lib/Analysis/ConstantFolding.cpp

   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>
  27 using 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.
  36 static 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)->getElementOffset(CI->getZExtValue());
  74       } else {
  75         const SequentialType *SQT = cast<SequentialType>(*GTI);
  76         Offset += TD.getTypeSize(SQT->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.
  90 static 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.
 119 static 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       uint64_t Offset = TD->getIndexedOffset(Ptr->getType(),
 137                                              (Value**)Ops+1, NumOps-1);
 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 ///
 157 Constant *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 ///
 192 Constant *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::Trunc:
 220   case Instruction::ZExt:
 221   case Instruction::SExt:
 222   case Instruction::FPTrunc:
 223   case Instruction::FPExt:
 224   case Instruction::UIToFP:
 225   case Instruction::SIToFP:
 226   case Instruction::FPToUI:
 227   case Instruction::FPToSI:
 228   case Instruction::PtrToInt:
 229   case Instruction::IntToPtr:
 230   case Instruction::BitCast:
 231     return ConstantExpr::getCast(Opc, Ops[0], DestTy);
 232   case Instruction::Select:
 233     return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]);
 234   case Instruction::ExtractElement:
 235     return ConstantExpr::getExtractElement(Ops[0], Ops[1]);
 236   case Instruction::InsertElement:
 237     return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]);
 238   case Instruction::ShuffleVector:
 239     return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]);
 240   case Instruction::GetElementPtr:
 241     if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, I->getType(), TD))
 242       return C;
 243
 244     return ConstantExpr::getGetElementPtr(Ops[0], Ops+1, NumOps-1);
 245   }
 246 }
 247
 248 /// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
 249 /// getelementptr constantexpr, return the constant value being addressed by the
 250 /// constant expression, or null if something is funny and we can't decide.
 251 Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
 252                                                        ConstantExpr *CE) {
 253   if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
 254     return 0;  // Do not allow stepping over the value!
 255
 256   // Loop over all of the operands, tracking down which value we are
 257   // addressing...
 258   gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
 259   for (++I; I != E; ++I)
 260     if (const StructType *STy = dyn_cast<StructType>(*I)) {
 261       ConstantInt *CU = cast<ConstantInt>(I.getOperand());
 262       assert(CU->getZExtValue() < STy->getNumElements() &&
 263              "Struct index out of range!");
 264       unsigned El = (unsigned)CU->getZExtValue();
 265       if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
 266         C = CS->getOperand(El);
 267       } else if (isa<ConstantAggregateZero>(C)) {
 268         C = Constant::getNullValue(STy->getElementType(El));
 269       } else if (isa<UndefValue>(C)) {
 270         C = UndefValue::get(STy->getElementType(El));
 271       } else {
 272         return 0;
 273       }
 274     } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
 275       if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) {
 276         if (CI->getZExtValue() >= ATy->getNumElements())
 277          return 0;
 278         if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
 279           C = CA->getOperand(CI->getZExtValue());
 280         else if (isa<ConstantAggregateZero>(C))
 281           C = Constant::getNullValue(ATy->getElementType());
 282         else if (isa<UndefValue>(C))
 283           C = UndefValue::get(ATy->getElementType());
 284         else
 285           return 0;
 286       } else if (const VectorType *PTy = dyn_cast<VectorType>(*I)) {
 287         if (CI->getZExtValue() >= PTy->getNumElements())
 288           return 0;
 289         if (ConstantVector *CP = dyn_cast<ConstantVector>(C))
 290           C = CP->getOperand(CI->getZExtValue());
 291         else if (isa<ConstantAggregateZero>(C))
 292           C = Constant::getNullValue(PTy->getElementType());
 293         else if (isa<UndefValue>(C))
 294           C = UndefValue::get(PTy->getElementType());
 295         else
 296           return 0;
 297       } else {
 298         return 0;
 299       }
 300     } else {
 301       return 0;
 302     }
 303   return C;
 304 }
 305
 306
 307 //===----------------------------------------------------------------------===//
 308 //  Constant Folding for Calls
 309 //
 310
 311 /// canConstantFoldCallTo - Return true if its even possible to fold a call to
 312 /// the specified function.
 313 bool
 314 llvm::canConstantFoldCallTo(Function *F) {
 315   const std::string &Name = F->getName();
 316
 317   switch (F->getIntrinsicID()) {
 318   case Intrinsic::sqrt_f32:
 319   case Intrinsic::sqrt_f64:
 320   case Intrinsic::powi_f32:
 321   case Intrinsic::powi_f64:
 322   case Intrinsic::bswap:
 323   case Intrinsic::ctpop:
 324   case Intrinsic::ctlz:
 325   case Intrinsic::cttz:
 326     return true;
 327   default: break;
 328   }
 329
 330   switch (Name[0])
 331   {
 332     case 'a':
 333       return Name == "acos" || Name == "asin" || Name == "atan" ||
 334              Name == "atan2";
 335     case 'c':
 336       return Name == "ceil" || Name == "cos" || Name == "cosf" ||
 337              Name == "cosh";
 338     case 'e':
 339       return Name == "exp";
 340     case 'f':
 341       return Name == "fabs" || Name == "fmod" || Name == "floor";
 342     case 'l':
 343       return Name == "log" || Name == "log10";
 344     case 'p':
 345       return Name == "pow";
 346     case 's':
 347       return Name == "sin" || Name == "sinh" ||
 348              Name == "sqrt" || Name == "sqrtf";
 349     case 't':
 350       return Name == "tan" || Name == "tanh";
 351     default:
 352       return false;
 353   }
 354 }
 355
 356 static Constant *ConstantFoldFP(double (*NativeFP)(double), double V,
 357                                 const Type *Ty) {
 358   errno = 0;
 359   V = NativeFP(V);
 360   if (errno == 0)
 361     return ConstantFP::get(Ty, V);
 362   errno = 0;
 363   return 0;
 364 }
 365
 366 /// ConstantFoldCall - Attempt to constant fold a call to the specified function
 367 /// with the specified arguments, returning null if unsuccessful.
