lib/CodeGen/PeepholeOptimizer.cpp

   1 //===-- PeepholeOptimizer.cpp - Peephole Optimizations --------------------===//
   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 // Perform peephole optimizations on the machine code:
  11 //
  12 // - Optimize Extensions
  13 //
  14 //     Optimization of sign / zero extension instructions. It may be extended to
  15 //     handle other instructions with similar properties.
  16 //
  17 //     On some targets, some instructions, e.g. X86 sign / zero extension, may
  18 //     leave the source value in the lower part of the result. This optimization
  19 //     will replace some uses of the pre-extension value with uses of the
  20 //     sub-register of the results.
  21 //
  22 // - Optimize Comparisons
  23 //
  24 //     Optimization of comparison instructions. For instance, in this code:
  25 //
  26 //       sub r1, 1
  27 //       cmp r1, 0
  28 //       bz  L1
  29 //
  30 //     If the "sub" instruction all ready sets (or could be modified to set) the
  31 //     same flag that the "cmp" instruction sets and that "bz" uses, then we can
  32 //     eliminate the "cmp" instruction.
  33 //
  34 // - Optimize Bitcast pairs:
  35 //
  36 //     v1 = bitcast v0
  37 //     v2 = bitcast v1
  38 //        = v2
  39 //   =>
  40 //     v1 = bitcast v0
  41 //        = v0
  42 //
  43 //===----------------------------------------------------------------------===//
  44
  45 #define DEBUG_TYPE "peephole-opt"
  46 #include "llvm/CodeGen/Passes.h"
  47 #include "llvm/CodeGen/MachineDominators.h"
  48 #include "llvm/CodeGen/MachineInstrBuilder.h"
  49 #include "llvm/CodeGen/MachineRegisterInfo.h"
  50 #include "llvm/Target/TargetInstrInfo.h"
  51 #include "llvm/Target/TargetRegisterInfo.h"
  52 #include "llvm/Support/CommandLine.h"
  53 #include "llvm/ADT/DenseMap.h"
  54 #include "llvm/ADT/SmallPtrSet.h"
  55 #include "llvm/ADT/SmallSet.h"
  56 #include "llvm/ADT/Statistic.h"
  57 using namespace llvm;
  58
  59 // Optimize Extensions
  60 static cl::opt<bool>
  61 Aggressive("aggressive-ext-opt", cl::Hidden,
  62            cl::desc("Aggressive extension optimization"));
  63
  64 static cl::opt<bool>
  65 DisablePeephole("disable-peephole", cl::Hidden, cl::init(false),
  66                 cl::desc("Disable the peephole optimizer"));
  67
  68 STATISTIC(NumReuse,      "Number of extension results reused");
  69 STATISTIC(NumBitcasts,   "Number of bitcasts eliminated");
  70 STATISTIC(NumCmps,       "Number of compares eliminated");
  71 STATISTIC(NumImmFold,    "Number of move immediate foled");
  72
  73 namespace {
  74   class PeepholeOptimizer : public MachineFunctionPass {
  75     const TargetMachine   *TM;
  76     const TargetInstrInfo *TII;
  77     MachineRegisterInfo   *MRI;
  78     MachineDominatorTree  *DT;  // Machine dominator tree
  79
  80   public:
  81     static char ID; // Pass identification
  82     PeepholeOptimizer() : MachineFunctionPass(ID) {
  83       initializePeepholeOptimizerPass(*PassRegistry::getPassRegistry());
  84     }
  85
  86     virtual bool runOnMachineFunction(MachineFunction &MF);
  87
  88     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  89       AU.setPreservesCFG();
  90       MachineFunctionPass::getAnalysisUsage(AU);
  91       if (Aggressive) {
  92         AU.addRequired<MachineDominatorTree>();
  93         AU.addPreserved<MachineDominatorTree>();
  94       }
  95     }
  96
  97   private:
  98     bool OptimizeBitcastInstr(MachineInstr *MI, MachineBasicBlock *MBB);
