lib/Analysis/DemandedBits.cpp

   1 //===- DemandedBits.cpp - Determine demanded bits -------------------------===//
   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 pass implements a demanded bits analysis. A demanded bit is one that
  11 // contributes to a result; bits that are not demanded can be either zero or
  12 // one without affecting control or data flow. For example in this sequence:
  13 //
  14 //   %1 = add i32 %x, %y
  15 //   %2 = trunc i32 %1 to i16
  16 //
  17 // Only the lowest 16 bits of %1 are demanded; the rest are removed by the
  18 // trunc.
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #include "llvm/Analysis/DemandedBits.h"
  23 #include "llvm/ADT/APInt.h"
  24 #include "llvm/ADT/SmallPtrSet.h"
  25 #include "llvm/ADT/SmallVector.h"
  26 #include "llvm/ADT/StringExtras.h"
  27 #include "llvm/Analysis/AssumptionCache.h"
  28 #include "llvm/Analysis/ValueTracking.h"
  29 #include "llvm/IR/BasicBlock.h"
  30 #include "llvm/IR/Constants.h"
  31 #include "llvm/IR/DataLayout.h"
  32 #include "llvm/IR/DerivedTypes.h"
  33 #include "llvm/IR/Dominators.h"
  34 #include "llvm/IR/InstIterator.h"
  35 #include "llvm/IR/InstrTypes.h"
  36 #include "llvm/IR/Instruction.h"
  37 #include "llvm/IR/IntrinsicInst.h"
  38 #include "llvm/IR/Intrinsics.h"
  39 #include "llvm/IR/Module.h"
  40 #include "llvm/IR/Operator.h"
  41 #include "llvm/IR/PassManager.h"
  42 #include "llvm/IR/Type.h"
  43 #include "llvm/IR/Use.h"
  44 #include "llvm/Pass.h"
  45 #include "llvm/Support/Casting.h"
  46 #include "llvm/Support/Debug.h"
  47 #include "llvm/Support/KnownBits.h"
  48 #include "llvm/Support/raw_ostream.h"
  49 #include <algorithm>
  50 #include <cstdint>
  51
  52 using namespace llvm;
  53
  54 #define DEBUG_TYPE "demanded-bits"
  55
  56 char DemandedBitsWrapperPass::ID = 0;
  57
  58 INITIALIZE_PASS_BEGIN(DemandedBitsWrapperPass, "demanded-bits",
  59                       "Demanded bits analysis", false, false)
  60 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
  61 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  62 INITIALIZE_PASS_END(DemandedBitsWrapperPass, "demanded-bits",
  63                     "Demanded bits analysis", false, false)
  64
  65 DemandedBitsWrapperPass::DemandedBitsWrapperPass() : FunctionPass(ID) {
  66   initializeDemandedBitsWrapperPassPass(*PassRegistry::getPassRegistry());
  67 }
  68
  69 void DemandedBitsWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
  70   AU.setPreservesCFG();
  71   AU.addRequired<AssumptionCacheTracker>();
  72   AU.addRequired<DominatorTreeWrapperPass>();
  73   AU.setPreservesAll();
  74 }
  75
  76 void DemandedBitsWrapperPass::print(raw_ostream &OS, const Module *M) const {
  77   DB->print(OS);
  78 }
  79
  80 static bool isAlwaysLive(Instruction *I) {
  81   return I->isTerminator() || isa<DbgInfoIntrinsic>(I) || I->isEHPad() ||
  82          I->mayHaveSideEffects();
  83 }
  84
  85 void DemandedBits::determineLiveOperandBits(
  86     const Instruction *UserI, const Instruction *I, unsigned OperandNo,
  87     const APInt &AOut, APInt &AB, KnownBits &Known, KnownBits &Known2) {
  88   unsigned BitWidth = AB.getBitWidth();
  89
  90   // We're called once per operand, but for some instructions, we need to
  91   // compute known bits of both operands in order to determine the live bits of
  92   // either (when both operands are instructions themselves). We don't,
  93   // however, want to do this twice, so we cache the result in APInts that live
  94   // in the caller. For the two-relevant-operands case, both operand values are
  95   // provided here.
