RegAllocGreedy.cpp revision f6dff84d4e44d6c4a46c4f8a18e13c78f804547c 
1//===-- RegAllocGreedy.cpp - greedy register allocator --------------------===//
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 file defines the RAGreedy function pass for register allocation in
11// optimized builds.
12//
13//===----------------------------------------------------------------------===//
14
15#define DEBUG_TYPE "regalloc"
16#include "AllocationOrder.h"
17#include "LiveIntervalUnion.h"
18#include "RegAllocBase.h"
19#include "Spiller.h"
20#include "VirtRegMap.h"
21#include "VirtRegRewriter.h"
22#include "llvm/Analysis/AliasAnalysis.h"
23#include "llvm/Function.h"
24#include "llvm/PassAnalysisSupport.h"
25#include "llvm/CodeGen/CalcSpillWeights.h"
26#include "llvm/CodeGen/LiveIntervalAnalysis.h"
27#include "llvm/CodeGen/LiveStackAnalysis.h"
28#include "llvm/CodeGen/MachineFunctionPass.h"
29#include "llvm/CodeGen/MachineLoopInfo.h"
30#include "llvm/CodeGen/MachineRegisterInfo.h"
31#include "llvm/CodeGen/Passes.h"
32#include "llvm/CodeGen/RegAllocRegistry.h"
33#include "llvm/CodeGen/RegisterCoalescer.h"
34#include "llvm/Target/TargetOptions.h"
35#include "llvm/Support/Debug.h"
36#include "llvm/Support/ErrorHandling.h"
37#include "llvm/Support/raw_ostream.h"
38
39using namespace llvm;
40
41static RegisterRegAlloc greedyRegAlloc("greedy", "greedy register allocator",
42                                       createGreedyRegisterAllocator);
43
44namespace {
45class RAGreedy : public MachineFunctionPass, public RegAllocBase {
46  // context
47  MachineFunction *MF;
48  const TargetMachine *TM;
49  MachineRegisterInfo *MRI;
50
51  BitVector ReservedRegs;
52
53  // analyses
54  LiveStacks *LS;
55
56  // state
57  std::auto_ptr<Spiller> SpillerInstance;
58
59public:
60  RAGreedy();
61
62  /// Return the pass name.
63  virtual const char* getPassName() const {
64    return "Basic Register Allocator";
65  }
66
67  /// RAGreedy analysis usage.
68  virtual void getAnalysisUsage(AnalysisUsage &AU) const;
69
70  virtual void releaseMemory();
71
72  virtual Spiller &spiller() { return *SpillerInstance; }
73
74  virtual float getPriority(LiveInterval *LI);
75
76  virtual unsigned selectOrSplit(LiveInterval &VirtReg,
77                                 SmallVectorImpl<LiveInterval*> &SplitVRegs);
78
79  /// Perform register allocation.
80  virtual bool runOnMachineFunction(MachineFunction &mf);
81
82  static char ID;
83
84private:
85  bool checkUncachedInterference(LiveInterval &, unsigned);
86  bool reassignVReg(LiveInterval &InterferingVReg, unsigned OldPhysReg);
87  bool reassignInterferences(LiveInterval &VirtReg, unsigned PhysReg);
88};
89} // end anonymous namespace
90
91char RAGreedy::ID = 0;
92
93FunctionPass* llvm::createGreedyRegisterAllocator() {
94  return new RAGreedy();
95}
96
97RAGreedy::RAGreedy(): MachineFunctionPass(ID) {
98  initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
99  initializeSlotIndexesPass(*PassRegistry::getPassRegistry());
100  initializeStrongPHIEliminationPass(*PassRegistry::getPassRegistry());
101  initializeRegisterCoalescerAnalysisGroup(*PassRegistry::getPassRegistry());
102  initializeCalculateSpillWeightsPass(*PassRegistry::getPassRegistry());
103  initializeLiveStacksPass(*PassRegistry::getPassRegistry());
104  initializeMachineDominatorTreePass(*PassRegistry::getPassRegistry());
105  initializeMachineLoopInfoPass(*PassRegistry::getPassRegistry());
106  initializeVirtRegMapPass(*PassRegistry::getPassRegistry());
107}
108
109void RAGreedy::getAnalysisUsage(AnalysisUsage &AU) const {
110  AU.setPreservesCFG();
111  AU.addRequired<AliasAnalysis>();
112  AU.addPreserved<AliasAnalysis>();
113  AU.addRequired<LiveIntervals>();
114  AU.addPreserved<SlotIndexes>();
115  if (StrongPHIElim)
116    AU.addRequiredID(StrongPHIEliminationID);
117  AU.addRequiredTransitive<RegisterCoalescer>();
118  AU.addRequired<CalculateSpillWeights>();
119  AU.addRequired<LiveStacks>();
120  AU.addPreserved<LiveStacks>();
121  AU.addRequiredID(MachineDominatorsID);
122  AU.addPreservedID(MachineDominatorsID);
123  AU.addRequired<MachineLoopInfo>();
124  AU.addPreserved<MachineLoopInfo>();
125  AU.addRequired<VirtRegMap>();
126  AU.addPreserved<VirtRegMap>();
127  MachineFunctionPass::getAnalysisUsage(AU);
128}
129
130void RAGreedy::releaseMemory() {
131  SpillerInstance.reset(0);
132  RegAllocBase::releaseMemory();
133}
134
135float RAGreedy::getPriority(LiveInterval *LI) {
136  float Priority = LI->weight;
137
138  // Prioritize hinted registers so they are allocated first.
