lib/Target/ARM/Thumb2ITBlockPass.cpp

   1 //===-- Thumb2ITBlockPass.cpp - Insert Thumb-2 IT blocks ------------------===//
   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 #include "ARM.h"
  10 #include "ARMMachineFunctionInfo.h"
  11 #include "ARMSubtarget.h"
  12 #include "MCTargetDesc/ARMBaseInfo.h"
  13 #include "Thumb2InstrInfo.h"
  14 #include "llvm/ADT/SmallSet.h"
  15 #include "llvm/ADT/SmallVector.h"
  16 #include "llvm/ADT/Statistic.h"
  17 #include "llvm/ADT/StringRef.h"
  18 #include "llvm/CodeGen/MachineBasicBlock.h"
  19 #include "llvm/CodeGen/MachineFunction.h"
  20 #include "llvm/CodeGen/MachineFunctionPass.h"
  21 #include "llvm/CodeGen/MachineInstr.h"
  22 #include "llvm/CodeGen/MachineInstrBuilder.h"
  23 #include "llvm/CodeGen/MachineInstrBundle.h"
  24 #include "llvm/CodeGen/MachineOperand.h"
  25 #include "llvm/IR/DebugLoc.h"
  26 #include "llvm/MC/MCInstrDesc.h"
  27 #include "llvm/MC/MCRegisterInfo.h"
  28 #include <cassert>
  29 #include <new>
  30
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "thumb2-it"
  34 #define PASS_NAME "Thumb IT blocks insertion pass"
  35
  36 STATISTIC(NumITs,        "Number of IT blocks inserted");
  37 STATISTIC(NumMovedInsts, "Number of predicated instructions moved");
  38
  39 using RegisterSet = SmallSet<unsigned, 4>;
  40
  41 namespace {
  42
  43   class Thumb2ITBlock : public MachineFunctionPass {
  44   public:
  45     static char ID;
  46
  47     bool restrictIT;
  48     const Thumb2InstrInfo *TII;
  49     const TargetRegisterInfo *TRI;
  50     ARMFunctionInfo *AFI;
  51
  52     Thumb2ITBlock() : MachineFunctionPass(ID) {}
  53
  54     bool runOnMachineFunction(MachineFunction &Fn) override;
  55
  56     MachineFunctionProperties getRequiredProperties() const override {
  57       return MachineFunctionProperties().set(
  58           MachineFunctionProperties::Property::NoVRegs);
  59     }
  60
  61     StringRef getPassName() const override {
  62       return PASS_NAME;
  63     }
  64
  65   private:
  66     bool MoveCopyOutOfITBlock(MachineInstr *MI,
  67                               ARMCC::CondCodes CC, ARMCC::CondCodes OCC,
  68                               RegisterSet &Defs, RegisterSet &Uses);
  69     bool InsertITInstructions(MachineBasicBlock &Block);
  70   };
  71
  72   char Thumb2ITBlock::ID = 0;
  73
  74 } // end anonymous namespace
  75
  76 INITIALIZE_PASS(Thumb2ITBlock, DEBUG_TYPE, PASS_NAME, false, false)
  77
  78 /// TrackDefUses - Tracking what registers are being defined and used by
  79 /// instructions in the IT block. This also tracks "dependencies", i.e. uses
  80 /// in the IT block that are defined before the IT instruction.
  81 static void TrackDefUses(MachineInstr *MI, RegisterSet &Defs, RegisterSet &Uses,
  82                          const TargetRegisterInfo *TRI) {
  83   using RegList = SmallVector<unsigned, 4>;
  84   RegList LocalDefs;
  85   RegList LocalUses;
  86
  87   for (auto &MO : MI->operands()) {
  88     if (!MO.isReg())
  89       continue;
  90     Register Reg = MO.getReg();
  91     if (!Reg || Reg == ARM::ITSTATE || Reg == ARM::SP)
  92       continue;
  93     if (MO.isUse())
  94       LocalUses.push_back(Reg);
  95     else
  96       LocalDefs.push_back(Reg);
  97   }
  98
  99   auto InsertUsesDefs = [&](RegList &Regs, RegisterSet &UsesDefs) {
 100     for (unsigned Reg : Regs)
 101       for (MCSubRegIterator Subreg(Reg, TRI, /*IncludeSelf=*/true);
 102            Subreg.isValid(); ++Subreg)
 103         UsesDefs.insert(*Subreg);
 104   };
 105
 106   InsertUsesDefs(LocalDefs, Defs);
 107   InsertUsesDefs(LocalUses, Uses);
 108 }
 109
 110 /// Clear kill flags for any uses in the given set.  This will likely
 111 /// conservatively remove more kill flags than are necessary, but removing them
 112 /// is safer than incorrect kill flags remaining on instructions.
