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llvm-project/llvm/lib/Transforms/Scalar/LoopRerollPass.cpp

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//===-- LoopReroll.cpp - Loop rerolling pass ------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop reroller.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-reroll"
STATISTIC(NumRerolledLoops, "Number of rerolled loops");
static cl::opt<unsigned>
MaxInc("max-reroll-increment", cl::init(2048), cl::Hidden,
cl::desc("The maximum increment for loop rerolling"));
// This loop re-rolling transformation aims to transform loops like this:
//
// int foo(int a);
// void bar(int *x) {
// for (int i = 0; i < 500; i += 3) {
// foo(i);
// foo(i+1);
// foo(i+2);
// }
// }
//
// into a loop like this:
//
// void bar(int *x) {
// for (int i = 0; i < 500; ++i)
// foo(i);
// }
//
// It does this by looking for loops that, besides the latch code, are composed
// of isomorphic DAGs of instructions, with each DAG rooted at some increment
// to the induction variable, and where each DAG is isomorphic to the DAG
// rooted at the induction variable (excepting the sub-DAGs which root the
// other induction-variable increments). In other words, we're looking for loop
// bodies of the form:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// f(%iv)
// %iv.1 = add %iv, 1 <-- a root increment
// f(%iv.1)
// %iv.2 = add %iv, 2 <-- a root increment
// f(%iv.2)
// %iv.scale_m_1 = add %iv, scale-1 <-- a root increment
// f(%iv.scale_m_1)
// ...
// %iv.next = add %iv, scale
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// where each f(i) is a set of instructions that, collectively, are a function
// only of i (and other loop-invariant values).
//
// As a special case, we can also reroll loops like this:
//
// int foo(int);
// void bar(int *x) {
// for (int i = 0; i < 500; ++i) {
// x[3*i] = foo(0);
// x[3*i+1] = foo(0);
// x[3*i+2] = foo(0);
// }
// }
//
// into this:
//
// void bar(int *x) {
// for (int i = 0; i < 1500; ++i)
// x[i] = foo(0);
// }
//
// in which case, we're looking for inputs like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// %scaled.iv = mul %iv, scale
// f(%scaled.iv)
// %scaled.iv.1 = add %scaled.iv, 1
// f(%scaled.iv.1)
// %scaled.iv.2 = add %scaled.iv, 2
// f(%scaled.iv.2)
// %scaled.iv.scale_m_1 = add %scaled.iv, scale-1
// f(%scaled.iv.scale_m_1)
// ...
// %iv.next = add %iv, 1
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
namespace {
enum IterationLimits {
/// The maximum number of iterations that we'll try and reroll. This
/// has to be less than 25 in order to fit into a SmallBitVector.
IL_MaxRerollIterations = 16,
/// The bitvector index used by loop induction variables and other
/// instructions that belong to no one particular iteration.
IL_LoopIncIdx,
IL_End
};
class LoopReroll : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopReroll() : LoopPass(ID) {
initializeLoopRerollPass(*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AliasAnalysis>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<ScalarEvolution>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
protected:
AliasAnalysis *AA;
LoopInfo *LI;
ScalarEvolution *SE;
const DataLayout *DL;
TargetLibraryInfo *TLI;
DominatorTree *DT;
typedef SmallVector<Instruction *, 16> SmallInstructionVector;
typedef SmallSet<Instruction *, 16> SmallInstructionSet;
// A chain of isomorphic instructions, indentified by a single-use PHI,
// representing a reduction. Only the last value may be used outside the
// loop.
struct SimpleLoopReduction {
SimpleLoopReduction(Instruction *P, Loop *L)
: Valid(false), Instructions(1, P) {
assert(isa<PHINode>(P) && "First reduction instruction must be a PHI");
add(L);
}
bool valid() const {
return Valid;
}
Instruction *getPHI() const {
assert(Valid && "Using invalid reduction");
return Instructions.front();
}
Instruction *getReducedValue() const {
assert(Valid && "Using invalid reduction");
return Instructions.back();
}
Instruction *get(size_t i) const {
assert(Valid && "Using invalid reduction");
return Instructions[i+1];
}
Instruction *operator [] (size_t i) const { return get(i); }
// The size, ignoring the initial PHI.
size_t size() const {
assert(Valid && "Using invalid reduction");
return Instructions.size()-1;
}
typedef SmallInstructionVector::iterator iterator;
typedef SmallInstructionVector::const_iterator const_iterator;
iterator begin() {
assert(Valid && "Using invalid reduction");
return std::next(Instructions.begin());
}
const_iterator begin() const {
assert(Valid && "Using invalid reduction");
return std::next(Instructions.begin());
}
iterator end() { return Instructions.end(); }
const_iterator end() const { return Instructions.end(); }
protected:
bool Valid;
SmallInstructionVector Instructions;
void add(Loop *L);
};
// The set of all reductions, and state tracking of possible reductions
// during loop instruction processing.
struct ReductionTracker {
typedef SmallVector<SimpleLoopReduction, 16> SmallReductionVector;
// Add a new possible reduction.
void addSLR(SimpleLoopReduction &SLR) { PossibleReds.push_back(SLR); }
// Setup to track possible reductions corresponding to the provided
// rerolling scale. Only reductions with a number of non-PHI instructions
// that is divisible by the scale are considered. Three instructions sets
// are filled in:
// - A set of all possible instructions in eligible reductions.
