ConstantFolding.cpp revision ad58eb34342f70f094008e6d08cb4ed814754e64 
1//===-- ConstantFolding.cpp - Analyze constant folding possibilities ------===//
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 determines the possibility of performing constant
11// folding.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Analysis/ConstantFolding.h"
16#include "llvm/Constants.h"
17#include "llvm/DerivedTypes.h"
18#include "llvm/Function.h"
19#include "llvm/Instructions.h"
20#include "llvm/Intrinsics.h"
21#include "llvm/ADT/SmallVector.h"
22#include "llvm/Target/TargetData.h"
23#include "llvm/Support/GetElementPtrTypeIterator.h"
24#include "llvm/Support/MathExtras.h"
25#include <cerrno>
26#include <cmath>
27using namespace llvm;
28
29//===----------------------------------------------------------------------===//
30// Constant Folding internal helper functions
31//===----------------------------------------------------------------------===//
32
33/// IsConstantOffsetFromGlobal - If this constant is actually a constant offset
34/// from a global, return the global and the constant.  Because of
35/// constantexprs, this function is recursive.
36static bool IsConstantOffsetFromGlobal(Constant *C, GlobalValue *&GV,
37                                       int64_t &Offset, const TargetData &TD) {
38  // Trivial case, constant is the global.
39  if ((GV = dyn_cast<GlobalValue>(C))) {
40    Offset = 0;
41    return true;
42  }
43
44  // Otherwise, if this isn't a constant expr, bail out.
45  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
46  if (!CE) return false;
47
48  // Look through ptr->int and ptr->ptr casts.
49  if (CE->getOpcode() == Instruction::PtrToInt ||
50      CE->getOpcode() == Instruction::BitCast)
51    return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD);
52
53  // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5)
54  if (CE->getOpcode() == Instruction::GetElementPtr) {
55    // Cannot compute this if the element type of the pointer is missing size
56    // info.
57    if (!cast<PointerType>(CE->getOperand(0)->getType())->getElementType()->isSized())
58      return false;
59
60    // If the base isn't a global+constant, we aren't either.
61    if (!IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD))
62      return false;
63
64    // Otherwise, add any offset that our operands provide.
65    gep_type_iterator GTI = gep_type_begin(CE);
66    for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i, ++GTI) {
67      ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(i));
68      if (!CI) return false;  // Index isn't a simple constant?
69      if (CI->getZExtValue() == 0) continue;  // Not adding anything.
70
71      if (const StructType *ST = dyn_cast<StructType>(*GTI)) {
72        // N = N + Offset
73        Offset += TD.getStructLayout(ST)->MemberOffsets[CI->getZExtValue()];
74      } else {
75        const SequentialType *ST = cast<SequentialType>(*GTI);
76        Offset += TD.getTypeSize(ST->getElementType())*CI->getSExtValue();
77      }
78    }
79    return true;
80  }
81
82  return false;
83}
84
85
86/// SymbolicallyEvaluateBinop - One of Op0/Op1 is a constant expression.
87/// Attempt to symbolically evaluate the result of  a binary operator merging
88/// these together.  If target data info is available, it is provided as TD,
89/// otherwise TD is null.
90static Constant *SymbolicallyEvaluateBinop(unsigned Opc, Constant *Op0,
91                                           Constant *Op1, const TargetData *TD){
92  // SROA
93
94  // Fold (and 0xffffffff00000000, (shl x, 32)) -> shl.
95  // Fold (lshr (or X, Y), 32) -> (lshr [X/Y], 32) if one doesn't contribute
96  // bits.
97
98
99  // If the constant expr is something like &A[123] - &A[4].f, fold this into a
100  // constant.  This happens frequently when iterating over a global array.
101  if (Opc == Instruction::Sub && TD) {
102    GlobalValue *GV1, *GV2;
103    int64_t Offs1, Offs2;
104
105    if (IsConstantOffsetFromGlobal(Op0, GV1, Offs1, *TD))
106      if (IsConstantOffsetFromGlobal(Op1, GV2, Offs2, *TD) &&
107          GV1 == GV2) {
108        // (&GV+C1) - (&GV+C2) -> C1-C2, pointer arithmetic cannot overflow.
109        return ConstantInt::get(Op0->getType(), Offs1-Offs2);
110      }
111  }
112
113  // TODO: Fold icmp setne/seteq as well.
114  return 0;
115}
116
117/// SymbolicallyEvaluateGEP - If we can symbolically evaluate the specified GEP
118/// constant expression, do so.
