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/MathExtras.h"
  24 #include "llvm/Target/TargetMachine.h"
  25 using namespace llvm;
  26
  27 #define DEBUG_TYPE "msp430-branch-select"
  28
  29 static cl::opt<bool>
  30     BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
  31                         cl::desc("Expand out of range branches"));
  32
  33 STATISTIC(NumSplit, "Number of machine basic blocks split");
  34 STATISTIC(NumExpanded, "Number of branches expanded to long format");
  35
  36 namespace {
  37 class MSP430BSel : public MachineFunctionPass {
  38
  39   typedef SmallVector<int, 16> OffsetVector;
  40
  41   MachineFunction *MF;
  42   const MSP430InstrInfo *TII;
  43
  44   unsigned measureFunction(OffsetVector &BlockOffsets,
  45                            MachineBasicBlock *FromBB = nullptr);
  46   bool expandBranches(OffsetVector &BlockOffsets);
  47
  48 public:
  49   static char ID;
  50   MSP430BSel() : MachineFunctionPass(ID) {}
  51
  52   bool runOnMachineFunction(MachineFunction &MF) override;
  53
  54   MachineFunctionProperties getRequiredProperties() const override {
  55     return MachineFunctionProperties().set(
  56         MachineFunctionProperties::Property::NoVRegs);
  57   }
  58
  59   StringRef getPassName() const override { return "MSP430 Branch Selector"; }
  60 };
  61 char MSP430BSel::ID = 0;
  62 }
  63
  64 static bool isInRage(int DistanceInBytes) {
  65   // According to CC430 Family User's Guide, Section 4.5.1.3, branch
  66   // instructions have the signed 10-bit word offset field, so first we need to
  67   // convert the distance from bytes to words, then check if it fits in 10-bit
  68   // signed integer.
  69   const int WordSize = 2;
  70
  71   assert((DistanceInBytes % WordSize == 0) &&
  72          "Branch offset should be word aligned!");
  73
  74   int Words = DistanceInBytes / WordSize;
  75   return isInt<10>(Words);
  76 }
  77
  78 /// Measure each basic block, fill the BlockOffsets, and return the size of
  79 /// the function, starting with BB
  80 unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
  81                                      MachineBasicBlock *FromBB) {
  82   // Give the blocks of the function a dense, in-order, numbering.
  83   MF->RenumberBlocks(FromBB);
  84
  85   MachineFunction::iterator Begin;
  86   if (FromBB == nullptr) {
  87     Begin = MF->begin();
  88   } else {
  89     Begin = FromBB->getIterator();
  90   }
  91
  92   BlockOffsets.resize(MF->getNumBlockIDs());
  93
  94   unsigned TotalSize = BlockOffsets[Begin->getNumber()];
  95   for (auto &MBB : make_range(Begin, MF->end())) {
  96     BlockOffsets[MBB.getNumber()] = TotalSize;
  97     for (MachineInstr &MI : MBB) {
  98       TotalSize += TII->getInstSizeInBytes(MI);
  99     }
 100   }
 101   return TotalSize;
 102 }
 103
 104 /// Do expand branches and split the basic blocks if necessary.
 105 /// Returns true if made any change.
 106 bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
 107   // For each conditional branch, if the offset to its destination is larger
 108   // than the offset field allows, transform it into a long branch sequence
 109   // like this:
 110   //   short branch:
 111   //     bCC MBB
 112   //   long branch:
 113   //     b!CC $PC+6
 114   //     b MBB
 115   //
 116   bool MadeChange = false;
 117   for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
 118     unsigned MBBStartOffset = 0;
 119     for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
 120       MBBStartOffset += TII->getInstSizeInBytes(*MI);
 121
 122       // If this instruction is not a short branch then skip it.
 123       if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
 124         continue;
 125       }
 126
 127       MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
 128       // Determine the distance from the current branch to the destination
 129       // block. MBBStartOffset already includes the size of the current branch
 130       // instruction.
 131       int BlockDistance =
 132           BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
 133       int BranchDistance = BlockDistance - MBBStartOffset;
 134
 135       // If this branch is in range, ignore it.
