lib/Analysis/CFG.cpp

   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 }