llvm/lib/Transforms/Scalar/Float2Int.cpp

   1 //===- Float2Int.cpp - Demote floating point ops to work on integers ------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the Float2Int pass, which aims to demote floating
  10 // point operations to work on integers, where that is losslessly possible.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Transforms/Scalar/Float2Int.h"
  15 #include "llvm/ADT/APInt.h"
  16 #include "llvm/ADT/APSInt.h"
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/Analysis/GlobalsModRef.h"
  19 #include "llvm/IR/Constants.h"
  20 #include "llvm/IR/Dominators.h"
  21 #include "llvm/IR/IRBuilder.h"
  22 #include "llvm/IR/Module.h"
  23 #include "llvm/Support/CommandLine.h"
  24 #include "llvm/Support/Debug.h"
  25 #include "llvm/Support/raw_ostream.h"
  26 #include <deque>
  27
  28 #define DEBUG_TYPE "float2int"
  29
  30 using namespace llvm;
  31
  32 // The algorithm is simple. Start at instructions that convert from the
  33 // float to the int domain: fptoui, fptosi and fcmp. Walk up the def-use
  34 // graph, using an equivalence datastructure to unify graphs that interfere.
  35 //
  36 // Mappable instructions are those with an integer corrollary that, given
  37 // integer domain inputs, produce an integer output; fadd, for example.
  38 //
  39 // If a non-mappable instruction is seen, this entire def-use graph is marked
  40 // as non-transformable. If we see an instruction that converts from the
  41 // integer domain to FP domain (uitofp,sitofp), we terminate our walk.
  42
  43 /// The largest integer type worth dealing with.
  44 static cl::opt<unsigned>
  45 MaxIntegerBW("float2int-max-integer-bw", cl::init(64), cl::Hidden,
  46              cl::desc("Max integer bitwidth to consider in float2int"
  47                       "(default=64)"));
  48
  49 // Given a FCmp predicate, return a matching ICmp predicate if one
  50 // exists, otherwise return BAD_ICMP_PREDICATE.
  51 static CmpInst::Predicate mapFCmpPred(CmpInst::Predicate P) {
  52   switch (P) {
  53   case CmpInst::FCMP_OEQ:
  54   case CmpInst::FCMP_UEQ:
  55     return CmpInst::ICMP_EQ;
  56   case CmpInst::FCMP_OGT:
  57   case CmpInst::FCMP_UGT:
  58     return CmpInst::ICMP_SGT;
  59   case CmpInst::FCMP_OGE:
  60   case CmpInst::FCMP_UGE:
  61     return CmpInst::ICMP_SGE;
  62   case CmpInst::FCMP_OLT:
  63   case CmpInst::FCMP_ULT:
  64     return CmpInst::ICMP_SLT;
  65   case CmpInst::FCMP_OLE:
  66   case CmpInst::FCMP_ULE:
  67     return CmpInst::ICMP_SLE;
  68   case CmpInst::FCMP_ONE:
  69   case CmpInst::FCMP_UNE:
  70     return CmpInst::ICMP_NE;
  71   default:
  72     return CmpInst::BAD_ICMP_PREDICATE;
  73   }
  74 }
  75
  76 // Given a floating point binary operator, return the matching
  77 // integer version.
  78 static Instruction::BinaryOps mapBinOpcode(unsigned Opcode) {
  79   switch (Opcode) {
  80   default: llvm_unreachable("Unhandled opcode!");
  81   case Instruction::FAdd: return Instruction::Add;
  82   case Instruction::FSub: return Instruction::Sub;
  83   case Instruction::FMul: return Instruction::Mul;
  84   }
  85 }
  86
  87 // Find the roots - instructions that convert from the FP domain to
  88 // integer domain.
  89 void Float2IntPass::findRoots(Function &F, const DominatorTree &DT) {
  90   for (BasicBlock &BB : F) {
  91     // Unreachable code can take on strange forms that we are not prepared to
  92     // handle. For example, an instruction may have itself as an operand.
