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//===- HexagonVectorLoopCarriedReuse.cpp ----------------------------------===//
//
// 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 removes the computation of provably redundant expressions that have
// been computed earlier in a previous iteration. It relies on the use of PHIs
// to identify loop carried dependences. This is scalar replacement for vector
// types.
//
//-----------------------------------------------------------------------------
// Motivation: Consider the case where we have the following loop structure.
//
// Loop:
// t0 = a[i];
// t1 = f(t0);
// t2 = g(t1);
// ...
// t3 = a[i+1];
// t4 = f(t3);
// t5 = g(t4);
// t6 = op(t2, t5)
// cond_branch <Loop>
//
// This can be converted to
// t00 = a[0];
// t10 = f(t00);
// t20 = g(t10);
// Loop:
// t2 = t20;
// t3 = a[i+1];
// t4 = f(t3);
// t5 = g(t4);
// t6 = op(t2, t5)
// t20 = t5
// cond_branch <Loop>
//
// SROA does a good job of reusing a[i+1] as a[i] in the next iteration.
// Such a loop comes to this pass in the following form.
//
// LoopPreheader:
// X0 = a[0];
// Loop:
// X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
// t1 = f(X2) <-- I1
// t2 = g(t1)
// ...
// X1 = a[i+1]
// t4 = f(X1) <-- I2
// t5 = g(t4)
// t6 = op(t2, t5)
// cond_branch <Loop>
//
// In this pass, we look for PHIs such as X2 whose incoming values come only
// from the Loop Preheader and over the backedge and additionaly, both these
// values are the results of the same operation in terms of opcode. We call such
// a PHI node a dependence chain or DepChain. In this case, the dependence of X2
// over X1 is carried over only one iteration and so the DepChain is only one
// PHI node long.
//
// Then, we traverse the uses of the PHI (X2) and the uses of the value of the
// PHI coming over the backedge (X1). We stop at the first pair of such users
// I1 (of X2) and I2 (of X1) that meet the following conditions.
// 1. I1 and I2 are the same operation, but with different operands.
// 2. X2 and X1 are used at the same operand number in the two instructions.
// 3. All other operands Op1 of I1 and Op2 of I2 are also such that there is a
// a DepChain from Op1 to Op2 of the same length as that between X2 and X1.
//
// We then make the following transformation
// LoopPreheader:
// X0 = a[0];
// Y0 = f(X0);
// Loop:
// X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
// Y2 = PHI<(Y0, LoopPreheader), (t4, Loop)>
// t1 = f(X2) <-- Will be removed by DCE.
// t2 = g(Y2)
// ...
// X1 = a[i+1]
// t4 = f(X1)
// t5 = g(t4)
// t6 = op(t2, t5)
// cond_branch <Loop>
//
// We proceed until we cannot find any more such instructions I1 and I2.
//
// --- DepChains & Loop carried dependences ---
// Consider a single basic block loop such as
//
// LoopPreheader:
// X0 = ...
// Y0 = ...
// Loop:
// X2 = PHI<(X0, LoopPreheader), (X1, Loop)>
// Y2 = PHI<(Y0, LoopPreheader), (X2, Loop)>
// ...
// X1 = ...
// ...
// cond_branch <Loop>
//
// Then there is a dependence between X2 and X1 that goes back one iteration,
// i.e. X1 is used as X2 in the very next iteration. We represent this as a
// DepChain from X2 to X1 (X2->X1).
// Similarly, there is a dependence between Y2 and X1 that goes back two
// iterations. X1 is used as Y2 two iterations after it is computed. This is
// represented by a DepChain as (Y2->X2->X1).
//
// A DepChain has the following properties.
// 1. Num of edges in DepChain = Number of Instructions in DepChain = Number of
// iterations of carried dependence + 1.
