Local.cpp revision 9fa038dc21e966dceb23f9410351e863e3ce1114 
1//===-- Local.cpp - Functions to perform local transformations ------------===//
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 family of functions perform various local transformations to the
11// program.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Transforms/Utils/Local.h"
16#include "llvm/Constants.h"
17#include "llvm/DerivedTypes.h"
18#include "llvm/Instructions.h"
19#include "llvm/Intrinsics.h"
20#include "llvm/Analysis/ConstantFolding.h"
21#include "llvm/Target/TargetData.h"
22#include "llvm/Support/GetElementPtrTypeIterator.h"
23#include "llvm/Support/MathExtras.h"
24#include "llvm/ADT/SmallVector.h"
25#include <cerrno>
26using namespace llvm;
27
28//===----------------------------------------------------------------------===//
29//  Local constant propagation...
30//
31
32/// doConstantPropagation - If an instruction references constants, try to fold
33/// them together...
34///
35bool llvm::doConstantPropagation(BasicBlock::iterator &II,
36                                 const TargetData *TD) {
37  if (Constant *C = ConstantFoldInstruction(II, TD)) {
38    // Replaces all of the uses of a variable with uses of the constant.
39    II->replaceAllUsesWith(C);
40
41    // Remove the instruction from the basic block...
42    II = II->getParent()->getInstList().erase(II);
43    return true;
44  }
45
46  return false;
47}
48
49/// ConstantFoldInstruction - Attempt to constant fold the specified
50/// instruction.  If successful, the constant result is returned, if not, null
51/// is returned.  Note that this function can only fail when attempting to fold
52/// instructions like loads and stores, which have no constant expression form.
53///
54Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
55  if (PHINode *PN = dyn_cast<PHINode>(I)) {
56    if (PN->getNumIncomingValues() == 0)
57      return Constant::getNullValue(PN->getType());
58
59    Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
60    if (Result == 0) return 0;
61
62    // Handle PHI nodes specially here...
63    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
64      if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
65        return 0;   // Not all the same incoming constants...
66
67    // If we reach here, all incoming values are the same constant.
68    return Result;
69  }
70
71  Constant *Op0 = 0, *Op1 = 0;
72  switch (I->getNumOperands()) {
73  default:
74  case 2:
75    Op1 = dyn_cast<Constant>(I->getOperand(1));
76    if (Op1 == 0) return 0;        // Not a constant?, can't fold
77    /* FALL THROUGH */
78  case 1:
79    Op0 = dyn_cast<Constant>(I->getOperand(0));
80    if (Op0 == 0) return 0;        // Not a constant?, can't fold
81    break;
82  case 0: return 0;
83  }
84
85  // Scan the operand list, checking to see if they are all constants, if so,
86  // hand off to ConstantFoldInstOperands.
87  SmallVector<Constant*, 8> Ops;
88  for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
89    if (Constant *Op = dyn_cast<Constant>(I->getOperand(i)))
90      Ops.push_back(Op);
91    else
92      return 0;  // All operands not constant!
93
94  return ConstantFoldInstOperands(I, &Ops[0], Ops.size());
95}
96
97/// ConstantFoldInstOperands - Attempt to constant fold an instruction with the
98/// specified opcode and operands.  If successful, the constant result is
99/// returned, if not, null is returned.  Note that this function can fail when
100/// attempting to fold instructions like loads and stores, which have no
101/// constant expression form.
