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

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//===- LoopStrengthReduce.cpp - Strength Reduce GEPs in Loops -------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by Nate Begeman and is distributed under the
// University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs a strength reduction on array references inside loops that
// have as one or more of their components the loop induction variable. This is
// accomplished by creating a new Value to hold the initial value of the array
// access for the first iteration, and then creating a new GEP instruction in
// the loop to increment the value by the appropriate amount.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-reduce"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include <algorithm>
#include <set>
using namespace llvm;
namespace {
Statistic<> NumReduced ("loop-reduce", "Number of GEPs strength reduced");
Statistic<> NumInserted("loop-reduce", "Number of PHIs inserted");
/// IVStrideUse - Keep track of one use of a strided induction variable, where
/// the stride is stored externally. The Offset member keeps track of the
/// offset from the IV, User is the actual user of the operand, and 'Operand'
/// is the operand # of the User that is the use.
struct IVStrideUse {
SCEVHandle Offset;
Instruction *User;
Value *OperandValToReplace;
// isUseOfPostIncrementedValue - True if this should use the
// post-incremented version of this IV, not the preincremented version.
// This can only be set in special cases, such as the terminating setcc
// instruction for a loop.
bool isUseOfPostIncrementedValue;
IVStrideUse(const SCEVHandle &Offs, Instruction *U, Value *O)
: Offset(Offs), User(U), OperandValToReplace(O),
isUseOfPostIncrementedValue(false) {}
};
/// IVUsersOfOneStride - This structure keeps track of all instructions that
/// have an operand that is based on the trip count multiplied by some stride.
/// The stride for all of these users is common and kept external to this
/// structure.
struct IVUsersOfOneStride {
/// Users - Keep track of all of the users of this stride as well as the
/// initial value and the operand that uses the IV.
std::vector<IVStrideUse> Users;
void addUser(const SCEVHandle &Offset,Instruction *User, Value *Operand) {
Users.push_back(IVStrideUse(Offset, User, Operand));
}
};
class LoopStrengthReduce : public FunctionPass {
LoopInfo *LI;
DominatorSet *DS;
ScalarEvolution *SE;
const TargetData *TD;
const Type *UIntPtrTy;
bool Changed;
/// MaxTargetAMSize - This is the maximum power-of-two scale value that the
/// target can handle for free with its addressing modes.
unsigned MaxTargetAMSize;
/// IVUsesByStride - Keep track of all uses of induction variables that we
/// are interested in. The key of the map is the stride of the access.
std::map<Value*, IVUsersOfOneStride> IVUsesByStride;
/// CastedValues - As we need to cast values to uintptr_t, this keeps track
/// of the casted version of each value. This is accessed by
/// getCastedVersionOf.
std::map<Value*, Value*> CastedPointers;
/// DeadInsts - Keep track of instructions we may have made dead, so that
/// we can remove them after we are done working.
std::set<Instruction*> DeadInsts;
public:
LoopStrengthReduce(unsigned MTAMS = 1)
: MaxTargetAMSize(MTAMS) {
}
virtual bool runOnFunction(Function &) {
LI = &getAnalysis<LoopInfo>();
DS = &getAnalysis<DominatorSet>();
SE = &getAnalysis<ScalarEvolution>();
TD = &getAnalysis<TargetData>();
UIntPtrTy = TD->getIntPtrType();
Changed = false;
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
runOnLoop(*I);
return Changed;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequiredID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorSet>();
AU.addRequired<TargetData>();
AU.addRequired<ScalarEvolution>();
}
/// getCastedVersionOf - Return the specified value casted to uintptr_t.
///
Value *getCastedVersionOf(Value *V);
private:
void runOnLoop(Loop *L);
bool AddUsersIfInteresting(Instruction *I, Loop *L,
std::set<Instruction*> &Processed);
SCEVHandle GetExpressionSCEV(Instruction *E, Loop *L);
void OptimizeIndvars(Loop *L);
void StrengthReduceStridedIVUsers(Value *Stride, IVUsersOfOneStride &Uses,
Loop *L, bool isOnlyStride);
void DeleteTriviallyDeadInstructions(std::set<Instruction*> &Insts);
};
RegisterOpt<LoopStrengthReduce> X("loop-reduce",
"Strength Reduce GEP Uses of Ind. Vars");
}
FunctionPass *llvm::createLoopStrengthReducePass(unsigned MaxTargetAMSize) {
return new LoopStrengthReduce(MaxTargetAMSize);
}
/// getCastedVersionOf - Return the specified value casted to uintptr_t.
