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a5c0038c25
llvm-mirror/lib/Transforms/Scalar/LoopStrengthReduce.cpp
Chris Lattner a5c0038c25 Fix a FIXME: if we are inserting code for a PHI argument, split the critical 
edge so that the code is not always executed for both operands.  This
prevents LSR from inserting code into loops whose exit blocks contain
PHI uses of IV expressions (which are outside of loops).  On gzip, for
example, we turn this ugly code:

.LBB_test_1:    ; loopentry
        add r27, r3, r28
        lhz r27, 3(r27)
        add r26, r4, r28
        lhz r26, 3(r26)
        add r25, r30, r28    ;; Only live if exiting the loop
        add r24, r29, r28    ;; Only live if exiting the loop
        cmpw cr0, r27, r26
        bne .LBB_test_5 ; loopexit

into this:

.LBB_test_1:    ; loopentry
        or r27, r28, r28
        add r28, r3, r27
        lhz r28, 3(r28)
        add r26, r4, r27
        lhz r26, 3(r26)
        cmpw cr0, r28, r26
        beq .LBB_test_3 ; shortcirc_next.0
.LBB_test_2:    ; loopentry.loopexit_crit_edge
        add r2, r30, r27
        add r8, r29, r27
        b .LBB_test_9   ; loopexit
.LBB_test_2:    ; shortcirc_next.0
        ...
        blt .LBB_test_1


into this:

.LBB_test_1:    ; loopentry
        or r27, r28, r28
        add r28, r3, r27
        lhz r28, 3(r28)
        add r26, r4, r27
        lhz r26, 3(r26)
        cmpw cr0, r28, r26
        beq .LBB_test_3 ; shortcirc_next.0
.LBB_test_2:    ; loopentry.loopexit_crit_edge
        add r2, r30, r27
        add r8, r29, r27
        b .LBB_t_3:    ; shortcirc_next.0
.LBB_test_3:    ; shortcirc_next.0
        ...
        blt .LBB_test_1


Next step: get the block out of the loop so that the loop is all
fall-throughs again.

llvm-svn: 22766
2005-08-12 22:06:11 +00:00

968 lines
39 KiB
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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 <algorithm>
#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.
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<SCEVHandle, 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(const SCEVHandle &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) {
// 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;
}
/// 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());
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) && 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 {
/// 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.
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, Pass *P);
// Sort by the Base field.
bool operator<(const BasedUser &BU) const { return Base < BU.Base; }
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;
}
// 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,
Pass *P) {
if (!isa<PHINode>(Inst)) {
SCEVHandle NewValSCEV = SCEVAddExpr::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. 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.
if (e != 1 &&
PN->getIncomingBlock(i)->getTerminator()->getNumSuccessors() > 1) {
TerminatorInst *PredTI = PN->getIncomingBlock(i)->getTerminator();
for (unsigned Succ = 0; ; ++Succ) {
assert(Succ != PredTI->getNumSuccessors() &&"Didn't find successor?");
if (PredTI->getSuccessor(Succ) == PN->getParent()) {
SplitCriticalEdge(PredTI, Succ, P);
break;
}
}
}
Value *&Code = InsertedCode[PN->getIncomingBlock(i)];
if (!Code) {
// Insert the code into the end of the predecessor block.
BasicBlock::iterator InsertPt =PN->getIncomingBlock(i)->getTerminator();
SCEVHandle NewValSCEV = SCEVAddExpr::get(NewBase, Imm);
Code = Rewriter.expandCodeFor(NewValSCEV, InsertPt,
OperandValToReplace->getType());
}
// 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) {
// FIXME: Look at the target to decide if &GV is a legal constant immediate.
if (SCEVConstant *SC = dyn_cast<SCEVConstant>(V)) {
// PPC allows a sign-extended 16-bit immediate field.
if ((int64_t)SC->getValue()->getRawValue() > -(1 << 16) &&
(int64_t)SC->getValue()->getRawValue() < (1 << 16)-1)
return true;
return false;
}
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;
}
/// 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(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.
}
/// 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;
for (unsigned i = 0; i != NumUses; ++i)
if (SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(Uses[i].Base)) {
for (unsigned j = 0, e = AE->getNumOperands(); j != e; ++j)
SubExpressionUseCounts[AE->getOperand(j)]++;
} else {
// If the base is zero (which is common), return zero now, there are no
// CSEs we can find.
if (Uses[i].Base == Zero) return Result;
SubExpressionUseCounts[Uses[i].Base]++;
}
// Now that we know how many times each is used, build Result.
for (std::map<SCEVHandle, unsigned>::iterator I =
SubExpressionUseCounts.begin(), E = SubExpressionUseCounts.end();
I != E; )
if (I->second == NumUses) { // Found CSE!
Result = SCEVAddExpr::get(Result, I->first);
++I;
} 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)
if (SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(Uses[i].Base)) {
std::vector<SCEVHandle> NewOps;
// Remove all of the values that are now in SubExpressionUseCounts.
for (unsigned j = 0, e = AE->getNumOperands(); j != e; ++j)
if (!SubExpressionUseCounts.count(AE->getOperand(j)))
NewOps.push_back(AE->getOperand(j));
if (NewOps.empty())
Uses[i].Base = Zero;
else
Uses[i].Base = SCEVAddExpr::get(NewOps);
} else {
// If the base is zero (which is common), return zero now, there are no
// CSEs we can find.
assert(Uses[i].Base == Result);
Uses[i].Base = Zero;
}
return Result;
}
/// 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!");
}
// 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) {
// 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(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();
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);
// Create a new Phi for this base, and stick it in the loop header.
const Type *ReplacedTy = CommonExprs->getType();
PHINode *NewPHI = new PHINode(ReplacedTy, "iv.", PhiInsertBefore);
++NumInserted;
// Insert the stride into the preheader.
Value *StrideV = PreheaderRewriter.expandCodeFor(Stride, PreInsertPt,
ReplacedTy);
if (!isa<ConstantInt>(StrideV)) ++NumVariable;
// Emit the initial base value into the loop preheader, and add it to the
// Phi node.
Value *PHIBaseV = PreheaderRewriter.expandCodeFor(CommonExprs, PreInsertPt,
ReplacedTy);
NewPHI->addIncoming(PHIBaseV, Preheader);
// 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));
Value *IncV = Rewriter.expandCodeFor(IncExp, LatchBlock->getTerminator(),
ReplacedTy);
IncV->setName(NewPHI->getName()+".inc");
NewPHI->addIncoming(IncV, LatchBlock);
// 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().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()) {
// 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.
while (!UsersToProcess.empty() && UsersToProcess.front().Base == Base) {
BasedUser &User = UsersToProcess.front();
// 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;
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();
// 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, 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.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;
const SCEVHandle *CondStride = 0;
for (std::map<SCEVHandle, 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.
// 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 coallesce the
// live ranges for the IV correctly.
CondUse->Offset = SCEV::getMinusSCEV(CondUse->Offset, *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;
// Note: this processes each stride/type pair individually. All users passed
// into StrengthReduceStridedIVUsers have the same type AND stride.
for (std::map<SCEVHandle, 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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