1 //===- HexagonVectorLoopCarriedReuse.cpp ----------------------------------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This pass removes the computation of provably redundant expressions that have
10 // been computed earlier in a previous iteration. It relies on the use of PHIs
11 // to identify loop carried dependences. This is scalar replacement for vector
14 //-----------------------------------------------------------------------------
15 // Motivation: Consider the case where we have the following loop structure.
28 // This can be converted to
41 // SROA does a good job of reusing a[i+1] as a[i] in the next iteration.
42 // Such a loop comes to this pass in the following form.
47 // X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
57 // In this pass, we look for PHIs such as X2 whose incoming values come only
58 // from the Loop Preheader and over the backedge and additionaly, both these
59 // values are the results of the same operation in terms of opcode. We call such
60 // a PHI node a dependence chain or DepChain. In this case, the dependence of X2
61 // over X1 is carried over only one iteration and so the DepChain is only one
64 // Then, we traverse the uses of the PHI (X2) and the uses of the value of the
65 // PHI coming over the backedge (X1). We stop at the first pair of such users
66 // I1 (of X2) and I2 (of X1) that meet the following conditions.
67 // 1. I1 and I2 are the same operation, but with different operands.
68 // 2. X2 and X1 are used at the same operand number in the two instructions.
69 // 3. All other operands Op1 of I1 and Op2 of I2 are also such that there is a
70 // a DepChain from Op1 to Op2 of the same length as that between X2 and X1.
72 // We then make the following transformation
77 // X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
78 // Y2 = PHI<(Y0, LoopPreheader), (t4, Loop)>
79 // t1 = f(X2) <-- Will be removed by DCE.
88 // We proceed until we cannot find any more such instructions I1 and I2.
90 // --- DepChains & Loop carried dependences ---
91 // Consider a single basic block loop such as
97 // X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
98 // Y2 = PHI<(Y0, LoopPreheader), (X2, Loop)>
102 // cond_branch <Loop>
104 // Then there is a dependence between X2 and X1 that goes back one iteration,
105 // i.e. X1 is used as X2 in the very next iteration. We represent this as a
106 // DepChain from X2 to X1 (X2->X1).
107 // Similarly, there is a dependence between Y2 and X1 that goes back two
108 // iterations. X1 is used as Y2 two iterations after it is computed. This is
109 // represented by a DepChain as (Y2->X2->X1).
111 // A DepChain has the following properties.
112 // 1. Num of edges in DepChain = Number of Instructions in DepChain = Number of
113 // iterations of carried dependence + 1.
114 // 2. All instructions in the DepChain except the last are PHIs.
116 //===----------------------------------------------------------------------===//
118 #include "llvm/ADT/SetVector.h"
119 #include "llvm/ADT/SmallVector.h"
120 #include "llvm/ADT/Statistic.h"
121 #include "llvm/Analysis/LoopInfo.h"
122 #include "llvm/Analysis/LoopPass.h"
123 #include "llvm/IR/BasicBlock.h"
124 #include "llvm/IR/DerivedTypes.h"
125 #include "llvm/IR/IRBuilder.h"
126 #include "llvm/IR/Instruction.h"
127 #include "llvm/IR/Instructions.h"
128 #include "llvm/IR/IntrinsicInst.h"
129 #include "llvm/IR/Intrinsics.h"
130 #include "llvm/IR/IntrinsicsHexagon.h"
131 #include "llvm/IR/Use.h"
132 #include "llvm/IR/User.h"
133 #include "llvm/IR/Value.h"
134 #include "llvm/InitializePasses.h"
135 #include "llvm/Pass.h"
136 #include "llvm/Support/Casting.h"
137 #include "llvm/Support/CommandLine.h"
138 #include "llvm/Support/Compiler.h"
139 #include "llvm/Support/Debug.h"
140 #include "llvm/Support/raw_ostream.h"
141 #include "llvm/Transforms/Scalar.h"
142 #include "llvm/Transforms/Utils.h"
150 using namespace llvm;
152 #define DEBUG_TYPE "hexagon-vlcr"
154 STATISTIC(HexagonNumVectorLoopCarriedReuse,
155 "Number of values that were reused from a previous iteration.");
157 static cl::opt<int> HexagonVLCRIterationLim("hexagon-vlcr-iteration-lim",
159 cl::desc("Maximum distance of loop carried dependences that are handled"),
160 cl::init(2), cl::ZeroOrMore);
164 void initializeHexagonVectorLoopCarriedReusePass(PassRegistry&);
165 Pass *createHexagonVectorLoopCarriedReusePass();
167 } // end namespace llvm
171 // See info about DepChain in the comments at the top of this file.
