lib/CodeGen/IntrinsicLowering.cpp

   1 //===-- IntrinsicLowering.cpp - Intrinsic Lowering default implementation -===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements the IntrinsicLowering class.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Constants.h"
  15 #include "llvm/DerivedTypes.h"
  16 #include "llvm/Module.h"
  17 #include "llvm/Type.h"
  18 #include "llvm/CodeGen/IntrinsicLowering.h"
  19 #include "llvm/Support/ErrorHandling.h"
  20 #include "llvm/Support/IRBuilder.h"
  21 #include "llvm/Support/raw_ostream.h"
  22 #include "llvm/Target/TargetData.h"
  23 #include "llvm/ADT/SmallVector.h"
  24 using namespace llvm;
  25
  26 template <class ArgIt>
  27 static void EnsureFunctionExists(Module &M, const char *Name,
  28                                  ArgIt ArgBegin, ArgIt ArgEnd,
  29                                  const Type *RetTy) {
  30   // Insert a correctly-typed definition now.
  31   std::vector<const Type *> ParamTys;
  32   for (ArgIt I = ArgBegin; I != ArgEnd; ++I)
  33     ParamTys.push_back(I->getType());
  34   M.getOrInsertFunction(Name, FunctionType::get(RetTy, ParamTys, false));
  35 }
  36
  37 static void EnsureFPIntrinsicsExist(Module &M, Function *Fn,
  38                                     const char *FName,
  39                                     const char *DName, const char *LDName) {
  40   // Insert definitions for all the floating point types.
  41   switch((int)Fn->arg_begin()->getType()->getTypeID()) {
  42   case Type::FloatTyID:
  43     EnsureFunctionExists(M, FName, Fn->arg_begin(), Fn->arg_end(),
  44                          Type::getFloatTy(M.getContext()));
  45     break;
  46   case Type::DoubleTyID:
  47     EnsureFunctionExists(M, DName, Fn->arg_begin(), Fn->arg_end(),
  48                          Type::getDoubleTy(M.getContext()));
  49     break;
  50   case Type::X86_FP80TyID:
  51   case Type::FP128TyID:
  52   case Type::PPC_FP128TyID:
  53     EnsureFunctionExists(M, LDName, Fn->arg_begin(), Fn->arg_end(),
  54                          Fn->arg_begin()->getType());
  55     break;
  56   }
  57 }
  58
  59 /// ReplaceCallWith - This function is used when we want to lower an intrinsic
  60 /// call to a call of an external function.  This handles hard cases such as
  61 /// when there was already a prototype for the external function, and if that
  62 /// prototype doesn't match the arguments we expect to pass in.
  63 template <class ArgIt>
  64 static CallInst *ReplaceCallWith(const char *NewFn, CallInst *CI,
  65                                  ArgIt ArgBegin, ArgIt ArgEnd,
  66                                  const Type *RetTy) {
  67   // If we haven't already looked up this function, check to see if the
  68   // program already contains a function with this name.
  69   Module *M = CI->getParent()->getParent()->getParent();
  70   // Get or insert the definition now.
