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Diffstat (limited to 'llvm/lib/Transforms/Utils/LoopSimplify.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Utils/LoopSimplify.cpp | 929 |
1 files changed, 929 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/LoopSimplify.cpp b/llvm/lib/Transforms/Utils/LoopSimplify.cpp new file mode 100644 index 000000000000..d0f89dc54bfb --- /dev/null +++ b/llvm/lib/Transforms/Utils/LoopSimplify.cpp @@ -0,0 +1,929 @@ +//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// +// +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. +// See https://llvm.org/LICENSE.txt for license information. +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception +// +//===----------------------------------------------------------------------===// +// +// 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. +// +// Indirectbr instructions introduce several complications. If the loop +// contains or is entered by an indirectbr instruction, it may not be possible +// to transform the loop and make these guarantees. Client code should check +// that these conditions are true before relying on them. +// +// Similar complications arise from callbr instructions, particularly in +// asm-goto where blockaddress expressions are used. +// +// 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. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/Utils/LoopSimplify.h" +#include "llvm/ADT/DepthFirstIterator.h" +#include "llvm/ADT/SetOperations.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/AssumptionCache.h" +#include "llvm/Analysis/BasicAliasAnalysis.h" +#include "llvm/Analysis/BranchProbabilityInfo.h" +#include "llvm/Analysis/DependenceAnalysis.h" +#include "llvm/Analysis/GlobalsModRef.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/MemorySSA.h" +#include "llvm/Analysis/MemorySSAUpdater.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" +#include "llvm/IR/CFG.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DataLayout.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/LLVMContext.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/Type.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Utils.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Transforms/Utils/LoopUtils.h" +using namespace llvm; + +#define DEBUG_TYPE "loop-simplify" + +STATISTIC(NumNested , "Number of nested loops split out"); + +// 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. +static void placeSplitBlockCarefully(BasicBlock *NewBB, + SmallVectorImpl<BasicBlock *> &SplitPreds, + Loop *L) { + // Check to see if NewBB is already well placed. + Function::iterator BBI = --NewBB->getIterator(); + 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 = nullptr; + for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { + Function::iterator BBI = SplitPreds[i]->getIterator(); + 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); +} + +/// 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. +/// +BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT, + LoopInfo *LI, MemorySSAUpdater *MSSAU, + bool PreserveLCSSA) { + 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) { + BasicBlock *P = *PI; + if (!L->contains(P)) { // Coming in from outside the loop? + // If the loop is branched to from an indirect terminator, we won't + // be able to fully transform the loop, because it prohibits + // edge splitting. + if (P->getTerminator()->isIndirectTerminator()) + return nullptr; + + // Keep track of it. + OutsideBlocks.push_back(P); + } + } + + // Split out the loop pre-header. + BasicBlock *PreheaderBB; + PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT, + LI, MSSAU, PreserveLCSSA); + if (!PreheaderBB) + return nullptr; + + LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header " + << PreheaderBB->getName() << "\n"); + + // Make sure that NewBB is put someplace intelligent, which doesn't mess up + // code layout too horribly. + placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L); + + return PreheaderBB; +} + +/// 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) { + SmallVector<BasicBlock *, 8> Worklist; + Worklist.push_back(InputBB); + do { + BasicBlock *BB = Worklist.pop_back_val(); + 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()); +} + +/// 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, + AssumptionCache *AC) { + const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); + for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { + PHINode *PN = cast<PHINode>(I); + ++I; + if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) { + // This is a degenerate PHI already, don't modify it! + PN->replaceAllUsesWith(V); + 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 nullptr; +} + +/// 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. +/// +static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader, + DominatorTree *DT, LoopInfo *LI, + ScalarEvolution *SE, bool PreserveLCSSA, + AssumptionCache *AC, MemorySSAUpdater *MSSAU) { + // Don't