119static Constant *SymbolicallyEvaluateGEP(Constant** Ops, unsigned NumOps,
120                                         const Type *ResultTy,
121                                         const TargetData *TD) {
122  Constant *Ptr = Ops[0];
123  if (!cast<PointerType>(Ptr->getType())->getElementType()->isSized())
124    return 0;
125
126  if (TD && Ptr->isNullValue()) {
127    // If this is a constant expr gep that is effectively computing an
128    // "offsetof", fold it into 'cast int Size to T*' instead of 'gep 0, 0, 12'
129    bool isFoldableGEP = true;
130    for (unsigned i = 1; i != NumOps; ++i)
131      if (!isa<ConstantInt>(Ops[i])) {
132        isFoldableGEP = false;
133        break;
134      }
135    if (isFoldableGEP) {
136      std::vector<Value*> NewOps(Ops+1, Ops+NumOps);
137      uint64_t Offset = TD->getIndexedOffset(Ptr->getType(), NewOps);
138      Constant *C = ConstantInt::get(TD->getIntPtrType(), Offset);
139      return ConstantExpr::getIntToPtr(C, ResultTy);
140    }
141  }
142
143  return 0;
144}
145
146
147//===----------------------------------------------------------------------===//
148// Constant Folding public APIs
149//===----------------------------------------------------------------------===//
150
151
152/// ConstantFoldInstruction - Attempt to constant fold the specified
153/// instruction.  If successful, the constant result is returned, if not, null
154/// is returned.  Note that this function can only fail when attempting to fold
155/// instructions like loads and stores, which have no constant expression form.
156///
157Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
158  if (PHINode *PN = dyn_cast<PHINode>(I)) {
159    if (PN->getNumIncomingValues() == 0)
160      return Constant::getNullValue(PN->getType());
161
162    Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
163    if (Result == 0) return 0;
164
165    // Handle PHI nodes specially here...
166    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
167      if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
168        return 0;   // Not all the same incoming constants...
169
170    // If we reach here, all incoming values are the same constant.
171    return Result;
172  }
173
174  // Scan the operand list, checking to see if they are all constants, if so,
175  // hand off to ConstantFoldInstOperands.
176  SmallVector<Constant*, 8> Ops;
177  for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
178    if (Constant *Op = dyn_cast<Constant>(I->getOperand(i)))
179      Ops.push_back(Op);
180    else
181      return 0;  // All operands not constant!
182
183  return ConstantFoldInstOperands(I, &Ops[0], Ops.size(), TD);
184}
185
186/// ConstantFoldInstOperands - Attempt to constant fold an instruction with the
187/// specified opcode and operands.  If successful, the constant result is
188/// returned, if not, null is returned.  Note that this function can fail when
189/// attempting to fold instructions like loads and stores, which have no
190/// constant expression form.
191///
192Constant *llvm::ConstantFoldInstOperands(const Instruction* I,
193                                         Constant** Ops, unsigned NumOps,
194                                         const TargetData *TD) {
195  unsigned Opc = I->getOpcode();
196  const Type *DestTy = I->getType();
197
198  // Handle easy binops first.
199  if (isa<BinaryOperator>(I)) {
200    if (isa<ConstantExpr>(Ops[0]) || isa<ConstantExpr>(Ops[1]))
201      if (Constant *C = SymbolicallyEvaluateBinop(I->getOpcode(), Ops[0],
202                                                  Ops[1], TD))
203        return C;
204
205    return ConstantExpr::get(Opc, Ops[0], Ops[1]);
206  }
207
208  switch (Opc) {
209  default: return 0;
210  case Instruction::Call:
211    if (Function *F = dyn_cast<Function>(Ops[0]))
212      if (canConstantFoldCallTo(F))
213        return ConstantFoldCall(F, Ops+1, NumOps-1);
214    return 0;
215  case Instruction::ICmp:
216  case Instruction::FCmp:
217    return ConstantExpr::getCompare(cast<CmpInst>(I)->getPredicate(), Ops[0],
218                                    Ops[1]);
219  case Instruction::Shl:
220  case Instruction::LShr:
221  case Instruction::AShr:
222    return ConstantExpr::get(Opc, Ops[0], Ops[1]);
223  case Instruction::Trunc:
224  case Instruction::ZExt:
225  case Instruction::SExt:
226  case Instruction::FPTrunc:
227  case Instruction::FPExt:
228  case Instruction::UIToFP:
229  case Instruction::SIToFP:
230  case Instruction::FPToUI:
231  case Instruction::FPToSI:
232  case Instruction::PtrToInt:
233  case Instruction::IntToPtr:
234  case Instruction::BitCast:
235    return ConstantExpr::getCast(Opc, Ops[0], DestTy);
236  case Instruction::Select:
237    return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]);
238  case Instruction::ExtractElement:
239    return ConstantExpr::getExtractElement(Ops[0], Ops[1]);
240  case Instruction::InsertElement:
241    return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]);
242  case Instruction::ShuffleVector:
243    return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]);
244  case Instruction::GetElementPtr:
245    if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, I->getType(), TD))
246      return C;
247
248    return ConstantExpr::getGetElementPtr(Ops[0], Ops+1, NumOps-1);
249  }
250}
251
252/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
253/// getelementptr constantexpr, return the constant value being addressed by the
254/// constant expression, or null if something is funny and we can't decide.
255Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
256                                                       ConstantExpr *CE) {
257  if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
258    return 0;  // Do not allow stepping over the value!
259
260  // Loop over all of the operands, tracking down which value we are
261  // addressing...
262  gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
263  for (++I; I != E; ++I)
264    if (const StructType *STy = dyn_cast<StructType>(*I)) {
265      ConstantInt *CU = cast<ConstantInt>(I.getOperand());
266      assert(CU->getZExtValue() < STy->getNumElements() &&
267             "Struct index out of range!");
268      unsigned El = (unsigned)CU->getZExtValue();
269      if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
270        C = CS->getOperand(El);
271      } else if (isa<ConstantAggregateZero>(C)) {
272        C = Constant::getNullValue(STy->getElementType(El));
273      } else if (isa<UndefValue>(C)) {
274        C = UndefValue::get(STy->getElementType(El));
275      } else {
276        return 0;
277      }
278    } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
279      if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
280        if (CI->getZExtValue() >= ATy->getNumElements())
281         return 0;
282        if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
283          C = CA->getOperand(CI->getZExtValue());
284        else if (isa<ConstantAggregateZero>(C))
285          C = Constant::getNullValue(ATy->getElementType());
286        else if (isa<UndefValue>(C))
287          C = UndefValue::get(ATy->getElementType());
288        else
289          return 0;
290      } else if (const PackedType *PTy = dyn_cast<PackedType>(*I)) {
291        if (CI->getZExtValue() >= PTy->getNumElements())
292          return 0;
293        if (ConstantPacked *CP = dyn_cast<ConstantPacked>(C))
294          C = CP->getOperand(CI->getZExtValue());
295        else if (isa<ConstantAggregateZero>(C))
296          C = Constant::getNullValue(PTy->getElementType());
297        else if (isa<UndefValue>(C))
298          C = UndefValue::get(PTy->getElementType());
299        else
300          return 0;
301      } else {
302        return 0;
303      }
304    } else {
305      return 0;
306    }
307  return C;
308}
309
310
311//===----------------------------------------------------------------------===//
312//  Constant Folding for Calls
313//
314
315/// canConstantFoldCallTo - Return true if its even possible to fold a call to
316/// the specified function.
317bool
318llvm::canConstantFoldCallTo(Function *F) {
319  const std::string &Name = F->getName();
320
321  switch (F->getIntrinsicID()) {
322  case Intrinsic::sqrt_f32:
323  case Intrinsic::sqrt_f64:
324  case Intrinsic::bswap_i16:
325  case Intrinsic::bswap_i32:
326  case Intrinsic::bswap_i64:
327  case Intrinsic::powi_f32:
328  case Intrinsic::powi_f64:
329  // FIXME: these should be constant folded as well
330  //case Intrinsic::ctpop_i8:
331  //case Intrinsic::ctpop_i16:
332  //case Intrinsic::ctpop_i32:
333  //case Intrinsic::ctpop_i64:
334  //case Intrinsic::ctlz_i8:
335  //case Intrinsic::ctlz_i16:
336  //case Intrinsic::ctlz_i32:
337  //case Intrinsic::ctlz_i64:
338  //case Intrinsic::cttz_i8:
339  //case Intrinsic::cttz_i16:
340  //case Intrinsic::cttz_i32:
341  //case Intrinsic::cttz_i64:
342    return true;
343  default: break;
344  }
345
346  switch (Name[0])
347  {
348    case 'a':
349      return Name == "acos" || Name == "asin" || Name == "atan" ||
350             Name == "atan2";
351    case 'c':
352      return Name == "ceil" || Name == "cos" || Name == "cosf" ||
353             Name == "cosh";
354    case 'e':
355      return Name == "exp";
356    case 'f':
357      return Name == "fabs" || Name == "fmod" || Name == "floor";
358    case 'l':
359      return Name == "log" || Name == "log10";
360    case 'p':
361      return Name == "pow";
362    case 's':
363      return Name == "sin" || Name == "sinh" ||
364             Name == "sqrt" || Name == "sqrtf";
365    case 't':
366      return Name == "tan" || Name == "tanh";
367    default:
368      return false;
369  }
370}
371
372static Constant *ConstantFoldFP(double (*NativeFP)(double), double V,
373                                const Type *Ty) {
374  errno = 0;
375  V = NativeFP(V);
376  if (errno == 0)
377    return ConstantFP::get(Ty, V);
378  errno = 0;
379  return 0;
380}
381
382/// ConstantFoldCall - Attempt to constant fold a call to the specified function
383/// with the specified arguments, returning null if unsuccessful.