  99     bool OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB);
 100     bool OptimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
 101                           SmallPtrSet<MachineInstr*, 8> &LocalMIs);
 102     bool isMoveImmediate(MachineInstr *MI,
 103                          SmallSet<unsigned, 4> &ImmDefRegs,
 104                          DenseMap<unsigned, MachineInstr*> &ImmDefMIs);
 105     bool FoldImmediate(MachineInstr *MI, MachineBasicBlock *MBB,
 106                        SmallSet<unsigned, 4> &ImmDefRegs,
 107                        DenseMap<unsigned, MachineInstr*> &ImmDefMIs);
 108   };
 109 }
 110
 111 char PeepholeOptimizer::ID = 0;
 112 INITIALIZE_PASS_BEGIN(PeepholeOptimizer, "peephole-opts",
 113                 "Peephole Optimizations", false, false)
 114 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
 115 INITIALIZE_PASS_END(PeepholeOptimizer, "peephole-opts",
 116                 "Peephole Optimizations", false, false)
 117
 118 FunctionPass *llvm::createPeepholeOptimizerPass() {
 119   return new PeepholeOptimizer();
 120 }
 121
 122 /// OptimizeExtInstr - If instruction is a copy-like instruction, i.e. it reads
 123 /// a single register and writes a single register and it does not modify the
 124 /// source, and if the source value is preserved as a sub-register of the
 125 /// result, then replace all reachable uses of the source with the subreg of the
 126 /// result.
 127 ///
 128 /// Do not generate an EXTRACT that is used only in a debug use, as this changes
 129 /// the code. Since this code does not currently share EXTRACTs, just ignore all
 130 /// debug uses.
 131 bool PeepholeOptimizer::
 132 OptimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
 133                  SmallPtrSet<MachineInstr*, 8> &LocalMIs) {
 134   unsigned SrcReg, DstReg, SubIdx;
 135   if (!TII->isCoalescableExtInstr(*MI, SrcReg, DstReg, SubIdx))
 136     return false;
 137
 138   if (TargetRegisterInfo::isPhysicalRegister(DstReg) ||
 139       TargetRegisterInfo::isPhysicalRegister(SrcReg))
 140     return false;
 141
 142   MachineRegisterInfo::use_nodbg_iterator UI = MRI->use_nodbg_begin(SrcReg);
 143   if (++UI == MRI->use_nodbg_end())
 144     // No other uses.
 145     return false;
 146
 147   // The source has other uses. See if we can replace the other uses with use of
 148   // the result of the extension.
 149   SmallPtrSet<MachineBasicBlock*, 4> ReachedBBs;
 150   UI = MRI->use_nodbg_begin(DstReg);
 151   for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
 152        UI != UE; ++UI)
 153     ReachedBBs.insert(UI->getParent());
 154
 155   // Uses that are in the same BB of uses of the result of the instruction.
 156   SmallVector<MachineOperand*, 8> Uses;
 157
 158   // Uses that the result of the instruction can reach.
 159   SmallVector<MachineOperand*, 8> ExtendedUses;
 160
 161   bool ExtendLife = true;
 162   UI = MRI->use_nodbg_begin(SrcReg);
 163   for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
 164        UI != UE; ++UI) {
 165     MachineOperand &UseMO = UI.getOperand();
 166     MachineInstr *UseMI = &*UI;
 167     if (UseMI == MI)
 168       continue;
 169
 170     if (UseMI->isPHI()) {
 171       ExtendLife = false;
 172       continue;
 173     }
 174
 175     // It's an error to translate this:
 176     //
 177     //    %reg1025 = <sext> %reg1024
 178     //     ...
 179     //    %reg1026 = SUBREG_TO_REG 0, %reg1024, 4
 180     //
 181     // into this:
 182     //
 183     //    %reg1025 = <sext> %reg1024
 184     //     ...