///
Value *LoopStrengthReduce::getCastedVersionOf(Value *V) {
if (V->getType() == UIntPtrTy) return V;
if (Constant *CB = dyn_cast<Constant>(V))
return ConstantExpr::getCast(CB, UIntPtrTy);
Value *&New = CastedPointers[V];
if (New) return New;
BasicBlock::iterator InsertPt;
if (Argument *Arg = dyn_cast<Argument>(V)) {
// Insert into the entry of the function, after any allocas.
InsertPt = Arg->getParent()->begin()->begin();
while (isa<AllocaInst>(InsertPt)) ++InsertPt;
} else {
if (InvokeInst *II = dyn_cast<InvokeInst>(V)) {
InsertPt = II->getNormalDest()->begin();
} else {
InsertPt = cast<Instruction>(V);
++InsertPt;
}
// Do not insert casts into the middle of PHI node blocks.
while (isa<PHINode>(InsertPt)) ++InsertPt;
}
New = new CastInst(V, UIntPtrTy, V->getName(), InsertPt);
DeadInsts.insert(cast<Instruction>(New));
return New;
}
/// DeleteTriviallyDeadInstructions - If any of the instructions is the
/// specified set are trivially dead, delete them and see if this makes any of
/// their operands subsequently dead.
void LoopStrengthReduce::
DeleteTriviallyDeadInstructions(std::set<Instruction*> &Insts) {
while (!Insts.empty()) {
Instruction *I = *Insts.begin();
Insts.erase(Insts.begin());
if (isInstructionTriviallyDead(I)) {
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *U = dyn_cast<Instruction>(I->getOperand(i)))
Insts.insert(U);
SE->deleteInstructionFromRecords(I);
I->eraseFromParent();
Changed = true;
}
}
}
/// GetExpressionSCEV - Compute and return the SCEV for the specified
/// instruction.
SCEVHandle LoopStrengthReduce::GetExpressionSCEV(Instruction *Exp, Loop *L) {
GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Exp);
if (!GEP)
return SE->getSCEV(Exp);
// Analyze all of the subscripts of this getelementptr instruction, looking
// for uses that are determined by the trip count of L. First, skip all
// operands the are not dependent on the IV.
// Build up the base expression. Insert an LLVM cast of the pointer to
// uintptr_t first.
SCEVHandle GEPVal = SCEVUnknown::get(getCastedVersionOf(GEP->getOperand(0)));
gep_type_iterator GTI = gep_type_begin(GEP);
for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i, ++GTI) {
// If this is a use of a recurrence that we can analyze, and it comes before
// Op does in the GEP operand list, we will handle this when we process this
// operand.
if (const StructType *STy = dyn_cast<StructType>(*GTI)) {
const StructLayout *SL = TD->getStructLayout(STy);
unsigned Idx = cast<ConstantUInt>(GEP->getOperand(i))->getValue();
uint64_t Offset = SL->MemberOffsets[Idx];
GEPVal = SCEVAddExpr::get(GEPVal,
SCEVUnknown::getIntegerSCEV(Offset, UIntPtrTy));
} else {
Value *OpVal = getCastedVersionOf(GEP->getOperand(i));
SCEVHandle Idx = SE->getSCEV(OpVal);
uint64_t TypeSize = TD->getTypeSize(GTI.getIndexedType());
if (TypeSize != 1)
Idx = SCEVMulExpr::get(Idx,
SCEVConstant::get(ConstantUInt::get(UIntPtrTy,
TypeSize)));
GEPVal = SCEVAddExpr::get(GEPVal, Idx);
}
}
return GEPVal;
}
/// getSCEVStartAndStride - Compute the start and stride of this expression,
/// returning false if the expression is not a start/stride pair, or true if it
/// is. The stride must be a loop invariant expression, but the start may be
/// a mix of loop invariant and loop variant expressions.
static bool getSCEVStartAndStride(const SCEVHandle &SH, Loop *L,
SCEVHandle &Start, Value *&Stride) {
SCEVHandle TheAddRec = Start; // Initialize to zero.
