1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This family of functions performs analyses on basic blocks, and instructions
11 // contained within basic blocks.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Analysis/CFG.h"
16 #include "llvm/ADT/SmallSet.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/IR/Dominators.h"
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.
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_begin(BB) == succ_end(BB))
33 SmallPtrSet<const BasicBlock*, 8> Visited;
34 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
35 SmallPtrSet<const BasicBlock*, 8> InStack;
38 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
41 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
42 const BasicBlock *ParentBB = Top.first;
43 succ_const_iterator &I = Top.second;
45 bool FoundNew = false;
46 while (I != succ_end(ParentBB)) {
48 if (Visited.insert(BB)) {
52 // Successor is in VisitStack, it's a back edge.
53 if (InStack.count(BB))
54 Result.push_back(std::make_pair(ParentBB, BB));
58 // Go down one level if there is a unvisited successor.
60 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
63 InStack.erase(VisitStack.pop_back_val().first);
65 } while (!VisitStack.empty());
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
72 unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
73 TerminatorInst *Term = BB->getTerminator();
75 unsigned e = Term->getNumSuccessors();
77 for (unsigned i = 0; ; ++i) {
78 assert(i != e && "Didn't find edge?");
79 if (Term->getSuccessor(i) == Succ)
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;
92 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
93 const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
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)
102 // If AllowIdenticalEdges is true, then we allow this edge to be considered
103 // non-critical iff all preds come from TI's block.
105 const BasicBlock *P = *I;
108 // Note: leave this as is until no one ever compiles with either gcc 4.0.1
109 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
116 // LoopInfo contains a mapping from basic block to the innermost loop. Find
117 // the outermost loop in the loop nest that contains BB.
118 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
119 const Loop *L = LI->getLoopFor(BB);
121 while (const Loop *Parent = L->getParentLoop())
127 // True if there is a loop which contains both BB1 and BB2.
128 static bool loopContainsBoth(const LoopInfo *LI,
129 const BasicBlock *BB1, const BasicBlock *BB2) {
130 const Loop *L1 = getOutermostLoop(LI, BB1);
131 const Loop *L2 = getOutermostLoop(LI, BB2);
132 return L1 != NULL && L1 == L2;
135 static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist,
137 const DominatorTree *DT,
138 const 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))
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.
147 SmallSet<const BasicBlock*, 64> Visited;
149 BasicBlock *BB = Worklist.pop_back_val();
150 if (!Visited.insert(BB))
154 if (DT && DT->dominates(BB, StopBB))
156 if (LI && loopContainsBoth(LI, BB, StopBB))
160 // We haven't been able to prove it one way or the other. Conservatively
161 // answer true -- that there is potentially a path.
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);
171 Worklist.append(succ_begin(BB), succ_end(BB));
173 } while (!Worklist.empty());
175 // We have exhausted all possible paths and are certain that 'To' can not be
176 // reached from 'From'.
180 bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
181 const DominatorTree *DT, const LoopInfo *LI) {
182 assert(A->getParent() == B->getParent() &&
183 "This analysis is function-local!");
185 SmallVector<BasicBlock*, 32> Worklist;
186 Worklist.push_back(const_cast<BasicBlock*>(A));
188 return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B),
192 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
193 const DominatorTree *DT, const LoopInfo *LI) {
194 assert(A->getParent()->getParent() == B->getParent()->getParent() &&
195 "This analysis is function-local!");
197 SmallVector<BasicBlock*, 32> Worklist;
199 if (A->getParent() == B->getParent()) {
200 // The same block case is special because it's the only time we're looking
201 // within a single block to see which instruction comes first. Once we
202 // start looking at multiple blocks, the first instruction of the block is
203 // reachable, so we only need to determine reachability between whole
205 BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
207 // If the block is in a loop then we can reach any instruction in the block
208 // from any other instruction in the block by going around a backedge.
209 if (LI && LI->getLoopFor(BB) != 0)
212 // Linear scan, start at 'A', see whether we hit 'B' or the end first.
213 for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
218 // Can't be in a loop if it's the entry block -- the entry block may not
219 // have predecessors.
220 if (BB == &BB->getParent()->getEntryBlock())
223 // Otherwise, continue doing the normal per-BB CFG walk.
224 Worklist.append(succ_begin(BB), succ_end(BB));
226 if (Worklist.empty()) {
227 // We've proven that there's no path!
231 Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
234 if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
236 if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
239 return isPotentiallyReachableInner(Worklist,
240 const_cast<BasicBlock*>(B->getParent()),