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//===- LoopRotation.cpp - Loop Rotation 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 Loop Rotation Pass.
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
using namespace llvm;
#define DEBUG_TYPE "loop-rotate"
static cl::opt<unsigned>
DefaultRotationThreshold("rotation-max-header-size", cl::init(16), cl::Hidden,
cl::desc("The default maximum header size for automatic loop rotation"));
STATISTIC(NumRotated, "Number of loops rotated");
namespace {
class LoopRotate : public LoopPass {
static char ID; // Pass ID, replacement for typeid
LoopRotate(int SpecifiedMaxHeaderSize = -1) : LoopPass(ID) {
if (SpecifiedMaxHeaderSize == -1)
MaxHeaderSize = DefaultRotationThreshold;
MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
// LCSSA form makes instruction renaming easier.
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
bool simplifyLoopLatch(Loop *L);
bool rotateLoop(Loop *L, bool SimplifiedLatch);
unsigned MaxHeaderSize;
LoopInfo *LI;
const TargetTransformInfo *TTI;
AssumptionCache *AC;
DominatorTree *DT;
char LoopRotate::ID = 0;
INITIALIZE_PASS_BEGIN(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
INITIALIZE_PASS_END(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
Pass *llvm::createLoopRotatePass(int MaxHeaderSize) {
return new LoopRotate(MaxHeaderSize);
/// Rotate Loop L as many times as possible. Return true if
/// the loop is rotated at least once.
bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipOptnoneFunction(L))
return false;
// Save the loop metadata.
MDNode *LoopMD = L->getLoopID();
Function &F = *L->getHeader()->getParent();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
// Simplify the loop latch before attempting to rotate the header
// upward. Rotation may not be needed if the loop tail can be folded into the
// loop exit.
bool SimplifiedLatch = simplifyLoopLatch(L);
// One loop can be rotated multiple times.
bool MadeChange = false;
while (rotateLoop(L, SimplifiedLatch)) {
MadeChange = true;
SimplifiedLatch = false;
// Restore the loop metadata.
// NB! We presume LoopRotation DOESN'T ADD its own metadata.
if ((MadeChange || SimplifiedLatch) && LoopMD)
return MadeChange;
/// RewriteUsesOfClonedInstructions - We just cloned the instructions from the
/// old header into the preheader. If there were uses of the values produced by
/// these instruction that were outside of the loop, we have to insert PHI nodes
/// to merge the two values. Do this now.
static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
BasicBlock *OrigPreheader,
ValueToValueMapTy &ValueMap) {
// Remove PHI node entries that are no longer live.
BasicBlock::iterator I, E = OrigHeader->end();
for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
// Now fix up users of the instructions in OrigHeader, inserting PHI nodes
// as necessary.
SSAUpdater SSA;
for (I = OrigHeader->begin(); I != E; ++I) {
Value *OrigHeaderVal = I;
// If there are no uses of the value (e.g. because it returns void), there
// is nothing to rewrite.
if (OrigHeaderVal->use_empty())
Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];
// The value now exits in two versions: the initial value in the preheader
// and the loop "next" value in the original header.
SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal);
// Visit each use of the OrigHeader instruction.
for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
UE = OrigHeaderVal->use_end(); UI != UE; ) {
// Grab the use before incrementing the iterator.
Use &U = *UI;
// Increment the iterator before removing the use from the list.
// SSAUpdater can't handle a non-PHI use in the same block as an
// earlier def. We can easily handle those cases manually.
Instruction *UserInst = cast<Instruction>(U.getUser());
if (!isa<PHINode>(UserInst)) {
BasicBlock *UserBB = UserInst->getParent();
// The original users in the OrigHeader are already using the
// original definitions.
if (UserBB == OrigHeader)
// Users in the OrigPreHeader need to use the value to which the
// original definitions are mapped.
if (UserBB == OrigPreheader) {
U = OrigPreHeaderVal;
// Anything else can be handled by SSAUpdater.
