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

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//===- LoopReroll.cpp - Loop rerolling pass -------------------------------===//
//
// 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 implements a simple loop reroller.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <iterator>
#include <map>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "loop-reroll"
STATISTIC(NumRerolledLoops, "Number of rerolled loops");
static cl::opt<unsigned>
NumToleratedFailedMatches("reroll-num-tolerated-failed-matches", cl::init(400),
cl::Hidden,
cl::desc("The maximum number of failures to tolerate"
" during fuzzy matching. (default: 400)"));
// 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.
IL_MaxRerollIterations = 32,
/// The bitvector index used by loop induction variables and other
/// instructions that belong to all iterations.
IL_All,
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<TargetLibraryInfoWrapperPass>();
getLoopAnalysisUsage(AU);
}
protected:
AliasAnalysis *AA;
LoopInfo *LI;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
DominatorTree *DT;
bool PreserveLCSSA;
using SmallInstructionVector = SmallVector<Instruction *, 16>;
using SmallInstructionSet = SmallPtrSet<Instruction *, 16>;
// Map between induction variable and its increment
DenseMap<Instruction *, int64_t> IVToIncMap;
// For loop with multiple induction variable, remember the one used only to
// control the loop.
Instruction *LoopControlIV;
// A chain of isomorphic instructions, identified 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) : 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;
}
using iterator = SmallInstructionVector::iterator;
using const_iterator = SmallInstructionVector::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 = false;
SmallInstructionVector Instructions;
void add(Loop *L);
};
// The set of all reductions, and state tracking of possible reductions
// during loop instruction processing.
struct ReductionTracker {
using SmallReductionVector = SmallVector<SimpleLoopReduction, 16>;
// 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.
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;
};
// A DAGRootSet models an induction variable being used in a rerollable
// loop. For example,
//
// x[i*3+0] = y1
// x[i*3+1] = y2
// x[i*3+2] = y3
//
// Base instruction -> i*3
// +---+----+
// / | \
// ST[y1] +1 +2 <-- Roots
// | |
// ST[y2] ST[y3]
//
// There may be multiple DAGRoots, for example:
//
// x[i*2+0] = ... (1)
// x[i*2+1] = ... (1)
// x[i*2+4] = ... (2)
// x[i*2+5] = ... (2)
// x[(i+1234)*2+5678] = ... (3)
// x[(i+1234)*2+5679] = ... (3)
//
// The loop will be rerolled by adding a new loop induction variable,
// one for the Base instruction in each DAGRootSet.
//
struct DAGRootSet {
Instruction *BaseInst;
SmallInstructionVector Roots;
// The instructions between IV and BaseInst (but not including BaseInst).
SmallInstructionSet SubsumedInsts;
};
// 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, DominatorTree *DT, LoopInfo *LI,
bool PreserveLCSSA,
DenseMap<Instruction *, int64_t> &IncrMap,
Instruction *LoopCtrlIV)
: Parent(Parent), L(L), SE(SE), AA(AA), TLI(TLI), DT(DT), LI(LI),
PreserveLCSSA(PreserveLCSSA), IV(IV), IVToIncMap(IncrMap),
LoopControlIV(LoopCtrlIV) {}
/// 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 BackedgeTakenCount The backedge-taken count of L.
void replace(const SCEV *BackedgeTakenCount);
protected:
using UsesTy = MapVector<Instruction *, BitVector>;
void findRootsRecursive(Instruction *IVU,
SmallInstructionSet SubsumedInsts);
bool findRootsBase(Instruction *IVU, SmallInstructionSet SubsumedInsts);
bool collectPossibleRoots(Instruction *Base,
std::map<int64_t,Instruction*> &Roots);
bool validateRootSet(DAGRootSet &DRS);
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,
const SmallInstructionSet &Exclude,
UsesTy::iterator *StartI=nullptr);
bool isBaseInst(Instruction *I);
bool isRootInst(Instruction *I);
bool instrDependsOn(Instruction *I,
UsesTy::iterator Start,
UsesTy::iterator End);
void replaceIV(DAGRootSet &DRS, const SCEV *Start, const SCEV *IncrExpr);
LoopReroll *Parent;
// Members of Parent, replicated here for brevity.
