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//===------ PPCLoopPreIncPrep.cpp - Loop Pre-Inc. AM Prep. Pass -----------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements a pass to prepare loops for pre-increment addressing
// modes. Additional PHIs are created for loop induction variables used by
// load/store instructions so that the pre-increment forms can be used.
// Generically, this means transforming loops like this:
// for (int i = 0; i < n; ++i)
// array[i] = c;
// to look like this:
// T *p = array[-1];
// for (int i = 0; i < n; ++i)
// *++p = c;
#define DEBUG_TYPE "ppc-loop-preinc-prep"
#include "PPC.h"
#include "PPCSubtarget.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include <cassert>
#include <iterator>
#include <utility>
using namespace llvm;
// By default, we limit this to creating 16 PHIs (which is a little over half
// of the allocatable register set).
static cl::opt<unsigned> MaxVars("ppc-preinc-prep-max-vars",
cl::Hidden, cl::init(16),
cl::desc("Potential PHI threshold for PPC preinc loop prep"));
STATISTIC(PHINodeAlreadyExists, "PHI node already in pre-increment form");
STATISTIC(UpdFormChainRewritten, "Num of update form chain rewritten");
namespace {
struct BucketElement {
BucketElement(const SCEVConstant *O, Instruction *I) : Offset(O), Instr(I) {}
BucketElement(Instruction *I) : Offset(nullptr), Instr(I) {}
const SCEVConstant *Offset;
Instruction *Instr;
struct Bucket {
Bucket(const SCEV *B, Instruction *I) : BaseSCEV(B),
Elements(1, BucketElement(I)) {}
const SCEV *BaseSCEV;
SmallVector<BucketElement, 16> Elements;
class PPCLoopPreIncPrep : public FunctionPass {
static char ID; // Pass ID, replacement for typeid
PPCLoopPreIncPrep() : FunctionPass(ID) {
PPCLoopPreIncPrep(PPCTargetMachine &TM) : FunctionPass(ID), TM(&TM) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnFunction(Function &F) override;
PPCTargetMachine *TM = nullptr;
const PPCSubtarget *ST;
DominatorTree *DT;
LoopInfo *LI;
ScalarEvolution *SE;
bool PreserveLCSSA;
bool runOnLoop(Loop *L);
/// Check if required PHI node is already exist in Loop \p L.
bool alreadyPrepared(Loop *L, Instruction* MemI,
const SCEV *BasePtrStartSCEV,
const SCEVConstant *BasePtrIncSCEV);
/// Collect condition matched(\p isValidCandidate() returns true)
/// candidates in Loop \p L.
SmallVector<Bucket, 16>
collectCandidates(Loop *L,
std::function<bool(const Instruction *, const Value *)>
unsigned MaxCandidateNum);
/// Add a candidate to candidates \p Buckets.
void addOneCandidate(Instruction *MemI, const SCEV *LSCEV,
SmallVector<Bucket, 16> &Buckets,
unsigned MaxCandidateNum);
/// Prepare all candidates in \p Buckets for update form.
bool updateFormPrep(Loop *L, SmallVector<Bucket, 16> &Buckets);
/// Prepare for one chain \p BucketChain, find the best base element and
/// update all other elements in \p BucketChain accordingly.
bool prepareBaseForUpdateFormChain(Bucket &BucketChain);
/// Rewrite load/store instructions in \p BucketChain according to
/// preparation.