// If the outer level is an AddExpr, the operands are all start values except
// for a nested AddRecExpr.
if (SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) {
for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i)
if (SCEVAddRecExpr *AddRec =
dyn_cast<SCEVAddRecExpr>(AE->getOperand(i))) {
if (AddRec->getLoop() == L)
TheAddRec = SCEVAddExpr::get(AddRec, TheAddRec);
else
return false; // Nested IV of some sort?
} else {
Start = SCEVAddExpr::get(Start, AE->getOperand(i));
}
} else if (SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(SH)) {
TheAddRec = SH;
} else {
return false; // not analyzable.
}
SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(TheAddRec);
if (!AddRec || AddRec->getLoop() != L) return false;
// FIXME: Generalize to non-affine IV's.
if (!AddRec->isAffine()) return false;
Start = SCEVAddExpr::get(Start, AddRec->getOperand(0));
// FIXME: generalize to IV's with more complex strides (must emit stride
// expression outside of loop!)
if (!isa<SCEVConstant>(AddRec->getOperand(1)))
return false;
SCEVConstant *StrideC = cast<SCEVConstant>(AddRec->getOperand(1));
Stride = StrideC->getValue();
assert(Stride->getType()->isUnsigned() &&
"Constants should be canonicalized to unsigned!");
return true;
}
/// AddUsersIfInteresting - Inspect the specified instruction. If it is a
/// reducible SCEV, recursively add its users to the IVUsesByStride set and
/// return true. Otherwise, return false.
bool LoopStrengthReduce::AddUsersIfInteresting(Instruction *I, Loop *L,
std::set<Instruction*> &Processed) {
if (I->getType() == Type::VoidTy) return false;
if (!Processed.insert(I).second)
return true; // Instruction already handled.
// Get the symbolic expression for this instruction.
SCEVHandle ISE = GetExpressionSCEV(I, L);
if (isa<SCEVCouldNotCompute>(ISE)) return false;
// Get the start and stride for this expression.
SCEVHandle Start = SCEVUnknown::getIntegerSCEV(0, ISE->getType());
Value *Stride = 0;
if (!getSCEVStartAndStride(ISE, L, Start, Stride))
return false; // Non-reducible symbolic expression, bail out.
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;++UI){
Instruction *User = cast<Instruction>(*UI);
// Do not infinitely recurse on PHI nodes.
if (isa<PHINode>(User) && User->getParent() == L->getHeader())
continue;
// If this is an instruction defined in a nested loop, or outside this loop,
// don't recurse into it.
bool AddUserToIVUsers = false;
if (LI->getLoopFor(User->getParent()) != L) {
DEBUG(std::cerr << "FOUND USER in nested loop: " << *User
<< " OF SCEV: " << *ISE << "\n");
AddUserToIVUsers = true;
} else if (!AddUsersIfInteresting(User, L, Processed)) {
DEBUG(std::cerr << "FOUND USER: " << *User
<< " OF SCEV: " << *ISE << "\n");
AddUserToIVUsers = true;
}
if (AddUserToIVUsers) {
// Okay, we found a user that we cannot reduce. Analyze the instruction
// and decide what to do with it.
IVUsesByStride[Stride].addUser(Start, User, I);
}
}
return true;
}
namespace {
/// BasedUser - For a particular base value, keep information about how we've
/// partitioned the expression so far.
struct BasedUser {
/// Inst - The instruction using the induction variable.
Instruction *Inst;
/// OperandValToReplace - The operand value of Inst to replace with the
/// EmittedBase.
Value *OperandValToReplace;
/// Imm - The immediate value that should be added to the base immediately
/// before Inst, because it will be folded into the imm field of the
/// instruction.
SCEVHandle Imm;
/// EmittedBase - The actual value* to use for the base value of this
/// operation. This is null if we should just use zero so far.
Value *EmittedBase;
// isUseOfPostIncrementedValue - True if this should use the
// post-incremented version of this IV, not the preincremented version.