Loop *L;
ScalarEvolution *SE;
AliasAnalysis *AA;
TargetLibraryInfo *TLI;
DominatorTree *DT;
LoopInfo *LI;
bool PreserveLCSSA;
// The loop induction variable.
Instruction *IV;
// Loop step amount.
int64_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;
// The roots themselves.
SmallVector<DAGRootSet,16> RootSets;
// 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_All for instructions
// used in the loop increment mechanism).
UsesTy Uses;
// Map between induction variable and its increment
DenseMap<Instruction *, int64_t> &IVToIncMap;
Instruction *LoopControlIV;
};
// Check if it is a compare-like instruction whose user is a branch
bool isCompareUsedByBranch(Instruction *I) {
auto *TI = I->getParent()->getTerminator();
if (!isa<BranchInst>(TI) || !isa<CmpInst>(I))
return false;
return I->hasOneUse() && TI->getOperand(0) == I;
};
bool isLoopControlIV(Loop *L, Instruction *IV);
void collectPossibleIVs(Loop *L, SmallInstructionVector &PossibleIVs);
void collectPossibleReductions(Loop *L,
ReductionTracker &Reductions);
bool reroll(Instruction *IV, Loop *L, BasicBlock *Header,
const SCEV *BackedgeTakenCount, ReductionTracker &Reductions);
};
} // end anonymous namespace
char LoopReroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopReroll, "loop-reroll", "Reroll loops", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
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;
}
// Check if an IV is only used to control the loop. There are two cases:
// 1. It only has one use which is loop increment, and the increment is only
// used by comparison and the PHI (could has sext with nsw in between), and the
// comparison is only used by branch.
// 2. It is used by loop increment and the comparison, the loop increment is
// only used by the PHI, and the comparison is used only by the branch.
bool LoopReroll::isLoopControlIV(Loop *L, Instruction *IV) {
unsigned IVUses = IV->getNumUses();
if (IVUses != 2 && IVUses != 1)
return false;
for (auto *User : IV->users()) {
int32_t IncOrCmpUses = User->getNumUses();
bool IsCompInst = isCompareUsedByBranch(cast<Instruction>(User));
// User can only have one or two uses.
if (IncOrCmpUses != 2 && IncOrCmpUses != 1)
return false;
// Case 1
if (IVUses == 1) {
// The only user must be the loop increment.
// The loop increment must have two uses.
if (IsCompInst || IncOrCmpUses != 2)
return false;
}
// Case 2
if (IVUses == 2 && IncOrCmpUses != 1)
return false;
// The users of the IV must be a binary operation or a comparison
if (auto *BO = dyn_cast<BinaryOperator>(User)) {
if (BO->getOpcode() == Instruction::Add) {
// Loop Increment
// User of Loop Increment should be either PHI or CMP
for (auto *UU : User->users()) {
if (PHINode *PN = dyn_cast<PHINode>(UU)) {
if (PN != IV)
return false;
}
// Must be a CMP or an ext (of a value with nsw) then CMP
else {
Instruction *UUser = dyn_cast<Instruction>(UU);
// Skip SExt if we are extending an nsw value
// TODO: Allow ZExt too
if (BO->hasNoSignedWrap() && UUser && UUser->hasOneUse() &&
isa<SExtInst>(UUser))
UUser = dyn_cast<Instruction>(*(UUser->user_begin()));
if (!isCompareUsedByBranch(UUser))
return false;
}
}
} else
return false;
// Compare : can only have one use, and must be branch
} else if (!IsCompInst)
return false;
}
return true;
}
// 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() && !I->getType()->isPointerTy())
continue;
if (const SCEVAddRecExpr *PHISCEV =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(&*I))) {
if (PHISCEV->getLoop() != L)
continue;
if (!PHISCEV->isAffine())
continue;
auto IncSCEV = dyn_cast<SCEVConstant>(PHISCEV->getStepRecurrence(*SE));
if (IncSCEV) {
IVToIncMap[&*I] = IncSCEV->getValue()->getSExtValue();
LLVM_DEBUG(dbgs() << "LRR: Possible IV: " << *I << " = " << *PHISCEV
<< "\n");
if (isLoopControlIV(L, &*I)) {
assert(!LoopControlIV && "Found two loop control only IV");
LoopControlIV = &(*I);
LLVM_DEBUG(dbgs() << "LRR: Possible loop control only IV: " << *I
<< " = " << *PHISCEV << "\n");
} else
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();
if (C->user_empty())
return;
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;
LLVM_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 (Instruction *Root : Roots)
collectInLoopUserSet(Root, Exclude, Final, Users);
}
static bool isUnorderedLoadStore(Instruction *I) {
if (LoadInst *LI = dyn_cast<LoadInst>(I))
return LI->isUnordered();
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
return !MI->isVolatile();
return false;
}
/// Return true if IVU is a "simple" arithmetic operation.
