MachineSink.cpp revision 6f0d024a534af18d9e60b3ea757376cd8a3a980e 
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((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, bool &SawStore);
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(TargetRegisterInfo::isVirtualRegister(Reg) &&
65         "Only makes sense for vregs");
66  for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
67       E = RegInfo->reg_end(); I != E; ++I) {
68    if (I.getOperand().isDef()) continue;  // ignore def.
69
70    // Determine the block of the use.
71    MachineInstr *UseInst = &*I;
72    MachineBasicBlock *UseBlock = UseInst->getParent();
73    if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
74      // PHI nodes use the operand in the predecessor block, not the block with
75      // the PHI.
76      UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
77    }
78    // Check that it dominates.
79    if (!DT->dominates(MBB, UseBlock))
80      return false;
81  }
82  return true;
83}
84
85
86
87bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
88  DOUT << "******** Machine Sinking ********\n";
89
90  CurMF = &MF;
91  TM = &CurMF->getTarget();
92  TII = TM->getInstrInfo();
93  RegInfo = &CurMF->getRegInfo();
94  DT = &getAnalysis<MachineDominatorTree>();
95
96  bool EverMadeChange = false;
97
98  while (1) {
99    bool MadeChange = false;
100
101    // Process all basic blocks.
102    for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end();
103         I != E; ++I)
104      MadeChange |= ProcessBlock(*I);
105
106    // If this iteration over the code changed anything, keep iterating.
107    if (!MadeChange) break;
108    EverMadeChange = true;
109  }
110  return EverMadeChange;
111}
112
113bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
114  bool MadeChange = false;
115
116  // Can't sink anything out of a block that has less than two successors.
117  if (MBB.succ_size() <= 1) return false;
118
119  // Walk the basic block bottom-up.  Remember if we saw a store.
120  bool SawStore = false;
121  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
122    MachineBasicBlock::iterator LastIt = I;
123    if (SinkInstruction(--I, SawStore)) {
124      I = LastIt;
125      ++NumSunk;
126    }
127  }
128
129  return MadeChange;
130}
131
132/// SinkInstruction - Determine whether it is safe to sink the specified machine
133/// instruction out of its current block into a successor.
134bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
135  const TargetInstrDesc &TID = MI->getDesc();
136
137  // Ignore stuff that we obviously can't sink.
138  if (TID.mayStore() || TID.isCall()) {
139    SawStore = true;
140    return false;
141  }
142  if (TID.isReturn() || TID.isBranch() || TID.hasUnmodeledSideEffects())
143    return false;
144
145  // See if this instruction does a load.  If so, we have to guarantee that the
146  // loaded value doesn't change between the load and the end of block.  The
147  // check for isInvariantLoad gives the targe the chance to classify the load
148  // as always returning a constant, e.g. a constant pool load.
149  if (TID.mayLoad() && !TII->isInvariantLoad(MI)) {
150    // Otherwise, this is a real load.  If there is a store between the load and
151    // end of block, we can't sink the load.
152    //
153    // FIXME: we can't do this transformation until we know that the load is
154    // not volatile, and machineinstrs don't keep this info. :(
155    //
156    //if (SawStore)
157    return false;
158  }
159
160  // FIXME: This should include support for sinking instructions within the
161  // block they are currently in to shorten the live ranges.  We often get
162  // instructions sunk into the top of a large block, but it would be better to
163  // also sink them down before their first use in the block.  This xform has to
164  // be careful not to *increase* register pressure though, e.g. sinking
165  // "x = y + z" down if it kills y and z would increase the live ranges of y
166  // and z only the shrink the live range of x.
167
168  // Loop over all the operands of the specified instruction.  If there is
169  // anything we can't handle, bail out.
170  MachineBasicBlock *ParentBlock = MI->getParent();
171
172  // SuccToSinkTo - This is the successor to sink this instruction to, once we
173  // decide.
174  MachineBasicBlock *SuccToSinkTo = 0;
175
176  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
177    const MachineOperand &MO = MI->getOperand(i);
178    if (!MO.isReg()) continue;  // Ignore non-register operands.
179
180    unsigned Reg = MO.getReg();
181    if (Reg == 0) continue;
182
183    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
184      // If this is a physical register use, we can't move it.  If it is a def,
185      // we can move it, but only if the def is dead.
186      if (MO.isUse() || !MO.isDead())
187        return false;
188    } else {
189      // Virtual register uses are always safe to sink.
190      if (MO.isUse()) continue;
191
192      // FIXME: This picks a successor to sink into based on having one
193      // successor that dominates all the uses.  However, there are cases where
194      // sinking can happen but where the sink point isn't a successor.  For
195      // example:
196      //   x = computation
197      //   if () {} else {}
198      //   use x
199      // the instruction could be sunk over the whole diamond for the
200      // if/then/else (or loop, etc), allowing it to be sunk into other blocks
201      // after that.
202
203      // Virtual register defs can only be sunk if all their uses are in blocks
204      // dominated by one of the successors.
205      if (SuccToSinkTo) {
206        // If a previous operand picked a block to sink to, then this operand
207        // must be sinkable to the same block.
208        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
209          return false;
210        continue;
211      }
212
213      // Otherwise, we should look at all the successors and decide which one
214      // we should sink to.
215      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
216           E = ParentBlock->succ_end(); SI != E; ++SI) {
217        if (AllUsesDominatedByBlock(Reg, *SI)) {
218          SuccToSinkTo = *SI;
219          break;
220        }
221      }
222
223      // If we couldn't find a block to sink to, ignore this instruction.
224      if (SuccToSinkTo == 0)
225        return false;
226    }
227  }
228
229  // If there are no outputs, it must have side-effects.
230  if (SuccToSinkTo == 0)
231    return false;
232
233  DEBUG(cerr << "Sink instr " << *MI);
234  DEBUG(cerr << "to block " << *SuccToSinkTo);
235
236  // If the block has multiple predecessors, this would introduce computation on
237  // a path that it doesn't already exist.  We could split the critical edge,
238  // but for now we just punt.
239  // FIXME: Split critical edges if not backedges.
240  if (SuccToSinkTo->pred_size() > 1) {
241    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
242    return false;
243  }
244
245  // Determine where to insert into.  Skip phi nodes.
246  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
247  while (InsertPos != SuccToSinkTo->end() &&
248         InsertPos->getOpcode() == TargetInstrInfo::PHI)
249    ++InsertPos;
250
251  // Move the instruction.
252  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
253                       ++MachineBasicBlock::iterator(MI));
254  return true;
255}
256