blob: 00622d360acbfdad79c01a2a28037ec86d9b5aad [file] [log] [blame]
//===- 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/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.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();
/// containsAddRecFromDifferentLoop - Determine whether expression S involves a
/// subexpression that is an AddRec from a loop other than L. An outer loop
/// of L is OK, but not an inner loop nor a disjoint loop.
static bool containsAddRecFromDifferentLoop(const SCEV *S, Loop *L) {
// This is very common, put it first.
if (isa<SCEVConstant>(S))
return false;
if (const SCEVCommutativeExpr *AE = dyn_cast<SCEVCommutativeExpr>(S)) {
for (unsigned int i=0; i< AE->getNumOperands(); i++)
if (containsAddRecFromDifferentLoop(AE->getOperand(i), L))
return true;
return false;
if (const SCEVAddRecExpr *AE = dyn_cast<SCEVAddRecExpr>(S)) {
if (const Loop *newLoop = AE->getLoop()) {
if (newLoop == L)
return false;
// if newLoop is an outer loop of L, this is OK.
if (!LoopInfo::isNotAlreadyContainedIn(L, newLoop))
return false;
return true;
if (const SCEVUDivExpr *DE = dyn_cast<SCEVUDivExpr>(S))
return containsAddRecFromDifferentLoop(DE->getLHS(), L) ||
containsAddRecFromDifferentLoop(DE->getRHS(), L);
#if 0
// SCEVSDivExpr has been backed out temporarily, but will be back; we'll
// need this when it is.
if (const SCEVSDivExpr *DE = dyn_cast<SCEVSDivExpr>(S))
return containsAddRecFromDifferentLoop(DE->getLHS(), L) ||
containsAddRecFromDifferentLoop(DE->getRHS(), L);
if (const SCEVCastExpr *CE = dyn_cast<SCEVCastExpr>(S))
return containsAddRecFromDifferentLoop(CE->getOperand(), L);
return false;
/// 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))) {
if (AddRec->getLoop() == L)
TheAddRec = SE->getAddExpr(AddRec, TheAddRec);
return false; // Nested IV of some sort?
} else {
Start = SE->getAddExpr(Start, AE->getOperand(i));
} else if (isa<SCEVAddRecExpr>(SH)) {
TheAddRec = SH;
} else {
return false; // not analyzable.
const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(TheAddRec);
if (!AddRec || AddRec->getLoop() != L) return false;
// Use getSCEVAtScope to attempt to simplify other loops out of
// the picture.
const SCEV *AddRecStart = AddRec->getStart();
AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop);
const SCEV *AddRecStride = AddRec->getStepRecurrence(*SE);
// FIXME: If Start contains an SCEVAddRecExpr from a different loop, other
// than an outer loop of the current loop, reject it. LSR has no concept of
// operating on more than one loop at a time so don't confuse it with such
// expressions.
if (containsAddRecFromDifferentLoop(AddRecStart, L))
return false;
Start = SE->getAddExpr(Start, AddRecStart);
// If stride is an instruction, make sure it dominates the loop preheader.
// Otherwise we could end up with a use before def situation.
if (!isa<SCEVConstant>(AddRecStride)) {
BasicBlock *Preheader = L->getLoopPreheader();
if (!AddRecStride->dominates(Preheader, DT))
return false;
DEBUG(errs() << "[" << L->getHeader()->getName()
<< "] Variable stride: " << *AddRec << "\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, LoopInfo *LI, DominatorTree *DT,
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 (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) {
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.
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))
// Do not infinitely recurse on PHI nodes.
if (isa<PHINode>(User) && Processed.count(User))
// 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 ouside 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)) {
DOUT << "FOUND USER in other loop: " << *User << '\n'
<< " OF SCEV: " << *ISE << "\n";
AddUserToIVUsers = true;
} else if (Processed.count(User) ||
!AddUsersIfInteresting(User)) {
DOUT << "FOUND USER: " << *User << '\n'
<< " OF SCEV: " << *ISE << "\n";
AddUserToIVUsers = true;
if (AddUserToIVUsers) {
IVUsersOfOneStride *StrideUses = IVUsesByStride[Stride];
if (!StrideUses) { // First occurrence of this stride?
StrideUses = new IVUsersOfOneStride(Stride);
IVUsesByStride[Stride] = StrideUses;
// 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, LI, DT, this)) {
// 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);
StrideUses->addUser(NewStart, User, I);
DOUT << " USING POSTINC SCEV, START=" << *NewStart<< "\n";
} else {
StrideUses->addUser(Start, User, I);
return true;
: LoopPass(&ID) {
void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
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)
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.getParent()->Stride->getType());
// Create the basic add recurrence.
RetVal = SE->getAddRecExpr(RetVal, U.getParent()->Stride, 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.getParent()->Stride);
// Evaluate the expression out of the loop, if possible.
if (!L->contains(U.getUser()->getParent())) {
const SCEV *ExitVal = SE->getSCEVAtScope(RetVal, L->getParentLoop());
if (ExitVal->isLoopInvariant(L))
RetVal = ExitVal;
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";
for (unsigned Stride = 0, e = StrideOrder.size(); Stride != e; ++Stride) {
std::map<const SCEV *, IVUsersOfOneStride*>::const_iterator SI =
assert(SI != IVUsesByStride.end() && "Stride doesn't exist!");
OS << " Stride " << *SI->first->getType() << " " << *SI->first << ":\n";
for (ilist<IVStrideUse>::const_iterator UI = SI->second->Users.begin(),
E = SI->second->Users.end(); UI != E; ++UI) {
OS << " ";
WriteAsOperand(OS, UI->getOperandValToReplace(), false);
OS << " = ";
OS << *getReplacementExpr(*UI);
if (UI->isUseOfPostIncrementedValue())
OS << " (post-inc)";
OS << " in ";
OS << '\n';
void IVUsers::print(std::ostream &o, const Module *M) const {
raw_os_ostream OS(o);
print(OS, M);
void IVUsers::dump() const {
void IVUsers::releaseMemory() {
void IVStrideUse::deleted() {
// Remove this user from the list.
// this now dangles!