 113 static void ClearKillFlags(MachineInstr *MI, RegisterSet &Uses) {
 114   for (MachineOperand &MO : MI->operands()) {
 115     if (!MO.isReg() || MO.isDef() || !MO.isKill())
 116       continue;
 117     if (!Uses.count(MO.getReg()))
 118       continue;
 119     MO.setIsKill(false);
 120   }
 121 }
 122
 123 static bool isCopy(MachineInstr *MI) {
 124   switch (MI->getOpcode()) {
 125   default:
 126     return false;
 127   case ARM::MOVr:
 128   case ARM::MOVr_TC:
 129   case ARM::tMOVr:
 130   case ARM::t2MOVr:
 131     return true;
 132   }
 133 }
 134
 135 bool
 136 Thumb2ITBlock::MoveCopyOutOfITBlock(MachineInstr *MI,
 137                                     ARMCC::CondCodes CC, ARMCC::CondCodes OCC,
 138                                     RegisterSet &Defs, RegisterSet &Uses) {
 139   if (!isCopy(MI))
 140     return false;
 141   // llvm models select's as two-address instructions. That means a copy
 142   // is inserted before a t2MOVccr, etc. If the copy is scheduled in
 143   // between selects we would end up creating multiple IT blocks.
 144   assert(MI->getOperand(0).getSubReg() == 0 &&
 145          MI->getOperand(1).getSubReg() == 0 &&
 146          "Sub-register indices still around?");
 147
 148   Register DstReg = MI->getOperand(0).getReg();
 149   Register SrcReg = MI->getOperand(1).getReg();
 150
 151   // First check if it's safe to move it.
 152   if (Uses.count(DstReg) || Defs.count(SrcReg))
 153     return false;
 154
 155   // If the CPSR is defined by this copy, then we don't want to move it. E.g.,
 156   // if we have:
 157   //
 158   //   movs  r1, r1
 159   //   rsb   r1, 0
 160   //   movs  r2, r2
 161   //   rsb   r2, 0
 162   //
 163   // we don't want this to be converted to:
 164   //
 165   //   movs  r1, r1
 166   //   movs  r2, r2
 167   //   itt   mi
 168   //   rsb   r1, 0
 169   //   rsb   r2, 0
 170   //
 171   const MCInstrDesc &MCID = MI->getDesc();
 172   if (MI->hasOptionalDef() &&
 173       MI->getOperand(MCID.getNumOperands() - 1).getReg() == ARM::CPSR)
 174     return false;
 175
 176   // Then peek at the next instruction to see if it's predicated on CC or OCC.
 177   // If not, then there is nothing to be gained by moving the copy.
 178   MachineBasicBlock::iterator I = MI;
 179   ++I;
 180   MachineBasicBlock::iterator E = MI->getParent()->end();
 181
 182   while (I != E && I->isDebugInstr())
 183     ++I;
 184
 185   if (I != E) {
 186     unsigned NPredReg = 0;
 187     ARMCC::CondCodes NCC = getITInstrPredicate(*I, NPredReg);
 188     if (NCC == CC || NCC == OCC)
 189       return true;
 190   }
 191   return false;
 192 }
 193
 194 bool Thumb2ITBlock::InsertITInstructions(MachineBasicBlock &MBB) {
 195   bool Modified = false;
 196   RegisterSet Defs, Uses;
 197   MachineBasicBlock::iterator MBBI = MBB.begin(), E = MBB.end();
 198
 199   while (MBBI != E) {
 200     MachineInstr *MI = &*MBBI;
 201     DebugLoc dl = MI->getDebugLoc();
 202     unsigned PredReg = 0;
 203     ARMCC::CondCodes CC = getITInstrPredicate(*MI, PredReg);
 204     if (CC == ARMCC::AL) {
 205       ++MBBI;
 206       continue;
 207     }
 208
 209     Defs.clear();
 210     Uses.clear();
 211     TrackDefUses(MI, Defs, Uses, TRI);
 212
 213     // Insert an IT instruction.
