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