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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 "MSP430Subtarget.h"
     21 #include "llvm/ADT/Statistic.h"
     22 #include "llvm/CodeGen/MachineFunctionPass.h"
     23 #include "llvm/CodeGen/MachineInstrBuilder.h"
     24 #include "llvm/Support/MathExtras.h"
     25 #include "llvm/Target/TargetMachine.h"
     26 using namespace llvm;
     27 
     28 #define DEBUG_TYPE "msp430-branch-select"
     29 
     30 STATISTIC(NumExpanded, "Number of branches expanded to long format");
     31 
     32 namespace {
     33   struct MSP430BSel : public MachineFunctionPass {
     34     static char ID;
     35     MSP430BSel() : MachineFunctionPass(ID) {}
     36 
     37     /// BlockSizes - The sizes of the basic blocks in the function.
     38     std::vector<unsigned> BlockSizes;
     39 
     40     bool runOnMachineFunction(MachineFunction &Fn) override;
     41 
     42     MachineFunctionProperties getRequiredProperties() const override {
     43       return MachineFunctionProperties().set(
     44           MachineFunctionProperties::Property::AllVRegsAllocated);
     45     }
     46 
     47     const char *getPassName() const override {
     48       return "MSP430 Branch Selector";
     49     }
     50   };
     51   char MSP430BSel::ID = 0;
     52 }
     53 
     54 /// createMSP430BranchSelectionPass - returns an instance of the Branch
     55 /// Selection Pass
     56 ///
     57 FunctionPass *llvm::createMSP430BranchSelectionPass() {
     58   return new MSP430BSel();
     59 }
     60 
     61 bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
     62   const MSP430InstrInfo *TII =
     63       static_cast<const MSP430InstrInfo *>(Fn.getSubtarget().getInstrInfo());
     64   // Give the blocks of the function a dense, in-order, numbering.
     65   Fn.RenumberBlocks();
     66   BlockSizes.resize(Fn.getNumBlockIDs());
     67 
     68   // Measure each MBB and compute a size for the entire function.
     69   unsigned FuncSize = 0;
     70   for (MachineBasicBlock &MBB : Fn) {
     71     unsigned BlockSize = 0;
     72     for (MachineInstr &MI : MBB)
     73       BlockSize += TII->GetInstSizeInBytes(MI);
     74 
     75     BlockSizes[MBB.getNumber()] = BlockSize;
     76     FuncSize += BlockSize;
     77   }
     78 
     79   // If the entire function is smaller than the displacement of a branch field,
     80   // we know we don't need to shrink any branches in this function.  This is a
     81   // common case.
     82   if (FuncSize < (1 << 9)) {
     83     BlockSizes.clear();
     84     return false;
     85   }
     86 
     87   // For each conditional branch, if the offset to its destination is larger
     88   // than the offset field allows, transform it into a long branch sequence
     89   // like this:
     90   //   short branch:
     91   //     bCC MBB
     92   //   long branch:
     93   //     b!CC $PC+6
     94   //     b MBB
     95   //
     96   bool MadeChange = true;
     97   bool EverMadeChange = false;
     98   while (MadeChange) {
     99     // Iteratively expand branches until we reach a fixed point.
    100     MadeChange = false;
    101 
    102     for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
    103          ++MFI) {
    104       MachineBasicBlock &MBB = *MFI;
    105       unsigned MBBStartOffset = 0;
    106       for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
    107            I != E; ++I) {
    108         if ((I->getOpcode() != MSP430::JCC || I->getOperand(0).isImm()) &&
    109             I->getOpcode() != MSP430::JMP) {
    110           MBBStartOffset += TII->GetInstSizeInBytes(*I);
    111           continue;
    112         }
    113 
    114         // Determine the offset from the current branch to the destination
    115         // block.
    116         MachineBasicBlock *Dest = I->getOperand(0).getMBB();
    117 
    118         int BranchSize;
    119         if (Dest->getNumber() <= MBB.getNumber()) {
    120           // If this is a backwards branch, the delta is the offset from the
    121           // start of this block to this branch, plus the sizes of all blocks
    122           // from this block to the dest.
    123           BranchSize = MBBStartOffset;
    124 
    125           for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
    126             BranchSize += BlockSizes[i];
    127         } else {
    128           // Otherwise, add the size of the blocks between this block and the
    129           // dest to the number of bytes left in this block.
    130           BranchSize = -MBBStartOffset;
    131 
    132           for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
    133             BranchSize += BlockSizes[i];
    134         }
    135 
    136         // If this branch is in range, ignore it.
    137         if (isInt<10>(BranchSize)) {
    138           MBBStartOffset += 2;
    139           continue;
    140         }
    141 
    142         // Otherwise, we have to expand it to a long branch.
    143         unsigned NewSize;
    144         MachineInstr &OldBranch = *I;
    145         DebugLoc dl = OldBranch.getDebugLoc();
    146 
    147         if (I->getOpcode() == MSP430::JMP) {
    148           NewSize = 4;
    149         } else {
    150           // The BCC operands are:
    151           // 0. MSP430 branch predicate
    152           // 1. Target MBB
    153           SmallVector<MachineOperand, 1> Cond;
    154           Cond.push_back(I->getOperand(1));
    155 
    156           // Jump over the uncond branch inst (i.e. $+6) on opposite condition.
    157           TII->ReverseBranchCondition(Cond);
    158           BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
    159             .addImm(4).addOperand(Cond[0]);
    160 
    161           NewSize = 6;
    162         }
    163         // Uncond branch to the real destination.
    164         I = BuildMI(MBB, I, dl, TII->get(MSP430::Bi)).addMBB(Dest);
    165 
    166         // Remove the old branch from the function.
    167         OldBranch.eraseFromParent();
    168 
    169         // Remember that this instruction is NewSize bytes, increase the size of the
    170         // block by NewSize-2, remember to iterate.
    171         BlockSizes[MBB.getNumber()] += NewSize-2;
    172         MBBStartOffset += NewSize;
    173 
    174         ++NumExpanded;
    175         MadeChange = true;
    176       }
    177     }
    178     EverMadeChange |= MadeChange;
    179   }
    180 
    181   BlockSizes.clear();
    182   return true;
    183 }
    184