llvm/lib/CodeGen/MIRCanonicalizerPass.cpp

   1 //===-------------- MIRCanonicalizer.cpp - MIR Canonicalizer --------------===//
   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 // The purpose of this pass is to employ a canonical code transformation so
  10 // that code compiled with slightly different IR passes can be diffed more
  11 // effectively than otherwise. This is done by renaming vregs in a given
  12 // LiveRange in a canonical way. This pass also does a pseudo-scheduling to
  13 // move defs closer to their use inorder to reduce diffs caused by slightly
  14 // different schedules.
  15 //
  16 // Basic Usage:
  17 //
  18 // llc -o - -run-pass mir-canonicalizer example.mir
  19 //
  20 // Reorders instructions canonically.
  21 // Renames virtual register operands canonically.
  22 // Strips certain MIR artifacts (optionally).
  23 //
  24 //===----------------------------------------------------------------------===//
  25
  26 #include "MIRVRegNamerUtils.h"
  27 #include "llvm/ADT/PostOrderIterator.h"
  28 #include "llvm/ADT/STLExtras.h"
  29 #include "llvm/CodeGen/MachineFunctionPass.h"
  30 #include "llvm/CodeGen/MachineRegisterInfo.h"
  31 #include "llvm/InitializePasses.h"
  32 #include "llvm/Pass.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/Support/raw_ostream.h"
  35
  36 using namespace llvm;
  37
  38 #define DEBUG_TYPE "mir-canonicalizer"
  39
  40 static cl::opt<unsigned>
  41     CanonicalizeFunctionNumber("canon-nth-function", cl::Hidden, cl::init(~0u),
  42                                cl::value_desc("N"),
  43                                cl::desc("Function number to canonicalize."));
  44
  45 namespace {
  46
  47 class MIRCanonicalizer : public MachineFunctionPass {
  48 public:
  49   static char ID;
  50   MIRCanonicalizer() : MachineFunctionPass(ID) {}
  51
  52   StringRef getPassName() const override {
  53     return "Rename register operands in a canonical ordering.";
  54   }
  55
  56   void getAnalysisUsage(AnalysisUsage &AU) const override {
  57     AU.setPreservesCFG();
  58     MachineFunctionPass::getAnalysisUsage(AU);
  59   }
  60
  61   bool runOnMachineFunction(MachineFunction &MF) override;
  62 };
  63
  64 } // end anonymous namespace
  65
  66 char MIRCanonicalizer::ID;
  67
  68 char &llvm::MIRCanonicalizerID = MIRCanonicalizer::ID;
  69
  70 INITIALIZE_PASS_BEGIN(MIRCanonicalizer, "mir-canonicalizer",
  71                       "Rename Register Operands Canonically", false, false)
  72
  73 INITIALIZE_PASS_END(MIRCanonicalizer, "mir-canonicalizer",
  74                     "Rename Register Operands Canonically", false, false)
  75
  76 static std::vector<MachineBasicBlock *> GetRPOList(MachineFunction &MF) {
  77   if (MF.empty())
  78     return {};
  79   ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
  80   std::vector<MachineBasicBlock *> RPOList;
  81   append_range(RPOList, RPOT);
  82
  83   return RPOList;
  84 }
  85
  86 static bool
  87 rescheduleLexographically(std::vector<MachineInstr *> instructions,
  88                           MachineBasicBlock *MBB,
  89                           std::function<MachineBasicBlock::iterator()> getPos) {
  90
  91   bool Changed = false;
  92   using StringInstrPair = std::pair<std::string, MachineInstr *>;
  93   std::vector<StringInstrPair> StringInstrMap;
  94
  95   for (auto *II : instructions) {
  96     std::string S;
  97     raw_string_ostream OS(S);
  98     II->print(OS);
  99     OS.flush();
 100
 101     // Trim the assignment, or start from the beginning in the case of a store.
