ConstantFolding.cpp revision a099b6c7bb574f22bc002e0b3c65c25abccca1d5 
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/ADT/StringMap.h"
23#include "llvm/Target/TargetData.h"
24#include "llvm/Support/GetElementPtrTypeIterator.h"
25#include "llvm/Support/MathExtras.h"
26#include <cerrno>
27#include <cmath>
28using namespace llvm;
29
30//===----------------------------------------------------------------------===//
31// Constant Folding internal helper functions
32//===----------------------------------------------------------------------===//
33
34/// IsConstantOffsetFromGlobal - If this constant is actually a constant offset
35/// from a global, return the global and the constant.  Because of
36/// constantexprs, this function is recursive.
37static bool IsConstantOffsetFromGlobal(Constant *C, GlobalValue *&GV,
38                                       int64_t &Offset, const TargetData &TD) {
39  // Trivial case, constant is the global.
40  if ((GV = dyn_cast<GlobalValue>(C))) {
41    Offset = 0;
42    return true;
43  }
44
45  // Otherwise, if this isn't a constant expr, bail out.
46  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
47  if (!CE) return false;
48
49  // Look through ptr->int and ptr->ptr casts.
50  if (CE->getOpcode() == Instruction::PtrToInt ||
51      CE->getOpcode() == Instruction::BitCast)
52    return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD);
53
54  // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5)
55  if (CE->getOpcode() == Instruction::GetElementPtr) {
56    // Cannot compute this if the element type of the pointer is missing size
57    // info.
58    if (!cast<PointerType>(CE->getOperand(0)->getType())->getElementType()->isSized())
59      return false;
60
61    // If the base isn't a global+constant, we aren't either.
62    if (!IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD))
63      return false;
64
65    // Otherwise, add any offset that our operands provide.
66    gep_type_iterator GTI = gep_type_begin(CE);
67    for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i, ++GTI) {
68      ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(i));
69      if (!CI) return false;  // Index isn't a simple constant?
70      if (CI->getZExtValue() == 0) continue;  // Not adding anything.
71
72      if (const StructType *ST = dyn_cast<StructType>(*GTI)) {
73        // N = N + Offset
74        Offset += TD.getStructLayout(ST)->getElementOffset(CI->getZExtValue());
75      } else {
76        const SequentialType *SQT = cast<SequentialType>(*GTI);
77        Offset += TD.getTypeSize(SQT->getElementType())*CI->getSExtValue();
78      }
79    }
80    return true;
81  }
82
83  return false;
84}
85
86
87/// SymbolicallyEvaluateBinop - One of Op0/Op1 is a constant expression.
88/// Attempt to symbolically evaluate the result of  a binary operator merging
89/// these together.  If target data info is available, it is provided as TD,
90/// otherwise TD is null.
91static Constant *SymbolicallyEvaluateBinop(unsigned Opc, Constant *Op0,
92                                           Constant *Op1, const TargetData *TD){
93  // SROA
94
95  // Fold (and 0xffffffff00000000, (shl x, 32)) -> shl.
96  // Fold (lshr (or X, Y), 32) -> (lshr [X/Y], 32) if one doesn't contribute
97  // bits.
98
99
100  // If the constant expr is something like &A[123] - &A[4].f, fold this into a
101  // constant.  This happens frequently when iterating over a global array.
102  if (Opc == Instruction::Sub && TD) {
103    GlobalValue *GV1, *GV2;
104    int64_t Offs1, Offs2;
105
106    if (IsConstantOffsetFromGlobal(Op0, GV1, Offs1, *TD))
107      if (IsConstantOffsetFromGlobal(Op1, GV2, Offs2, *TD) &&
108          GV1 == GV2) {
109        // (&GV+C1) - (&GV+C2) -> C1-C2, pointer arithmetic cannot overflow.
110        return ConstantInt::get(Op0->getType(), Offs1-Offs2);
111      }
112  }
113
114  // TODO: Fold icmp setne/seteq as well.
115  return 0;
116}
117
118/// SymbolicallyEvaluateGEP - If we can symbolically evaluate the specified GEP
119/// constant expression, do so.
