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Diffstat (limited to 'lib/Transforms/Utils/LoopSimplify.cpp')
| -rw-r--r-- | lib/Transforms/Utils/LoopSimplify.cpp | 600 |
1 files changed, 600 insertions, 0 deletions
diff --git a/lib/Transforms/Utils/LoopSimplify.cpp b/lib/Transforms/Utils/LoopSimplify.cpp new file mode 100644 index 0000000000000..03d273d25d791 --- /dev/null +++ b/lib/Transforms/Utils/LoopSimplify.cpp @@ -0,0 +1,600 @@ +//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This pass performs several transformations to transform natural loops into a +// simpler form, which makes subsequent analyses and transformations simpler and +// more effective. +// +// Loop pre-header insertion guarantees that there is a single, non-critical +// entry edge from outside of the loop to the loop header. This simplifies a +// number of analyses and transformations, such as LICM. +// +// Loop exit-block insertion guarantees that all exit blocks from the loop +// (blocks which are outside of the loop that have predecessors inside of the +// loop) only have predecessors from inside of the loop (and are thus dominated +// by the loop header). This simplifies transformations such as store-sinking +// that are built into LICM. +// +// This pass also guarantees that loops will have exactly one backedge. +// +// Note that the simplifycfg pass will clean up blocks which are split out but +// end up being unnecessary, so usage of this pass should not pessimize +// generated code. +// +// This pass obviously modifies the CFG, but updates loop information and +// dominator information. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "loopsimplify" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Constants.h" +#include "llvm/Instructions.h" +#include "llvm/Function.h" +#include "llvm/Type.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Support/CFG.h" +#include "llvm/Support/Compiler.h" +#include "llvm/ADT/SetOperations.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/DepthFirstIterator.h" +using namespace llvm; + +STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted"); +STATISTIC(NumNested , "Number of nested loops split out"); + +namespace { + struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + LoopSimplify() : FunctionPass(&ID) {} + + // AA - If we have an alias analysis object to update, this is it, otherwise + // this is null. + AliasAnalysis *AA; + LoopInfo *LI; + DominatorTree *DT; + virtual bool runOnFunction(Function &F); + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + // We need loop information to identify the loops... + AU.addRequired<LoopInfo>(); + AU.addRequired<DominatorTree>(); + + AU.addPreserved<LoopInfo>(); + AU.addPreserved<DominatorTree>(); + AU.addPreserved<DominanceFrontier>(); + AU.addPreserved<AliasAnalysis>(); + AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. + } + + /// verifyAnalysis() - Verify loop nest. + void verifyAnalysis() const { +#ifndef NDEBUG + LoopInfo *NLI = &getAnalysis<LoopInfo>(); + for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I) + (*I)->verifyLoop(); +#endif + } + + private: + bool ProcessLoop(Loop *L); + BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit); + void InsertPreheaderForLoop(Loop *L); + Loop *SeparateNestedLoop(Loop *L); + void InsertUniqueBackedgeBlock(Loop *L); + void PlaceSplitBlockCarefully(BasicBlock *NewBB, + SmallVectorImpl<BasicBlock*> &SplitPreds, + Loop *L); + }; +} + +char LoopSimplify::ID = 0; +static RegisterPass<LoopSimplify> +X("loopsimplify", "Canonicalize natural loops", true); + +// Publically exposed interface to pass... +const PassInfo *const llvm::LoopSimplifyID = &X; +FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } + +/// runOnFunction - Run down all loops in the CFG (recursively, but we could do +/// it in any convenient order) inserting preheaders... +/// +bool LoopSimplify::runOnFunction(Function &F) { + bool Changed = false; + LI = &getAnalysis<LoopInfo>(); + AA = getAnalysisIfAvailable<AliasAnalysis>(); + DT = &getAnalysis<DominatorTree>(); + + // Check to see that no blocks (other than the header) in loops have + // predecessors that are not in loops. This is not valid for natural loops, + // but can occur if the blocks are unreachable. Since they are unreachable we + // can just shamelessly destroy