/// Determine whether the instructions in this range may be safely and cheaply
/// speculated. This is not an important enough situation to develop complex
/// heuristics. We handle a single arithmetic instruction along with any type
/// conversions.
static bool shouldSpeculateInstrs(BasicBlock::iterator Begin,
BasicBlock::iterator End, Loop *L) {
bool seenIncrement = false;
bool MultiExitLoop = false;
if (!L->getExitingBlock())
MultiExitLoop = true;
for (BasicBlock::iterator I = Begin; I != End; ++I) {
if (!isSafeToSpeculativelyExecute(I))
return false;
if (isa<DbgInfoIntrinsic>(I))
switch (I->getOpcode()) {
return false;
case Instruction::GetElementPtr:
// GEPs are cheap if all indices are constant.
if (!cast<GEPOperator>(I)->hasAllConstantIndices())
return false;
// fall-thru to increment case
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr: {
Value *IVOpnd = !isa<Constant>(I->getOperand(0))
? I->getOperand(0)
: !isa<Constant>(I->getOperand(1))
? I->getOperand(1)
: nullptr;
if (!IVOpnd)
return false;
// If increment operand is used outside of the loop, this speculation
// could cause extra live range interference.
if (MultiExitLoop) {
for (User *UseI : IVOpnd->users()) {
auto *UserInst = cast<Instruction>(UseI);
if (!L->contains(UserInst))
return false;
if (seenIncrement)
return false;
seenIncrement = true;
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
// ignore type conversions
return true;
/// Fold the loop tail into the loop exit by speculating the loop tail
/// instructions. Typically, this is a single post-increment. In the case of a
/// simple 2-block loop, hoisting the increment can be much better than
/// duplicating the entire loop header. In the case of loops with early exits,
/// rotation will not work anyway, but simplifyLoopLatch will put the loop in
/// canonical form so downstream passes can handle it.
/// I don't believe this invalidates SCEV.
bool LoopRotate::simplifyLoopLatch(Loop *L) {
BasicBlock *Latch = L->getLoopLatch();
if (!Latch || Latch->hasAddressTaken())
return false;
BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
if (!Jmp || !Jmp->isUnconditional())
return false;
BasicBlock *LastExit = Latch->getSinglePredecessor();
if (!LastExit || !L->isLoopExiting(LastExit))
return false;
BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
if (!BI)
return false;
if (!shouldSpeculateInstrs(Latch->begin(), Jmp, L))
return false;
DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
<< LastExit->getName() << "\n");
// Hoist the instructions from Latch into LastExit.
LastExit->getInstList().splice(BI, Latch->getInstList(), Latch->begin(), Jmp);
unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
BasicBlock *Header = Jmp->getSuccessor(0);
assert(Header == L->getHeader() && "expected a backward branch");
// Remove Latch from the CFG so that LastExit becomes the new Latch.
BI->setSuccessor(FallThruPath, Header);
// Nuke the Latch block.
assert(Latch->empty() && "unable to evacuate Latch");
if (DT)
return true;
/// Rotate loop LP. Return true if the loop is rotated.
/// \param SimplifiedLatch is true if the latch was just folded into the final
/// loop exit. In this case we may want to rotate even though the new latch is
/// now an exiting branch. This rotation would have happened had the latch not
/// been simplified. However, if SimplifiedLatch is false, then we avoid
/// rotating loops in which the latch exits to avoid excessive or endless
/// rotation. LoopRotate should be repeatable and converge to a canonical
/// form. This property is satisfied because simplifying the loop latch can only
/// happen once across multiple invocations of the LoopRotate pass.
bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
// If the loop has only one block then there is not much to rotate.
if (L->getBlocks().size() == 1)
return false;
BasicBlock *OrigHeader = L->getHeader();
BasicBlock *OrigLatch = L->getLoopLatch();
BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
if (!BI || BI->isUnconditional())
return false;
// If the loop header is not one of the loop exiting blocks then
// either this loop is already rotated or it is not
// suitable for loop rotation transformations.
if (!L->isLoopExiting(OrigHeader))
return false;
// If the loop latch already contains a branch that leaves the loop then the
// loop is already rotated.
if (!OrigLatch)
return false;
// Rotate if either the loop latch does *not* exit the loop, or if the loop
// latch was just simplified.
if (L->isLoopExiting(OrigLatch) && !SimplifiedLatch)
return false;
// Check size of original header and reject loop if it is very big or we can't
// duplicate blocks inside it.
