From 72dba254ae65b06062106910a70d46f21e19d55a Mon Sep 17 00:00:00 2001 From: Nick Lewycky Date: Tue, 13 Aug 2013 00:03:47 +0000 Subject: [PATCH] Fix an oversight in isPotentiallyReachable where we wouldn't do any CFG-walking to find loops if the From and To instructions were in the same block. Refactor the code a little now that we need to fill to start the CFG-walking algorithm with more than one starting basic block sometimes. Special thanks to Andrew Trick for catching an error in my understanding of natural loops in code review. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188236 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/llvm/Analysis/CFG.h | 12 ++++ lib/Analysis/CFG.cpp | 123 +++++++++++++++++++-------------- unittests/Analysis/CFGTest.cpp | 17 +++++ 3 files changed, 99 insertions(+), 53 deletions(-) diff --git a/include/llvm/Analysis/CFG.h b/include/llvm/Analysis/CFG.h index 08fdc2689d4..979926b18b3 100644 --- a/include/llvm/Analysis/CFG.h +++ b/include/llvm/Analysis/CFG.h @@ -49,6 +49,9 @@ unsigned GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ); bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, bool AllowIdenticalEdges = false); +/// \brief Determine whether instruction 'To' is reachable from 'From', +/// returning true if uncertain. +/// /// Determine whether there is a path from From to To within a single function. /// Returns false only if we can prove that once 'From' has been executed then /// 'To' can not be executed. Conservatively returns true. @@ -64,6 +67,15 @@ bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, bool isPotentiallyReachable(const Instruction *From, const Instruction *To, DominatorTree *DT = 0, LoopInfo *LI = 0); +/// \brief Determine whether block 'To' is reachable from 'From', returning +/// true if uncertain. +/// +/// Determine whether there is a path from From to To within a single function. +/// Returns false only if we can prove that once 'From' has been reached then +/// 'To' can not be executed. Conservatively returns true. +bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To, + DominatorTree *DT = 0, LoopInfo *LI = 0); + } // End llvm namespace #endif diff --git a/lib/Analysis/CFG.cpp b/lib/Analysis/CFG.cpp index a5ed21afbff..be7e9fce63c 100644 --- a/lib/Analysis/CFG.cpp +++ b/lib/Analysis/CFG.cpp @@ -133,53 +133,9 @@ static bool loopContainsBoth(LoopInfo *LI, 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; - +static bool isPotentiallyReachableInner(SmallVectorImpl &Worklist, + BasicBlock *StopBB, + DominatorTree *DT, LoopInfo *LI) { // 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)) @@ -188,11 +144,7 @@ bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, // 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 Visited; - SmallVector Worklist; - Worklist.push_back(const_cast(A->getParent())); - do { BasicBlock *BB = Worklist.pop_back_val(); if (!Visited.insert(BB)) @@ -221,7 +173,72 @@ bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B, } } while (!Worklist.empty()); - // We have exhaustived all possible paths and are certain that 'To' can not - // be reached from 'From'. + // We have exhausted all possible paths and are certain that 'To' can not be + // reached from 'From'. return false; } + +bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B, + DominatorTree *DT, LoopInfo *LI) { + assert(A->getParent() == B->getParent() && + "This analysis is function-local!"); + + SmallVector Worklist; + Worklist.push_back(const_cast(A)); + + return isPotentiallyReachableInner(Worklist, const_cast(B), + DT, LI); +} + +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!"); + + SmallVector Worklist; + + if (A->getParent() == B->getParent()) { + // The same block case is special because it's the only time we're looking + // within a single block to see which instruction 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. + BasicBlock *BB = const_cast(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 a backedge. + if (LI && LI->getLoopFor(BB) != 0) + 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; + } + + // Can't be in a loop if it's the entry block -- the entry block may not + // have predecessors. + if (BB == &BB->getParent()->getEntryBlock()) + return false; + + // Otherwise, continue doing the normal per-BB CFG walk. + for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) + Worklist.push_back(*I); + + if (Worklist.empty()) { + // We've proven that there's no path! + return false; + } + } else { + Worklist.push_back(const_cast(A->getParent())); + } + + if (A->getParent() == &A->getParent()->getParent()->getEntryBlock()) + return true; + if (B->getParent() == &A->getParent()->getParent()->getEntryBlock()) + return false; + + return isPotentiallyReachableInner(Worklist, + const_cast(B->getParent()), + DT, LI); +} diff --git a/unittests/Analysis/CFGTest.cpp b/unittests/Analysis/CFGTest.cpp index 23588037784..e931709620f 100644 --- a/unittests/Analysis/CFGTest.cpp +++ b/unittests/Analysis/CFGTest.cpp @@ -147,6 +147,23 @@ TEST_F(IsPotentiallyReachableTest, SameBlockPath) { ExpectPath(true); } +TEST_F(IsPotentiallyReachableTest, SameBlockNoLoop) { + ParseAssembly( + "define void @test() {\n" + "entry:\n" + " br label %middle\n" + "middle:\n" + " %B = bitcast i8 undef to i8\n" + " bitcast i8 undef to i8\n" + " bitcast i8 undef to i8\n" + " %A = bitcast i8 undef to i8\n" + " br label %nextblock\n" + "nextblock:\n" + " ret void\n" + "}\n"); + ExpectPath(false); +} + TEST_F(IsPotentiallyReachableTest, StraightNoPath) { ParseAssembly( "define void @test() {\n" -- 2.34.1