PPCBranchSelector.cpp revision 8aa797aa51cd4ea1ec6f46f4891a6897944b75b2 
1//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=//
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 psuedo 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 "PPCInstrBuilder.h"
21#include "PPCInstrInfo.h"
22#include "PPCPredicates.h"
23#include "llvm/CodeGen/MachineFunctionPass.h"
24#include "llvm/Target/TargetMachine.h"
25#include "llvm/Target/TargetAsmInfo.h"
26#include "llvm/ADT/Statistic.h"
27#include "llvm/Support/Compiler.h"
28#include "llvm/Support/MathExtras.h"
29using namespace llvm;
30
31STATISTIC(NumExpanded, "Number of branches expanded to long format");
32
33namespace {
34  struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
35    static char ID;
36    PPCBSel() : MachineFunctionPass((intptr_t)&ID) {}
37
38    /// BlockSizes - The sizes of the basic blocks in the function.
39    std::vector<unsigned> BlockSizes;
40
41    virtual bool runOnMachineFunction(MachineFunction &Fn);
42
43    virtual const char *getPassName() const {
44      return "PowerPC Branch Selector";
45    }
46  };
47  char PPCBSel::ID = 0;
48}
49
50/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
51/// Pass
52///
53FunctionPass *llvm::createPPCBranchSelectionPass() {
54  return new PPCBSel();
55}
56
57/// getNumBytesForInstruction - Return the number of bytes of code the specified
58/// instruction may be.  This returns the maximum number of bytes.
59///
60static unsigned getNumBytesForInstruction(MachineInstr *MI) {
61  switch (MI->getOpcode()) {
62  case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
63  case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
64  case PPC::IMPLICIT_DEF_F4:   // no asm emitted
65  case PPC::IMPLICIT_DEF_F8:   // no asm emitted
66  case PPC::IMPLICIT_DEF_VRRC: // no asm emitted
67    return 0;
68  case PPC::INLINEASM: {       // Inline Asm: Variable size.
69    MachineFunction *MF = MI->getParent()->getParent();
70    const char *AsmStr = MI->getOperand(0).getSymbolName();
71    return MF->getTarget().getTargetAsmInfo()->getInlineAsmLength(AsmStr);
72  }
73  case PPC::LABEL: {
74    return 0;
75  }
76  default:
77    return 4; // PowerPC instructions are all 4 bytes
78  }
79}
80
81
82bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
83  const TargetInstrInfo *TII = Fn.getTarget().getInstrInfo();
84  // Give the blocks of the function a dense, in-order, numbering.
85  Fn.RenumberBlocks();
86  BlockSizes.resize(Fn.getNumBlockIDs());
87
88  // Measure each MBB and compute a size for the entire function.
89  unsigned FuncSize = 0;
90  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
91       ++MFI) {
92    MachineBasicBlock *MBB = MFI;
93
94    unsigned BlockSize = 0;
95    for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
96         MBBI != EE; ++MBBI)
97      BlockSize += getNumBytesForInstruction(MBBI);
98
99    BlockSizes[MBB->getNumber()] = BlockSize;
100    FuncSize += BlockSize;
101  }
102
103  // If the entire function is smaller than the displacement of a branch field,
104  // we know we don't need to shrink any branches in this function.  This is a
105  // common case.
106  if (FuncSize < (1 << 15)) {
107    BlockSizes.clear();
108    return false;
109  }
110
111  // For each conditional branch, if the offset to its destination is larger
112  // than the offset field allows, transform it into a long branch sequence
113  // like this:
114  //   short branch:
115  //     bCC MBB
116  //   long branch:
117  //     b!CC $PC+8
118  //     b MBB
119  //
120  bool MadeChange = true;
121  bool EverMadeChange = false;
122  while (MadeChange) {
123    // Iteratively expand branches until we reach a fixed point.
124    MadeChange = false;
125
126    for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
127         ++MFI) {
128      MachineBasicBlock &MBB = *MFI;
129      unsigned MBBStartOffset = 0;
130      for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
131           I != E; ++I) {
132        if (I->getOpcode() != PPC::BCC || I->getOperand(2).isImmediate()) {
133          MBBStartOffset += getNumBytesForInstruction(I);
134          continue;
135        }
136
137        // Determine the offset from the current branch to the destination
138        // block.
139        MachineBasicBlock *Dest = I->getOperand(2).getMBB();
140
141        int BranchSize;
142        if (Dest->getNumber() <= MBB.getNumber()) {
143          // If this is a backwards branch, the delta is the offset from the
144          // start of this block to this branch, plus the sizes of all blocks
145          // from this block to the dest.
146          BranchSize = MBBStartOffset;
147
148          for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
149            BranchSize += BlockSizes[i];
150        } else {
151          // Otherwise, add the size of the blocks between this block and the
152          // dest to the number of bytes left in this block.
153          BranchSize = -MBBStartOffset;
154
155          for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
156            BranchSize += BlockSizes[i];
157        }
158
159        // If this branch is in range, ignore it.
160        if (isInt16(BranchSize)) {
161          MBBStartOffset += 4;
162          continue;
163        }
164
165        // Otherwise, we have to expand it to a long branch.
166        // The BCC operands are:
167        // 0. PPC branch predicate
168        // 1. CR register
169        // 2. Target MBB
170        PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
171        unsigned CRReg = I->getOperand(1).getReg();
172
173        MachineInstr *OldBranch = I;
174
175        // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
176        BuildMI(MBB, I, TII->get(PPC::BCC))
177          .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
178
179        // Uncond branch to the real destination.
180        I = BuildMI(MBB, I, TII->get(PPC::B)).addMBB(Dest);
181
182        // Remove the old branch from the function.
183        OldBranch->eraseFromParent();
184
185        // Remember that this instruction is 8-bytes, increase the size of the
186        // block by 4, remember to iterate.
187        BlockSizes[MBB.getNumber()] += 4;
188        MBBStartOffset += 8;
189        ++NumExpanded;
190        MadeChange = true;
191      }
192    }
193    EverMadeChange |= MadeChange;
194  }
195
196  BlockSizes.clear();
197  return true;
198}
199
200