// - A set of all PHIs in eligible reductions
// - A set of all reduced values (last instructions) in eligible
// reductions.
void restrictToScale(uint64_t Scale,
SmallInstructionSet &PossibleRedSet,
SmallInstructionSet &PossibleRedPHISet,
SmallInstructionSet &PossibleRedLastSet) {
PossibleRedIdx.clear();
PossibleRedIter.clear();
Reds.clear();
for (unsigned i = 0, e = PossibleReds.size(); i != e; ++i)
if (PossibleReds[i].size() % Scale == 0) {
PossibleRedLastSet.insert(PossibleReds[i].getReducedValue());
PossibleRedPHISet.insert(PossibleReds[i].getPHI());
PossibleRedSet.insert(PossibleReds[i].getPHI());
PossibleRedIdx[PossibleReds[i].getPHI()] = i;
for (Instruction *J : PossibleReds[i]) {
PossibleRedSet.insert(J);
PossibleRedIdx[J] = i;
}
}
}
// The functions below are used while processing the loop instructions.
// Are the two instructions both from reductions, and furthermore, from
// the same reduction?
bool isPairInSame(Instruction *J1, Instruction *J2) {
DenseMap<Instruction *, int>::iterator J1I = PossibleRedIdx.find(J1);
if (J1I != PossibleRedIdx.end()) {
DenseMap<Instruction *, int>::iterator J2I = PossibleRedIdx.find(J2);
if (J2I != PossibleRedIdx.end() && J1I->second == J2I->second)
return true;
}
return false;
}
// The two provided instructions, the first from the base iteration, and
// the second from iteration i, form a matched pair. If these are part of
// a reduction, record that fact.
void recordPair(Instruction *J1, Instruction *J2, unsigned i) {
if (PossibleRedIdx.count(J1)) {
assert(PossibleRedIdx.count(J2) &&
"Recording reduction vs. non-reduction instruction?");
PossibleRedIter[J1] = 0;
PossibleRedIter[J2] = i;
int Idx = PossibleRedIdx[J1];
assert(Idx == PossibleRedIdx[J2] &&
"Recording pair from different reductions?");
Reds.insert(Idx);
}
}
// The functions below can be called after we've finished processing all
// instructions in the loop, and we know which reductions were selected.
// Is the provided instruction the PHI of a reduction selected for
// rerolling?
bool isSelectedPHI(Instruction *J) {
if (!isa<PHINode>(J))
return false;
for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
RI != RIE; ++RI) {
int i = *RI;
if (cast<Instruction>(J) == PossibleReds[i].getPHI())
return true;
}
return false;
}
bool validateSelected();
void replaceSelected();
protected:
// The vector of all possible reductions (for any scale).
SmallReductionVector PossibleReds;
DenseMap<Instruction *, int> PossibleRedIdx;
DenseMap<Instruction *, int> PossibleRedIter;
DenseSet<int> Reds;
};
// The set of all DAG roots, and state tracking of all roots
// for a particular induction variable.
struct DAGRootTracker {
DAGRootTracker(LoopReroll *Parent, Loop *L, Instruction *IV,
ScalarEvolution *SE, AliasAnalysis *AA,
TargetLibraryInfo *TLI, const DataLayout *DL)
: Parent(Parent), L(L), SE(SE), AA(AA), TLI(TLI),
DL(DL), IV(IV) {
}
/// Stage 1: Find all the DAG roots for the induction variable.
bool findRoots();
/// Stage 2: Validate if the found roots are valid.
bool validate(ReductionTracker &Reductions);
/// Stage 3: Assuming validate() returned true, perform the
/// replacement.
/// @param IterCount The maximum iteration count of L.
void replace(const SCEV *IterCount);
protected:
typedef MapVector<Instruction*, SmallBitVector> UsesTy;
bool findScaleFromMul();
bool collectAllRoots();
bool collectUsedInstructions(SmallInstructionSet &PossibleRedSet);
void collectInLoopUserSet(const SmallInstructionVector &Roots,
const SmallInstructionSet &Exclude,
const SmallInstructionSet &Final,
DenseSet<Instruction *> &Users);
void collectInLoopUserSet(Instruction *Root,
const SmallInstructionSet &Exclude,
const SmallInstructionSet &Final,
DenseSet<Instruction *> &Users);
UsesTy::iterator nextInstr(int Val, UsesTy &In, UsesTy::iterator I);
LoopReroll *Parent;
// Members of Parent, replicated here for brevity.
Loop *L;
ScalarEvolution *SE;
AliasAnalysis *AA;
TargetLibraryInfo *TLI;
const DataLayout *DL;
// The loop induction variable.
Instruction *IV;
// Loop step amount.
uint64_t Inc;
// Loop reroll count; if Inc == 1, this records the scaling applied
// to the indvar: a[i*2+0] = ...; a[i*2+1] = ... ;
// If Inc is not 1, Scale = Inc.
uint64_t Scale;
// If Scale != Inc, then RealIV is IV after its multiplication.