 185     //    %reg1027 = COPY %reg1025:4
 186     //    %reg1026 = SUBREG_TO_REG 0, %reg1027, 4
 187     //
 188     // The problem here is that SUBREG_TO_REG is there to assert that an
 189     // implicit zext occurs. It doesn't insert a zext instruction. If we allow
 190     // the COPY here, it will give us the value after the <sext>, not the
 191     // original value of %reg1024 before <sext>.
 192     if (UseMI->getOpcode() == TargetOpcode::SUBREG_TO_REG)
 193       continue;
 194
 195     MachineBasicBlock *UseMBB = UseMI->getParent();
 196     if (UseMBB == MBB) {
 197       // Local uses that come after the extension.
 198       if (!LocalMIs.count(UseMI))
 199         Uses.push_back(&UseMO);
 200     } else if (ReachedBBs.count(UseMBB)) {
 201       // Non-local uses where the result of the extension is used. Always
 202       // replace these unless it's a PHI.
 203       Uses.push_back(&UseMO);
 204     } else if (Aggressive && DT->dominates(MBB, UseMBB)) {
 205       // We may want to extend the live range of the extension result in order
 206       // to replace these uses.
 207       ExtendedUses.push_back(&UseMO);
 208     } else {
 209       // Both will be live out of the def MBB anyway. Don't extend live range of
 210       // the extension result.
 211       ExtendLife = false;
 212       break;
 213     }
 214   }
 215
 216   if (ExtendLife && !ExtendedUses.empty())
 217     // Extend the liveness of the extension result.
 218     std::copy(ExtendedUses.begin(), ExtendedUses.end(),
 219               std::back_inserter(Uses));
 220
 221   // Now replace all uses.
 222   bool Changed = false;
 223   if (!Uses.empty()) {
 224     SmallPtrSet<MachineBasicBlock*, 4> PHIBBs;
 225
 226     // Look for PHI uses of the extended result, we don't want to extend the
 227     // liveness of a PHI input. It breaks all kinds of assumptions down
 228     // stream. A PHI use is expected to be the kill of its source values.
 229     UI = MRI->use_nodbg_begin(DstReg);
 230     for (MachineRegisterInfo::use_nodbg_iterator
 231            UE = MRI->use_nodbg_end(); UI != UE; ++UI)
 232       if (UI->isPHI())
 233         PHIBBs.insert(UI->getParent());
 234
 235     const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
 236     for (unsigned i = 0, e = Uses.size(); i != e; ++i) {
 237       MachineOperand *UseMO = Uses[i];
 238       MachineInstr *UseMI = UseMO->getParent();
 239       MachineBasicBlock *UseMBB = UseMI->getParent();
 240       if (PHIBBs.count(UseMBB))
 241         continue;
 242
 243       unsigned NewVR = MRI->createVirtualRegister(RC);
 244       BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
 245               TII->get(TargetOpcode::COPY), NewVR)
 246         .addReg(DstReg, 0, SubIdx);
 247
 248       UseMO->setReg(NewVR);
 249       ++NumReuse;
 250       Changed = true;
 251     }
 252   }
 253
 254   return Changed;
 255 }
 256
 257 /// OptimizeBitcastInstr - If the instruction is a bitcast instruction A that
 258 /// cannot be optimized away during isel (e.g. ARM::VMOVSR, which bitcast
 259 /// a value cross register classes), and the source is defined by another
 260 /// bitcast instruction B. And if the register class of source of B matches
 261 /// the register class of instruction A, then it is legal to replace all uses
 262 /// of the def of A with source of B. e.g.
 263 ///   %vreg0<def> = VMOVSR %vreg1
 264 ///   %vreg3<def> = VMOVRS %vreg0
 265 ///   Replace all uses of vreg3 with vreg1.