// This can only be set in special cases, such as the terminating setcc
// instruction for a loop.
bool isUseOfPostIncrementedValue;
BasedUser(Instruction *I, Value *Op, const SCEVHandle &IMM, bool iUOPIV)
: Inst(I), OperandValToReplace(Op), Imm(IMM), EmittedBase(0),
isUseOfPostIncrementedValue(iUOPIV) {}
// Once we rewrite the code to insert the new IVs we want, update the
// operands of Inst to use the new expression 'NewBase', with 'Imm' added
// to it.
void RewriteInstructionToUseNewBase(Value *NewBase, SCEVExpander &Rewriter);
// No need to compare these.
bool operator<(const BasedUser &BU) const { return 0; }
void dump() const;
};
}
void BasedUser::dump() const {
std::cerr << " Imm=" << *Imm;
if (EmittedBase)
std::cerr << " EB=" << *EmittedBase;
std::cerr << " Inst: " << *Inst;
}
// Once we rewrite the code to insert the new IVs we want, update the
// operands of Inst to use the new expression 'NewBase', with 'Imm' added
// to it.
void BasedUser::RewriteInstructionToUseNewBase(Value *NewBase,
SCEVExpander &Rewriter) {
if (!isa<PHINode>(Inst)) {
SCEVHandle NewValSCEV = SCEVAddExpr::get(SCEVUnknown::get(NewBase), Imm);
Value *NewVal = Rewriter.expandCodeFor(NewValSCEV, Inst,
OperandValToReplace->getType());
// Replace the use of the operand Value with the new Phi we just created.
Inst->replaceUsesOfWith(OperandValToReplace, NewVal);
DEBUG(std::cerr << " CHANGED: IMM =" << *Imm << " Inst = " << *Inst);
return;
}
// PHI nodes are more complex. We have to insert one copy of the NewBase+Imm
// expression into each operand block that uses it.
PHINode *PN = cast<PHINode>(Inst);
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if (PN->getIncomingValue(i) == OperandValToReplace) {
// FIXME: this should split any critical edges.
// Insert the code into the end of the predecessor block.
BasicBlock::iterator InsertPt = PN->getIncomingBlock(i)->getTerminator();
SCEVHandle NewValSCEV = SCEVAddExpr::get(SCEVUnknown::get(NewBase), Imm);
Value *NewVal = Rewriter.expandCodeFor(NewValSCEV, InsertPt,
OperandValToReplace->getType());
// Replace the use of the operand Value with the new Phi we just created.
PN->setIncomingValue(i, NewVal);
Rewriter.clear();
}
}
DEBUG(std::cerr << " CHANGED: IMM =" << *Imm << " Inst = " << *Inst);
}
/// isTargetConstant - Return true if the following can be referenced by the
/// immediate field of a target instruction.
static bool isTargetConstant(const SCEVHandle &V) {
// FIXME: Look at the target to decide if &GV is a legal constant immediate.
if (isa<SCEVConstant>(V)) return true;
return false; // ENABLE this for x86
if (SCEVUnknown *SU = dyn_cast<SCEVUnknown>(V))
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(SU->getValue()))
if (CE->getOpcode() == Instruction::Cast)
if (isa<GlobalValue>(CE->getOperand(0)))
// FIXME: should check to see that the dest is uintptr_t!
return true;
return false;
}
/// MoveImmediateValues - Look at Val, and pull out any additions of constants
/// that can fit into the immediate field of instructions in the target.
/// Accumulate these immediate values into the Imm value.
static void MoveImmediateValues(SCEVHandle &Val, SCEVHandle &Imm,
bool isAddress, Loop *L) {
if (SCEVAddExpr *SAE = dyn_cast<SCEVAddExpr>(Val)) {
std::vector<SCEVHandle> NewOps;
NewOps.reserve(SAE->getNumOperands());
for (unsigned i = 0; i != SAE->getNumOperands(); ++i)
if (isAddress && isTargetConstant(SAE->getOperand(i))) {
Imm = SCEVAddExpr::get(Imm, SAE->getOperand(i));
} else if (!SAE->getOperand(i)->isLoopInvariant(L)) {
// If this is a loop-variant expression, it must stay in the immediate
// field of the expression.
Imm = SCEVAddExpr::get(Imm, SAE->getOperand(i));
} else {
NewOps.push_back(SAE->getOperand(i));
}
if (NewOps.empty())
Val = SCEVUnknown::getIntegerSCEV(0, Val->getType());
else
Val = SCEVAddExpr::get(NewOps);
return;
} else if (SCEVAddRecExpr *SARE = dyn_cast<SCEVAddRecExpr>(Val)) {
// Try to pull immediates out of the start value of nested addrec's.
