LLVM  9.0.0svn
PPCBranchSelector.cpp
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1 //===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file contains a pass that scans a machine function to determine which
10 // conditional branches need more than 16 bits of displacement to reach their
11 // target basic block. It does this in two passes; a calculation of basic block
12 // positions pass, and a branch pseudo op to machine branch opcode pass. This
13 // pass should be run last, just before the assembly printer.
14 //
15 //===----------------------------------------------------------------------===//
16 
18 #include "PPC.h"
19 #include "PPCInstrBuilder.h"
20 #include "PPCInstrInfo.h"
21 #include "PPCSubtarget.h"
22 #include "llvm/ADT/Statistic.h"
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "ppc-branch-select"
31 
32 STATISTIC(NumExpanded, "Number of branches expanded to long format");
33 
34 namespace llvm {
36 }
37 
38 namespace {
39  struct PPCBSel : public MachineFunctionPass {
40  static char ID;
41  PPCBSel() : MachineFunctionPass(ID) {
43  }
44 
45  // The sizes of the basic blocks in the function (the first
46  // element of the pair); the second element of the pair is the amount of the
47  // size that is due to potential padding.
48  std::vector<std::pair<unsigned, unsigned>> BlockSizes;
49 
50  bool runOnMachineFunction(MachineFunction &Fn) override;
51 
52  MachineFunctionProperties getRequiredProperties() const override {
55  }
56 
57  StringRef getPassName() const override { return "PowerPC Branch Selector"; }
58  };
59  char PPCBSel::ID = 0;
60 }
61 
62 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
63  false, false)
64 
65 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
66 /// Pass
67 ///
69  return new PPCBSel();
70 }
71 
72 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
73  const PPCInstrInfo *TII =
74  static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
75  // Give the blocks of the function a dense, in-order, numbering.
76  Fn.RenumberBlocks();
77  BlockSizes.resize(Fn.getNumBlockIDs());
78 
79  auto GetAlignmentAdjustment =
80  [](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
81  unsigned Align = MBB.getAlignment();
82  if (!Align)
83  return 0;
84 
85  unsigned AlignAmt = 1 << Align;
86  unsigned ParentAlign = MBB.getParent()->getAlignment();
87 
88  if (Align <= ParentAlign)
89  return OffsetToAlignment(Offset, AlignAmt);
90 
91  // The alignment of this MBB is larger than the function's alignment, so we
92  // can't tell whether or not it will insert nops. Assume that it will.
93  return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
94  };
95 
96  // We need to be careful about the offset of the first block in the function
97  // because it might not have the function's alignment. This happens because,
98  // under the ELFv2 ABI, for functions which require a TOC pointer, we add a
99  // two-instruction sequence to the start of the function.
100  // Note: This needs to be synchronized with the check in
101  // PPCLinuxAsmPrinter::EmitFunctionBodyStart.
102  unsigned InitialOffset = 0;
103  if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
104  !Fn.getRegInfo().use_empty(PPC::X2))
105  InitialOffset = 8;
106 
107  // Measure each MBB and compute a size for the entire function.
108  unsigned FuncSize = InitialOffset;
109  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
110  ++MFI) {
111  MachineBasicBlock *MBB = &*MFI;
112 
113  // The end of the previous block may have extra nops if this block has an
114  // alignment requirement.
115  if (MBB->getNumber() > 0) {
116  unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
117 
118  auto &BS = BlockSizes[MBB->getNumber()-1];
119  BS.first += AlignExtra;
120  BS.second = AlignExtra;
121 
122  FuncSize += AlignExtra;
123  }
124 
125  unsigned BlockSize = 0;
126  for (MachineInstr &MI : *MBB)
127  BlockSize += TII->getInstSizeInBytes(MI);
128 
129  BlockSizes[MBB->getNumber()].first = BlockSize;
130  FuncSize += BlockSize;
131  }
132 
133  // If the entire function is smaller than the displacement of a branch field,
134  // we know we don't need to shrink any branches in this function. This is a
135  // common case.