// 2. All instructions in the DepChain except the last are PHIs.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <map>
#include <memory>
#include <set>
using namespace llvm;
#define DEBUG_TYPE "hexagon-vlcr"
STATISTIC(HexagonNumVectorLoopCarriedReuse,
"Number of values that were reused from a previous iteration.");
static cl::opt<int> HexagonVLCRIterationLim("hexagon-vlcr-iteration-lim",
cl::Hidden,
cl::desc("Maximum distance of loop carried dependences that are handled"),
cl::init(2), cl::ZeroOrMore);
namespace llvm {
void initializeHexagonVectorLoopCarriedReusePass(PassRegistry&);
Pass *createHexagonVectorLoopCarriedReusePass();
} // end namespace llvm
namespace {
// See info about DepChain in the comments at the top of this file.
using ChainOfDependences = SmallVector<Instruction *, 4>;
class DepChain {
ChainOfDependences Chain;
public:
bool isIdentical(DepChain &Other) const {
if (Other.size() != size())
return false;
ChainOfDependences &OtherChain = Other.getChain();
for (int i = 0; i < size(); ++i) {
if (Chain[i] != OtherChain[i])
return false;
}
return true;
}
ChainOfDependences &getChain() {
return Chain;
}
int size() const {
return Chain.size();
}
void clear() {
Chain.clear();
}
void push_back(Instruction *I) {
Chain.push_back(I);
}
int iterations() const {
return size() - 1;
}
Instruction *front() const {
return Chain.front();
}
Instruction *back() const {
return Chain.back();
}
Instruction *&operator[](const int index) {
return Chain[index];
}
friend raw_ostream &operator<< (raw_ostream &OS, const DepChain &D);
};
LLVM_ATTRIBUTE_UNUSED
raw_ostream &operator<<(raw_ostream &OS, const DepChain &D) {
const ChainOfDependences &CD = D.Chain;
int ChainSize = CD.size();
OS << "**DepChain Start::**\n";
for (int i = 0; i < ChainSize -1; ++i) {
OS << *(CD[i]) << " -->\n";
}
OS << *CD[ChainSize-1] << "\n";
return OS;
}
struct ReuseValue {
Instruction *Inst2Replace = nullptr;
// In the new PHI node that we'll construct this is the value that'll be
// used over the backedge. This is teh value that gets reused from a
// previous iteration.
Instruction *BackedgeInst = nullptr;
ReuseValue() = default;
void reset() { Inst2Replace = nullptr; BackedgeInst = nullptr; }
bool isDefined() { return Inst2Replace != nullptr; }
};
LLVM_ATTRIBUTE_UNUSED
raw_ostream &operator<<(raw_ostream &OS, const ReuseValue &RU) {
OS << "** ReuseValue ***\n";
OS << "Instruction to Replace: " << *(RU.Inst2Replace) << "\n";
OS << "Backedge Instruction: " << *(RU.BackedgeInst) << "\n";
return OS;
}
class HexagonVectorLoopCarriedReuse : public LoopPass {
public:
static char ID;
explicit HexagonVectorLoopCarriedReuse() : LoopPass(ID) {
PassRegistry *PR = PassRegistry::getPassRegistry();
initializeHexagonVectorLoopCarriedReusePass(*PR);
}
StringRef getPassName() const override {
return "Hexagon-specific loop carried reuse for HVX vectors";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LoopInfoWrapperPass>();
AU.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.setPreservesCFG();
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
private:
SetVector<DepChain *> Dependences;
std::set<Instruction *> ReplacedInsts;
Loop *CurLoop;
ReuseValue ReuseCandidate;
bool doVLCR();
void findLoopCarriedDeps();
void findValueToReuse();
void findDepChainFromPHI(Instruction *I, DepChain &D);
void reuseValue();
Value *findValueInBlock(Value *Op, BasicBlock *BB);
bool isDepChainBtwn(Instruction *I1, Instruction *I2, int Iters);
DepChain *getDepChainBtwn(Instruction *I1, Instruction *I2);
bool isEquivalentOperation(Instruction *I1, Instruction *I2);
bool canReplace(Instruction *I);
};
} // end anonymous namespace
char HexagonVectorLoopCarriedReuse::ID = 0;
INITIALIZE_PASS_BEGIN(HexagonVectorLoopCarriedReuse, "hexagon-vlcr",
"Hexagon-specific predictive commoning for HVX vectors", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSAWrapperPass)
INITIALIZE_PASS_END(HexagonVectorLoopCarriedReuse, "hexagon-vlcr",
"Hexagon-specific predictive commoning for HVX vectors", false, false)
bool HexagonVectorLoopCarriedReuse::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipLoop(L))
return false;
if (!L->getLoopPreheader())
return false;
// Work only on innermost loops.
