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 "llvm/CodeGen/MachineBasicBlock.h"
  11 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
  12 #include "llvm/CodeGen/MachineFunction.h"
  13 #include "llvm/CodeGen/MachineFunctionPass.h"
  14 #include "llvm/CodeGen/MachineInstr.h"
  15 #include "llvm/CodeGen/MachineOperand.h"
  16 #include "llvm/CodeGen/TargetInstrInfo.h"
  17 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  18 #include "llvm/Pass.h"
  19 #include "llvm/Support/ErrorHandling.h"
  20 #include <cassert>
  21 #include <vector>
  22
  23 using namespace llvm;
  24
  25 #define DEBUG_TYPE "hexagon_cfg"
  26
  27 namespace llvm {
  28
  29 FunctionPass *createHexagonCFGOptimizer();
  30 void initializeHexagonCFGOptimizerPass(PassRegistry&);
  31
  32 } // end namespace llvm
  33
  34 namespace {
  35
  36 class HexagonCFGOptimizer : public MachineFunctionPass {
  37 private:
  38   void InvertAndChangeJumpTarget(MachineInstr &, MachineBasicBlock *);
  39   bool isOnFallThroughPath(MachineBasicBlock *MBB);
  40
  41 public:
  42   static char ID;
  43
  44   HexagonCFGOptimizer() : MachineFunctionPass(ID) {
  45     initializeHexagonCFGOptimizerPass(*PassRegistry::getPassRegistry());
  46   }
  47
  48   StringRef getPassName() const override { return "Hexagon CFG Optimizer"; }
  49   bool runOnMachineFunction(MachineFunction &Fn) override;
  50
  51   MachineFunctionProperties getRequiredProperties() const override {
  52     return MachineFunctionProperties().set(
  53         MachineFunctionProperties::Property::NoVRegs);
  54   }
  55 };
  56
  57 } // end anonymous namespace
  58
  59 char HexagonCFGOptimizer::ID = 0;
  60
  61 static bool IsConditionalBranch(int Opc) {
  62   switch (Opc) {
  63     case Hexagon::J2_jumpt:
  64     case Hexagon::J2_jumptpt:
  65     case Hexagon::J2_jumpf:
  66     case Hexagon::J2_jumpfpt:
  67     case Hexagon::J2_jumptnew:
  68     case Hexagon::J2_jumpfnew:
  69     case Hexagon::J2_jumptnewpt:
  70     case Hexagon::J2_jumpfnewpt:
  71       return true;
  72   }
  73   return false;
  74 }
  75
  76 static bool IsUnconditionalJump(int Opc) {
  77   return (Opc == Hexagon::J2_jump);
  78 }
  79
  80 void HexagonCFGOptimizer::InvertAndChangeJumpTarget(
  81     MachineInstr &MI, MachineBasicBlock *NewTarget) {
  82   const TargetInstrInfo *TII =
  83       MI.getParent()->getParent()->getSubtarget().getInstrInfo();
  84   int NewOpcode = 0;
  85   switch (MI.getOpcode()) {
  86   case Hexagon::J2_jumpt:
  87     NewOpcode = Hexagon::J2_jumpf;
  88     break;
  89   case Hexagon::J2_jumpf:
  90     NewOpcode = Hexagon::J2_jumpt;
  91     break;
  92   case Hexagon::J2_jumptnewpt:
  93     NewOpcode = Hexagon::J2_jumpfnewpt;
  94     break;
  95   case Hexagon::J2_jumpfnewpt:
  96     NewOpcode = Hexagon::J2_jumptnewpt;
  97     break;
  98   default:
  99     llvm_unreachable("Cannot handle this case");
 100   }
 101
 102   MI.setDesc(TII->get(NewOpcode));
 103   MI.getOperand(1).setMBB(NewTarget);
 104 }
 105
 106 bool HexagonCFGOptimizer::isOnFallThroughPath(MachineBasicBlock *MBB) {
 107   if (MBB->canFallThrough())
 108     return true;
 109   for (MachineBasicBlock *PB : MBB->predecessors())
 110     if (PB->isLayoutSuccessor(MBB) && PB->canFallThrough())
 111       return true;
 112   return false;
 113 }
 114
 115 bool HexagonCFGOptimizer::runOnMachineFunction(MachineFunction &Fn) {
 116   if (skipFunction(Fn.getFunction()))
 117     return false;
 118
 119   // Loop over all of the basic blocks.
 120   for (MachineFunction::iterator MBBb = Fn.begin(), MBBe = Fn.end();
 121        MBBb != MBBe; ++MBBb) {
 122     MachineBasicBlock *MBB = &*MBBb;
 123
 124     // Traverse the basic block.
 125     MachineBasicBlock::iterator MII = MBB->getFirstTerminator();
 126     if (MII != MBB->end()) {
 127       MachineInstr &MI = *MII;
 128       int Opc = MI.getOpcode();
 129       if (IsConditionalBranch(Opc)) {
 130         // (Case 1) Transform the code if the following condition occurs:
 131         //   BB1: if (p0) jump BB3
 132         //   ...falls-through to BB2 ...
 133         //   BB2: jump BB4
 134         //   ...next block in layout is BB3...
 135         //   BB3: ...
 136         //
 137         //  Transform this to:
 138         //  BB1: if (!p0) jump BB4
 139         //  Remove BB2
 140         //  BB3: ...
