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llvm-6502/lib/Transforms/Scalar/LoopStrengthReduce.cpp
Reid Spencer ad2072643a Get rid of a signed/unsigned compare warning. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27625 91177308-0d34-0410-b5e6-96231b3b80d8
2006-04-12 19:28:15 +00:00

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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/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetLowering.h"
#include <algorithm>
#include <iostream>
#include <set>
using namespace llvm;
namespace {
Statistic<> NumReduced ("loop-reduce", "Number of GEPs strength reduced");
Statistic<> NumInserted("loop-reduce", "Number of PHIs inserted");
Statistic<> NumVariable("loop-reduce","Number of PHIs with variable strides");
/// 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 or uses dominated by the 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));
}
};
/// IVInfo - This structure keeps track of one IV expression inserted during
/// StrengthReduceStridedIVUsers. It contains the stride, the common base, as
/// well as the PHI node and increment value created for rewrite.
struct IVExpr {
SCEVHandle Stride;
SCEVHandle Base;
PHINode *PHI;
Value *IncV;
IVExpr()
: Stride(SCEVUnknown::getIntegerSCEV(0, Type::UIntTy)),
Base (SCEVUnknown::getIntegerSCEV(0, Type::UIntTy)) {}
IVExpr(const SCEVHandle &stride, const SCEVHandle &base, PHINode *phi,
Value *incv)
: Stride(stride), Base(base), PHI(phi), IncV(incv) {}
};
/// IVsOfOneStride - This structure keeps track of all IV expression inserted
/// during StrengthReduceStridedIVUsers for a particular stride of the IV.
struct IVsOfOneStride {
std::vector<IVExpr> IVs;
void addIV(const SCEVHandle &Stride, const SCEVHandle &Base, PHINode *PHI,
Value *IncV) {
IVs.push_back(IVExpr(Stride, Base, PHI, IncV));
}
};
class LoopStrengthReduce : public FunctionPass {
LoopInfo *LI;
ETForest *EF;
ScalarEvolution *SE;
const TargetData *TD;
const Type *UIntPtrTy;
bool Changed;
/// 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<SCEVHandle, IVUsersOfOneStride> IVUsesByStride;
/// IVsByStride - Keep track of all IVs that have been inserted for a
/// particular stride.
std::map<SCEVHandle, IVsOfOneStride> IVsByStride;
/// StrideOrder - An ordering of the keys in IVUsesByStride that is stable:
/// We use this to iterate over the IVUsesByStride collection without being
/// dependent on random ordering of pointers in the process.
std::vector<SCEVHandle> StrideOrder;
/// 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;
/// TLI - Keep a pointer of a TargetLowering to consult for determining
/// transformation profitability.
const TargetLowering *TLI;
public:
LoopStrengthReduce(const TargetLowering *tli = NULL)
: TLI(tli) {
}
virtual bool runOnFunction(Function &) {
LI = &getAnalysis<LoopInfo>();
EF = &getAnalysis<ETForest>();
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 {
// We split critical edges, so we change the CFG. However, we do update
// many analyses if they are around.
AU.addPreservedID(LoopSimplifyID);
AU.addPreserved<LoopInfo>();
AU.addPreserved<DominatorSet>();
AU.addPreserved<ETForest>();
AU.addPreserved<ImmediateDominators>();
AU.addPreserved<DominanceFrontier>();
AU.addPreserved<DominatorTree>();
AU.addRequiredID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
AU.addRequired<ETForest>();
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);
unsigned CheckForIVReuse(const SCEVHandle &Stride, IVExpr &IV);
void StrengthReduceStridedIVUsers(const SCEVHandle &Stride,
IVUsersOfOneStride &Uses,
Loop *L, bool isOnlyStride);
void DeleteTriviallyDeadInstructions(std::set<Instruction*> &Insts);
};
RegisterOpt<LoopStrengthReduce> X("loop-reduce",
"Loop Strength Reduction");
}
FunctionPass *llvm::createLoopStrengthReducePass(const TargetLowering *TLI) {
return new LoopStrengthReduce(TLI);
}
/// 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;
New = SCEVExpander::InsertCastOfTo(V, UIntPtrTy);
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) {
// Scalar Evolutions doesn't know how to compute SCEV's for GEP instructions.