SCEVHandle Start = SARE->getStart();
MoveImmediateValues(Start, Imm, isAddress, L);
if (Start != SARE->getStart()) {
std::vector<SCEVHandle> Ops(SARE->op_begin(), SARE->op_end());
Ops[0] = Start;
Val = SCEVAddRecExpr::get(Ops, SARE->getLoop());
}
return;
}
// Loop-variant expressions must stay in the immediate field of the
// expression.
if ((isAddress && isTargetConstant(Val)) ||
!Val->isLoopInvariant(L)) {
Imm = SCEVAddExpr::get(Imm, Val);
Val = SCEVUnknown::getIntegerSCEV(0, Val->getType());
return;
}
// Otherwise, no immediates to move.
}
/// StrengthReduceStridedIVUsers - Strength reduce all of the users of a single
/// stride of IV. All of the users may have different starting values, and this
/// may not be the only stride (we know it is if isOnlyStride is true).
void LoopStrengthReduce::StrengthReduceStridedIVUsers(Value *Stride,
IVUsersOfOneStride &Uses,
Loop *L,
bool isOnlyStride) {
// Transform our list of users and offsets to a bit more complex table. In
// this new vector, the first entry for each element is the base of the
// strided access, and the second is the BasedUser object for the use. We
// progressively move information from the first to the second entry, until we
// eventually emit the object.
std::vector<std::pair<SCEVHandle, BasedUser> > UsersToProcess;
UsersToProcess.reserve(Uses.Users.size());
SCEVHandle ZeroBase = SCEVUnknown::getIntegerSCEV(0,
Uses.Users[0].Offset->getType());
for (unsigned i = 0, e = Uses.Users.size(); i != e; ++i)
UsersToProcess.push_back(std::make_pair(Uses.Users[i].Offset,
BasedUser(Uses.Users[i].User,
Uses.Users[i].OperandValToReplace,
ZeroBase,
Uses.Users[i].isUseOfPostIncrementedValue)));
// First pass, figure out what we can represent in the immediate fields of
// instructions. If we can represent anything there, move it to the imm
// fields of the BasedUsers.
for (unsigned i = 0, e = UsersToProcess.size(); i != e; ++i) {
// Addressing modes can be folded into loads and stores. Be careful that
// the store is through the expression, not of the expression though.
bool isAddress = isa<LoadInst>(UsersToProcess[i].second.Inst);
if (StoreInst *SI = dyn_cast<StoreInst>(UsersToProcess[i].second.Inst))
if (SI->getOperand(1) == UsersToProcess[i].second.OperandValToReplace)
isAddress = true;
MoveImmediateValues(UsersToProcess[i].first, UsersToProcess[i].second.Imm,
isAddress, L);
assert(UsersToProcess[i].first->isLoopInvariant(L) &&
"Base value is not loop invariant!");
}
SCEVExpander Rewriter(*SE, *LI);
SCEVExpander PreheaderRewriter(*SE, *LI);
BasicBlock *Preheader = L->getLoopPreheader();
Instruction *PreInsertPt = Preheader->getTerminator();
Instruction *PhiInsertBefore = L->getHeader()->begin();
assert(isa<PHINode>(PhiInsertBefore) &&
"How could this loop have IV's without any phis?");
PHINode *SomeLoopPHI = cast<PHINode>(PhiInsertBefore);
assert(SomeLoopPHI->getNumIncomingValues() == 2 &&
"This loop isn't canonicalized right");
BasicBlock *LatchBlock =
SomeLoopPHI->getIncomingBlock(SomeLoopPHI->getIncomingBlock(0) == Preheader);
DEBUG(std::cerr << "INSERTING IVs of STRIDE " << *Stride << ":\n");
// FIXME: This loop needs increasing levels of intelligence.
// STAGE 0: just emit everything as its own base.
// STAGE 1: factor out common vars from bases, and try and push resulting
// constants into Imm field. <-- We are here
// STAGE 2: factor out large constants to try and make more constants
// acceptable for target loads and stores.
