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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         MachineBasicBlock *Dest = 0;
    116         if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
    117           Dest = I->getOperand(2).getMBB();
    118         else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
    119                   I->getOpcode() == PPC::BDZ8  || I->getOpcode() == PPC::BDZ) &&
    120                  !I->getOperand(0).isImm())
    121           Dest = I->getOperand(0).getMBB();
    122 
    123         if (!Dest) {
    124           MBBStartOffset += TII->GetInstSizeInBytes(I);
    125           continue;
    126         }
    127 
    128         // Determine the offset from the current branch to the destination
    129         // block.
    130         int BranchSize;
    131         if (Dest->getNumber() <= MBB.getNumber()) {
    132           // If this is a backwards branch, the delta is the offset from the
    133           // start of this block to this branch, plus the sizes of all blocks
    134           // from this block to the dest.
    135           BranchSize = MBBStartOffset;
    136 
    137           for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
    138             BranchSize += BlockSizes[i];
    139         } else {
    140           // Otherwise, add the size of the blocks between this block and the
    141           // dest to the number of bytes left in this block.
    142           BranchSize = -MBBStartOffset;
    143 
    144           for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
    145             BranchSize += BlockSizes[i];
    146         }
    147 
    148         // If this branch is in range, ignore it.
    149         if (isInt<16>(BranchSize)) {
    150           MBBStartOffset += 4;
    151           continue;
    152         }
    153 
    154         // Otherwise, we have to expand it to a long branch.
    155         MachineInstr *OldBranch = I;
    156         DebugLoc dl = OldBranch->getDebugLoc();
    157 
    158         if (I->getOpcode() == PPC::BCC) {
    159           // The BCC operands are:
    160           // 0. PPC branch predicate
    161           // 1. CR register
    162           // 2. Target MBB
    163           PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
    164           unsigned CRReg = I->getOperand(1).getReg();
    165 
    166           // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
    167           BuildMI(MBB, I, dl, TII->get(PPC::BCC))
    168             .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
    169         } else if (I->getOpcode() == PPC::BDNZ) {
    170           BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
    171         } else if (I->getOpcode() == PPC::BDNZ8) {
    172           BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
    173         } else if (I->getOpcode() == PPC::BDZ) {
    174           BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
    175         } else if (I->getOpcode() == PPC::BDZ8) {
    176           BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
    177         } else {
    178            llvm_unreachable("Unhandled branch type!");
    179         }
    180 
    181         // Uncond branch to the real destination.
    182         I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
    183 
    184         // Remove the old branch from the function.
    185         OldBranch->eraseFromParent();
    186 
    187         // Remember that this instruction is 8-bytes, increase the size of the
    188         // block by 4, remember to iterate.
    189         BlockSizes[MBB.getNumber()] += 4;
    190         MBBStartOffset += 8;
    191         ++NumExpanded;
    192         MadeChange = true;
    193       }
    194     }
    195     EverMadeChange |= MadeChange;
    196   }
    197 
    198   BlockSizes.clear();
    199   return true;
    200 }
    201 
    202