1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This family of functions performs analyses on basic blocks, and instructions
10 // contained within basic blocks.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Analysis/CFG.h"
15 #include "llvm/ADT/SmallSet.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/IR/Dominators.h"
18 
19 using namespace llvm;
20 
21 /// FindFunctionBackedges - Analyze the specified function to find all of the
22 /// loop backedges in the function and return them.  This is a relatively cheap
23 /// (compared to computing dominators and loop info) analysis.
24 ///
25 /// The output is added to Result, as pairs of <from,to> edge info.
26 void llvm::FindFunctionBackedges(const Function &F,
27      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
28   const BasicBlock *BB = &F.getEntryBlock();
29   if (succ_empty(BB))
30     return;
31 
32   SmallPtrSet<const BasicBlock*, 8> Visited;
33   SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
34   SmallPtrSet<const BasicBlock*, 8> InStack;
35 
36   Visited.insert(BB);
37   VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
38   InStack.insert(BB);
39   do {
40     std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
41     const BasicBlock *ParentBB = Top.first;
42     succ_const_iterator &I = Top.second;
43 
44     bool FoundNew = false;
45     while (I != succ_end(ParentBB)) {
46       BB = *I++;
47       if (Visited.insert(BB).second) {
48         FoundNew = true;
49         break;
50       }
51       // Successor is in VisitStack, it's a back edge.
52       if (InStack.count(BB))
53         Result.push_back(std::make_pair(ParentBB, BB));
54     }
55 
56     if (FoundNew) {
57       // Go down one level if there is a unvisited successor.
58       InStack.insert(BB);
59       VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
60     } else {
61       // Go up one level.
62       InStack.erase(VisitStack.pop_back_val().first);
63     }
64   } while (!VisitStack.empty());
65 }
66 
67 /// GetSuccessorNumber - Search for the specified successor of basic block BB
68 /// and return its position in the terminator instruction's list of
69 /// successors.  It is an error to call this with a block that is not a
70 /// successor.
71 unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
72     const BasicBlock *Succ) {
73   const Instruction *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 Instruction *TI, unsigned SuccNum,
88                           bool AllowIdenticalEdges) {
89   assert(TI->isTerminator() && "Must be a terminator to have successors!");
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.
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.
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 
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 
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 
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