// Sort by the base value, so that all IVs with identical bases are next to
// each other.
std::sort(UsersToProcess.begin(), UsersToProcess.end());
while (!UsersToProcess.empty()) {
SCEVHandle Base = UsersToProcess.front().first;
DEBUG(std::cerr << " INSERTING PHI with BASE = " << *Base << ":\n");
// Create a new Phi for this base, and stick it in the loop header.
const Type *ReplacedTy = Base->getType();
PHINode *NewPHI = new PHINode(ReplacedTy, "iv.", PhiInsertBefore);
++NumInserted;
// Emit the initial base value into the loop preheader, and add it to the
// Phi node.
Value *BaseV = PreheaderRewriter.expandCodeFor(Base, PreInsertPt,
ReplacedTy);
NewPHI->addIncoming(BaseV, Preheader);
// Emit the increment of the base value before the terminator of the loop
// latch block, and add it to the Phi node.
SCEVHandle Inc = SCEVAddExpr::get(SCEVUnknown::get(NewPHI),
SCEVUnknown::get(Stride));
Value *IncV = Rewriter.expandCodeFor(Inc, LatchBlock->getTerminator(),
ReplacedTy);
IncV->setName(NewPHI->getName()+".inc");
NewPHI->addIncoming(IncV, LatchBlock);
// Emit the code to add the immediate offset to the Phi value, just before
// the instructions that we identified as using this stride and base.
while (!UsersToProcess.empty() && UsersToProcess.front().first == Base) {
BasedUser &User = UsersToProcess.front().second;
// Clear the SCEVExpander's expression map so that we are guaranteed
// to have the code emitted where we expect it.
Rewriter.clear();
// Now that we know what we need to do, insert code before User for the
// immediate and any loop-variant expressions.
Value *NewBase = NewPHI;
// If this instruction wants to use the post-incremented value, move it
// after the post-inc and use its value instead of the PHI.
if (User.isUseOfPostIncrementedValue) {
NewBase = IncV;
User.Inst->moveBefore(LatchBlock->getTerminator());
}
User.RewriteInstructionToUseNewBase(NewBase, Rewriter);
// Mark old value we replaced as possibly dead, so that it is elminated
// if we just replaced the last use of that value.
DeadInsts.insert(cast<Instruction>(User.OperandValToReplace));
UsersToProcess.erase(UsersToProcess.begin());
++NumReduced;
}
// TODO: Next, find out which base index is the most common, pull it out.
}
// IMPORTANT TODO: Figure out how to partition the IV's with this stride, but
// different starting values, into different PHIs.
}
// OptimizeIndvars - Now that IVUsesByStride is set up with all of the indvar
// uses in the loop, look to see if we can eliminate some, in favor of using
// common indvars for the different uses.
void LoopStrengthReduce::OptimizeIndvars(Loop *L) {
// TODO: implement optzns here.
// Finally, get the terminating condition for the loop if possible. If we
// can, we want to change it to use a post-incremented version of its
// induction variable, to allow coallescing the live ranges for the IV into
// one register value.
PHINode *SomePHI = cast<PHINode>(L->getHeader()->begin());
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *LatchBlock =
SomePHI->getIncomingBlock(SomePHI->getIncomingBlock(0) == Preheader);
BranchInst *TermBr = dyn_cast<BranchInst>(LatchBlock->getTerminator());
if (!TermBr || TermBr->isUnconditional() ||
!isa<SetCondInst>(TermBr->getCondition()))
return;
SetCondInst *Cond = cast<SetCondInst>(TermBr->getCondition());
// Search IVUsesByStride to find Cond's IVUse if there is one.
IVStrideUse *CondUse = 0;
Value *CondStride = 0;
for (std::map<Value*, IVUsersOfOneStride>::iterator I =IVUsesByStride.begin(),
E = IVUsesByStride.end(); I != E && !CondUse; ++I)
for (std::vector<IVStrideUse>::iterator UI = I->second.Users.begin(),
E = I->second.Users.end(); UI != E; ++UI)
if (UI->User == Cond) {
CondUse = &*UI;
CondStride = I->first;
// NOTE: we could handle setcc instructions with multiple uses here, but
// InstCombine does it as well for simple uses, it's not clear that it
// occurs enough in real life to handle.
break;
}
if (!CondUse) return; // setcc doesn't use the IV.