if (!L->getSubLoops().empty())
return false;
// Work only on single basic blocks loops.
if (L->getNumBlocks() != 1)
return false;
CurLoop = L;
return doVLCR();
}
bool HexagonVectorLoopCarriedReuse::isEquivalentOperation(Instruction *I1,
Instruction *I2) {
if (!I1->isSameOperationAs(I2))
return false;
// This check is in place specifically for intrinsics. isSameOperationAs will
// return two for any two hexagon intrinsics because they are essentially the
// same instruciton (CallInst). We need to scratch the surface to see if they
// are calls to the same function.
if (CallInst *C1 = dyn_cast<CallInst>(I1)) {
if (CallInst *C2 = dyn_cast<CallInst>(I2)) {
if (C1->getCalledFunction() != C2->getCalledFunction())
return false;
}
}
// If both the Instructions are of Vector Type and any of the element
// is integer constant, check their values too for equivalence.
if (I1->getType()->isVectorTy() && I2->getType()->isVectorTy()) {
unsigned NumOperands = I1->getNumOperands();
for (unsigned i = 0; i < NumOperands; ++i) {
ConstantInt *C1 = dyn_cast<ConstantInt>(I1->getOperand(i));
ConstantInt *C2 = dyn_cast<ConstantInt>(I2->getOperand(i));
if(!C1) continue;
assert(C2);
if (C1->getSExtValue() != C2->getSExtValue())
return false;
}
}
return true;
}
bool HexagonVectorLoopCarriedReuse::canReplace(Instruction *I) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
if (II &&
(II->getIntrinsicID() == Intrinsic::hexagon_V6_hi ||
II->getIntrinsicID() == Intrinsic::hexagon_V6_lo)) {
LLVM_DEBUG(dbgs() << "Not considering for reuse: " << *II << "\n");
return false;
}
return true;
}
void HexagonVectorLoopCarriedReuse::findValueToReuse() {
for (auto *D : Dependences) {
LLVM_DEBUG(dbgs() << "Processing dependence " << *(D->front()) << "\n");
if (D->iterations() > HexagonVLCRIterationLim) {
LLVM_DEBUG(
dbgs()
<< ".. Skipping because number of iterations > than the limit\n");
continue;
}
PHINode *PN = cast<PHINode>(D->front());
Instruction *BEInst = D->back();
int Iters = D->iterations();
BasicBlock *BB = PN->getParent();
LLVM_DEBUG(dbgs() << "Checking if any uses of " << *PN
<< " can be reused\n");
SmallVector<Instruction *, 4> PNUsers;
for (auto UI = PN->use_begin(), E = PN->use_end(); UI != E; ++UI) {
Use &U = *UI;
Instruction *User = cast<Instruction>(U.getUser());
if (User->getParent() != BB)
continue;
if (ReplacedInsts.count(User)) {
LLVM_DEBUG(dbgs() << *User
<< " has already been replaced. Skipping...\n");
continue;
}
if (isa<PHINode>(User))
continue;
if (User->mayHaveSideEffects())
continue;
if (!canReplace(User))
continue;
PNUsers.push_back(User);
}
LLVM_DEBUG(dbgs() << PNUsers.size() << " use(s) of the PHI in the block\n");
// For each interesting use I of PN, find an Instruction BEUser that
// performs the same operation as I on BEInst and whose other operands,
// if any, can also be rematerialized in OtherBB. We stop when we find the
// first such Instruction BEUser. This is because once BEUser is
// rematerialized in OtherBB, we may find more such "fixup" opportunities
// in this block. So, we'll start over again.