172 using ChainOfDependences = SmallVector<Instruction *, 4>;
175 ChainOfDependences Chain;
178 bool isIdentical(DepChain &Other) const {
179 if (Other.size() != size())
181 ChainOfDependences &OtherChain = Other.getChain();
182 for (int i = 0; i < size(); ++i) {
183 if (Chain[i] != OtherChain[i])
189 ChainOfDependences &getChain() {
201 void push_back(Instruction *I) {
205 int iterations() const {
209 Instruction *front() const {
210 return Chain.front();
213 Instruction *back() const {
217 Instruction *&operator[](const int index) {
221 friend raw_ostream &operator<< (raw_ostream &OS, const DepChain &D);
224 LLVM_ATTRIBUTE_UNUSED
225 raw_ostream &operator<<(raw_ostream &OS, const DepChain &D) {
226 const ChainOfDependences &CD = D.Chain;
227 int ChainSize = CD.size();
228 OS << "**DepChain Start::**\n";
229 for (int i = 0; i < ChainSize -1; ++i) {
230 OS << *(CD[i]) << " -->\n";
232 OS << *CD[ChainSize-1] << "\n";
237 Instruction *Inst2Replace = nullptr;
239 // In the new PHI node that we'll construct this is the value that'll be
240 // used over the backedge. This is the value that gets reused from a
241 // previous iteration.
242 Instruction *BackedgeInst = nullptr;
243 std::map<Instruction *, DepChain *> DepChains;
246 ReuseValue() = default;
249 Inst2Replace = nullptr;
250 BackedgeInst = nullptr;
254 bool isDefined() { return Inst2Replace != nullptr; }
257 LLVM_ATTRIBUTE_UNUSED
258 raw_ostream &operator<<(raw_ostream &OS, const ReuseValue &RU) {
259 OS << "** ReuseValue ***\n";
260 OS << "Instruction to Replace: " << *(RU.Inst2Replace) << "\n";
261 OS << "Backedge Instruction: " << *(RU.BackedgeInst) << "\n";
265 class HexagonVectorLoopCarriedReuse : public LoopPass {
269 explicit HexagonVectorLoopCarriedReuse() : LoopPass(ID) {
270 PassRegistry *PR = PassRegistry::getPassRegistry();
271 initializeHexagonVectorLoopCarriedReusePass(*PR);
274 StringRef getPassName() const override {
275 return "Hexagon-specific loop carried reuse for HVX vectors";
278 void getAnalysisUsage(AnalysisUsage &AU) const override {
279 AU.addRequired<LoopInfoWrapperPass>();
280 AU.addRequiredID(LoopSimplifyID);
281 AU.addRequiredID(LCSSAID);
282 AU.addPreservedID(LCSSAID);
283 AU.setPreservesCFG();
286 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
289 SetVector<DepChain *> Dependences;
290 std::set<Instruction *> ReplacedInsts;
292 ReuseValue ReuseCandidate;
295 void findLoopCarriedDeps();
296 void findValueToReuse();
297 void findDepChainFromPHI(Instruction *I, DepChain &D);
299 Value *findValueInBlock(Value *Op, BasicBlock *BB);
300 DepChain *getDepChainBtwn(Instruction *I1, Instruction *I2, int Iters);
301 bool isEquivalentOperation(Instruction *I1, Instruction *I2);
302 bool canReplace(Instruction *I);
303 bool isCallInstCommutative(CallInst *C);
306 } // end anonymous namespace
308 char HexagonVectorLoopCarriedReuse::ID = 0;
310 INITIALIZE_PASS_BEGIN(HexagonVectorLoopCarriedReuse, "hexagon-vlcr",
311 "Hexagon-specific predictive commoning for HVX vectors", false, false)
312 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
313 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
314 INITIALIZE_PASS_DEPENDENCY(LCSSAWrapperPass)
315 INITIALIZE_PASS_END(HexagonVectorLoopCarriedReuse, "hexagon-vlcr",
316 "Hexagon-specific predictive commoning for HVX vectors", false, false)
318 bool HexagonVectorLoopCarriedReuse::runOnLoop(Loop *L, LPPassManager &LPM) {
322 if (!L->getLoopPreheader())
325 // Work only on innermost loops.