136  if (FuncSize < (1 << 15)) {
137  BlockSizes.clear();
138  return false;
139  }
140 
141  // For each conditional branch, if the offset to its destination is larger
142  // than the offset field allows, transform it into a long branch sequence
143  // like this:
144  // short branch:
145  // bCC MBB
146  // long branch:
147  // b!CC $PC+8
148  // b MBB
149  //
150  bool MadeChange = true;
151  bool EverMadeChange = false;
152  while (MadeChange) {
153  // Iteratively expand branches until we reach a fixed point.
154  MadeChange = false;
155 
156  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
157  ++MFI) {
158  MachineBasicBlock &MBB = *MFI;
159  unsigned MBBStartOffset = 0;
160  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
161  I != E; ++I) {
162  MachineBasicBlock *Dest = nullptr;
163  if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
164  Dest = I->getOperand(2).getMBB();
165  else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
166  !I->getOperand(1).isImm())
167  Dest = I->getOperand(1).getMBB();
168  else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
169  I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
170  !I->getOperand(0).isImm())
171  Dest = I->getOperand(0).getMBB();
172 
173  if (!Dest) {
174  MBBStartOffset += TII->getInstSizeInBytes(*I);
175  continue;
176  }
177 
178  // Determine the offset from the current branch to the destination
179  // block.
180  int BranchSize;
181  if (Dest->getNumber() <= MBB.getNumber()) {
182  // If this is a backwards branch, the delta is the offset from the
183  // start of this block to this branch, plus the sizes of all blocks
184  // from this block to the dest.
185  BranchSize = MBBStartOffset;
186 
187  for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
188  BranchSize += BlockSizes[i].first;
189  } else {
190  // Otherwise, add the size of the blocks between this block and the
191  // dest to the number of bytes left in this block.
192  BranchSize = -MBBStartOffset;
193 
194  for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
195  BranchSize += BlockSizes[i].first;
196  }
197 
198  // If this branch is in range, ignore it.
199  if (isInt<16>(BranchSize)) {
200  MBBStartOffset += 4;
201  continue;
202  }
203 
204  // Otherwise, we have to expand it to a long branch.
205  MachineInstr &OldBranch = *I;
206  DebugLoc dl = OldBranch.getDebugLoc();
207 
208  if (I->getOpcode() == PPC::BCC) {
209  // The BCC operands are:
210  // 0. PPC branch predicate
211  // 1. CR register
212  // 2. Target MBB
213  PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
214  unsigned CRReg = I->getOperand(1).getReg();
215 
216  // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
217  BuildMI(MBB, I, dl, TII->get(PPC::BCC))
218  .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
219  } else if (I->getOpcode() == PPC::BC) {
220  unsigned CRBit = I->getOperand(0).getReg();
221  BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
222  } else if (I->getOpcode() == PPC::BCn) {
223  unsigned CRBit = I->getOperand(0).getReg();
224  BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
225  } else if (I->getOpcode() == PPC::BDNZ) {
226  BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
227  } else if (I->getOpcode() == PPC::BDNZ8) {
228  BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
229  } else if (I->getOpcode() == PPC::BDZ) {
230  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
231  } else if (I->getOpcode() == PPC::BDZ8) {
232  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
233  } else {
234  llvm_unreachable("Unhandled branch type!");
235  }
236 
237  // Uncond branch to the real destination.
238  I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
239 
240  // Remove the old branch from the function.
241  OldBranch.eraseFromParent();
242 
243  // Remember that this instruction is 8-bytes, increase the size of the
244  // block by 4, remember to iterate.
245  BlockSizes[MBB.getNumber()].first += 4;
246  MBBStartOffset += 8;
247  ++NumExpanded;
248  MadeChange = true;
249  }
250  }
251 
252  if (MadeChange) {
253  // If we're going to iterate again, make sure we've updated our
254  // padding-based contributions to the block sizes.