/// This is used for narrowing the search space for DAGRoots; only arithmetic
/// and GEPs can be part of a DAGRoot.
static bool isSimpleArithmeticOp(User *IVU) {
if (Instruction *I = dyn_cast<Instruction>(IVU)) {
switch (I->getOpcode()) {
default: return false;
case Instruction::Add:
case Instruction::Sub:
case Instruction::Mul:
case Instruction::Shl:
case Instruction::AShr:
case Instruction::LShr:
case Instruction::GetElementPtr:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
return true;
}
}
return false;
}
static bool isLoopIncrement(User *U, Instruction *IV) {
BinaryOperator *BO = dyn_cast<BinaryOperator>(U);
if ((BO && BO->getOpcode() != Instruction::Add) ||
(!BO && !isa<GetElementPtrInst>(U)))
return false;
for (auto *UU : U->users()) {
PHINode *PN = dyn_cast<PHINode>(UU);
if (PN && PN == IV)
return true;
}
return false;
}
bool LoopReroll::DAGRootTracker::
collectPossibleRoots(Instruction *Base, std::map<int64_t,Instruction*> &Roots) {
SmallInstructionVector BaseUsers;
for (auto *I : Base->users()) {
ConstantInt *CI = nullptr;
if (isLoopIncrement(I, IV)) {
LoopIncs.push_back(cast<Instruction>(I));
continue;
}
// The root nodes must be either GEPs, ORs or ADDs.
if (auto *BO = dyn_cast<BinaryOperator>(I)) {
if (BO->getOpcode() == Instruction::Add ||
BO->getOpcode() == Instruction::Or)
CI = dyn_cast<ConstantInt>(BO->getOperand(1));
} else if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
Value *LastOperand = GEP->getOperand(GEP->getNumOperands()-1);
CI = dyn_cast<ConstantInt>(LastOperand);
}
if (!CI) {
if (Instruction *II = dyn_cast<Instruction>(I)) {
BaseUsers.push_back(II);
continue;
} else {
LLVM_DEBUG(dbgs() << "LRR: Aborting due to non-instruction: " << *I
<< "\n");
return false;
}
}
int64_t V = std::abs(CI->getValue().getSExtValue());
if (Roots.find(V) != Roots.end())
// No duplicates, please.
return false;
Roots[V] = cast<Instruction>(I);
}
// Make sure we have at least two roots.
if (Roots.empty() || (Roots.size() == 1 && BaseUsers.empty()))
return false;
// If we found non-loop-inc, non-root users of Base, assume they are
// for the zeroth root index. This is because "add %a, 0" gets optimized
// away.
if (BaseUsers.size()) {
if (Roots.find(0) != Roots.end()) {
LLVM_DEBUG(dbgs() << "LRR: Multiple roots found for base - aborting!\n");
return false;
}
Roots[0] = Base;
}
// Calculate the number of users of the base, or lowest indexed, iteration.
unsigned NumBaseUses = BaseUsers.size();
if (NumBaseUses == 0)
NumBaseUses = Roots.begin()->second->getNumUses();
// Check that every node has the same number of users.
for (auto &KV : Roots) {
if (KV.first == 0)
continue;
if (!KV.second->hasNUses(NumBaseUses)) {
LLVM_DEBUG(dbgs() << "LRR: Aborting - Root and Base #users not the same: "
<< "#Base=" << NumBaseUses
<< ", #Root=" << KV.second->getNumUses() << "\n");
return false;
}
}
return true;
}
void LoopReroll::DAGRootTracker::
findRootsRecursive(Instruction *I, SmallInstructionSet SubsumedInsts) {
// Does the user look like it could be part of a root set?
