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//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file implements Loop Rotation Pass.
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-rotate"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Function.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/SmallVector.h"
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#define MAX_HEADER_SIZE 16
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STATISTIC(NumRotated, "Number of loops rotated");
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class LoopRotate : public LoopPass {
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static char ID; // Pass ID, replacement for typeid
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LoopRotate() : LoopPass(&ID) {}
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// Rotate Loop L as many times as possible. Return true if
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// loop is rotated at least once.
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bool runOnLoop(Loop *L, LPPassManager &LPM);
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// LCSSA form makes instruction renaming easier.
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredID(LoopSimplifyID);
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AU.addPreservedID(LoopSimplifyID);
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AU.addRequiredID(LCSSAID);
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AU.addPreservedID(LCSSAID);
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AU.addPreserved<ScalarEvolution>();
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AU.addRequired<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<DominanceFrontier>();
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bool rotateLoop(Loop *L, LPPassManager &LPM);
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/// Initialize local data
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/// After loop rotation, loop pre-header has multiple sucessors.
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/// Insert one forwarding basic block to ensure that loop pre-header
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/// has only one successor.
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void preserveCanonicalLoopForm(LPPassManager &LPM);
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BasicBlock *OrigHeader;
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BasicBlock *OrigPreHeader;
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BasicBlock *OrigLatch;
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BasicBlock *NewHeader;
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LPPassManager *LPM_Ptr;
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char LoopRotate::ID = 0;
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static RegisterPass<LoopRotate> X("loop-rotate", "Rotate Loops");
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Pass *llvm::createLoopRotatePass() { return new LoopRotate(); }
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/// Rotate Loop L as many times as possible. Return true if
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/// the loop is rotated at least once.
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bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
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bool RotatedOneLoop = false;
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// One loop can be rotated multiple times.
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while (rotateLoop(Lp,LPM)) {
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RotatedOneLoop = true;
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return RotatedOneLoop;
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/// Rotate loop LP. Return true if the loop is rotated.
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bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
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OrigPreHeader = L->getLoopPreheader();
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if (!OrigPreHeader) return false;
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OrigLatch = L->getLoopLatch();
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if (!OrigLatch) return false;
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OrigHeader = L->getHeader();
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// If the loop has only one block then there is not much to rotate.
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if (L->getBlocks().size() == 1)
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// If the loop header is not one of the loop exiting blocks then
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// either this loop is already rotated or it is not
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// suitable for loop rotation transformations.
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if (!L->isLoopExiting(OrigHeader))
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BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
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assert(BI->isConditional() && "Branch Instruction is not conditional");
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// Updating PHInodes in loops with multiple exits adds complexity.
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// Keep it simple, and restrict loop rotation to loops with one exit only.
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// In future, lift this restriction and support for multiple exits if
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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if (ExitBlocks.size() > 1)
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// Check size of original header and reject
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// loop if it is very big.
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// FIXME: Use common api to estimate size.
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for (BasicBlock::const_iterator OI = OrigHeader->begin(),
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OE = OrigHeader->end(); OI != OE; ++OI) {
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if (isa<PHINode>(OI))
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continue; // PHI nodes don't count.
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if (isa<DbgInfoIntrinsic>(OI))
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continue; // Debug intrinsics don't count as size.
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if (Size > MAX_HEADER_SIZE)
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// Now, this loop is suitable for rotation.
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// Anything ScalarEvolution may know about this loop or the PHI nodes
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// in its header will soon be invalidated.
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if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
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// Find new Loop header. NewHeader is a Header's one and only successor
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// that is inside loop. Header's other successor is outside the
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// loop. Otherwise loop is not suitable for rotation.
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Exit = BI->getSuccessor(0);
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NewHeader = BI->getSuccessor(1);
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if (L->contains(Exit))
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std::swap(Exit, NewHeader);
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assert(NewHeader && "Unable to determine new loop header");
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assert(L->contains(NewHeader) && !L->contains(Exit) &&
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"Unable to determine loop header and exit blocks");
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// This code assumes that the new header has exactly one predecessor.
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// Remove any single-entry PHI nodes in it.
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assert(NewHeader->getSinglePredecessor() &&
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"New header doesn't have one pred!");
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FoldSingleEntryPHINodes(NewHeader);
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// Begin by walking OrigHeader and populating ValueMap with an entry for
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BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
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DenseMap<const Value *, Value *> ValueMap;
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// For PHI nodes, the value available in OldPreHeader is just the
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// incoming value from OldPreHeader.
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for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
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ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));
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// For the rest of the instructions, create a clone in the OldPreHeader.
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TerminatorInst *LoopEntryBranch = OrigPreHeader->getTerminator();
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for (; I != E; ++I) {
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Instruction *C = I->clone();
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C->setName(I->getName());
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C->insertBefore(LoopEntryBranch);
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// Along with all the other instructions, we just cloned OrigHeader's
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// terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
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// successors by duplicating their incoming values for OrigHeader.
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TerminatorInst *TI = OrigHeader->getTerminator();
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
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for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin();
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PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
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PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreHeader);
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// Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
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// OrigPreHeader's old terminator (the original branch into the loop), and
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// remove the corresponding incoming values from the PHI nodes in OrigHeader.
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LoopEntryBranch->eraseFromParent();
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for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
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PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));
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// Now fix up users of the instructions in OrigHeader, inserting PHI nodes
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for (I = OrigHeader->begin(); I != E; ++I) {
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Value *OrigHeaderVal = I;
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Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];
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// The value now exits in two versions: the initial value in the preheader
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// and the loop "next" value in the original header.
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SSA.Initialize(OrigHeaderVal);
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SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
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SSA.AddAvailableValue(OrigPreHeader, OrigPreHeaderVal);
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// Visit each use of the OrigHeader instruction.