bool rewriteLoadStores(Loop *L, Bucket &BucketChain,
SmallSet<BasicBlock *, 16> &BBChanged);
} // end anonymous namespace
char PPCLoopPreIncPrep::ID = 0;
static const char *name = "Prepare loop for pre-inc. addressing modes";
INITIALIZE_PASS_BEGIN(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
INITIALIZE_PASS_END(PPCLoopPreIncPrep, DEBUG_TYPE, name, false, false)
static const std::string PHINodeNameSuffix = ".phi";
static const std::string CastNodeNameSuffix = ".cast";
static const std::string GEPNodeIncNameSuffix = ".inc";
static const std::string GEPNodeOffNameSuffix = ".off";
FunctionPass *llvm::createPPCLoopPreIncPrepPass(PPCTargetMachine &TM) {
return new PPCLoopPreIncPrep(TM);
static bool IsPtrInBounds(Value *BasePtr) {
Value *StrippedBasePtr = BasePtr;
while (BitCastInst *BC = dyn_cast<BitCastInst>(StrippedBasePtr))
StrippedBasePtr = BC->getOperand(0);
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(StrippedBasePtr))
return GEP->isInBounds();
return false;
static std::string getInstrName(const Value *I, const std::string Suffix) {
assert(I && "Invalid paramater!");
if (I->hasName())
return (I->getName() + Suffix).str();
return "";
static Value *GetPointerOperand(Value *MemI) {
if (LoadInst *LMemI = dyn_cast<LoadInst>(MemI)) {
return LMemI->getPointerOperand();
} else if (StoreInst *SMemI = dyn_cast<StoreInst>(MemI)) {
return SMemI->getPointerOperand();
} else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(MemI)) {
if (IMemI->getIntrinsicID() == Intrinsic::prefetch)
return IMemI->getArgOperand(0);
return nullptr;
bool PPCLoopPreIncPrep::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
ST = TM ? TM->getSubtargetImpl(F) : nullptr;
bool MadeChange = false;
for (auto I = LI->begin(), IE = LI->end(); I != IE; ++I)
for (auto L = df_begin(*I), LE = df_end(*I); L != LE; ++L)
MadeChange |= runOnLoop(*L);
return MadeChange;
void PPCLoopPreIncPrep::addOneCandidate(Instruction *MemI, const SCEV *LSCEV,
SmallVector<Bucket, 16> &Buckets,
unsigned MaxCandidateNum) {
assert((MemI && GetPointerOperand(MemI)) &&
"Candidate should be a memory instruction.");
assert(LSCEV && "Invalid SCEV for Ptr value.");
bool FoundBucket = false;
for (auto &B : Buckets) {
const SCEV *Diff = SE->getMinusSCEV(LSCEV, B.BaseSCEV);
if (const auto *CDiff = dyn_cast<SCEVConstant>(Diff)) {
B.Elements.push_back(BucketElement(CDiff, MemI));
FoundBucket = true;
if (!FoundBucket) {
if (Buckets.size() == MaxCandidateNum)
Buckets.push_back(Bucket(LSCEV, MemI));
SmallVector<Bucket, 16> PPCLoopPreIncPrep::collectCandidates(
Loop *L,
std::function<bool(const Instruction *, const Value *)> isValidCandidate,
unsigned MaxCandidateNum) {
SmallVector<Bucket, 16> Buckets;
for (const auto &BB : L->blocks())
for (auto &J : *BB) {
Value *PtrValue;
Instruction *MemI;
if (LoadInst *LMemI = dyn_cast<LoadInst>(&J)) {
MemI = LMemI;
PtrValue = LMemI->getPointerOperand();
} else if (StoreInst *SMemI = dyn_cast<StoreInst>(&J)) {
MemI = SMemI;
PtrValue = SMemI->getPointerOperand();
} else if (IntrinsicInst *IMemI = dyn_cast<IntrinsicInst>(&J)) {
if (IMemI->getIntrinsicID() == Intrinsic::prefetch) {
MemI = IMemI;
PtrValue = IMemI->getArgOperand(0);
} else continue;
} else continue;
unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
if (PtrAddrSpace)
if (L->isLoopInvariant(PtrValue))
const SCEV *LSCEV = SE->getSCEVAtScope(PtrValue, L);
const SCEVAddRecExpr *LARSCEV = dyn_cast<SCEVAddRecExpr>(LSCEV);
if (!LARSCEV || LARSCEV->getLoop() != L)
if (isValidCandidate(&J, PtrValue))
addOneCandidate(MemI, LSCEV, Buckets, MaxCandidateNum);
return Buckets;
// TODO: implement a more clever base choosing policy.