// If this is a GEP that SE doesn't know about, compute it now and insert it.
// If this is not a GEP, or if we have already done this computation, just let
// SE figure it out.
GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Exp);
if (!GEP || SE->hasSCEV(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);
}
}
SE->setSCEV(GEP, GEPVal);
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, SCEVHandle &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));
if (!isa<SCEVConstant>(AddRec->getOperand(1)))
DEBUG(std::cerr << "[" << L->getHeader()->getName()
<< "] Variable stride: " << *AddRec << "\n");
Stride = AddRec->getOperand(1);
// Check that all constant strides are the unsigned type, we don't want to
// have two IV's one of signed stride 4 and one of unsigned stride 4 to not be
// merged.
assert((!isa<SCEVConstant>(Stride) || Stride->getType()->isUnsigned()) &&
"Constants should be canonicalized to unsigned!");
return true;
}
/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
/// and now we need to decide whether the user should use the preinc or post-inc
/// value. If this user should use the post-inc version of the IV, return true.
///
/// Choosing wrong here can break dominance properties (if we choose to use the
/// post-inc value when we cannot) or it can end up adding extra live-ranges to
/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
/// should use the post-inc value).
static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV,
Loop *L, ETForest *EF, Pass *P) {
// If the user is in the loop, use the preinc value.
if (L->contains(User->getParent())) return false;
BasicBlock *LatchBlock = L->getLoopLatch();
// Ok, the user is outside of the loop. If it is dominated by the latch
// block, use the post-inc value.
if (EF->dominates(LatchBlock, User->getParent()))
return true;
// There is one case we have to be careful of: PHI nodes. These little guys
// can live in blocks that do not dominate the latch block, but (since their
// uses occur in the predecessor block, not the block the PHI lives in) should
// still use the post-inc value. Check for this case now.
PHINode *PN = dyn_cast<PHINode>(User);
if (!PN) return false; // not a phi, not dominated by latch block.
// Look at all of the uses of IV by the PHI node. If any use corresponds to
// a block that is not dominated by the latch block, give up and use the
// preincremented value.
unsigned NumUses = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == IV) {
++NumUses;
if (!EF->dominates(LatchBlock, PN->getIncomingBlock(i)))
return false;
}
// Okay, all uses of IV by PN are in predecessor blocks that really are
// dominated by the latch block. Split the critical edges and use the
// post-incremented value.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == IV) {
SplitCriticalEdge(PN->getIncomingBlock(i), PN->getParent(), P);
if (--NumUses == 0) break;
}
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()->isInteger() && !isa<PointerType>(I->getType()))
return false; // Void and FP expressions cannot be reduced.
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());
SCEVHandle Stride = Start;
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) && Processed.count(User))
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 other 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) {
IVUsersOfOneStride &StrideUses = IVUsesByStride[Stride];
if (StrideUses.Users.empty()) // First occurance of this stride?
StrideOrder.push_back(Stride);
// Okay, we found a user that we cannot reduce. Analyze the instruction
// and decide what to do with it. If we are a use inside of the loop, use
// the value before incrementation, otherwise use it after incrementation.
if (IVUseShouldUsePostIncValue(User, I, L, EF, this)) {
// The value used will be incremented by the stride more than we are
// expecting, so subtract this off.
SCEVHandle NewStart = SCEV::getMinusSCEV(Start, Stride);
StrideUses.addUser(NewStart, User, I);
StrideUses.Users.back().isUseOfPostIncrementedValue = true;
DEBUG(std::cerr << " USING POSTINC SCEV, START=" << *NewStart<< "\n");
} else {
StrideUses.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 {
/// Base - The Base value for the PHI node that needs to be inserted for
/// this use. As the use is processed, information gets moved from this
/// field to the Imm field (below). BasedUser values are sorted by this
/// field.