their terminators to make them not branch into + // the loop! + for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { + // This case can only occur for unreachable blocks. Blocks that are + // unreachable can't be in loops, so filter those blocks out. + if (LI->getLoopFor(BB)) continue; + + bool BlockUnreachable = false; + TerminatorInst *TI = BB->getTerminator(); + + // Check to see if any successors of this block are non-loop-header loops + // that are not the header. + for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { + // If this successor is not in a loop, BB is clearly ok. + Loop *L = LI->getLoopFor(TI->getSuccessor(i)); + if (!L) continue; + + // If the succ is the loop header, and if L is a top-level loop, then this + // is an entrance into a loop through the header, which is also ok. + if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0) + continue; + + // Otherwise, this is an entrance into a loop from some place invalid. + // Either the loop structure is invalid and this is not a natural loop (in + // which case the compiler is buggy somewhere else) or BB is unreachable. + BlockUnreachable = true; + break; + } + + // If this block is ok, check the next one. + if (!BlockUnreachable) continue; + + // Otherwise, this block is dead. To clean up the CFG and to allow later + // loop transformations to ignore this case, we delete the edges into the + // loop by replacing the terminator. + + // Remove PHI entries from the successors. + for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) + TI->getSuccessor(i)->removePredecessor(BB); + + // Add a new unreachable instruction before the old terminator. + new UnreachableInst(TI); + + // Delete the dead terminator. + if (AA) AA->deleteValue(TI); + if (!TI->use_empty()) + TI->replaceAllUsesWith(UndefValue::get(TI->getType())); + TI->eraseFromParent(); + Changed |= true; + } + + for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) + Changed |= ProcessLoop(*I); + + return Changed; +} + +/// ProcessLoop - Walk the loop structure in depth first order, ensuring that +/// all loops have preheaders. +/// +bool LoopSimplify::ProcessLoop(Loop *L) { + bool Changed = false; +ReprocessLoop: + + // Canonicalize inner loops before outer loops. Inner loop canonicalization + // can provide work for the outer loop to canonicalize. + for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) + Changed |= ProcessLoop(*I); + + assert(L->getBlocks()[0] == L->getHeader() && + "Header isn't first block in loop?"); + + // Does the loop already have a preheader? If so, don't insert one. + if (L->getLoopPreheader() == 0) { + InsertPreheaderForLoop(L); + NumInserted++; + Changed = true; + } + + // Next, check to make sure that all exit nodes of the loop only have + // predecessors that are inside of the loop. This check guarantees that the + // loop preheader/header will dominate the exit blocks. If the exit block has + // predecessors from outside of the loop, split the edge now. + SmallVector<BasicBlock*, 8> ExitBlocks; + L->getExitBlocks(ExitBlocks); + + SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end()); + for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(), + E = ExitBlockSet.end(); I != E; ++I) { + BasicBlock *ExitBlock = *I; + for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); + PI != PE; ++PI) + // Must be exactly this loop: no subloops, parent loops, or non-loop preds + // allowed. + if (!L->contains(*PI)) { + RewriteLoopExitBlock(L, ExitBlock); + NumInserted++; + Changed = true; + break; + } + } + + // If the header has more than two predecessors at this point (from the + // preheader and from multiple backedges), we must adjust the loop. + unsigned NumBackedges = L->getNumBackEdges(); + if (NumBackedges != 1) { + // If this is really a nested loop, rip it out into a child loop. Don't do + // this for loops with a giant number of backedges, just factor them into a + // common backedge instead. + if (NumBackedges < 8) { + if (Loop *NL = SeparateNestedLoop(L)) { + ++NumNested; + // This is a big restructuring change, reprocess the whole loop. + ProcessLoop(NL); + Changed = true; + // GCC doesn't tail recursion eliminate this. + goto ReprocessLoop; + } + } + + // If we either couldn't, or didn't want to, identify nesting of the loops, + // insert a new block that all