// Currently we always choose an exist load/store offset. This maybe lead to
// suboptimal code sequences. For example, for one DS chain with offsets
// {-32769, 2003, 2007, 2011}, we choose -32769 as base offset, and left disp
// for load/stores are {0, 34772, 34776, 34780}. Though each offset now is a
// multipler of 4, it cannot be represented by sint16.
bool PPCLoopPreIncPrep::prepareBaseForUpdateFormChain(Bucket &BucketChain) {
// We have a choice now of which instruction's memory operand we use as the
// base for the generated PHI. Always picking the first instruction in each
// bucket does not work well, specifically because that instruction might
// be a prefetch (and there are no pre-increment dcbt variants). Otherwise,
// the choice is somewhat arbitrary, because the backend will happily
// generate direct offsets from both the pre-incremented and
// post-incremented pointer values. Thus, we'll pick the first non-prefetch
// instruction in each bucket, and adjust the recurrence and other offsets
// accordingly.
for (int j = 0, je = BucketChain.Elements.size(); j != je; ++j) {
if (auto *II = dyn_cast<IntrinsicInst>(BucketChain.Elements[j].Instr))
if (II->getIntrinsicID() == Intrinsic::prefetch)
// If we'd otherwise pick the first element anyway, there's nothing to do.
if (j == 0)
// If our chosen element has no offset from the base pointer, there's
// nothing to do.
if (!BucketChain.Elements[j].Offset ||
const SCEV *Offset = BucketChain.Elements[j].Offset;
BucketChain.BaseSCEV = SE->getAddExpr(BucketChain.BaseSCEV, Offset);
for (auto &E : BucketChain.Elements) {
if (E.Offset)
E.Offset = cast<SCEVConstant>(SE->getMinusSCEV(E.Offset, Offset));
E.Offset = cast<SCEVConstant>(SE->getNegativeSCEV(Offset));
std::swap(BucketChain.Elements[j], BucketChain.Elements[0]);
return true;
bool PPCLoopPreIncPrep::rewriteLoadStores(
Loop *L, Bucket &BucketChain, SmallSet<BasicBlock *, 16> &BBChanged) {
bool MadeChange = false;
const SCEVAddRecExpr *BasePtrSCEV =
if (!BasePtrSCEV->isAffine())
return MadeChange;
LLVM_DEBUG(dbgs() << "PIP: Transforming: " << *BasePtrSCEV << "\n");
assert(BasePtrSCEV->getLoop() == L && "AddRec for the wrong loop?");
// The instruction corresponding to the Bucket's BaseSCEV must be the first
// in the vector of elements.
Instruction *MemI = BucketChain.Elements.begin()->Instr;
Value *BasePtr = GetPointerOperand(MemI);
assert(BasePtr && "No pointer operand");
Type *I8Ty = Type::getInt8Ty(MemI->getParent()->getContext());
Type *I8PtrTy = Type::getInt8PtrTy(MemI->getParent()->getContext(),
const SCEV *BasePtrStartSCEV = BasePtrSCEV->getStart();
if (!SE->isLoopInvariant(BasePtrStartSCEV, L))
return MadeChange;
const SCEVConstant *BasePtrIncSCEV =
if (!BasePtrIncSCEV)
return MadeChange;
BasePtrStartSCEV = SE->getMinusSCEV(BasePtrStartSCEV, BasePtrIncSCEV);
if (!isSafeToExpand(BasePtrStartSCEV, *SE))
return MadeChange;
if (alreadyPrepared(L, MemI, BasePtrStartSCEV, BasePtrIncSCEV))
return MadeChange;
LLVM_DEBUG(dbgs() << "PIP: New start is: " << *BasePtrStartSCEV << "\n");
BasicBlock *Header = L->getHeader();
unsigned HeaderLoopPredCount = pred_size(Header);
BasicBlock *LoopPredecessor = L->getLoopPredecessor();
PHINode *NewPHI =
PHINode::Create(I8PtrTy, HeaderLoopPredCount,
getInstrName(MemI, PHINodeNameSuffix),
SCEVExpander SCEVE(*SE, Header->getModule()->getDataLayout(), "pistart");
Value *BasePtrStart = SCEVE.expandCodeFor(BasePtrStartSCEV, I8PtrTy,
// Note that LoopPredecessor might occur in the predecessor list multiple
// times, and we need to add it the right number of times.