Instruction *RealIV;
// The roots themselves.
SmallInstructionVector Roots;
// All increment instructions for IV.
SmallInstructionVector LoopIncs;
// Map of all instructions in the loop (in order) to the iterations
// they are used in (or specially, IL_LoopIncIdx for instructions
// used in the loop increment mechanism).
UsesTy Uses;
};
void collectPossibleIVs(Loop *L, SmallInstructionVector &PossibleIVs);
void collectPossibleReductions(Loop *L,
ReductionTracker &Reductions);
bool reroll(Instruction *IV, Loop *L, BasicBlock *Header, const SCEV *IterCount,
ReductionTracker &Reductions);
};
}
char LoopReroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopReroll, "loop-reroll", "Reroll loops", false, false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(LoopReroll, "loop-reroll", "Reroll loops", false, false)
Pass *llvm::createLoopRerollPass() {
return new LoopReroll;
}
// Returns true if the provided instruction is used outside the given loop.
// This operates like Instruction::isUsedOutsideOfBlock, but considers PHIs in
// non-loop blocks to be outside the loop.
static bool hasUsesOutsideLoop(Instruction *I, Loop *L) {
for (User *U : I->users()) {
if (!L->contains(cast<Instruction>(U)))
return true;
}
return false;
}
// Collect the list of loop induction variables with respect to which it might
// be possible to reroll the loop.
void LoopReroll::collectPossibleIVs(Loop *L,
SmallInstructionVector &PossibleIVs) {
BasicBlock *Header = L->getHeader();
for (BasicBlock::iterator I = Header->begin(),
IE = Header->getFirstInsertionPt(); I != IE; ++I) {
if (!isa<PHINode>(I))
continue;
if (!I->getType()->isIntegerTy())
continue;
if (const SCEVAddRecExpr *PHISCEV =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(I))) {
if (PHISCEV->getLoop() != L)
continue;
if (!PHISCEV->isAffine())
continue;
if (const SCEVConstant *IncSCEV =
dyn_cast<SCEVConstant>(PHISCEV->getStepRecurrence(*SE))) {
if (!IncSCEV->getValue()->getValue().isStrictlyPositive())
continue;
if (IncSCEV->getValue()->uge(MaxInc))
continue;
DEBUG(dbgs() << "LRR: Possible IV: " << *I << " = " <<
*PHISCEV << "\n");
PossibleIVs.push_back(I);
}
}
}
}
// Add the remainder of the reduction-variable chain to the instruction vector
// (the initial PHINode has already been added). If successful, the object is
// marked as valid.
void LoopReroll::SimpleLoopReduction::add(Loop *L) {
assert(!Valid && "Cannot add to an already-valid chain");
// The reduction variable must be a chain of single-use instructions
// (including the PHI), except for the last value (which is used by the PHI
// and also outside the loop).
Instruction *C = Instructions.front();
do {
C = cast<Instruction>(*C->user_begin());
if (C->hasOneUse()) {
if (!C->isBinaryOp())
return;
if (!(isa<PHINode>(Instructions.back()) ||
C->isSameOperationAs(Instructions.back())))
return;
Instructions.push_back(C);
}
} while (C->hasOneUse());
if (Instructions.size() < 2 ||
!C->isSameOperationAs(Instructions.back()) ||
C->use_empty())
return;
// C is now the (potential) last instruction in the reduction chain.
for (User *U : C->users()) {
// The only in-loop user can be the initial PHI.
if (L->contains(cast<Instruction>(U)))
if (cast<Instruction>(U) != Instructions.front())
return;
}
Instructions.push_back(C);
Valid = true;
}
// Collect the vector of possible reduction variables.
void LoopReroll::collectPossibleReductions(Loop *L,
ReductionTracker &Reductions) {
BasicBlock *Header = L->getHeader();
for (BasicBlock::iterator I = Header->begin(),
IE = Header->getFirstInsertionPt(); I != IE; ++I) {
if (!isa<PHINode>(I))
continue;
if (!I->getType()->isSingleValueType())
continue;
SimpleLoopReduction SLR(I, L);
if (!SLR.valid())
continue;
DEBUG(dbgs() << "LRR: Possible reduction: " << *I << " (with " <<
SLR.size() << " chained instructions)\n");
Reductions.addSLR(SLR);
}
}
// Collect the set of all users of the provided root instruction. This set of
// users contains not only the direct users of the root instruction, but also
// all users of those users, and so on. There are two exceptions:
//
// 1. Instructions in the set of excluded instructions are never added to the
// use set (even if they are users). This is used, for example, to exclude
// including root increments in the use set of the primary IV.
