Mercurial > hg > Members > tobaru > cbc > CbC_llvm
view lib/Transforms/Scalar/LoopDeletion.cpp @ 83:60c9769439b8
LLVM 3.7
author | Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp> |
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date | Wed, 18 Feb 2015 14:55:36 +0900 |
parents | 54457678186b |
children | afa8332a0e37 |
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//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This file implements the Dead Loop Deletion Pass. This pass is responsible // for eliminating loops with non-infinite computable trip counts that have no // side effects or volatile instructions, and do not contribute to the // computation of the function's return value. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/ScalarEvolution.h" #include "llvm/IR/Dominators.h" using namespace llvm; #define DEBUG_TYPE "loop-delete" STATISTIC(NumDeleted, "Number of loops deleted"); namespace { class LoopDeletion : public LoopPass { public: static char ID; // Pass ID, replacement for typeid LoopDeletion() : LoopPass(ID) { initializeLoopDeletionPass(*PassRegistry::getPassRegistry()); } // Possibly eliminate loop L if it is dead. bool runOnLoop(Loop *L, LPPassManager &LPM) override; void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired<DominatorTreeWrapperPass>(); AU.addRequired<LoopInfoWrapperPass>(); AU.addRequired<ScalarEvolution>(); AU.addRequiredID(LoopSimplifyID); AU.addRequiredID(LCSSAID); AU.addPreserved<ScalarEvolution>(); AU.addPreserved<DominatorTreeWrapperPass>(); AU.addPreserved<LoopInfoWrapperPass>(); AU.addPreservedID(LoopSimplifyID); AU.addPreservedID(LCSSAID); } private: bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks, SmallVectorImpl<BasicBlock *> &exitBlocks, bool &Changed, BasicBlock *Preheader); }; } char LoopDeletion::ID = 0; INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion", "Delete dead loops", false, false) INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) INITIALIZE_PASS_DEPENDENCY(LoopSimplify) INITIALIZE_PASS_DEPENDENCY(LCSSA) INITIALIZE_PASS_END(LoopDeletion, "loop-deletion", "Delete dead loops", false, false) Pass *llvm::createLoopDeletionPass() { return new LoopDeletion(); } /// isLoopDead - Determined if a loop is dead. This assumes that we've already /// checked for unique exit and exiting blocks, and that the code is in LCSSA /// form. bool LoopDeletion::isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks, SmallVectorImpl<BasicBlock *> &exitBlocks, bool &Changed, BasicBlock *Preheader) { BasicBlock *exitBlock = exitBlocks[0]; // Make sure that all PHI entries coming from the loop are loop invariant. // Because the code is in LCSSA form, any values used outside of the loop // must pass through a PHI in the exit block, meaning that this check is // sufficient to guarantee that no loop-variant values are used outside // of the loop. BasicBlock::iterator BI = exitBlock->begin(); while (PHINode *P = dyn_cast<PHINode>(BI)) { Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]); // Make sure all exiting blocks produce the same incoming value for the exit // block. If there are different incoming values for different exiting // blocks, then it is impossible to statically determine which value should // be used. for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) { if (incoming != P->getIncomingValueForBlock(exitingBlocks[i])) return false; } if (Instruction *I = dyn_cast<Instruction>(incoming)) if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) return false; ++BI; } // Make sure that no instructions in the block have potential side-effects. // This includes instructions that could write to memory, and loads that are // marked volatile. This could be made more aggressive by using aliasing // information to identify readonly and readnone calls. for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); LI != LE; ++LI) { for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); BI != BE; ++BI) { if (BI->mayHaveSideEffects()) return false; } } return true; } /// runOnLoop - Remove dead loops, by which we mean loops that do not impact the /// observable behavior of the program other than finite running time. Note /// we do ensure that this never remove a loop that might be infinite, as doing /// so could change the halting/non-halting nature of a program. /// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA /// in order to make