SCEVHandle Base;
/// 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 and uses outside the loop that are dominated by
// the loop.
bool isUseOfPostIncrementedValue;
BasedUser(IVStrideUse &IVSU)
: Base(IVSU.Offset), Inst(IVSU.User),
OperandValToReplace(IVSU.OperandValToReplace),
Imm(SCEVUnknown::getIntegerSCEV(0, Base->getType())), EmittedBase(0),
isUseOfPostIncrementedValue(IVSU.isUseOfPostIncrementedValue) {}
// 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(const SCEVHandle &NewBase,
SCEVExpander &Rewriter, Loop *L,
Pass *P);
Value *InsertCodeForBaseAtPosition(const SCEVHandle &NewBase,
SCEVExpander &Rewriter,
Instruction *IP, Loop *L);
void dump() const;
};
}
void BasedUser::dump() const {
std::cerr << " Base=" << *Base;
std::cerr << " Imm=" << *Imm;
if (EmittedBase)
std::cerr << " EB=" << *EmittedBase;
std::cerr << " Inst: " << *Inst;
}
Value *BasedUser::InsertCodeForBaseAtPosition(const SCEVHandle &NewBase,
SCEVExpander &Rewriter,
Instruction *IP, Loop *L) {
// Figure out where we *really* want to insert this code. In particular, if
// the user is inside of a loop that is nested inside of L, we really don't
// want to insert this expression before the user, we'd rather pull it out as
// many loops as possible.
LoopInfo &LI = Rewriter.getLoopInfo();
Instruction *BaseInsertPt = IP;
// Figure out the most-nested loop that IP is in.
Loop *InsertLoop = LI.getLoopFor(IP->getParent());
// If InsertLoop is not L, and InsertLoop is nested inside of L, figure out
// the preheader of the outer-most loop where NewBase is not loop invariant.
while (InsertLoop && NewBase->isLoopInvariant(InsertLoop)) {
BaseInsertPt = InsertLoop->getLoopPreheader()->getTerminator();
InsertLoop = InsertLoop->getParentLoop();
}
// If there is no immediate value, skip the next part.
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(Imm))
if (SC->getValue()->isNullValue())
return Rewriter.expandCodeFor(NewBase, BaseInsertPt,
OperandValToReplace->getType());
Value *Base = Rewriter.expandCodeFor(NewBase, BaseInsertPt);
// Always emit the immediate (if non-zero) into the same block as the user.
SCEVHandle NewValSCEV = SCEVAddExpr::get(SCEVUnknown::get(Base), Imm);
return Rewriter.expandCodeFor(NewValSCEV, IP,
OperandValToReplace->getType());
}
// 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(const SCEVHandle &NewBase,
SCEVExpander &Rewriter,
Loop *L, Pass *P) {
if (!isa<PHINode>(Inst)) {
Value *NewVal = InsertCodeForBaseAtPosition(NewBase, Rewriter, Inst, L);
// 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. Note that PHI nodes can
// have multiple entries for the same predecessor. We use a map to make sure
// that a PHI node only has a single Value* for each predecessor (which also
// prevents us from inserting duplicate code in some blocks).
std::map<BasicBlock*, Value*> InsertedCode;
PHINode *PN = cast<PHINode>(Inst);
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if (PN->getIncomingValue(i) == OperandValToReplace) {
// If this is a critical edge, split the edge so that we do not insert the
// code on all predecessor/successor paths. We do this unless this is the
// canonical backedge for this loop, as this can make some inserted code
// be in an illegal position.
BasicBlock *PHIPred = PN->getIncomingBlock(i);
if (e != 1 && PHIPred->getTerminator()->getNumSuccessors() > 1 &&
(PN->getParent() != L->getHeader() || !L->contains(PHIPred))) {
// First step, split the critical edge.
SplitCriticalEdge(PHIPred, PN->getParent(), P);
// Next step: move the basic block. In particular, if the PHI node
// is outside of the loop, and PredTI is in the loop, we want to
// move the block to be immediately before the PHI block, not
// immediately after PredTI.
if (L->contains(PHIPred) && !L->contains(PN->getParent())) {
BasicBlock *NewBB = PN->getIncomingBlock(i);
NewBB->moveBefore(PN->getParent());
}
}
Value *&Code = InsertedCode[PN->getIncomingBlock(i)];
if (!Code) {
// Insert the code into the end of the predecessor block.
Instruction *InsertPt = PN->getIncomingBlock(i)->getTerminator();
Code = InsertCodeForBaseAtPosition(NewBase, Rewriter, InsertPt, L);
}
// Replace the use of the operand Value with the new Phi we just created.