//
// 2. Instructions in the set of final instructions are added to the use set
// if they are users, but their users are not added. This is used, for
// example, to prevent a reduction update from forcing all later reduction
// updates into the use set.
void LoopReroll::DAGRootTracker::collectInLoopUserSet(
Instruction *Root, const SmallInstructionSet &Exclude,
const SmallInstructionSet &Final,
DenseSet<Instruction *> &Users) {
SmallInstructionVector Queue(1, Root);
while (!Queue.empty()) {
Instruction *I = Queue.pop_back_val();
if (!Users.insert(I).second)
continue;
if (!Final.count(I))
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *PN = dyn_cast<PHINode>(User)) {
// Ignore "wrap-around" uses to PHIs of this loop's header.
if (PN->getIncomingBlock(U) == L->getHeader())
continue;
}
if (L->contains(User) && !Exclude.count(User)) {
Queue.push_back(User);
}
}
// We also want to collect single-user "feeder" values.
for (User::op_iterator OI = I->op_begin(),
OIE = I->op_end(); OI != OIE; ++OI) {
if (Instruction *Op = dyn_cast<Instruction>(*OI))
if (Op->hasOneUse() && L->contains(Op) && !Exclude.count(Op) &&
!Final.count(Op))
Queue.push_back(Op);
}
}
}
// Collect all of the users of all of the provided root instructions (combined
// into a single set).
void LoopReroll::DAGRootTracker::collectInLoopUserSet(
const SmallInstructionVector &Roots,
const SmallInstructionSet &Exclude,
const SmallInstructionSet &Final,
DenseSet<Instruction *> &Users) {
for (SmallInstructionVector::const_iterator I = Roots.begin(),
IE = Roots.end(); I != IE; ++I)
collectInLoopUserSet(*I, Exclude, Final, Users);
}
static bool isSimpleLoadStore(Instruction *I) {
if (LoadInst *LI = dyn_cast<LoadInst>(I))
return LI->isSimple();
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isSimple();
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
return !MI->isVolatile();
return false;
}
bool LoopReroll::DAGRootTracker::findRoots() {
const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(IV));
Inc = cast<SCEVConstant>(RealIVSCEV->getOperand(1))->
getValue()->getZExtValue();
// The effective induction variable, IV, is normally also the real induction
// variable. When we're dealing with a loop like:
// for (int i = 0; i < 500; ++i)
// x[3*i] = ...;
// x[3*i+1] = ...;
// x[3*i+2] = ...;
// then the real IV is still i, but the effective IV is (3*i).
Scale = Inc;
RealIV = IV;
if (Inc == 1 && !findScaleFromMul())
return false;
// The set of increment instructions for each increment value.
if (!collectAllRoots())
return false;
if (Roots.size() > IL_MaxRerollIterations) {
DEBUG(dbgs() << "LRR: Aborting - too many iterations found. "
<< "#Found=" << Roots.size() << ", #Max=" << IL_MaxRerollIterations
<< "\n");
return false;
}
return true;
}
// Recognize loops that are setup like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// %scaled.iv = mul %iv, scale
// f(%scaled.iv)
// %scaled.iv.1 = add %scaled.iv, 1
// f(%scaled.iv.1)
// %scaled.iv.2 = add %scaled.iv, 2
// f(%scaled.iv.2)
// %scaled.iv.scale_m_1 = add %scaled.iv, scale-1
// f(%scaled.iv.scale_m_1)
// ...
// %iv.next = add %iv, 1
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// and, if found, set IV = %scaled.iv, and add %iv.next to LoopIncs.
bool LoopReroll::DAGRootTracker::findScaleFromMul() {
// This is a special case: here we're looking for all uses (except for
// the increment) to be multiplied by a common factor. The increment must
// be by one. This is to capture loops like:
// for (int i = 0; i < 500; ++i) {
// foo(3*i); foo(3*i+1); foo(3*i+2);
// }
if (RealIV->getNumUses() != 2)
return false;
const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(RealIV));
Instruction *User1 = cast<Instruction>(*RealIV->user_begin()),
*User2 = cast<Instruction>(*std::next(RealIV->user_begin()));
if (!SE->isSCEVable(User1->getType()) || !SE->isSCEVable(User2->getType()))
return false;
const SCEVAddRecExpr *User1SCEV =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(User1)),
*User2SCEV =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(User2));
if (!User1SCEV || !User1SCEV->isAffine() ||
!User2SCEV || !User2SCEV->isAffine())
return false;
// We assume below that User1 is the scale multiply and User2 is the
// increment. If this can't be true, then swap them.
if (User1SCEV == RealIVSCEV->getPostIncExpr(*SE)) {
std::swap(User1, User2);
std::swap(User1SCEV, User2SCEV);
}
if (User2SCEV != RealIVSCEV->getPostIncExpr(*SE))
return false;
assert(User2SCEV->getStepRecurrence(*SE)->isOne() &&
"Invalid non-unit step for multiplicative scaling");
LoopIncs.push_back(User2);
if (const SCEVConstant *MulScale =
dyn_cast<SCEVConstant>(User1SCEV->getStepRecurrence(*SE))) {
// Make sure that both the start and step have the same multiplier.
if (RealIVSCEV->getStart()->getType() != MulScale->getType())
return false;
if (SE->getMulExpr(RealIVSCEV->getStart(), MulScale) !=
User1SCEV->getStart())
return false;
ConstantInt *MulScaleCI = MulScale->getValue();
if (!MulScaleCI->uge(2) || MulScaleCI->uge(MaxInc))
return false;
Scale = MulScaleCI->getZExtValue();
IV = User1;
} else
return false;
DEBUG(dbgs() << "LRR: Found possible scaling " << *User1 << "\n");
assert(Scale <= MaxInc && "Scale is too large");
assert(Scale > 1 && "Scale must be at least 2");
return true;
}
// Collect all root increments with respect to the provided induction variable
// (normally the PHI, but sometimes a multiply). A root increment is an
// instruction, normally an add, with a positive constant less than Scale. In a
// rerollable loop, each of these increments is the root of an instruction
// graph isomorphic to the others. Also, we collect the final induction
// increment (the increment equal to the Scale), and its users in LoopIncs.