326 if (!L->getSubLoops().empty())
329 // Work only on single basic blocks loops.
330 if (L->getNumBlocks() != 1)
338 bool HexagonVectorLoopCarriedReuse::isCallInstCommutative(CallInst *C) {
339 switch (C->getCalledFunction()->getIntrinsicID()) {
340 case Intrinsic::hexagon_V6_vaddb:
341 case Intrinsic::hexagon_V6_vaddb_128B:
342 case Intrinsic::hexagon_V6_vaddh:
343 case Intrinsic::hexagon_V6_vaddh_128B:
344 case Intrinsic::hexagon_V6_vaddw:
345 case Intrinsic::hexagon_V6_vaddw_128B:
346 case Intrinsic::hexagon_V6_vaddubh:
347 case Intrinsic::hexagon_V6_vaddubh_128B:
348 case Intrinsic::hexagon_V6_vadduhw:
349 case Intrinsic::hexagon_V6_vadduhw_128B:
350 case Intrinsic::hexagon_V6_vaddhw:
351 case Intrinsic::hexagon_V6_vaddhw_128B:
352 case Intrinsic::hexagon_V6_vmaxb:
353 case Intrinsic::hexagon_V6_vmaxb_128B:
354 case Intrinsic::hexagon_V6_vmaxh:
355 case Intrinsic::hexagon_V6_vmaxh_128B:
356 case Intrinsic::hexagon_V6_vmaxw:
357 case Intrinsic::hexagon_V6_vmaxw_128B:
358 case Intrinsic::hexagon_V6_vmaxub:
359 case Intrinsic::hexagon_V6_vmaxub_128B:
360 case Intrinsic::hexagon_V6_vmaxuh:
361 case Intrinsic::hexagon_V6_vmaxuh_128B:
362 case Intrinsic::hexagon_V6_vminub:
363 case Intrinsic::hexagon_V6_vminub_128B:
364 case Intrinsic::hexagon_V6_vminuh:
365 case Intrinsic::hexagon_V6_vminuh_128B:
366 case Intrinsic::hexagon_V6_vminb:
367 case Intrinsic::hexagon_V6_vminb_128B:
368 case Intrinsic::hexagon_V6_vminh:
369 case Intrinsic::hexagon_V6_vminh_128B:
370 case Intrinsic::hexagon_V6_vminw:
371 case Intrinsic::hexagon_V6_vminw_128B:
372 case Intrinsic::hexagon_V6_vmpyub:
373 case Intrinsic::hexagon_V6_vmpyub_128B:
374 case Intrinsic::hexagon_V6_vmpyuh:
375 case Intrinsic::hexagon_V6_vmpyuh_128B:
376 case Intrinsic::hexagon_V6_vavgub:
377 case Intrinsic::hexagon_V6_vavgub_128B:
378 case Intrinsic::hexagon_V6_vavgh:
379 case Intrinsic::hexagon_V6_vavgh_128B:
380 case Intrinsic::hexagon_V6_vavguh:
381 case Intrinsic::hexagon_V6_vavguh_128B:
382 case Intrinsic::hexagon_V6_vavgw:
383 case Intrinsic::hexagon_V6_vavgw_128B:
384 case Intrinsic::hexagon_V6_vavgb:
385 case Intrinsic::hexagon_V6_vavgb_128B:
386 case Intrinsic::hexagon_V6_vavguw:
387 case Intrinsic::hexagon_V6_vavguw_128B:
388 case Intrinsic::hexagon_V6_vabsdiffh:
389 case Intrinsic::hexagon_V6_vabsdiffh_128B:
390 case Intrinsic::hexagon_V6_vabsdiffub:
391 case Intrinsic::hexagon_V6_vabsdiffub_128B:
392 case Intrinsic::hexagon_V6_vabsdiffuh:
393 case Intrinsic::hexagon_V6_vabsdiffuh_128B:
394 case Intrinsic::hexagon_V6_vabsdiffw:
395 case Intrinsic::hexagon_V6_vabsdiffw_128B:
402 bool HexagonVectorLoopCarriedReuse::isEquivalentOperation(Instruction *I1,
404 if (!I1->isSameOperationAs(I2))
406 // This check is in place specifically for intrinsics. isSameOperationAs will
407 // return two for any two hexagon intrinsics because they are essentially the
408 // same instruciton (CallInst). We need to scratch the surface to see if they
409 // are calls to the same function.
