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      1 //===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//
      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 // This file contains a pass that scans a machine function to determine which
     11 // conditional branches need more than 16 bits of displacement to reach their
     12 // target basic block.  It does this in two passes; a calculation of basic block
     13 // positions pass, and a branch pseudo op to machine branch opcode pass.  This
     14 // pass should be run last, just before the assembly printer.
     15 //
     16 //===----------------------------------------------------------------------===//
     17 
     18 #include "PPC.h"
     19 #include "MCTargetDesc/PPCPredicates.h"
     20 #include "PPCInstrBuilder.h"
     21 #include "PPCInstrInfo.h"
     22 #include "llvm/ADT/Statistic.h"
     23 #include "llvm/CodeGen/MachineFunctionPass.h"
     24 #include "llvm/Support/MathExtras.h"
     25 #include "llvm/Target/TargetMachine.h"
     26 using namespace llvm;
     27 
     28 #define DEBUG_TYPE "ppc-branch-select"
     29 
     30 STATISTIC(NumExpanded, "Number of branches expanded to long format");
     31 
     32 namespace llvm {
     33   void initializePPCBSelPass(PassRegistry&);
     34 }
     35 
     36 namespace {
     37   struct PPCBSel : public MachineFunctionPass {
     38     static char ID;
     39     PPCBSel() : MachineFunctionPass(ID) {
     40       initializePPCBSelPass(*PassRegistry::getPassRegistry());
     41     }
     42 
     43     /// BlockSizes - The sizes of the basic blocks in the function.
     44     std::vector<unsigned> BlockSizes;
     45 
     46     bool runOnMachineFunction(MachineFunction &Fn) override;
     47 
     48     const char *getPassName() const override {
     49       return "PowerPC Branch Selector";
     50     }
     51   };
     52   char PPCBSel::ID = 0;
     53 }
     54 
     55 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
     56                 false, false)
     57 
     58 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
     59 /// Pass
     60 ///
     61 FunctionPass *llvm::createPPCBranchSelectionPass() {
     62   return new PPCBSel();
     63 }
     64 
     65 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
     66   const PPCInstrInfo *TII =
     67                 static_cast<const PPCInstrInfo*>(Fn.getTarget().getInstrInfo());
     68   // Give the blocks of the function a dense, in-order, numbering.
     69   Fn.RenumberBlocks();
     70   BlockSizes.resize(Fn.getNumBlockIDs());
     71 
     72   // Measure each MBB and compute a size for the entire function.
     73   unsigned FuncSize = 0;
     74   for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
     75        ++MFI) {
     76     MachineBasicBlock *MBB = MFI;
     77 
     78     unsigned BlockSize = 0;
     79     for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
     80          MBBI != EE; ++MBBI)
     81       BlockSize += TII->GetInstSizeInBytes(MBBI);
     82 
     83     BlockSizes[MBB->getNumber()] = BlockSize;
     84     FuncSize += BlockSize;
     85   }
     86 
     87   // If the entire function is smaller than the displacement of a branch field,
     88   // we know we don't need to shrink any branches in this function.  This is a
     89   // common case.
     90   if (FuncSize < (1 << 15)) {
     91     BlockSizes.clear();
     92     return false;
     93   }
     94 
     95   // For each conditional branch, if the offset to its destination is larger
     96   // than the offset field allows, transform it into a long branch sequence
     97   // like this:
     98   //   short branch:
     99   //     bCC MBB
    100   //   long branch:
    101   //     b!CC $PC+8
    102   //     b MBB
    103   //
    104   bool MadeChange = true;
    105   bool EverMadeChange = false;
    106   while (MadeChange) {
    107     // Iteratively expand branches until we reach a fixed point.
    108     MadeChange = false;
    109 
    110     for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
    111          ++MFI) {
    112       MachineBasicBlock &MBB = *MFI;
    113       unsigned MBBStartOffset = 0;
    114       for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
    115            I != E; ++I) {
    116         MachineBasicBlock *Dest = nullptr;
    117         if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
    118           Dest = I->getOperand(2).getMBB();
    119         else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
    120                  !I->getOperand(1).isImm())
    121           Dest = I->getOperand(1).getMBB();
    122         else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
    123                   I->getOpcode() == PPC::BDZ8  || I->getOpcode() == PPC::BDZ) &&
    124                  !I->getOperand(0).isImm())
    125           Dest = I->getOperand(0).getMBB();
    126 
    127         if (!Dest) {
    128           MBBStartOffset += TII->GetInstSizeInBytes(I);
    129           continue;
    130         }
    131 
    132         // Determine the offset from the current branch to the destination
    133         // block.
    134         int BranchSize;
    135         if (Dest->getNumber() <= MBB.getNumber()) {
    136           // If this is a backwards branch, the delta is the offset from the
    137           // start of this block to this branch, plus the sizes of all blocks
    138           // from this block to the dest.
    139           BranchSize = MBBStartOffset;
    140 
    141           for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
    142             BranchSize += BlockSizes[i];
    143         } else {
    144           // Otherwise, add the size of the blocks between this block and the
    145           // dest to the number of bytes left in this block.
    146           BranchSize = -MBBStartOffset;
    147 
    148           for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
    149             BranchSize += BlockSizes[i];
    150         }
    151 
    152         // If this branch is in range, ignore it.
    153         if (isInt<16>(BranchSize)) {
    154           MBBStartOffset += 4;
    155           continue;
    156         }
    157 
    158         // Otherwise, we have to expand it to a long branch.
    159         MachineInstr *OldBranch = I;
    160         DebugLoc dl = OldBranch->getDebugLoc();
    161 
    162         if (I->getOpcode() == PPC::BCC) {
    163           // The BCC operands are:
    164           // 0. PPC branch predicate
    165           // 1. CR register
    166           // 2. Target MBB
    167           PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
    168           unsigned CRReg = I->getOperand(1).getReg();
    169 
    170           // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
    171           BuildMI(MBB, I, dl, TII->get(PPC::BCC))
    172             .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
    173         } else if (I->getOpcode() == PPC::BC) {
    174           unsigned CRBit = I->getOperand(0).getReg();
    175           BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
    176         } else if (I->getOpcode() == PPC::BCn) {
    177           unsigned CRBit = I->getOperand(0).getReg();
    178           BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
    179         } else if (I->getOpcode() == PPC::BDNZ) {
    180           BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
    181         } else if (I->getOpcode() == PPC::BDNZ8) {
    182           BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
    183         } else if (I->getOpcode() == PPC::BDZ) {
    184           BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
    185         } else if (I->getOpcode() == PPC::BDZ8) {
    186           BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
    187         } else {
    188            llvm_unreachable("Unhandled branch type!");
    189         }
    190 
    191         // Uncond branch to the real destination.
    192         I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
    193 
    194         // Remove the old branch from the function.
    195         OldBranch->eraseFromParent();
    196 
    197         // Remember that this instruction is 8-bytes, increase the size of the
    198         // block by 4, remember to iterate.
    199         BlockSizes[MBB.getNumber()] += 4;
    200         MBBStartOffset += 8;
    201         ++NumExpanded;
    202         MadeChange = true;
    203       }
    204     }
    205     EverMadeChange |= MadeChange;
    206   }
    207 
    208   BlockSizes.clear();
    209   return true;
    210 }
    211 
    212