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