try to separate loops without a preheader. + if (!Preheader) + return nullptr; + + // The header is not a landing pad; preheader insertion should ensure this. + BasicBlock *Header = L->getHeader(); + assert(!Header->isEHPad() && "Can't insert backedge to EH pad"); + + PHINode *PN = findPHIToPartitionLoops(L, DT, AC); + if (!PN) return nullptr; // 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))) { + // We can't split indirect control flow edges. + if (PN->getIncomingBlock(i)->getTerminator()->isIndirectTerminator()) + return nullptr; + OuterLoopPreds.push_back(PN->getIncomingBlock(i)); + } + } + LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n"); + + // If ScalarEvolution is around and knows anything about values in + // this loop, tell it to forget them, because we're about to + // substantially change it. + if (SE) + SE->forgetLoop(L); + + BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", + DT, LI, MSSAU, PreserveLCSSA); + + // 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 = LI->AllocateLoop(); + + // 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); + + // 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); + + // Now reset the header in L, which had been moved by + // SplitBlockPredecessors for the outer loop. + L->moveToHeader(Header); + + // 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) { + BasicBlock *P = *PI; + if (DT->dominates(Header, P)) + addBlockAndPredsToSet(P, 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)); + + SmallVector<BasicBlock *, 8> OuterLoopBlocks; + OuterLoopBlocks.push_back(NewBB); + // 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); + OuterLoopBlocks.push_back(BB); + } + --i; + } + } + + // Split edges to exit blocks from the inner loop, if they emerged in the + // process of separating the outer one. + formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA); + + if (PreserveLCSSA) { + // Fix LCSSA form for L. Some values, which previously were only used inside + // L, can now be used in NewOuter loop. We need to insert phi-nodes for them + // in corresponding exit blocks. + // We don't need to form LCSSA recursively, because there cannot be uses + // inside a newly created loop of defs from inner loops as those would + // already be a use of an LCSSA phi node. + formLCSSA(*L, *DT, LI, SE); + + assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) && + "LCSSA is broken after separating nested loops!"); + } + + return NewOuter; +} + +/// 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. +static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader, + DominatorTree *DT, LoopInfo *LI, + MemorySSAUpdater *MSSAU) { + assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); + + // Get information about the loop + BasicBlock *Header = L->getHeader(); + Function *F = Header->getParent(); + + // Unique backedge insertion currently depends on having a preheader. + if (!Preheader) + return nullptr; + + // The header is not an EH pad; preheader insertion should ensure this. + assert(!Header->isEHPad() && "Can't insert backedge to EH pad"); + + // 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){ + BasicBlock *P = *I; + + // Indirect edges cannot be split, so we must fail if we find one. + if (P->getTerminator()->isIndirectTerminator()) + return nullptr; + + if (P != Preheader) BackedgeBlocks.push_back(P); + } + + // Create and insert the new backedge block... + BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(), + Header->getName() + ".backedge", F); + BranchInst *BETerminator = BranchInst::Create(Header, BEBlock); + BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc()); + + LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block " + << BEBlock->getName() << "\n"); + + // Move the new backedge block to right after the last backedge block. + Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator(); + 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(), BackedgeBlocks.size(), + PN->getName()+".be", BETerminator); + + // 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 = nullptr; + 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) + 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); + BEBlock->getInstList().erase(NewPN); + } + } + + // Now that all of the PHI nodes have been inserted and adjusted, modify the + // backedge blocks to jump to the BEBlock instead of the header. + // If one of the backedges has llvm.loop metadata attached, we remove + // it from the backedge and add it to BEBlock. + unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop"); + MDNode *LoopMD = nullptr; + for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { + Instruction *TI = BackedgeBlocks[i]->getTerminator(); + if (!LoopMD) + LoopMD = TI->getMetadata(LoopMDKind); + TI->setMetadata(LoopMDKind, nullptr); + TI->replaceSuccessorWith(Header, BEBlock); + } + BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD); + + //===--- 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); + + // Update dominator