PN->setIncomingValue(i, Code);
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, const TargetLowering *TLI) {
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(V)) {
int64_t V = SC->getValue()->getSExtValue();
if (TLI)
return TLI->isLegalAddressImmediate(V);
else
// Defaults to PPC. PPC allows a sign-extended 16-bit immediate field.
return (V > -(1 << 16) && V < (1 << 16)-1);
}
if (SCEVUnknown *SU = dyn_cast<SCEVUnknown>(V))
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(SU->getValue()))
if (CE->getOpcode() == Instruction::Cast) {
Constant *Op0 = CE->getOperand(0);
if (isa<GlobalValue>(Op0) &&
TLI &&
TLI->isLegalAddressImmediate(cast<GlobalValue>(Op0)))
return true;
}
return false;
}
/// MoveLoopVariantsToImediateField - Move any subexpressions from Val that are
/// loop varying to the Imm operand.
static void MoveLoopVariantsToImediateField(SCEVHandle &Val, SCEVHandle &Imm,
Loop *L) {
if (Val->isLoopInvariant(L)) return; // Nothing to do.
if (SCEVAddExpr *SAE = dyn_cast<SCEVAddExpr>(Val)) {
std::vector<SCEVHandle> NewOps;
NewOps.reserve(SAE->getNumOperands());
for (unsigned i = 0; i != SAE->getNumOperands(); ++i)
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);
} 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();
MoveLoopVariantsToImediateField(Start, Imm, L);
std::vector<SCEVHandle> Ops(SARE->op_begin(), SARE->op_end());
Ops[0] = Start;
Val = SCEVAddRecExpr::get(Ops, SARE->getLoop());
} else {
// Otherwise, all of Val is variant, move the whole thing over.
Imm = SCEVAddExpr::get(Imm, Val);
Val = SCEVUnknown::getIntegerSCEV(0, Val->getType());
}
}
/// 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(const TargetLowering *TLI,
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) {
SCEVHandle NewOp = SAE->getOperand(i);
MoveImmediateValues(TLI, NewOp, Imm, isAddress, L);
if (!NewOp->isLoopInvariant(L)) {
// If this is a loop-variant expression, it must stay in the immediate
// field of the expression.
Imm = SCEVAddExpr::get(Imm, NewOp);
} else {
NewOps.push_back(NewOp);
}
}
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(TLI, 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;
} else if (SCEVMulExpr *SME = dyn_cast<SCEVMulExpr>(Val)) {
// Transform "8 * (4 + v)" -> "32 + 8*V" if "32" fits in the immed field.
if (isAddress && isTargetConstant(SME->getOperand(0), TLI) &&
SME->getNumOperands() == 2 && SME->isLoopInvariant(L)) {
SCEVHandle SubImm = SCEVUnknown::getIntegerSCEV(0, Val->getType());
SCEVHandle NewOp = SME->getOperand(1);
MoveImmediateValues(TLI, NewOp, SubImm, isAddress, L);
// If we extracted something out of the subexpressions, see if we can
// simplify this!
if (NewOp != SME->getOperand(1)) {
// Scale SubImm up by "8". If the result is a target constant, we are
// good.
SubImm = SCEVMulExpr::get(SubImm, SME->getOperand(0));
if (isTargetConstant(SubImm, TLI)) {
// Accumulate the immediate.
Imm = SCEVAddExpr::get(Imm, SubImm);
// Update what is left of 'Val'.
Val = SCEVMulExpr::get(SME->getOperand(0), NewOp);
return;
}
}
}
}
// Loop-variant expressions must stay in the immediate field of the
// expression.
if ((isAddress && isTargetConstant(Val, TLI)) ||
!Val->isLoopInvariant(L)) {
Imm = SCEVAddExpr::get(Imm, Val);
Val = SCEVUnknown::getIntegerSCEV(0, Val->getType());
return;
}
// Otherwise, no immediates to move.
}
/// IncrementAddExprUses - Decompose the specified expression into its added
/// subexpressions, and increment SubExpressionUseCounts for each of these
/// decomposed parts.
static void SeparateSubExprs(std::vector<SCEVHandle> &SubExprs,
SCEVHandle Expr) {
if (SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(Expr)) {
for (unsigned j = 0, e = AE->getNumOperands(); j != e; ++j)
SeparateSubExprs(SubExprs, AE->getOperand(j));
} else if (SCEVAddRecExpr *SARE = dyn_cast<SCEVAddRecExpr>(Expr)) {
SCEVHandle Zero = SCEVUnknown::getIntegerSCEV(0, Expr->getType());
if (SARE->getOperand(0) == Zero) {
SubExprs.push_back(Expr);
} else {
// Compute the addrec with zero as its base.
std::vector<SCEVHandle> Ops(SARE->op_begin(), SARE->op_end());
Ops[0] = Zero; // Start with zero base.