  71   std::vector<const Type *> ParamTys;
  72   for (ArgIt I = ArgBegin; I != ArgEnd; ++I)
  73     ParamTys.push_back((*I)->getType());
  74   Constant* FCache = M->getOrInsertFunction(NewFn,
  75                                   FunctionType::get(RetTy, ParamTys, false));
  76
  77   IRBuilder<> Builder(CI->getParent(), CI);
  78   SmallVector<Value *, 8> Args(ArgBegin, ArgEnd);
  79   CallInst *NewCI = Builder.CreateCall(FCache, Args.begin(), Args.end());
  80   NewCI->setName(CI->getName());
  81   if (!CI->use_empty())
  82     CI->replaceAllUsesWith(NewCI);
  83   return NewCI;
  84 }
  85
  86 void IntrinsicLowering::AddPrototypes(Module &M) {
  87   LLVMContext &Context = M.getContext();
  88   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
  89     if (I->isDeclaration() && !I->use_empty())
  90       switch (I->getIntrinsicID()) {
  91       default: break;
  92       case Intrinsic::setjmp:
  93         EnsureFunctionExists(M, "setjmp", I->arg_begin(), I->arg_end(),
  94                              Type::getInt32Ty(M.getContext()));
  95         break;
  96       case Intrinsic::longjmp:
  97         EnsureFunctionExists(M, "longjmp", I->arg_begin(), I->arg_end(),
  98                              Type::getVoidTy(M.getContext()));
  99         break;
 100       case Intrinsic::siglongjmp:
 101         EnsureFunctionExists(M, "abort", I->arg_end(), I->arg_end(),
 102                              Type::getVoidTy(M.getContext()));
 103         break;
 104       case Intrinsic::memcpy:
 105         M.getOrInsertFunction("memcpy",
 106           Type::getInt8PtrTy(Context),
 107                               Type::getInt8PtrTy(Context),
 108                               Type::getInt8PtrTy(Context),
 109                               TD.getIntPtrType(Context), (Type *)0);
 110         break;
 111       case Intrinsic::memmove:
 112         M.getOrInsertFunction("memmove",
 113           Type::getInt8PtrTy(Context),
 114                               Type::getInt8PtrTy(Context),
 115                               Type::getInt8PtrTy(Context),
 116                               TD.getIntPtrType(Context), (Type *)0);
 117         break;
 118       case Intrinsic::memset:
 119         M.getOrInsertFunction("memset",
 120           Type::getInt8PtrTy(Context),
 121                               Type::getInt8PtrTy(Context),
 122                               Type::getInt32Ty(M.getContext()),
 123                               TD.getIntPtrType(Context), (Type *)0);
 124         break;
 125       case Intrinsic::sqrt:
 126         EnsureFPIntrinsicsExist(M, I, "sqrtf", "sqrt", "sqrtl");
 127         break;
 128       case Intrinsic::sin:
 129         EnsureFPIntrinsicsExist(M, I, "sinf", "sin", "sinl");
 130         break;
 131       case Intrinsic::cos:
 132         EnsureFPIntrinsicsExist(M, I, "cosf", "cos", "cosl");
 133         break;
 134       case Intrinsic::pow:
 135         EnsureFPIntrinsicsExist(M, I, "powf", "pow", "powl");
 136         break;
 137       case Intrinsic::log:
 138         EnsureFPIntrinsicsExist(M, I, "logf", "log", "logl");
 139         break;
 140       case Intrinsic::log2:
 141         EnsureFPIntrinsicsExist(M, I, "log2f", "log2", "log2l");
 142         break;
 143       case Intrinsic::log10:
 144         EnsureFPIntrinsicsExist(M, I, "log10f", "log10", "log10l");
 145         break;
 146       case Intrinsic::exp:
 147         EnsureFPIntrinsicsExist(M, I, "expf", "exp", "expl");
 148         break;
 149       case Intrinsic::exp2:
 150         EnsureFPIntrinsicsExist(M, I, "exp2f", "exp2", "exp2l");
 151         break;
 152       }
 153 }
 154
 155 /// LowerBSWAP - Emit the code to lower bswap of V before the specified
 156 /// instruction IP.
 157 static Value *LowerBSWAP(LLVMContext &Context, Value *V, Instruction *IP) {
 158   assert(V->getType()->isIntegerTy() && "Can't bswap a non-integer type!");
 159
 160   unsigned BitSize = V->getType()->getPrimitiveSizeInBits();
 161
 162   IRBuilder<> Builder(IP->getParent(), IP);
 163
 164   switch(BitSize) {
 165   default: llvm_unreachable("Unhandled type size of value to byteswap!");
 166   case 16: {
 167     Value *Tmp1 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 8),
 168                                     "bswap.2");
 169     Value *Tmp2 = Builder.CreateLShr(V, ConstantInt::get(V->getType(), 8),