  93     if (!DT.isReachableFromEntry(&BB))
  94       continue;
  95
  96     for (Instruction &I : BB) {
  97       if (isa<VectorType>(I.getType()))
  98         continue;
  99       switch (I.getOpcode()) {
 100       default: break;
 101       case Instruction::FPToUI:
 102       case Instruction::FPToSI:
 103         Roots.insert(&I);
 104         break;
 105       case Instruction::FCmp:
 106         if (mapFCmpPred(cast<CmpInst>(&I)->getPredicate()) !=
 107             CmpInst::BAD_ICMP_PREDICATE)
 108           Roots.insert(&I);
 109         break;
 110       }
 111     }
 112   }
 113 }
 114
 115 // Helper - mark I as having been traversed, having range R.
 116 void Float2IntPass::seen(Instruction *I, ConstantRange R) {
 117   LLVM_DEBUG(dbgs() << "F2I: " << *I << ":" << R << "\n");
 118   auto IT = SeenInsts.find(I);
 119   if (IT != SeenInsts.end())
 120     IT->second = std::move(R);
 121   else
 122     SeenInsts.insert(std::make_pair(I, std::move(R)));
 123 }
 124
 125 // Helper - get a range representing a poison value.
 126 ConstantRange Float2IntPass::badRange() {
 127   return ConstantRange::getFull(MaxIntegerBW + 1);
 128 }
 129 ConstantRange Float2IntPass::unknownRange() {
 130   return ConstantRange::getEmpty(MaxIntegerBW + 1);
 131 }
 132 ConstantRange Float2IntPass::validateRange(ConstantRange R) {
 133   if (R.getBitWidth() > MaxIntegerBW + 1)
 134     return badRange();
 135   return R;
 136 }
 137
 138 // The most obvious way to structure the search is a depth-first, eager
 139 // search from each root. However, that require direct recursion and so
 140 // can only handle small instruction sequences. Instead, we split the search
 141 // up into two phases:
 142 //   - walkBackwards:  A breadth-first walk of the use-def graph starting from
 143 //                     the roots. Populate "SeenInsts" with interesting
 144 //                     instructions and poison values if they're obvious and
 145 //                     cheap to compute. Calculate the equivalance set structure
 146 //                     while we're here too.
 147 //   - walkForwards:  Iterate over SeenInsts in reverse order, so we visit
 148 //                     defs before their uses. Calculate the real range info.
 149
 150 // Breadth-first walk of the use-def graph; determine the set of nodes
 151 // we care about and eagerly determine if some of them are poisonous.
 152 void Float2IntPass::walkBackwards() {
 153   std::deque<Instruction*> Worklist(Roots.begin(), Roots.end());
 154   while (!Worklist.empty()) {
 155     Instruction *I = Worklist.back();
 156     Worklist.pop_back();
 157
 158     if (SeenInsts.contains(I))
 159       // Seen already.
 160       continue;
 161
 162     switch (I->getOpcode()) {
 163       // FIXME: Handle select and phi nodes.
 164     default:
 165       // Path terminated uncleanly.
 166       seen(I, badRange());
 167       break;
 168
 169     case Instruction::UIToFP:
 170     case Instruction::SIToFP: {
 171       // Path terminated cleanly - use the type of the integer input to seed
 172       // the analysis.
 173       unsigned BW = I->getOperand(0)->getType()->getPrimitiveSizeInBits();
 174       auto Input = ConstantRange::getFull(BW);
 175       auto CastOp = (Instruction::CastOps)I->getOpcode();
 176       seen(I, validateRange(Input.castOp(CastOp, MaxIntegerBW+1)));
 177       continue;
 178     }
 179
 180     case Instruction::FNeg:
 181     case Instruction::FAdd:
 182     case Instruction::FSub:
 183     case Instruction::FMul:
 184     case Instruction::FPToUI:
 185     case Instruction::FPToSI:
 186     case Instruction::FCmp:
 187       seen(I, unknownRange());
 188       break;
 189     }
 190
 191     for (Value *O : I->operands()) {
 192       if (Instruction *OI = dyn_cast<Instruction>(O)) {
 193         // Unify def-use chains if they interfere.
 194         ECs.unionSets(I, OI);
 195         if (SeenInsts.find(I)->second != badRange())
 196           Worklist.push_back(OI);
 197       } else if (!isa<ConstantFP>(O)) {
 198         // Not an instruction or ConstantFP? we can't do anything.
