llvm/lib/Target/MSP430/MSP430BranchSelector.cpp

   1 //===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file contains a pass that scans a machine function to determine which
  10 // conditional branches need more than 10 bits of displacement to reach their
  11 // target basic block.  It does this in two passes; a calculation of basic block
  12 // positions pass, and a branch pseudo op to machine branch opcode pass.  This
  13 // pass should be run last, just before the assembly printer.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "MSP430.h"
  18 #include "MSP430InstrInfo.h"
  19 #include "MSP430Subtarget.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/CodeGen/MachineFunctionPass.h"
  22 #include "llvm/CodeGen/MachineInstrBuilder.h"
  23 #include "llvm/Support/Debug.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 static cl::opt<bool>
  31     BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
  32                         cl::desc("Expand out of range branches"));
  33
  34 STATISTIC(NumSplit, "Number of machine basic blocks split");
  35 STATISTIC(NumExpanded, "Number of branches expanded to long format");
  36
  37 namespace {
  38 class MSP430BSel : public MachineFunctionPass {
  39
  40   typedef SmallVector<int, 16> OffsetVector;
  41
  42   MachineFunction *MF;
  43   const MSP430InstrInfo *TII;
  44
  45   unsigned measureFunction(OffsetVector &BlockOffsets,
  46                            MachineBasicBlock *FromBB = nullptr);
  47   bool expandBranches(OffsetVector &BlockOffsets);
  48
  49 public:
  50   static char ID;
  51   MSP430BSel() : MachineFunctionPass(ID) {}
  52
  53   bool runOnMachineFunction(MachineFunction &MF) override;
  54
  55   MachineFunctionProperties getRequiredProperties() const override {
  56     return MachineFunctionProperties().set(
  57         MachineFunctionProperties::Property::NoVRegs);
  58   }
  59
  60   StringRef getPassName() const override { return "MSP430 Branch Selector"; }
  61 };
  62 char MSP430BSel::ID = 0;
  63 }
  64
  65 static bool isInRage(int DistanceInBytes) {
  66   // According to CC430 Family User's Guide, Section 4.5.1.3, branch
  67   // instructions have the signed 10-bit word offset field, so first we need to
  68   // convert the distance from bytes to words, then check if it fits in 10-bit
  69   // signed integer.
  70   const int WordSize = 2;
  71
  72   assert((DistanceInBytes % WordSize == 0) &&
  73          "Branch offset should be word aligned!");
  74
  75   int Words = DistanceInBytes / WordSize;
  76   return isInt<10>(Words);
  77 }
  78
  79 /// Measure each basic block, fill the BlockOffsets, and return the size of
  80 /// the function, starting with BB
  81 unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
  82                                      MachineBasicBlock *FromBB) {
  83   // Give the blocks of the function a dense, in-order, numbering.
  84   MF->RenumberBlocks(FromBB);
  85
  86   MachineFunction::iterator Begin;
  87   if (FromBB == nullptr) {
  88     Begin = MF->begin();
  89   } else {
  90     Begin = FromBB->getIterator();
  91   }
  92
  93   BlockOffsets.resize(MF->getNumBlockIDs());
  94
  95   unsigned TotalSize = BlockOffsets[Begin->getNumber()];
  96   for (auto &MBB : make_range(Begin, MF->end())) {
  97     BlockOffsets[MBB.getNumber()] = TotalSize;
  98     for (MachineInstr &MI : MBB) {
  99       TotalSize += TII->getInstSizeInBytes(MI);
 100     }
 101   }
 102   return TotalSize;
 103 }
 104
 105 /// Do expand branches and split the basic blocks if necessary.
 106 /// Returns true if made any change.
 107 bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
 108   // For each conditional branch, if the offset to its destination is larger
 109   // than the offset field allows, transform it into a long branch sequence
 110   // like this:
 111   //   short branch:
 112   //     bCC MBB
 113   //   long branch:
 114   //     b!CC $PC+6
 115   //     b MBB
 116   //
 117   bool MadeChange = false;
 118   for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
 119     unsigned MBBStartOffset = 0;
 120     for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
 121       MBBStartOffset += TII->getInstSizeInBytes(*MI);
 122
 123       // If this instruction is not a short branch then skip it.
 124       if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
 125         continue;
 126       }
 127
 128       MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
 129       // Determine the distance from the current branch to the destination
 130       // block. MBBStartOffset already includes the size of the current branch
 131       // instruction.