bool LoopReroll::DAGRootTracker::collectAllRoots() {
Roots.resize(Scale-1);
for (User *U : IV->users()) {
Instruction *UI = cast<Instruction>(U);
if (!SE->isSCEVable(UI->getType()))
continue;
if (UI->getType() != IV->getType())
continue;
if (!L->contains(UI))
continue;
if (hasUsesOutsideLoop(UI, L))
continue;
if (const SCEVConstant *Diff = dyn_cast<SCEVConstant>(SE->getMinusSCEV(
SE->getSCEV(UI), SE->getSCEV(IV)))) {
uint64_t Idx = Diff->getValue()->getValue().getZExtValue();
if (Idx > 0 && Idx < Scale) {
if (Roots[Idx-1])
// No duplicates allowed.
return false;
Roots[Idx-1] = UI;
} else if (Idx == Scale && Inc > 1) {
LoopIncs.push_back(UI);
}
}
}
for (unsigned i = 0; i < Scale-1; ++i) {
if (!Roots[i])
return false;
}
return true;
}
bool LoopReroll::DAGRootTracker::collectUsedInstructions(SmallInstructionSet &PossibleRedSet) {
// Populate the MapVector with all instructions in the block, in order first,
// so we can iterate over the contents later in perfect order.
for (auto &I : *L->getHeader()) {
Uses[&I].resize(IL_End);
}
SmallInstructionSet Exclude;
Exclude.insert(Roots.begin(), Roots.end());
Exclude.insert(LoopIncs.begin(), LoopIncs.end());
DenseSet<Instruction*> VBase;
collectInLoopUserSet(IV, Exclude, PossibleRedSet, VBase);
for (auto *I : VBase) {
Uses[I].set(0);
}
unsigned Idx = 1;
for (auto *Root : Roots) {
DenseSet<Instruction*> V;
collectInLoopUserSet(Root, Exclude, PossibleRedSet, V);
// While we're here, check the use sets are the same size.
if (V.size() != VBase.size()) {
DEBUG(dbgs() << "LRR: Aborting - use sets are different sizes\n");
return false;
}
for (auto *I : V) {
Uses[I].set(Idx);
}
++Idx;
}
// Make sure the loop increments are also accounted for.
Exclude.clear();
Exclude.insert(Roots.begin(), Roots.end());
DenseSet<Instruction*> V;
collectInLoopUserSet(LoopIncs, Exclude, PossibleRedSet, V);
for (auto *I : V) {
Uses[I].set(IL_LoopIncIdx);
}
if (IV != RealIV)
Uses[RealIV].set(IL_LoopIncIdx);
return true;
}
LoopReroll::DAGRootTracker::UsesTy::iterator
LoopReroll::DAGRootTracker::nextInstr(int Val, UsesTy &In,
UsesTy::iterator I) {
while (I != In.end() && I->second.test(Val) == 0)
++I;
return I;
}
bool LoopReroll::DAGRootTracker::validate(ReductionTracker &Reductions) {
// We now need to check for equivalence of the use graph of each root with
// that of the primary induction variable (excluding the roots). Our goal
// here is not to solve the full graph isomorphism problem, but rather to
// catch common cases without a lot of work. As a result, we will assume
// that the relative order of the instructions in each unrolled iteration
// is the same (although we will not make an assumption about how the
// different iterations are intermixed). Note that while the order must be
// the same, the instructions may not be in the same basic block.
// An array of just the possible reductions for this scale factor. When we
// collect the set of all users of some root instructions, these reduction
// instructions are treated as 'final' (their uses are not considered).
// This is important because we don't want the root use set to search down
// the reduction chain.
SmallInstructionSet PossibleRedSet;
SmallInstructionSet PossibleRedLastSet;
SmallInstructionSet PossibleRedPHISet;
Reductions.restrictToScale(Scale, PossibleRedSet,
PossibleRedPHISet, PossibleRedLastSet);
// Populate "Uses" with where each instruction is used.
if (!collectUsedInstructions(PossibleRedSet))
return false;
// Make sure we mark the reduction PHIs as used in all iterations.
for (auto *I : PossibleRedPHISet) {
Uses[I].set(IL_LoopIncIdx);
}
// Make sure all instructions in the loop are in one and only one
// set.
for (auto &KV : Uses) {
if (KV.second.count() != 1) {
DEBUG(dbgs() << "LRR: Aborting - instruction is not used in 1 iteration: "
<< *KV.first << " (#uses=" << KV.second.count() << ")\n");
return false;
}
}
DEBUG(
for (auto &KV : Uses) {
dbgs() << "LRR: " << KV.second.find_first() << "\t" << *KV.first << "\n";
}
);
for (unsigned Iter = 1; Iter < Scale; ++Iter) {
// In addition to regular aliasing information, we need to look for
// instructions from later (future) iterations that have side effects
// preventing us from reordering them past other instructions with side
// effects.
bool FutureSideEffects = false;
AliasSetTracker AST(*AA);
// The map between instructions in f(%iv.(i+1)) and f(%iv).