 199         seen(I, badRange());
 200       }
 201     }
 202   }
 203 }
 204
 205 // Calculate result range from operand ranges.
 206 // Return std::nullopt if the range cannot be calculated yet.
 207 std::optional<ConstantRange> Float2IntPass::calcRange(Instruction *I) {
 208   SmallVector<ConstantRange, 4> OpRanges;
 209   for (Value *O : I->operands()) {
 210     if (Instruction *OI = dyn_cast<Instruction>(O)) {
 211       auto OpIt = SeenInsts.find(OI);
 212       assert(OpIt != SeenInsts.end() && "def not seen before use!");
 213       if (OpIt->second == unknownRange())
 214         return std::nullopt; // Wait until operand range has been calculated.
 215       OpRanges.push_back(OpIt->second);
 216     } else if (ConstantFP *CF = dyn_cast<ConstantFP>(O)) {
 217       // Work out if the floating point number can be losslessly represented
 218       // as an integer.
 219       // APFloat::convertToInteger(&Exact) purports to do what we want, but
 220       // the exactness can be too precise. For example, negative zero can
 221       // never be exactly converted to an integer.
 222       //
 223       // Instead, we ask APFloat to round itself to an integral value - this
 224       // preserves sign-of-zero - then compare the result with the original.
 225       //
 226       const APFloat &F = CF->getValueAPF();
 227
 228       // First, weed out obviously incorrect values. Non-finite numbers
 229       // can't be represented and neither can negative zero, unless
 230       // we're in fast math mode.
 231       if (!F.isFinite() ||
 232           (F.isZero() && F.isNegative() && isa<FPMathOperator>(I) &&
 233            !I->hasNoSignedZeros()))
 234         return badRange();
 235
 236       APFloat NewF = F;
 237       auto Res = NewF.roundToIntegral(APFloat::rmNearestTiesToEven);
 238       if (Res != APFloat::opOK || NewF != F)
 239         return badRange();
 240
 241       // OK, it's representable. Now get it.
 242       APSInt Int(MaxIntegerBW+1, false);
 243       bool Exact;
 244       CF->getValueAPF().convertToInteger(Int,
 245                                          APFloat::rmNearestTiesToEven,
 246                                          &Exact);
 247       OpRanges.push_back(ConstantRange(Int));
 248     } else {
 249       llvm_unreachable("Should have already marked this as badRange!");
 250     }
 251   }
 252
 253   switch (I->getOpcode()) {
 254   // FIXME: Handle select and phi nodes.
 255   default:
 256   case Instruction::UIToFP:
 257   case Instruction::SIToFP:
 258     llvm_unreachable("Should have been handled in walkForwards!");
 259
 260   case Instruction::FNeg: {
 261     assert(OpRanges.size() == 1 && "FNeg is a unary operator!");
 262     unsigned Size = OpRanges[0].getBitWidth();
 263     auto Zero = ConstantRange(APInt::getZero(Size));
 264     return Zero.sub(OpRanges[0]);
 265   }
 266
 267   case Instruction::FAdd:
 268   case Instruction::FSub:
 269   case Instruction::FMul: {
 270     assert(OpRanges.size() == 2 && "its a binary operator!");
 271     auto BinOp = (Instruction::BinaryOps) I->getOpcode();
 272     return OpRanges[0].binaryOp(BinOp, OpRanges[1]);
 273   }
 274
 275   //
 276   // Root-only instructions - we'll only see these if they're the
 277   //                          first node in a walk.
 278   //
 279   case Instruction::FPToUI:
 280   case Instruction::FPToSI: {
 281     assert(OpRanges.size() == 1 && "FPTo[US]I is a unary operator!");
 282     // Note: We're ignoring the casts output size here as that's what the
 283     // caller expects.
 284     auto CastOp = (Instruction::CastOps)I->getOpcode();
 285     return OpRanges[0].castOp(CastOp, MaxIntegerBW+1);
 286   }
 287
 288   case Instruction::FCmp:
 289     assert(OpRanges.size() == 2 && "FCmp is a binary operator!");
 290     return OpRanges[0].unionWith(OpRanges[1]);
 291   }
 292 }
 293
 294 // Walk forwards down the list of seen instructions, so we visit defs before
 295 // uses.
