LLVM  3.7.0
PPCBranchSelector.cpp
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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 #include "PPC.h"
20 #include "PPCInstrBuilder.h"
21 #include "PPCInstrInfo.h"
22 #include "llvm/ADT/Statistic.h"
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "ppc-branch-select"
30 
31 STATISTIC(NumExpanded, "Number of branches expanded to long format");
32 
33 namespace llvm {
35 }
36 
37 namespace {
38  struct PPCBSel : public MachineFunctionPass {
39  static char ID;
40  PPCBSel() : MachineFunctionPass(ID) {
42  }
43 
44  /// BlockSizes - The sizes of the basic blocks in the function.
45  std::vector<unsigned> BlockSizes;
46 
47  bool runOnMachineFunction(MachineFunction &Fn) override;
48 
49  const char *getPassName() const override {
50  return "PowerPC Branch Selector";
51  }
52  };
53  char PPCBSel::ID = 0;
54 }
55 
56 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
57  false, false)
58 
59 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
60 /// Pass
61 ///
63  return new PPCBSel();
64 }
65 
66 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
67  const PPCInstrInfo *TII =
68  static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
69  // Give the blocks of the function a dense, in-order, numbering.
70  Fn.RenumberBlocks();
71  BlockSizes.resize(Fn.getNumBlockIDs());
72 
73  auto GetAlignmentAdjustment =
74  [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
75  unsigned Align = MBB.getAlignment();
76  if (!Align)
77  return 0;
78 
79  unsigned AlignAmt = 1 << Align;
80  unsigned ParentAlign = MBB.getParent()->getAlignment();
81 
82  if (Align <= ParentAlign)
83  return OffsetToAlignment(Offset, AlignAmt);
84 
85  // The alignment of this MBB is larger than the function's alignment, so we
86  // can't tell whether or not it will insert nops. Assume that it will.
87  return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
88  };
89 
90  // Measure each MBB and compute a size for the entire function.
91  unsigned FuncSize = 0;
92  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
93  ++MFI) {
94  MachineBasicBlock *MBB = MFI;
95 
96  // The end of the previous block may have extra nops if this block has an
97  // alignment requirement.
98  if (MBB->getNumber() > 0) {
99  unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
100  BlockSizes[MBB->getNumber()-1] += AlignExtra;
101  FuncSize += AlignExtra;
102  }
103 
104  unsigned BlockSize = 0;
105  for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
106  MBBI != EE; ++MBBI)
107  BlockSize += TII->GetInstSizeInBytes(MBBI);
108 
109  BlockSizes[MBB->getNumber()] = BlockSize;
110  FuncSize += BlockSize;
111  }
112 
113  // If the entire function is smaller than the displacement of a branch field,
114  // we know we don't need to shrink any branches in this function. This is a
115  // common case.
116  if (FuncSize < (1 << 15)) {
117  BlockSizes.clear();
118  return false;
119  }
120 
121  // For each conditional branch, if the offset to its destination is larger
122  // than the offset field allows, transform it into a long branch sequence
123  // like this:
124  // short branch:
125  // bCC MBB
126  // long branch:
127  // b!CC $PC+8
128  // b MBB
129  //
130  bool MadeChange = true;
131  bool EverMadeChange = false;
132  while (MadeChange) {
133  // Iteratively expand branches until we reach a fixed point.
134  MadeChange = false;
135 
136  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
137  ++MFI) {
138  MachineBasicBlock &MBB = *MFI;
139  unsigned MBBStartOffset = 0;
140  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
141  I != E; ++I) {
142  MachineBasicBlock *Dest = nullptr;
143  if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
144  Dest = I->getOperand(2).getMBB();
145  else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
146  !I->getOperand(1).isImm())
147  Dest = I->getOperand(1).getMBB();
148  else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
149  I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
150  !I->getOperand(0).isImm())
151  Dest = I->getOperand(0).getMBB();
152 
153  if (!Dest) {
154  MBBStartOffset += TII->GetInstSizeInBytes(I);
155  continue;
156  }
157 
158  // Determine the offset from the current branch to the destination
159  // block.
160  int BranchSize;
161  if (Dest->getNumber() <= MBB.getNumber()) {
162  // If this is a backwards branch, the delta is the offset from the
163  // start of this block to this branch, plus the sizes of all blocks
164  // from this block to the dest.
165  BranchSize = MBBStartOffset;
166 
167  for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
168  BranchSize += BlockSizes[i];
169  } else {
170  // Otherwise, add the size of the blocks between this block and the
171  // dest to the number of bytes left in this block.
172  BranchSize = -MBBStartOffset;
173 
174  for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
175  BranchSize += BlockSizes[i];
176  }
177 
178  // If this branch is in range, ignore it.
179  if (isInt<16>(BranchSize)) {
180  MBBStartOffset += 4;
181  continue;
182  }
183 
184  // Otherwise, we have to expand it to a long branch.
