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Diffstat (limited to 'lib/Analysis/CFG.cpp')
| -rw-r--r-- | lib/Analysis/CFG.cpp | 227 |
1 files changed, 227 insertions, 0 deletions
diff --git a/lib/Analysis/CFG.cpp b/lib/Analysis/CFG.cpp new file mode 100644 index 0000000000..a5ed21afbf --- /dev/null +++ b/lib/Analysis/CFG.cpp @@ -0,0 +1,227 @@ +//===-- CFG.cpp - BasicBlock analysis --------------------------------------==// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This family of functions performs analyses on basic blocks, and instructions +// contained within basic blocks. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Analysis/CFG.h" + +#include "llvm/ADT/SmallSet.h" +#include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/LoopInfo.h" + +using namespace llvm; + +/// FindFunctionBackedges - Analyze the specified function to find all of the +/// loop backedges in the function and return them. This is a relatively cheap +/// (compared to computing dominators and loop info) analysis. +/// +/// The output is added to Result, as pairs of <from,to> edge info. +void llvm::FindFunctionBackedges(const Function &F, + SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { + const BasicBlock *BB = &F.getEntryBlock(); + if (succ_begin(BB) == succ_end(BB)) + return; + + SmallPtrSet<const BasicBlock*, 8> Visited; + SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; + SmallPtrSet<const BasicBlock*, 8> InStack; + + Visited.insert(BB); + VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); + InStack.insert(BB); + do { + std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); + const BasicBlock *ParentBB = Top.first; + succ_const_iterator &I = Top.second; + + bool FoundNew = false; + while (I != succ_end(ParentBB)) { + BB = *I++; + if (Visited.insert(BB)) { + FoundNew = true; + break; + } + // Successor is in VisitStack, it's a back edge. + if (InStack.count(BB)) + Result.push_back(std::make_pair(ParentBB, BB)); + } + + if (FoundNew) { + // Go down one level if there is a unvisited successor. + InStack.insert(BB); + VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); + } else { + // Go up one level. + InStack.erase(VisitStack.pop_back_val().first); + } + } while (!VisitStack.empty()); +} + +/// GetSuccessorNumber - Search for the specified successor of basic block BB +/// and return its position in the terminator instruction's list of +/// successors. It is an error to call this with a block that is not a +/// successor. +unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) { + TerminatorInst *Term = BB->getTerminator(); +#ifndef NDEBUG + unsigned e = Term->getNumSuccessors(); +#endif + for (unsigned i = 0; ; ++i) { + assert(i != e && "Didn't find edge?"); + if (Term->getSuccessor(i) == Succ) + return i; + } +} + +/// isCriticalEdge - Return true if the specified edge is a critical edge. +/// Critical edges are edges from a block with multiple successors to a block +/// with multiple predecessors. +bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, + bool AllowIdenticalEdges) { + assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!"); + if (TI->getNumSuccessors() == 1) return false; + + const BasicBlock *Dest = TI->getSuccessor(SuccNum); + const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest); + + // If there is more than one predecessor, this is a critical edge... + assert(I != E && "No preds, but we have an edge to the block?"); + const BasicBlock *FirstPred = *I; + ++I; // Skip one edge due to the incoming arc from TI. + if (!AllowIdenticalEdges) + return I != E; + + // If AllowIdenticalEdges is true, then we allow this edge to be considered + // non-critical iff all preds come from TI's block. + while (I != E) { + const BasicBlock *P = *I; + if (P != FirstPred) + return true; + // Note: leave this as is until no one ever compiles with either gcc 4.0.1 + // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207 + E = pred_end(P); + ++I; + } + return false; +} + +// LoopInfo contains a mapping from basic block to the innermost loop. Find +// the outermost loop in the loop nest that contains BB. +static const Loop *getOutermostLoop(LoopInfo *LI, const BasicBlock *BB) { + const Loop *L = LI->getLoopFor(BB); + if (L) { + while (const Loop *Parent = L->getParentLoop()) + L = Parent; + } + return L; +} + +// True if there is a loop which contains both BB1 and BB2. +static bool loopContainsBoth(LoopInfo *LI, + const BasicBlock *BB1, const BasicBlock *BB2) { + const Loop *L1 = getOutermostLoop(LI, BB1); + const Loop *L2 = getOutermostLoop(LI, BB2); + return L1 != NULL && L1 == L2; +} + +static bool isPotentiallyReachableSameBlock(const Instruction *A, + const Instruction *B, + LoopInfo *LI) { + // The same block case is special because it's the only time we're looking + // within a single block to see which comes first. Once we start looking at + // multiple blocks, the first instruction of the block is reachable, so we + // only need to determine reachability between whole blocks. + + const BasicBlock *BB = A->getParent(); + // If the block is in a loop then we can reach any instruction in the block + // from any other instruction in the block by going around the backedge. + // Check whether we're in a loop (or aren't sure). + + // Can't be in a loop if it's the entry block -- the entry block may not + // have predecessors. + bool HasLoop = BB != &BB->getParent()->getEntryBlock(); + + // Can't be in a loop if LoopInfo doesn't know about it. + if (LI && HasLoop) { + HasLoop = LI->getLoopFor(BB) != 0; + } + if (HasLoop) + return true; + + // Linear scan, start at 'A', see whether we hit 'B' or the end first. + for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) { + if (&*I == B) + return true; + } + return false; +} + +bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, + DominatorTree *DT, LoopInfo *LI) { + assert(A->getParent()->getParent() == B->getParent()->getParent() && + "This analysis is function-local!"); + + const BasicBlock *StopBB = B->getParent(); + + if (A->getParent() == B->getParent()) + return isPotentiallyReachableSameBlock(A, B, LI); + + if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) + return true; + if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) + return false; + + // When the stop block is unreachable, it's dominated from everywhere, + // regardless of whether there's a path between the two blocks. + if (DT && !DT->isReachableFromEntry(StopBB)) + DT = 0; + + // Limit the number of blocks we visit. The goal is to avoid run-away compile + // times on large CFGs without hampering sensible code. Arbitrarily chosen. + unsigned Limit = 32; + + SmallSet<const BasicBlock*, 64> Visited; + SmallVector<BasicBlock*, 32> Worklist; + Worklist.push_back(const_cast<BasicBlock*>(A->getParent())); + + do { + BasicBlock *BB = Worklist.pop_back_val(); + if (!Visited.insert(BB)) + continue; + if (BB == StopBB) + return true; + if (DT && DT->dominates(BB, StopBB)) + return true; + if (LI && loopContainsBoth(LI, BB, StopBB)) + return true; + + if (!--Limit) { + // We haven't been able to prove it one way or the other. Conservatively + // answer true -- that there is potentially a path. + return true; + } + + if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) { + // All blocks in a single loop are reachable from all other blocks. From + // any of these blocks, we can skip directly to the exits of the loop, + // ignoring any other blocks inside the loop body. + Outer->getExitBlocks(Worklist); + } else { + for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) + Worklist.push_back(*I); + } + } while (!Worklist.empty()); + + // We have exhaustived all possible paths and are certain that 'To' can not + // be reached from 'From'. + return false; +} |