llvm/lib/Target/Hexagon/HexagonCFGOptimizer.cpp

   1 //===- HexagonCFGOptimizer.cpp - CFG optimizations ------------------------===//
   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 "Hexagon.h"
  10 #include "MCTargetDesc/HexagonMCTargetDesc.h"
  11 #include "llvm/CodeGen/MachineBasicBlock.h"
  12 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
  13 #include "llvm/CodeGen/MachineFunction.h"
  14 #include "llvm/CodeGen/MachineFunctionPass.h"
  15 #include "llvm/CodeGen/MachineInstr.h"
  16 #include "llvm/CodeGen/MachineOperand.h"
  17 #include "llvm/CodeGen/TargetInstrInfo.h"
  18 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  19 #include "llvm/Pass.h"
  20 #include "llvm/Support/ErrorHandling.h"
  21 #include <cassert>
  22 #include <vector>
  23
  24 using namespace llvm;
  25
  26 #define DEBUG_TYPE "hexagon_cfg"
  27
  28 namespace llvm {
  29
  30 FunctionPass *createHexagonCFGOptimizer();
  31 void initializeHexagonCFGOptimizerPass(PassRegistry&);
  32
  33 } // end namespace llvm
  34
  35 namespace {
  36
  37 class HexagonCFGOptimizer : public MachineFunctionPass {
  38 private:
  39   void InvertAndChangeJumpTarget(MachineInstr &, MachineBasicBlock *);
  40   bool isOnFallThroughPath(MachineBasicBlock *MBB);
  41
  42 public:
  43   static char ID;
  44
  45   HexagonCFGOptimizer() : MachineFunctionPass(ID) {
  46     initializeHexagonCFGOptimizerPass(*PassRegistry::getPassRegistry());
  47   }
  48
  49   StringRef getPassName() const override { return "Hexagon CFG Optimizer"; }
  50   bool runOnMachineFunction(MachineFunction &Fn) override;
  51
  52   MachineFunctionProperties getRequiredProperties() const override {
  53     return MachineFunctionProperties().set(
  54         MachineFunctionProperties::Property::NoVRegs);
  55   }
  56 };
  57
  58 } // end anonymous namespace
  59
  60 char HexagonCFGOptimizer::ID = 0;
  61
  62 static bool IsConditionalBranch(int Opc) {
  63   switch (Opc) {
  64     case Hexagon::J2_jumpt:
  65     case Hexagon::J2_jumptpt:
  66     case Hexagon::J2_jumpf:
  67     case Hexagon::J2_jumpfpt:
  68     case Hexagon::J2_jumptnew:
  69     case Hexagon::J2_jumpfnew:
  70     case Hexagon::J2_jumptnewpt:
  71     case Hexagon::J2_jumpfnewpt:
  72       return true;
  73   }
  74   return false;
  75 }
  76
  77 static bool IsUnconditionalJump(int Opc) {
  78   return (Opc == Hexagon::J2_jump);
  79 }
  80
  81 void HexagonCFGOptimizer::InvertAndChangeJumpTarget(
  82     MachineInstr &MI, MachineBasicBlock *NewTarget) {
  83   const TargetInstrInfo *TII =
  84       MI.getParent()->getParent()->getSubtarget().getInstrInfo();
  85   int NewOpcode = 0;
  86   switch (MI.getOpcode()) {
  87   case Hexagon::J2_jumpt:
  88     NewOpcode = Hexagon::J2_jumpf;
  89     break;
  90   case Hexagon::J2_jumpf:
  91     NewOpcode = Hexagon::J2_jumpt;
  92     break;
  93   case Hexagon::J2_jumptnewpt:
  94     NewOpcode = Hexagon::J2_jumpfnewpt;
  95     break;
  96   case Hexagon::J2_jumpfnewpt:
  97     NewOpcode = Hexagon::J2_jumptnewpt;
  98     break;
  99   default:
 100     llvm_unreachable("Cannot handle this case");
 101   }
 102
 103   MI.setDesc(TII->get(NewOpcode));
 104   MI.getOperand(1).setMBB(NewTarget);
 105 }
 106
 107 bool HexagonCFGOptimizer::isOnFallThroughPath(MachineBasicBlock *MBB) {
 108   if (MBB->canFallThrough())
 109     return true;
 110   for (MachineBasicBlock *PB : MBB->predecessors())
 111     if (PB->isLayoutSuccessor(MBB) && PB->canFallThrough())
 112       return true;
 113   return false;
 114 }
 115
 116 bool HexagonCFGOptimizer::runOnMachineFunction(MachineFunction &Fn) {
 117   if (skipFunction(Fn.getFunction()))
 118     return false;
 119
 120   // Loop over all of the basic blocks.
 121   for (MachineBasicBlock &MBB : Fn) {
 122     // Traverse the basic block.
 123     MachineBasicBlock::iterator MII = MBB.getFirstTerminator();
 124     if (MII != MBB.end()) {
 125       MachineInstr &MI = *MII;
 126       int Opc = MI.getOpcode();
 127       if (IsConditionalBranch(Opc)) {
 128         // (Case 1) Transform the code if the following condition occurs:
 129         //   BB1: if (p0) jump BB3
 130         //   ...falls-through to BB2 ...
 131         //   BB2: jump BB4
 132         //   ...next block in layout is BB3...
 133         //   BB3: ...