 368 Constant *
 369 llvm::ConstantFoldCall(Function *F, Constant** Operands, unsigned NumOperands) {
 370   const std::string &Name = F->getName();
 371   const Type *Ty = F->getReturnType();
 372
 373   if (NumOperands == 1) {
 374     if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
 375       double V = Op->getValue();
 376       switch (Name[0])
 377       {
 378         case 'a':
 379           if (Name == "acos")
 380             return ConstantFoldFP(acos, V, Ty);
 381           else if (Name == "asin")
 382             return ConstantFoldFP(asin, V, Ty);
 383           else if (Name == "atan")
 384             return ConstantFP::get(Ty, atan(V));
 385           break;
 386         case 'c':
 387           if (Name == "ceil")
 388             return ConstantFoldFP(ceil, V, Ty);
 389           else if (Name == "cos")
 390             return ConstantFP::get(Ty, cos(V));
 391           else if (Name == "cosh")
 392             return ConstantFP::get(Ty, cosh(V));
 393           break;
 394         case 'e':
 395           if (Name == "exp")
 396             return ConstantFP::get(Ty, exp(V));
 397           break;
 398         case 'f':
 399           if (Name == "fabs")
 400             return ConstantFP::get(Ty, fabs(V));
 401           else if (Name == "floor")
 402             return ConstantFoldFP(floor, V, Ty);
 403           break;
 404         case 'l':
 405           if (Name == "log" && V > 0)
 406             return ConstantFP::get(Ty, log(V));
 407           else if (Name == "log10" && V > 0)
 408             return ConstantFoldFP(log10, V, Ty);
 409           else if (Name == "llvm.sqrt.f32" || Name == "llvm.sqrt.f64") {
 410             if (V >= -0.0)
 411               return ConstantFP::get(Ty, sqrt(V));
 412             else // Undefined
 413               return ConstantFP::get(Ty, 0.0);
 414           }
 415           break;
 416         case 's':
 417           if (Name == "sin")
 418             return ConstantFP::get(Ty, sin(V));
 419           else if (Name == "sinh")
 420             return ConstantFP::get(Ty, sinh(V));
 421           else if (Name == "sqrt" && V >= 0)
 422             return ConstantFP::get(Ty, sqrt(V));
 423           else if (Name == "sqrtf" && V >= 0)
 424             return ConstantFP::get(Ty, sqrt((float)V));
 425           break;
 426         case 't':
 427           if (Name == "tan")
 428             return ConstantFP::get(Ty, tan(V));
 429           else if (Name == "tanh")
 430             return ConstantFP::get(Ty, tanh(V));
 431           break;
 432         default:
 433           break;
 434       }
 435     } else if (ConstantInt *Op = dyn_cast<ConstantInt>(Operands[0])) {
 436       if (Name.size() > 11 && !memcmp(&Name[0], "llvm.bswap", 10)) {
 437         return ConstantInt::get(Op->getValue().byteSwap());
 438       } else if (Name.size() > 11 && !memcmp(&Name[0],"llvm.ctpop",10)) {
 439         uint64_t ctpop = Op->getValue().countPopulation();
 440         return ConstantInt::get(Type::Int32Ty, ctpop);
 441       } else if (Name.size() > 10 && !memcmp(&Name[0], "llvm.cttz", 9)) {
 442         uint64_t cttz = Op->getValue().countTrailingZeros();
 443         return ConstantInt::get(Type::Int32Ty, cttz);
 444       } else if (Name.size() > 10 && !memcmp(&Name[0], "llvm.ctlz", 9)) {
 445         uint64_t ctlz = Op->getValue().countLeadingZeros();
 446         return ConstantInt::get(Type::Int32Ty, ctlz);
 447       }
 448     }
 449   } else if (NumOperands == 2) {
 450     if (ConstantFP *Op1 = dyn_cast<ConstantFP>(Operands[0])) {
 451       double Op1V = Op1->getValue();
 452       if (ConstantFP *Op2 = dyn_cast<ConstantFP>(Operands[1])) {
 453         double Op2V = Op2->getValue();
 454
 455         if (Name == "pow") {
 456           errno = 0;
 457           double V = pow(Op1V, Op2V);
 458           if (errno == 0)
 459             return ConstantFP::get(Ty, V);
 460         } else if (Name == "fmod") {
 461           errno = 0;
 462           double V = fmod(Op1V, Op2V);
 463           if (errno == 0)
 464             return ConstantFP::get(Ty, V);
 465         } else if (Name == "atan2") {
 466           return ConstantFP::get(Ty, atan2(Op1V,Op2V));
 467         }
 468       } else if (ConstantInt *Op2C = dyn_cast<ConstantInt>(Operands[1])) {
 469         if (Name == "llvm.powi.f32") {
 470           return ConstantFP::get(Ty, std::pow((float)Op1V,
 471                                               (int)Op2C->getZExtValue()));
 472         } else if (Name == "llvm.powi.f64") {
 473           return ConstantFP::get(Ty, std::pow((double)Op1V,
 474                                               (int)Op2C->getZExtValue()));
 475         }
 476       }
 477     }
 478   }
 479   return 0;
 480 }
 481