lib/Analysis/ScalarEvolutionExpander.cpp

   1 //===- ScalarEvolutionExpander.cpp - Scalar Evolution Analysis --*- C++ -*-===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by the LLVM research group and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file contains the implementation of the scalar evolution expander,
  11 // which is used to generate the code corresponding to a given scalar evolution
  12 // expression.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/Analysis/ScalarEvolutionExpander.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 using namespace llvm;
  19
  20 /// InsertCastOfTo - Insert a cast of V to the specified type, doing what
  21 /// we can to share the casts.
  22 Value *SCEVExpander::InsertCastOfTo(Instruction::CastOps opcode, Value *V,
  23                                     const Type *Ty) {
  24   // FIXME: keep track of the cast instruction.
  25   if (Constant *C = dyn_cast<Constant>(V))
  26     return ConstantExpr::getCast(opcode, C, Ty);
  27
  28   if (Argument *A = dyn_cast<Argument>(V)) {
  29     // Check to see if there is already a cast!
  30     for (Value::use_iterator UI = A->use_begin(), E = A->use_end();
  31          UI != E; ++UI) {
  32       if ((*UI)->getType() == Ty)
  33         if (CastInst *CI = dyn_cast<CastInst>(cast<Instruction>(*UI))) {
  34           // If the cast isn't the first instruction of the function, move it.
  35           if (BasicBlock::iterator(CI) !=
  36               A->getParent()->getEntryBlock().begin()) {
  37             CI->moveBefore(A->getParent()->getEntryBlock().begin());
  38           }
  39           return CI;
  40         }
  41     }
  42     return CastInst::create(opcode, V, Ty, V->getName(),
  43                             A->getParent()->getEntryBlock().begin());
  44   }
  45
  46   Instruction *I = cast<Instruction>(V);
  47
  48   // Check to see if there is already a cast.  If there is, use it.
  49   for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
  50        UI != E; ++UI) {
  51     if ((*UI)->getType() == Ty)
  52       if (CastInst *CI = dyn_cast<CastInst>(cast<Instruction>(*UI))) {
  53         BasicBlock::iterator It = I; ++It;
  54         if (isa<InvokeInst>(I))
  55           It = cast<InvokeInst>(I)->getNormalDest()->begin();
  56         while (isa<PHINode>(It)) ++It;
  57         if (It != BasicBlock::iterator(CI)) {
  58           // Splice the cast immediately after the operand in question.
  59           CI->moveBefore(It);
  60         }
  61         return CI;
  62       }
  63   }
  64   BasicBlock::iterator IP = I; ++IP;
  65   if (InvokeInst *II = dyn_cast<InvokeInst>(I))
  66     IP = II->getNormalDest()->begin();
  67   while (isa<PHINode>(IP)) ++IP;
  68   return CastInst::create(opcode, V, Ty, V->getName(), IP);
  69 }
  70
  71 /// InsertBinop - Insert the specified binary operator, doing a small amount
  72 /// of work to avoid inserting an obviously redundant operation.
  73 Value *SCEVExpander::InsertBinop(Instruction::BinaryOps Opcode, Value *LHS,
  74                                  Value *RHS, Instruction *&InsertPt) {
  75   // Fold a binop with constant operands.
  76   if (Constant *CLHS = dyn_cast<Constant>(LHS))
  77     if (Constant *CRHS = dyn_cast<Constant>(RHS))
  78       return ConstantExpr::get(Opcode, CLHS, CRHS);
  79
  80   // Do a quick scan to see if we have this binop nearby.  If so, reuse it.
  81   unsigned ScanLimit = 6;
  82   for (BasicBlock::iterator IP = InsertPt, E = InsertPt->getParent()->begin();
  83        ScanLimit; --IP, --ScanLimit) {
  84     if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(IP))
  85       if (BinOp->getOpcode() == Opcode && BinOp->getOperand(0) == LHS &&
  86           BinOp->getOperand(1) == RHS) {
  87         // If we found the instruction *at* the insert point, insert later
  88         // instructions after it.
  89         if (BinOp == InsertPt)
  90           InsertPt = ++IP;
  91         return BinOp;
  92       }
  93     if (IP == E) break;
  94   }
  95
  96   // If we don't have
  97   return BinaryOperator::create(Opcode, LHS, RHS, "tmp.", InsertPt);
  98 }
  99
 100 Value *SCEVExpander::visitMulExpr(SCEVMulExpr *S) {
 101   int FirstOp = 0;  // Set if we should emit a subtract.
 102   if (SCEVConstant *SC = dyn_cast<SCEVConstant>(S->getOperand(0)))
 103     if (SC->getValue()->isAllOnesValue())
 104       FirstOp = 1;
 105
 106   int i = S->getNumOperands()-2;
 107   Value *V = expand(S->getOperand(i+1));
 108
 109   // Emit a bunch of multiply instructions
 110   for (; i >= FirstOp; --i)
 111     V = InsertBinop(Instruction::Mul, V, expand(S->getOperand(i)),
 112                     InsertPt);
 113   // -1 * ...  --->  0 - ...