backedges target, then make it jump to the + // loop header. + InsertUniqueBackedgeBlock(L); + NumInserted++; + Changed = true; + } + + // Scan over the PHI nodes in the loop header. Since they now have only two + // incoming values (the loop is canonicalized), we may have simplified the PHI + // down to 'X = phi [X, Y]', which should be replaced with 'Y'. + PHINode *PN; + for (BasicBlock::iterator I = L->getHeader()->begin(); + (PN = dyn_cast<PHINode>(I++)); ) + if (Value *V = PN->hasConstantValue()) { + if (AA) AA->deleteValue(PN); + PN->replaceAllUsesWith(V); + PN->eraseFromParent(); + } + + return Changed; +} + +/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a +/// preheader, this method is called to insert one. This method has two phases: +/// preheader insertion and analysis updating. +/// +void LoopSimplify::InsertPreheaderForLoop(Loop *L) { + BasicBlock *Header = L->getHeader(); + + // Compute the set of predecessors of the loop that are not in the loop. + SmallVector<BasicBlock*, 8> OutsideBlocks; + for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); + PI != PE; ++PI) + if (!L->contains(*PI)) // Coming in from outside the loop? + OutsideBlocks.push_back(*PI); // Keep track of it... + + // Split out the loop pre-header. + BasicBlock *NewBB = + SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(), + ".preheader", this); + + + //===--------------------------------------------------------------------===// + // Update analysis results now that we have performed the transformation + // + + // We know that we have loop information to update... update it now. + if (Loop *Parent = L->getParentLoop()) + Parent->addBasicBlockToLoop(NewBB, LI->getBase()); + + // Make sure that NewBB is put someplace intelligent, which doesn't mess up + // code layout too horribly. + PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L); +} + +/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit +/// blocks. This method is used to split exit blocks that have predecessors +/// outside of the loop. +BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) { + SmallVector<BasicBlock*, 8> LoopBlocks; + for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) + if (L->contains(*I)) + LoopBlocks.push_back(*I); + + assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); + BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0], + LoopBlocks.size(), ".loopexit", + this); + + // Update Loop Information - we know that the new block will be in whichever + // loop the Exit block is in. Note that it may not be in that immediate loop, + // if the successor is some other loop header. In that case, we continue + // walking up the loop tree to find a loop that contains both the successor + // block and the predecessor block. + Loop *SuccLoop = LI->getLoopFor(Exit); + while (SuccLoop && !SuccLoop->contains(L->getHeader())) + SuccLoop = SuccLoop->getParentLoop(); + if (SuccLoop) + SuccLoop->addBasicBlockToLoop(NewBB, LI->getBase()); + + return NewBB; +} + +/// AddBlockAndPredsToSet - Add the specified block, and all of its +/// predecessors, to the specified set, if it's not already in there. Stop +/// predecessor traversal when we reach StopBlock. +static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock, + std::set<BasicBlock*> &Blocks) { + std::vector<BasicBlock *> WorkList; + WorkList.push_back(InputBB); + do { + BasicBlock *BB = WorkList.back(); WorkList.pop_back(); + if (Blocks.insert(BB).second && BB != StopBlock) + // If BB is not already processed and it is not a stop block then + // insert its predecessor in the work list + for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { + BasicBlock *WBB = *I; + WorkList.push_back(WBB); + } + } while(!WorkList.empty()); +} + +/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a +/// PHI node that tells us how to partition the loops. +static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT, + AliasAnalysis *AA) { + for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { + PHINode *PN = cast<PHINode>(I); + ++I; + if (Value *V = PN->hasConstantValue()) + if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) { + // This is a degenerate PHI already, don't modify it! + PN->replaceAllUsesWith(V); + if (AA) AA->deleteValue(PN); + PN->eraseFromParent(); + continue; + } + + // Scan this PHI