MachineSink.cpp revision 193323 
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/Statistic.h"
22#include "llvm/Support/Compiler.h"
23#include "llvm/Support/Debug.h"
24using namespace llvm;
25
26STATISTIC(NumSunk, "Number of machine instructions sunk");
27
28namespace {
29  class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
30    const TargetMachine   *TM;
31    const TargetInstrInfo *TII;
32    MachineFunction       *CurMF; // Current MachineFunction
33    MachineRegisterInfo  *RegInfo; // Machine register information
34    MachineDominatorTree *DT;   // Machine dominator tree for the current Loop
35
36  public:
37    static char ID; // Pass identification
38    MachineSinking() : MachineFunctionPass(&ID) {}
39
40    virtual bool runOnMachineFunction(MachineFunction &MF);
41
42    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43      MachineFunctionPass::getAnalysisUsage(AU);
44      AU.addRequired<MachineDominatorTree>();
45      AU.addPreserved<MachineDominatorTree>();
46    }
47  private:
48    bool ProcessBlock(MachineBasicBlock &MBB);
49    bool SinkInstruction(MachineInstr *MI, bool &SawStore);
50    bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
51  };
52} // end anonymous namespace
53
54char MachineSinking::ID = 0;
55static RegisterPass<MachineSinking>
56X("machine-sink", "Machine code sinking");
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  // Can't sink anything out of a block that has less than two successors.
115  if (MBB.succ_size() <= 1 || MBB.empty()) return false;
116
117  bool MadeChange = false;
118
119  // Walk the basic block bottom-up.  Remember if we saw a store.
120  MachineBasicBlock::iterator I = MBB.end();
121  --I;
122  bool ProcessedBegin, SawStore = false;
123  do {
124    MachineInstr *MI = I;  // The instruction to sink.
125
126    // Predecrement I (if it's not begin) so that it isn't invalidated by
127    // sinking.
128    ProcessedBegin = I == MBB.begin();
129    if (!ProcessedBegin)
130      --I;
131
132    if (SinkInstruction(MI, SawStore))
133      ++NumSunk, MadeChange = true;
134
135    // If we just processed the first instruction in the block, we're done.
136  } while (!ProcessedBegin);
137
138  return MadeChange;
139}
140
141/// SinkInstruction - Determine whether it is safe to sink the specified machine
142/// instruction out of its current block into a successor.
143bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
144  // Check if it's safe to move the instruction.
145  if (!MI->isSafeToMove(TII, SawStore))
146    return false;
147
148  // FIXME: This should include support for sinking instructions within the
149  // block they are currently in to shorten the live ranges.  We often get
150  // instructions sunk into the top of a large block, but it would be better to
151  // also sink them down before their first use in the block.  This xform has to
152  // be careful not to *increase* register pressure though, e.g. sinking
153  // "x = y + z" down if it kills y and z would increase the live ranges of y
154  // and z only the shrink the live range of x.
155
156  // Loop over all the operands of the specified instruction.  If there is
157  // anything we can't handle, bail out.
158  MachineBasicBlock *ParentBlock = MI->getParent();
159
160  // SuccToSinkTo - This is the successor to sink this instruction to, once we
161  // decide.
162  MachineBasicBlock *SuccToSinkTo = 0;
163
164  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
165    const MachineOperand &MO = MI->getOperand(i);
166    if (!MO.isReg()) continue;  // Ignore non-register operands.
167
168    unsigned Reg = MO.getReg();
169    if (Reg == 0) continue;
170
171    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
172      // If this is a physical register use, we can't move it.  If it is a def,
173      // we can move it, but only if the def is dead.
174      if (MO.isUse() || !MO.isDead())
175        return false;
176    } else {
177      // Virtual register uses are always safe to sink.
178      if (MO.isUse()) continue;
179
180      // If it's not safe to move defs of the register class, then abort.
181      if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
182        return false;
183
184      // FIXME: This picks a successor to sink into based on having one
185      // successor that dominates all the uses.  However, there are cases where
186      // sinking can happen but where the sink point isn't a successor.  For
187      // example:
188      //   x = computation
189      //   if () {} else {}
190      //   use x
191      // the instruction could be sunk over the whole diamond for the
192      // if/then/else (or loop, etc), allowing it to be sunk into other blocks
193      // after that.
194
195      // Virtual register defs can only be sunk if all their uses are in blocks
196      // dominated by one of the successors.
197      if (SuccToSinkTo) {
198        // If a previous operand picked a block to sink to, then this operand
199        // must be sinkable to the same block.
200        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
201          return false;
202        continue;
203      }
204
205      // Otherwise, we should look at all the successors and decide which one
206      // we should sink to.
207      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
208           E = ParentBlock->succ_end(); SI != E; ++SI) {
209        if (AllUsesDominatedByBlock(Reg, *SI)) {
210          SuccToSinkTo = *SI;
211          break;
212        }
213      }
214
215      // If we couldn't find a block to sink to, ignore this instruction.
216      if (SuccToSinkTo == 0)
217        return false;
218    }
219  }
220
221  // If there are no outputs, it must have side-effects.
222  if (SuccToSinkTo == 0)
223    return false;
224
225  // It's not safe to sink instructions to EH landing pad. Control flow into
226  // landing pad is implicitly defined.
227  if (SuccToSinkTo->isLandingPad())
228    return false;
229
230  // If is not possible to sink an instruction into its own block.  This can
231  // happen with loops.
232  if (MI->getParent() == SuccToSinkTo)
233    return false;
234
235  DEBUG(cerr << "Sink instr " << *MI);
236  DEBUG(cerr << "to block " << *SuccToSinkTo);
237
238  // If the block has multiple predecessors, this would introduce computation on
239  // a path that it doesn't already exist.  We could split the critical edge,
240  // but for now we just punt.
241  // FIXME: Split critical edges if not backedges.
242  if (SuccToSinkTo->pred_size() > 1) {
243    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
244    return false;
245  }
246
247  // Determine where to insert into.  Skip phi nodes.
248  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
249  while (InsertPos != SuccToSinkTo->end() &&
250         InsertPos->getOpcode() == TargetInstrInfo::PHI)
251    ++InsertPos;
252
253  // Move the instruction.
254  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
255                       ++MachineBasicBlock::iterator(MI));
256  return true;
257}
258