 132       int BlockDistance =
 133           BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
 134       int BranchDistance = BlockDistance - MBBStartOffset;
 135
 136       // If this branch is in range, ignore it.
 137       if (isInRage(BranchDistance)) {
 138         continue;
 139       }
 140
 141       LLVM_DEBUG(dbgs() << "  Found a branch that needs expanding, "
 142                         << printMBBReference(*DestBB) << ", Distance "
 143                         << BranchDistance << "\n");
 144
 145       // If JCC is not the last instruction we need to split the MBB.
 146       if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
 147
 148         LLVM_DEBUG(dbgs() << "  Found a basic block that needs to be split, "
 149                           << printMBBReference(*MBB) << "\n");
 150
 151         // Create a new basic block.
 152         MachineBasicBlock *NewBB =
 153             MF->CreateMachineBasicBlock(MBB->getBasicBlock());
 154         MF->insert(std::next(MBB), NewBB);
 155
 156         // Splice the instructions following MI over to the NewBB.
 157         NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
 158
 159         // Update the successor lists.
 160         for (MachineBasicBlock *Succ : MBB->successors()) {
 161           if (Succ == DestBB) {
 162             continue;
 163           }
 164           MBB->replaceSuccessor(Succ, NewBB);
 165           NewBB->addSuccessor(Succ);
 166         }
 167
 168         // We introduced a new MBB so all following blocks should be numbered
 169         // and measured again.
 170         measureFunction(BlockOffsets, &*MBB);
 171
 172         ++NumSplit;
 173
 174         // It may be not necessary to start all over at this point, but it's
 175         // safer do this anyway.
 176         return true;
 177       }
 178
 179       MachineInstr &OldBranch = *MI;
 180       DebugLoc dl = OldBranch.getDebugLoc();
 181       int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
 182
 183       if (MI->getOpcode() == MSP430::JCC) {
 184         MachineBasicBlock *NextMBB = &*std::next(MBB);
 185         assert(MBB->isSuccessor(NextMBB) &&
 186                "This block must have a layout successor!");
 187
 188         // The BCC operands are:
 189         // 0. Target MBB
 190         // 1. MSP430 branch predicate
 191         SmallVector<MachineOperand, 1> Cond;
 192         Cond.push_back(MI->getOperand(1));
 193
 194         // Jump over the long branch on the opposite condition
 195         TII->reverseBranchCondition(Cond);
 196         MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
 197                  .addMBB(NextMBB)
 198                  .add(Cond[0]);
 199         InstrSizeDiff += TII->getInstSizeInBytes(*MI);
 200         ++MI;
 201       }
 202
 203       // Unconditional branch to the real destination.
 204       MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
 205       InstrSizeDiff += TII->getInstSizeInBytes(*MI);
 206
 207       // Remove the old branch from the function.
 208       OldBranch.eraseFromParent();
 209
 210       // The size of a new instruction is different from the old one, so we need
 211       // to correct all block offsets.
 212       for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
 213         BlockOffsets[i] += InstrSizeDiff;
 214       }
 215       MBBStartOffset += InstrSizeDiff;
 216
 217       ++NumExpanded;
 218       MadeChange = true;
 219     }
 220   }
 221   return MadeChange;
 222 }
 223
 224 bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
 225   MF = &mf;
 226   TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
 227
 228   // If the pass is disabled, just bail early.
 229   if (!BranchSelectEnabled)
 230     return false;
 231
 232   LLVM_DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
 233
 234   // BlockOffsets - Contains the distance from the beginning of the function to
 235   // the beginning of each basic block.
 236   OffsetVector BlockOffsets;
 237
 238   unsigned FunctionSize = measureFunction(BlockOffsets);
 239   // If the entire function is smaller than the displacement of a branch field,
 240   // we know we don't need to expand any branches in this
 241   // function. This is a common case.
 242   if (isInRage(FunctionSize)) {
 243     return false;
 244   }
 245
 246   // Iteratively expand branches until we reach a fixed point.
 247   bool MadeChange = false;
 248   while (expandBranches(BlockOffsets))
 249     MadeChange = true;
 250
 251   return MadeChange;
 252 }
 253
 254 /// Returns an instance of the Branch Selection Pass
 255 FunctionPass *llvm::createMSP430BranchSelectionPass() {
 256   return new MSP430BSel();
 257 }