 170                                      "bswap.1");
 171     V = Builder.CreateOr(Tmp1, Tmp2, "bswap.i16");
 172     break;
 173   }
 174   case 32: {
 175     Value *Tmp4 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 24),
 176                                     "bswap.4");
 177     Value *Tmp3 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 8),
 178                                     "bswap.3");
 179     Value *Tmp2 = Builder.CreateLShr(V, ConstantInt::get(V->getType(), 8),
 180                                      "bswap.2");
 181     Value *Tmp1 = Builder.CreateLShr(V,ConstantInt::get(V->getType(), 24),
 182                                      "bswap.1");
 183     Tmp3 = Builder.CreateAnd(Tmp3,
 184                          ConstantInt::get(Type::getInt32Ty(Context), 0xFF0000),
 185                              "bswap.and3");
 186     Tmp2 = Builder.CreateAnd(Tmp2,
 187                            ConstantInt::get(Type::getInt32Ty(Context), 0xFF00),
 188                              "bswap.and2");
 189     Tmp4 = Builder.CreateOr(Tmp4, Tmp3, "bswap.or1");
 190     Tmp2 = Builder.CreateOr(Tmp2, Tmp1, "bswap.or2");
 191     V = Builder.CreateOr(Tmp4, Tmp2, "bswap.i32");
 192     break;
 193   }
 194   case 64: {
 195     Value *Tmp8 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 56),
 196                                     "bswap.8");
 197     Value *Tmp7 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 40),
 198                                     "bswap.7");
 199     Value *Tmp6 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 24),
 200                                     "bswap.6");
 201     Value *Tmp5 = Builder.CreateShl(V, ConstantInt::get(V->getType(), 8),
 202                                     "bswap.5");
 203     Value* Tmp4 = Builder.CreateLShr(V, ConstantInt::get(V->getType(), 8),
 204                                      "bswap.4");
 205     Value* Tmp3 = Builder.CreateLShr(V,
 206                                      ConstantInt::get(V->getType(), 24),
 207                                      "bswap.3");
 208     Value* Tmp2 = Builder.CreateLShr(V,
 209                                      ConstantInt::get(V->getType(), 40),
 210                                      "bswap.2");
 211     Value* Tmp1 = Builder.CreateLShr(V,
 212                                      ConstantInt::get(V->getType(), 56),
 213                                      "bswap.1");
 214     Tmp7 = Builder.CreateAnd(Tmp7,
 215                              ConstantInt::get(Type::getInt64Ty(Context),
 216                                               0xFF000000000000ULL),
 217                              "bswap.and7");
 218     Tmp6 = Builder.CreateAnd(Tmp6,
 219                              ConstantInt::get(Type::getInt64Ty(Context),
 220                                               0xFF0000000000ULL),
 221                              "bswap.and6");
 222     Tmp5 = Builder.CreateAnd(Tmp5,
 223                         ConstantInt::get(Type::getInt64Ty(Context),
 224                              0xFF00000000ULL),
 225                              "bswap.and5");
 226     Tmp4 = Builder.CreateAnd(Tmp4,
 227                         ConstantInt::get(Type::getInt64Ty(Context),
 228                              0xFF000000ULL),
 229                              "bswap.and4");
 230     Tmp3 = Builder.CreateAnd(Tmp3,
 231                              ConstantInt::get(Type::getInt64Ty(Context),
 232                              0xFF0000ULL),
 233                              "bswap.and3");
 234     Tmp2 = Builder.CreateAnd(Tmp2,
 235                              ConstantInt::get(Type::getInt64Ty(Context),
 236                              0xFF00ULL),
 237                              "bswap.and2");
 238     Tmp8 = Builder.CreateOr(Tmp8, Tmp7, "bswap.or1");
 239     Tmp6 = Builder.CreateOr(Tmp6, Tmp5, "bswap.or2");
 240     Tmp4 = Builder.CreateOr(Tmp4, Tmp3, "bswap.or3");
 241     Tmp2 = Builder.CreateOr(Tmp2, Tmp1, "bswap.or4");
 242     Tmp8 = Builder.CreateOr(Tmp8, Tmp6, "bswap.or5");
 243     Tmp4 = Builder.CreateOr(Tmp4, Tmp2, "bswap.or6");
 244     V = Builder.CreateOr(Tmp8, Tmp4, "bswap.i64");
 245     break;
 246   }
 247   }
 248   return V;
 249 }
 250
 251 /// LowerCTPOP - Emit the code to lower ctpop of V before the specified
 252 /// instruction IP.