for (const auto &PI : predecessors(Header)) {
if (PI != LoopPredecessor)
NewPHI->addIncoming(BasePtrStart, LoopPredecessor);
Instruction *InsPoint = &*Header->getFirstInsertionPt();
GetElementPtrInst *PtrInc = GetElementPtrInst::Create(
I8Ty, NewPHI, BasePtrIncSCEV->getValue(),
getInstrName(MemI, GEPNodeIncNameSuffix), InsPoint);
for (const auto &PI : predecessors(Header)) {
if (PI == LoopPredecessor)
NewPHI->addIncoming(PtrInc, PI);
Instruction *NewBasePtr;
if (PtrInc->getType() != BasePtr->getType())
NewBasePtr = new BitCastInst(PtrInc, BasePtr->getType(),
getInstrName(PtrInc, CastNodeNameSuffix), InsPoint);
NewBasePtr = PtrInc;
if (Instruction *IDel = dyn_cast<Instruction>(BasePtr))
// Keep track of the replacement pointer values we've inserted so that we
// don't generate more pointer values than necessary.
SmallPtrSet<Value *, 16> NewPtrs;
for (auto I = std::next(BucketChain.Elements.begin()),
IE = BucketChain.Elements.end(); I != IE; ++I) {
Value *Ptr = GetPointerOperand(I->Instr);
assert(Ptr && "No pointer operand");
if (NewPtrs.count(Ptr))
Instruction *RealNewPtr;
if (!I->Offset || I->Offset->getValue()->isZero()) {
RealNewPtr = NewBasePtr;
} else {
Instruction *PtrIP = dyn_cast<Instruction>(Ptr);
if (PtrIP && isa<Instruction>(NewBasePtr) &&
cast<Instruction>(NewBasePtr)->getParent() == PtrIP->getParent())
PtrIP = nullptr;
else if (PtrIP && isa<PHINode>(PtrIP))
PtrIP = &*PtrIP->getParent()->getFirstInsertionPt();
else if (!PtrIP)
PtrIP = I->Instr;
GetElementPtrInst *NewPtr = GetElementPtrInst::Create(
I8Ty, PtrInc, I->Offset->getValue(),
getInstrName(I->Instr, GEPNodeOffNameSuffix), PtrIP);
if (!PtrIP)
RealNewPtr = NewPtr;
if (Instruction *IDel = dyn_cast<Instruction>(Ptr))
Instruction *ReplNewPtr;
if (Ptr->getType() != RealNewPtr->getType()) {
ReplNewPtr = new BitCastInst(RealNewPtr, Ptr->getType(),
getInstrName(Ptr, CastNodeNameSuffix));
} else
ReplNewPtr = RealNewPtr;
MadeChange = true;
return MadeChange;
bool PPCLoopPreIncPrep::updateFormPrep(Loop *L,
SmallVector<Bucket, 16> &Buckets) {
bool MadeChange = false;
if (Buckets.empty())
return MadeChange;
SmallSet<BasicBlock *, 16> BBChanged;
for (auto &Bucket : Buckets)
// The base address of each bucket is transformed into a phi and the others
// are rewritten based on new base.
if (prepareBaseForUpdateFormChain(Bucket))
MadeChange |= rewriteLoadStores(L, Bucket, BBChanged);
if (MadeChange)
for (auto &BB : L->blocks())
if (BBChanged.count(BB))
return MadeChange;
// In order to prepare for the pre-increment a PHI is added.