SubExprs.push_back(SCEVAddRecExpr::get(Ops, SARE->getLoop()));
SeparateSubExprs(SubExprs, SARE->getOperand(0));
}
} else if (!isa<SCEVConstant>(Expr) ||
!cast<SCEVConstant>(Expr)->getValue()->isNullValue()) {
// Do not add zero.
SubExprs.push_back(Expr);
}
}
/// RemoveCommonExpressionsFromUseBases - Look through all of the uses in Bases,
/// removing any common subexpressions from it. Anything truly common is
/// removed, accumulated, and returned. This looks for things like (a+b+c) and
/// (a+c+d) -> (a+c). The common expression is *removed* from the Bases.
static SCEVHandle
RemoveCommonExpressionsFromUseBases(std::vector<BasedUser> &Uses) {
unsigned NumUses = Uses.size();
// Only one use? Use its base, regardless of what it is!
SCEVHandle Zero = SCEVUnknown::getIntegerSCEV(0, Uses[0].Base->getType());
SCEVHandle Result = Zero;
if (NumUses == 1) {
std::swap(Result, Uses[0].Base);
return Result;
}
// To find common subexpressions, count how many of Uses use each expression.
// If any subexpressions are used Uses.size() times, they are common.
std::map<SCEVHandle, unsigned> SubExpressionUseCounts;
// UniqueSubExprs - Keep track of all of the subexpressions we see in the
// order we see them.
std::vector<SCEVHandle> UniqueSubExprs;
std::vector<SCEVHandle> SubExprs;
for (unsigned i = 0; i != NumUses; ++i) {
// If the base is zero (which is common), return zero now, there are no
// CSEs we can find.
if (Uses[i].Base == Zero) return Zero;
// Split the expression into subexprs.
SeparateSubExprs(SubExprs, Uses[i].Base);
// Add one to SubExpressionUseCounts for each subexpr present.
for (unsigned j = 0, e = SubExprs.size(); j != e; ++j)
if (++SubExpressionUseCounts[SubExprs[j]] == 1)
UniqueSubExprs.push_back(SubExprs[j]);
SubExprs.clear();
}
// Now that we know how many times each is used, build Result. Iterate over
// UniqueSubexprs so that we have a stable ordering.
for (unsigned i = 0, e = UniqueSubExprs.size(); i != e; ++i) {
std::map<SCEVHandle, unsigned>::iterator I =
SubExpressionUseCounts.find(UniqueSubExprs[i]);
assert(I != SubExpressionUseCounts.end() && "Entry not found?");
if (I->second == NumUses) { // Found CSE!
Result = SCEVAddExpr::get(Result, I->first);
} else {
// Remove non-cse's from SubExpressionUseCounts.
SubExpressionUseCounts.erase(I);
}
}
// If we found no CSE's, return now.
if (Result == Zero) return Result;
// Otherwise, remove all of the CSE's we found from each of the base values.
for (unsigned i = 0; i != NumUses; ++i) {
// Split the expression into subexprs.
SeparateSubExprs(SubExprs, Uses[i].Base);
// Remove any common subexpressions.
for (unsigned j = 0, e = SubExprs.size(); j != e; ++j)
if (SubExpressionUseCounts.count(SubExprs[j])) {
SubExprs.erase(SubExprs.begin()+j);
--j; --e;
}
// Finally, the non-shared expressions together.
if (SubExprs.empty())
Uses[i].Base = Zero;
else
Uses[i].Base = SCEVAddExpr::get(SubExprs);
SubExprs.clear();
}
return Result;
}
/// isZero - returns true if the scalar evolution expression is zero.