 296 void Float2IntPass::walkForwards() {
 297   std::deque<Instruction *> Worklist;
 298   for (const auto &Pair : SeenInsts)
 299     if (Pair.second == unknownRange())
 300       Worklist.push_back(Pair.first);
 301
 302   while (!Worklist.empty()) {
 303     Instruction *I = Worklist.back();
 304     Worklist.pop_back();
 305
 306     if (std::optional<ConstantRange> Range = calcRange(I))
 307       seen(I, *Range);
 308     else
 309       Worklist.push_front(I); // Reprocess later.
 310   }
 311 }
 312
 313 // If there is a valid transform to be done, do it.
 314 bool Float2IntPass::validateAndTransform() {
 315   bool MadeChange = false;
 316
 317   // Iterate over every disjoint partition of the def-use graph.
 318   for (auto It = ECs.begin(), E = ECs.end(); It != E; ++It) {
 319     ConstantRange R(MaxIntegerBW + 1, false);
 320     bool Fail = false;
 321     Type *ConvertedToTy = nullptr;
 322
 323     // For every member of the partition, union all the ranges together.
 324     for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
 325          MI != ME; ++MI) {
 326       Instruction *I = *MI;
 327       auto SeenI = SeenInsts.find(I);
 328       if (SeenI == SeenInsts.end())
 329         continue;
 330
 331       R = R.unionWith(SeenI->second);
 332       // We need to ensure I has no users that have not been seen.
 333       // If it does, transformation would be illegal.
 334       //
 335       // Don't count the roots, as they terminate the graphs.
 336       if (!Roots.contains(I)) {
 337         // Set the type of the conversion while we're here.
 338         if (!ConvertedToTy)
 339           ConvertedToTy = I->getType();
 340         for (User *U : I->users()) {
 341           Instruction *UI = dyn_cast<Instruction>(U);
 342           if (!UI || !SeenInsts.contains(UI)) {
 343             LLVM_DEBUG(dbgs() << "F2I: Failing because of " << *U << "\n");
 344             Fail = true;
 345             break;
 346           }
 347         }
 348       }
 349       if (Fail)
 350         break;
 351     }
 352
 353     // If the set was empty, or we failed, or the range is poisonous,
 354     // bail out.
 355     if (ECs.member_begin(It) == ECs.member_end() || Fail ||
 356         R.isFullSet() || R.isSignWrappedSet())
 357       continue;
 358     assert(ConvertedToTy && "Must have set the convertedtoty by this point!");
 359
 360     // The number of bits required is the maximum of the upper and
 361     // lower limits, plus one so it can be signed.
 362     unsigned MinBW = std::max(R.getLower().getSignificantBits(),
 363                               R.getUpper().getSignificantBits()) +
 364                      1;
 365     LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");
 366
 367     // If we've run off the realms of the exactly representable integers,
 368     // the floating point result will differ from an integer approximation.
 369
 370     // Do we need more bits than are in the mantissa of the type we converted
 371     // to? semanticsPrecision returns the number of mantissa bits plus one
 372     // for the sign bit.
 373     unsigned MaxRepresentableBits
 374       = APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
 375     if (MinBW > MaxRepresentableBits) {
 376       LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
 377       continue;
 378     }
 379     if (MinBW > 64) {
 380       LLVM_DEBUG(
 381           dbgs() << "F2I: Value requires more than 64 bits to represent!\n");
 382       continue;
 383     }
 384
 385     // OK, R is known to be representable. Now pick a type for it.
 386     // FIXME: Pick the smallest legal type that will fit.
 387     Type *Ty = (MinBW > 32) ? Type::getInt64Ty(*Ctx) : Type::getInt32Ty(*Ctx);
 388
 389     for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
 390          MI != ME; ++MI)
 391       convert(*MI, Ty);
 392     MadeChange = true;
 393   }
 394
 395   return MadeChange;
 396 }
 397
 398 Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
 399   if (ConvertedInsts.contains(I))
 400     // Already converted this instruction.