120static Constant *SymbolicallyEvaluateGEP(Constant** Ops, unsigned NumOps,
121                                         const Type *ResultTy,
122                                         const TargetData *TD) {
123  Constant *Ptr = Ops[0];
124  if (!cast<PointerType>(Ptr->getType())->getElementType()->isSized())
125    return 0;
126
127  if (TD && Ptr->isNullValue()) {
128    // If this is a constant expr gep that is effectively computing an
129    // "offsetof", fold it into 'cast int Size to T*' instead of 'gep 0, 0, 12'
130    bool isFoldableGEP = true;
131    for (unsigned i = 1; i != NumOps; ++i)
132      if (!isa<ConstantInt>(Ops[i])) {
133        isFoldableGEP = false;
134        break;
135      }
136    if (isFoldableGEP) {
137      uint64_t Offset = TD->getIndexedOffset(Ptr->getType(),
138                                             (Value**)Ops+1, NumOps-1);
139      Constant *C = ConstantInt::get(TD->getIntPtrType(), Offset);
140      return ConstantExpr::getIntToPtr(C, ResultTy);
141    }
142  }
143
144  return 0;
145}
146
147
148//===----------------------------------------------------------------------===//
149// Constant Folding public APIs
150//===----------------------------------------------------------------------===//
151
152
153/// ConstantFoldInstruction - Attempt to constant fold the specified
154/// instruction.  If successful, the constant result is returned, if not, null
155/// is returned.  Note that this function can only fail when attempting to fold
156/// instructions like loads and stores, which have no constant expression form.
157///
158Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
159  if (PHINode *PN = dyn_cast<PHINode>(I)) {
160    if (PN->getNumIncomingValues() == 0)
161      return Constant::getNullValue(PN->getType());
162
163    Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
164    if (Result == 0) return 0;
165
166    // Handle PHI nodes specially here...
167    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
168      if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
169        return 0;   // Not all the same incoming constants...
170
171    // If we reach here, all incoming values are the same constant.
172    return Result;
173  }
174
175  // Scan the operand list, checking to see if they are all constants, if so,
176  // hand off to ConstantFoldInstOperands.
177  SmallVector<Constant*, 8> Ops;
178  for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
179    if (Constant *Op = dyn_cast<Constant>(I->getOperand(i)))
180      Ops.push_back(Op);
181    else
182      return 0;  // All operands not constant!
183
184  return ConstantFoldInstOperands(I, &Ops[0], Ops.size(), TD);
185}
186
187/// ConstantFoldInstOperands - Attempt to constant fold an instruction with the
188/// specified opcode and operands.  If successful, the constant result is
189/// returned, if not, null is returned.  Note that this function can fail when
190/// attempting to fold instructions like loads and stores, which have no
191/// constant expression form.
192///
193Constant *llvm::ConstantFoldInstOperands(const Instruction* I,
194                                         Constant** Ops, unsigned NumOps,
195                                         const TargetData *TD) {
196  unsigned Opc = I->getOpcode();
197  const Type *DestTy = I->getType();
198
199  // Handle easy binops first.