  96   auto ComputeKnownBits =
  97       [&](unsigned BitWidth, const Value *V1, const Value *V2) {
  98         const DataLayout &DL = I->getModule()->getDataLayout();
  99         Known = KnownBits(BitWidth);
 100         computeKnownBits(V1, Known, DL, 0, &AC, UserI, &DT);
 101
 102         if (V2) {
 103           Known2 = KnownBits(BitWidth);
 104           computeKnownBits(V2, Known2, DL, 0, &AC, UserI, &DT);
 105         }
 106       };
 107
 108   switch (UserI->getOpcode()) {
 109   default: break;
 110   case Instruction::Call:
 111   case Instruction::Invoke:
 112     if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
 113       switch (II->getIntrinsicID()) {
 114       default: break;
 115       case Intrinsic::bswap:
 116         // The alive bits of the input are the swapped alive bits of
 117         // the output.
 118         AB = AOut.byteSwap();
 119         break;
 120       case Intrinsic::bitreverse:
 121         // The alive bits of the input are the reversed alive bits of
 122         // the output.
 123         AB = AOut.reverseBits();
 124         break;
 125       case Intrinsic::ctlz:
 126         if (OperandNo == 0) {
 127           // We need some output bits, so we need all bits of the
 128           // input to the left of, and including, the leftmost bit
 129           // known to be one.
 130           ComputeKnownBits(BitWidth, I, nullptr);
 131           AB = APInt::getHighBitsSet(BitWidth,
 132                  std::min(BitWidth, Known.countMaxLeadingZeros()+1));
 133         }
 134         break;
 135       case Intrinsic::cttz:
 136         if (OperandNo == 0) {
 137           // We need some output bits, so we need all bits of the
 138           // input to the right of, and including, the rightmost bit
 139           // known to be one.
 140           ComputeKnownBits(BitWidth, I, nullptr);
 141           AB = APInt::getLowBitsSet(BitWidth,
 142                  std::min(BitWidth, Known.countMaxTrailingZeros()+1));
 143         }
 144         break;
 145       }
 146     break;
 147   case Instruction::Add:
 148   case Instruction::Sub:
 149   case Instruction::Mul:
 150     // Find the highest live output bit. We don't need any more input
 151     // bits than that (adds, and thus subtracts, ripple only to the
 152     // left).
 153     AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
 154     break;
 155   case Instruction::Shl:
 156     if (OperandNo == 0)
 157       if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
 158         uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
 159         AB = AOut.lshr(ShiftAmt);
 160
 161         // If the shift is nuw/nsw, then the high bits are not dead
 162         // (because we've promised that they *must* be zero).
 163         const ShlOperator *S = cast<ShlOperator>(UserI);
 164         if (S->hasNoSignedWrap())
 165           AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
 166         else if (S->hasNoUnsignedWrap())
 167           AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
 168       }
 169     break;
 170   case Instruction::LShr:
 171     if (OperandNo == 0)
 172       if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
 173         uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
 174         AB = AOut.shl(ShiftAmt);
 175
 176         // If the shift is exact, then the low bits are not dead
 177         // (they must be zero).
 178         if (cast<LShrOperator>(UserI)->isExact())
 179           AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
 180       }
 181     break;
 182   case Instruction::AShr:
 183     if (OperandNo == 0)
 184       if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
 185         uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
 186         AB = AOut.shl(ShiftAmt);
 187         // Because the high input bit is replicated into the
 188         // high-order bits of the result, if we need any of those
 189         // bits, then we must keep the highest input bit.
 190         if ((AOut & APInt::getHighBitsSet(BitWidth, ShiftAmt))
 191             .getBoolValue())
 192           AB.setSignBit();
 193
 194         // If the shift is exact, then the low bits are not dead
 195         // (they must be zero).
 196         if (cast<AShrOperator>(UserI)->isExact())
 197           AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
 198       }
 199     break;
 200   case Instruction::And:
 201     AB = AOut;
 202
 203     // For bits that are known zero, the corresponding bits in the
 204     // other operand are dead (unless they're both zero, in which
 205     // case they can't both be dead, so just mark the LHS bits as
 206     // dead).