SmallPtrSet<const Value *, 32> EphValues;
CodeMetrics::collectEphemeralValues(L, AC, EphValues);
CodeMetrics Metrics;
Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues);
if (Metrics.notDuplicatable) {
DEBUG(dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable"
<< " instructions: "; L->dump());
return false;
if (Metrics.NumInsts > MaxHeaderSize)
return false;
// Now, this loop is suitable for rotation.
BasicBlock *OrigPreheader = L->getLoopPreheader();
// If the loop could not be converted to canonical form, it must have an
// indirectbr in it, just give up.
if (!OrigPreheader)
return false;
// Anything ScalarEvolution may know about this loop or the PHI nodes
// in its header will soon be invalidated.
if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
DEBUG(dbgs() << "LoopRotation: rotating "; L->dump());
// Find new Loop header. NewHeader is a Header's one and only successor
// that is inside loop. Header's other successor is outside the
// loop. Otherwise loop is not suitable for rotation.
BasicBlock *Exit = BI->getSuccessor(0);
BasicBlock *NewHeader = BI->getSuccessor(1);
if (L->contains(Exit))
std::swap(Exit, NewHeader);
assert(NewHeader && "Unable to determine new loop header");
assert(L->contains(NewHeader) && !L->contains(Exit) &&
"Unable to determine loop header and exit blocks");
// This code assumes that the new header has exactly one predecessor.
// Remove any single-entry PHI nodes in it.
assert(NewHeader->getSinglePredecessor() &&
"New header doesn't have one pred!");
// Begin by walking OrigHeader and populating ValueMap with an entry for
// each Instruction.
BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
ValueToValueMapTy ValueMap;
// For PHI nodes, the value available in OldPreHeader is just the
// incoming value from OldPreHeader.
for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
ValueMap[PN] = PN->getIncomingValueForBlock(OrigPreheader);
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
// For the rest of the instructions, either hoist to the OrigPreheader if
// possible or create a clone in the OldPreHeader if not.
TerminatorInst *LoopEntryBranch = OrigPreheader->getTerminator();
while (I != E) {
Instruction *Inst = I++;
// If the instruction's operands are invariant and it doesn't read or write
// memory, then it is safe to hoist. Doing this doesn't change the order of
// execution in the preheader, but does prevent the instruction from
// executing in each iteration of the loop. This means it is safe to hoist
// something that might trap, but isn't safe to hoist something that reads
// memory (without proving that the loop doesn't write).
if (L->hasLoopInvariantOperands(Inst) &&
!Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
!isa<TerminatorInst>(Inst) && !isa<DbgInfoIntrinsic>(Inst) &&
!isa<AllocaInst>(Inst)) {
// Otherwise, create a duplicate of the instruction.
Instruction *C = Inst->clone();
// Eagerly remap the operands of the instruction.
RemapInstruction(C, ValueMap,
// With the operands remapped, see if the instruction constant folds or is
// otherwise simplifyable. This commonly occurs because the entry from PHI
// nodes allows icmps and other instructions to fold.
// FIXME: Provide TLI, DT, AC to SimplifyInstruction.
Value *V = SimplifyInstruction(C, DL);
if (V && LI->replacementPreservesLCSSAForm(C, V)) {
// If so, then delete the temporary instruction and stick the folded value
// in the map.
delete C;
ValueMap[Inst] = V;
} else {
// Otherwise, stick the new instruction into the new block!
ValueMap[Inst] = C;
// Along with all the other instructions, we just cloned OrigHeader's
// terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
// successors by duplicating their incoming values for OrigHeader.
TerminatorInst *TI = OrigHeader->getTerminator();
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin();
PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreheader);
// Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
// OrigPreHeader's old terminator (the original branch into the loop), and
// remove the corresponding incoming values from the PHI nodes in OrigHeader.
// If there were any uses of instructions in the duplicated block outside the
// loop, update them, inserting PHI nodes as required
RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap);
// NewHeader is now the header of the loop.
assert(L->getHeader() == NewHeader && "Latch block is our new header");
// At this point, we've finished our major CFG changes. As part of cloning
// the loop into the preheader we've simplified instructions and the
// duplicated conditional branch may now be branching on a constant. If it is
// branching on a constant and if that constant means that we enter the loop,
// then we fold away the cond branch to an uncond branch. This simplifies the
// loop in cases important for nested loops, and it also means we don't have
// to split as many edges.
BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator());
assert(PHBI->isConditional() && "Should be clone of BI condbr!");
if (!isa<ConstantInt>(PHBI->getCondition()) ||
!= NewHeader) {
// The conditional branch can't be folded, handle the general case.
// Update DominatorTree to reflect the CFG change we just made. Then split
// edges as necessary to preserve LoopSimplify form.
if (DT) {
// Everything that was dominated by the old loop header is now dominated
// by the original loop preheader. Conceptually the header was merged
// into the preheader, even though we reuse the actual block as a new
// loop latch.
DomTreeNode *OrigHeaderNode = DT->getNode(OrigHeader);
SmallVector<DomTreeNode *, 8> HeaderChildren(OrigHeaderNode->begin(),
DomTreeNode *OrigPreheaderNode = DT->getNode(OrigPreheader);
for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I)
DT->changeImmediateDominator(HeaderChildren[I], OrigPreheaderNode);
assert(DT->getNode(Exit)->getIDom() == OrigPreheaderNode);
assert(DT->getNode(NewHeader)->getIDom() == OrigPreheaderNode);
// Update OrigHeader to be dominated by the new header block.
DT->changeImmediateDominator(OrigHeader, OrigLatch);
// Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
// thus is not a preheader anymore.
// Split the edge to form a real preheader.
BasicBlock *NewPH = SplitCriticalEdge(
OrigPreheader, NewHeader,
CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA());
NewPH->setName(NewHeader->getName() + "");
// Preserve canonical loop form, which means that 'Exit' should have only
// one predecessor. Note that Exit could be an exit block for multiple
// nested loops, causing both of the edges to now be critical and need to
// be split.
SmallVector<BasicBlock *, 4> ExitPreds(pred_begin(Exit), pred_end(Exit));
bool SplitLatchEdge = false;
for (SmallVectorImpl<BasicBlock *>::iterator PI = ExitPreds.begin(),
PE = ExitPreds.end();
PI != PE; ++PI) {
// We only need to split loop exit edges.
Loop *PredLoop = LI->getLoopFor(*PI);
if (!PredLoop || PredLoop->contains(Exit))
if (isa<IndirectBrInst>((*PI)->getTerminator()))
SplitLatchEdge |= L->getLoopLatch() == *PI;
BasicBlock *ExitSplit = SplitCriticalEdge(
*PI, Exit, CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA());
assert(SplitLatchEdge &&
"Despite splitting all preds, failed to split latch exit?");
} else {
// We can fold the conditional branch in the preheader, this makes things
// simpler. The first step is to remove the extra edge to the Exit block.
Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/);
BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI);
// With our CFG finalized, update DomTree if it is available.
if (DT) {
// Update OrigHeader to be dominated by the new header block.
DT->changeImmediateDominator(NewHeader, OrigPreheader);
DT->changeImmediateDominator(OrigHeader, OrigLatch);
// Brute force incremental dominator tree update. Call
// findNearestCommonDominator on all CFG predecessors of each child of the
// original header.
DomTreeNode *OrigHeaderNode = DT->getNode(OrigHeader);
SmallVector<DomTreeNode *, 8> HeaderChildren(OrigHeaderNode->begin(),
bool Changed;
do {
Changed = false;
for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I) {
DomTreeNode *Node = HeaderChildren[I];
BasicBlock *BB = Node->getBlock();
pred_iterator PI = pred_begin(BB);
BasicBlock *NearestDom = *PI;
for (pred_iterator PE = pred_end(BB); PI != PE; ++PI)
NearestDom = DT->findNearestCommonDominator(NearestDom, *PI);
// Remember if this changes the DomTree.
if (Node->getIDom()->getBlock() != NearestDom) {
DT->changeImmediateDominator(BB, NearestDom);
Changed = true;
// If the dominator changed, this may have an effect on other
// predecessors, continue until we reach a fixpoint.
} while (Changed);
assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation");
assert(L->getLoopLatch() && "Invalid loop latch after loop rotation");
// Now that the CFG and DomTree are in a consistent state again, try to merge
// the OrigHeader block into OrigLatch. This will succeed if they are
// connected by an unconditional branch. This is just a cleanup so the
// emitted code isn't too gross in this common case.
MergeBlockIntoPredecessor(OrigHeader, DT, LI);
DEBUG(dbgs() << "LoopRotation: into "; L->dump());
return true;