///
static bool isZero(SCEVHandle &V) {
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(V))
return SC->getValue()->getRawValue() == 0;
return false;
}
/// CheckForIVReuse - Returns the multiple if the stride is the multiple
/// of a previous stride and it is a legal value for the target addressing
/// mode scale component. This allows the users of this stride to be rewritten
/// as prev iv * factor. It returns 0 if no reuse is possible.
unsigned LoopStrengthReduce::CheckForIVReuse(const SCEVHandle &Stride,
IVExpr &IV) {
if (!TLI) return 0;
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(Stride)) {
int64_t SInt = SC->getValue()->getSExtValue();
if (SInt == 1) return 0;
for (TargetLowering::legal_am_scale_iterator
I = TLI->legal_am_scale_begin(), E = TLI->legal_am_scale_end();
I != E; ++I) {
unsigned Scale = *I;
if (unsigned(abs(SInt)) < Scale || (SInt % Scale) != 0)
continue;
std::map<SCEVHandle, IVsOfOneStride>::iterator SI =
IVsByStride.find(SCEVUnknown::getIntegerSCEV(SInt/Scale, Type::UIntTy));
if (SI == IVsByStride.end())
continue;
for (std::vector<IVExpr>::iterator II = SI->second.IVs.begin(),
IE = SI->second.IVs.end(); II != IE; ++II)
// FIXME: Only handle base == 0 for now.
if (isZero(II->Base)) {
IV = *II;
return Scale;
}
}
}
return 0;
}
/// 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(const SCEVHandle &Stride,
IVUsersOfOneStride &Uses,
Loop *L,
bool isOnlyStride) {
// Transform our list of users and offsets to a bit more complex table. In
// this new vector, each 'BasedUser' contains 'Base' the base of the
// strided accessas well as the old information from Uses. We progressively
// move information from the Base field to the Imm field, until we eventually
// have the full access expression to rewrite the use.
std::vector<BasedUser> UsersToProcess;
UsersToProcess.reserve(Uses.Users.size());
for (unsigned i = 0, e = Uses.Users.size(); i != e; ++i) {
UsersToProcess.push_back(Uses.Users[i]);
// Move any loop invariant operands from the offset field to the immediate
// field of the use, so that we don't try to use something before it is
// computed.
MoveLoopVariantsToImediateField(UsersToProcess.back().Base,
UsersToProcess.back().Imm, L);
assert(UsersToProcess.back().Base->isLoopInvariant(L) &&
"Base value is not loop invariant!");
}
// Check if it is possible to reuse a IV with stride that is factor of this
// stride. And the multiple is a number that can be encoded in the scale
// field of the target addressing mode.
PHINode *NewPHI = NULL;
Value *IncV = NULL;
IVExpr ReuseIV;
unsigned RewriteFactor = CheckForIVReuse(Stride, ReuseIV);
if (RewriteFactor != 0) {
DEBUG(std::cerr << "BASED ON IV of STRIDE " << *ReuseIV.Stride
<< " and BASE " << *ReuseIV.Base << " :\n");
NewPHI = ReuseIV.PHI;
IncV = ReuseIV.IncV;
}
// We now have a whole bunch of uses of like-strided induction variables, but
// they might all have different bases. We want to emit one PHI node for this
// stride which we fold as many common expressions (between the IVs) into as
// possible. Start by identifying the common expressions in the base values
// for the strides (e.g. if we have "A+C+B" and "A+B+D" as our bases, find
// "A+B"), emit it to the preheader, then remove the expression from the
// UsersToProcess base values.
SCEVHandle CommonExprs =
RemoveCommonExpressionsFromUseBases(UsersToProcess);
// Next, 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. We do this so that it increases the commonality
// of the remaining uses.
for (unsigned i = 0, e = UsersToProcess.size(); i != e; ++i) {
// If the user is not in the current loop, this means it is using the exit
// value of the IV. Do not put anything in the base, make sure it's all in
// the immediate field to allow as much factoring as possible.
if (!L->contains(UsersToProcess[i].Inst->getParent())) {
UsersToProcess[i].Imm = SCEVAddExpr::get(UsersToProcess[i].Imm,
UsersToProcess[i].Base);
UsersToProcess[i].Base =
SCEVUnknown::getIntegerSCEV(0, UsersToProcess[i].Base->getType());
} else {
// 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].Inst);
if (StoreInst *SI = dyn_cast<StoreInst>(UsersToProcess[i].Inst))
if (SI->getOperand(1) == UsersToProcess[i].OperandValToReplace)
isAddress = true;
MoveImmediateValues(TLI, UsersToProcess[i].Base, UsersToProcess[i].Imm,
isAddress, L);
}
}
// Now that we know what we need to do, insert the PHI node itself.
