MachineSink.cpp revision ecb436f195e55e6bf84713b54efd6c621a9ddbfe 
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      AU.setPreservesCFG();
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  // Can't sink anything out of a block that has less than two successors.
116  if (MBB.succ_size() <= 1 || MBB.empty()) return false;
117
118  bool MadeChange = false;
119
120  // Walk the basic block bottom-up.  Remember if we saw a store.
121  MachineBasicBlock::iterator I = MBB.end();
122  --I;
123  bool ProcessedBegin, SawStore = false;
124  do {
125    MachineInstr *MI = I;  // The instruction to sink.
126
127    // Predecrement I (if it's not begin) so that it isn't invalidated by
128    // sinking.
129    ProcessedBegin = I == MBB.begin();
130    if (!ProcessedBegin)
131      --I;
132
133    if (SinkInstruction(MI, SawStore))
134      ++NumSunk, MadeChange = true;
135
136    // If we just processed the first instruction in the block, we're done.
137  } while (!ProcessedBegin);
138
139  return MadeChange;
140}
141
142/// SinkInstruction - Determine whether it is safe to sink the specified machine
143/// instruction out of its current block into a successor.
144bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
145  // Check if it's safe to move the instruction.
146  if (!MI->isSafeToMove(TII, SawStore))
147    return false;
148
149  // FIXME: This should include support for sinking instructions within the
150  // block they are currently in to shorten the live ranges.  We often get
151  // instructions sunk into the top of a large block, but it would be better to
152  // also sink them down before their first use in the block.  This xform has to
153  // be careful not to *increase* register pressure though, e.g. sinking
154  // "x = y + z" down if it kills y and z would increase the live ranges of y
155  // and z only the shrink the live range of x.
156
157  // Loop over all the operands of the specified instruction.  If there is
158  // anything we can't handle, bail out.
159  MachineBasicBlock *ParentBlock = MI->getParent();
160
161  // SuccToSinkTo - This is the successor to sink this instruction to, once we
162  // decide.
163  MachineBasicBlock *SuccToSinkTo = 0;
164
165  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
166    const MachineOperand &MO = MI->getOperand(i);
167    if (!MO.isReg()) continue;  // Ignore non-register operands.
168
169    unsigned Reg = MO.getReg();
170    if (Reg == 0) continue;
171
172    if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
173      // If this is a physical register use, we can't move it.  If it is a def,
174      // we can move it, but only if the def is dead.
175      if (MO.isUse() || !MO.isDead())
176        return false;
177    } else {
178      // Virtual register uses are always safe to sink.
179      if (MO.isUse()) continue;
180
181      // If it's not safe to move defs of the register class, then abort.
182      if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
183        return false;
184
185      // FIXME: This picks a successor to sink into based on having one
186      // successor that dominates all the uses.  However, there are cases where
187      // sinking can happen but where the sink point isn't a successor.  For
188      // example:
189      //   x = computation
190      //   if () {} else {}
191      //   use x
192      // the instruction could be sunk over the whole diamond for the
193      // if/then/else (or loop, etc), allowing it to be sunk into other blocks
194      // after that.
195
196      // Virtual register defs can only be sunk if all their uses are in blocks
197      // dominated by one of the successors.
198      if (SuccToSinkTo) {
199        // If a previous operand picked a block to sink to, then this operand
200        // must be sinkable to the same block.
201        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
202          return false;
203        continue;
204      }
205
206      // Otherwise, we should look at all the successors and decide which one
207      // we should sink to.
208      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
209           E = ParentBlock->succ_end(); SI != E; ++SI) {
210        if (AllUsesDominatedByBlock(Reg, *SI)) {
211          SuccToSinkTo = *SI;
212          break;
213        }
214      }
215
216      // If we couldn't find a block to sink to, ignore this instruction.
217      if (SuccToSinkTo == 0)
218        return false;
219    }
220  }
221
222  // If there are no outputs, it must have side-effects.
223  if (SuccToSinkTo == 0)
224    return false;
225
226  // It's not safe to sink instructions to EH landing pad. Control flow into
227  // landing pad is implicitly defined.
228  if (SuccToSinkTo->isLandingPad())
229    return false;
230
231  // If is not possible to sink an instruction into its own block.  This can
232  // happen with loops.
233  if (MI->getParent() == SuccToSinkTo)
234    return false;
235
236  DEBUG(cerr << "Sink instr " << *MI);
237  DEBUG(cerr << "to block " << *SuccToSinkTo);
238
239  // If the block has multiple predecessors, this would introduce computation on
240  // a path that it doesn't already exist.  We could split the critical edge,
241  // but for now we just punt.
242  // FIXME: Split critical edges if not backedges.
243  if (SuccToSinkTo->pred_size() > 1) {
244    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
245    return false;
246  }
247
248  // Determine where to insert into.  Skip phi nodes.
249  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
250  while (InsertPos != SuccToSinkTo->end() &&
251         InsertPos->getOpcode() == TargetInstrInfo::PHI)
252    ++InsertPos;
253
254  // Move the instruction.
255  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
256                       ++MachineBasicBlock::iterator(MI));
257  return true;
258}
259