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