ScalarEvolutionExpander.cpp revision 246b2564d3bbbafe06ebf6a67745cd24141b5cb4 
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"
18using namespace llvm;
19
20/// InsertCastOfTo - Insert a cast of V to the specified type, doing what
21/// we can to share the casts.
22Value *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.
73Value *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
100Value *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
120Value *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] = SE.getIntegerSCEV(0, Ty);
132    Value *Rest = expand(SE.getAddRecExpr(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) == SE.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 = SE.getUnknown(I);   // Get I as a "symbolic" SCEV.
204
205  SCEVHandle V = S->evaluateAtIteration(IH, SE);
206  //cerr << "Evaluated: " << *this << "\n     to: " << *V << "\n";
207
208  return expand(V);
209}
210
211Value *SCEVExpander::expand(SCEV *S) {
212  // Check to see if we already expanded this.
213  std::map<SCEVHandle, Value*>::iterator I = InsertedExpressions.find(S);
214  if (I != InsertedExpressions.end())
215    return I->second;
216
217  Value *V = visit(S);
218  InsertedExpressions[S] = V;
219  return V;
220}
221
222