DenseMap<Value *, Value *> BaseMap;
// Compare iteration Iter to the base.
auto BaseIt = nextInstr(0, Uses, Uses.begin());
auto RootIt = nextInstr(Iter, Uses, Uses.begin());
auto LastRootIt = Uses.begin();
while (BaseIt != Uses.end() && RootIt != Uses.end()) {
Instruction *BaseInst = BaseIt->first;
Instruction *RootInst = RootIt->first;
// Skip over the IV or root instructions; only match their users.
bool Continue = false;
if (BaseInst == RealIV || BaseInst == IV) {
BaseIt = nextInstr(0, Uses, ++BaseIt);
Continue = true;
}
if (std::find(Roots.begin(), Roots.end(), RootInst) != Roots.end()) {
LastRootIt = RootIt;
RootIt = nextInstr(Iter, Uses, ++RootIt);
Continue = true;
}
if (Continue) continue;
// All instructions between the last root and this root
// belong to some other iteration. If they belong to a
// future iteration, then they're dangerous to alias with.
for (; LastRootIt != RootIt; ++LastRootIt) {
Instruction *I = LastRootIt->first;
if (LastRootIt->second.find_first() < (int)Iter)
continue;
if (I->mayWriteToMemory())
AST.add(I);
// Note: This is specifically guarded by a check on isa<PHINode>,
// which while a valid (somewhat arbitrary) micro-optimization, is
// needed because otherwise isSafeToSpeculativelyExecute returns
// false on PHI nodes.
if (!isa<PHINode>(I) && !isSimpleLoadStore(I) &&
!isSafeToSpeculativelyExecute(I, DL))
// Intervening instructions cause side effects.
FutureSideEffects = true;
}
if (!BaseInst->isSameOperationAs(RootInst)) {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst <<
" vs. " << *RootInst << "\n");
return false;
}
// Make sure that this instruction, which is in the use set of this
// root instruction, does not also belong to the base set or the set of
// some other root instruction.
if (RootIt->second.count() > 1) {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst <<
" vs. " << *RootInst << " (prev. case overlap)\n");
return false;
}
// Make sure that we don't alias with any instruction in the alias set
// tracker. If we do, then we depend on a future iteration, and we
// can't reroll.
if (RootInst->mayReadFromMemory())
for (auto &K : AST) {
if (K.aliasesUnknownInst(RootInst, *AA)) {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst <<
" vs. " << *RootInst << " (depends on future store)\n");
return false;
}
}
// If we've past an instruction from a future iteration that may have
// side effects, and this instruction might also, then we can't reorder
// them, and this matching fails. As an exception, we allow the alias
// set tracker to handle regular (simple) load/store dependencies.
if (FutureSideEffects &&
((!isSimpleLoadStore(BaseInst) &&
!isSafeToSpeculativelyExecute(BaseInst, DL)) ||
(!isSimpleLoadStore(RootInst) &&
!isSafeToSpeculativelyExecute(RootInst, DL)))) {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst <<
" vs. " << *RootInst <<
" (side effects prevent reordering)\n");
return false;
}
// For instructions that are part of a reduction, if the operation is
// associative, then don't bother matching the operands (because we
// already know that the instructions are isomorphic, and the order
// within the iteration does not matter). For non-associative reductions,
// we do need to match the operands, because we need to reject
// out-of-order instructions within an iteration!
// For example (assume floating-point addition), we need to reject this:
// x += a[i]; x += b[i];
// x += a[i+1]; x += b[i+1];
// x += b[i+2]; x += a[i+2];
bool InReduction = Reductions.isPairInSame(BaseInst, RootInst);
if (!(InReduction && BaseInst->isAssociative())) {
bool Swapped = false, SomeOpMatched = false;
for (unsigned j = 0; j < BaseInst->getNumOperands(); ++j) {
Value *Op2 = RootInst->getOperand(j);
// If this is part of a reduction (and the operation is not
// associatve), then we match all operands, but not those that are
// part of the reduction.
if (InReduction)
if (Instruction *Op2I = dyn_cast<Instruction>(Op2))
if (Reductions.isPairInSame(RootInst, Op2I))
continue;
DenseMap<Value *, Value *>::iterator BMI = BaseMap.find(Op2);
if (BMI != BaseMap.end())
Op2 = BMI->second;
else if (Roots[Iter-1] == (Instruction*) Op2)
Op2 = IV;
if (BaseInst->getOperand(Swapped ? unsigned(!j) : j) != Op2) {
// If we've not already decided to swap the matched operands, and
// we've not already matched our first operand (note that we could
// have skipped matching the first operand because it is part of a
// reduction above), and the instruction is commutative, then try
// the swapped match.
