llvm-6502/lib/Analysis/IVUsers.cpp
Dan Gohman 3f46a3abee Spelling fixes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97453 91177308-0d34-0410-b5e6-96231b3b80d8
2010-03-01 17:49:51 +00:00

364 lines
13 KiB
C++

//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements bookkeeping for "interesting" users of expressions
// computed from induction variables.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "iv-users"
#include "llvm/Analysis/IVUsers.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/LoopPass.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Assembly/AsmAnnotationWriter.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
char IVUsers::ID = 0;
static RegisterPass<IVUsers>
X("iv-users", "Induction Variable Users", false, true);
Pass *llvm::createIVUsersPass() {
return new IVUsers();
}
/// CollectSubexprs - Split S into subexpressions which can be pulled out into
/// separate registers.
static void CollectSubexprs(const SCEV *S,
SmallVectorImpl<const SCEV *> &Ops,
ScalarEvolution &SE) {
if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
// Break out add operands.
for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
I != E; ++I)
CollectSubexprs(*I, Ops, SE);
return;
} else if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// Split a non-zero base out of an addrec.
if (!AR->getStart()->isZero()) {
CollectSubexprs(AR->getStart(), Ops, SE);
CollectSubexprs(SE.getAddRecExpr(SE.getIntegerSCEV(0, AR->getType()),
AR->getStepRecurrence(SE),
AR->getLoop()), Ops, SE);
return;
}
}
// Otherwise use the value itself.
Ops.push_back(S);
}
/// 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. The start cannot,
/// however, contain an AddRec from a different loop, unless that loop is an
/// outer loop of the current loop.
static bool getSCEVStartAndStride(const SCEV *&SH, Loop *L, Loop *UseLoop,
const SCEV *&Start, const SCEV *&Stride,
ScalarEvolution *SE, DominatorTree *DT) {
const SCEV *TheAddRec = Start; // Initialize to zero.
// If the outer level is an AddExpr, the operands are all start values except
// for a nested AddRecExpr.
if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) {
for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i)
if (const SCEVAddRecExpr *AddRec =
dyn_cast<SCEVAddRecExpr>(AE->getOperand(i)))
TheAddRec = SE->getAddExpr(AddRec, TheAddRec);
else
Start = SE->getAddExpr(Start, AE->getOperand(i));
} else if (isa<SCEVAddRecExpr>(SH)) {
TheAddRec = SH;
} else {
return false; // not analyzable.
}
// Break down TheAddRec into its component parts.
SmallVector<const SCEV *, 4> Subexprs;
CollectSubexprs(TheAddRec, Subexprs, *SE);
// Look for an addrec on the current loop among the parts.
const SCEV *AddRecStride = 0;
for (SmallVectorImpl<const SCEV *>::iterator I = Subexprs.begin(),
E = Subexprs.end(); I != E; ++I) {
const SCEV *S = *I;
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S))
if (AR->getLoop() == L) {
*I = AR->getStart();
AddRecStride = AR->getStepRecurrence(*SE);
break;
}
}
if (!AddRecStride)
return false;
// Add up everything else into a start value (which may not be
// loop-invariant).
const SCEV *AddRecStart = SE->getAddExpr(Subexprs);
// Use getSCEVAtScope to attempt to simplify other loops out of
// the picture.
AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop);
Start = SE->getAddExpr(Start, AddRecStart);
// If stride is an instruction, make sure it properly dominates the header.
// Otherwise we could end up with a use before def situation.
if (!isa<SCEVConstant>(AddRecStride)) {
BasicBlock *Header = L->getHeader();
if (!AddRecStride->properlyDominates(Header, DT))
return false;
DEBUG(dbgs() << "[";
WriteAsOperand(dbgs(), L->getHeader(), /*PrintType=*/false);
dbgs() << "] Variable stride: " << *AddRecStride << "\n");
}
Stride = AddRecStride;
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, DominatorTree *DT) {
// If the user is in the loop, use the preinc value.
if (L->contains(User)) return false;
BasicBlock *LatchBlock = L->getLoopLatch();
if (!LatchBlock)
return false;
// Ok, the user is outside of the loop. If it is dominated by the latch
// block, use the post-inc value.
if (DT->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 are not dominated by 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 (!DT->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. Use the post-incremented value.
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 IVUsers::AddUsersIfInteresting(Instruction *I) {
if (!SE->isSCEVable(I->getType()))
return false; // Void and FP expressions cannot be reduced.
// LSR is not APInt clean, do not touch integers bigger than 64-bits.
if (SE->getTypeSizeInBits(I->getType()) > 64)
return false;
if (!Processed.insert(I))
return true; // Instruction already handled.
// Get the symbolic expression for this instruction.
const SCEV *ISE = SE->getSCEV(I);
if (isa<SCEVCouldNotCompute>(ISE)) return false;
// Get the start and stride for this expression.
