Local.cpp revision d900c6a8b3de655bac138cfdf4b9a85b7e64ec1c 
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/Support/GetElementPtrTypeIterator.h"
22#include "llvm/Support/MathExtras.h"
23#include <cerrno>
24#include <cmath>
25using namespace llvm;
26
27//===----------------------------------------------------------------------===//
28//  Local constant propagation...
29//
30
31/// doConstantPropagation - If an instruction references constants, try to fold
32/// them together...
33///
34bool llvm::doConstantPropagation(BasicBlock::iterator &II) {
35  if (Constant *C = ConstantFoldInstruction(II)) {
36    // Replaces all of the uses of a variable with uses of the constant.
37    II->replaceAllUsesWith(C);
38
39    // Remove the instruction from the basic block...
40    II = II->getParent()->getInstList().erase(II);
41    return true;
42  }
43
44  return false;
45}
46
47/// ConstantFoldInstruction - Attempt to constant fold the specified
48/// instruction.  If successful, the constant result is returned, if not, null
49/// is returned.  Note that this function can only fail when attempting to fold
50/// instructions like loads and stores, which have no constant expression form.
51///
52Constant *llvm::ConstantFoldInstruction(Instruction *I) {
53  if (PHINode *PN = dyn_cast<PHINode>(I)) {
54    if (PN->getNumIncomingValues() == 0)
55      return Constant::getNullValue(PN->getType());
56
57    Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
58    if (Result == 0) return 0;
59
60    // Handle PHI nodes specially here...
61    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
62      if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
63        return 0;   // Not all the same incoming constants...
64
65    // If we reach here, all incoming values are the same constant.
66    return Result;
67  } else if (CallInst *CI = dyn_cast<CallInst>(I)) {
68    if (Function *F = CI->getCalledFunction())
69      if (canConstantFoldCallTo(F)) {
70        std::vector<Constant*> Args;
71        for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
72          if (Constant *Op = dyn_cast<Constant>(CI->getOperand(i)))
73            Args.push_back(Op);
74          else
75            return 0;
76        return ConstantFoldCall(F, Args);
77      }
78    return 0;
79  }
80
81  Constant *Op0 = 0, *Op1 = 0;
82  switch (I->getNumOperands()) {
83  default:
84  case 2:
85    Op1 = dyn_cast<Constant>(I->getOperand(1));
86    if (Op1 == 0) return 0;        // Not a constant?, can't fold
87  case 1:
88    Op0 = dyn_cast<Constant>(I->getOperand(0));
89    if (Op0 == 0) return 0;        // Not a constant?, can't fold
90    break;
91  case 0: return 0;
92  }
93
94  if (isa<BinaryOperator>(I) || isa<ShiftInst>(I))
95    return ConstantExpr::get(I->getOpcode(), Op0, Op1);
96
97  switch (I->getOpcode()) {
98  default: return 0;
99  case Instruction::Cast:
100    return ConstantExpr::getCast(Op0, I->getType());
101  case Instruction::Select:
102    if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(2)))
103      return ConstantExpr::getSelect(Op0, Op1, Op2);
104    return 0;
105  case Instruction::ExtractElement:
106    return ConstantExpr::getExtractElement(Op0, Op1);
107  case Instruction::InsertElement:
108    if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(2)))
109      return ConstantExpr::getInsertElement(Op0, Op1, Op2);
110  case Instruction::GetElementPtr:
111    std::vector<Constant*> IdxList;
112    IdxList.reserve(I->getNumOperands()-1);
113    if (Op1) IdxList.push_back(Op1);
114    for (unsigned i = 2, e = I->getNumOperands(); i != e; ++i)
115      if (Constant *C = dyn_cast<Constant>(I->getOperand(i)))
116        IdxList.push_back(C);
117      else
118        return 0;  // Non-constant operand
119    return ConstantExpr::getGetElementPtr(Op0, IdxList);
120  }
121}
122
123// ConstantFoldTerminator - If a terminator instruction is predicated on a
124// constant value, convert it into an unconditional branch to the constant
125// destination.