// All its users must be simple arithmetic ops.
if (I->hasNUsesOrMore(IL_MaxRerollIterations + 1))
return;
if (I != IV && findRootsBase(I, SubsumedInsts))
return;
SubsumedInsts.insert(I);
for (User *V : I->users()) {
Instruction *I = cast<Instruction>(V);
if (is_contained(LoopIncs, I))
continue;
if (!isSimpleArithmeticOp(I))
continue;
// The recursive call makes a copy of SubsumedInsts.
findRootsRecursive(I, SubsumedInsts);
}
}
bool LoopReroll::DAGRootTracker::validateRootSet(DAGRootSet &DRS) {
if (DRS.Roots.empty())
return false;
// If the value of the base instruction is used outside the loop, we cannot
// reroll the loop. Check for other root instructions is unnecessary because
// they don't match any base instructions if their values are used outside.
if (hasUsesOutsideLoop(DRS.BaseInst, L))
return false;
// Consider a DAGRootSet with N-1 roots (so N different values including
// BaseInst).
// Define d = Roots[0] - BaseInst, which should be the same as
// Roots[I] - Roots[I-1] for all I in [1..N).
// Define D = BaseInst@J - BaseInst@J-1, where "@J" means the value at the
// loop iteration J.
//
// Now, For the loop iterations to be consecutive:
// D = d * N
const auto *ADR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(DRS.BaseInst));
if (!ADR)
return false;
// Check that the first root is evenly spaced.
unsigned N = DRS.Roots.size() + 1;
const SCEV *StepSCEV = SE->getMinusSCEV(SE->getSCEV(DRS.Roots[0]), ADR);
const SCEV *ScaleSCEV = SE->getConstant(StepSCEV->getType(), N);
if (ADR->getStepRecurrence(*SE) != SE->getMulExpr(StepSCEV, ScaleSCEV))
return false;
// Check that the remainling roots are evenly spaced.
for (unsigned i = 1; i < N - 1; ++i) {
const SCEV *NewStepSCEV = SE->getMinusSCEV(SE->getSCEV(DRS.Roots[i]),
SE->getSCEV(DRS.Roots[i-1]));
if (NewStepSCEV != StepSCEV)
return false;
}
return true;
}
bool LoopReroll::DAGRootTracker::
findRootsBase(Instruction *IVU, SmallInstructionSet SubsumedInsts) {
// The base of a RootSet must be an AddRec, so it can be erased.
const auto *IVU_ADR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IVU));
if (!IVU_ADR || IVU_ADR->getLoop() != L)
return false;
std::map<int64_t, Instruction*> V;
if (!collectPossibleRoots(IVU, V))
return false;
// If we didn't get a root for index zero, then IVU must be
// subsumed.
if (V.find(0) == V.end())
SubsumedInsts.insert(IVU);
// Partition the vector into monotonically increasing indexes.
DAGRootSet DRS;
DRS.BaseInst = nullptr;
SmallVector<DAGRootSet, 16> PotentialRootSets;
for (auto &KV : V) {
if (!DRS.BaseInst) {
DRS.BaseInst = KV.second;
DRS.SubsumedInsts = SubsumedInsts;
} else if (DRS.Roots.empty()) {
DRS.Roots.push_back(KV.second);
} else if (V.find(KV.first - 1) != V.end()) {
DRS.Roots.push_back(KV.second);
} else {
// Linear sequence terminated.
if (!validateRootSet(DRS))
return false;
// Construct a new DAGRootSet with the next sequence.
