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.
FindFunctionBackedges(const Function & F,SmallVectorImpl<std::pair<const BasicBlock *,const BasicBlock * >> & Result)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.
GetSuccessorNumber(BasicBlock * BB,BasicBlock * Succ)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.
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 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.
getOutermostLoop(LoopInfo * LI,const BasicBlock * 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.
loopContainsBoth(LoopInfo * LI,const BasicBlock * BB1,const BasicBlock * 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
isPotentiallyReachableSameBlock(const Instruction * A,const Instruction * B,LoopInfo * LI)136 static bool isPotentiallyReachableSameBlock(const Instruction *A,
137 const Instruction *B,
138 LoopInfo *LI) {
139 // The same block case is special because it's the only time we're looking
140 // within a single block to see which comes first. Once we start looking at
141 // multiple blocks, the first instruction of the block is reachable, so we
142 // only need to determine reachability between whole blocks.
143
144 const BasicBlock *BB = A->getParent();
145 // If the block is in a loop then we can reach any instruction in the block
146 // from any other instruction in the block by going around the backedge.
147 // Check whether we're in a loop (or aren't sure).
148
149 // Can't be in a loop if it's the entry block -- the entry block may not
150 // have predecessors.
151 bool HasLoop = BB != &BB->getParent()->getEntryBlock();
152
153 // Can't be in a loop if LoopInfo doesn't know about it.
154 if (LI && HasLoop) {
155 HasLoop = LI->getLoopFor(BB) != 0;
156 }
157 if (HasLoop)
158 return true;
159
160 // Linear scan, start at 'A', see whether we hit 'B' or the end first.
161 for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
162 if (&*I == B)
163 return true;
164 }
165 return false;
166 }
167
isPotentiallyReachable(const Instruction * A,const Instruction * B,DominatorTree * DT,LoopInfo * LI)168 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
169 DominatorTree *DT, LoopInfo *LI) {
170 assert(A->getParent()->getParent() == B->getParent()->getParent() &&
171 "This analysis is function-local!");
172
173 const BasicBlock *StopBB = B->getParent();
174
175 if (A->getParent() == B->getParent())
176 return isPotentiallyReachableSameBlock(A, B, LI);
177
178 if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
179 return true;
180 if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
181 return false;
182
183 // When the stop block is unreachable, it's dominated from everywhere,
184 // regardless of whether there's a path between the two blocks.
185 if (DT && !DT->isReachableFromEntry(StopBB))
186 DT = 0;
187
188 // Limit the number of blocks we visit. The goal is to avoid run-away compile
189 // times on large CFGs without hampering sensible code. Arbitrarily chosen.
190 unsigned Limit = 32;
191
192 SmallSet<const BasicBlock*, 64> Visited;
193 SmallVector<BasicBlock*, 32> Worklist;
194 Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
195
196 do {
197 BasicBlock *BB = Worklist.pop_back_val();
198 if (!Visited.insert(BB))
199 continue;
200 if (BB == StopBB)
201 return true;
202 if (DT && DT->dominates(BB, StopBB))
203 return true;
204 if (LI && loopContainsBoth(LI, BB, StopBB))
205 return true;
206
207 if (!--Limit) {
208 // We haven't been able to prove it one way or the other. Conservatively
209 // answer true -- that there is potentially a path.
210 return true;
211 }
212
213 if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) {
214 // All blocks in a single loop are reachable from all other blocks. From
215 // any of these blocks, we can skip directly to the exits of the loop,
216 // ignoring any other blocks inside the loop body.
217 Outer->getExitBlocks(Worklist);
218 } else {
219 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
220 Worklist.push_back(*I);
221 }
222 } while (!Worklist.empty());
223
224 // We have exhaustived all possible paths and are certain that 'To' can not
225 // be reached from 'From'.
226 return false;
227 }
228