llvm/lib/Transforms/Utils/BypassSlowDivision.cpp

   1 //===- BypassSlowDivision.cpp - Bypass slow division ----------------------===//
   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 contains an optimization for div and rem on architectures that
  10 // execute short instructions significantly faster than longer instructions.
  11 // For example, on Intel Atom 32-bit divides are slow enough that during
  12 // runtime it is profitable to check the value of the operands, and if they are
  13 // positive and less than 256 use an unsigned 8-bit divide.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/Transforms/Utils/BypassSlowDivision.h"
  18 #include "llvm/ADT/DenseMap.h"
  19 #include "llvm/ADT/STLExtras.h"
  20 #include "llvm/ADT/SmallPtrSet.h"
  21 #include "llvm/Transforms/Utils/Local.h"
  22 #include "llvm/Analysis/ValueTracking.h"
  23 #include "llvm/IR/BasicBlock.h"
  24 #include "llvm/IR/Constants.h"
  25 #include "llvm/IR/DerivedTypes.h"
  26 #include "llvm/IR/Function.h"
  27 #include "llvm/IR/IRBuilder.h"
  28 #include "llvm/IR/Instruction.h"
  29 #include "llvm/IR/Instructions.h"
  30 #include "llvm/IR/Module.h"
  31 #include "llvm/IR/Type.h"
  32 #include "llvm/IR/Value.h"
  33 #include "llvm/Support/Casting.h"
  34 #include "llvm/Support/KnownBits.h"
  35 #include <cassert>
  36 #include <cstdint>
  37
  38 using namespace llvm;
  39
  40 #define DEBUG_TYPE "bypass-slow-division"
  41
  42 namespace {
  43
  44   struct QuotRemPair {
  45     Value *Quotient;
  46     Value *Remainder;
  47
  48     QuotRemPair(Value *InQuotient, Value *InRemainder)
  49         : Quotient(InQuotient), Remainder(InRemainder) {}
  50   };
  51
  52   /// A quotient and remainder, plus a BB from which they logically "originate".
  53   /// If you use Quotient or Remainder in a Phi node, you should use BB as its
  54   /// corresponding predecessor.
  55   struct QuotRemWithBB {
  56     BasicBlock *BB = nullptr;
  57     Value *Quotient = nullptr;
  58     Value *Remainder = nullptr;
  59   };
  60
  61 using DivCacheTy = DenseMap<DivRemMapKey, QuotRemPair>;
  62 using BypassWidthsTy = DenseMap<unsigned, unsigned>;
  63 using VisitedSetTy = SmallPtrSet<Instruction *, 4>;
  64
  65 enum ValueRange {
  66   /// Operand definitely fits into BypassType. No runtime checks are needed.
  67   VALRNG_KNOWN_SHORT,
  68   /// A runtime check is required, as value range is unknown.
  69   VALRNG_UNKNOWN,
  70   /// Operand is unlikely to fit into BypassType. The bypassing should be
  71   /// disabled.
