LLVM  3.7.0
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 STATISTIC(NumExpanded, "Number of branches expanded to long format");
31 
32 namespace {
33  struct MSP430BSel : public MachineFunctionPass {
34  static char ID;
35  MSP430BSel() : MachineFunctionPass(ID) {}
36 
37  /// BlockSizes - The sizes of the basic blocks in the function.
38  std::vector<unsigned> BlockSizes;
39 
40  bool runOnMachineFunction(MachineFunction &Fn) override;
41 
42  const char *getPassName() const override {
43  return "MSP430 Branch Selector";
44  }
45  };
46  char MSP430BSel::ID = 0;
47 }
48 
49 /// createMSP430BranchSelectionPass - returns an instance of the Branch
50 /// Selection Pass
51 ///
53  return new MSP430BSel();
54 }
55 
56 bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
57  const MSP430InstrInfo *TII =
58  static_cast<const MSP430InstrInfo *>(Fn.getSubtarget().getInstrInfo());
59  // Give the blocks of the function a dense, in-order, numbering.
60  Fn.RenumberBlocks();
61  BlockSizes.resize(Fn.getNumBlockIDs());
62 
63  // Measure each MBB and compute a size for the entire function.
64  unsigned FuncSize = 0;
65  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
66  ++MFI) {
67  MachineBasicBlock *MBB = MFI;
68 
69  unsigned BlockSize = 0;
70  for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
71  MBBI != EE; ++MBBI)
72  BlockSize += TII->GetInstSizeInBytes(MBBI);
73 
74  BlockSizes[MBB->getNumber()] = BlockSize;
75  FuncSize += BlockSize;
76  }
77 
78  // If the entire function is smaller than the displacement of a branch field,
79  // we know we don't need to shrink any branches in this function. This is a
80  // common case.
81  if (FuncSize < (1 << 9)) {
82  BlockSizes.clear();
83  return false;
84  }
85 
86  // For each conditional branch, if the offset to its destination is larger
87  // than the offset field allows, transform it into a long branch sequence
88  // like this:
89  // short branch:
90  // bCC MBB
91  // long branch:
92  // b!CC $PC+6
93  // b MBB
94  //
95  bool MadeChange = true;
96  bool EverMadeChange = false;
97  while (MadeChange) {
98  // Iteratively expand branches until we reach a fixed point.
99  MadeChange = false;
100 
101  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
102  ++MFI) {
103  MachineBasicBlock &MBB = *MFI;
104  unsigned MBBStartOffset = 0;
105  for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
106  I != E; ++I) {
107  if ((I->getOpcode() != MSP430::JCC || I->getOperand(0).isImm()) &&
108  I->getOpcode() != MSP430::JMP) {
109  MBBStartOffset += TII->GetInstSizeInBytes(I);
110  continue;
111  }
112 
113  // Determine the offset from the current branch to the destination
114  // block.
115  MachineBasicBlock *Dest = I->getOperand(0).getMBB();
116 
117  int BranchSize;
118  if (Dest->getNumber() <= MBB.getNumber()) {
119  // If this is a backwards branch, the delta is the offset from the
120  // start of this block to this branch, plus the sizes of all blocks
121  // from this block to the dest.
122  BranchSize = MBBStartOffset;
123 
124  for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
125  BranchSize += BlockSizes[i];
126  } else {
127  // Otherwise, add the size of the blocks between this block and the
128  // dest to the number of bytes left in this block.
129  BranchSize = -MBBStartOffset;
130 
131  for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
132  BranchSize += BlockSizes[i];
133  }
134 
135  // If this branch is in range, ignore it.
136  if (isInt<10>(BranchSize)) {
137  MBBStartOffset += 2;
138  continue;
139  }
140 
141  // Otherwise, we have to expand it to a long branch.
142  unsigned NewSize;
143  MachineInstr *OldBranch = I;
144  DebugLoc dl = OldBranch->getDebugLoc();
145 
146  if (I->getOpcode() == MSP430::JMP) {
147  NewSize = 4;
148  } else {
149  // The BCC operands are:
150  // 0. MSP430 branch predicate
151  // 1. Target MBB
153  Cond.push_back(I->getOperand(1));
154 
155  // Jump over the uncond branch inst (i.e. $+6) on opposite condition.
156  TII->ReverseBranchCondition(Cond);
157  BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
158  .addImm(4).addOperand(Cond[0]);
159 
160  NewSize = 6;
161  }
162  // Uncond branch to the real destination.
163  I = BuildMI(MBB, I, dl, TII->get(MSP430::Bi)).addMBB(Dest);
164 
165  // Remove the old branch from the function.
166  OldBranch->eraseFromParent();
167 
168  // Remember that this instruction is NewSize bytes, increase the size of the
169  // block by NewSize-2, remember to iterate.
170  BlockSizes[MBB.getNumber()] += NewSize-2;
171  MBBStartOffset += NewSize;
172 
173  ++NumExpanded;
174  MadeChange = true;
175  }
176  }
177  EverMadeChange |= MadeChange;
178  }
179 
180  BlockSizes.clear();
181  return true;
182 }
bool ReverseBranchCondition(SmallVectorImpl< MachineOperand > &Cond) const override
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 eraseFromParent()
Unlink 'this' from the containing basic block and delete it.
bundle_iterator< MachineInstr, instr_iterator > iterator
unsigned GetInstSizeInBytes(const MachineInstr *MI) const
GetInstSize - Return the number of bytes of code the specified instruction may be.
FunctionPass * createMSP430BranchSelectionPass()
createMSP430BranchSelectionPass - 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.
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
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
#define I(x, y, z)
Definition: MD5.cpp:54
virtual const TargetInstrInfo * getInstrInfo() const
const MachineInstrBuilder & addOperand(const MachineOperand &MO) const
BasicBlockListType::iterator iterator