//
DEBUG(std::cerr << "INSERTING IV of STRIDE " << *Stride << " and BASE "
<< *CommonExprs << " :\n");
SCEVExpander Rewriter(*SE, *LI);
SCEVExpander PreheaderRewriter(*SE, *LI);
BasicBlock *Preheader = L->getLoopPreheader();
Instruction *PreInsertPt = Preheader->getTerminator();
Instruction *PhiInsertBefore = L->getHeader()->begin();
BasicBlock *LatchBlock = L->getLoopLatch();
const Type *ReplacedTy = CommonExprs->getType();
// Emit the initial base value into the loop preheader.
Value *CommonBaseV
= PreheaderRewriter.expandCodeFor(CommonExprs, PreInsertPt,
ReplacedTy);
if (RewriteFactor == 0) {
// Create a new Phi for this base, and stick it in the loop header.
NewPHI = new PHINode(ReplacedTy, "iv.", PhiInsertBefore);
++NumInserted;
// Add common base to the new Phi node.
NewPHI->addIncoming(CommonBaseV, Preheader);
// Insert the stride into the preheader.
Value *StrideV = PreheaderRewriter.expandCodeFor(Stride, PreInsertPt,
ReplacedTy);
if (!isa<ConstantInt>(StrideV)) ++NumVariable;
// Emit the increment of the base value before the terminator of the loop
// latch block, and add it to the Phi node.
SCEVHandle IncExp = SCEVAddExpr::get(SCEVUnknown::get(NewPHI),
SCEVUnknown::get(StrideV));
IncV = Rewriter.expandCodeFor(IncExp, LatchBlock->getTerminator(),
ReplacedTy);
IncV->setName(NewPHI->getName()+".inc");
NewPHI->addIncoming(IncV, LatchBlock);
// Remember this in case a later stride is multiple of this.
IVsByStride[Stride].addIV(Stride, CommonExprs, NewPHI, IncV);
} else {
Constant *C = dyn_cast<Constant>(CommonBaseV);
if (!C ||
(!C->isNullValue() &&
!isTargetConstant(SCEVUnknown::get(CommonBaseV), TLI)))
// We want the common base emitted into the preheader!
CommonBaseV = new CastInst(CommonBaseV, CommonBaseV->getType(),
"commonbase", PreInsertPt);
}
// Sort by the base value, so that all IVs with identical bases are next to
// each other.
while (!UsersToProcess.empty()) {
SCEVHandle Base = UsersToProcess.back().Base;
DEBUG(std::cerr << " INSERTING code for BASE = " << *Base << ":\n");
// Emit the code for Base into the preheader.
Value *BaseV = PreheaderRewriter.expandCodeFor(Base, PreInsertPt,
ReplacedTy);
// If BaseV is a constant other than 0, make sure that it gets inserted into
// the preheader, instead of being forward substituted into the uses. We do
// this by forcing a noop cast to be inserted into the preheader in this
// case.
if (Constant *C = dyn_cast<Constant>(BaseV))
if (!C->isNullValue() && !isTargetConstant(Base, TLI)) {
// We want this constant emitted into the preheader!
BaseV = new CastInst(BaseV, BaseV->getType(), "preheaderinsert",
PreInsertPt);
}
// 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.
unsigned ScanPos = 0;
do {
BasedUser &User = UsersToProcess.back();
// If this instruction wants to use the post-incremented value, move it
// after the post-inc and use its value instead of the PHI.
Value *RewriteOp = NewPHI;
if (User.isUseOfPostIncrementedValue) {
RewriteOp = IncV;
// If this user is in the loop, make sure it is the last thing in the
// loop to ensure it is dominated by the increment.
if (L->contains(User.Inst->getParent()))
User.Inst->moveBefore(LatchBlock->getTerminator());
}
SCEVHandle RewriteExpr = SCEVUnknown::get(RewriteOp);
// Clear the SCEVExpander's expression map so that we are guaranteed
// to have the code emitted where we expect it.