 141         //
 142         // (Case 2) A variation occurs when BB3 contains a JMP to BB4:
 143         //   BB1: if (p0) jump BB3
 144         //   ...falls-through to BB2 ...
 145         //   BB2: jump BB4
 146         //   ...other basic blocks ...
 147         //   BB4:
 148         //   ...not a fall-thru
 149         //   BB3: ...
 150         //     jump BB4
 151         //
 152         // Transform this to:
 153         //   BB1: if (!p0) jump BB4
 154         //   Remove BB2
 155         //   BB3: ...
 156         //   BB4: ...
 157         unsigned NumSuccs = MBB->succ_size();
 158         MachineBasicBlock::succ_iterator SI = MBB->succ_begin();
 159         MachineBasicBlock* FirstSucc = *SI;
 160         MachineBasicBlock* SecondSucc = *(++SI);
 161         MachineBasicBlock* LayoutSucc = nullptr;
 162         MachineBasicBlock* JumpAroundTarget = nullptr;
 163
 164         if (MBB->isLayoutSuccessor(FirstSucc)) {
 165           LayoutSucc = FirstSucc;
 166           JumpAroundTarget = SecondSucc;
 167         } else if (MBB->isLayoutSuccessor(SecondSucc)) {
 168           LayoutSucc = SecondSucc;
 169           JumpAroundTarget = FirstSucc;
 170         } else {
 171           // Odd case...cannot handle.
 172         }
 173
 174         // The target of the unconditional branch must be JumpAroundTarget.
 175         // TODO: If not, we should not invert the unconditional branch.
 176         MachineBasicBlock* CondBranchTarget = nullptr;
 177         if (MI.getOpcode() == Hexagon::J2_jumpt ||
 178             MI.getOpcode() == Hexagon::J2_jumpf) {
 179           CondBranchTarget = MI.getOperand(1).getMBB();
 180         }
 181
 182         if (!LayoutSucc || (CondBranchTarget != JumpAroundTarget)) {
 183           continue;
 184         }
 185
 186         if ((NumSuccs == 2) && LayoutSucc && (LayoutSucc->pred_size() == 1)) {
 187           // Ensure that BB2 has one instruction -- an unconditional jump.
 188           if ((LayoutSucc->size() == 1) &&
 189               IsUnconditionalJump(LayoutSucc->front().getOpcode())) {
 190             assert(JumpAroundTarget && "jump target is needed to process second basic block");
 191             MachineBasicBlock* UncondTarget =
 192               LayoutSucc->front().getOperand(0).getMBB();
 193             // Check if the layout successor of BB2 is BB3.
 194             bool case1 = LayoutSucc->isLayoutSuccessor(JumpAroundTarget);
 195             bool case2 = JumpAroundTarget->isSuccessor(UncondTarget) &&
 196               !JumpAroundTarget->empty() &&
 197               IsUnconditionalJump(JumpAroundTarget->back().getOpcode()) &&
 198               JumpAroundTarget->pred_size() == 1 &&
 199               JumpAroundTarget->succ_size() == 1;
 200
 201             if (case1 || case2) {
 202               InvertAndChangeJumpTarget(MI, UncondTarget);
 203               MBB->replaceSuccessor(JumpAroundTarget, UncondTarget);
 204
 205               // Remove the unconditional branch in LayoutSucc.
 206               LayoutSucc->erase(LayoutSucc->begin());
 207               LayoutSucc->replaceSuccessor(UncondTarget, JumpAroundTarget);
 208
 209               // This code performs the conversion for case 2, which moves
 210               // the block to the fall-thru case (BB3 in the code above).
 211               if (case2 && !case1) {
 212                 JumpAroundTarget->moveAfter(LayoutSucc);
 213                 // only move a block if it doesn't have a fall-thru. otherwise
 214                 // the CFG will be incorrect.
 215                 if (!isOnFallThroughPath(UncondTarget))
 216                   UncondTarget->moveAfter(JumpAroundTarget);
 217               }
 218
 219               // Correct live-in information. Is used by post-RA scheduler
 220               // The live-in to LayoutSucc is now all values live-in to
 221               // JumpAroundTarget.
 222               std::vector<MachineBasicBlock::RegisterMaskPair> OrigLiveIn(
 223                   LayoutSucc->livein_begin(), LayoutSucc->livein_end());
 224               std::vector<MachineBasicBlock::RegisterMaskPair> NewLiveIn(
 225                   JumpAroundTarget->livein_begin(),
 226                   JumpAroundTarget->livein_end());
 227               for (const auto &OrigLI : OrigLiveIn)
 228                 LayoutSucc->removeLiveIn(OrigLI.PhysReg);
 229               for (const auto &NewLI : NewLiveIn)
 230                 LayoutSucc->addLiveIn(NewLI);
 231             }
 232           }
 233         }
 234       }
 235     }
 236   }
 237   return true;
 238 }
 239
 240 //===----------------------------------------------------------------------===//
 241 //                         Public Constructor Functions
 242 //===----------------------------------------------------------------------===//
 243
 244 INITIALIZE_PASS(HexagonCFGOptimizer, "hexagon-cfg", "Hexagon CFG Optimizer",
 245                 false, false)
 246
 247 FunctionPass *llvm::createHexagonCFGOptimizer() {
 248   return new HexagonCFGOptimizer();
 249 }