102///
103Constant *llvm::ConstantFoldInstOperands(const Instruction* I,
104                                         Constant** Ops, unsigned NumOps,
105                                         const TargetData *TD) {
106  unsigned Opc = I->getOpcode();
107  const Type *DestTy = I->getType();
108
109  // Handle easy binops first
110  if (isa<BinaryOperator>(I))
111    return ConstantExpr::get(Opc, Ops[0], Ops[1]);
112
113  switch (Opc) {
114  default: return 0;
115  case Instruction::Call:
116    if (Function *F = dyn_cast<Function>(Ops[0]))
117      if (canConstantFoldCallTo(F))
118        return ConstantFoldCall(F, Ops+1, NumOps);
119    return 0;
120  case Instruction::ICmp:
121  case Instruction::FCmp:
122    return ConstantExpr::getCompare(cast<CmpInst>(I)->getPredicate(), Ops[0],
123                                    Ops[1]);
124  case Instruction::Shl:
125  case Instruction::LShr:
126  case Instruction::AShr:
127    return ConstantExpr::get(Opc, Ops[0], Ops[1]);
128  case Instruction::Trunc:
129  case Instruction::ZExt:
130  case Instruction::SExt:
131  case Instruction::FPTrunc:
132  case Instruction::FPExt:
133  case Instruction::UIToFP:
134  case Instruction::SIToFP:
135  case Instruction::FPToUI:
136  case Instruction::FPToSI:
137  case Instruction::PtrToInt:
138  case Instruction::IntToPtr:
139  case Instruction::BitCast:
140    return ConstantExpr::getCast(Opc, Ops[0], DestTy);
141  case Instruction::Select:
142    return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]);
143  case Instruction::ExtractElement:
144    return ConstantExpr::getExtractElement(Ops[0], Ops[1]);
145  case Instruction::InsertElement:
146    return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]);
147  case Instruction::ShuffleVector:
148    return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]);
149  case Instruction::GetElementPtr:
150    return ConstantExpr::getGetElementPtr(Ops[0],
151                                          std::vector<Constant*>(Ops+1,
152                                                                 Ops+NumOps));
153  }
154}
155
156// ConstantFoldTerminator - If a terminator instruction is predicated on a
157// constant value, convert it into an unconditional branch to the constant
158// destination.
159//
160bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
161  TerminatorInst *T = BB->getTerminator();
162
163  // Branch - See if we are conditional jumping on constant
164  if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
165    if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
166    BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
167    BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
168
169    if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
170      // Are we branching on constant?
171      // YES.  Change to unconditional branch...
172      BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
173      BasicBlock *OldDest     = Cond->getZExtValue() ? Dest2 : Dest1;
174
175      //cerr << "Function: " << T->getParent()->getParent()
176      //     << "\nRemoving branch from " << T->getParent()
177      //     << "\n\nTo: " << OldDest << endl;
178
179      // Let the basic block know that we are letting go of it.  Based on this,
180      // it will adjust it's PHI nodes.
181      assert(BI->getParent() && "Terminator not inserted in block!");
182      OldDest->removePredecessor(BI->getParent());
183
184      // Set the unconditional destination, and change the insn to be an
185      // unconditional branch.
186      BI->setUnconditionalDest(Destination);
187      return true;
188    } else if (Dest2 == Dest1) {       // Conditional branch to same location?
189      // This branch matches something like this:
190      //     br bool %cond, label %Dest, label %Dest
191      // and changes it into:  br label %Dest
192
193      // Let the basic block know that we are letting go of one copy of it.
194      assert(BI->getParent() && "Terminator not inserted in block!");
195      Dest1->removePredecessor(BI->getParent());
196
197      // Change a conditional branch to unconditional.
198      BI->setUnconditionalDest(Dest1);
199      return true;
200    }
201  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
202    // If we are switching on a constant, we can convert the switch into a
203    // single branch instruction!
204    ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
205    BasicBlock *TheOnlyDest = SI->getSuccessor(0);  // The default dest
206    BasicBlock *DefaultDest = TheOnlyDest;
207    assert(TheOnlyDest == SI->getDefaultDest() &&
208           "Default destination is not successor #0?");
209
210    // Figure out which case it goes to...
211    for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
212      // Found case matching a constant operand?
213      if (SI->getSuccessorValue(i) == CI) {
214        TheOnlyDest = SI->getSuccessor(i);
215        break;
216      }
217
218      // Check to see if this branch is going to the same place as the default
219      // dest.  If so, eliminate it as an explicit compare.
220      if (SI->getSuccessor(i) == DefaultDest) {
221        // Remove this entry...
222        DefaultDest->removePredecessor(SI->getParent());
223        SI->removeCase(i);
224        --i; --e;  // Don't skip an entry...
225        continue;
226      }
227
228      // Otherwise, check to see if the switch only branches to one destination.
229      // We do this by reseting "TheOnlyDest" to null when we find two non-equal
230      // destinations.
231      if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
232    }
233
234    if (CI && !TheOnlyDest) {
235      // Branching on a constant, but not any of the cases, go to the default
236      // successor.
237      TheOnlyDest = SI->getDefaultDest();
238    }
239
240    // If we found a single destination that we can fold the switch into, do so
241    // now.
242    if (TheOnlyDest) {
243      // Insert the new branch..