410 if (CallInst *C1 = dyn_cast<CallInst>(I1)) {
411 if (CallInst *C2 = dyn_cast<CallInst>(I2)) {
412 if (C1->getCalledFunction() != C2->getCalledFunction())
417 // If both the Instructions are of Vector Type and any of the element
418 // is integer constant, check their values too for equivalence.
419 if (I1->getType()->isVectorTy() && I2->getType()->isVectorTy()) {
420 unsigned NumOperands = I1->getNumOperands();
421 for (unsigned i = 0; i < NumOperands; ++i) {
422 ConstantInt *C1 = dyn_cast<ConstantInt>(I1->getOperand(i));
423 ConstantInt *C2 = dyn_cast<ConstantInt>(I2->getOperand(i));
426 if (C1->getSExtValue() != C2->getSExtValue())
434 bool HexagonVectorLoopCarriedReuse::canReplace(Instruction *I) {
435 const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
439 switch (II->getIntrinsicID()) {
440 case Intrinsic::hexagon_V6_hi:
441 case Intrinsic::hexagon_V6_lo:
442 case Intrinsic::hexagon_V6_hi_128B:
443 case Intrinsic::hexagon_V6_lo_128B:
444 LLVM_DEBUG(dbgs() << "Not considering for reuse: " << *II << "\n");
450 void HexagonVectorLoopCarriedReuse::findValueToReuse() {
451 for (auto *D : Dependences) {
452 LLVM_DEBUG(dbgs() << "Processing dependence " << *(D->front()) << "\n");
453 if (D->iterations() > HexagonVLCRIterationLim) {
456 << ".. Skipping because number of iterations > than the limit\n");
460 PHINode *PN = cast<PHINode>(D->front());
461 Instruction *BEInst = D->back();
462 int Iters = D->iterations();
463 BasicBlock *BB = PN->getParent();
464 LLVM_DEBUG(dbgs() << "Checking if any uses of " << *PN
465 << " can be reused\n");
467 SmallVector<Instruction *, 4> PNUsers;
468 for (auto UI = PN->use_begin(), E = PN->use_end(); UI != E; ++UI) {
470 Instruction *User = cast<Instruction>(U.getUser());
472 if (User->getParent() != BB)
474 if (ReplacedInsts.count(User)) {
475 LLVM_DEBUG(dbgs() << *User
476 << " has already been replaced. Skipping...\n");
479 if (isa<PHINode>(User))
481 if (User->mayHaveSideEffects())
483 if (!canReplace(User))
486 PNUsers.push_back(User);
488 LLVM_DEBUG(dbgs() << PNUsers.size() << " use(s) of the PHI in the block\n");
490 // For each interesting use I of PN, find an Instruction BEUser that
491 // performs the same operation as I on BEInst and whose other operands,
492 // if any, can also be rematerialized in OtherBB. We stop when we find the
493 // first such Instruction BEUser. This is because once BEUser is
494 // rematerialized in OtherBB, we may find more such "fixup" opportunities
495 // in this block. So, we'll start over again.