 207     if (OperandNo == 0) {
 208       ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
 209       AB &= ~Known2.Zero;
 210     } else {
 211       if (!isa<Instruction>(UserI->getOperand(0)))
 212         ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
 213       AB &= ~(Known.Zero & ~Known2.Zero);
 214     }
 215     break;
 216   case Instruction::Or:
 217     AB = AOut;
 218
 219     // For bits that are known one, the corresponding bits in the
 220     // other operand are dead (unless they're both one, in which
 221     // case they can't both be dead, so just mark the LHS bits as
 222     // dead).
 223     if (OperandNo == 0) {
 224       ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
 225       AB &= ~Known2.One;
 226     } else {
 227       if (!isa<Instruction>(UserI->getOperand(0)))
 228         ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
 229       AB &= ~(Known.One & ~Known2.One);
 230     }
 231     break;
 232   case Instruction::Xor:
 233   case Instruction::PHI:
 234     AB = AOut;
 235     break;
 236   case Instruction::Trunc:
 237     AB = AOut.zext(BitWidth);
 238     break;
 239   case Instruction::ZExt:
 240     AB = AOut.trunc(BitWidth);
 241     break;
 242   case Instruction::SExt:
 243     AB = AOut.trunc(BitWidth);
 244     // Because the high input bit is replicated into the
 245     // high-order bits of the result, if we need any of those
 246     // bits, then we must keep the highest input bit.
 247     if ((AOut & APInt::getHighBitsSet(AOut.getBitWidth(),
 248                                       AOut.getBitWidth() - BitWidth))
 249         .getBoolValue())
 250       AB.setSignBit();
 251     break;
 252   case Instruction::Select:
 253     if (OperandNo != 0)
 254       AB = AOut;
 255     break;
 256   }
 257 }
 258
 259 bool DemandedBitsWrapperPass::runOnFunction(Function &F) {
 260   auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
 261   auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 262   DB.emplace(F, AC, DT);
 263   return false;
 264 }
 265
 266 void DemandedBitsWrapperPass::releaseMemory() {
 267   DB.reset();
 268 }
 269
 270 void DemandedBits::performAnalysis() {
 271   if (Analyzed)
 272     // Analysis already completed for this function.
 273     return;
 274   Analyzed = true;
 275
 276   Visited.clear();
 277   AliveBits.clear();
 278
 279   SmallVector<Instruction*, 128> Worklist;
 280
 281   // Collect the set of "root" instructions that are known live.
 282   for (Instruction &I : instructions(F)) {
 283     if (!isAlwaysLive(&I))
 284       continue;
 285
 286     LLVM_DEBUG(dbgs() << "DemandedBits: Root: " << I << "\n");
 287     // For integer-valued instructions, set up an initial empty set of alive
 288     // bits and add the instruction to the work list. For other instructions
 289     // add their operands to the work list (for integer values operands, mark
 290     // all bits as live).
 291     if (IntegerType *IT = dyn_cast<IntegerType>(I.getType())) {
 292       if (AliveBits.try_emplace(&I, IT->getBitWidth(), 0).second)
 293         Worklist.push_back(&I);
 294
 295       continue;
 296     }
 297
 298     // Non-integer-typed instructions...
 299     for (Use &OI : I.operands()) {
 300       if (Instruction *J = dyn_cast<Instruction>(OI)) {
 301         if (IntegerType *IT = dyn_cast<IntegerType>(J->getType()))
 302           AliveBits[J] = APInt::getAllOnesValue(IT->getBitWidth());
 303         Worklist.push_back(J);
 304       }
 305     }
 306     // To save memory, we don't add I to the Visited set here. Instead, we
 307     // check isAlwaysLive on every instruction when searching for dead
 308     // instructions later (we need to check isAlwaysLive for the
 309     // integer-typed instructions anyway).
 310   }
 311
 312   // Propagate liveness backwards to operands.