  72   VALRNG_LIKELY_LONG
  73 };
  74
  75 class FastDivInsertionTask {
  76   bool IsValidTask = false;
  77   Instruction *SlowDivOrRem = nullptr;
  78   IntegerType *BypassType = nullptr;
  79   BasicBlock *MainBB = nullptr;
  80
  81   bool isHashLikeValue(Value *V, VisitedSetTy &Visited);
  82   ValueRange getValueRange(Value *Op, VisitedSetTy &Visited);
  83   QuotRemWithBB createSlowBB(BasicBlock *Successor);
  84   QuotRemWithBB createFastBB(BasicBlock *Successor);
  85   QuotRemPair createDivRemPhiNodes(QuotRemWithBB &LHS, QuotRemWithBB &RHS,
  86                                    BasicBlock *PhiBB);
  87   Value *insertOperandRuntimeCheck(Value *Op1, Value *Op2);
  88   std::optional<QuotRemPair> insertFastDivAndRem();
  89
  90   bool isSignedOp() {
  91     return SlowDivOrRem->getOpcode() == Instruction::SDiv ||
  92            SlowDivOrRem->getOpcode() == Instruction::SRem;
  93   }
  94
  95   bool isDivisionOp() {
  96     return SlowDivOrRem->getOpcode() == Instruction::SDiv ||
  97            SlowDivOrRem->getOpcode() == Instruction::UDiv;
  98   }
  99
 100   Type *getSlowType() { return SlowDivOrRem->getType(); }
 101
 102 public:
 103   FastDivInsertionTask(Instruction *I, const BypassWidthsTy &BypassWidths);
 104
 105   Value *getReplacement(DivCacheTy &Cache);
 106 };
 107
 108 } // end anonymous namespace
 109
 110 FastDivInsertionTask::FastDivInsertionTask(Instruction *I,
 111                                            const BypassWidthsTy &BypassWidths) {
 112   switch (I->getOpcode()) {
 113   case Instruction::UDiv:
 114   case Instruction::SDiv:
 115   case Instruction::URem:
 116   case Instruction::SRem:
 117     SlowDivOrRem = I;
 118     break;
 119   default:
 120     // I is not a div/rem operation.
 121     return;
 122   }
 123
 124   // Skip division on vector types. Only optimize integer instructions.
 125   IntegerType *SlowType = dyn_cast<IntegerType>(SlowDivOrRem->getType());
 126   if (!SlowType)
 127     return;
 128
 129   // Skip if this bitwidth is not bypassed.
 130   auto BI = BypassWidths.find(SlowType->getBitWidth());
 131   if (BI == BypassWidths.end())
 132     return;
 133
 134   // Get type for div/rem instruction with bypass bitwidth.
 135   IntegerType *BT = IntegerType::get(I->getContext(), BI->second);
 136   BypassType = BT;
 137
 138   // The original basic block.
 139   MainBB = I->getParent();
 140
 141   // The instruction is indeed a slow div or rem operation.
 142   IsValidTask = true;
 143 }
 144
 145 /// Reuses previously-computed dividend or remainder from the current BB if
 146 /// operands and operation are identical. Otherwise calls insertFastDivAndRem to
 147 /// perform the optimization and caches the resulting dividend and remainder.
 148 /// If no replacement can be generated, nullptr is returned.
 149 Value *FastDivInsertionTask::getReplacement(DivCacheTy &Cache) {
 150   // First, make sure that the task is valid.
 151   if (!IsValidTask)
 152     return nullptr;
 153
 154   // Then, look for a value in Cache.
 155   Value *Dividend = SlowDivOrRem->getOperand(0);
 156   Value *Divisor = SlowDivOrRem->getOperand(1);
 157   DivRemMapKey Key(isSignedOp(), Dividend, Divisor);
 158   auto CacheI = Cache.find(Key);
 159
 160   if (CacheI == Cache.end()) {
 161     // If previous instance does not exist, try to insert fast div.
 162     std::optional<QuotRemPair> OptResult = insertFastDivAndRem();
 163     // Bail out if insertFastDivAndRem has failed.
 164     if (!OptResult)
 165       return nullptr;
 166     CacheI = Cache.insert({Key, *OptResult}).first;
 167   }
 168
 169   QuotRemPair &Value = CacheI->second;
 170   return isDivisionOp() ? Value.Quotient : Value.Remainder;
 171 }
 172
 173 /// Check if a value looks like a hash.
 174 ///
 175 /// The routine is expected to detect values computed using the most common hash
 176 /// algorithms. Typically, hash computations end with one of the following
 177 /// instructions:
 178 ///
 179 /// 1) MUL with a constant wider than BypassType
 180 /// 2) XOR instruction
 181 ///
 182 /// And even if we are wrong and the value is not a hash, it is still quite
 183 /// unlikely that such values will fit into BypassType.