200  if (isa<BinaryOperator>(I)) {
201    if (isa<ConstantExpr>(Ops[0]) || isa<ConstantExpr>(Ops[1]))
202      if (Constant *C = SymbolicallyEvaluateBinop(I->getOpcode(), Ops[0],
203                                                  Ops[1], TD))
204        return C;
205
206    return ConstantExpr::get(Opc, Ops[0], Ops[1]);
207  }
208
209  switch (Opc) {
210  default: return 0;
211  case Instruction::Call:
212    if (Function *F = dyn_cast<Function>(Ops[0]))
213      if (canConstantFoldCallTo(F))
214        return ConstantFoldCall(F, Ops+1, NumOps-1);
215    return 0;
216  case Instruction::ICmp:
217  case Instruction::FCmp:
218    return ConstantExpr::getCompare(cast<CmpInst>(I)->getPredicate(), Ops[0],
219                                    Ops[1]);
220  case Instruction::Trunc:
221  case Instruction::ZExt:
222  case Instruction::SExt:
223  case Instruction::FPTrunc:
224  case Instruction::FPExt:
225  case Instruction::UIToFP:
226  case Instruction::SIToFP:
227  case Instruction::FPToUI:
228  case Instruction::FPToSI:
229  case Instruction::PtrToInt:
230  case Instruction::IntToPtr:
231  case Instruction::BitCast:
232    return ConstantExpr::getCast(Opc, Ops[0], DestTy);
233  case Instruction::Select:
234    return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]);
235  case Instruction::ExtractElement:
236    return ConstantExpr::getExtractElement(Ops[0], Ops[1]);
237  case Instruction::InsertElement:
238    return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]);
239  case Instruction::ShuffleVector:
240    return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]);
241  case Instruction::GetElementPtr:
242    if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, I->getType(), TD))
243      return C;
244
245    return ConstantExpr::getGetElementPtr(Ops[0], Ops+1, NumOps-1);
246  }
247}
248
249/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
250/// getelementptr constantexpr, return the constant value being addressed by the
251/// constant expression, or null if something is funny and we can't decide.
252Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
253                                                       ConstantExpr *CE) {
254  if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
255    return 0;  // Do not allow stepping over the value!
256
257  // Loop over all of the operands, tracking down which value we are
258  // addressing...
259  gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
260  for (++I; I != E; ++I)
261    if (const StructType *STy = dyn_cast<StructType>(*I)) {
262      ConstantInt *CU = cast<ConstantInt>(I.getOperand());
263      assert(CU->getZExtValue() < STy->getNumElements() &&
264             "Struct index out of range!");
265      unsigned El = (unsigned)CU->getZExtValue();
266      if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
267        C = CS->getOperand(El);
268      } else if (isa<ConstantAggregateZero>(C)) {
269        C = Constant::getNullValue(STy->getElementType(El));
270      } else if (isa<UndefValue>(C)) {
271        C = UndefValue::get(STy->getElementType(El));
272      } else {
273        return 0;
274      }
275    } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
276      if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
277        if (CI->getZExtValue() >= ATy->getNumElements())
278         return 0;
279        if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
280          C = CA->getOperand(CI->getZExtValue());
281        else if (isa<ConstantAggregateZero>(C))
282          C = Constant::getNullValue(ATy->getElementType());
283        else if (isa<UndefValue>(C))
284          C = UndefValue::get(ATy->getElementType());
285        else
286          return 0;
287      } else if (const VectorType *PTy = dyn_cast<VectorType>(*I)) {
288        if (CI->getZExtValue() >= PTy->getNumElements())
289          return 0;
290        if (ConstantVector *CP = dyn_cast<ConstantVector>(C))
291          C = CP->getOperand(CI->getZExtValue());
292        else if (isa<ConstantAggregateZero>(C))
293          C = Constant::getNullValue(PTy->getElementType());
294        else if (isa<UndefValue>(C))
295          C = UndefValue::get(PTy->getElementType());
296        else
297          return 0;
298      } else {
299        return 0;
300      }
301    } else {
302      return 0;
303    }
304  return C;
305}
306
307
308//===----------------------------------------------------------------------===//
309//  Constant Folding for Calls
310//
311
312/// canConstantFoldCallTo - Return true if its even possible to fold a call to
313/// the specified function.
314bool
315llvm::canConstantFoldCallTo(Function *F) {
316  switch (F->getIntrinsicID()) {
317  case Intrinsic::sqrt_f32:
318  case Intrinsic::sqrt_f64:
319  case Intrinsic::powi_f32:
320  case Intrinsic::powi_f64:
321  case Intrinsic::bswap:
322  case Intrinsic::ctpop:
323  case Intrinsic::ctlz:
324  case Intrinsic::cttz:
325    return true;
326  default: break;
327  }
328
329  const ValueName *NameVal = F->getValueName();
330  if (NameVal == 0) return false;
331  const char *Str = NameVal->getKeyData();
332  unsigned Len = NameVal->getKeyLength();
333
334  // In these cases, the check of the length is required.  We don't want to
335  // return true for a name like "cos\0blah" which strcmp would return equal to
336  // "cos", but has length 8.