 102     const size_t i = S.find('=');
 103     StringInstrMap.push_back({(i == std::string::npos) ? S : S.substr(i), II});
 104   }
 105
 106   llvm::sort(StringInstrMap, llvm::less_first());
 107
 108   for (auto &II : StringInstrMap) {
 109
 110     LLVM_DEBUG({
 111       dbgs() << "Splicing ";
 112       II.second->dump();
 113       dbgs() << " right before: ";
 114       getPos()->dump();
 115     });
 116
 117     Changed = true;
 118     MBB->splice(getPos(), MBB, II.second);
 119   }
 120
 121   return Changed;
 122 }
 123
 124 static bool rescheduleCanonically(unsigned &PseudoIdempotentInstCount,
 125                                   MachineBasicBlock *MBB) {
 126
 127   bool Changed = false;
 128
 129   // Calculates the distance of MI from the beginning of its parent BB.
 130   auto getInstrIdx = [](const MachineInstr &MI) {
 131     unsigned i = 0;
 132     for (const auto &CurMI : *MI.getParent()) {
 133       if (&CurMI == &MI)
 134         return i;
 135       i++;
 136     }
 137     return ~0U;
 138   };
 139
 140   // Pre-Populate vector of instructions to reschedule so that we don't
 141   // clobber the iterator.
 142   std::vector<MachineInstr *> Instructions;
 143   for (auto &MI : *MBB) {
 144     Instructions.push_back(&MI);
 145   }
 146
 147   std::map<MachineInstr *, std::vector<MachineInstr *>> MultiUsers;
 148   std::map<unsigned, MachineInstr *> MultiUserLookup;
 149   unsigned UseToBringDefCloserToCount = 0;
 150   std::vector<MachineInstr *> PseudoIdempotentInstructions;
 151   std::vector<unsigned> PhysRegDefs;
 152   for (auto *II : Instructions) {
 153     for (unsigned i = 1; i < II->getNumOperands(); i++) {
 154       MachineOperand &MO = II->getOperand(i);
 155       if (!MO.isReg())
 156         continue;
 157
 158       if (MO.getReg().isVirtual())
 159         continue;
 160
 161       if (!MO.isDef())
 162         continue;
 163
 164       PhysRegDefs.push_back(MO.getReg());
 165     }
 166   }
 167
 168   for (auto *II : Instructions) {
 169     if (II->getNumOperands() == 0)
 170       continue;
 171     if (II->mayLoadOrStore())
 172       continue;
 173
 174     MachineOperand &MO = II->getOperand(0);
 175     if (!MO.isReg() || !MO.getReg().isVirtual())
 176       continue;
 177     if (!MO.isDef())
 178       continue;
 179
 180     bool IsPseudoIdempotent = true;
 181     for (unsigned i = 1; i < II->getNumOperands(); i++) {
 182
 183       if (II->getOperand(i).isImm()) {
 184         continue;
 185       }
 186
 187       if (II->getOperand(i).isReg()) {
 188         if (!II->getOperand(i).getReg().isVirtual())
 189           if (!llvm::is_contained(PhysRegDefs, II->getOperand(i).getReg())) {
 190             continue;
 191           }
 192       }
 193
 194       IsPseudoIdempotent = false;
 195       break;
 196     }
 197
 198     if (IsPseudoIdempotent) {
 199       PseudoIdempotentInstructions.push_back(II);
 200       continue;
 201     }
 202
 203     LLVM_DEBUG(dbgs() << "Operand " << 0 << " of "; II->dump(); MO.dump(););
 204
 205     MachineInstr *Def = II;
 206     unsigned Distance = ~0U;
 207     MachineInstr *UseToBringDefCloserTo = nullptr;
 208     MachineRegisterInfo *MRI = &MBB->getParent()->getRegInfo();
 209     for (auto &UO : MRI->use_nodbg_operands(MO.getReg())) {
 210       MachineInstr *UseInst = UO.getParent();
 211
 212       const unsigned DefLoc = getInstrIdx(*Def);
