach any instruction in the block
201     // from any other instruction in the block by going around a backedge.
202     if (LI && LI->getLoopFor(BB) != nullptr)
203       return true;
204 
205     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
206     for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
207       if (&*I == B)
208         return true;
209     }
210 
211     // Can't be in a loop if it's the entry block -- the entry block may not
212     // have predecessors.
213     if (BB == &BB->getParent()->getEntryBlock())
214       return false;
215 
216     // Otherwise, continue doing the normal per-BB CFG walk.
217     Worklist.append(succ_begin(BB), succ_end(BB));
218 
219     if (Worklist.empty()) {
220       // We've proven that there's no path!
221       return false;
222     }
223   } else {
224     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
225   }
226 
227   if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
228     return true;
229   if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
230     return false;
231 
232   return isPotentiallyReachableFromMany(
233       Worklist, const_cast<BasicBlock *>(B->getParent()), DT, LI);
234 }
235