 136       if (isInRage(BranchDistance)) {
 137         continue;
 138       }
 139
 140       LLVM_DEBUG(dbgs() << "  Found a branch that needs expanding, "
 141                         << printMBBReference(*DestBB) << ", Distance "
 142                         << BranchDistance << "\n");
 143
 144       // If JCC is not the last instruction we need to split the MBB.
 145       if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
 146
 147         LLVM_DEBUG(dbgs() << "  Found a basic block that needs to be split, "
 148                           << printMBBReference(*MBB) << "\n");
 149
 150         // Create a new basic block.
 151         MachineBasicBlock *NewBB =
 152             MF->CreateMachineBasicBlock(MBB->getBasicBlock());
 153         MF->insert(std::next(MBB), NewBB);
 154
 155         // Splice the instructions following MI over to the NewBB.
 156         NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
 157
 158         // Update the successor lists.
 159         for (MachineBasicBlock *Succ : MBB->successors()) {
 160           if (Succ == DestBB) {
 161             continue;
 162           }
 163           MBB->replaceSuccessor(Succ, NewBB);
 164           NewBB->addSuccessor(Succ);
 165         }
 166
 167         // We introduced a new MBB so all following blocks should be numbered
 168         // and measured again.
 169         measureFunction(BlockOffsets, &*MBB);
 170
 171         ++NumSplit;
 172
 173         // It may be not necessary to start all over at this point, but it's
 174         // safer do this anyway.
 175         return true;
 176       }
 177
 178       MachineInstr &OldBranch = *MI;
 179       DebugLoc dl = OldBranch.getDebugLoc();
 180       int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
 181
 182       if (MI->getOpcode() == MSP430::JCC) {
 183         MachineBasicBlock *NextMBB = &*std::next(MBB);
 184         assert(MBB->isSuccessor(NextMBB) &&
 185                "This block must have a layout successor!");
 186
 187         // The BCC operands are:
 188         // 0. Target MBB
 189         // 1. MSP430 branch predicate
 190         SmallVector<MachineOperand, 1> Cond;
 191         Cond.push_back(MI->getOperand(1));
 192
 193         // Jump over the long branch on the opposite condition
 194         TII->reverseBranchCondition(Cond);
 195         MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
 196                  .addMBB(NextMBB)
 197                  .add(Cond[0]);
 198         InstrSizeDiff += TII->getInstSizeInBytes(*MI);
 199         ++MI;
 200       }
 201
 202       // Unconditional branch to the real destination.
 203       MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
 204       InstrSizeDiff += TII->getInstSizeInBytes(*MI);
 205
 206       // Remove the old branch from the function.
 207       OldBranch.eraseFromParent();
 208
 209       // The size of a new instruction is different from the old one, so we need
 210       // to correct all block offsets.
 211       for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
 212         BlockOffsets[i] += InstrSizeDiff;
 213       }
 214       MBBStartOffset += InstrSizeDiff;
 215
 216       ++NumExpanded;
 217       MadeChange = true;
 218     }
 219   }
 220   return MadeChange;
 221 }
 222
 223 bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
 224   MF = &mf;
 225   TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
 226
 227   // If the pass is disabled, just bail early.
 228   if (!BranchSelectEnabled)
 229     return false;
 230
 231   LLVM_DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
 232
 233   // BlockOffsets - Contains the distance from the beginning of the function to
 234   // the beginning of each basic block.
 235   OffsetVector BlockOffsets;
 236
 237   unsigned FunctionSize = measureFunction(BlockOffsets);
 238   // If the entire function is smaller than the displacement of a branch field,
 239   // we know we don't need to expand any branches in this
 240   // function. This is a common case.
 241   if (isInRage(FunctionSize)) {
 242     return false;
 243   }
 244
 245   // Iteratively expand branches until we reach a fixed point.
 246   bool MadeChange = false;
 247   while (expandBranches(BlockOffsets))
 248     MadeChange = true;
 249
 250   return MadeChange;
 251 }
 252
 253 /// Returns an instance of the Branch Selection Pass
 254 FunctionPass *llvm::createMSP430BranchSelectionPass() {
 255   return new MSP430BSel();
 256 }