// This function will check to see if that PHI already exists and will return
// true if it found an existing PHI with the same start and increment as the
// one we wanted to create.
bool PPCLoopPreIncPrep::alreadyPrepared(Loop *L, Instruction* MemI,
const SCEV *BasePtrStartSCEV,
const SCEVConstant *BasePtrIncSCEV) {
BasicBlock *BB = MemI->getParent();
if (!BB)
return false;
BasicBlock *PredBB = L->getLoopPredecessor();
BasicBlock *LatchBB = L->getLoopLatch();
if (!PredBB || !LatchBB)
return false;
// Run through the PHIs and see if we have some that looks like a preparation
iterator_range<BasicBlock::phi_iterator> PHIIter = BB->phis();
for (auto & CurrentPHI : PHIIter) {
PHINode *CurrentPHINode = dyn_cast<PHINode>(&CurrentPHI);
if (!CurrentPHINode)
if (!SE->isSCEVable(CurrentPHINode->getType()))
const SCEV *PHISCEV = SE->getSCEVAtScope(CurrentPHINode, L);
const SCEVAddRecExpr *PHIBasePtrSCEV = dyn_cast<SCEVAddRecExpr>(PHISCEV);
if (!PHIBasePtrSCEV)
const SCEVConstant *PHIBasePtrIncSCEV =
if (!PHIBasePtrIncSCEV)
if (CurrentPHINode->getNumIncomingValues() == 2) {
if ((CurrentPHINode->getIncomingBlock(0) == LatchBB &&
CurrentPHINode->getIncomingBlock(1) == PredBB) ||
(CurrentPHINode->getIncomingBlock(1) == LatchBB &&
CurrentPHINode->getIncomingBlock(0) == PredBB)) {
if (PHIBasePtrSCEV->getStart() == BasePtrStartSCEV &&
PHIBasePtrIncSCEV == BasePtrIncSCEV) {
// The existing PHI (CurrentPHINode) has the same start and increment
// as the PHI that we wanted to create.
return true;
return false;
bool PPCLoopPreIncPrep::runOnLoop(Loop *L) {
bool MadeChange = false;
// Only prep. the inner-most loop
if (!L->empty())
return MadeChange;
LLVM_DEBUG(dbgs() << "PIP: Examining: " << *L << "\n");
BasicBlock *LoopPredecessor = L->getLoopPredecessor();
// If there is no loop predecessor, or the loop predecessor's terminator
// returns a value (which might contribute to determining the loop's
// iteration space), insert a new preheader for the loop.
if (!LoopPredecessor ||
!LoopPredecessor->getTerminator()->getType()->isVoidTy()) {
LoopPredecessor = InsertPreheaderForLoop(L, DT, LI, nullptr, PreserveLCSSA);
if (LoopPredecessor)
MadeChange = true;
if (!LoopPredecessor) {
LLVM_DEBUG(dbgs() << "PIP fails since no predecessor for current loop.\n");
return MadeChange;
// Check if a load/store has update form. This lambda is used by function
// collectCandidates which can collect candidates for types defined by lambda.
auto isUpdateFormCandidate = [&] (const Instruction *I,
const Value *PtrValue) {
assert((PtrValue && I) && "Invalid parameter!");
// There are no update forms for Altivec vector load/stores.
if (ST && ST->hasAltivec() &&
return false;
// See getPreIndexedAddressParts, the displacement for LDU/STDU has to
// be 4's multiple (DS-form). For i64 loads/stores when the displacement
// fits in a 16-bit signed field but isn't a multiple of 4, it will be
// useless and possible to break some original well-form addressing mode
// to make this pre-inc prep for it.
if (PtrValue->getType()->getPointerElementType()->isIntegerTy(64)) {
const SCEV *LSCEV = SE->getSCEVAtScope(const_cast<Value *>(PtrValue), L);
const SCEVAddRecExpr *LARSCEV = dyn_cast<SCEVAddRecExpr>(LSCEV);
if (!LARSCEV || LARSCEV->getLoop() != L)
return false;
if (const SCEVConstant *StepConst =
dyn_cast<SCEVConstant>(LARSCEV->getStepRecurrence(*SE))) {
const APInt &ConstInt = StepConst->getValue()->getValue();
if (ConstInt.isSignedIntN(16) && ConstInt.srem(4) != 0)
return false;
return true;
// Collect buckets of comparable addresses used by loads, stores and prefetch
// intrinsic for update form.
SmallVector<Bucket, 16> UpdateFormBuckets =
collectCandidates(L, isUpdateFormCandidate, MaxVars);
// Prepare for update form.
if (!UpdateFormBuckets.empty())
MadeChange |= updateFormPrep(L, UpdateFormBuckets);
return MadeChange;