node looking for a use of the PHI node by itself. + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) + if (PN->getIncomingValue(i) == PN && + L->contains(PN->getIncomingBlock(i))) + // We found something tasty to remove. + return PN; + } + return 0; +} + +// PlaceSplitBlockCarefully - If the block isn't already, move the new block to +// right after some 'outside block' block. This prevents the preheader from +// being placed inside the loop body, e.g. when the loop hasn't been rotated. +void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB, + SmallVectorImpl<BasicBlock*> &SplitPreds, + Loop *L) { + // Check to see if NewBB is already well placed. + Function::iterator BBI = NewBB; --BBI; + for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { + if (&*BBI == SplitPreds[i]) + return; + } + + // If it isn't already after an outside block, move it after one. This is + // always good as it makes the uncond branch from the outside block into a + // fall-through. + + // Figure out *which* outside block to put this after. Prefer an outside + // block that neighbors a BB actually in the loop. + BasicBlock *FoundBB = 0; + for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { + Function::iterator BBI = SplitPreds[i]; + if (++BBI != NewBB->getParent()->end() && + L->contains(BBI)) { + FoundBB = SplitPreds[i]; + break; + } + } + + // If our heuristic for a *good* bb to place this after doesn't find + // anything, just pick something. It's likely better than leaving it within + // the loop. + if (!FoundBB) + FoundBB = SplitPreds[0]; + NewBB->moveAfter(FoundBB); +} + + +/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of +/// them out into a nested loop. This is important for code that looks like +/// this: +/// +/// Loop: +/// ... +/// br cond, Loop, Next +/// ... +/// br cond2, Loop, Out +/// +/// To identify this common case, we look at the PHI nodes in the header of the +/// loop. PHI nodes with unchanging values on one backedge correspond to values +/// that change in the "outer" loop, but not in the "inner" loop. +/// +/// If we are able to separate out a loop, return the new outer loop that was +/// created. +/// +Loop *LoopSimplify::SeparateNestedLoop(Loop *L) { + PHINode *PN = FindPHIToPartitionLoops(L, DT, AA); + if (PN == 0) return 0; // No known way to partition. + + // Pull out all predecessors that have varying values in the loop. This + // handles the case when a PHI node has multiple instances of itself as + // arguments. + SmallVector<BasicBlock*, 8> OuterLoopPreds; + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) + if (PN->getIncomingValue(i) != PN || + !L->contains(PN->getIncomingBlock(i))) + OuterLoopPreds.push_back(PN->getIncomingBlock(i)); + + BasicBlock *Header = L->getHeader(); + BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0], + OuterLoopPreds.size(), + ".outer", this); + + // Make sure that NewBB is put someplace intelligent, which doesn't mess up + // code layout too horribly. + PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L); + + // Create the new outer loop. + Loop *NewOuter = new Loop(); + + // Change the parent loop to use the outer loop as its child now. + if (Loop *Parent = L->getParentLoop()) + Parent->replaceChildLoopWith(L, NewOuter); + else + LI->changeTopLevelLoop(L, NewOuter); + + // This block is going to be our new header block: add it to this loop and all + // parent loops. + NewOuter->addBasicBlockToLoop(NewBB, LI->getBase()); + + // L is now a subloop of our outer loop. + NewOuter->addChildLoop(L); + + for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); + I != E; ++I) + NewOuter->addBlockEntry(*I); + + // Determine which blocks should stay in L and which should be moved out to + // the Outer loop now. + std::set<BasicBlock*> BlocksInL; + for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) + if (DT->dominates(Header, *PI)) + AddBlockAndPredsToSet(*PI, Header, BlocksInL); + + + // Scan all of the loop children of L, moving them to OuterLoop if they are + // not part of the inner loop. + const std::vector<Loop*> &SubLoops = L->getSubLoops(); + for (size_t I = 0; I != SubLoops.size(); ) + if (BlocksInL.count(SubLoops[I]->getHeader())) + ++I; // Loop remains in L + else + NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I)); + + // Now that we know which