LLVM  6.0.0svn
MSP430BranchSelector.cpp
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1 //===-- MSP430BranchSelector.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 10 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 "MSP430.h"
19 #include "MSP430InstrInfo.h"
20 #include "MSP430Subtarget.h"
21 #include "llvm/ADT/Statistic.h"
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "msp430-branch-select"
29 
30 static cl::opt<bool>
31  BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
32  cl::desc("Expand out of range branches"));
33 
34 STATISTIC(NumSplit, "Number of machine basic blocks split");
35 STATISTIC(NumExpanded, "Number of branches expanded to long format");
36 
37 namespace {
38 class MSP430BSel : public MachineFunctionPass {
39 
40  typedef SmallVector<int, 16> OffsetVector;
41 
42  MachineFunction *MF;
43  const MSP430InstrInfo *TII;
44 
45  unsigned measureFunction(OffsetVector &BlockOffsets,
46  MachineBasicBlock *FromBB = nullptr);
47  bool expandBranches(OffsetVector &BlockOffsets);
48 
49 public:
50  static char ID;
51  MSP430BSel() : MachineFunctionPass(ID) {}
52 
53  bool runOnMachineFunction(MachineFunction &MF) override;
54 
55  MachineFunctionProperties getRequiredProperties() const override {
58  }
59 
60  StringRef getPassName() const override { return "MSP430 Branch Selector"; }
61 };
62 char MSP430BSel::ID = 0;
63 }
64 
65 static bool isInRage(int DistanceInBytes) {
66  // According to CC430 Family User's Guide, Section 4.5.1.3, branch
67  // instructions have the signed 10-bit word offset field, so first we need to
68  // convert the distance from bytes to words, then check if it fits in 10-bit
69  // signed integer.
70  const int WordSize = 2;
71 
72  assert((DistanceInBytes % WordSize == 0) &&
73  "Branch offset should be word aligned!");
74 
75  int Words = DistanceInBytes / WordSize;
76  return isInt<10>(Words);
77 }
78 
79 /// Measure each basic block, fill the BlockOffsets, and return the size of
80 /// the function, starting with BB
81 unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
82  MachineBasicBlock *FromBB) {
83  // Give the blocks of the function a dense, in-order, numbering.
84  MF->RenumberBlocks(FromBB);
85 
87  if (FromBB == nullptr) {
88  Begin = MF->begin();
89  } else {
90  Begin = FromBB->getIterator();
91  }
92 
93  BlockOffsets.resize(MF->getNumBlockIDs());
94 
95  unsigned TotalSize = BlockOffsets[Begin->getNumber()];
96  for (auto &MBB : make_range(Begin, MF->end())) {
97  BlockOffsets[MBB.getNumber()] = TotalSize;
98  for (MachineInstr &MI : MBB) {
99  TotalSize += TII->getInstSizeInBytes(MI);
100  }
101  }
102  return TotalSize;
103 }
104 
105 /// Do expand branches and split the basic blocks if necessary.
106 /// Returns true if made any change.
107 bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
108  // For each conditional branch, if the offset to its destination is larger
109  // than the offset field allows, transform it into a long branch sequence
110  // like this:
111  // short branch:
112  // bCC MBB
113  // long branch:
114  // b!CC $PC+6
115  // b MBB
116  //
117  bool MadeChange = false;
118  for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
119  unsigned MBBStartOffset = 0;
120  for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
121  MBBStartOffset += TII->getInstSizeInBytes(*MI);
122 
123  // If this instruction is not a short branch then skip it.
124  if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
125  continue;
126  }
127 
128  MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
129  // Determine the distance from the current branch to the destination
130  // block. MBBStartOffset already includes the size of the current branch
131  // instruction.
132  int BlockDistance =
133  BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
134  int BranchDistance = BlockDistance - MBBStartOffset;
135 
136  // If this branch is in range, ignore it.
137  if (isInRage(BranchDistance)) {
138  continue;
139  }
140 
141  DEBUG(dbgs() << " Found a branch that needs expanding, BB#"
142  << DestBB->getNumber() << ", Distance " << BranchDistance
143  << "\n");
144 
145  // If JCC is not the last instruction we need to split the MBB.
146  if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
147 
148  DEBUG(dbgs() << " Found a basic block that needs to be split, BB#"
149  << MBB->getNumber() << "\n");
150 
151  // Create a new basic block.
152  MachineBasicBlock *NewBB =
153  MF->CreateMachineBasicBlock(MBB->getBasicBlock());
154  MF->insert(std::next(MBB), NewBB);
155 
156  // Splice the instructions following MI over to the NewBB.
157  NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
158 
159  // Update the successor lists.