 213       const unsigned UseLoc = getInstrIdx(*UseInst);
 214       const unsigned Delta = (UseLoc - DefLoc);
 215
 216       if (UseInst->getParent() != Def->getParent())
 217         continue;
 218       if (DefLoc >= UseLoc)
 219         continue;
 220
 221       if (Delta < Distance) {
 222         Distance = Delta;
 223         UseToBringDefCloserTo = UseInst;
 224         MultiUserLookup[UseToBringDefCloserToCount++] = UseToBringDefCloserTo;
 225       }
 226     }
 227
 228     const auto BBE = MBB->instr_end();
 229     MachineBasicBlock::iterator DefI = BBE;
 230     MachineBasicBlock::iterator UseI = BBE;
 231
 232     for (auto BBI = MBB->instr_begin(); BBI != BBE; ++BBI) {
 233
 234       if (DefI != BBE && UseI != BBE)
 235         break;
 236
 237       if (&*BBI == Def) {
 238         DefI = BBI;
 239         continue;
 240       }
 241
 242       if (&*BBI == UseToBringDefCloserTo) {
 243         UseI = BBI;
 244         continue;
 245       }
 246     }
 247
 248     if (DefI == BBE || UseI == BBE)
 249       continue;
 250
 251     LLVM_DEBUG({
 252       dbgs() << "Splicing ";
 253       DefI->dump();
 254       dbgs() << " right before: ";
 255       UseI->dump();
 256     });
 257
 258     MultiUsers[UseToBringDefCloserTo].push_back(Def);
 259     Changed = true;
 260     MBB->splice(UseI, MBB, DefI);
 261   }
 262
 263   // Sort the defs for users of multiple defs lexographically.
 264   for (const auto &E : MultiUserLookup) {
 265
 266     auto UseI = llvm::find_if(MBB->instrs(), [&](MachineInstr &MI) -> bool {
 267       return &MI == E.second;
 268     });
 269
 270     if (UseI == MBB->instr_end())
 271       continue;
 272
 273     LLVM_DEBUG(
 274         dbgs() << "Rescheduling Multi-Use Instructions Lexographically.";);
 275     Changed |= rescheduleLexographically(
 276         MultiUsers[E.second], MBB,
 277         [&]() -> MachineBasicBlock::iterator { return UseI; });
 278   }
 279
 280   PseudoIdempotentInstCount = PseudoIdempotentInstructions.size();
 281   LLVM_DEBUG(
 282       dbgs() << "Rescheduling Idempotent Instructions Lexographically.";);
 283   Changed |= rescheduleLexographically(
 284       PseudoIdempotentInstructions, MBB,
 285       [&]() -> MachineBasicBlock::iterator { return MBB->begin(); });
 286
 287   return Changed;
 288 }
 289
 290 static bool propagateLocalCopies(MachineBasicBlock *MBB) {
 291   bool Changed = false;
 292   MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
 293
 294   std::vector<MachineInstr *> Copies;
 295   for (MachineInstr &MI : MBB->instrs()) {
 296     if (MI.isCopy())
 297       Copies.push_back(&MI);
 298   }
 299
 300   for (MachineInstr *MI : Copies) {
 301
 302     if (!MI->getOperand(0).isReg())
 303       continue;
 304     if (!MI->getOperand(1).isReg())
 305       continue;
 306
 307     const Register Dst = MI->getOperand(0).getReg();
 308     const Register Src = MI->getOperand(1).getReg();
 309
 310     if (!Dst.isVirtual())
 311       continue;
 312     if (!Src.isVirtual())
 313       continue;
 314     // Not folding COPY instructions if regbankselect has not set the RCs.
 315     // Why are we only considering Register Classes? Because the verifier
 316     // sometimes gets upset if the register classes don't match even if the
 317     // types do. A future patch might add COPY folding for matching types in
 318     // pre-registerbankselect code.
