Added a SimplifyIndVar utility to simplify induction variable users

based on ScalarEvolution without changing the induction variable phis.

This utility is the main tool of IndVarSimplifyPass, but the pass also
restructures induction variables in strange ways that are sensitive to
pass ordering. This provides a way for other loop passes to simplify
new uses of induction variables created during transformation. The
utility may be used by any pass that preserves ScalarEvolution. Soon
LoopUnroll will use it.

The net effect in this checkin is to cleanup the IndVarSimplify pass
by factoring out the SimplifyIndVar algorithm into a standalone utility.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137197 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 413 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/SimplifyIndVar.cpp b/lib/Transforms/Utils/SimplifyIndVar.cpp
new file mode 100644
index 0000000000..a3cf79694c
--- /dev/null
+++ b/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -0,0 +1,413 @@
+//===-- SimplifyIndVar.cpp - Induction variable simplification ------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements induction variable simplification. It does
+// not define any actual pass or policy, but provides a single function to
+// simplify a loop's induction variables based on ScalarEvolution.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "indvars"
+
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/IVUsers.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/SimplifyIndVar.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+
+using namespace llvm;
+
+STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
+STATISTIC(NumElimOperand,  "Number of IV operands folded into a use");
+STATISTIC(NumElimRem     , "Number of IV remainder operations eliminated");
+STATISTIC(NumElimCmp     , "Number of IV comparisons eliminated");
+
+namespace {
+  /// SimplifyIndvar - This is a utility for simplifying induction variables
+  /// based on ScalarEvolution. It is the primary instrument of the
+  /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
+  /// other loop passes that preserve SCEV.
+  class SimplifyIndvar {
+    Loop             *L;
+    LoopInfo         *LI;
+    DominatorTree    *DT;
+    ScalarEvolution  *SE;
+    IVUsers          *IU; // NULL for DisableIVRewrite
+    const TargetData *TD; // May be NULL
+
+    SmallVectorImpl<WeakVH> &DeadInsts;
+
+    bool Changed;
+
+  public:
+    SimplifyIndvar(Loop *Loop, LPPassManager *LPM,
+                   SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = NULL) :
+      L(Loop),
+      LI(LPM->getAnalysisIfAvailable<LoopInfo>()),
+      SE(LPM->getAnalysisIfAvailable<ScalarEvolution>()),
+      IU(IVU),
+      TD(LPM->getAnalysisIfAvailable<TargetData>()),
+      DeadInsts(Dead),
+      Changed(false) {
+      assert(LI && SE && "IV simplification requires ScalarEvolution");
+    }
+
+    bool hasChanged() const { return Changed; }
+
+    /// Iteratively perform simplification on a worklist of users of the
+    /// specified induction variable. This is the top-level driver that applies
+    /// all simplicitions to users of an IV.
+    void simplifyUsers(PHINode *CurrIV, IVVisitor *V = NULL);
+
+    bool foldIVUser(Instruction *UseInst, Instruction *IVOperand);
+
+    bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
+    void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
+    void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
+                              bool IsSigned);
+  };
+}
+
+/// foldIVUser - Fold an IV operand into its use.  This removes increments of an
+/// aligned IV when used by a instruction that ignores the low bits.
+bool SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
+  Value *IVSrc = 0;
+  unsigned OperIdx = 0;
+  const SCEV *FoldedExpr = 0;
+  switch (UseInst->getOpcode()) {
+  default:
+    return false;
+  case Instruction::UDiv:
+  case Instruction::LShr:
+    // We're only interested in the case where we know something about
+    // the numerator and have a constant denominator.
+    if (IVOperand != UseInst->getOperand(OperIdx) ||
+        !isa<ConstantInt>(UseInst->getOperand(1)))
+      return false;
+
+    // Attempt to fold a binary operator with constant operand.
+    // e.g. ((I + 1) >> 2) => I >> 2
+    if (IVOperand->getNumOperands() != 2 ||
+        !isa<ConstantInt>(IVOperand->getOperand(1)))
+      return false;
+
+    IVSrc = IVOperand->getOperand(0);
+    // IVSrc must be the (SCEVable) IV, since the other operand is const.
+    assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
+
+    ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
+    if (UseInst->getOpcode() == Instruction::LShr) {
+      // Get a constant for the divisor. See createSCEV.
+      uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
+      if (D->getValue().uge(BitWidth))
+        return false;
+
+      D = ConstantInt::get(UseInst->getContext(),
+                           APInt(BitWidth, 1).shl(D->getZExtValue()));
+    }
+    FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
+  }
+  // We have something that might fold it's operand. Compare SCEVs.
+  if (!SE->isSCEVable(UseInst->getType()))
+    return false;
+
+  // Bypass the operand if SCEV can prove it has no effect.