 266
 267 bool PeepholeOptimizer::OptimizeBitcastInstr(MachineInstr *MI,
 268                                              MachineBasicBlock *MBB) {
 269   unsigned NumDefs = MI->getDesc().getNumDefs();
 270   unsigned NumSrcs = MI->getDesc().getNumOperands() - NumDefs;
 271   if (NumDefs != 1)
 272     return false;
 273
 274   unsigned Def = 0;
 275   unsigned Src = 0;
 276   for (unsigned i = 0, e = NumDefs + NumSrcs; i != e; ++i) {
 277     const MachineOperand &MO = MI->getOperand(i);
 278     if (!MO.isReg())
 279       continue;
 280     unsigned Reg = MO.getReg();
 281     if (!Reg)
 282       continue;
 283     if (MO.isDef())
 284       Def = Reg;
 285     else if (Src)
 286       // Multiple sources?
 287       return false;
 288     else
 289       Src = Reg;
 290   }
 291
 292   assert(Def && Src && "Malformed bitcast instruction!");
 293
 294   MachineInstr *DefMI = MRI->getVRegDef(Src);
 295   if (!DefMI || !DefMI->getDesc().isBitcast())
 296     return false;
 297
 298   unsigned SrcDef = 0;
 299   unsigned SrcSrc = 0;
 300   NumDefs = DefMI->getDesc().getNumDefs();
 301   NumSrcs = DefMI->getDesc().getNumOperands() - NumDefs;
 302   if (NumDefs != 1)
 303     return false;
 304   for (unsigned i = 0, e = NumDefs + NumSrcs; i != e; ++i) {
 305     const MachineOperand &MO = DefMI->getOperand(i);
 306     if (!MO.isReg() || MO.isDef())
 307       continue;
 308     unsigned Reg = MO.getReg();
 309     if (!Reg)
 310       continue;
 311     if (MO.isDef())
 312       SrcDef = Reg;
 313     else if (SrcSrc)
 314       // Multiple sources?
 315       return false;
 316     else
 317       SrcSrc = Reg;
 318   }
 319
 320   if (MRI->getRegClass(SrcSrc) != MRI->getRegClass(Def))
 321     return false;
 322
 323   MRI->replaceRegWith(Def, SrcSrc);
 324   MRI->clearKillFlags(SrcSrc);
 325   MI->eraseFromParent();
 326   ++NumBitcasts;
 327   return true;
 328 }
 329
 330 /// OptimizeCmpInstr - If the instruction is a compare and the previous
 331 /// instruction it's comparing against all ready sets (or could be modified to
 332 /// set) the same flag as the compare, then we can remove the comparison and use
 333 /// the flag from the previous instruction.
 334 bool PeepholeOptimizer::OptimizeCmpInstr(MachineInstr *MI,
 335                                          MachineBasicBlock *MBB) {
 336   // If this instruction is a comparison against zero and isn't comparing a
 337   // physical register, we can try to optimize it.
 338   unsigned SrcReg;
 339   int CmpMask, CmpValue;
 340   if (!TII->AnalyzeCompare(MI, SrcReg, CmpMask, CmpValue) ||
 341       TargetRegisterInfo::isPhysicalRegister(SrcReg))
 342     return false;
 343
 344   // Attempt to optimize the comparison instruction.
 345   if (TII->OptimizeCompareInstr(MI, SrcReg, CmpMask, CmpValue, MRI)) {
 346     ++NumCmps;
 347     return true;
 348   }
 349
 350   return false;
 351 }
 352
 353 bool PeepholeOptimizer::isMoveImmediate(MachineInstr *MI,
 354                                         SmallSet<unsigned, 4> &ImmDefRegs,
 355                                  DenseMap<unsigned, MachineInstr*> &ImmDefMIs) {
 356   const TargetInstrDesc &TID = MI->getDesc();
 357   if (!TID.isMoveImmediate())
 358     return false;
 359   if (TID.getNumDefs() != 1)
 360     return false;
 361   unsigned Reg = MI->getOperand(0).getReg();
 362   if (TargetRegisterInfo::isVirtualRegister(Reg)) {
 363     ImmDefMIs.insert(std::make_pair(Reg, MI));
 364     ImmDefRegs.insert(Reg);
 365     return true;
 366   }
 367
 368   return false;
 369 }
 370
 371 /// FoldImmediate - Try folding register operands that are defined by move
 372 /// immediate instructions, i.e. a trivial constant folding optimization, if
 373 /// and only if the def and use are in the same BB.