 319     if (!MRI.getRegClassOrNull(Dst))
 320       continue;
 321     if (MRI.getRegClass(Dst) != MRI.getRegClass(Src))
 322       continue;
 323
 324     std::vector<MachineOperand *> Uses;
 325     for (MachineOperand &MO : MRI.use_operands(Dst))
 326       Uses.push_back(&MO);
 327     for (auto *MO : Uses)
 328       MO->setReg(Src);
 329
 330     Changed = true;
 331     MI->eraseFromParent();
 332   }
 333
 334   return Changed;
 335 }
 336
 337 static bool doDefKillClear(MachineBasicBlock *MBB) {
 338   bool Changed = false;
 339
 340   for (auto &MI : *MBB) {
 341     for (auto &MO : MI.operands()) {
 342       if (!MO.isReg())
 343         continue;
 344       if (!MO.isDef() && MO.isKill()) {
 345         Changed = true;
 346         MO.setIsKill(false);
 347       }
 348
 349       if (MO.isDef() && MO.isDead()) {
 350         Changed = true;
 351         MO.setIsDead(false);
 352       }
 353     }
 354   }
 355
 356   return Changed;
 357 }
 358
 359 static bool runOnBasicBlock(MachineBasicBlock *MBB,
 360                             unsigned BasicBlockNum, VRegRenamer &Renamer) {
 361   LLVM_DEBUG({
 362     dbgs() << "\n\n  NEW BASIC BLOCK: " << MBB->getName() << "  \n\n";
 363     dbgs() << "\n\n================================================\n\n";
 364   });
 365
 366   bool Changed = false;
 367
 368   LLVM_DEBUG(dbgs() << "\n\n NEW BASIC BLOCK: " << MBB->getName() << "\n\n";);
 369
 370   LLVM_DEBUG(dbgs() << "MBB Before Canonical Copy Propagation:\n";
 371              MBB->dump(););
 372   Changed |= propagateLocalCopies(MBB);
 373   LLVM_DEBUG(dbgs() << "MBB After Canonical Copy Propagation:\n"; MBB->dump(););
 374
 375   LLVM_DEBUG(dbgs() << "MBB Before Scheduling:\n"; MBB->dump(););
 376   unsigned IdempotentInstCount = 0;
 377   Changed |= rescheduleCanonically(IdempotentInstCount, MBB);
 378   LLVM_DEBUG(dbgs() << "MBB After Scheduling:\n"; MBB->dump(););
 379
 380   Changed |= Renamer.renameVRegs(MBB, BasicBlockNum);
 381
 382   // TODO: Consider dropping this. Dropping kill defs is probably not
 383   // semantically sound.
 384   Changed |= doDefKillClear(MBB);
 385
 386   LLVM_DEBUG(dbgs() << "Updated MachineBasicBlock:\n"; MBB->dump();
 387              dbgs() << "\n";);
 388   LLVM_DEBUG(
 389       dbgs() << "\n\n================================================\n\n");
 390   return Changed;
 391 }
 392
 393 bool MIRCanonicalizer::runOnMachineFunction(MachineFunction &MF) {
 394
 395   static unsigned functionNum = 0;
 396   if (CanonicalizeFunctionNumber != ~0U) {
 397     if (CanonicalizeFunctionNumber != functionNum++)
 398       return false;
 399     LLVM_DEBUG(dbgs() << "\n Canonicalizing Function " << MF.getName()
 400                       << "\n";);
 401   }
 402
 403   // we need a valid vreg to create a vreg type for skipping all those
 404   // stray vreg numbers so reach alignment/canonical vreg values.
 405   std::vector<MachineBasicBlock *> RPOList = GetRPOList(MF);
 406
 407   LLVM_DEBUG(
 408       dbgs() << "\n\n  NEW MACHINE FUNCTION: " << MF.getName() << "  \n\n";
 409       dbgs() << "\n\n================================================\n\n";
 410       dbgs() << "Total Basic Blocks: " << RPOList.size() << "\n";
 411       for (auto MBB
 412            : RPOList) { dbgs() << MBB->getName() << "\n"; } dbgs()
 413       << "\n\n================================================\n\n";);
 414
 415   unsigned BBNum = 0;
 416   bool Changed = false;
 417   MachineRegisterInfo &MRI = MF.getRegInfo();
 418   VRegRenamer Renamer(MRI);
 419   for (auto *MBB : RPOList)
 420     Changed |= runOnBasicBlock(MBB, BBNum++, Renamer);
 421
 422   return Changed;
 423 }