337  switch (Str[0]) {
338  default: return false;
339  case 'a':
340    if (Len == 4)
341      return !strcmp(Str, "acos") || !strcmp(Str, "asin") ||
342             !strcmp(Str, "atan");
343    else if (Len == 5)
344      return !strcmp(Str, "atan2");
345    return false;
346  case 'c':
347    if (Len == 3)
348      return !strcmp(Str, "cos");
349    else if (Len == 4)
350      return !strcmp(Str, "ceil") || !strcmp(Str, "cosf") ||
351             !strcmp(Str, "cosh");
352    return false;
353  case 'e':
354    if (Len == 3)
355      return !strcmp(Str, "exp");
356    return false;
357  case 'f':
358    if (Len == 4)
359      return !strcmp(Str, "fabs") || !strcmp(Str, "fmod");
360    else if (Len == 5)
361      return !strcmp(Str, "floor");
362    return false;
363    break;
364  case 'l':
365    if (Len == 3 && !strcmp(Str, "log"))
366      return true;
367    if (Len == 5 && !strcmp(Str, "log10"))
368      return true;
369    return false;
370  case 'p':
371    if (Len == 3 && !strcmp(Str, "pow"))
372      return true;
373    return false;
374  case 's':
375    if (Len == 3)
376      return !strcmp(Str, "sin");
377    if (Len == 4)
378      return !strcmp(Str, "sinh") || !strcmp(Str, "sqrt");
379    if (Len == 5)
380      return !strcmp(Str, "sqrtf");
381    return false;
382  case 't':
383    if (Len == 3 && !strcmp(Str, "tan"))
384      return true;
385    else if (Len == 4 && !strcmp(Str, "tanh"))
386      return true;
387    return false;
388  }
389}
390
391static Constant *ConstantFoldFP(double (*NativeFP)(double), double V,
392                                const Type *Ty) {
393  errno = 0;
394  V = NativeFP(V);
395  if (errno == 0)
396    return ConstantFP::get(Ty, V);
397  errno = 0;
398  return 0;
399}
400
401static Constant *ConstantFoldBinaryFP(double (*NativeFP)(double, double),
402                                      double V, double W,
403                                      const Type *Ty) {
404  errno = 0;
405  V = NativeFP(V, W);
406  if (errno == 0)
407    return ConstantFP::get(Ty, V);
408  errno = 0;
409  return 0;
410}
411
412/// ConstantFoldCall - Attempt to constant fold a call to the specified function
413/// with the specified arguments, returning null if unsuccessful.
414Constant *
415llvm::ConstantFoldCall(Function *F, Constant** Operands, unsigned NumOperands) {
416  const ValueName *NameVal = F->getValueName();
417  if (NameVal == 0) return 0;
418  const char *Str = NameVal->getKeyData();
419  unsigned Len = NameVal->getKeyLength();
420
421  const Type *Ty = F->getReturnType();
422  if (NumOperands == 1) {
423    if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
424      double V = Op->getValue();
425      switch (Str[0]) {
426      case 'a':
427        if (Len == 4 && !strcmp(Str, "acos"))
428          return ConstantFoldFP(acos, V, Ty);
429        else if (Len == 4 && !strcmp(Str, "asin"))
430          return ConstantFoldFP(asin, V, Ty);
431        else if (Len == 4 && !strcmp(Str, "atan"))
432          return ConstantFoldFP(atan, V, Ty);
433        break;
434      case 'c':
435        if (Len == 4 && !strcmp(Str, "ceil"))
436          return ConstantFoldFP(ceil, V, Ty);
437        else if (Len == 3 && !strcmp(Str, "cos"))
438          return ConstantFoldFP(cos, V, Ty);
439        else if (Len == 4 && !strcmp(Str, "cosh"))
440          return ConstantFoldFP(cosh, V, Ty);
441        break;
442      case 'e':
443        if (Len == 3 && !strcmp(Str, "exp"))