244      new BranchInst(TheOnlyDest, SI);
245      BasicBlock *BB = SI->getParent();
246
247      // Remove entries from PHI nodes which we no longer branch to...
248      for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
249        // Found case matching a constant operand?
250        BasicBlock *Succ = SI->getSuccessor(i);
251        if (Succ == TheOnlyDest)
252          TheOnlyDest = 0;  // Don't modify the first branch to TheOnlyDest
253        else
254          Succ->removePredecessor(BB);
255      }
256
257      // Delete the old switch...
258      BB->getInstList().erase(SI);
259      return true;
260    } else if (SI->getNumSuccessors() == 2) {
261      // Otherwise, we can fold this switch into a conditional branch
262      // instruction if it has only one non-default destination.
263      Value *Cond = new ICmpInst(ICmpInst::ICMP_EQ, SI->getCondition(),
264                                 SI->getSuccessorValue(1), "cond", SI);
265      // Insert the new branch...
266      new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
267
268      // Delete the old switch...
269      SI->getParent()->getInstList().erase(SI);
270      return true;
271    }
272  }
273  return false;
274}
275
276/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
277/// getelementptr constantexpr, return the constant value being addressed by the
278/// constant expression, or null if something is funny and we can't decide.
279Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
280                                                       ConstantExpr *CE) {
281  if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
282    return 0;  // Do not allow stepping over the value!
283
284  // Loop over all of the operands, tracking down which value we are
285  // addressing...
286  gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
287  for (++I; I != E; ++I)
288    if (const StructType *STy = dyn_cast<StructType>(*I)) {
289      ConstantInt *CU = cast<ConstantInt>(I.getOperand());
290      assert(CU->getZExtValue() < STy->getNumElements() &&
291             "Struct index out of range!");
292      unsigned El = (unsigned)CU->getZExtValue();
293      if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
294        C = CS->getOperand(El);
295      } else if (isa<ConstantAggregateZero>(C)) {
296        C = Constant::getNullValue(STy->getElementType(El));
297      } else if (isa<UndefValue>(C)) {
298        C = UndefValue::get(STy->getElementType(El));
299      } else {
300        return 0;
301      }
302    } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
303      if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
304        if (CI->getZExtValue() >= ATy->getNumElements())
305         return 0;
306        if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
307          C = CA->getOperand(CI->getZExtValue());
308        else if (isa<ConstantAggregateZero>(C))
309          C = Constant::getNullValue(ATy->getElementType());
310        else if (isa<UndefValue>(C))
311          C = UndefValue::get(ATy->getElementType());
312        else
313          return 0;
314      } else if (const PackedType *PTy = dyn_cast<PackedType>(*I)) {
315        if (CI->getZExtValue() >= PTy->getNumElements())
316          return 0;
317        if (ConstantPacked *CP = dyn_cast<ConstantPacked>(C))
318          C = CP->getOperand(CI->getZExtValue());
319        else if (isa<ConstantAggregateZero>(C))
320          C = Constant::getNullValue(PTy->getElementType());
321        else if (isa<UndefValue>(C))
322          C = UndefValue::get(PTy->getElementType());
323        else
324          return 0;
325      } else {
326        return 0;
327      }
328    } else {
329      return 0;
330    }
331  return C;
332}
333
334
335//===----------------------------------------------------------------------===//
336//  Local dead code elimination...
337//
338
339bool llvm::isInstructionTriviallyDead(Instruction *I) {
340  if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
341
342  if (!I->mayWriteToMemory()) return true;
343
344  if (CallInst *CI = dyn_cast<CallInst>(I))
345    if (Function *F = CI->getCalledFunction()) {
346      unsigned IntrinsicID = F->getIntrinsicID();
347#define GET_SIDE_EFFECT_INFO
348#include "llvm/Intrinsics.gen"
349#undef GET_SIDE_EFFECT_INFO
350    }
351  return false;
352}
353
354// dceInstruction - Inspect the instruction at *BBI and figure out if it's
355// [trivially] dead.  If so, remove the instruction and update the iterator
356// to point to the instruction that immediately succeeded the original
357// instruction.
358//
359bool llvm::dceInstruction(BasicBlock::iterator &BBI) {
360  // Look for un"used" definitions...
361  if (isInstructionTriviallyDead(BBI)) {
362    BBI = BBI->getParent()->getInstList().erase(BBI);   // Bye bye
363    return true;
364  }
365  return false;
366}
367