126//
127bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
128  TerminatorInst *T = BB->getTerminator();
129
130  // Branch - See if we are conditional jumping on constant
131  if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
132    if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
133    BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
134    BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
135
136    if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
137      // Are we branching on constant?
138      // YES.  Change to unconditional branch...
139      BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
140      BasicBlock *OldDest     = Cond->getValue() ? Dest2 : Dest1;
141
142      //cerr << "Function: " << T->getParent()->getParent()
143      //     << "\nRemoving branch from " << T->getParent()
144      //     << "\n\nTo: " << OldDest << endl;
145
146      // Let the basic block know that we are letting go of it.  Based on this,
147      // it will adjust it's PHI nodes.
148      assert(BI->getParent() && "Terminator not inserted in block!");
149      OldDest->removePredecessor(BI->getParent());
150
151      // Set the unconditional destination, and change the insn to be an
152      // unconditional branch.
153      BI->setUnconditionalDest(Destination);
154      return true;
155    } else if (Dest2 == Dest1) {       // Conditional branch to same location?
156      // This branch matches something like this:
157      //     br bool %cond, label %Dest, label %Dest
158      // and changes it into:  br label %Dest
159
160      // Let the basic block know that we are letting go of one copy of it.
161      assert(BI->getParent() && "Terminator not inserted in block!");
162      Dest1->removePredecessor(BI->getParent());
163
164      // Change a conditional branch to unconditional.
165      BI->setUnconditionalDest(Dest1);
166      return true;
167    }
168  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
169    // If we are switching on a constant, we can convert the switch into a
170    // single branch instruction!
171    ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
172    BasicBlock *TheOnlyDest = SI->getSuccessor(0);  // The default dest
173    BasicBlock *DefaultDest = TheOnlyDest;
174    assert(TheOnlyDest == SI->getDefaultDest() &&
175           "Default destination is not successor #0?");
176
177    // Figure out which case it goes to...
178    for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
179      // Found case matching a constant operand?
180      if (SI->getSuccessorValue(i) == CI) {
181        TheOnlyDest = SI->getSuccessor(i);
182        break;
183      }
184
185      // Check to see if this branch is going to the same place as the default
186      // dest.  If so, eliminate it as an explicit compare.
187      if (SI->getSuccessor(i) == DefaultDest) {
188        // Remove this entry...
189        DefaultDest->removePredecessor(SI->getParent());
190        SI->removeCase(i);
191        --i; --e;  // Don't skip an entry...
192        continue;
193      }
194
195      // Otherwise, check to see if the switch only branches to one destination.
196      // We do this by reseting "TheOnlyDest" to null when we find two non-equal
197      // destinations.
198      if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
199    }
200
201    if (CI && !TheOnlyDest) {
202      // Branching on a constant, but not any of the cases, go to the default
203      // successor.
204      TheOnlyDest = SI->getDefaultDest();
205    }
206
207    // If we found a single destination that we can fold the switch into, do so
208    // now.
209    if (TheOnlyDest) {
210      // Insert the new branch..
211      new BranchInst(TheOnlyDest, SI);
212      BasicBlock *BB = SI->getParent();
213
214      // Remove entries from PHI nodes which we no longer branch to...
215      for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
216        // Found case matching a constant operand?
217        BasicBlock *Succ = SI->getSuccessor(i);
218        if (Succ == TheOnlyDest)
219          TheOnlyDest = 0;  // Don't modify the first branch to TheOnlyDest
220        else
221          Succ->removePredecessor(BB);
222      }
223
224      // Delete the old switch...
225      BB->getInstList().erase(SI);
226      return true;
227    } else if (SI->getNumSuccessors() == 2) {
228      // Otherwise, we can fold this switch into a conditional branch
229      // instruction if it has only one non-default destination.
230      Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(),
231                                    SI->getSuccessorValue(1), "cond", SI);
232      // Insert the new branch...
233      new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
234
235      // Delete the old switch...
236      SI->getParent()->getInstList().erase(SI);
237      return true;
238    }
239  }
240  return false;
241}
242
243/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
244/// getelementptr constantexpr, return the constant value being addressed by the
245/// constant expression, or null if something is funny and we can't decide.
246Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
247                                                       ConstantExpr *CE) {
248  if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
249    return 0;  // Do not allow stepping over the value!
250
251  // Loop over all of the operands, tracking down which value we are
252  // addressing...
253  gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
254  for (++I; I != E; ++I)
255    if (const StructType *STy = dyn_cast<StructType>(*I)) {
256      ConstantUInt *CU = cast<ConstantUInt>(I.getOperand());
257      assert(CU->getValue() < STy->getNumElements() &&
258             "Struct index out of range!");
259      unsigned El = (unsigned)CU->getValue();
260      if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
261        C = CS->getOperand(El);
262      } else if (isa<ConstantAggregateZero>(C)) {
263        C = Constant::getNullValue(STy->getElementType(El));
264      } else if (isa<UndefValue>(C)) {
265        C = UndefValue::get(STy->getElementType(El));
266      } else {
267        return 0;
268      }
269    } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
270      if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
271        if ((uint64_t)CI->getRawValue() >= ATy->getNumElements()) return 0;
272        if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
273          C = CA->getOperand((unsigned)CI->getRawValue());
274        else if (isa<ConstantAggregateZero>(C))
275          C = Constant::getNullValue(ATy->getElementType());
276        else if (isa<UndefValue>(C))
277          C = UndefValue::get(ATy->getElementType());
278        else
279          return 0;
280      } else if (const PackedType *PTy = dyn_cast<PackedType>(*I)) {
281        if ((uint64_t)CI->getRawValue() >= PTy->getNumElements()) return 0;
282        if (ConstantPacked *CP = dyn_cast<ConstantPacked>(C))
283          C = CP->getOperand((unsigned)CI->getRawValue());
284        else if (isa<ConstantAggregateZero>(C))
285          C = Constant::getNullValue(PTy->getElementType());
286        else if (isa<UndefValue>(C))
287          C = UndefValue::get(PTy->getElementType());
288        else
289          return 0;
290      } else {
291        return 0;
292      }
293    } else {
294      return 0;
295    }
296  return C;
297}
298
299
300//===----------------------------------------------------------------------===//
301//  Local dead code elimination...
302//
303
304bool llvm::isInstructionTriviallyDead(Instruction *I) {
305  if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
306
307  if (!I->mayWriteToMemory()) return true;
308
309  if (CallInst *CI = dyn_cast<CallInst>(I))
310    if (Function *F = CI->getCalledFunction())
311      switch (F->getIntrinsicID()) {
312      default: break;
313      case Intrinsic::returnaddress:
314      case Intrinsic::frameaddress:
315      case Intrinsic::stacksave:
316      case Intrinsic::isunordered_f32:
317      case Intrinsic::isunordered_f64:
318      case Intrinsic::bswap_i16:
319      case Intrinsic::bswap_i32:
320      case Intrinsic::bswap_i64:
321      case Intrinsic::ctpop_i8:
322      case Intrinsic::ctpop_i16:
323      case Intrinsic::ctpop_i32:
324      case Intrinsic::ctpop_i64:
325      case Intrinsic::ctlz_i8:
326      case Intrinsic::ctlz_i16:
327      case Intrinsic::ctlz_i32:
328      case Intrinsic::ctlz_i64:
329      case Intrinsic::cttz_i8:
330      case Intrinsic::cttz_i16:
331      case Intrinsic::cttz_i32:
332      case Intrinsic::cttz_i64:
333      case Intrinsic::sqrt_f32:
334      case Intrinsic::sqrt_f64:
335        return true;             // These intrinsics have no side effects.
336      }
337  return false;
338}
339
340// dceInstruction - Inspect the instruction at *BBI and figure out if it's
341// [trivially] dead.  If so, remove the instruction and update the iterator
342// to point to the instruction that immediately succeeded the original
343// instruction.
344//
345bool llvm::dceInstruction(BasicBlock::iterator &BBI) {
346  // Look for un"used" definitions...
347  if (isInstructionTriviallyDead(BBI)) {
348    BBI = BBI->getParent()->getInstList().erase(BBI);   // Bye bye
349    return true;
350  }
351  return false;
352}
353