MachineSink.cpp revision 7f7f365665a22e1b4397d0924f24b929607f9690 
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/TargetRegisterInfo.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(&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, bool &SawStore);
51    bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
52  };
53} // end anonymous namespace
54
55char MachineSinking::ID = 0;
56static RegisterPass<MachineSinking>
57X("machine-sink", "Machine code sinking");
58
59FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
60
61/// AllUsesDominatedByBlock - Return true if all uses of the specified register
62/// occur in blocks dominated by the specified block.
63bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
64                                             MachineBasicBlock *MBB) const {
65  assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
66         "Only makes sense for vregs");
67  for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
68       E = RegInfo->reg_end(); I != E; ++I) {
69    if (I.getOperand().isDef()) continue;  // ignore def.
70
71    // Determine the block of the use.
72    MachineInstr *UseInst = &*I;
73    MachineBasicBlock *UseBlock = UseInst->getParent();
74    if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
75      // PHI nodes use the operand in the predecessor block, not the block with
76      // the PHI.
77      UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
78    }
79    // Check that it dominates.
80    if (!DT->dominates(MBB, UseBlock))
81      return false;
82  }
83  return true;
84}
85
86
87
88bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
89  DOUT << "******** Machine Sinking ********\n";
90
91  CurMF = &MF;
92  TM = &CurMF->getTarget();
93  TII = TM->getInstrInfo();
94  RegInfo = &CurMF->getRegInfo();
95  DT = &getAnalysis<MachineDominatorTree>();
96
97  bool EverMadeChange = false;
98
99  while (1) {
100    bool MadeChange = false;
101
102    // Process all basic blocks.
103    for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end();
104         I != E; ++I)
105      MadeChange |= ProcessBlock(*I);
106
107    // If this iteration over the code changed anything, keep iterating.
108    if (!MadeChange) break;
109    EverMadeChange = true;
110  }
111  return EverMadeChange;
112}
113
114bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
115  bool MadeChange = false;
116
117  // Can't sink anything out of a block that has less than two successors.
118  if (MBB.succ_size() <= 1) return false;
119
120  // Walk the basic block bottom-up.  Remember if we saw a store.
121  bool SawStore = false;
122  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
123    MachineBasicBlock::iterator LastIt = I;
124    if (SinkInstruction(--I, SawStore)) {
125      I = LastIt;
126      ++NumSunk;
127    }
128  }
129
130  return MadeChange;
131}
132
133/// SinkInstruction - Determine whether it is safe to sink the specified machine
134/// instruction out of its current block into a successor.
135bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
136  // Check if it's safe to move the instruction.
137  if (!MI->isSafeToMove(TII, SawStore))
138    return false;
139
140  // FIXME: This should include support for sinking instructions within the
141  // block they are currently in to shorten the live ranges.  We often get
142  // instructions sunk into the top of a large block, but it would be better to
143  // also sink them down before their first use in the block.  This xform has to
144  // be careful not to *increase* register pressure though, e.g. sinking
145  // "x = y + z" down if it kills y and z would increase the live ranges of y
146  // and z only the shrink the live range of x.
147
148  // Loop over all the operands of the specified instruction.  If there is
149  // anything we can't handle, bail out.
150  MachineBasicBlock *ParentBlock = MI->getParent();
151
152  // SuccToSinkTo - This is the successor to sink this instruction to, once we
153  // decide.
154  MachineBasicBlock *SuccToSinkTo = 0;
155
156  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
157    const MachineOperand &MO = MI->getOperand(i);
158    if (!MO.isRegister()) continue;  // Ignore non-register operands.
159
160    unsigned Reg = MO.getReg();
161    if (Reg == 0) continue;
162
163    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
164      // If this is a physical register use, we can't move it.  If it is a def,
165      // we can move it, but only if the def is dead.
166      if (MO.isUse() || !MO.isDead())
167        return false;
168    } else {
169      // Virtual register uses are always safe to sink.
170      if (MO.isUse()) continue;
171
172      // FIXME: This picks a successor to sink into based on having one
173      // successor that dominates all the uses.  However, there are cases where
174      // sinking can happen but where the sink point isn't a successor.  For
175      // example:
176      //   x = computation
177      //   if () {} else {}
178      //   use x
179      // the instruction could be sunk over the whole diamond for the
180      // if/then/else (or loop, etc), allowing it to be sunk into other blocks
181      // after that.
182
183      // Virtual register defs can only be sunk if all their uses are in blocks
184      // dominated by one of the successors.
185      if (SuccToSinkTo) {
186        // If a previous operand picked a block to sink to, then this operand
187        // must be sinkable to the same block.
188        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
189          return false;
190        continue;
191      }
192
193      // Otherwise, we should look at all the successors and decide which one
194      // we should sink to.
195      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
196           E = ParentBlock->succ_end(); SI != E; ++SI) {
197        if (AllUsesDominatedByBlock(Reg, *SI)) {
198          SuccToSinkTo = *SI;
199          break;
200        }
201      }
202
203      // If we couldn't find a block to sink to, ignore this instruction.
204      if (SuccToSinkTo == 0)
205        return false;
206    }
207  }
208
209  // If there are no outputs, it must have side-effects.
210  if (SuccToSinkTo == 0)
211    return false;
212
213  DEBUG(cerr << "Sink instr " << *MI);
214  DEBUG(cerr << "to block " << *SuccToSinkTo);
215
216  // If the block has multiple predecessors, this would introduce computation on
217  // a path that it doesn't already exist.  We could split the critical edge,
218  // but for now we just punt.
219  // FIXME: Split critical edges if not backedges.
220  if (SuccToSinkTo->pred_size() > 1) {
221    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
222    return false;
223  }
224
225  // Determine where to insert into.  Skip phi nodes.
226  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
227  while (InsertPos != SuccToSinkTo->end() &&
228         InsertPos->getOpcode() == TargetInstrInfo::PHI)
229    ++InsertPos;
230
231  // Move the instruction.
232  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
233                       ++MachineBasicBlock::iterator(MI));
234  return true;
235}
236