496 for (Instruction *I : PNUsers) {
497 for (auto UI = BEInst->use_begin(), E = BEInst->use_end(); UI != E;
500 Instruction *BEUser = cast<Instruction>(U.getUser());
502 if (BEUser->getParent() != BB)
504 if (!isEquivalentOperation(I, BEUser))
507 int NumOperands = I->getNumOperands();
509 // Take operands of each PNUser one by one and try to find DepChain
510 // with every operand of the BEUser. If any of the operands of BEUser
511 // has DepChain with current operand of the PNUser, break the matcher
512 // loop. Keep doing this for Every PNUser operand. If PNUser operand
513 // does not have DepChain with any of the BEUser operand, break the
514 // outer matcher loop, mark the BEUser as null and reset the ReuseCandidate.
515 // This ensures that DepChain exist for all the PNUser operand with
516 // BEUser operand. This also ensures that DepChains are independent of
517 // the positions in PNUser and BEUser.
518 std::map<Instruction *, DepChain *> DepChains;
519 CallInst *C1 = dyn_cast<CallInst>(I);
520 if ((I && I->isCommutative()) || (C1 && isCallInstCommutative(C1))) {
522 for (int OpNo = 0; OpNo < NumOperands; ++OpNo) {
523 Value *Op = I->getOperand(OpNo);
524 Instruction *OpInst = dyn_cast<Instruction>(Op);
526 for (int T = 0; T < NumOperands; ++T) {
527 Value *BEOp = BEUser->getOperand(T);
528 Instruction *BEOpInst = dyn_cast<Instruction>(BEOp);
529 if (!OpInst && !BEOpInst) {
536 if ((OpInst && !BEOpInst) || (!OpInst && BEOpInst))
539 DepChain *D = getDepChainBtwn(OpInst, BEOpInst, Iters);
543 DepChains[OpInst] = D;
554 for (int OpNo = 0; OpNo < NumOperands; ++OpNo) {
555 Value *Op = I->getOperand(OpNo);
556 Value *BEOp = BEUser->getOperand(OpNo);
558 Instruction *OpInst = dyn_cast<Instruction>(Op);
562 // Do not allow reuse to occur when the operands may be different
568 Instruction *BEOpInst = dyn_cast<Instruction>(BEOp);
569 DepChain *D = getDepChainBtwn(OpInst, BEOpInst, Iters);
572 DepChains[OpInst] = D;
580 LLVM_DEBUG(dbgs() << "Found Value for reuse.\n");
581 ReuseCandidate.Inst2Replace = I;
582 ReuseCandidate.BackedgeInst = BEUser;
583 ReuseCandidate.DepChains = DepChains;
584 ReuseCandidate.Iterations = Iters;
587 ReuseCandidate.reset();
591 ReuseCandidate.reset();
594 Value *HexagonVectorLoopCarriedReuse::findValueInBlock(Value *Op,
596 PHINode *PN = dyn_cast<PHINode>(Op);
598 Value *ValueInBlock = PN->getIncomingValueForBlock(BB);
602 void HexagonVectorLoopCarriedReuse::reuseValue() {
603 LLVM_DEBUG(dbgs() << ReuseCandidate);
604 Instruction *Inst2Replace = ReuseCandidate.Inst2Replace;
605 Instruction *BEInst = ReuseCandidate.BackedgeInst;
606 int NumOperands = Inst2Replace->getNumOperands();
607 std::map<Instruction *, DepChain *> &DepChains = ReuseCandidate.DepChains;
608 int Iterations = ReuseCandidate.Iterations;
609 BasicBlock *LoopPH = CurLoop->getLoopPreheader();
610 assert(!DepChains.empty() && "No DepChains");
611 LLVM_DEBUG(dbgs() << "reuseValue is making the following changes\n");
613 SmallVector<Instruction *, 4> InstsInPreheader;
614 for (int i = 0; i < Iterations; ++i) {
615 Instruction *InstInPreheader = Inst2Replace->clone();
616 SmallVector<Value *, 4> Ops;
617 for (int j = 0; j < NumOperands; ++j) {
618 Instruction *I = dyn_cast<Instruction>(Inst2Replace->getOperand(j));
621 // Get the DepChain corresponding to this operand.