Loop *UseLoop = LI->getLoopFor(I->getParent());
const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType());
const SCEV *Stride = Start;
if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT))
return false; // Non-reducible symbolic expression, bail out.
// Keep things simple. Don't touch loop-variant strides.
if (!Stride->isLoopInvariant(L) && L->contains(I))
return false;
SmallPtrSet<Instruction *, 4> UniqueUsers;
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI) {
Instruction *User = cast<Instruction>(*UI);
if (!UniqueUsers.insert(User))
continue;
// Do not infinitely recurse on PHI nodes.
if (isa<PHINode>(User) && Processed.count(User))
continue;
// Descend recursively, but not into PHI nodes outside the current loop.
// It's important to see the entire expression outside the loop to get
// choices that depend on addressing mode use right, although we won't
// consider references outside the loop in all cases.
// If User is already in Processed, we don't want to recurse into it again,
// but do want to record a second reference in the same instruction.
bool AddUserToIVUsers = false;
if (LI->getLoopFor(User->getParent()) != L) {
if (isa<PHINode>(User) || Processed.count(User) ||
!AddUsersIfInteresting(User)) {
DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
<< " OF SCEV: " << *ISE << '\n');
AddUserToIVUsers = true;
}
} else if (Processed.count(User) ||
!AddUsersIfInteresting(User)) {
DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
<< " 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. If we are a use inside of the loop, use
// the value before incrementation, otherwise use it after incrementation.
if (IVUseShouldUsePostIncValue(User, I, L, DT)) {
// The value used will be incremented by the stride more than we are
// expecting, so subtract this off.
const SCEV *NewStart = SE->getMinusSCEV(Start, Stride);
IVUses.push_back(new IVStrideUse(this, Stride, NewStart, User, I));
IVUses.back().setIsUseOfPostIncrementedValue(true);
DEBUG(dbgs() << " USING POSTINC SCEV, START=" << *NewStart<< "\n");
} else {
IVUses.push_back(new IVStrideUse(this, Stride, Start, User, I));
}
}
}
return true;
}
IVStrideUse &IVUsers::AddUser(const SCEV *Stride, const SCEV *Offset,
Instruction *User, Value *Operand) {
IVUses.push_back(new IVStrideUse(this, Stride, Offset, User, Operand));
return IVUses.back();
}
IVUsers::IVUsers()
: LoopPass(&ID) {
}
void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorTree>();
AU.addRequired<ScalarEvolution>();
AU.setPreservesAll();
}
bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
L = l;
LI = &getAnalysis<LoopInfo>();
DT = &getAnalysis<DominatorTree>();
SE = &getAnalysis<ScalarEvolution>();
// Find all uses of induction variables in this loop, and categorize
// 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.
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
AddUsersIfInteresting(I);
return false;
}
/// getReplacementExpr - Return a SCEV expression which computes the
/// value of the OperandValToReplace of the given IVStrideUse.
const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const {
// Start with zero.
const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
// Create the basic add recurrence.
RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
// Add the offset in a separate step, because it may be loop-variant.
RetVal = SE->getAddExpr(RetVal, U.getOffset());
// For uses of post-incremented values, add an extra stride to compute
// the actual replacement value.
if (U.isUseOfPostIncrementedValue())
RetVal = SE->getAddExpr(RetVal, U.getStride());
return RetVal;
}
/// getCanonicalExpr - Return a SCEV expression which computes the
/// value of the SCEV of the given IVStrideUse, ignoring the
/// isUseOfPostIncrementedValue flag.
const SCEV *IVUsers::getCanonicalExpr(const IVStrideUse &U) const {
// Start with zero.
const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
// Create the basic add recurrence.
RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
// Add the offset in a separate step, because it may be loop-variant.
RetVal = SE->getAddExpr(RetVal, U.getOffset());
return RetVal;
}
void IVUsers::print(raw_ostream &OS, const Module *M) const {
OS << "IV Users for loop ";
WriteAsOperand(OS, L->getHeader(), false);
if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
OS << " with backedge-taken count "
<< *SE->getBackedgeTakenCount(L);
}
OS << ":\n";
// Use a default AssemblyAnnotationWriter to suppress the default info
// comments, which aren't relevant here.
AssemblyAnnotationWriter Annotator;
for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
E = IVUses.end(); UI != E; ++UI) {
OS << " ";
WriteAsOperand(OS, UI->getOperandValToReplace(), false);
OS << " = "
<< *getReplacementExpr(*UI);
if (UI->isUseOfPostIncrementedValue())
OS << " (post-inc)";
OS << " in ";
UI->getUser()->print(OS, &Annotator);
OS << '\n';
}
}
void IVUsers::dump() const {
print(dbgs());
}
void IVUsers::releaseMemory() {
Processed.clear();
IVUses.clear();
}
void IVStrideUse::deleted() {
// Remove this user from the list.
Parent->IVUses.erase(this);
// this now dangles!
}