 253 static Value *LowerCTPOP(LLVMContext &Context, Value *V, Instruction *IP) {
 254   assert(V->getType()->isIntegerTy() && "Can't ctpop a non-integer type!");
 255
 256   static const uint64_t MaskValues[6] = {
 257     0x5555555555555555ULL, 0x3333333333333333ULL,
 258     0x0F0F0F0F0F0F0F0FULL, 0x00FF00FF00FF00FFULL,
 259     0x0000FFFF0000FFFFULL, 0x00000000FFFFFFFFULL
 260   };
 261
 262   IRBuilder<> Builder(IP->getParent(), IP);
 263
 264   unsigned BitSize = V->getType()->getPrimitiveSizeInBits();
 265   unsigned WordSize = (BitSize + 63) / 64;
 266   Value *Count = ConstantInt::get(V->getType(), 0);
 267
 268   for (unsigned n = 0; n < WordSize; ++n) {
 269     Value *PartValue = V;
 270     for (unsigned i = 1, ct = 0; i < (BitSize>64 ? 64 : BitSize);
 271          i <<= 1, ++ct) {
 272       Value *MaskCst = ConstantInt::get(V->getType(), MaskValues[ct]);
 273       Value *LHS = Builder.CreateAnd(PartValue, MaskCst, "cppop.and1");
 274       Value *VShift = Builder.CreateLShr(PartValue,
 275                                         ConstantInt::get(V->getType(), i),
 276                                          "ctpop.sh");
 277       Value *RHS = Builder.CreateAnd(VShift, MaskCst, "cppop.and2");
 278       PartValue = Builder.CreateAdd(LHS, RHS, "ctpop.step");
 279     }
 280     Count = Builder.CreateAdd(PartValue, Count, "ctpop.part");
 281     if (BitSize > 64) {
 282       V = Builder.CreateLShr(V, ConstantInt::get(V->getType(), 64),
 283                              "ctpop.part.sh");
 284       BitSize -= 64;
 285     }
 286   }
 287
 288   return Count;
 289 }
 290
 291 /// LowerCTLZ - Emit the code to lower ctlz of V before the specified
 292 /// instruction IP.
 293 static Value *LowerCTLZ(LLVMContext &Context, Value *V, Instruction *IP) {
 294
 295   IRBuilder<> Builder(IP->getParent(), IP);
 296
 297   unsigned BitSize = V->getType()->getPrimitiveSizeInBits();
 298   for (unsigned i = 1; i < BitSize; i <<= 1) {
 299     Value *ShVal = ConstantInt::get(V->getType(), i);
 300     ShVal = Builder.CreateLShr(V, ShVal, "ctlz.sh");
 301     V = Builder.CreateOr(V, ShVal, "ctlz.step");
 302   }
 303
 304   V = Builder.CreateNot(V);
 305   return LowerCTPOP(Context, V, IP);
 306 }
 307
 308 static void ReplaceFPIntrinsicWithCall(CallInst *CI, const char *Fname,
 309                                        const char *Dname,
 310                                        const char *LDname) {
 311   switch (CI->getOperand(1)->getType()->getTypeID()) {
 312   default: llvm_unreachable("Invalid type in intrinsic");
 313   case Type::FloatTyID:
 314     ReplaceCallWith(Fname, CI, CI->op_begin() + 1, CI->op_end(),
 315                   Type::getFloatTy(CI->getContext()));
 316     break;
 317   case Type::DoubleTyID:
 318     ReplaceCallWith(Dname, CI, CI->op_begin() + 1, CI->op_end(),
 319                   Type::getDoubleTy(CI->getContext()));
 320     break;
 321   case Type::X86_FP80TyID:
 322   case Type::FP128TyID:
 323   case Type::PPC_FP128TyID:
 324     ReplaceCallWith(LDname, CI, CI->op_begin() + 1, CI->op_end(),
 325                   CI->getOperand(1)->getType());
 326     break;
 327   }
 328 }
 329
 330 void IntrinsicLowering::LowerIntrinsicCall(CallInst *CI) {
 331   IRBuilder<> Builder(CI->getParent(), CI);
 332   LLVMContext &Context = CI->getContext();
 333
 334   const Function *Callee = CI->getCalledFunction();
 335   assert(Callee && "Cannot lower an indirect call!");
 336
 337   switch (Callee->getIntrinsicID()) {
 338   case Intrinsic::not_intrinsic:
 339     report_fatal_error("Cannot lower a call to a non-intrinsic function '"+
 340                       Callee->getName() + "'!");
 341   default:
 342     report_fatal_error("Code generator does not support intrinsic function '"+
 343                       Callee->getName()+"'!");
 344
 345     // The setjmp/longjmp intrinsics should only exist in the code if it was
 346     // never optimized (ie, right out of the CFE), or if it has been hacked on
 347     // by the lowerinvoke pass.  In both cases, the right thing to do is to
 348     // convert the call to an explicit setjmp or longjmp call.
