PPCBranchSelector.cpp revision a4f0b3a084d120cfc5b5bb06f64b222f5cb72740 
1//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by Nate Baegeman and is distributed under the
6// University of Illinois Open Source 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 psuedo op to machine branch opcode pass.  This
14// pass should be run last, just before the assembly printer.
15//
16//===----------------------------------------------------------------------===//
17
18#include "PPC.h"
19#include "PPCInstrBuilder.h"
20#include "PPCInstrInfo.h"
21#include "llvm/CodeGen/MachineFunctionPass.h"
22#include "llvm/Support/Compiler.h"
23#include <map>
24using namespace llvm;
25
26namespace {
27  struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
28    // OffsetMap - Mapping between BB and byte offset from start of function
29    std::map<MachineBasicBlock*, unsigned> OffsetMap;
30
31    virtual bool runOnMachineFunction(MachineFunction &Fn);
32
33    virtual const char *getPassName() const {
34      return "PowerPC Branch Selection";
35    }
36  };
37}
38
39/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
40/// Pass
41///
42FunctionPass *llvm::createPPCBranchSelectionPass() {
43  return new PPCBSel();
44}
45
46/// getNumBytesForInstruction - Return the number of bytes of code the specified
47/// instruction may be.  This returns the maximum number of bytes.
48///
49static unsigned getNumBytesForInstruction(MachineInstr *MI) {
50  switch (MI->getOpcode()) {
51  case PPC::COND_BRANCH:
52    // while this will be 4 most of the time, if we emit 8 it is just a
53    // minor pessimization that saves us from having to worry about
54    // keeping the offsets up to date later when we emit long branch glue.
55    return 8;
56  case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
57  case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
58  case PPC::IMPLICIT_DEF_F4: // no asm emitted
59  case PPC::IMPLICIT_DEF_F8: // no asm emitted
60    return 0;
61  case PPC::INLINEASM:    // Inline Asm: Variable size.
62    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i)
63      if (MI->getOperand(i).isExternalSymbol()) {
64        const char *AsmStr = MI->getOperand(i).getSymbolName();
65        // Count the number of newline's in the asm string.
66        unsigned NumInstrs = 0;
67        for (; *AsmStr; ++AsmStr)
68          NumInstrs += *AsmStr == '\n';
69        return NumInstrs*4;
70      }
71    assert(0 && "INLINEASM didn't have format string??");
72  default:
73    return 4; // PowerPC instructions are all 4 bytes
74  }
75}
76
77
78bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
79  // Running total of instructions encountered since beginning of function
80  unsigned ByteCount = 0;
81
82  // For each MBB, add its offset to the offset map, and count up its
83  // instructions
84  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
85       ++MFI) {
86    MachineBasicBlock *MBB = MFI;
87    OffsetMap[MBB] = ByteCount;
88
89    for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
90         MBBI != EE; ++MBBI)
91      ByteCount += getNumBytesForInstruction(MBBI);
92  }
93
94  // We're about to run over the MBB's again, so reset the ByteCount
95  ByteCount = 0;
96
97  // For each MBB, find the conditional branch pseudo instructions, and
98  // calculate the difference between the target MBB and the current ICount
99  // to decide whether or not to emit a short or long branch.
100  //
101  // short branch:
102  // bCC .L_TARGET_MBB
103  //
104  // long branch:
105  // bInverseCC $PC+8
106  // b .L_TARGET_MBB
107  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
108       ++MFI) {
109    MachineBasicBlock *MBB = MFI;
110
111    for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
112         MBBI != EE; ++MBBI) {
113      // We may end up deleting the MachineInstr that MBBI points to, so
114      // remember its opcode now so we can refer to it after calling erase()
115      unsigned ByteSize = getNumBytesForInstruction(MBBI);
116      if (MBBI->getOpcode() == PPC::COND_BRANCH) {
117        MachineBasicBlock::iterator MBBJ = MBBI;
118        ++MBBJ;
119
120        // condbranch operands:
121        // 0. CR0 register
122        // 1. bc opcode
123        // 2. target MBB
124        // 3. fallthrough MBB
125        MachineBasicBlock *trueMBB =
126          MBBI->getOperand(2).getMachineBasicBlock();
127
128        int Displacement = OffsetMap[trueMBB] - ByteCount;
129        unsigned Opcode = MBBI->getOperand(1).getImmedValue();
130        unsigned CRReg = MBBI->getOperand(0).getReg();
131        unsigned Inverted = PPCInstrInfo::invertPPCBranchOpcode(Opcode);
132
133        if (Displacement >= -32768 && Displacement <= 32767) {
134          BuildMI(*MBB, MBBJ, Opcode, 2).addReg(CRReg).addMBB(trueMBB);
135        } else {
136          // Long branch, skip next branch instruction (i.e. $PC+8).
137          BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addImm(2);
138          BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB);
139        }
140
141        // Erase the psuedo COND_BRANCH instruction, and then back up the
142        // iterator so that when the for loop increments it, we end up in
143        // the correct place rather than iterating off the end.
144        MBB->erase(MBBI);
145        MBBI = --MBBJ;
146      }
147      ByteCount += ByteSize;
148    }
149  }
150
151  OffsetMap.clear();
152  return true;
153}
154
155