 184 ///
 185 /// To detect string hash algorithms like FNV we have to look through PHI-nodes.
 186 /// It is implemented as a depth-first search for values that look neither long
 187 /// nor hash-like.
 188 bool FastDivInsertionTask::isHashLikeValue(Value *V, VisitedSetTy &Visited) {
 189   Instruction *I = dyn_cast<Instruction>(V);
 190   if (!I)
 191     return false;
 192
 193   switch (I->getOpcode()) {
 194   case Instruction::Xor:
 195     return true;
 196   case Instruction::Mul: {
 197     // After Constant Hoisting pass, long constants may be represented as
 198     // bitcast instructions. As a result, some constants may look like an
 199     // instruction at first, and an additional check is necessary to find out if
 200     // an operand is actually a constant.
 201     Value *Op1 = I->getOperand(1);
 202     ConstantInt *C = dyn_cast<ConstantInt>(Op1);
 203     if (!C && isa<BitCastInst>(Op1))
 204       C = dyn_cast<ConstantInt>(cast<BitCastInst>(Op1)->getOperand(0));
 205     return C && C->getValue().getSignificantBits() > BypassType->getBitWidth();
 206   }
 207   case Instruction::PHI:
 208     // Stop IR traversal in case of a crazy input code. This limits recursion
 209     // depth.
 210     if (Visited.size() >= 16)
 211       return false;
 212     // Do not visit nodes that have been visited already. We return true because
 213     // it means that we couldn't find any value that doesn't look hash-like.
 214     if (!Visited.insert(I).second)
 215       return true;
 216     return llvm::all_of(cast<PHINode>(I)->incoming_values(), [&](Value *V) {
 217       // Ignore undef values as they probably don't affect the division
 218       // operands.
 219       return getValueRange(V, Visited) == VALRNG_LIKELY_LONG ||
 220              isa<UndefValue>(V);
 221     });
 222   default:
 223     return false;
 224   }
 225 }
 226
 227 /// Check if an integer value fits into our bypass type.
 228 ValueRange FastDivInsertionTask::getValueRange(Value *V,
 229                                                VisitedSetTy &Visited) {
 230   unsigned ShortLen = BypassType->getBitWidth();
 231   unsigned LongLen = V->getType()->getIntegerBitWidth();
 232
 233   assert(LongLen > ShortLen && "Value type must be wider than BypassType");
 234   unsigned HiBits = LongLen - ShortLen;
 235
 236   const DataLayout &DL = SlowDivOrRem->getModule()->getDataLayout();
 237   KnownBits Known(LongLen);
 238
 239   computeKnownBits(V, Known, DL);
 240
 241   if (Known.countMinLeadingZeros() >= HiBits)
 242     return VALRNG_KNOWN_SHORT;
 243
 244   if (Known.countMaxLeadingZeros() < HiBits)
 245     return VALRNG_LIKELY_LONG;
 246
 247   // Long integer divisions are often used in hashtable implementations. It's
 248   // not worth bypassing such divisions because hash values are extremely
 249   // unlikely to have enough leading zeros. The call below tries to detect
 250   // values that are unlikely to fit BypassType (including hashes).
 251   if (isHashLikeValue(V, Visited))
 252     return VALRNG_LIKELY_LONG;
 253
 254   return VALRNG_UNKNOWN;
 255 }
 256
 257 /// Add new basic block for slow div and rem operations and put it before
 258 /// SuccessorBB.