160  for (MachineBasicBlock *Succ : MBB->successors()) {
161  if (Succ == DestBB) {
162  continue;
163  }
164  MBB->replaceSuccessor(Succ, NewBB);
165  NewBB->addSuccessor(Succ);
166  }
167 
168  // We introduced a new MBB so all following blocks should be numbered
169  // and measured again.
170  measureFunction(BlockOffsets, &*MBB);
171 
172  ++NumSplit;
173 
174  // It may be not necessary to start all over at this point, but it's
175  // safer do this anyway.
176  return true;
177  }
178 
179  MachineInstr &OldBranch = *MI;
180  DebugLoc dl = OldBranch.getDebugLoc();
181  int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
182 
183  if (MI->getOpcode() == MSP430::JCC) {
184  MachineBasicBlock *NextMBB = &*std::next(MBB);
185  assert(MBB->isSuccessor(NextMBB) &&
186  "This block must have a layout successor!");
187 
188  // The BCC operands are:
189  // 0. Target MBB
190  // 1. MSP430 branch predicate
192  Cond.push_back(MI->getOperand(1));
193 
194  // Jump over the long branch on the opposite condition
196  MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
197  .addMBB(NextMBB)
198  .add(Cond[0]);
199  InstrSizeDiff += TII->getInstSizeInBytes(*MI);
200  ++MI;
201  }
202 
203  // Unconditional branch to the real destination.
204  MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
205  InstrSizeDiff += TII->getInstSizeInBytes(*MI);
206 
207  // Remove the old branch from the function.
208  OldBranch.eraseFromParent();
209 
210  // The size of a new instruction is different from the old one, so we need
211  // to correct all block offsets.
212  for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
213  BlockOffsets[i] += InstrSizeDiff;
214  }
215  MBBStartOffset += InstrSizeDiff;
216 
217  ++NumExpanded;
218  MadeChange = true;
219  }
220  }
221  return MadeChange;
222 }
223 
224 bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
225  MF = &mf;
226  TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
227 
228  // If the pass is disabled, just bail early.
229  if (!BranchSelectEnabled)
230  return false;
231 
232  DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
233 
234  // BlockOffsets - Contains the distance from the beginning of the function to
235  // the beginning of each basic block.
236  OffsetVector BlockOffsets;
237 
238  unsigned FunctionSize = measureFunction(BlockOffsets);
239  // If the entire function is smaller than the displacement of a branch field,
240  // we know we don't need to expand any branches in this
241  // function. This is a common case.
242  if (isInRage(FunctionSize)) {
243  return false;
244  }
245 
246  // Iteratively expand branches until we reach a fixed point.
247  bool MadeChange = false;
248  while (expandBranches(BlockOffsets))
249  MadeChange = true;
250 
251  return MadeChange;
252 }
253 
254 /// Returns an instance of the Branch Selection Pass
256  return new MSP430BSel();
257 }
void push_back(const T &Elt)
Definition: SmallVector.h:212
const MachineInstrBuilder & add(const MachineOperand &MO) const
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
const DebugLoc & getDebugLoc() const
Returns the debug location id of this MachineInstr.
Definition: MachineInstr.h:268
bool reverseBranchCondition(SmallVectorImpl< MachineOperand > &Cond) const override
Reverses the branch condition of the specified condition list, returning false on success and true if...
STATISTIC(NumFunctions, "Total number of functions")
A debug info location.
Definition: DebugLoc.h:34
iterator_range< succ_iterator > successors()
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
void eraseFromParent()
Unlink &#39;this&#39; from the containing basic block and delete it.
int getNumber() const
MachineBasicBlocks are uniquely numbered at the function level, unless they&#39;re not in a MachineFuncti...
instr_iterator insert(instr_iterator I, MachineInstr *M)
Insert MI into the instruction list before I, possibly inside a bundle.
MachineInstrBuilder BuildMI(MachineFunction &MF, const DebugLoc &DL, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:406
FunctionPass * createMSP430BranchSelectionPass()
Returns an instance of the Branch Selection Pass.
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
self_iterator getIterator()
Definition: ilist_node.h:82
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
Iterator for intrusive lists based on ilist_node.
This is a &#39;vector&#39; (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:864
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:132
MachineFunctionProperties & set(Property P)
Representation of each machine instruction.
Definition: MachineInstr.h:59
static bool isInRage(int DistanceInBytes)
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
#define DEBUG(X)
Definition: Debug.h:118
IRTranslator LLVM IR MI
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:49
static cl::opt< bool > BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true), cl::desc("Expand out of range branches"))
Properties which a MachineFunction may have at a given point in time.