information + DT->splitBlock(BEBlock); + + if (MSSAU) + MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(Header, Preheader, + BEBlock); + + return BEBlock; +} + +/// Simplify one loop and queue further loops for simplification. +static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist, + DominatorTree *DT, LoopInfo *LI, + ScalarEvolution *SE, AssumptionCache *AC, + MemorySSAUpdater *MSSAU, bool PreserveLCSSA) { + bool Changed = false; + if (MSSAU && VerifyMemorySSA) + MSSAU->getMemorySSA()->verifyMemorySSA(); + +ReprocessLoop: + + // Check to see that no blocks (other than the header) in this loop have + // predecessors that are not in the loop. This is not valid for natural + // loops, but can occur if the blocks are unreachable. Since they are + // unreachable we can just shamelessly delete those CFG edges! + for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); + BB != E; ++BB) { + if (*BB == L->getHeader()) continue; + + SmallPtrSet<BasicBlock*, 4> BadPreds; + for (pred_iterator PI = pred_begin(*BB), + PE = pred_end(*BB); PI != PE; ++PI) { + BasicBlock *P = *PI; + if (!L->contains(P)) + BadPreds.insert(P); + } + + // Delete each unique out-of-loop (and thus dead) predecessor. + for (BasicBlock *P : BadPreds) { + + LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor " + << P->getName() << "\n"); + + // Zap the dead pred's terminator and replace it with unreachable. + Instruction *TI = P->getTerminator(); + changeToUnreachable(TI, /*UseLLVMTrap=*/false, PreserveLCSSA, + /*DTU=*/nullptr, MSSAU); + Changed = true; + } + } + + if (MSSAU && VerifyMemorySSA) + MSSAU->getMemorySSA()->verifyMemorySSA(); + + // If there are exiting blocks with branches on undef, resolve the undef in + // the direction which will exit the loop. This will help simplify loop + // trip count computations. + SmallVector<BasicBlock*, 8> ExitingBlocks; + L->getExitingBlocks(ExitingBlocks); + for (BasicBlock *ExitingBlock : ExitingBlocks) + if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator())) + if (BI->isConditional()) { + if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) { + + LLVM_DEBUG(dbgs() + << "LoopSimplify: Resolving \"br i1 undef\" to exit in " + << ExitingBlock->getName() << "\n"); + + BI->setCondition(ConstantInt::get(Cond->getType(), + !L->contains(BI->getSuccessor(0)))); + + Changed = true; + } + } + + // Does the loop already have a preheader? If so, don't insert one. + BasicBlock *Preheader = L->getLoopPreheader(); + if (!Preheader) { + Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA); + if (Preheader) + 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. + if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA)) + Changed = true; + + if (MSSAU && VerifyMemorySSA) + MSSAU->getMemorySSA()->verifyMemorySSA(); + + // If the header has more than two predecessors at this point (from the + // preheader and from multiple backedges), we must adjust the loop. + BasicBlock *LoopLatch = L->getLoopLatch(); + if (!LoopLatch) { + // 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 (L->getNumBackEdges() < 8) { + if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE, + PreserveLCSSA, AC, MSSAU)) { + ++NumNested; + // Enqueue the outer loop as it should be processed next in our + // depth-first nest walk. + Worklist.push_back(OuterL); + + // This is a big restructuring change, reprocess the whole loop. + Changed = true; + // GCC doesn't tail recursion eliminate this. + // FIXME: It isn't clear we can't rely on LLVM to TRE 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. + LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU); + if (LoopLatch) + Changed = true; + } + + if (MSSAU && VerifyMemorySSA) + MSSAU->getMemorySSA()->verifyMemorySSA(); + + const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); + + // 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 = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) { + if (SE) SE->forgetValue(PN); + if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) { + PN->replaceAllUsesWith(V); + PN->eraseFromParent(); + } + } + + // If this loop has multiple exits and the exits all go to the same + // block, attempt to merge the exits. This helps several passes, such + // as LoopRotation, which do not support loops with multiple exits. + // SimplifyCFG also does this (and this code uses the same utility + // function), however this code is loop-aware, where SimplifyCFG is + // not. That gives it the advantage of being able to hoist + // loop-invariant instructions out of the way to open up more + // opportunities, and the disadvantage of having the responsibility + // to preserve dominator information. + auto HasUniqueExitBlock = [&]() { + BasicBlock *UniqueExit = nullptr; + for (auto *ExitingBB : ExitingBlocks) + for (auto *SuccBB : successors(ExitingBB)) { + if (L->contains(SuccBB)) + continue; + + if (!UniqueExit) + UniqueExit = SuccBB; + else if (UniqueExit != SuccBB) + return false; + } + + return true; + }; + if (HasUniqueExitBlock()) { + for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { + BasicBlock *ExitingBlock = ExitingBlocks[i]; + if (!ExitingBlock->getSinglePredecessor()) continue; + BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()); + if (!BI || !BI->isConditional()) continue; + CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition()); + if (!CI || CI->getParent() != ExitingBlock) continue; + + // Attempt to hoist out all instructions except for the + // comparison and the branch. + bool AllInvariant = true; + bool AnyInvariant = false; + for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) { + Instruction *Inst = &*I++; + if (Inst == CI) + continue; + if (!L->makeLoopInvariant( + Inst, AnyInvariant, + Preheader ? Preheader->getTerminator() : nullptr, MSSAU)) { + AllInvariant = false; + break; + } + } + if (AnyInvariant) { + Changed = true; + // The loop disposition of all SCEV expressions that depend on any + // hoisted values have also changed. + if (SE) + SE->forgetLoopDispositions(L); + } + if (!AllInvariant) continue; + + // The block has now been cleared of all instructions except for + // a comparison and a conditional branch. SimplifyCFG may be able + // to fold it now. + if (!FoldBranchToCommonDest(BI, MSSAU)) + continue; + + // Success. The block is now dead, so remove it from the loop, + // update the dominator tree and delete it. + LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block " + << ExitingBlock->getName() << "\n"); + + assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock)); + Changed = true; + LI->removeBlock(ExitingBlock); + + DomTreeNode *Node = DT->getNode(ExitingBlock); + const std::vector<DomTreeNodeBase<BasicBlock> *> &Children = + Node->getChildren(); + while (!Children.empty()) { + DomTreeNode *Child = Children.front(); + DT->changeImmediateDominator(Child, Node->getIDom()); + } + DT->eraseNode(ExitingBlock); + if (MSSAU) { + SmallSetVector<BasicBlock *, 8> ExitBlockSet; + ExitBlockSet.insert(ExitingBlock); + MSSAU->removeBlocks(ExitBlockSet); + } + + BI->getSuccessor(0)->removePredecessor( + ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA); + BI->getSuccessor(1)->removePredecessor( + ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA); + ExitingBlock->eraseFromParent(); + } + } + + // Changing exit conditions for blocks may affect exit counts of this loop and + // any of its paretns, so we must invalidate the entire subtree if we've made + // any changes. + if (Changed && SE) + SE->forgetTopmostLoop(L); + + if (MSSAU && VerifyMemorySSA) + MSSAU->getMemorySSA()->verifyMemorySSA(); + + return Changed; +} + +bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, + ScalarEvolution *SE, AssumptionCache *AC, + MemorySSAUpdater *MSSAU, bool PreserveLCSSA) { + bool Changed = false; + +#ifndef NDEBUG + // If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA + // form. + if (PreserveLCSSA) { + assert(DT && "DT not available."); + assert(LI && "LI not available."); + assert(L->isRecursivelyLCSSAForm(*DT, *LI) && + "Requested to preserve LCSSA, but it's already broken."); + } +#endif + + // Worklist maintains our depth-first queue of loops in this nest to process. + SmallVector<Loop *, 4> Worklist; + Worklist.push_back(L); + + // Walk the worklist from front to back, pushing newly found sub loops onto + // the back. This will let us process loops from back to front in depth-first + // order. We can use this simple process because loops form a tree. + for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) { + Loop *L2 = Worklist[Idx]; + Worklist.append(L2->begin(), L2->end()); + } + + while (!Worklist.empty()) + Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE, + AC, MSSAU, PreserveLCSSA); + + return Changed; +} + +namespace { + struct LoopSimplify : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + LoopSimplify() : FunctionPass(ID) { + initializeLoopSimplifyPass(*PassRegistry::getPassRegistry()); + } + + bool runOnFunction(Function &F) override; + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.addRequired<AssumptionCacheTracker>(); + + // We need loop information to identify the loops... + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addPreserved<DominatorTreeWrapperPass>(); + + AU.addRequired<LoopInfoWrapperPass>(); + AU.addPreserved<LoopInfoWrapperPass>(); + + AU.addPreserved<BasicAAWrapperPass>(); + AU.addPreserved<AAResultsWrapperPass>(); + AU.addPreserved<GlobalsAAWrapperPass>(); + AU.addPreserved<ScalarEvolutionWrapperPass>(); + AU.addPreserved<SCEVAAWrapperPass>(); + AU.addPreservedID(LCSSAID); + AU.addPreserved<DependenceAnalysisWrapperPass>(); + AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. + AU.addPreserved<BranchProbabilityInfoWrapperPass>(); + if (EnableMSSALoopDependency) + AU.addPreserved<MemorySSAWrapperPass>(); + } + + /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees. + void verifyAnalysis() const override; + }; +} + +char LoopSimplify::ID = 0; +INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify", + "Canonicalize natural