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for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
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UE = OrigHeaderVal->use_end(); UI != UE; ) {
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// Grab the use before incrementing the iterator.
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Use &U = UI.getUse();
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// Increment the iterator before removing the use from the list.
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// SSAUpdater can't handle a non-PHI use in the same block as an
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// earlier def. We can easily handle those cases manually.
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Instruction *UserInst = cast<Instruction>(U.getUser());
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if (!isa<PHINode>(UserInst)) {
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BasicBlock *UserBB = UserInst->getParent();
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// The original users in the OrigHeader are already using the
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// original definitions.
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if (UserBB == OrigHeader)
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// Users in the OrigPreHeader need to use the value to which the
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// original definitions are mapped.
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if (UserBB == OrigPreHeader) {
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U = OrigPreHeaderVal;
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// Anything else can be handled by SSAUpdater.
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// NewHeader is now the header of the loop.
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L->moveToHeader(NewHeader);
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preserveCanonicalLoopForm(LPM);
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/// Initialize local data
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void LoopRotate::initialize() {
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OrigPreHeader = NULL;
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/// After loop rotation, loop pre-header has multiple sucessors.
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/// Insert one forwarding basic block to ensure that loop pre-header
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/// has only one successor.
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void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) {
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// Right now original pre-header has two successors, new header and
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// exit block. Insert new block between original pre-header and
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// new header such that loop's new pre-header has only one successor.
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BasicBlock *NewPreHeader = BasicBlock::Create(OrigHeader->getContext(),
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OrigHeader->getParent(),
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LoopInfo &LI = getAnalysis<LoopInfo>();
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if (Loop *PL = LI.getLoopFor(OrigPreHeader))
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PL->addBasicBlockToLoop(NewPreHeader, LI.getBase());
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BranchInst::Create(NewHeader, NewPreHeader);
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BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
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if (OrigPH_BI->getSuccessor(0) == NewHeader)
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OrigPH_BI->setSuccessor(0, NewPreHeader);
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assert(OrigPH_BI->getSuccessor(1) == NewHeader &&
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"Unexpected original pre-header terminator");
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OrigPH_BI->setSuccessor(1, NewPreHeader);
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for (BasicBlock::iterator I = NewHeader->begin();
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(PN = dyn_cast<PHINode>(I)); ++I) {
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int index = PN->getBasicBlockIndex(OrigPreHeader);
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assert(index != -1 && "Expected incoming value from Original PreHeader");
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PN->setIncomingBlock(index, NewPreHeader);
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assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 &&
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"Expected only one incoming value from Original PreHeader");
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if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
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DT->addNewBlock(NewPreHeader, OrigPreHeader);
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DT->changeImmediateDominator(L->getHeader(), NewPreHeader);
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DT->changeImmediateDominator(Exit, OrigPreHeader);
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for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
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if (L->getHeader() != B) {
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DomTreeNode *Node = DT->getNode(B);
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if (Node && Node->getBlock() == OrigHeader)
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DT->changeImmediateDominator(*BI, L->getHeader());
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DT->changeImmediateDominator(OrigHeader, OrigLatch);
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if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) {
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// New Preheader's dominance frontier is Exit block.
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DominanceFrontier::DomSetType NewPHSet;
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NewPHSet.insert(Exit);
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DF->addBasicBlock(NewPreHeader, NewPHSet);
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// New Header's dominance frontier now includes itself and Exit block
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DominanceFrontier::iterator HeadI = DF->find(L->getHeader());
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if (HeadI != DF->end()) {
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DominanceFrontier::DomSetType & HeaderSet = HeadI->second;
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HeaderSet.insert(L->getHeader());
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HeaderSet.insert(Exit);
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DominanceFrontier::DomSetType HeaderSet;
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HeaderSet.insert(L->getHeader());
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HeaderSet.insert(Exit);
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DF->addBasicBlock(L->getHeader(), HeaderSet);
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// Original header (new Loop Latch)'s dominance frontier is Exit.
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DominanceFrontier::iterator LatchI = DF->find(L->getLoopLatch());
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if (LatchI != DF->end()) {
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DominanceFrontier::DomSetType &LatchSet = LatchI->second;
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LatchSet = LatchI->second;
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LatchSet.insert(Exit);
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DominanceFrontier::DomSetType LatchSet;
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LatchSet.insert(Exit);
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DF->addBasicBlock(L->getHeader(), LatchSet);
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// If a loop block dominates new loop latch then add to its frontiers
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// new header and Exit and remove new latch (which is equal to original
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BasicBlock *NewLatch = L->getLoopLatch();
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assert(NewLatch == OrigHeader && "NewLatch is inequal to OrigHeader");
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if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
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for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
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if (DT->dominates(B, NewLatch)) {
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DominanceFrontier::iterator BDFI = DF->find(B);
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if (BDFI != DF->end()) {
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DominanceFrontier::DomSetType &BSet = BDFI->second;
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BSet.erase(NewLatch);
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BSet.insert(L->getHeader());
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DominanceFrontier::DomSetType BSet;
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BSet.insert(L->getHeader());
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DF->addBasicBlock(B, BSet);
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// Preserve canonical loop form, which means Exit block should
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// have only one predecessor.
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SplitEdge(L->getLoopLatch(), Exit, this);
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assert(NewHeader && L->getHeader() == NewHeader &&
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"Invalid loop header after loop rotation");
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assert(NewPreHeader && L->getLoopPreheader() == NewPreHeader &&
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"Invalid loop preheader after loop rotation");
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assert(L->getLoopLatch() &&
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"Invalid loop latch after loop rotation");