 349   case Intrinsic::setjmp: {
 350     Value *V = ReplaceCallWith("setjmp", CI, CI->op_begin() + 1, CI->op_end(),
 351                                Type::getInt32Ty(Context));
 352     if (!CI->getType()->isVoidTy())
 353       CI->replaceAllUsesWith(V);
 354     break;
 355   }
 356   case Intrinsic::sigsetjmp:
 357      if (!CI->getType()->isVoidTy())
 358        CI->replaceAllUsesWith(Constant::getNullValue(CI->getType()));
 359      break;
 360
 361   case Intrinsic::longjmp: {
 362     ReplaceCallWith("longjmp", CI, CI->op_begin() + 1, CI->op_end(),
 363                     Type::getVoidTy(Context));
 364     break;
 365   }
 366
 367   case Intrinsic::siglongjmp: {
 368     // Insert the call to abort
 369     ReplaceCallWith("abort", CI, CI->op_end(), CI->op_end(),
 370                     Type::getVoidTy(Context));
 371     break;
 372   }
 373   case Intrinsic::ctpop:
 374     CI->replaceAllUsesWith(LowerCTPOP(Context, CI->getOperand(1), CI));
 375     break;
 376
 377   case Intrinsic::bswap:
 378     CI->replaceAllUsesWith(LowerBSWAP(Context, CI->getOperand(1), CI));
 379     break;
 380
 381   case Intrinsic::ctlz:
 382     CI->replaceAllUsesWith(LowerCTLZ(Context, CI->getOperand(1), CI));
 383     break;
 384
 385   case Intrinsic::cttz: {
 386     // cttz(x) -> ctpop(~X & (X-1))
 387     Value *Src = CI->getOperand(1);
 388     Value *NotSrc = Builder.CreateNot(Src);
 389     NotSrc->setName(Src->getName() + ".not");
 390     Value *SrcM1 = ConstantInt::get(Src->getType(), 1);
 391     SrcM1 = Builder.CreateSub(Src, SrcM1);
 392     Src = LowerCTPOP(Context, Builder.CreateAnd(NotSrc, SrcM1), CI);
 393     CI->replaceAllUsesWith(Src);
 394     break;
 395   }
 396
 397   case Intrinsic::stacksave:
 398   case Intrinsic::stackrestore: {
 399     if (!Warned)
 400       errs() << "WARNING: this target does not support the llvm.stack"
 401              << (Callee->getIntrinsicID() == Intrinsic::stacksave ?
 402                "save" : "restore") << " intrinsic.\n";
 403     Warned = true;
 404     if (Callee->getIntrinsicID() == Intrinsic::stacksave)
 405       CI->replaceAllUsesWith(Constant::getNullValue(CI->getType()));
 406     break;
 407   }
 408
 409   case Intrinsic::returnaddress:
 410   case Intrinsic::frameaddress:
 411     errs() << "WARNING: this target does not support the llvm."
 412            << (Callee->getIntrinsicID() == Intrinsic::returnaddress ?
 413              "return" : "frame") << "address intrinsic.\n";
 414     CI->replaceAllUsesWith(ConstantPointerNull::get(
 415                                             cast<PointerType>(CI->getType())));
 416     break;
 417
 418   case Intrinsic::prefetch:
 419     break;    // Simply strip out prefetches on unsupported architectures
 420
 421   case Intrinsic::pcmarker:
 422     break;    // Simply strip out pcmarker on unsupported architectures
 423   case Intrinsic::readcyclecounter: {
 424     errs() << "WARNING: this target does not support the llvm.readcyclecoun"
 425            << "ter intrinsic.  It is being lowered to a constant 0\n";
 426     CI->replaceAllUsesWith(ConstantInt::get(Type::getInt64Ty(Context), 0));
 427     break;
 428   }
 429
 430   case Intrinsic::dbg_declare:
 431     break;    // Simply strip out debugging intrinsics
 432
 433   case Intrinsic::eh_exception:
 434   case Intrinsic::eh_selector:
 435     CI->replaceAllUsesWith(Constant::getNullValue(CI->getType()));
 436     break;
 437
 438   case Intrinsic::eh_typeid_for:
 439     // Return something different to eh_selector.