PotentialRootSets.push_back(DRS);
DRS.BaseInst = KV.second;
DRS.Roots.clear();
}
}
if (!validateRootSet(DRS))
return false;
PotentialRootSets.push_back(DRS);
RootSets.append(PotentialRootSets.begin(), PotentialRootSets.end());
return true;
}
bool LoopReroll::DAGRootTracker::findRoots() {
Inc = IVToIncMap[IV];
assert(RootSets.empty() && "Unclean state!");
if (std::abs(Inc) == 1) {
for (auto *IVU : IV->users()) {
if (isLoopIncrement(IVU, IV))
LoopIncs.push_back(cast<Instruction>(IVU));
}
findRootsRecursive(IV, SmallInstructionSet());
LoopIncs.push_back(IV);
} else {
if (!findRootsBase(IV, SmallInstructionSet()))
return false;
}
// Ensure all sets have the same size.
if (RootSets.empty()) {
LLVM_DEBUG(dbgs() << "LRR: Aborting because no root sets found!\n");
return false;
}
for (auto &V : RootSets) {
if (V.Roots.empty() || V.Roots.size() != RootSets[0].Roots.size()) {
LLVM_DEBUG(
dbgs()
<< "LRR: Aborting because not all root sets have the same size\n");
return false;
}
}
Scale = RootSets[0].Roots.size() + 1;
if (Scale > IL_MaxRerollIterations) {
LLVM_DEBUG(dbgs() << "LRR: Aborting - too many iterations found. "
<< "#Found=" << Scale
<< ", #Max=" << IL_MaxRerollIterations << "\n");
return false;
}
LLVM_DEBUG(dbgs() << "LRR: Successfully found roots: Scale=" << Scale
<< "\n");
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;
for (auto &DRS : RootSets) {
Exclude.insert(DRS.Roots.begin(), DRS.Roots.end());
Exclude.insert(DRS.SubsumedInsts.begin(), DRS.SubsumedInsts.end());
Exclude.insert(DRS.BaseInst);
}
Exclude.insert(LoopIncs.begin(), LoopIncs.end());
for (auto &DRS : RootSets) {
DenseSet<Instruction*> VBase;
collectInLoopUserSet(DRS.BaseInst, Exclude, PossibleRedSet, VBase);
for (auto *I : VBase) {
Uses[I].set(0);
}
unsigned Idx = 1;
for (auto *Root : DRS.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()) {
LLVM_DEBUG(dbgs() << "LRR: Aborting - use sets are different sizes\n");
return false;
}
for (auto *I : V) {
Uses[I].set(Idx);
}
++Idx;
}
// Make sure our subsumed instructions are remembered too.
for (auto *I : DRS.SubsumedInsts) {
Uses[I].set(IL_All);
}
}
// Make sure the loop increments are also accounted for.
Exclude.clear();
for (auto &DRS : RootSets) {
Exclude.insert(DRS.Roots.begin(), DRS.Roots.end());
Exclude.insert(DRS.SubsumedInsts.begin(), DRS.SubsumedInsts.end());
Exclude.insert(DRS.BaseInst);
}
DenseSet<Instruction*> V;
collectInLoopUserSet(LoopIncs, Exclude, PossibleRedSet, V);
for (auto *I : V) {
Uses[I].set(IL_All);
}
return true;
}
/// Get the next instruction in "In" that is a member of set Val.
/// Start searching from StartI, and do not return anything in Exclude.
/// If StartI is not given, start from In.begin().