 114   if (FirstOp == 1)
 115     V = InsertBinop(Instruction::Sub, Constant::getNullValue(V->getType()), V,
 116                     InsertPt);
 117   return V;
 118 }
 119
 120 Value *SCEVExpander::visitAddRecExpr(SCEVAddRecExpr *S) {
 121   const Type *Ty = S->getType();
 122   const Loop *L = S->getLoop();
 123   // We cannot yet do fp recurrences, e.g. the xform of {X,+,F} --> X+{0,+,F}
 124   assert(Ty->isInteger() && "Cannot expand fp recurrences yet!");
 125
 126   // {X,+,F} --> X + {0,+,F}
 127   if (!isa<SCEVConstant>(S->getStart()) ||
 128       !cast<SCEVConstant>(S->getStart())->getValue()->isZero()) {
 129     Value *Start = expand(S->getStart());
 130     std::vector<SCEVHandle> NewOps(S->op_begin(), S->op_end());
 131     NewOps[0] = SCEVUnknown::getIntegerSCEV(0, Ty);
 132     Value *Rest = expand(SCEVAddRecExpr::get(NewOps, L));
 133
 134     // FIXME: look for an existing add to use.
 135     return InsertBinop(Instruction::Add, Rest, Start, InsertPt);
 136   }
 137
 138   // {0,+,1} --> Insert a canonical induction variable into the loop!
 139   if (S->getNumOperands() == 2 &&
 140       S->getOperand(1) == SCEVUnknown::getIntegerSCEV(1, Ty)) {
 141     // Create and insert the PHI node for the induction variable in the
 142     // specified loop.
 143     BasicBlock *Header = L->getHeader();
 144     PHINode *PN = new PHINode(Ty, "indvar", Header->begin());
 145     PN->addIncoming(Constant::getNullValue(Ty), L->getLoopPreheader());
 146
 147     pred_iterator HPI = pred_begin(Header);
 148     assert(HPI != pred_end(Header) && "Loop with zero preds???");
 149     if (!L->contains(*HPI)) ++HPI;
 150     assert(HPI != pred_end(Header) && L->contains(*HPI) &&
 151            "No backedge in loop?");
 152
 153     // Insert a unit add instruction right before the terminator corresponding
 154     // to the back-edge.
 155     Constant *One = ConstantInt::get(Ty, 1);
 156     Instruction *Add = BinaryOperator::createAdd(PN, One, "indvar.next",
 157                                                  (*HPI)->getTerminator());
 158
 159     pred_iterator PI = pred_begin(Header);
 160     if (*PI == L->getLoopPreheader())
 161       ++PI;
 162     PN->addIncoming(Add, *PI);
 163     return PN;
 164   }
 165
 166   // Get the canonical induction variable I for this loop.
 167   Value *I = getOrInsertCanonicalInductionVariable(L, Ty);
 168
 169   // If this is a simple linear addrec, emit it now as a special case.
 170   if (S->getNumOperands() == 2) {   // {0,+,F} --> i*F
 171     Value *F = expand(S->getOperand(1));
 172
 173     // IF the step is by one, just return the inserted IV.
 174     if (ConstantInt *CI = dyn_cast<ConstantInt>(F))
 175       if (CI->getValue() == 1)
 176         return I;
 177
 178     // If the insert point is directly inside of the loop, emit the multiply at
 179     // the insert point.  Otherwise, L is a loop that is a parent of the insert
 180     // point loop.  If we can, move the multiply to the outer most loop that it
 181     // is safe to be in.
 182     Instruction *MulInsertPt = InsertPt;
 183     Loop *InsertPtLoop = LI.getLoopFor(MulInsertPt->getParent());
 184     if (InsertPtLoop != L && InsertPtLoop &&
 185         L->contains(InsertPtLoop->getHeader())) {
 186       while (InsertPtLoop != L) {
 187         // If we cannot hoist the multiply out of this loop, don't.
 188         if (!InsertPtLoop->isLoopInvariant(F)) break;
 189
 190         // Otherwise, move the insert point to the preheader of the loop.
 191         MulInsertPt = InsertPtLoop->getLoopPreheader()->getTerminator();
 192         InsertPtLoop = InsertPtLoop->getParentLoop();
 193       }
 194     }
 195
 196     return InsertBinop(Instruction::Mul, I, F, MulInsertPt);
 197   }
 198
 199   // If this is a chain of recurrences, turn it into a closed form, using the
 200   // folders, then expandCodeFor the closed form.  This allows the folders to
 201   // simplify the expression without having to build a bunch of special code
 202   // into this folder.
 203   SCEVHandle IH = SCEVUnknown::get(I);   // Get I as a "symbolic" SCEV.
 204
 205   SCEVHandle V = S->evaluateAtIteration(IH);
 206   //cerr << "Evaluated: " << *this << "\n     to: " << *V << "\n";
 207
 208   return expand(V);
 209 }