 259 QuotRemWithBB FastDivInsertionTask::createSlowBB(BasicBlock *SuccessorBB) {
 260   QuotRemWithBB DivRemPair;
 261   DivRemPair.BB = BasicBlock::Create(MainBB->getParent()->getContext(), "",
 262                                      MainBB->getParent(), SuccessorBB);
 263   IRBuilder<> Builder(DivRemPair.BB, DivRemPair.BB->begin());
 264   Builder.SetCurrentDebugLocation(SlowDivOrRem->getDebugLoc());
 265
 266   Value *Dividend = SlowDivOrRem->getOperand(0);
 267   Value *Divisor = SlowDivOrRem->getOperand(1);
 268
 269   if (isSignedOp()) {
 270     DivRemPair.Quotient = Builder.CreateSDiv(Dividend, Divisor);
 271     DivRemPair.Remainder = Builder.CreateSRem(Dividend, Divisor);
 272   } else {
 273     DivRemPair.Quotient = Builder.CreateUDiv(Dividend, Divisor);
 274     DivRemPair.Remainder = Builder.CreateURem(Dividend, Divisor);
 275   }
 276
 277   Builder.CreateBr(SuccessorBB);
 278   return DivRemPair;
 279 }
 280
 281 /// Add new basic block for fast div and rem operations and put it before
 282 /// SuccessorBB.
 283 QuotRemWithBB FastDivInsertionTask::createFastBB(BasicBlock *SuccessorBB) {
 284   QuotRemWithBB DivRemPair;
 285   DivRemPair.BB = BasicBlock::Create(MainBB->getParent()->getContext(), "",
 286                                      MainBB->getParent(), SuccessorBB);
 287   IRBuilder<> Builder(DivRemPair.BB, DivRemPair.BB->begin());
 288   Builder.SetCurrentDebugLocation(SlowDivOrRem->getDebugLoc());
 289
 290   Value *Dividend = SlowDivOrRem->getOperand(0);
 291   Value *Divisor = SlowDivOrRem->getOperand(1);
 292   Value *ShortDivisorV =
 293       Builder.CreateCast(Instruction::Trunc, Divisor, BypassType);
 294   Value *ShortDividendV =
 295       Builder.CreateCast(Instruction::Trunc, Dividend, BypassType);
 296
 297   // udiv/urem because this optimization only handles positive numbers.
 298   Value *ShortQV = Builder.CreateUDiv(ShortDividendV, ShortDivisorV);
 299   Value *ShortRV = Builder.CreateURem(ShortDividendV, ShortDivisorV);
 300   DivRemPair.Quotient =
 301       Builder.CreateCast(Instruction::ZExt, ShortQV, getSlowType());
 302   DivRemPair.Remainder =
 303       Builder.CreateCast(Instruction::ZExt, ShortRV, getSlowType());
 304   Builder.CreateBr(SuccessorBB);
 305
 306   return DivRemPair;
 307 }
 308
 309 /// Creates Phi nodes for result of Div and Rem.
 310 QuotRemPair FastDivInsertionTask::createDivRemPhiNodes(QuotRemWithBB &LHS,
 311                                                        QuotRemWithBB &RHS,
 312                                                        BasicBlock *PhiBB) {
 313   IRBuilder<> Builder(PhiBB, PhiBB->begin());
 314   Builder.SetCurrentDebugLocation(SlowDivOrRem->getDebugLoc());
 315   PHINode *QuoPhi = Builder.CreatePHI(getSlowType(), 2);
 316   QuoPhi->addIncoming(LHS.Quotient, LHS.BB);
 317   QuoPhi->addIncoming(RHS.Quotient, RHS.BB);
 318   PHINode *RemPhi = Builder.CreatePHI(getSlowType(), 2);
 319   RemPhi->addIncoming(LHS.Remainder, LHS.BB);
 320   RemPhi->addIncoming(RHS.Remainder, RHS.BB);
 321   return QuotRemPair(QuoPhi, RemPhi);
 322 }
 323
 324 /// Creates a runtime check to test whether both the divisor and dividend fit
 325 /// into BypassType. The check is inserted at the end of MainBB. True return
 326 /// value means that the operands fit. Either of the operands may be NULL if it
 327 /// doesn't need a runtime check.