LoopReroll::DAGRootTracker::UsesTy::iterator
LoopReroll::DAGRootTracker::nextInstr(int Val, UsesTy &In,
const SmallInstructionSet &Exclude,
UsesTy::iterator *StartI) {
UsesTy::iterator I = StartI ? *StartI : In.begin();
while (I != In.end() && (I->second.test(Val) == 0 ||
Exclude.count(I->first) != 0))
++I;
return I;
}
bool LoopReroll::DAGRootTracker::isBaseInst(Instruction *I) {
for (auto &DRS : RootSets) {
if (DRS.BaseInst == I)
return true;
}
return false;
}
bool LoopReroll::DAGRootTracker::isRootInst(Instruction *I) {
for (auto &DRS : RootSets) {
if (is_contained(DRS.Roots, I))
return true;
}
return false;
}
/// Return true if instruction I depends on any instruction between
/// Start and End.
bool LoopReroll::DAGRootTracker::instrDependsOn(Instruction *I,
UsesTy::iterator Start,
UsesTy::iterator End) {
for (auto *U : I->users()) {
for (auto It = Start; It != End; ++It)
if (U == It->first)
return true;
}
return false;
}
static bool isIgnorableInst(const Instruction *I) {
if (isa<DbgInfoIntrinsic>(I))
return true;
const IntrinsicInst* II = dyn_cast<IntrinsicInst>(I);
if (!II)
return false;
switch (II->getIntrinsicID()) {
default:
return false;
case Intrinsic::annotation:
case Intrinsic::ptr_annotation:
case Intrinsic::var_annotation:
// TODO: the following intrinsics may also be whitelisted:
// lifetime_start, lifetime_end, invariant_start, invariant_end
return true;
}
return false;
}
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_All);
}
// Make sure we mark loop-control-only PHIs as used in all iterations. See
// comment above LoopReroll::isLoopControlIV for more information.
BasicBlock *Header = L->getHeader();
if (LoopControlIV && LoopControlIV != IV) {
for (auto *U : LoopControlIV->users()) {
Instruction *IVUser = dyn_cast<Instruction>(U);
// IVUser could be loop increment or compare
Uses[IVUser].set(IL_All);
for (auto *UU : IVUser->users()) {
Instruction *UUser = dyn_cast<Instruction>(UU);
// UUser could be compare, PHI or branch
Uses[UUser].set(IL_All);
// Skip SExt
if (isa<SExtInst>(UUser)) {
UUser = dyn_cast<Instruction>(*(UUser->user_begin()));
Uses[UUser].set(IL_All);
}
// Is UUser a compare instruction?
if (UU->hasOneUse()) {
Instruction *BI = dyn_cast<BranchInst>(*UUser->user_begin());
if (BI == cast<BranchInst>(Header->getTerminator()))
Uses[BI].set(IL_All);
}
}
}
}
// Make sure all instructions in the loop are in one and only one
// set.
for (auto &KV : Uses) {
if (KV.second.count() != 1 && !isIgnorableInst(KV.first)) {
LLVM_DEBUG(
dbgs() << "LRR: Aborting - instruction is not used in 1 iteration: "
<< *KV.first << " (#uses=" << KV.second.count() << ")\n");
return false;
}
}
LLVM_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.
SmallInstructionSet Visited;
auto BaseIt = nextInstr(0, Uses, Visited);
auto RootIt = nextInstr(Iter, Uses, Visited);
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 (isBaseInst(BaseInst)) {
Visited.insert(BaseInst);
BaseIt = nextInstr(0, Uses, Visited);
Continue = true;
}
if (isRootInst(RootInst)) {
LastRootIt = RootIt;
Visited.insert(RootInst);
RootIt = nextInstr(Iter, Uses, Visited);
Continue = true;
}
if (Continue) continue;
if (!BaseInst->isSameOperationAs(RootInst)) {
// Last chance saloon. We don't try and solve the full isomorphism
// problem, but try and at least catch the case where two instructions
// *of different types* are round the wrong way. We won't be able to
// efficiently tell, given two ADD instructions, which way around we
// should match them, but given an ADD and a SUB, we can at least infer
// which one is which.
//
// This should allow us to deal with a greater subset of the isomorphism
// problem. It does however change a linear algorithm into a quadratic
// one, so limit the number of probes we do.
auto TryIt = RootIt;
unsigned N = NumToleratedFailedMatches;
while (TryIt != Uses.end() &&
!BaseInst->isSameOperationAs(TryIt->first) &&
N--) {
++TryIt;
TryIt = nextInstr(Iter, Uses, Visited, &TryIt);
}
if (TryIt == Uses.end() || TryIt == RootIt ||
instrDependsOn(TryIt->first, RootIt, TryIt)) {
LLVM_DEBUG(dbgs() << "LRR: iteration root match failed at "
<< *BaseInst << " vs. " << *RootInst << "\n");
return false;
}
RootIt = TryIt;
RootInst = TryIt->first;
}
// All instructions between the last root and this root
// may belong to some other iteration. If they belong to a
// future iteration, then they're dangerous to alias with.