384Constant *
385llvm::ConstantFoldCall(Function *F, Constant** Operands, unsigned NumOperands) {
386  const std::string &Name = F->getName();
387  const Type *Ty = F->getReturnType();
388
389  if (NumOperands == 1) {
390    if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
391      double V = Op->getValue();
392      switch (Name[0])
393      {
394        case 'a':
395          if (Name == "acos")
396            return ConstantFoldFP(acos, V, Ty);
397          else if (Name == "asin")
398            return ConstantFoldFP(asin, V, Ty);
399          else if (Name == "atan")
400            return ConstantFP::get(Ty, atan(V));
401          break;
402        case 'c':
403          if (Name == "ceil")
404            return ConstantFoldFP(ceil, V, Ty);
405          else if (Name == "cos")
406            return ConstantFP::get(Ty, cos(V));
407          else if (Name == "cosh")
408            return ConstantFP::get(Ty, cosh(V));
409          break;
410        case 'e':
411          if (Name == "exp")
412            return ConstantFP::get(Ty, exp(V));
413          break;
414        case 'f':
415          if (Name == "fabs")
416            return ConstantFP::get(Ty, fabs(V));
417          else if (Name == "floor")
418            return ConstantFoldFP(floor, V, Ty);
419          break;
420        case 'l':
421          if (Name == "log" && V > 0)
422            return ConstantFP::get(Ty, log(V));
423          else if (Name == "log10" && V > 0)
424            return ConstantFoldFP(log10, V, Ty);
425          else if (Name == "llvm.sqrt.f32" || Name == "llvm.sqrt.f64") {
426            if (V >= -0.0)
427              return ConstantFP::get(Ty, sqrt(V));
428            else // Undefined
429              return ConstantFP::get(Ty, 0.0);
430          }
431          break;
432        case 's':
433          if (Name == "sin")
434            return ConstantFP::get(Ty, sin(V));
435          else if (Name == "sinh")
436            return ConstantFP::get(Ty, sinh(V));
437          else if (Name == "sqrt" && V >= 0)
438            return ConstantFP::get(Ty, sqrt(V));
439          else if (Name == "sqrtf" && V >= 0)
440            return ConstantFP::get(Ty, sqrt((float)V));
441          break;
442        case 't':
443          if (Name == "tan")
444            return ConstantFP::get(Ty, tan(V));
445          else if (Name == "tanh")
446            return ConstantFP::get(Ty, tanh(V));
447          break;
448        default:
449          break;
450      }
451    } else if (ConstantInt *Op = dyn_cast<ConstantInt>(Operands[0])) {
452      uint64_t V = Op->getZExtValue();
453      if (Name == "llvm.bswap.i16")
454        return ConstantInt::get(Ty, ByteSwap_16(V));
455      else if (Name == "llvm.bswap.i32")
456        return ConstantInt::get(Ty, ByteSwap_32(V));
457      else if (Name == "llvm.bswap.i64")
458        return ConstantInt::get(Ty, ByteSwap_64(V));
459    }
460  } else if (NumOperands == 2) {
461    if (ConstantFP *Op1 = dyn_cast<ConstantFP>(Operands[0])) {
462      double Op1V = Op1->getValue();
463      if (ConstantFP *Op2 = dyn_cast<ConstantFP>(Operands[1])) {
464        double Op2V = Op2->getValue();
465
466        if (Name == "pow") {
467          errno = 0;
468          double V = pow(Op1V, Op2V);
469          if (errno == 0)
470            return ConstantFP::get(Ty, V);
471        } else if (Name == "fmod") {
472          errno = 0;
473          double V = fmod(Op1V, Op2V);
474          if (errno == 0)
475            return ConstantFP::get(Ty, V);
476        } else if (Name == "atan2") {
477          return ConstantFP::get(Ty, atan2(Op1V,Op2V));
478        }
479      } else if (ConstantInt *Op2C = dyn_cast<ConstantInt>(Operands[1])) {
480        if (Name == "llvm.powi.f32") {
481          return ConstantFP::get(Ty, std::pow((float)Op1V,
482                                              (int)Op2C->getZExtValue()));
483        } else if (Name == "llvm.powi.f64") {
484          return ConstantFP::get(Ty, std::pow((double)Op1V,
485                                              (int)Op2C->getZExtValue()));
486        }
487      }
488    }
489  }
490  return 0;
491}
492
493