if (!Swapped && BaseInst->isCommutative() && !SomeOpMatched &&
BaseInst->getOperand(!j) == Op2) {
Swapped = true;
} else {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst
<< " vs. " << *RootInst << " (operand " << j << ")\n");
return false;
}
}
SomeOpMatched = true;
}
}
if ((!PossibleRedLastSet.count(BaseInst) &&
hasUsesOutsideLoop(BaseInst, L)) ||
(!PossibleRedLastSet.count(RootInst) &&
hasUsesOutsideLoop(RootInst, L))) {
DEBUG(dbgs() << "LRR: iteration root match failed at " << *BaseInst <<
" vs. " << *RootInst << " (uses outside loop)\n");
return false;
}
Reductions.recordPair(BaseInst, RootInst, Iter);
BaseMap.insert(std::make_pair(RootInst, BaseInst));
LastRootIt = RootIt;
BaseIt = nextInstr(0, Uses, ++BaseIt);
RootIt = nextInstr(Iter, Uses, ++RootIt);
}
assert (BaseIt == Uses.end() && RootIt == Uses.end() &&
"Mismatched set sizes!");
}
DEBUG(dbgs() << "LRR: Matched all iteration increments for " <<
*RealIV << "\n");
return true;
}
void LoopReroll::DAGRootTracker::replace(const SCEV *IterCount) {
BasicBlock *Header = L->getHeader();
// Remove instructions associated with non-base iterations.
for (BasicBlock::reverse_iterator J = Header->rbegin();
J != Header->rend();) {
unsigned I = Uses[&*J].find_first();
if (I > 0 && I < IL_LoopIncIdx) {
Instruction *D = &*J;
DEBUG(dbgs() << "LRR: removing: " << *D << "\n");
D->eraseFromParent();
continue;
}
++J;
}
// Insert the new induction variable.
const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(RealIV));
const SCEV *Start = RealIVSCEV->getStart();
if (Inc == 1)
Start = SE->getMulExpr(Start,
SE->getConstant(Start->getType(), Scale));
const SCEVAddRecExpr *H =
cast<SCEVAddRecExpr>(SE->getAddRecExpr(Start,
SE->getConstant(RealIVSCEV->getType(), 1),
L, SCEV::FlagAnyWrap));
{ // Limit the lifetime of SCEVExpander.
SCEVExpander Expander(*SE, "reroll");
Value *NewIV = Expander.expandCodeFor(H, IV->getType(), Header->begin());
for (auto &KV : Uses) {
if (KV.second.find_first() == 0)
KV.first->replaceUsesOfWith(IV, NewIV);
}
if (BranchInst *BI = dyn_cast<BranchInst>(Header->getTerminator())) {
// FIXME: Why do we need this check?
if (Uses[BI].find_first() == IL_LoopIncIdx) {
const SCEV *ICSCEV = RealIVSCEV->evaluateAtIteration(IterCount, *SE);
if (Inc == 1)
ICSCEV =
SE->getMulExpr(ICSCEV, SE->getConstant(ICSCEV->getType(), Scale));
// Iteration count SCEV minus 1
const SCEV *ICMinus1SCEV =
SE->getMinusSCEV(ICSCEV, SE->getConstant(ICSCEV->getType(), 1));
Value *ICMinus1; // Iteration count minus 1
if (isa<SCEVConstant>(ICMinus1SCEV)) {
ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(), BI);
} else {
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader)
Preheader = InsertPreheaderForLoop(L, Parent);
ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(),
Preheader->getTerminator());
}
Value *Cond =
new ICmpInst(BI, CmpInst::ICMP_EQ, NewIV, ICMinus1, "exitcond");
BI->setCondition(Cond);
if (BI->getSuccessor(1) != Header)
BI->swapSuccessors();
}
}
}
SimplifyInstructionsInBlock(Header, DL, TLI);
DeleteDeadPHIs(Header, TLI);
}
// Validate the selected reductions. All iterations must have an isomorphic
// part of the reduction chain and, for non-associative reductions, the chain
// entries must appear in order.
bool LoopReroll::ReductionTracker::validateSelected() {
// For a non-associative reduction, the chain entries must appear in order.
for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
RI != RIE; ++RI) {
int i = *RI;
int PrevIter = 0, BaseCount = 0, Count = 0;
for (Instruction *J : PossibleReds[i]) {
// Note that all instructions in the chain must have been found because
// all instructions in the function must have been assigned to some
// iteration.
int Iter = PossibleRedIter[J];
if (Iter != PrevIter && Iter != PrevIter + 1 &&
!PossibleReds[i].getReducedValue()->isAssociative()) {
DEBUG(dbgs() << "LRR: Out-of-order non-associative reduction: " <<
J << "\n");
return false;
}
if (Iter != PrevIter) {
if (Count != BaseCount) {
DEBUG(dbgs() << "LRR: Iteration " << PrevIter <<
" reduction use count " << Count <<
" is not equal to the base use count " <<
BaseCount << "\n");
return false;
}
Count = 0;
}
++Count;
if (Iter == 0)
++BaseCount;
PrevIter = Iter;
}
}
return true;
}
// For all selected reductions, remove all parts except those in the first
// iteration (and the PHI). Replace outside uses of the reduced value with uses
// of the first-iteration reduced value (in other words, reroll the selected
// reductions).
void LoopReroll::ReductionTracker::replaceSelected() {
// Fixup reductions to refer to the last instruction associated with the
// first iteration (not the last).
for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
RI != RIE; ++RI) {
int i = *RI;
int j = 0;
for (int e = PossibleReds[i].size(); j != e; ++j)
if (PossibleRedIter[PossibleReds[i][j]] != 0) {
--j;
break;
}
// Replace users with the new end-of-chain value.