 328 Value *FastDivInsertionTask::insertOperandRuntimeCheck(Value *Op1, Value *Op2) {
 329   assert((Op1 || Op2) && "Nothing to check");
 330   IRBuilder<> Builder(MainBB, MainBB->end());
 331   Builder.SetCurrentDebugLocation(SlowDivOrRem->getDebugLoc());
 332
 333   Value *OrV;
 334   if (Op1 && Op2)
 335     OrV = Builder.CreateOr(Op1, Op2);
 336   else
 337     OrV = Op1 ? Op1 : Op2;
 338
 339   // BitMask is inverted to check if the operands are
 340   // larger than the bypass type
 341   uint64_t BitMask = ~BypassType->getBitMask();
 342   Value *AndV = Builder.CreateAnd(OrV, BitMask);
 343
 344   // Compare operand values
 345   Value *ZeroV = ConstantInt::getSigned(getSlowType(), 0);
 346   return Builder.CreateICmpEQ(AndV, ZeroV);
 347 }
 348
 349 /// Substitutes the div/rem instruction with code that checks the value of the
 350 /// operands and uses a shorter-faster div/rem instruction when possible.
 351 std::optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
 352   Value *Dividend = SlowDivOrRem->getOperand(0);
 353   Value *Divisor = SlowDivOrRem->getOperand(1);
 354
 355   VisitedSetTy SetL;
 356   ValueRange DividendRange = getValueRange(Dividend, SetL);
 357   if (DividendRange == VALRNG_LIKELY_LONG)
 358     return std::nullopt;
 359
 360   VisitedSetTy SetR;
 361   ValueRange DivisorRange = getValueRange(Divisor, SetR);
 362   if (DivisorRange == VALRNG_LIKELY_LONG)
 363     return std::nullopt;
 364
 365   bool DividendShort = (DividendRange == VALRNG_KNOWN_SHORT);
 366   bool DivisorShort = (DivisorRange == VALRNG_KNOWN_SHORT);
 367
 368   if (DividendShort && DivisorShort) {
 369     // If both operands are known to be short then just replace the long
 370     // division with a short one in-place.  Since we're not introducing control
 371     // flow in this case, narrowing the division is always a win, even if the
 372     // divisor is a constant (and will later get replaced by a multiplication).
 373
 374     IRBuilder<> Builder(SlowDivOrRem);
 375     Value *TruncDividend = Builder.CreateTrunc(Dividend, BypassType);
 376     Value *TruncDivisor = Builder.CreateTrunc(Divisor, BypassType);
 377     Value *TruncDiv = Builder.CreateUDiv(TruncDividend, TruncDivisor);
 378     Value *TruncRem = Builder.CreateURem(TruncDividend, TruncDivisor);
 379     Value *ExtDiv = Builder.CreateZExt(TruncDiv, getSlowType());
 380     Value *ExtRem = Builder.CreateZExt(TruncRem, getSlowType());
 381     return QuotRemPair(ExtDiv, ExtRem);
 382   }
 383
 384   if (isa<ConstantInt>(Divisor)) {
 385     // If the divisor is not a constant, DAGCombiner will convert it to a
 386     // multiplication by a magic constant.  It isn't clear if it is worth
 387     // introducing control flow to get a narrower multiply.
 388     return std::nullopt;
 389   }
 390
 391   // After Constant Hoisting pass, long constants may be represented as
 392   // bitcast instructions. As a result, some constants may look like an
 393   // instruction at first, and an additional check is necessary to find out if
 394   // an operand is actually a constant.
 395   if (auto *BCI = dyn_cast<BitCastInst>(Divisor))
 396     if (BCI->getParent() == SlowDivOrRem->getParent() &&
 397         isa<ConstantInt>(BCI->getOperand(0)))
 398       return std::nullopt;
 399
 400   IRBuilder<> Builder(MainBB, MainBB->end());
 401   Builder.SetCurrentDebugLocation(SlowDivOrRem->getDebugLoc());
 402
 403   if (DividendShort && !isSignedOp()) {
 404     // If the division is unsigned and Dividend is known to be short, then
 405     // either
 406     // 1) Divisor is less or equal to Dividend, and the result can be computed
 407     //    with a short division.