// setcc stride is complex, don't mess with users.
if (!isa<ConstantInt>(CondStride)) return;
// It's possible for the setcc instruction to be anywhere in the loop, and
// possible for it to have multiple users. If it is not immediately before
// the latch block branch, move it.
if (&*++BasicBlock::iterator(Cond) != (Instruction*)TermBr) {
if (Cond->hasOneUse()) { // Condition has a single use, just move it.
Cond->moveBefore(TermBr);
} else {
// Otherwise, clone the terminating condition and insert into the loopend.
Cond = cast<SetCondInst>(Cond->clone());
Cond->setName(L->getHeader()->getName() + ".termcond");
LatchBlock->getInstList().insert(TermBr, Cond);
// Clone the IVUse, as the old use still exists!
IVUsesByStride[CondStride].addUser(CondUse->Offset, Cond,
CondUse->OperandValToReplace);
CondUse = &IVUsesByStride[CondStride].Users.back();
}
}
// If we get to here, we know that we can transform the setcc instruction to
// use the post-incremented version of the IV, allowing us to coallesce the
// live ranges for the IV correctly.
CondUse->Offset = SCEV::getMinusSCEV(CondUse->Offset,
SCEVUnknown::get(CondStride));
CondUse->isUseOfPostIncrementedValue = true;
}
void LoopStrengthReduce::runOnLoop(Loop *L) {
// First step, transform all loops nesting inside of this loop.
for (LoopInfo::iterator I = L->begin(), E = L->end(); I != E; ++I)
runOnLoop(*I);
// Next, find all uses of induction variables in this loop, and catagorize
// them by stride. Start by finding all of the PHI nodes in the header for
// this loop. If they are induction variables, inspect their uses.
std::set<Instruction*> Processed; // Don't reprocess instructions.
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
AddUsersIfInteresting(I, L, Processed);
// If we have nothing to do, return.
if (IVUsesByStride.empty()) return;
// Optimize induction variables. Some indvar uses can be transformed to use
// strides that will be needed for other purposes. A common example of this
// is the exit test for the loop, which can often be rewritten to use the
// computation of some other indvar to decide when to terminate the loop.
OptimizeIndvars(L);
// FIXME: We can widen subreg IV's here for RISC targets. e.g. instead of
// doing computation in byte values, promote to 32-bit values if safe.
// FIXME: Attempt to reuse values across multiple IV's. In particular, we
// could have something like "for(i) { foo(i*8); bar(i*16) }", which should be
// codegened as "for (j = 0;; j+=8) { foo(j); bar(j+j); }" on X86/PPC. Need
// to be careful that IV's are all the same type. Only works for intptr_t
// indvars.
// If we only have one stride, we can more aggressively eliminate some things.
bool HasOneStride = IVUsesByStride.size() == 1;
for (std::map<Value*, IVUsersOfOneStride>::iterator SI
= IVUsesByStride.begin(), E = IVUsesByStride.end(); SI != E; ++SI)
StrengthReduceStridedIVUsers(SI->first, SI->second, L, HasOneStride);
// Clean up after ourselves
if (!DeadInsts.empty()) {
DeleteTriviallyDeadInstructions(DeadInsts);
BasicBlock::iterator I = L->getHeader()->begin();
PHINode *PN;
while ((PN = dyn_cast<PHINode>(I))) {
++I; // Preincrement iterator to avoid invalidating it when deleting PN.
// At this point, we know that we have killed one or more GEP instructions.
// It is worth checking to see if the cann indvar is also dead, so that we
// can remove it as well. The requirements for the cann indvar to be
// considered dead are:
// 1. the cann indvar has one use
// 2. the use is an add instruction
// 3. the add has one use
// 4. the add is used by the cann indvar
// If all four cases above are true, then we can remove both the add and
// the cann indvar.
// FIXME: this needs to eliminate an induction variable even if it's being
// compared against some value to decide loop termination.
if (PN->hasOneUse()) {
BinaryOperator *BO = dyn_cast<BinaryOperator>(*(PN->use_begin()));
if (BO && BO->hasOneUse()) {
if (PN == *(BO->use_begin())) {
DeadInsts.insert(BO);
// Break the cycle, then delete the PHI.
PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
SE->deleteInstructionFromRecords(PN);
PN->eraseFromParent();
}
}
}
}
DeleteTriviallyDeadInstructions(DeadInsts);
}
CastedPointers.clear();
IVUsesByStride.clear();
return;
}
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