Rewriter.clear();
// If we are reusing the iv, then it must be multiplied by a constant
// factor take advantage of addressing mode scale component.
if (RewriteFactor != 0) {
RewriteExpr =
SCEVMulExpr::get(SCEVUnknown::getIntegerSCEV(RewriteFactor,
RewriteExpr->getType()),
RewriteExpr);
// The common base is emitted in the loop preheader. But since we
// are reusing an IV, it has not been used to initialize the PHI node.
// Add it to the expression used to rewrite the uses.
if (!isa<ConstantInt>(CommonBaseV) ||
!cast<ConstantInt>(CommonBaseV)->isNullValue())
RewriteExpr = SCEVAddExpr::get(RewriteExpr,
SCEVUnknown::get(CommonBaseV));
}
// Now that we know what we need to do, insert code before User for the
// immediate and any loop-variant expressions.
if (!isa<ConstantInt>(BaseV) || !cast<ConstantInt>(BaseV)->isNullValue())
// Add BaseV to the PHI value if needed.
RewriteExpr = SCEVAddExpr::get(RewriteExpr, SCEVUnknown::get(BaseV));
User.RewriteInstructionToUseNewBase(RewriteExpr, Rewriter, L, this);
// 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.pop_back();
++NumReduced;
// If there are any more users to process with the same base, move one of
// them to the end of the list so that we will process it.
if (!UsersToProcess.empty()) {
for (unsigned e = UsersToProcess.size(); ScanPos != e; ++ScanPos)
if (UsersToProcess[ScanPos].Base == Base) {
std::swap(UsersToProcess[ScanPos], UsersToProcess.back());
break;
}
}
} while (!UsersToProcess.empty() && UsersToProcess.back().Base == Base);
// 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 coalescing 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;
const SCEVHandle *CondStride = 0;
for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e && !CondUse;
++Stride) {
std::map<SCEVHandle, IVUsersOfOneStride>::iterator SI =
IVUsesByStride.find(StrideOrder[Stride]);
assert(SI != IVUsesByStride.end() && "Stride doesn't exist!");
for (std::vector<IVStrideUse>::iterator UI = SI->second.Users.begin(),
E = SI->second.Users.end(); UI != E; ++UI)
if (UI->User == Cond) {
CondUse = &*UI;
CondStride = &SI->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.
// FIXME: Evaluate whether this is a good idea or not.
if (!isa<SCEVConstant>(*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 coalesce the
// live ranges for the IV correctly.
CondUse->Offset = SCEV::getMinusSCEV(CondUse->Offset, *CondStride);
CondUse->isUseOfPostIncrementedValue = true;
}
namespace {
// Constant strides come first which in turns are sorted by their absolute
// values. If absolute values are the same, then positive strides comes first.
// e.g.
// 4, -1, X, 1, 2 ==> 1, -1, 2, 4, X
struct StrideCompare {
bool operator()(const SCEVHandle &LHS, const SCEVHandle &RHS) {
SCEVConstant *LHSC = dyn_cast<SCEVConstant>(LHS);
SCEVConstant *RHSC = dyn_cast<SCEVConstant>(RHS);
if (LHSC && RHSC) {
int64_t LV = LHSC->getValue()->getSExtValue();
int64_t RV = RHSC->getValue()->getSExtValue();
uint64_t ALV = (LV < 0) ? -LV : LV;
uint64_t ARV = (RV < 0) ? -RV : RV;
if (ALV == ARV)
return LV > RV;
else
return ALV < ARV;
}
return (LHSC && !RHSC);
}
};
}
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;
#ifndef NDEBUG
DEBUG(std::cerr << "\nLSR on ");
DEBUG(L->dump());
#endif
// IVsByStride keeps IVs for one particular loop.
IVsByStride.clear();
// Sort the StrideOrder so we process larger strides first.
std::stable_sort(StrideOrder.begin(), StrideOrder.end(), StrideCompare());
// Note: this processes each stride/type pair individually. All users passed
// into StrengthReduceStridedIVUsers have the same type AND stride. Also,
// node that we iterate over IVUsesByStride indirectly by using StrideOrder.
// This extra layer of indirection makes the ordering of strides deterministic
// - not dependent on map order.
for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) {
std::map<SCEVHandle, IVUsersOfOneStride>::iterator SI =
IVUsesByStride.find(StrideOrder[Stride]);
assert(SI != IVUsesByStride.end() && "Stride doesn't exist!");
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();
StrideOrder.clear();
return;
}
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