lib/Target/MSP430/MSP430BranchSelector.cpp

   1 //===-- MSP430BranchSelector.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 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/CodeGen/MachineInstrBuilder.h"
  22 #include "llvm/CodeGen/MachineFunctionPass.h"
  23 #include "llvm/Target/TargetMachine.h"
  24 #include "llvm/ADT/Statistic.h"
  25 #include "llvm/Support/MathExtras.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 }