Home | History | Annotate | Line # | Download | only in Hexagon 
      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 (MachineFunction::iterator MBBb = Fn.begin(), MBBe = Fn.end();
    122        MBBb != MBBe; ++MBBb) {
    123     MachineBasicBlock *MBB = &*MBBb;
    124 
    125     // Traverse the basic block.
    126     MachineBasicBlock::iterator MII = MBB->getFirstTerminator();
    127     if (MII != MBB->end()) {
    128       MachineInstr &MI = *MII;
    129       int Opc = MI.getOpcode();
    130       if (IsConditionalBranch(Opc)) {
    131         // (Case 1) Transform the code if the following condition occurs:
    132         //   BB1: if (p0) jump BB3
    133         //   ...falls-through to BB2 ...
    134         //   BB2: jump BB4
    135         //   ...next block in layout is BB3...
    136         //   BB3: ...
    137         //
    138         //  Transform this to:
    139         //  BB1: if (!p0) jump BB4
    140         //  Remove BB2
    141         //  BB3: ...
    142         //
    143         // (Case 2) A variation occurs when BB3 contains a JMP to BB4:
    144         //   BB1: if (p0) jump BB3
    145         //   ...falls-through to BB2 ...
    146         //   BB2: jump BB4
    147         //   ...other basic blocks ...
    148         //   BB4:
    149         //   ...not a fall-thru
    150         //   BB3: ...
    151         //     jump BB4
    152         //
    153         // Transform this to:
    154         //   BB1: if (!p0) jump BB4
    155         //   Remove BB2
    156         //   BB3: ...
    157         //   BB4: ...
    158         unsigned NumSuccs = MBB->succ_size();
    159         MachineBasicBlock::succ_iterator SI = MBB->succ_begin();
    160         MachineBasicBlock* FirstSucc = *SI;
    161         MachineBasicBlock* SecondSucc = *(++SI);
    162         MachineBasicBlock* LayoutSucc = nullptr;
    163         MachineBasicBlock* JumpAroundTarget = nullptr;
    164 
    165         if (MBB->isLayoutSuccessor(FirstSucc)) {
    166           LayoutSucc = FirstSucc;
    167           JumpAroundTarget = SecondSucc;
    168         } else if (MBB->isLayoutSuccessor(SecondSucc)) {
    169           LayoutSucc = SecondSucc;
    170           JumpAroundTarget = FirstSucc;
    171         } else {
    172           // Odd case...cannot handle.
    173         }
    174 
    175         // The target of the unconditional branch must be JumpAroundTarget.
    176         // TODO: If not, we should not invert the unconditional branch.
    177         MachineBasicBlock* CondBranchTarget = nullptr;
    178         if (MI.getOpcode() == Hexagon::J2_jumpt ||
    179             MI.getOpcode() == Hexagon::J2_jumpf) {
    180           CondBranchTarget = MI.getOperand(1).getMBB();
    181         }
    182 
    183         if (!LayoutSucc || (CondBranchTarget != JumpAroundTarget)) {
    184           continue;
    185         }
    186 
    187         if ((NumSuccs == 2) && LayoutSucc && (LayoutSucc->pred_size() == 1)) {
    188           // Ensure that BB2 has one instruction -- an unconditional jump.
    189           if ((LayoutSucc->size() == 1) &&
    190               IsUnconditionalJump(LayoutSucc->front().getOpcode())) {
    191             assert(JumpAroundTarget && "jump target is needed to process second basic block");
    192             MachineBasicBlock* UncondTarget =
    193               LayoutSucc->front().getOperand(0).getMBB();
    194             // Check if the layout successor of BB2 is BB3.
    195             bool case1 = LayoutSucc->isLayoutSuccessor(JumpAroundTarget);
    196             bool case2 = JumpAroundTarget->isSuccessor(UncondTarget) &&
    197               !JumpAroundTarget->empty() &&
    198               IsUnconditionalJump(JumpAroundTarget->back().getOpcode()) &&
    199               JumpAroundTarget->pred_size() == 1 &&
    200               JumpAroundTarget->succ_size() == 1;
    201 
    202             if (case1 || case2) {
    203               InvertAndChangeJumpTarget(MI, UncondTarget);
    204               MBB->replaceSuccessor(JumpAroundTarget, UncondTarget);
    205 
    206               // Remove the unconditional branch in LayoutSucc.
    207               LayoutSucc->erase(LayoutSucc->begin());
    208               LayoutSucc->replaceSuccessor(UncondTarget, JumpAroundTarget);
    209 
    210               // This code performs the conversion for case 2, which moves
    211               // the block to the fall-thru case (BB3 in the code above).
    212               if (case2 && !case1) {
    213                 JumpAroundTarget->moveAfter(LayoutSucc);
    214                 // only move a block if it doesn't have a fall-thru. otherwise
    215                 // the CFG will be incorrect.
    216                 if (!isOnFallThroughPath(UncondTarget))
    217                   UncondTarget->moveAfter(JumpAroundTarget);
    218               }
    219 
    220               // Correct live-in information. Is used by post-RA scheduler
    221               // The live-in to LayoutSucc is now all values live-in to
    222               // JumpAroundTarget.
    223               std::vector<MachineBasicBlock::RegisterMaskPair> OrigLiveIn(
    224                   LayoutSucc->livein_begin(), LayoutSucc->livein_end());
    225               std::vector<MachineBasicBlock::RegisterMaskPair> NewLiveIn(
    226                   JumpAroundTarget->livein_begin(),
    227                   JumpAroundTarget->livein_end());
    228               for (const auto &OrigLI : OrigLiveIn)
    229                 LayoutSucc->removeLiveIn(OrigLI.PhysReg);
    230               for (const auto &NewLI : NewLiveIn)
    231                 LayoutSucc->addLiveIn(NewLI);
    232             }
    233           }
    234         }
    235       }
    236     }
    237   }
    238   return true;
    239 }
    240 
    241 //===----------------------------------------------------------------------===//
    242 //                         Public Constructor Functions
    243 //===----------------------------------------------------------------------===//
    244 
    245 INITIALIZE_PASS(HexagonCFGOptimizer, "hexagon-cfg", "Hexagon CFG Optimizer",
    246                 false, false)
    247 
    248 FunctionPass *llvm::createHexagonCFGOptimizer() {
    249   return new HexagonCFGOptimizer();
    250 }
    251