444          return ConstantFoldFP(exp, V, Ty);
445        break;
446      case 'f':
447        if (Len == 4 && !strcmp(Str, "fabs"))
448          return ConstantFP::get(Ty, fabs(V));
449        else if (Len == 5 && !strcmp(Str, "floor"))
450          return ConstantFoldFP(floor, V, Ty);
451        break;
452      case 'l':
453        if (Len == 3 && !strcmp(Str, "log") && V > 0)
454          return ConstantFoldFP(log, V, Ty);
455        else if (Len == 5 && !strcmp(Str, "log10") && V > 0)
456          return ConstantFoldFP(log10, V, Ty);
457        else if (!strcmp(Str, "llvm.sqrt.f32") ||
458                 !strcmp(Str, "llvm.sqrt.f64")) {
459          if (V >= -0.0)
460            return ConstantFP::get(Ty, sqrt(V));
461          else // Undefined
462            return ConstantFP::get(Ty, 0.0);
463        }
464        break;
465      case 's':
466        if (Len == 3 && !strcmp(Str, "sin"))
467          return ConstantFoldFP(sin, V, Ty);
468        else if (Len == 4 && !strcmp(Str, "sinh"))
469          return ConstantFoldFP(sinh, V, Ty);
470        else if (Len == 4 && !strcmp(Str, "sqrt") && V >= 0)
471          return ConstantFoldFP(sqrt, V, Ty);
472        else if (Len == 5 && !strcmp(Str, "sqrtf") && V >= 0)
473          return ConstantFoldFP(sqrt, V, Ty);
474        break;
475      case 't':
476        if (Len == 3 && !strcmp(Str, "tan"))
477          return ConstantFoldFP(tan, V, Ty);
478        else if (Len == 4 && !strcmp(Str, "tanh"))
479          return ConstantFoldFP(tanh, V, Ty);
480        break;
481      default:
482        break;
483      }
484    } else if (ConstantInt *Op = dyn_cast<ConstantInt>(Operands[0])) {
485      if (Len > 11 && !memcmp(Str, "llvm.bswap", 10)) {
486        return ConstantInt::get(Op->getValue().byteSwap());
487      } else if (Len > 11 && !memcmp(Str, "llvm.ctpop", 10)) {
488        uint64_t ctpop = Op->getValue().countPopulation();
489        return ConstantInt::get(Ty, ctpop);
490      } else if (Len > 10 && !memcmp(Str, "llvm.cttz", 9)) {
491        uint64_t cttz = Op->getValue().countTrailingZeros();
492        return ConstantInt::get(Ty, cttz);
493      } else if (Len > 10 && !memcmp(Str, "llvm.ctlz", 9)) {
494        uint64_t ctlz = Op->getValue().countLeadingZeros();
495        return ConstantInt::get(Ty, ctlz);
496      }
497    }
498  } else if (NumOperands == 2) {
499    if (ConstantFP *Op1 = dyn_cast<ConstantFP>(Operands[0])) {
500      double Op1V = Op1->getValue();
501      if (ConstantFP *Op2 = dyn_cast<ConstantFP>(Operands[1])) {
502        double Op2V = Op2->getValue();
503
504        if (Len == 3 && !strcmp(Str, "pow")) {
505          return ConstantFoldBinaryFP(pow, Op1V, Op2V, Ty);
506        } else if (Len == 4 && !strcmp(Str, "fmod")) {
507          return ConstantFoldBinaryFP(fmod, Op1V, Op2V, Ty);
508        } else if (Len == 5 && !strcmp(Str, "atan2")) {
509          return ConstantFoldBinaryFP(atan2, Op1V, Op2V, Ty);
510        }
511      } else if (ConstantInt *Op2C = dyn_cast<ConstantInt>(Operands[1])) {
512        if (!strcmp(Str, "llvm.powi.f32")) {
513          return ConstantFP::get(Ty, std::pow((float)Op1V,
514                                              (int)Op2C->getZExtValue()));
515        } else if (!strcmp(Str, "llvm.powi.f64")) {
516          return ConstantFP::get(Ty, std::pow((double)Op1V,
517                                              (int)Op2C->getZExtValue()));
518        }
519      }
520    }
521  }
522  return 0;
523}
524
525