PPCBranchSelector.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338 
1//===-- PPCBranchSelector.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 16 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 "ppc-branch-select"
19#include "PPC.h"
20#include "MCTargetDesc/PPCPredicates.h"
21#include "PPCInstrBuilder.h"
22#include "PPCInstrInfo.h"
23#include "llvm/ADT/Statistic.h"
24#include "llvm/CodeGen/MachineFunctionPass.h"
25#include "llvm/Support/MathExtras.h"
26#include "llvm/Target/TargetMachine.h"
27using namespace llvm;
28
29STATISTIC(NumExpanded, "Number of branches expanded to long format");
30
31namespace llvm {
32  void initializePPCBSelPass(PassRegistry&);
33}
34
35namespace {
36  struct PPCBSel : public MachineFunctionPass {
37    static char ID;
38    PPCBSel() : MachineFunctionPass(ID) {
39      initializePPCBSelPass(*PassRegistry::getPassRegistry());
40    }
41
42    /// BlockSizes - The sizes of the basic blocks in the function.
43    std::vector<unsigned> BlockSizes;
44
45    virtual bool runOnMachineFunction(MachineFunction &Fn);
46
47    virtual const char *getPassName() const {
48      return "PowerPC Branch Selector";
49    }
50  };
51  char PPCBSel::ID = 0;
52}
53
54INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
55                false, false)
56
57/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
58/// Pass
59///
60FunctionPass *llvm::createPPCBranchSelectionPass() {
61  return new PPCBSel();
62}
63
64bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
65  const PPCInstrInfo *TII =
66                static_cast<const PPCInstrInfo*>(Fn.getTarget().getInstrInfo());
67  // Give the blocks of the function a dense, in-order, numbering.
68  Fn.RenumberBlocks();
69  BlockSizes.resize(Fn.getNumBlockIDs());
70
71  // Measure each MBB and compute a size for the entire function.
72  unsigned FuncSize = 0;
73  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
74       ++MFI) {
75    MachineBasicBlock *MBB = MFI;
76
77    unsigned BlockSize = 0;
78    for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
79         MBBI != EE; ++MBBI)
80      BlockSize += TII->GetInstSizeInBytes(MBBI);
81
82    BlockSizes[MBB->getNumber()] = BlockSize;
83    FuncSize += BlockSize;
84  }
85
86  // If the entire function is smaller than the displacement of a branch field,
87  // we know we don't need to shrink any branches in this function.  This is a
88  // common case.
89  if (FuncSize < (1 << 15)) {
90    BlockSizes.clear();
91    return false;
92  }
93
94  // For each conditional branch, if the offset to its destination is larger
95  // than the offset field allows, transform it into a long branch sequence
96  // like this:
97  //   short branch:
98  //     bCC MBB
99  //   long branch:
100  //     b!CC $PC+8
101  //     b MBB
102  //
103  bool MadeChange = true;
104  bool EverMadeChange = false;
105  while (MadeChange) {
106    // Iteratively expand branches until we reach a fixed point.
107    MadeChange = false;
108
109    for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
110         ++MFI) {
111      MachineBasicBlock &MBB = *MFI;
112      unsigned MBBStartOffset = 0;
113      for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
114           I != E; ++I) {
115        MachineBasicBlock *Dest = 0;
116        if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
117          Dest = I->getOperand(2).getMBB();
118        else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
119                 !I->getOperand(1).isImm())
120          Dest = I->getOperand(1).getMBB();
121        else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
122                  I->getOpcode() == PPC::BDZ8  || I->getOpcode() == PPC::BDZ) &&
123                 !I->getOperand(0).isImm())
124          Dest = I->getOperand(0).getMBB();
125
126        if (!Dest) {
127          MBBStartOffset += TII->GetInstSizeInBytes(I);
128          continue;
129        }
130
131        // Determine the offset from the current branch to the destination
132        // block.
133        int BranchSize;
134        if (Dest->getNumber() <= MBB.getNumber()) {
135          // If this is a backwards branch, the delta is the offset from the
136          // start of this block to this branch, plus the sizes of all blocks
137          // from this block to the dest.
138          BranchSize = MBBStartOffset;
139
140          for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
141            BranchSize += BlockSizes[i];
142        } else {
143          // Otherwise, add the size of the blocks between this block and the
144          // dest to the number of bytes left in this block.
145          BranchSize = -MBBStartOffset;
146
147          for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
148            BranchSize += BlockSizes[i];
149        }
150
151        // If this branch is in range, ignore it.
152        if (isInt<16>(BranchSize)) {
153          MBBStartOffset += 4;
154          continue;
155        }
156
157        // Otherwise, we have to expand it to a long branch.
158        MachineInstr *OldBranch = I;
159        DebugLoc dl = OldBranch->getDebugLoc();
160
161        if (I->getOpcode() == PPC::BCC) {
162          // The BCC operands are:
163          // 0. PPC branch predicate
164          // 1. CR register
165          // 2. Target MBB
166          PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
167          unsigned CRReg = I->getOperand(1).getReg();
168
169          // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
170          BuildMI(MBB, I, dl, TII->get(PPC::BCC))
171            .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
172        } else if (I->getOpcode() == PPC::BC) {
173          unsigned CRBit = I->getOperand(0).getReg();
174          BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
175        } else if (I->getOpcode() == PPC::BCn) {
176          unsigned CRBit = I->getOperand(0).getReg();
177          BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
178        } else if (I->getOpcode() == PPC::BDNZ) {
179          BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
180        } else if (I->getOpcode() == PPC::BDNZ8) {
181          BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
182        } else if (I->getOpcode() == PPC::BDZ) {
183          BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
184        } else if (I->getOpcode() == PPC::BDZ8) {
185          BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
186        } else {
187           llvm_unreachable("Unhandled branch type!");
188        }
189
190        // Uncond branch to the real destination.
191        I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
192
193        // Remove the old branch from the function.
194        OldBranch->eraseFromParent();
195
196        // Remember that this instruction is 8-bytes, increase the size of the
197        // block by 4, remember to iterate.
198        BlockSizes[MBB.getNumber()] += 4;
199        MBBStartOffset += 8;
200        ++NumExpanded;
201        MadeChange = true;
202      }
203    }
204    EverMadeChange |= MadeChange;
205  }
206
207  BlockSizes.clear();
208  return true;
209}
210
211