for (Instruction *I : PNUsers) {
for (auto UI = BEInst->use_begin(), E = BEInst->use_end(); UI != E;
++UI) {
Use &U = *UI;
Instruction *BEUser = cast<Instruction>(U.getUser());
if (BEUser->getParent() != BB)
continue;
if (!isEquivalentOperation(I, BEUser))
continue;
int NumOperands = I->getNumOperands();
for (int OpNo = 0; OpNo < NumOperands; ++OpNo) {
Value *Op = I->getOperand(OpNo);
Instruction *OpInst = dyn_cast<Instruction>(Op);
if (!OpInst)
continue;
Value *BEOp = BEUser->getOperand(OpNo);
Instruction *BEOpInst = dyn_cast<Instruction>(BEOp);
if (!isDepChainBtwn(OpInst, BEOpInst, Iters)) {
BEUser = nullptr;
break;
}
}
if (BEUser) {
LLVM_DEBUG(dbgs() << "Found Value for reuse.\n");
ReuseCandidate.Inst2Replace = I;
ReuseCandidate.BackedgeInst = BEUser;
return;
} else
ReuseCandidate.reset();
}
}
}
ReuseCandidate.reset();
}
Value *HexagonVectorLoopCarriedReuse::findValueInBlock(Value *Op,
BasicBlock *BB) {
PHINode *PN = dyn_cast<PHINode>(Op);
assert(PN);
Value *ValueInBlock = PN->getIncomingValueForBlock(BB);
return ValueInBlock;
}
void HexagonVectorLoopCarriedReuse::reuseValue() {
LLVM_DEBUG(dbgs() << ReuseCandidate);
Instruction *Inst2Replace = ReuseCandidate.Inst2Replace;
Instruction *BEInst = ReuseCandidate.BackedgeInst;
int NumOperands = Inst2Replace->getNumOperands();
std::map<Instruction *, DepChain *> DepChains;
int Iterations = -1;
BasicBlock *LoopPH = CurLoop->getLoopPreheader();
for (int i = 0; i < NumOperands; ++i) {
Instruction *I = dyn_cast<Instruction>(Inst2Replace->getOperand(i));
if(!I)
continue;
else {
Instruction *J = cast<Instruction>(BEInst->getOperand(i));
DepChain *D = getDepChainBtwn(I, J);
assert(D &&
"No DepChain between corresponding operands in ReuseCandidate\n");
if (Iterations == -1)
Iterations = D->iterations();
assert(Iterations == D->iterations() && "Iterations mismatch");
DepChains[I] = D;
}
}
LLVM_DEBUG(dbgs() << "reuseValue is making the following changes\n");
SmallVector<Instruction *, 4> InstsInPreheader;
for (int i = 0; i < Iterations; ++i) {
Instruction *InstInPreheader = Inst2Replace->clone();
SmallVector<Value *, 4> Ops;
for (int j = 0; j < NumOperands; ++j) {
Instruction *I = dyn_cast<Instruction>(Inst2Replace->getOperand(j));
if (!I)
continue;
// Get the DepChain corresponding to this operand.
DepChain &D = *DepChains[I];
// Get the PHI for the iteration number and find
// the incoming value from the Loop Preheader for
// that PHI.
Value *ValInPreheader = findValueInBlock(D[i], LoopPH);
InstInPreheader->setOperand(j, ValInPreheader);
}
InstsInPreheader.push_back(InstInPreheader);
InstInPreheader->setName(Inst2Replace->getName() + ".hexagon.vlcr");
InstInPreheader->insertBefore(LoopPH->getTerminator());
LLVM_DEBUG(dbgs() << "Added " << *InstInPreheader << " to "
<< LoopPH->getName() << "\n");
}
BasicBlock *BB = BEInst->getParent();
IRBuilder<> IRB(BB);
IRB.SetInsertPoint(BB->getFirstNonPHI());
Value *BEVal = BEInst;
PHINode *NewPhi;
for (int i = Iterations-1; i >=0 ; --i) {
Instruction *InstInPreheader = InstsInPreheader[i];
NewPhi = IRB.CreatePHI(InstInPreheader->getType(), 2);
NewPhi->addIncoming(InstInPreheader, LoopPH);
NewPhi->addIncoming(BEVal, BB);
LLVM_DEBUG(dbgs() << "Adding " << *NewPhi << " to " << BB->getName()
<< "\n");
BEVal = NewPhi;
}
// We are in LCSSA form. So, a value defined inside the Loop is used only
// inside the loop. So, the following is safe.
