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 
17 #include "llvm/ADT/SmallSet.h"
18 #include "llvm/Analysis/Dominators.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 
21 using namespace llvm;
22 
23 /// FindFunctionBackedges - Analyze the specified function to find all of the
24 /// loop backedges in the function and return them.  This is a relatively cheap
25 /// (compared to computing dominators and loop info) analysis.
26 ///
27 /// The output is added to Result, as pairs of <from,to> edge info.
28 void llvm::FindFunctionBackedges(const Function &F,
29      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
30   const BasicBlock *BB = &F.getEntryBlock();
31   if (succ_begin(BB) == succ_end(BB))
32     return;
33 
34   SmallPtrSet<const BasicBlock*, 8> Visited;
35   SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
36   SmallPtrSet<const BasicBlock*, 8> InStack;
37 
38   Visited.insert(BB);
39   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
40   InStack.insert(BB);
41   do {
42     std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
43     const BasicBlock *ParentBB = Top.first;
44     succ_const_iterator &I = Top.second;
45 
46     bool FoundNew = false;
47     while (I != succ_end(ParentBB)) {
48       BB = *I++;
49       if (Visited.insert(BB)) {
50         FoundNew = true;
51         break;
52       }
53       // Successor is in VisitStack, it's a back edge.
54       if (InStack.count(BB))
55         Result.push_back(std::make_pair(ParentBB, BB));
56     }
57 
58     if (FoundNew) {
59       // Go down one level if there is a unvisited successor.
60       InStack.insert(BB);
61       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
62     } else {
63       // Go up one level.
64       InStack.erase(VisitStack.pop_back_val().first);
65     }
66   } while (!VisitStack.empty());
67 }
68 
69 /// GetSuccessorNumber - Search for the specified successor of basic block BB
70 /// and return its position in the terminator instruction's list of
71 /// successors.  It is an error to call this with a block that is not a
72 /// successor.
73 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
74   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.
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   while (I != E) {
106     const BasicBlock *P = *I;
107     if (P != FirstPred)
108       return true;
109     // Note: leave this as is until no one ever compiles with either gcc 4.0.1
110     // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
111     E = pred_end(P);
112     ++I;
113   }
114   return false;
115 }
116 
117 // LoopInfo contains a mapping from basic block to the innermost loop. Find
118 // the outermost loop in the loop nest that contains BB.
119 static const Loop *getOutermostLoop(LoopInfo *LI, const BasicBlock *BB) {
120   const Loop *L = LI->getLoopFor(BB);
121   if (L) {
122     while (const Loop *Parent = L->getParentLoop())
123       L = Parent;
124   }
125   return L;
126 }
127 
128 // True if there is a loop which contains both BB1 and BB2.
129 static bool loopContainsBoth(LoopInfo *LI,
130                              const BasicBlock *BB1, const BasicBlock *BB2) {
131   const Loop *L1 = getOutermostLoop(LI, BB1);
132   const Loop *L2 = getOutermostLoop(LI, BB2);
133   return L1 != NULL && L1 == L2;
134 }
135 
136 static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist,
137                                         BasicBlock *StopBB,
138                                         DominatorTree *DT, LoopInfo *LI) {
139   // When the stop block is unreachable, it's dominated from everywhere,
140   // regardless of whether there's a path between the two blocks.
141   if (DT && !DT->isReachableFromEntry(StopBB))
142     DT = 0;
143 
144   // Limit the number of blocks we visit. The goal is to avoid run-away compile
145   // times on large CFGs without hampering sensible code. Arbitrarily chosen.
146   unsigned Limit = 32;
147   SmallSet<const BasicBlock*, 64> Visited;
148   do {
149     BasicBlock *BB = Worklist.pop_back_val();
150     if (!Visited.insert(BB))
151       continue;
152     if (BB == StopBB)
153       return true;
154     if (DT && DT->dominates(BB, StopBB))
155       return true;
156     if (LI && loopContainsBoth(LI, BB, StopBB))
157       return true;
158 
159     if (!--Limit) {
160       // We haven't been able to prove it one way or the other. Conservatively
161       // answer true -- that there is potentially a path.
162       return true;
163     }
164 
165     if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) {
166       // All blocks in a single loop are reachable from all other blocks. From
167       // any of these blocks, we can skip directly to the exits of the loop,
168       // ignoring any other blocks inside the loop body.
169       Outer->getExitBlocks(Worklist);
170     } else {
171       for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
172         Worklist.push_back(*I);
173     }
174   } while (!Worklist.empty());
175 
176   // We have exhausted all possible paths and are certain that 'To' can not be
177   // reached from 'From'.
178   return false;
179 }
180 
181 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
182                                   DominatorTree *DT, LoopInfo *LI) {
183   assert(A->getParent() == B->getParent() &&
184          "This analysis is function-local!");
185 
186   SmallVector<BasicBlock*, 32> Worklist;
187   Worklist.push_back(const_cast<BasicBlock*>(A));
188 
189   return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B),
190                                      DT, LI);
191 }
192 
193 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
194                                   DominatorTree *DT, LoopInfo *LI) {
195   assert(A->getParent()->getParent() == B->getParent()->getParent() &&
196          "This analysis is function-local!");
197 
198   SmallVector<BasicBlock*, 32> Worklist;
199 
200   if (A->getParent() == B->getParent()) {
201     // The same block case is special because it's the only time we're looking
202     // within a single block to see which instruction comes first. Once we
203     // start looking at multiple blocks, the first instruction of the block is
204     // reachable, so we only need to determine reachability between whole
205     // blocks.
206     BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
207 
208     // If the block is in a loop then we can reach any instruction in the block
209     // from any other instruction in the block by going around a backedge.
210     if (LI && LI->getLoopFor(BB) != 0)
211       return true;
212 
213     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
214     for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
215       if (&*I == B)
216         return true;
217     }
218 
219     // Can't be in a loop if it's the entry block -- the entry block may not
220     // have predecessors.
221     if (BB == &BB->getParent()->getEntryBlock())
222       return false;
223 
224     // Otherwise, continue doing the normal per-BB CFG walk.
225     for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
226       Worklist.push_back(*I);
227 
228     if (Worklist.empty()) {
229       // We've proven that there's no path!
230       return false;
231     }
232   } else {
233     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
234   }
235 
236   if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
237     return true;
238   if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
239     return false;
240 
241   return isPotentiallyReachableInner(Worklist,
242                                      const_cast<BasicBlock*>(B->getParent()),
243                                      DT, LI);
244 }
245