 401     return ConvertedInsts[I];
 402
 403   SmallVector<Value*,4> NewOperands;
 404   for (Value *V : I->operands()) {
 405     // Don't recurse if we're an instruction that terminates the path.
 406     if (I->getOpcode() == Instruction::UIToFP ||
 407         I->getOpcode() == Instruction::SIToFP) {
 408       NewOperands.push_back(V);
 409     } else if (Instruction *VI = dyn_cast<Instruction>(V)) {
 410       NewOperands.push_back(convert(VI, ToTy));
 411     } else if (ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
 412       APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
 413       bool Exact;
 414       CF->getValueAPF().convertToInteger(Val,
 415                                          APFloat::rmNearestTiesToEven,
 416                                          &Exact);
 417       NewOperands.push_back(ConstantInt::get(ToTy, Val));
 418     } else {
 419       llvm_unreachable("Unhandled operand type?");
 420     }
 421   }
 422
 423   // Now create a new instruction.
 424   IRBuilder<> IRB(I);
 425   Value *NewV = nullptr;
 426   switch (I->getOpcode()) {
 427   default: llvm_unreachable("Unhandled instruction!");
 428
 429   case Instruction::FPToUI:
 430     NewV = IRB.CreateZExtOrTrunc(NewOperands[0], I->getType());
 431     break;
 432
 433   case Instruction::FPToSI:
 434     NewV = IRB.CreateSExtOrTrunc(NewOperands[0], I->getType());
 435     break;
 436
 437   case Instruction::FCmp: {
 438     CmpInst::Predicate P = mapFCmpPred(cast<CmpInst>(I)->getPredicate());
 439     assert(P != CmpInst::BAD_ICMP_PREDICATE && "Unhandled predicate!");
 440     NewV = IRB.CreateICmp(P, NewOperands[0], NewOperands[1], I->getName());
 441     break;
 442   }
 443
 444   case Instruction::UIToFP:
 445     NewV = IRB.CreateZExtOrTrunc(NewOperands[0], ToTy);
 446     break;
 447
 448   case Instruction::SIToFP:
 449     NewV = IRB.CreateSExtOrTrunc(NewOperands[0], ToTy);
 450     break;
 451
 452   case Instruction::FNeg:
 453     NewV = IRB.CreateNeg(NewOperands[0], I->getName());
 454     break;
 455
 456   case Instruction::FAdd:
 457   case Instruction::FSub:
 458   case Instruction::FMul:
 459     NewV = IRB.CreateBinOp(mapBinOpcode(I->getOpcode()),
 460                            NewOperands[0], NewOperands[1],
 461                            I->getName());
 462     break;
 463   }
 464
 465   // If we're a root instruction, RAUW.
 466   if (Roots.count(I))
 467     I->replaceAllUsesWith(NewV);
 468
 469   ConvertedInsts[I] = NewV;
 470   return NewV;
 471 }
 472
 473 // Perform dead code elimination on the instructions we just modified.
 474 void Float2IntPass::cleanup() {
 475   for (auto &I : reverse(ConvertedInsts))
 476     I.first->eraseFromParent();
 477 }
 478
 479 bool Float2IntPass::runImpl(Function &F, const DominatorTree &DT) {
 480   LLVM_DEBUG(dbgs() << "F2I: Looking at function " << F.getName() << "\n");
 481   // Clear out all state.
 482   ECs = EquivalenceClasses<Instruction*>();
 483   SeenInsts.clear();
 484   ConvertedInsts.clear();
 485   Roots.clear();
 486
 487   Ctx = &F.getParent()->getContext();
 488
 489   findRoots(F, DT);
 490
 491   walkBackwards();
 492   walkForwards();
 493
 494   bool Modified = validateAndTransform();
 495   if (Modified)
 496     cleanup();
 497   return Modified;
 498 }
 499
 500 PreservedAnalyses Float2IntPass::run(Function &F, FunctionAnalysisManager &AM) {
 501   const DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
 502   if (!runImpl(F, DT))
 503     return PreservedAnalyses::all();
 504
 505   PreservedAnalyses PA;
 506   PA.preserveSet<CFGAnalyses>();
 507   return PA;
 508 }