+  if (SE->getSCEV(UseInst) != FoldedExpr)
+    return false;
+
+  DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
+        << " -> " << *UseInst << '\n');
+
+  UseInst->setOperand(OperIdx, IVSrc);
+  assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
+
+  ++NumElimOperand;
+  Changed = true;
+  if (IVOperand->use_empty())
+    DeadInsts.push_back(IVOperand);
+  return true;
+}
+
+/// eliminateIVComparison - SimplifyIVUsers helper for eliminating useless
+/// comparisons against an induction variable.
+void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
+  unsigned IVOperIdx = 0;
+  ICmpInst::Predicate Pred = ICmp->getPredicate();
+  if (IVOperand != ICmp->getOperand(0)) {
+    // Swapped
+    assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
+    IVOperIdx = 1;
+    Pred = ICmpInst::getSwappedPredicate(Pred);
+  }
+
+  // Get the SCEVs for the ICmp operands.
+  const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
+  const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
+
+  // Simplify unnecessary loops away.
+  const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
+  S = SE->getSCEVAtScope(S, ICmpLoop);
+  X = SE->getSCEVAtScope(X, ICmpLoop);
+
+  // If the condition is always true or always false, replace it with
+  // a constant value.
+  if (SE->isKnownPredicate(Pred, S, X))
+    ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
+  else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
+    ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
+  else
+    return;
+
+  DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
+  ++NumElimCmp;
+  Changed = true;
+  DeadInsts.push_back(ICmp);
+}
+
+/// eliminateIVRemainder - SimplifyIVUsers helper for eliminating useless
+/// remainder operations operating on an induction variable.
+void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
+                                      Value *IVOperand,
+                                      bool IsSigned) {
+  // We're only interested in the case where we know something about
+  // the numerator.
+  if (IVOperand != Rem->getOperand(0))
+    return;
+
+  // Get the SCEVs for the ICmp operands.
+  const SCEV *S = SE->getSCEV(Rem->getOperand(0));
+  const SCEV *X = SE->getSCEV(Rem->getOperand(1));
+
+  // Simplify unnecessary loops away.
+  const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
+  S = SE->getSCEVAtScope(S, ICmpLoop);
+  X = SE->getSCEVAtScope(X, ICmpLoop);
+
+  // i % n  -->  i  if i is in [0,n).
+  if ((!IsSigned || SE->isKnownNonNegative(S)) &&
+      SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
+                           S, X))
+    Rem->replaceAllUsesWith(Rem->getOperand(0));
+  else {
+    // (i+1) % n  -->  (i+1)==n?0:(i+1)  if i is in [0,n).
+    const SCEV *LessOne =
+      SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
+    if (IsSigned && !SE->isKnownNonNegative(LessOne))
+      return;
+
+    if (!SE->isKnownPredicate(IsSigned ?
+                              ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
+                              LessOne, X))
+      return;
+
+    ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
+                                  Rem->getOperand(0), Rem->getOperand(1),
+                                  "tmp");
+    SelectInst *Sel =
+      SelectInst::Create(ICmp,
+                         ConstantInt::get(Rem->getType(), 0),
+                         Rem->getOperand(0), "tmp", Rem);
+    Rem->replaceAllUsesWith(Sel);
+  }
+
+  // Inform IVUsers about the new users.
+  if (IU) {
+    if (Instruction *I = dyn_cast<Instruction>(Rem->getOperand(0)))
+      IU->AddUsersIfInteresting(I);
+  }
+  DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
+  ++NumElimRem;
+  Changed = true;
+  DeadInsts.push_back(Rem);
+}
+
+/// eliminateIVUser - Eliminate an operation that consumes a simple IV and has
+/// no observable side-effect given the range of IV values.
+bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
+                                     Instruction *IVOperand) {
+  if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
+    eliminateIVComparison(ICmp, IVOperand);
+    return true;
+  }
+  if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
+    bool IsSigned = Rem->getOpcode() == Instruction::SRem;
+    if (IsSigned || Rem->getOpcode() == Instruction::URem) {
+      eliminateIVRemainder(Rem, IVOperand, IsSigned);
+      return true;
+    }
+  }
+
+  // Eliminate any operation that SCEV can prove is an identity function.
+  if (!SE->isSCEVable(UseInst->getType()) ||
+      (UseInst->getType() != IVOperand->getType()) ||
+      (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
+    return false;
+
+  DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
+
+  UseInst->replaceAllUsesWith(IVOperand);
+  ++NumElimIdentity;
+  Changed = true;
+  DeadInsts.push_back(UseInst);
+  return true;
+}
+
+/// pushIVUsers - Add all uses of Def to the current IV's worklist.
+///
+static void pushIVUsers(
+  Instruction *Def,
+  SmallPtrSet<Instruction*,16> &Simplified,
+  SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
+
+  for (Value::use_iterator UI = Def->use_begin(), E = Def->use_end();
+       UI != E; ++UI) {
+    Instruction *User = cast<Instruction>(*UI);
+
+    // Avoid infinite or exponential worklist processing.