 134         //
 135         //  Transform this to:
 136         //  BB1: if (!p0) jump BB4
 137         //  Remove BB2
 138         //  BB3: ...
 139         //
 140         // (Case 2) A variation occurs when BB3 contains a JMP to BB4:
 141         //   BB1: if (p0) jump BB3
 142         //   ...falls-through to BB2 ...
 143         //   BB2: jump BB4
 144         //   ...other basic blocks ...
 145         //   BB4:
 146         //   ...not a fall-thru
 147         //   BB3: ...
 148         //     jump BB4
 149         //
 150         // Transform this to:
 151         //   BB1: if (!p0) jump BB4
 152         //   Remove BB2
 153         //   BB3: ...
 154         //   BB4: ...
 155         unsigned NumSuccs = MBB.succ_size();
 156         MachineBasicBlock::succ_iterator SI = MBB.succ_begin();
 157         MachineBasicBlock* FirstSucc = *SI;
 158         MachineBasicBlock* SecondSucc = *(++SI);
 159         MachineBasicBlock* LayoutSucc = nullptr;
 160         MachineBasicBlock* JumpAroundTarget = nullptr;
 161
 162         if (MBB.isLayoutSuccessor(FirstSucc)) {
 163           LayoutSucc = FirstSucc;
 164           JumpAroundTarget = SecondSucc;
 165         } else if (MBB.isLayoutSuccessor(SecondSucc)) {
 166           LayoutSucc = SecondSucc;
 167           JumpAroundTarget = FirstSucc;
 168         } else {
 169           // Odd case...cannot handle.
 170         }
 171
 172         // The target of the unconditional branch must be JumpAroundTarget.
 173         // TODO: If not, we should not invert the unconditional branch.
 174         MachineBasicBlock* CondBranchTarget = nullptr;
 175         if (MI.getOpcode() == Hexagon::J2_jumpt ||
 176             MI.getOpcode() == Hexagon::J2_jumpf) {
 177           CondBranchTarget = MI.getOperand(1).getMBB();
 178         }
 179
 180         if (!LayoutSucc || (CondBranchTarget != JumpAroundTarget)) {
 181           continue;
 182         }
 183
 184         if ((NumSuccs == 2) && LayoutSucc && (LayoutSucc->pred_size() == 1)) {
 185           // Ensure that BB2 has one instruction -- an unconditional jump.
 186           if ((LayoutSucc->size() == 1) &&
 187               IsUnconditionalJump(LayoutSucc->front().getOpcode())) {
 188             assert(JumpAroundTarget && "jump target is needed to process second basic block");
 189             MachineBasicBlock* UncondTarget =
 190               LayoutSucc->front().getOperand(0).getMBB();
 191             // Check if the layout successor of BB2 is BB3.
 192             bool case1 = LayoutSucc->isLayoutSuccessor(JumpAroundTarget);
 193             bool case2 = JumpAroundTarget->isSuccessor(UncondTarget) &&
 194               !JumpAroundTarget->empty() &&
 195               IsUnconditionalJump(JumpAroundTarget->back().getOpcode()) &&
 196               JumpAroundTarget->pred_size() == 1 &&
 197               JumpAroundTarget->succ_size() == 1;
 198
 199             if (case1 || case2) {
 200               InvertAndChangeJumpTarget(MI, UncondTarget);
 201               MBB.replaceSuccessor(JumpAroundTarget, UncondTarget);
 202
 203               // Remove the unconditional branch in LayoutSucc.
 204               LayoutSucc->erase(LayoutSucc->begin());
 205               LayoutSucc->replaceSuccessor(UncondTarget, JumpAroundTarget);
 206
 207               // This code performs the conversion for case 2, which moves
 208               // the block to the fall-thru case (BB3 in the code above).
 209               if (case2 && !case1) {
 210                 JumpAroundTarget->moveAfter(LayoutSucc);
 211                 // only move a block if it doesn't have a fall-thru. otherwise
 212                 // the CFG will be incorrect.
 213                 if (!isOnFallThroughPath(UncondTarget))
 214                   UncondTarget->moveAfter(JumpAroundTarget);
 215               }
 216
 217               // Correct live-in information. Is used by post-RA scheduler
 218               // The live-in to LayoutSucc is now all values live-in to
 219               // JumpAroundTarget.
 220               std::vector<MachineBasicBlock::RegisterMaskPair> OrigLiveIn(
 221                   LayoutSucc->livein_begin(), LayoutSucc->livein_end());
 222               std::vector<MachineBasicBlock::RegisterMaskPair> NewLiveIn(
 223                   JumpAroundTarget->livein_begin(),
 224                   JumpAroundTarget->livein_end());
 225               for (const auto &OrigLI : OrigLiveIn)
 226                 LayoutSucc->removeLiveIn(OrigLI.PhysReg);
 227               for (const auto &NewLI : NewLiveIn)
 228                 LayoutSucc->addLiveIn(NewLI);
 229             }
 230           }
 231         }
 232       }
 233     }
 234   }
 235   return true;
 236 }
 237
 238 //===----------------------------------------------------------------------===//
 239 //                         Public Constructor Functions
 240 //===----------------------------------------------------------------------===//
 241
 242 INITIALIZE_PASS(HexagonCFGOptimizer, "hexagon-cfg", "Hexagon CFG Optimizer",
 243                 false, false)
 244
 245 FunctionPass *llvm::createHexagonCFGOptimizer() {
 246   return new HexagonCFGOptimizer();
 247 }