various safety checks work. bool LoopDeletion::runOnLoop(Loop *L, LPPassManager &LPM) { if (skipOptnoneFunction(L)) return false; // We can only remove the loop if there is a preheader that we can // branch from after removing it. BasicBlock *preheader = L->getLoopPreheader(); if (!preheader) return false; // If LoopSimplify form is not available, stay out of trouble. if (!L->hasDedicatedExits()) return false; // We can't remove loops that contain subloops. If the subloops were dead, // they would already have been removed in earlier executions of this pass. if (L->begin() != L->end()) return false; SmallVector<BasicBlock*, 4> exitingBlocks; L->getExitingBlocks(exitingBlocks); SmallVector<BasicBlock*, 4> exitBlocks; L->getUniqueExitBlocks(exitBlocks); // We require that the loop only have a single exit block. Otherwise, we'd // be in the situation of needing to be able to solve statically which exit // block will be branched to, or trying to preserve the branching logic in // a loop invariant manner. if (exitBlocks.size() != 1) return false; // Finally, we have to check that the loop really is dead. bool Changed = false; if (!isLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) return Changed; // Don't remove loops for which we can't solve the trip count. // They could be infinite, in which case we'd be changing program behavior. ScalarEvolution &SE = getAnalysis<ScalarEvolution>(); const SCEV *S = SE.getMaxBackedgeTakenCount(L); if (isa<SCEVCouldNotCompute>(S)) return Changed; // Now that we know the removal is safe, remove the loop by changing the // branch from the preheader to go to the single exit block. BasicBlock *exitBlock = exitBlocks[0]; // Because we're deleting a large chunk of code at once, the sequence in which // we remove things is very important to avoid invalidation issues. Don't // mess with this unless you have good reason and know what you're doing. // Tell ScalarEvolution that the loop is deleted. Do this before // deleting the loop so that ScalarEvolution can look at the loop // to determine what it needs to clean up. SE.forgetLoop(L); // Connect the preheader directly to the exit block. TerminatorInst *TI = preheader->getTerminator(); TI->replaceUsesOfWith(L->getHeader(), exitBlock); // Rewrite phis in the exit block to get their inputs from // the preheader instead of the exiting block. BasicBlock *exitingBlock = exitingBlocks[0]; BasicBlock::iterator BI = exitBlock->begin(); while (PHINode *P = dyn_cast<PHINode>(BI)) { int j = P->getBasicBlockIndex(exitingBlock); assert(j >= 0 && "Can't find exiting block in exit block's phi node!"); P->setIncomingBlock(j, preheader); for (unsigned i = 1; i < exitingBlocks.size(); ++i) P->removeIncomingValue(exitingBlocks[i]); ++BI; } // Update the dominator tree and remove the instructions and blocks that will // be deleted from the reference counting scheme. DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); SmallVector<DomTreeNode*, 8> ChildNodes; for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); LI != LE; ++LI) { // Move all of the block's children to be children of the preheader, which // allows us to remove the domtree entry for the block. ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end()); for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(), DE = ChildNodes.end(); DI != DE; ++DI) { DT.changeImmediateDominator(*DI, DT[preheader]); } ChildNodes.clear(); DT.eraseNode(*LI); // Remove the block from the reference counting scheme, so that we can // delete it freely later. (*LI)->dropAllReferences(); } // Erase the instructions and the blocks without having to worry // about ordering because we already dropped the references. // NOTE: This iteration is safe because erasing the block does not remove its // entry from the loop's block list. We do that in the next section. for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); LI != LE; ++LI) (*LI)->eraseFromParent(); // Finally, the blocks from loopinfo. This has to happen late because // otherwise our loop iterators won't work. LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); SmallPtrSet<BasicBlock*, 8> blocks; blocks.insert(L->block_begin(), L->block_end()); for (BasicBlock *BB : blocks) loopInfo.removeBlock(BB); // The last step is to inform the loop pass manager that we've // eliminated this loop. LPM.deleteLoopFromQueue(L); Changed = true; ++NumDeleted; return Changed; }