Inst2Replace->replaceAllUsesWith(NewPhi);
ReplacedInsts.insert(Inst2Replace);
++HexagonNumVectorLoopCarriedReuse;
}
bool HexagonVectorLoopCarriedReuse::doVLCR() {
assert(CurLoop->getSubLoops().empty() &&
"Can do VLCR on the innermost loop only");
assert((CurLoop->getNumBlocks() == 1) &&
"Can do VLCR only on single block loops");
bool Changed = false;
bool Continue;
LLVM_DEBUG(dbgs() << "Working on Loop: " << *CurLoop->getHeader() << "\n");
do {
// Reset datastructures.
Dependences.clear();
Continue = false;
findLoopCarriedDeps();
findValueToReuse();
if (ReuseCandidate.isDefined()) {
reuseValue();
Changed = true;
Continue = true;
}
llvm::for_each(Dependences, std::default_delete<DepChain>());
} while (Continue);
return Changed;
}
void HexagonVectorLoopCarriedReuse::findDepChainFromPHI(Instruction *I,
DepChain &D) {
PHINode *PN = dyn_cast<PHINode>(I);
if (!PN) {
D.push_back(I);
return;
} else {
auto NumIncomingValues = PN->getNumIncomingValues();
if (NumIncomingValues != 2) {
D.clear();
return;
}
BasicBlock *BB = PN->getParent();
if (BB != CurLoop->getHeader()) {
D.clear();
return;
}
Value *BEVal = PN->getIncomingValueForBlock(BB);
Instruction *BEInst = dyn_cast<Instruction>(BEVal);
// This is a single block loop with a preheader, so at least
// one value should come over the backedge.
assert(BEInst && "There should be a value over the backedge");
Value *PreHdrVal =
PN->getIncomingValueForBlock(CurLoop->getLoopPreheader());
if(!PreHdrVal || !isa<Instruction>(PreHdrVal)) {
D.clear();
return;
}
D.push_back(PN);
findDepChainFromPHI(BEInst, D);
}
}
bool HexagonVectorLoopCarriedReuse::isDepChainBtwn(Instruction *I1,
Instruction *I2,
int Iters) {
for (auto *D : Dependences) {
if (D->front() == I1 && D->back() == I2 && D->iterations() == Iters)
return true;
}
return false;
}
DepChain *HexagonVectorLoopCarriedReuse::getDepChainBtwn(Instruction *I1,
Instruction *I2) {
for (auto *D : Dependences) {
if (D->front() == I1 && D->back() == I2)
return D;
}
return nullptr;
}
void HexagonVectorLoopCarriedReuse::findLoopCarriedDeps() {
BasicBlock *BB = CurLoop->getHeader();
for (auto I = BB->begin(), E = BB->end(); I != E && isa<PHINode>(I); ++I) {
auto *PN = cast<PHINode>(I);
if (!isa<VectorType>(PN->getType()))
continue;
DepChain *D = new DepChain();
findDepChainFromPHI(PN, *D);
if (D->size() != 0)
Dependences.insert(D);
else
delete D;
}
LLVM_DEBUG(dbgs() << "Found " << Dependences.size() << " dependences\n");
LLVM_DEBUG(for (size_t i = 0; i < Dependences.size();
++i) { dbgs() << *Dependences[i] << "\n"; });
}
Pass *llvm::createHexagonVectorLoopCarriedReusePass() {
return new HexagonVectorLoopCarriedReuse();
}
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