//
// Note that because we allow a limited amount of flexibility in the order
// that we visit nodes, LastRootIt might be *before* RootIt, in which
// case we've already checked this set of instructions so we shouldn't
// do anything.
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) && !isUnorderedLoadStore(I) &&
!isSafeToSpeculativelyExecute(I))
// Intervening instructions cause side effects.
FutureSideEffects = true;
}
// 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) {
LLVM_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)) {
LLVM_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 (unordered) load/store dependencies.
if (FutureSideEffects && ((!isUnorderedLoadStore(BaseInst) &&
!isSafeToSpeculativelyExecute(BaseInst)) ||
(!isUnorderedLoadStore(RootInst) &&
!isSafeToSpeculativelyExecute(RootInst)))) {
LLVM_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 {
for (auto &DRS : RootSets) {
if (DRS.Roots[Iter-1] == (Instruction*) Op2) {
Op2 = DRS.BaseInst;
break;
}
}
}
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 {
LLVM_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))) {
LLVM_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;
Visited.insert(BaseInst);
Visited.insert(RootInst);
BaseIt = nextInstr(0, Uses, Visited);
RootIt = nextInstr(Iter, Uses, Visited);
}
assert(BaseIt == Uses.end() && RootIt == Uses.end() &&
"Mismatched set sizes!");
}
LLVM_DEBUG(dbgs() << "LRR: Matched all iteration increments for " << *IV
<< "\n");
return true;
}
void LoopReroll::DAGRootTracker::replace(const SCEV *BackedgeTakenCount) {
BasicBlock *Header = L->getHeader();
// Compute the start and increment for each BaseInst before we start erasing
// instructions.
SmallVector<const SCEV *, 8> StartExprs;
SmallVector<const SCEV *, 8> IncrExprs;
for (auto &DRS : RootSets) {
const SCEVAddRecExpr *IVSCEV =
cast<SCEVAddRecExpr>(SE->getSCEV(DRS.BaseInst));
StartExprs.push_back(IVSCEV->getStart());
IncrExprs.push_back(SE->getMinusSCEV(SE->getSCEV(DRS.Roots[0]), IVSCEV));
}
// Remove instructions associated with non-base iterations.
for (BasicBlock::reverse_iterator J = Header->rbegin(), JE = Header->rend();
J != JE;) {
unsigned I = Uses[&*J].find_first();
if (I > 0 && I < IL_All) {
LLVM_DEBUG(dbgs() << "LRR: removing: " << *J << "\n");
J++->eraseFromParent();
continue;
}
++J;
}
// Rewrite each BaseInst using SCEV.
for (size_t i = 0, e = RootSets.size(); i != e; ++i)
// Insert the new induction variable.
replaceIV(RootSets[i], StartExprs[i], IncrExprs[i]);
{ // Limit the lifetime of SCEVExpander.
BranchInst *BI = cast<BranchInst>(Header->getTerminator());
const DataLayout &DL = Header->getModule()->getDataLayout();
SCEVExpander Expander(*SE, DL, "reroll");
auto Zero = SE->getZero(BackedgeTakenCount->getType());
auto One = SE->getOne(BackedgeTakenCount->getType());
auto NewIVSCEV = SE->getAddRecExpr(Zero, One, L, SCEV::FlagAnyWrap);
Value *NewIV =
Expander.expandCodeFor(NewIVSCEV, BackedgeTakenCount->getType(),
Header->getFirstNonPHIOrDbg());
// FIXME: This arithmetic can overflow.