SmallInstructionVector Users;
for (User *U : PossibleReds[i].getReducedValue()->users()) {
Users.push_back(cast<Instruction>(U));
}
for (SmallInstructionVector::iterator J = Users.begin(),
JE = Users.end(); J != JE; ++J)
(*J)->replaceUsesOfWith(PossibleReds[i].getReducedValue(),
PossibleReds[i][j]);
}
}
// Reroll the provided loop with respect to the provided induction variable.
// Generally, we're looking for a loop like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// f(%iv)
// %iv.1 = add %iv, 1 <-- a root increment
// f(%iv.1)
// %iv.2 = add %iv, 2 <-- a root increment
// f(%iv.2)
// %iv.scale_m_1 = add %iv, scale-1 <-- a root increment
// f(%iv.scale_m_1)
// ...
// %iv.next = add %iv, scale
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// Notably, we do not require that f(%iv), f(%iv.1), etc. be isolated groups of
// instructions. In other words, the instructions in f(%iv), f(%iv.1), etc. can
// be intermixed with eachother. The restriction imposed by this algorithm is
// that the relative order of the isomorphic instructions in f(%iv), f(%iv.1),
// etc. be the same.
//
// First, we collect the use set of %iv, excluding the other increment roots.
// This gives us f(%iv). Then we iterate over the loop instructions (scale-1)
// times, having collected the use set of f(%iv.(i+1)), during which we:
// - Ensure that the next unmatched instruction in f(%iv) is isomorphic to
// the next unmatched instruction in f(%iv.(i+1)).
// - Ensure that both matched instructions don't have any external users
// (with the exception of last-in-chain reduction instructions).
// - Track the (aliasing) write set, and other side effects, of all
// instructions that belong to future iterations that come before the matched
// instructions. If the matched instructions read from that write set, then
// f(%iv) or f(%iv.(i+1)) has some dependency on instructions in
// f(%iv.(j+1)) for some j > i, and we cannot reroll the loop. Similarly,
// if any of these future instructions had side effects (could not be
// speculatively executed), and so do the matched instructions, when we
// cannot reorder those side-effect-producing instructions, and rerolling
// fails.
//
// Finally, we make sure that all loop instructions are either loop increment
// roots, belong to simple latch code, parts of validated reductions, part of
// f(%iv) or part of some f(%iv.i). If all of that is true (and all reductions
// have been validated), then we reroll the loop.
bool LoopReroll::reroll(Instruction *IV, Loop *L, BasicBlock *Header,
const SCEV *IterCount,
ReductionTracker &Reductions) {
DAGRootTracker DAGRoots(this, L, IV, SE, AA, TLI, DL);
if (!DAGRoots.findRoots())
return false;
DEBUG(dbgs() << "LRR: Found all root induction increments for: " <<
*IV << "\n");
if (!DAGRoots.validate(Reductions))
return false;
if (!Reductions.validateSelected())
return false;
// At this point, we've validated the rerolling, and we're committed to
// making changes!
Reductions.replaceSelected();
DAGRoots.replace(IterCount);
++NumRerolledLoops;
return true;
}
bool LoopReroll::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipOptnoneFunction(L))
return false;
AA = &getAnalysis<AliasAnalysis>();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
SE = &getAnalysis<ScalarEvolution>();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
DL = DLP ? &DLP->getDataLayout() : nullptr;
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
BasicBlock *Header = L->getHeader();
DEBUG(dbgs() << "LRR: F[" << Header->getParent()->getName() <<
"] Loop %" << Header->getName() << " (" <<
L->getNumBlocks() << " block(s))\n");
bool Changed = false;
// For now, we'll handle only single BB loops.
if (L->getNumBlocks() > 1)
return Changed;
if (!SE->hasLoopInvariantBackedgeTakenCount(L))
return Changed;
const SCEV *LIBETC = SE->getBackedgeTakenCount(L);
const SCEV *IterCount =
SE->getAddExpr(LIBETC, SE->getConstant(LIBETC->getType(), 1));
DEBUG(dbgs() << "LRR: iteration count = " << *IterCount << "\n");
// First, we need to find the induction variable with respect to which we can
// reroll (there may be several possible options).
SmallInstructionVector PossibleIVs;
collectPossibleIVs(L, PossibleIVs);
if (PossibleIVs.empty()) {
DEBUG(dbgs() << "LRR: No possible IVs found\n");
return Changed;
}
ReductionTracker Reductions;
collectPossibleReductions(L, Reductions);
// For each possible IV, collect the associated possible set of 'root' nodes
// (i+1, i+2, etc.).
for (SmallInstructionVector::iterator I = PossibleIVs.begin(),
IE = PossibleIVs.end(); I != IE; ++I)
if (reroll(*I, L, Header, IterCount, Reductions)) {
Changed = true;
break;
}
return Changed;
}
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