 408     // 2) Divisor is greater than Dividend. In this case, no division is needed
 409     //    at all: The quotient is 0 and the remainder is equal to Dividend.
 410     //
 411     // So instead of checking at runtime whether Divisor fits into BypassType,
 412     // we emit a runtime check to differentiate between these two cases. This
 413     // lets us entirely avoid a long div.
 414
 415     // Split the basic block before the div/rem.
 416     BasicBlock *SuccessorBB = MainBB->splitBasicBlock(SlowDivOrRem);
 417     // Remove the unconditional branch from MainBB to SuccessorBB.
 418     MainBB->back().eraseFromParent();
 419     QuotRemWithBB Long;
 420     Long.BB = MainBB;
 421     Long.Quotient = ConstantInt::get(getSlowType(), 0);
 422     Long.Remainder = Dividend;
 423     QuotRemWithBB Fast = createFastBB(SuccessorBB);
 424     QuotRemPair Result = createDivRemPhiNodes(Fast, Long, SuccessorBB);
 425     Value *CmpV = Builder.CreateICmpUGE(Dividend, Divisor);
 426     Builder.CreateCondBr(CmpV, Fast.BB, SuccessorBB);
 427     return Result;
 428   } else {
 429     // General case. Create both slow and fast div/rem pairs and choose one of
 430     // them at runtime.
 431
 432     // Split the basic block before the div/rem.
 433     BasicBlock *SuccessorBB = MainBB->splitBasicBlock(SlowDivOrRem);
 434     // Remove the unconditional branch from MainBB to SuccessorBB.
 435     MainBB->back().eraseFromParent();
 436     QuotRemWithBB Fast = createFastBB(SuccessorBB);
 437     QuotRemWithBB Slow = createSlowBB(SuccessorBB);
 438     QuotRemPair Result = createDivRemPhiNodes(Fast, Slow, SuccessorBB);
 439     Value *CmpV = insertOperandRuntimeCheck(DividendShort ? nullptr : Dividend,
 440                                             DivisorShort ? nullptr : Divisor);
 441     Builder.CreateCondBr(CmpV, Fast.BB, Slow.BB);
 442     return Result;
 443   }
 444 }
 445
 446 /// This optimization identifies DIV/REM instructions in a BB that can be
 447 /// profitably bypassed and carried out with a shorter, faster divide.
 448 bool llvm::bypassSlowDivision(BasicBlock *BB,
 449                               const BypassWidthsTy &BypassWidths) {
 450   DivCacheTy PerBBDivCache;
 451
 452   bool MadeChange = false;
 453   Instruction *Next = &*BB->begin();
 454   while (Next != nullptr) {
 455     // We may add instructions immediately after I, but we want to skip over
 456     // them.
 457     Instruction *I = Next;
 458     Next = Next->getNextNode();
 459
 460     // Ignore dead code to save time and avoid bugs.
 461     if (I->hasNUses(0))
 462       continue;
 463
 464     FastDivInsertionTask Task(I, BypassWidths);
 465     if (Value *Replacement = Task.getReplacement(PerBBDivCache)) {
 466       I->replaceAllUsesWith(Replacement);
 467       I->eraseFromParent();
 468       MadeChange = true;
 469     }
 470   }
 471
 472   // Above we eagerly create divs and rems, as pairs, so that we can efficiently
 473   // create divrem machine instructions.  Now erase any unused divs / rems so we
 474   // don't leave extra instructions sitting around.
 475   for (auto &KV : PerBBDivCache)
 476     for (Value *V : {KV.second.Quotient, KV.second.Remainder})
 477       RecursivelyDeleteTriviallyDeadInstructions(V);
 478
 479   return MadeChange;
 480 }