255  unsigned Offset = InitialOffset;
256  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
257  ++MFI) {
258  MachineBasicBlock *MBB = &*MFI;
259 
260  if (MBB->getNumber() > 0) {
261  auto &BS = BlockSizes[MBB->getNumber()-1];
262  BS.first -= BS.second;
263  Offset -= BS.second;
264 
265  unsigned AlignExtra = GetAlignmentAdjustment(*MBB, Offset);
266 
267  BS.first += AlignExtra;
268  BS.second = AlignExtra;
269 
270  Offset += AlignExtra;
271  }
272 
273  Offset += BlockSizes[MBB->getNumber()].first;
274  }
275  }
276 
277  EverMadeChange |= MadeChange;
278  }
279 
280  BlockSizes.clear();
281  return true;
282 }
constexpr char Align[]
Key for Kernel::Arg::Metadata::mAlign.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
This class represents lattice values for constants.
Definition: AllocatorList.h:23
void RenumberBlocks(MachineBasicBlock *MBBFrom=nullptr)
RenumberBlocks - This discards all of the MachineBasicBlock numbers and recomputes them...
unsigned getNumBlockIDs() const
getNumBlockIDs - Return the number of MBB ID&#39;s allocated.
const DebugLoc & getDebugLoc() const
Returns the debug location id of this MachineInstr.
Definition: MachineInstr.h:382
constexpr bool isInt< 16 >(int64_t x)
Definition: MathExtras.h:305
STATISTIC(NumFunctions, "Total number of functions")
A debug info location.
Definition: DebugLoc.h:33
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
void initializePPCBSelPass(PassRegistry &)
void eraseFromParent()
Unlink &#39;this&#39; from the containing basic block and delete it.
unsigned getAlignment() const
getAlignment - Return the alignment (log2, not bytes) of the function.
CHAIN = BDNZ CHAIN, DESTBB - These are used to create counter-based loops.
bool isELFv2ABI() const
int getNumber() const
MachineBasicBlocks are uniquely numbered at the function level, unless they&#39;re not in a MachineFuncti...
virtual const TargetInstrInfo * getInstrInfo() const
FunctionPass * createPPCBranchSelectionPass()
MachineInstrBuilder BuildMI(MachineFunction &MF, const DebugLoc &DL, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
unsigned getAlignment() const
Return alignment of the basic block.
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:284
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
unsigned first
static const int BlockSize
Definition: TarWriter.cpp:33
Iterator for intrusive lists based on ilist_node.
Predicate
Predicate - These are "(BI << 5) | BO" for various predicates.
Definition: PPCPredicates.h:26
bool use_empty(unsigned RegNo) const
use_empty - Return true if there are no instructions using the specified register.
MachineFunctionProperties & set(Property P)
Representation of each machine instruction.
Definition: MachineInstr.h:63
const MachineFunction * getParent() const
Return the MachineFunction containing this basic block.
Predicate InvertPredicate(Predicate Opcode)
Invert the specified predicate. != -> ==, < -> >=.
const MachineInstrBuilder & addImm(int64_t Val) const
Add a new immediate operand.
MachineRegisterInfo & getRegInfo()
getRegInfo - Return information about the registers currently in use.
#define I(x, y, z)
Definition: MD5.cpp:58
unsigned getInstSizeInBytes(const MachineInstr &MI) const override
GetInstSize - Return the number of bytes of code the specified instruction may be.
INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector", false, false) FunctionPass *llvm
createPPCBranchSelectionPass - returns an instance of the Branch Selection Pass
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:726
IRTranslator LLVM IR MI
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:48
PassRegistry - This class manages the registration and intitialization of the pass subsystem as appli...
Definition: PassRegistry.h:38
Properties which a MachineFunction may have at a given point in time.