loops", false, false) +INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) +INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) +INITIALIZE_PASS_END(LoopSimplify, "loop-simplify", + "Canonicalize natural loops", false, false) + +// Publicly exposed interface to pass... +char &llvm::LoopSimplifyID = LoopSimplify::ID; +Pass *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; + LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); + DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>(); + ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr; + AssumptionCache *AC = + &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); + MemorySSA *MSSA = nullptr; + std::unique_ptr<MemorySSAUpdater> MSSAU; + if (EnableMSSALoopDependency) { + auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>(); + if (MSSAAnalysis) { + MSSA = &MSSAAnalysis->getMSSA(); + MSSAU = std::make_unique<MemorySSAUpdater>(MSSA); + } + } + + bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID); + + // Simplify each loop nest in the function. + for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) + Changed |= simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), PreserveLCSSA); + +#ifndef NDEBUG + if (PreserveLCSSA) { + bool InLCSSA = all_of( + *LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); }); + assert(InLCSSA && "LCSSA is broken after loop-simplify."); + } +#endif + return Changed; +} + +PreservedAnalyses LoopSimplifyPass::run(Function &F, + FunctionAnalysisManager &AM) { + bool Changed = false; + LoopInfo *LI = &AM.getResult<LoopAnalysis>(F); + DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F); + ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F); + AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F); + auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(F); + std::unique_ptr<MemorySSAUpdater> MSSAU; + if (MSSAAnalysis) { + auto *MSSA = &MSSAAnalysis->getMSSA(); + MSSAU = std::make_unique<MemorySSAUpdater>(MSSA); + } + + + // Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA + // after simplifying the loops. MemorySSA is preserved if it exists. + for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) + Changed |= + simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), /*PreserveLCSSA*/ false); + + if (!Changed) + return PreservedAnalyses::all(); + + PreservedAnalyses PA; + PA.preserve<DominatorTreeAnalysis>(); + PA.preserve<LoopAnalysis>(); + PA.preserve<BasicAA>(); + PA.preserve<GlobalsAA>(); + PA.preserve<SCEVAA>(); + PA.preserve<ScalarEvolutionAnalysis>(); + PA.preserve<DependenceAnalysis>(); + if (MSSAAnalysis) + PA.preserve<MemorySSAAnalysis>(); + // BPI maps conditional terminators to probabilities, LoopSimplify can insert + // blocks, but it does so only by splitting existing blocks and edges. This + // results in the interesting property that all new terminators inserted are + // unconditional branches which do not appear in BPI. All deletions are + // handled via ValueHandle callbacks w/in BPI. + PA.preserve<BranchProbabilityAnalysis>(); + return PA; +} + +// FIXME: Restore this code when we re-enable verification in verifyAnalysis +// below. +#if 0 +static void verifyLoop(Loop *L) { + // Verify subloops. + for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) + verifyLoop(*I); + + // It used to be possible to just assert L->isLoopSimplifyForm(), however + // with the introduction of indirectbr, there are now cases where it's + // not possible to transform a loop as necessary. We can at least check + // that there is an indirectbr near any time there's trouble. + + // Indirectbr can interfere with preheader and unique backedge insertion. + if (!L->getLoopPreheader() || !L->getLoopLatch()) { + bool HasIndBrPred = false; + for (pred_iterator PI = pred_begin(L->getHeader()), + PE = pred_end(L->getHeader()); PI != PE; ++PI) + if (isa<IndirectBrInst>((*PI)->getTerminator())) { + HasIndBrPred = true; + break; + } + assert(HasIndBrPred && + "LoopSimplify has no excuse for missing loop header info!"); + (void)HasIndBrPred; + } + + // Indirectbr can interfere with exit block canonicalization. + if (!L->hasDedicatedExits()) { + bool HasIndBrExiting = false; + SmallVector<BasicBlock*, 8> ExitingBlocks; + L->getExitingBlocks(ExitingBlocks); + for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { + if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) { + HasIndBrExiting = true; + break; + } + } + + assert(HasIndBrExiting && + "LoopSimplify has no excuse for missing exit block info!"); + (void)HasIndBrExiting; + } +} +#endif + +void LoopSimplify::verifyAnalysis() const { + // FIXME: This routine is being called mid-way through the loop pass manager + // as loop passes destroy this analysis. That's actually fine, but we have no + // way of expressing that here. Once all of the passes that destroy this are + // hoisted out of the loop pass manager we can add back verification here. +#if 0 + for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) + verifyLoop(*I); +#endif +} |