blocks are in L and which need to be moved to + // OuterLoop, move any blocks that need it. + for (unsigned i = 0; i != L->getBlocks().size(); ++i) { + BasicBlock *BB = L->getBlocks()[i]; + if (!BlocksInL.count(BB)) { + // Move this block to the parent, updating the exit blocks sets + L->removeBlockFromLoop(BB); + if ((*LI)[BB] == L) + LI->changeLoopFor(BB, NewOuter); + --i; + } + } + + return NewOuter; +} + + + +/// InsertUniqueBackedgeBlock - This method is called when the specified loop +/// has more than one backedge in it. If this occurs, revector all of these +/// backedges to target a new basic block and have that block branch to the loop +/// header. This ensures that loops have exactly one backedge. +/// +void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) { + assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); + + // Get information about the loop + BasicBlock *Preheader = L->getLoopPreheader(); + BasicBlock *Header = L->getHeader(); + Function *F = Header->getParent(); + + // Figure out which basic blocks contain back-edges to the loop header. + std::vector<BasicBlock*> BackedgeBlocks; + for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I) + if (*I != Preheader) BackedgeBlocks.push_back(*I); + + // Create and insert the new backedge block... + BasicBlock *BEBlock = BasicBlock::Create(Header->getName()+".backedge", F); + BranchInst *BETerminator = BranchInst::Create(Header, BEBlock); + + // Move the new backedge block to right after the last backedge block. + Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; + F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); + + // Now that the block has been inserted into the function, create PHI nodes in + // the backedge block which correspond to any PHI nodes in the header block. + for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { + PHINode *PN = cast<PHINode>(I); + PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be", + BETerminator); + NewPN->reserveOperandSpace(BackedgeBlocks.size()); + if (AA) AA->copyValue(PN, NewPN); + + // Loop over the PHI node, moving all entries except the one for the + // preheader over to the new PHI node. + unsigned PreheaderIdx = ~0U; + bool HasUniqueIncomingValue = true; + Value *UniqueValue = 0; + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + BasicBlock *IBB = PN->getIncomingBlock(i); + Value *IV = PN->getIncomingValue(i); + if (IBB == Preheader) { + PreheaderIdx = i; + } else { + NewPN->addIncoming(IV, IBB); + if (HasUniqueIncomingValue) { + if (UniqueValue == 0) + UniqueValue = IV; + else if (UniqueValue != IV) + HasUniqueIncomingValue = false; + } + } + } + + // Delete all of the incoming values from the old PN except the preheader's + assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); + if (PreheaderIdx != 0) { + PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); + PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); + } + // Nuke all entries except the zero'th. + for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) + PN->removeIncomingValue(e-i, false); + + // Finally, add the newly constructed PHI node as the entry for the BEBlock. + PN->addIncoming(NewPN, BEBlock); + + // As an optimization, if all incoming values in the new PhiNode (which is a + // subset of the incoming values of the old PHI node) have the same value, + // eliminate the PHI Node. + if (HasUniqueIncomingValue) { + NewPN->replaceAllUsesWith(UniqueValue); + if (AA) AA->deleteValue(NewPN); + BEBlock->getInstList().erase(NewPN); + } + } + + // Now that all of the PHI nodes have been inserted and adjusted, modify the + // backedge blocks to just to the BEBlock instead of the header. + for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { + TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); + for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) + if (TI->getSuccessor(Op) == Header) + TI->setSuccessor(Op, BEBlock); + } + + //===--- Update all analyses which we must preserve now -----------------===// + + // Update Loop Information - we know that this block is now in the current + // loop and all parent loops. + L->addBasicBlockToLoop(BEBlock, LI->getBase()); + + // Update dominator information + DT->splitBlock(BEBlock); + if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) + DF->splitBlock(BEBlock); +} |