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      1 //===-- MSP430BranchSelector.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 10 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 "MSP430.h"
     19 #include "MSP430InstrInfo.h"
     20 #include "llvm/ADT/Statistic.h"
     21 #include "llvm/CodeGen/MachineFunctionPass.h"
     22 #include "llvm/CodeGen/MachineInstrBuilder.h"
     23 #include "llvm/Support/MathExtras.h"
     24 #include "llvm/Target/TargetMachine.h"
     25 using namespace llvm;
     26 
     27 #define DEBUG_TYPE "msp430-branch-select"
     28 
     29 STATISTIC(NumExpanded, "Number of branches expanded to long format");
     30 
     31 namespace {
     32   struct MSP430BSel : public MachineFunctionPass {
     33     static char ID;
     34     MSP430BSel() : MachineFunctionPass(ID) {}
     35 
     36     /// BlockSizes - The sizes of the basic blocks in the function.
     37     std::vector<unsigned> BlockSizes;
     38 
     39     bool runOnMachineFunction(MachineFunction &Fn) override;
     40 
     41     const char *getPassName() const override {
     42       return "MSP430 Branch Selector";
     43     }
     44   };
     45   char MSP430BSel::ID = 0;
     46 }
     47 
     48 /// createMSP430BranchSelectionPass - returns an instance of the Branch
     49 /// Selection Pass
     50 ///
     51 FunctionPass *llvm::createMSP430BranchSelectionPass() {
     52   return new MSP430BSel();
     53 }
     54 
     55 bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
     56   const MSP430InstrInfo *TII =
     57              static_cast<const MSP430InstrInfo*>(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 << 9)) {
     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+6
     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() != MSP430::JCC || I->getOperand(0).isImm()) &&
    107             I->getOpcode() != MSP430::JMP) {
    108           MBBStartOffset += TII->GetInstSizeInBytes(I);
    109           continue;
    110         }
    111 
    112         // Determine the offset from the current branch to the destination
    113         // block.
    114         MachineBasicBlock *Dest = I->getOperand(0).getMBB();
    115 
    116         int BranchSize;
    117         if (Dest->getNumber() <= MBB.getNumber()) {
    118           // If this is a backwards branch, the delta is the offset from the
    119           // start of this block to this branch, plus the sizes of all blocks
    120           // from this block to the dest.
    121           BranchSize = MBBStartOffset;
    122 
    123           for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
    124             BranchSize += BlockSizes[i];
    125         } else {
    126           // Otherwise, add the size of the blocks between this block and the
    127           // dest to the number of bytes left in this block.
    128           BranchSize = -MBBStartOffset;
    129 
    130           for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
    131             BranchSize += BlockSizes[i];
    132         }
    133 
    134         // If this branch is in range, ignore it.
    135         if (isInt<10>(BranchSize)) {
    136           MBBStartOffset += 2;
    137           continue;
    138         }
    139 
    140         // Otherwise, we have to expand it to a long branch.
    141         unsigned NewSize;
    142         MachineInstr *OldBranch = I;
    143         DebugLoc dl = OldBranch->getDebugLoc();
    144 
    145         if (I->getOpcode() == MSP430::JMP) {
    146           NewSize = 4;
    147         } else {
    148           // The BCC operands are:
    149           // 0. MSP430 branch predicate
    150           // 1. Target MBB
    151           SmallVector<MachineOperand, 1> Cond;
    152           Cond.push_back(I->getOperand(1));
    153 
    154           // Jump over the uncond branch inst (i.e. $+6) on opposite condition.
    155           TII->ReverseBranchCondition(Cond);
    156           BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
    157             .addImm(4).addOperand(Cond[0]);
    158 
    159           NewSize = 6;
    160         }
    161         // Uncond branch to the real destination.
    162         I = BuildMI(MBB, I, dl, TII->get(MSP430::Bi)).addMBB(Dest);
    163 
    164         // Remove the old branch from the function.
    165         OldBranch->eraseFromParent();
    166 
    167         // Remember that this instruction is NewSize bytes, increase the size of the
    168         // block by NewSize-2, remember to iterate.
    169         BlockSizes[MBB.getNumber()] += NewSize-2;
    170         MBBStartOffset += NewSize;
    171 
    172         ++NumExpanded;
    173         MadeChange = true;
    174       }
    175     }
    176     EverMadeChange |= MadeChange;
    177   }
    178 
    179   BlockSizes.clear();
    180   return true;
    181 }
    182