 440     CI->replaceAllUsesWith(ConstantInt::get(CI->getType(), 1));
 441     break;
 442
 443   case Intrinsic::var_annotation:
 444     break;   // Strip out annotate intrinsic
 445
 446   case Intrinsic::memcpy: {
 447     const IntegerType *IntPtr = TD.getIntPtrType(Context);
 448     Value *Size = Builder.CreateIntCast(CI->getOperand(3), IntPtr,
 449                                         /* isSigned */ false);
 450     Value *Ops[3];
 451     Ops[0] = CI->getOperand(1);
 452     Ops[1] = CI->getOperand(2);
 453     Ops[2] = Size;
 454     ReplaceCallWith("memcpy", CI, Ops, Ops+3, CI->getOperand(1)->getType());
 455     break;
 456   }
 457   case Intrinsic::memmove: {
 458     const IntegerType *IntPtr = TD.getIntPtrType(Context);
 459     Value *Size = Builder.CreateIntCast(CI->getOperand(3), IntPtr,
 460                                         /* isSigned */ false);
 461     Value *Ops[3];
 462     Ops[0] = CI->getOperand(1);
 463     Ops[1] = CI->getOperand(2);
 464     Ops[2] = Size;
 465     ReplaceCallWith("memmove", CI, Ops, Ops+3, CI->getOperand(1)->getType());
 466     break;
 467   }
 468   case Intrinsic::memset: {
 469     const IntegerType *IntPtr = TD.getIntPtrType(Context);
 470     Value *Size = Builder.CreateIntCast(CI->getOperand(3), IntPtr,
 471                                         /* isSigned */ false);
 472     Value *Ops[3];
 473     Ops[0] = CI->getOperand(1);
 474     // Extend the amount to i32.
 475     Ops[1] = Builder.CreateIntCast(CI->getOperand(2), Type::getInt32Ty(Context),
 476                                    /* isSigned */ false);
 477     Ops[2] = Size;
 478     ReplaceCallWith("memset", CI, Ops, Ops+3, CI->getOperand(1)->getType());
 479     break;
 480   }
 481   case Intrinsic::sqrt: {
 482     ReplaceFPIntrinsicWithCall(CI, "sqrtf", "sqrt", "sqrtl");
 483     break;
 484   }
 485   case Intrinsic::log: {
 486     ReplaceFPIntrinsicWithCall(CI, "logf", "log", "logl");
 487     break;
 488   }
 489   case Intrinsic::log2: {
 490     ReplaceFPIntrinsicWithCall(CI, "log2f", "log2", "log2l");
 491     break;
 492   }
 493   case Intrinsic::log10: {
 494     ReplaceFPIntrinsicWithCall(CI, "log10f", "log10", "log10l");
 495     break;
 496   }
 497   case Intrinsic::exp: {
 498     ReplaceFPIntrinsicWithCall(CI, "expf", "exp", "expl");
 499     break;
 500   }
 501   case Intrinsic::exp2: {
 502     ReplaceFPIntrinsicWithCall(CI, "exp2f", "exp2", "exp2l");
 503     break;
 504   }
 505   case Intrinsic::pow: {
 506     ReplaceFPIntrinsicWithCall(CI, "powf", "pow", "powl");
 507     break;
 508   }
 509   case Intrinsic::flt_rounds:
 510      // Lower to "round to the nearest"
 511      if (!CI->getType()->isVoidTy())
 512        CI->replaceAllUsesWith(ConstantInt::get(CI->getType(), 1));
 513      break;
 514   case Intrinsic::invariant_start:
 515   case Intrinsic::lifetime_start:
 516     // Discard region information.
 517     CI->replaceAllUsesWith(UndefValue::get(CI->getType()));
 518     break;
 519   case Intrinsic::invariant_end:
 520   case Intrinsic::lifetime_end:
 521     // Discard region information.
 522     break;
 523   }
 524
 525   assert(CI->use_empty() &&
 526          "Lowering should have eliminated any uses of the intrinsic call!");
 527   CI->eraseFromParent();
 528 }