185  MachineInstr *OldBranch = I;
186  DebugLoc dl = OldBranch->getDebugLoc();
187 
188  if (I->getOpcode() == PPC::BCC) {
189  // The BCC operands are:
190  // 0. PPC branch predicate
191  // 1. CR register
192  // 2. Target MBB
193  PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
194  unsigned CRReg = I->getOperand(1).getReg();
195 
196  // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
197  BuildMI(MBB, I, dl, TII->get(PPC::BCC))
198  .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
199  } else if (I->getOpcode() == PPC::BC) {
200  unsigned CRBit = I->getOperand(0).getReg();
201  BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
202  } else if (I->getOpcode() == PPC::BCn) {
203  unsigned CRBit = I->getOperand(0).getReg();
204  BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
205  } else if (I->getOpcode() == PPC::BDNZ) {
206  BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
207  } else if (I->getOpcode() == PPC::BDNZ8) {
208  BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
209  } else if (I->getOpcode() == PPC::BDZ) {
210  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
211  } else if (I->getOpcode() == PPC::BDZ8) {
212  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
213  } else {
214  llvm_unreachable("Unhandled branch type!");
215  }
216 
217  // Uncond branch to the real destination.
218  I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
219 
220  // Remove the old branch from the function.
221  OldBranch->eraseFromParent();
222 
223  // Remember that this instruction is 8-bytes, increase the size of the
224  // block by 4, remember to iterate.
225  BlockSizes[MBB.getNumber()] += 4;
226  MBBStartOffset += 8;
227  ++NumExpanded;
228  MadeChange = true;
229  }
230  }
231  EverMadeChange |= MadeChange;
232  }
233 
234  BlockSizes.clear();
235  return true;
236 }
237 
const MachineFunction * getParent() const
getParent - Return the MachineFunction containing this basic block.
unsigned getAlignment() const
getAlignment - Return the alignment (log2, not bytes) of the function.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
STATISTIC(NumFunctions,"Total number of functions")
void RenumberBlocks(MachineBasicBlock *MBBFrom=nullptr)
RenumberBlocks - This discards all of the MachineBasicBlock numbers and recomputes them...
int getNumber() const
getNumber - MachineBasicBlocks are uniquely numbered at the function level, unless they're not in a M...
A debug info location.
Definition: DebugLoc.h:34
unsigned getNumBlockIDs() const
getNumBlockIDs - Return the number of MBB ID's allocated.
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
void initializePPCBSelPass(PassRegistry &)
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
Definition: ErrorHandling.h:98
void eraseFromParent()
Unlink 'this' from the containing basic block and delete it.
const MachineInstrBuilder & addImm(int64_t Val) const
addImm - Add a new immediate operand.
CHAIN = BDNZ CHAIN, DESTBB - These are used to create counter-based loops.
FunctionPass * createPPCBranchSelectionPass()
bundle_iterator< MachineInstr, instr_iterator > iterator
INITIALIZE_PASS(PPCBSel,"ppc-branch-select","PowerPC Branch Selector", false, false) FunctionPass *llvm
createPPCBranchSelectionPass - returns an instance of the Branch Selection Pass
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:294
MachineInstrBuilder BuildMI(MachineFunction &MF, DebugLoc DL, const MCInstrDesc &MCID)
BuildMI - Builder interface.
Predicate
Predicate - These are "(BI << 5) | BO" for various predicates.
Definition: PPCPredicates.h:27
static cl::opt< AlignMode > Align(cl::desc("Load/store alignment support"), cl::Hidden, cl::init(NoStrictAlign), cl::values(clEnumValN(StrictAlign,"aarch64-strict-align","Disallow all unaligned memory accesses"), clEnumValN(NoStrictAlign,"aarch64-no-strict-align","Allow unaligned memory accesses"), clEnumValEnd))
const DebugLoc & getDebugLoc() const
Returns the debug location id of this MachineInstr.
Definition: MachineInstr.h:238
Representation of each machine instruction.
Definition: MachineInstr.h:51
Predicate InvertPredicate(Predicate Opcode)
Invert the specified predicate. != -> ==, < -> >=.
#define I(x, y, z)
Definition: MD5.cpp:54
bool isInt< 16 >(int64_t x)
Definition: MathExtras.h:272
unsigned GetInstSizeInBytes(const MachineInstr *MI) const
GetInstSize - Return the number of bytes of code the specified instruction may be.
virtual const TargetInstrInfo * getInstrInfo() const
uint64_t OffsetToAlignment(uint64_t Value, uint64_t Align)
Returns the offset to the next integer (mod 2**64) that is greater than or equal to Value and is a mu...
Definition: MathExtras.h:616
BasicBlockListType::iterator iterator
PassRegistry - This class manages the registration and intitialization of the pass subsystem as appli...
Definition: PassRegistry.h:41
unsigned getAlignment() const
getAlignment - Return alignment of the basic block.