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