auto TripCount = SE->getAddExpr(BackedgeTakenCount, One);
auto ScaledTripCount = SE->getMulExpr(
TripCount, SE->getConstant(BackedgeTakenCount->getType(), Scale));
auto ScaledBECount = SE->getMinusSCEV(ScaledTripCount, One);
Value *TakenCount =
Expander.expandCodeFor(ScaledBECount, BackedgeTakenCount->getType(),
Header->getFirstNonPHIOrDbg());
Value *Cond =
new ICmpInst(BI, CmpInst::ICMP_EQ, NewIV, TakenCount, "exitcond");
BI->setCondition(Cond);
if (BI->getSuccessor(1) != Header)
BI->swapSuccessors();
}
SimplifyInstructionsInBlock(Header, TLI);
DeleteDeadPHIs(Header, TLI);
}
void LoopReroll::DAGRootTracker::replaceIV(DAGRootSet &DRS,
const SCEV *Start,
const SCEV *IncrExpr) {
BasicBlock *Header = L->getHeader();
Instruction *Inst = DRS.BaseInst;
const SCEV *NewIVSCEV =
SE->getAddRecExpr(Start, IncrExpr, L, SCEV::FlagAnyWrap);
{ // Limit the lifetime of SCEVExpander.
const DataLayout &DL = Header->getModule()->getDataLayout();
SCEVExpander Expander(*SE, DL, "reroll");
Value *NewIV = Expander.expandCodeFor(NewIVSCEV, Inst->getType(),
Header->getFirstNonPHIOrDbg());
for (auto &KV : Uses)
if (KV.second.find_first() == 0)
KV.first->replaceUsesOfWith(Inst, NewIV);
}
}
// 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 (int i : Reds) {
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()) {
LLVM_DEBUG(dbgs() << "LRR: Out-of-order non-associative reduction: "
<< J << "\n");
return false;
}
if (Iter != PrevIter) {
if (Count != BaseCount) {
LLVM_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 (int i : Reds) {
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 (Instruction *User : Users)
User->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 *BackedgeTakenCount,
ReductionTracker &Reductions) {
DAGRootTracker DAGRoots(this, L, IV, SE, AA, TLI, DT, LI, PreserveLCSSA,
IVToIncMap, LoopControlIV);
if (!DAGRoots.findRoots())
return false;
LLVM_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(BackedgeTakenCount);
++NumRerolledLoops;
return true;
}
bool LoopReroll::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipLoop(L))
return false;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(
*L->getHeader()->getParent());
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
BasicBlock *Header = L->getHeader();
LLVM_DEBUG(dbgs() << "LRR: F[" << Header->getParent()->getName() << "] Loop %"
<< Header->getName() << " (" << L->getNumBlocks()
<< " block(s))\n");
// For now, we'll handle only single BB loops.
if (L->getNumBlocks() > 1)
return false;
if (!SE->hasLoopInvariantBackedgeTakenCount(L))
return false;
const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
LLVM_DEBUG(dbgs() << "\n Before Reroll:\n" << *(L->getHeader()) << "\n");
LLVM_DEBUG(dbgs() << "LRR: backedge-taken count = " << *BackedgeTakenCount
<< "\n");
// First, we need to find the induction variable with respect to which we can
// reroll (there may be several possible options).
SmallInstructionVector PossibleIVs;
IVToIncMap.clear();
LoopControlIV = nullptr;
collectPossibleIVs(L, PossibleIVs);
if (PossibleIVs.empty()) {
LLVM_DEBUG(dbgs() << "LRR: No possible IVs found\n");
return false;
}
ReductionTracker Reductions;
collectPossibleReductions(L, Reductions);
bool Changed = false;
// For each possible IV, collect the associated possible set of 'root' nodes
// (i+1, i+2, etc.).
for (Instruction *PossibleIV : PossibleIVs)
if (reroll(PossibleIV, L, Header, BackedgeTakenCount, Reductions)) {
Changed = true;
break;
}
LLVM_DEBUG(dbgs() << "\n After Reroll:\n" << *(L->getHeader()) << "\n");
// Trip count of L has changed so SE must be re-evaluated.
if (Changed)
SE->forgetLoop(L);
return Changed;
}
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