MachineSink.cpp revision 9bb459b55411c45175e599f6f421b7a57060ee57 
1//===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
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 pass
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "machine-sink"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/CodeGen/MachineRegisterInfo.h"
17#include "llvm/CodeGen/MachineDominators.h"
18#include "llvm/Target/MRegisterInfo.h"
19#include "llvm/Target/TargetInstrInfo.h"
20#include "llvm/Target/TargetMachine.h"
21#include "llvm/ADT/SmallVector.h"
22#include "llvm/ADT/Statistic.h"
23#include "llvm/Support/Compiler.h"
24#include "llvm/Support/Debug.h"
25using namespace llvm;
26
27STATISTIC(NumSunk, "Number of machine instructions sunk");
28
29namespace {
30  class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
31    const TargetMachine   *TM;
32    const TargetInstrInfo *TII;
33    MachineFunction       *CurMF; // Current MachineFunction
34    MachineRegisterInfo  *RegInfo; // Machine register information
35    MachineDominatorTree *DT;   // Machine dominator tree for the current Loop
36
37  public:
38    static char ID; // Pass identification
39    MachineSinking() : MachineFunctionPass((intptr_t)&ID) {}
40
41    virtual bool runOnMachineFunction(MachineFunction &MF);
42
43    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44      MachineFunctionPass::getAnalysisUsage(AU);
45      AU.addRequired<MachineDominatorTree>();
46      AU.addPreserved<MachineDominatorTree>();
47    }
48  private:
49    bool ProcessBlock(MachineBasicBlock &MBB);
50    bool SinkInstruction(MachineInstr *MI);
51    bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
52  };
53
54  char MachineSinking::ID = 0;
55  RegisterPass<MachineSinking> X("machine-sink", "Machine code sinking");
56} // end anonymous namespace
57
58FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
59
60/// AllUsesDominatedByBlock - Return true if all uses of the specified register
61/// occur in blocks dominated by the specified block.
62bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
63                                             MachineBasicBlock *MBB) const {
64  assert(MRegisterInfo::isVirtualRegister(Reg) && "Only makes sense for vregs");
65  for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
66       E = RegInfo->reg_end(); I != E; ++I) {
67    if (I.getOperand().isDef()) continue;  // ignore def.
68
69    // Determine the block of the use.
70    MachineInstr *UseInst = &*I;
71    MachineBasicBlock *UseBlock = UseInst->getParent();
72    if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
73      // PHI nodes use the operand in the predecessor block, not the block with
74      // the PHI.
75      UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
76    }
77    // Check that it dominates.
78    if (!DT->dominates(MBB, UseBlock))
79      return false;
80  }
81  return true;
82}
83
84
85
86bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
87  DOUT << "******** Machine Sinking ********\n";
88
89  CurMF = &MF;
90  TM = &CurMF->getTarget();
91  TII = TM->getInstrInfo();
92  RegInfo = &CurMF->getRegInfo();
93  DT = &getAnalysis<MachineDominatorTree>();
94
95  bool EverMadeChange = false;
96
97  while (1) {
98    bool MadeChange = false;
99
100    // Process all basic blocks.
101    for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end();
102         I != E; ++I)
103      MadeChange |= ProcessBlock(*I);
104
105    // If this iteration over the code changed anything, keep iterating.
106    if (!MadeChange) break;
107    EverMadeChange = true;
108  }
109  return EverMadeChange;
110}
111
112bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
113  bool MadeChange = false;
114
115  // Can't sink anything out of a block that has less than two successors.
116  if (MBB.succ_size() <= 1) return false;
117
118  // Walk the basic block bottom-up
119  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
120    MachineBasicBlock::iterator LastIt = I;
121    if (SinkInstruction(--I)) {
122      I = LastIt;
123      ++NumSunk;
124    }
125  }
126
127  return MadeChange;
128}
129
130/// SinkInstruction - Determine whether it is safe to sink the specified machine
131/// instruction out of its current block into a successor.
132bool MachineSinking::SinkInstruction(MachineInstr *MI) {
133  // Loop over all the operands of the specified instruction.  If there is
134  // anything we can't handle, bail out.
135  MachineBasicBlock *ParentBlock = MI->getParent();
136
137  // SuccToSinkTo - This is the successor to sink this instruction to, once we
138  // decide.
139  MachineBasicBlock *SuccToSinkTo = 0;
140
141  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
142    const MachineOperand &MO = MI->getOperand(i);
143    if (!MO.isReg()) continue;  // Ignore non-register operands.
144
145    unsigned Reg = MO.getReg();
146    if (Reg == 0) continue;
147
148    if (MRegisterInfo::isPhysicalRegister(Reg)) {
149      // If this is a physical register use, we can't move it.  If it is a def,
150      // we can move it, but only if the def is dead.
151      if (MO.isUse() || !MO.isDead())
152        return false;
153    } else {
154      // Virtual register uses are always safe to sink.
155      if (MO.isUse()) continue;
156
157      // Virtual register defs can only be sunk if all their uses are in blocks
158      // dominated by one of the successors.
159      if (SuccToSinkTo) {
160        // If a previous operand picked a block to sink to, then this operand
161        // must be sinkable to the same block.
162        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
163          return false;
164        continue;
165      }
166
167      // Otherwise, we should look at all the successors and decide which one
168      // we should sink to.
169      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
170           E = ParentBlock->succ_end(); SI != E; ++SI) {
171        if (AllUsesDominatedByBlock(Reg, *SI)) {
172          SuccToSinkTo = *SI;
173          break;
174        }
175      }
176
177      // If we couldn't find a block to sink to, ignore this instruction.
178      if (SuccToSinkTo == 0)
179        return false;
180    }
181  }
182
183  // If there are no outputs, it must have side-effects.
184  if (SuccToSinkTo == 0)
185    return false;
186
187  // FIXME: Check that the instr doesn't have side effects etc.
188
189  DEBUG(cerr << "Sink instr " << *MI);
190  DEBUG(cerr << "to block " << *SuccToSinkTo);
191
192  // If the block has multiple predecessors, this would introduce computation on
193  // a path that it doesn't already exist.  We could split the critical edge,
194  // but for now we just punt.
195  if (SuccToSinkTo->pred_size() > 1) {
196    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
197    return false;
198  }
199
200  // Determine where to insert into.  Skip phi nodes.
201  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
202  while (InsertPos != SuccToSinkTo->end() &&
203         InsertPos->getOpcode() == TargetInstrInfo::PHI)
204    ++InsertPos;
205
206  // Move the instruction.
207  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
208                       ++MachineBasicBlock::iterator(MI));
209  return true;
210}
211