 374 bool PeepholeOptimizer::FoldImmediate(MachineInstr *MI, MachineBasicBlock *MBB,
 375                                       SmallSet<unsigned, 4> &ImmDefRegs,
 376                                  DenseMap<unsigned, MachineInstr*> &ImmDefMIs) {
 377   for (unsigned i = 0, e = MI->getDesc().getNumOperands(); i != e; ++i) {
 378     MachineOperand &MO = MI->getOperand(i);
 379     if (!MO.isReg() || MO.isDef())
 380       continue;
 381     unsigned Reg = MO.getReg();
 382     if (!TargetRegisterInfo::isVirtualRegister(Reg))
 383       continue;
 384     if (ImmDefRegs.count(Reg) == 0)
 385       continue;
 386     DenseMap<unsigned, MachineInstr*>::iterator II = ImmDefMIs.find(Reg);
 387     assert(II != ImmDefMIs.end());
 388     if (TII->FoldImmediate(MI, II->second, Reg, MRI)) {
 389       ++NumImmFold;
 390       return true;
 391     }
 392   }
 393   return false;
 394 }
 395
 396 bool PeepholeOptimizer::runOnMachineFunction(MachineFunction &MF) {
 397   if (DisablePeephole)
 398     return false;
 399
 400   TM  = &MF.getTarget();
 401   TII = TM->getInstrInfo();
 402   MRI = &MF.getRegInfo();
 403   DT  = Aggressive ? &getAnalysis<MachineDominatorTree>() : 0;
 404
 405   bool Changed = false;
 406
 407   SmallPtrSet<MachineInstr*, 8> LocalMIs;
 408   SmallSet<unsigned, 4> ImmDefRegs;
 409   DenseMap<unsigned, MachineInstr*> ImmDefMIs;
 410   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
 411     MachineBasicBlock *MBB = &*I;
 412
 413     bool SeenMoveImm = false;
 414     LocalMIs.clear();
 415     ImmDefRegs.clear();
 416     ImmDefMIs.clear();
 417
 418     bool First = true;
 419     MachineBasicBlock::iterator PMII;
 420     for (MachineBasicBlock::iterator
 421            MII = I->begin(), MIE = I->end(); MII != MIE; ) {
 422       MachineInstr *MI = &*MII;
 423       LocalMIs.insert(MI);
 424
 425       if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
 426           MI->isKill() || MI->isInlineAsm() || MI->isDebugValue() ||
 427           MI->hasUnmodeledSideEffects()) {
 428         ++MII;
 429         continue;
 430       }
 431
 432       const TargetInstrDesc &TID = MI->getDesc();
 433
 434       if (TID.isBitcast()) {
 435         if (OptimizeBitcastInstr(MI, MBB)) {
 436           // MI is deleted.
 437           Changed = true;
 438           MII = First ? I->begin() : llvm::next(PMII);
 439           continue;
 440         }
 441       } else if (TID.isCompare()) {
 442         if (OptimizeCmpInstr(MI, MBB)) {
 443           // MI is deleted.
 444           Changed = true;
 445           MII = First ? I->begin() : llvm::next(PMII);
 446           continue;
 447         }
 448       }
 449
 450       if (isMoveImmediate(MI, ImmDefRegs, ImmDefMIs)) {
 451         SeenMoveImm = true;
 452       } else {
 453         Changed |= OptimizeExtInstr(MI, MBB, LocalMIs);
 454         if (SeenMoveImm)
 455           Changed |= FoldImmediate(MI, MBB, ImmDefRegs, ImmDefMIs);
 456       }
 457
 458       First = false;
 459       PMII = MII;
 460       ++MII;
 461     }
 462   }
 463
 464   return Changed;
 465 }