+    // Also ensure unique worklist users.
+    // If Def is a LoopPhi, it may not be in the Simplified set, so check for
+    // self edges first.
+    if (User != Def && Simplified.insert(User))
+      SimpleIVUsers.push_back(std::make_pair(User, Def));
+  }
+}
+
+/// isSimpleIVUser - Return true if this instruction generates a simple SCEV
+/// expression in terms of that IV.
+///
+/// This is similar to IVUsers' isInsteresting() but processes each instruction
+/// non-recursively when the operand is already known to be a simpleIVUser.
+///
+static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
+  if (!SE->isSCEVable(I->getType()))
+    return false;
+
+  // Get the symbolic expression for this instruction.
+  const SCEV *S = SE->getSCEV(I);
+
+  // Only consider affine recurrences.
+  const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
+  if (AR && AR->getLoop() == L)
+    return true;
+
+  return false;
+}
+
+/// simplifyUsers - Iteratively perform simplification on a worklist of users
+/// of the specified induction variable. Each successive simplification may push
+/// more users which may themselves be candidates for simplification.
+///
+/// This algorithm does not require IVUsers analysis. Instead, it simplifies
+/// instructions in-place during analysis. Rather than rewriting induction
+/// variables bottom-up from their users, it transforms a chain of IVUsers
+/// top-down, updating the IR only when it encouters a clear optimization
+/// opportunitiy.
+///
+/// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
+///
+void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
+  // Instructions processed by SimplifyIndvar for CurrIV.
+  SmallPtrSet<Instruction*,16> Simplified;
+
+  // Use-def pairs if IV users waiting to be processed for CurrIV.
+  SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
+
+  // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
+  // called multiple times for the same LoopPhi. This is the proper thing to
+  // do for loop header phis that use each other.
+  pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
+
+  while (!SimpleIVUsers.empty()) {
+    std::pair<Instruction*, Instruction*> UseOper =
+      SimpleIVUsers.pop_back_val();
+    // Bypass back edges to avoid extra work.
+    if (UseOper.first == CurrIV) continue;
+
+    foldIVUser(UseOper.first, UseOper.second);
+
+    if (eliminateIVUser(UseOper.first, UseOper.second)) {
+      pushIVUsers(UseOper.second, Simplified, SimpleIVUsers);
+      continue;
+    }
+    CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
+    if (V && Cast) {
+      V->visitCast(Cast);
+      continue;
+    }
+    if (isSimpleIVUser(UseOper.first, L, SE)) {
+      pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
+    }
+  }
+}
+
+namespace llvm {
+
+/// simplifyUsersOfIV - Simplify instructions that use this induction variable
+/// by using ScalarEvolution to analyze the IV's recurrence.
+bool simplifyUsersOfIV(PHINode *CurrIV, LPPassManager *LPM,
+                       SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
+{
+  LoopInfo *LI = &LPM->getAnalysis<LoopInfo>();
+  SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), LPM, Dead);
+  SIV.simplifyUsers(CurrIV, V);
+  return SIV.hasChanged();
+}
+
+/// simplifyLoopIVs - Simplify users of induction variables within this
+/// loop. This does not actually change or add IVs.
+bool simplifyLoopIVs(Loop *L, LPPassManager *LPM,
+                     SmallVectorImpl<WeakVH> &Dead) {
+  bool Changed = false;
+  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
+    Changed |= simplifyUsersOfIV(cast<PHINode>(I), LPM, Dead);
+  }
+  return Changed;
+}
+
+/// simplifyIVUsers - Perform simplification on instructions recorded by the
+/// IVUsers pass.
+///
+/// This is the old approach to IV simplification to be replaced by
+/// SimplifyLoopIVs.
+bool simplifyIVUsers(IVUsers *IU, LPPassManager *LPM,
+                     SmallVectorImpl<WeakVH> &Dead) {
+  SimplifyIndvar SIV(IU->getLoop(), LPM, Dead);
+
+  // Each round of simplification involves a round of eliminating operations
+  // followed by a round of widening IVs. A single IVUsers worklist is used
+  // across all rounds. The inner loop advances the user. If widening exposes
+  // more uses, then another pass through the outer loop is triggered.
+  for (IVUsers::iterator I = IU->begin(); I != IU->end(); ++I) {
+    Instruction *UseInst = I->getUser();
+    Value *IVOperand = I->getOperandValToReplace();
+
+    if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
+      SIV.eliminateIVComparison(ICmp, IVOperand);
+      continue;
+    }
+    if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
+      bool IsSigned = Rem->getOpcode() == Instruction::SRem;
+      if (IsSigned || Rem->getOpcode() == Instruction::URem) {
+        SIV.eliminateIVRemainder(Rem, IVOperand, IsSigned);
+        continue;
+      }
+    }
+  }
+  return SIV.hasChanged();
+}
+
+} // namespace llvm