 214     MachineInstrBuilder MIB = BuildMI(MBB, MBBI, dl, TII->get(ARM::t2IT))
 215       .addImm(CC);
 216
 217     // Add implicit use of ITSTATE to IT block instructions.
 218     MI->addOperand(MachineOperand::CreateReg(ARM::ITSTATE, false/*ifDef*/,
 219                                              true/*isImp*/, false/*isKill*/));
 220
 221     MachineInstr *LastITMI = MI;
 222     MachineBasicBlock::iterator InsertPos = MIB.getInstr();
 223     ++MBBI;
 224
 225     // Form IT block.
 226     ARMCC::CondCodes OCC = ARMCC::getOppositeCondition(CC);
 227     unsigned Mask = 0, Pos = 3;
 228
 229     // v8 IT blocks are limited to one conditional op unless -arm-no-restrict-it
 230     // is set: skip the loop
 231     if (!restrictIT) {
 232       // Branches, including tricky ones like LDM_RET, need to end an IT
 233       // block so check the instruction we just put in the block.
 234       for (; MBBI != E && Pos &&
 235              (!MI->isBranch() && !MI->isReturn()) ; ++MBBI) {
 236         if (MBBI->isDebugInstr())
 237           continue;
 238
 239         MachineInstr *NMI = &*MBBI;
 240         MI = NMI;
 241
 242         unsigned NPredReg = 0;
 243         ARMCC::CondCodes NCC = getITInstrPredicate(*NMI, NPredReg);
 244         if (NCC == CC || NCC == OCC) {
 245           Mask |= ((NCC ^ CC) & 1) << Pos;
 246           // Add implicit use of ITSTATE.
 247           NMI->addOperand(MachineOperand::CreateReg(ARM::ITSTATE, false/*ifDef*/,
 248                                                  true/*isImp*/, false/*isKill*/));
 249           LastITMI = NMI;
 250         } else {
 251           if (NCC == ARMCC::AL &&
 252               MoveCopyOutOfITBlock(NMI, CC, OCC, Defs, Uses)) {
 253             --MBBI;
 254             MBB.remove(NMI);
 255             MBB.insert(InsertPos, NMI);
 256             ClearKillFlags(MI, Uses);
 257             ++NumMovedInsts;
 258             continue;
 259           }
 260           break;
 261         }
 262         TrackDefUses(NMI, Defs, Uses, TRI);
 263         --Pos;
 264       }
 265     }
 266
 267     // Finalize IT mask.
 268     Mask |= (1 << Pos);
 269     MIB.addImm(Mask);
 270
 271     // Last instruction in IT block kills ITSTATE.
 272     LastITMI->findRegisterUseOperand(ARM::ITSTATE)->setIsKill();
 273
 274     // Finalize the bundle.
 275     finalizeBundle(MBB, InsertPos.getInstrIterator(),
 276                    ++LastITMI->getIterator());
 277
 278     Modified = true;
 279     ++NumITs;
 280   }
 281
 282   return Modified;
 283 }
 284
 285 bool Thumb2ITBlock::runOnMachineFunction(MachineFunction &Fn) {
 286   const ARMSubtarget &STI =
 287       static_cast<const ARMSubtarget &>(Fn.getSubtarget());
 288   if (!STI.isThumb2())
 289     return false;
 290   AFI = Fn.getInfo<ARMFunctionInfo>();
 291   TII = static_cast<const Thumb2InstrInfo *>(STI.getInstrInfo());
 292   TRI = STI.getRegisterInfo();
 293   restrictIT = STI.restrictIT();
 294
 295   if (!AFI->isThumbFunction())
 296     return false;
 297
 298   bool Modified = false;
 299   for (auto &MBB : Fn )
 300     Modified |= InsertITInstructions(MBB);
 301
 302   if (Modified)
 303     AFI->setHasITBlocks(true);
 304
 305   return Modified;
 306 }
 307
 308 /// createThumb2ITBlockPass - Returns an instance of the Thumb2 IT blocks
 309 /// insertion pass.
 310 FunctionPass *llvm::createThumb2ITBlockPass() { return new Thumb2ITBlock(); }