 313   while (!Worklist.empty()) {
 314     Instruction *UserI = Worklist.pop_back_val();
 315
 316     LLVM_DEBUG(dbgs() << "DemandedBits: Visiting: " << *UserI);
 317     APInt AOut;
 318     if (UserI->getType()->isIntegerTy()) {
 319       AOut = AliveBits[UserI];
 320       LLVM_DEBUG(dbgs() << " Alive Out: " << AOut);
 321     }
 322     LLVM_DEBUG(dbgs() << "\n");
 323
 324     if (!UserI->getType()->isIntegerTy())
 325       Visited.insert(UserI);
 326
 327     KnownBits Known, Known2;
 328     // Compute the set of alive bits for each operand. These are anded into the
 329     // existing set, if any, and if that changes the set of alive bits, the
 330     // operand is added to the work-list.
 331     for (Use &OI : UserI->operands()) {
 332       if (Instruction *I = dyn_cast<Instruction>(OI)) {
 333         if (IntegerType *IT = dyn_cast<IntegerType>(I->getType())) {
 334           unsigned BitWidth = IT->getBitWidth();
 335           APInt AB = APInt::getAllOnesValue(BitWidth);
 336           if (UserI->getType()->isIntegerTy() && !AOut &&
 337               !isAlwaysLive(UserI)) {
 338             AB = APInt(BitWidth, 0);
 339           } else {
 340             // If all bits of the output are dead, then all bits of the input
 341             // Bits of each operand that are used to compute alive bits of the
 342             // output are alive, all others are dead.
 343             determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
 344                                      Known, Known2);
 345           }
 346
 347           // If we've added to the set of alive bits (or the operand has not
 348           // been previously visited), then re-queue the operand to be visited
 349           // again.
 350           APInt ABPrev(BitWidth, 0);
 351           auto ABI = AliveBits.find(I);
 352           if (ABI != AliveBits.end())
 353             ABPrev = ABI->second;
 354
 355           APInt ABNew = AB | ABPrev;
 356           if (ABNew != ABPrev || ABI == AliveBits.end()) {
 357             AliveBits[I] = std::move(ABNew);
 358             Worklist.push_back(I);
 359           }
 360         } else if (!Visited.count(I)) {
 361           Worklist.push_back(I);
 362         }
 363       }
 364     }
 365   }
 366 }
 367
 368 APInt DemandedBits::getDemandedBits(Instruction *I) {
 369   performAnalysis();
 370
 371   const DataLayout &DL = I->getModule()->getDataLayout();
 372   auto Found = AliveBits.find(I);
 373   if (Found != AliveBits.end())
 374     return Found->second;
 375   return APInt::getAllOnesValue(DL.getTypeSizeInBits(I->getType()));
 376 }
 377
 378 bool DemandedBits::isInstructionDead(Instruction *I) {
 379   performAnalysis();
 380
 381   return !Visited.count(I) && AliveBits.find(I) == AliveBits.end() &&
 382     !isAlwaysLive(I);
 383 }
 384
 385 void DemandedBits::print(raw_ostream &OS) {
 386   performAnalysis();
 387   for (auto &KV : AliveBits) {
 388     OS << "DemandedBits: 0x" << Twine::utohexstr(KV.second.getLimitedValue())
 389        << " for " << *KV.first << '\n';
 390   }
 391 }
 392
 393 FunctionPass *llvm::createDemandedBitsWrapperPass() {
 394   return new DemandedBitsWrapperPass();
 395 }
 396
 397 AnalysisKey DemandedBitsAnalysis::Key;
 398
 399 DemandedBits DemandedBitsAnalysis::run(Function &F,
 400                                              FunctionAnalysisManager &AM) {
 401   auto &AC = AM.getResult<AssumptionAnalysis>(F);
 402   auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
 403   return DemandedBits(F, AC, DT);
 404 }
 405
 406 PreservedAnalyses DemandedBitsPrinterPass::run(Function &F,
 407                                                FunctionAnalysisManager &AM) {
 408   AM.getResult<DemandedBitsAnalysis>(F).print(OS);
 409   return PreservedAnalyses::all();
 410 }