622 DepChain &D = *DepChains[I];
623 // Get the PHI for the iteration number and find
624 // the incoming value from the Loop Preheader for
626 Value *ValInPreheader = findValueInBlock(D[i], LoopPH);
627 InstInPreheader->setOperand(j, ValInPreheader);
629 InstsInPreheader.push_back(InstInPreheader);
630 InstInPreheader->setName(Inst2Replace->getName() + ".hexagon.vlcr");
631 InstInPreheader->insertBefore(LoopPH->getTerminator());
632 LLVM_DEBUG(dbgs() << "Added " << *InstInPreheader << " to "
633 << LoopPH->getName() << "\n");
635 BasicBlock *BB = BEInst->getParent();
637 IRB.SetInsertPoint(BB->getFirstNonPHI());
638 Value *BEVal = BEInst;
640 for (int i = Iterations-1; i >=0 ; --i) {
641 Instruction *InstInPreheader = InstsInPreheader[i];
642 NewPhi = IRB.CreatePHI(InstInPreheader->getType(), 2);
643 NewPhi->addIncoming(InstInPreheader, LoopPH);
644 NewPhi->addIncoming(BEVal, BB);
645 LLVM_DEBUG(dbgs() << "Adding " << *NewPhi << " to " << BB->getName()
649 // We are in LCSSA form. So, a value defined inside the Loop is used only
650 // inside the loop. So, the following is safe.
651 Inst2Replace->replaceAllUsesWith(NewPhi);
652 ReplacedInsts.insert(Inst2Replace);
653 ++HexagonNumVectorLoopCarriedReuse;
656 bool HexagonVectorLoopCarriedReuse::doVLCR() {
657 assert(CurLoop->getSubLoops().empty() &&
658 "Can do VLCR on the innermost loop only");
659 assert((CurLoop->getNumBlocks() == 1) &&
660 "Can do VLCR only on single block loops");
662 bool Changed = false;
665 LLVM_DEBUG(dbgs() << "Working on Loop: " << *CurLoop->getHeader() << "\n");
667 // Reset datastructures.
671 findLoopCarriedDeps();
673 if (ReuseCandidate.isDefined()) {
678 llvm::for_each(Dependences, std::default_delete<DepChain>());
683 void HexagonVectorLoopCarriedReuse::findDepChainFromPHI(Instruction *I,
685 PHINode *PN = dyn_cast<PHINode>(I);
690 auto NumIncomingValues = PN->getNumIncomingValues();
691 if (NumIncomingValues != 2) {
696 BasicBlock *BB = PN->getParent();
697 if (BB != CurLoop->getHeader()) {
702 Value *BEVal = PN->getIncomingValueForBlock(BB);
703 Instruction *BEInst = dyn_cast<Instruction>(BEVal);
704 // This is a single block loop with a preheader, so at least
705 // one value should come over the backedge.
706 assert(BEInst && "There should be a value over the backedge");
709 PN->getIncomingValueForBlock(CurLoop->getLoopPreheader());
710 if(!PreHdrVal || !isa<Instruction>(PreHdrVal)) {
715 findDepChainFromPHI(BEInst, D);
719 DepChain *HexagonVectorLoopCarriedReuse::getDepChainBtwn(Instruction *I1,
722 for (auto *D : Dependences) {
723 if (D->front() == I1 && D->back() == I2 && D->iterations() == Iters)
729 void HexagonVectorLoopCarriedReuse::findLoopCarriedDeps() {
730 BasicBlock *BB = CurLoop->getHeader();
731 for (auto I = BB->begin(), E = BB->end(); I != E && isa<PHINode>(I); ++I) {
732 auto *PN = cast<PHINode>(I);
733 if (!isa<VectorType>(PN->getType()))
736 DepChain *D = new DepChain();
737 findDepChainFromPHI(PN, *D);
739 Dependences.insert(D);
743 LLVM_DEBUG(dbgs() << "Found " << Dependences.size() << " dependences\n");
744 LLVM_DEBUG(for (size_t i = 0; i < Dependences.size();
745 ++i) { dbgs() << *Dependences[i] << "\n"; });
748 Pass *llvm::createHexagonVectorLoopCarriedReusePass() {
749 return new HexagonVectorLoopCarriedReuse();