139  std::pair<unsigned, unsigned> Hint;
140  if (Hint.first || Hint.second) {
141    // The hint can be target specific, a virtual register, or a physreg.
142    Priority *= 2;
143
144    // Prefer physreg hints above anything else.
145    if (Hint.first == 0 && TargetRegisterInfo::isPhysicalRegister(Hint.second))
146      Priority *= 2;
147  }
148  return Priority;
149}
150
151// Check interference without using the cache.
152bool RAGreedy::checkUncachedInterference(LiveInterval &VirtReg,
153                                         unsigned PhysReg) {
154  LiveIntervalUnion::Query subQ(&VirtReg, &PhysReg2LiveUnion[PhysReg]);
155  if (subQ.checkInterference())
156      return true;
157  for (const unsigned *AliasI = TRI->getAliasSet(PhysReg); *AliasI; ++AliasI) {
158    subQ.init(&VirtReg, &PhysReg2LiveUnion[*AliasI]);
159    if (subQ.checkInterference())
160      return true;
161  }
162  return false;
163}
164
165// Attempt to reassign this virtual register to a different physical register.
166//
167// FIXME: we are not yet caching these "second-level" interferences discovered
168// in the sub-queries. These interferences can change with each call to
169// selectOrSplit. However, we could implement a "may-interfere" cache that
170// could be conservatively dirtied when we reassign or split.
171//
172// FIXME: This may result in a lot of alias queries. We could summarize alias
173// live intervals in their parent register's live union, but it's messy.
174bool RAGreedy::reassignVReg(LiveInterval &InterferingVReg,
175                            unsigned OldPhysReg) {
176  assert(OldPhysReg == VRM->getPhys(InterferingVReg.reg) &&
177         "inconsistent phys reg assigment");
178
179  AllocationOrder Order(InterferingVReg.reg, *VRM, ReservedRegs);
180  while (unsigned PhysReg = Order.next()) {
181    if (PhysReg == OldPhysReg)
182      continue;
183
184    if (checkUncachedInterference(InterferingVReg, PhysReg))
185      continue;
186
187    DEBUG(dbgs() << "reassigning: " << InterferingVReg << " from " <<
188          TRI->getName(OldPhysReg) << " to " << TRI->getName(PhysReg) << '\n');
189
190    // Reassign the interfering virtual reg to this physical reg.
191    PhysReg2LiveUnion[OldPhysReg].extract(InterferingVReg);
192    VRM->clearVirt(InterferingVReg.reg);
193    VRM->assignVirt2Phys(InterferingVReg.reg, PhysReg);
194    PhysReg2LiveUnion[PhysReg].unify(InterferingVReg);
195
196    return true;
197  }
198  return false;
199}
200
201// Collect all virtual regs currently assigned to PhysReg that interfere with
202// VirtReg.
203//
204// Currently, for simplicity, we only attempt to reassign a single interference
205// within the same register class.
206bool RAGreedy::reassignInterferences(LiveInterval &VirtReg, unsigned PhysReg) {
207  LiveIntervalUnion::Query &Q = query(VirtReg, PhysReg);
208
209  // Limit the interference search to one interference.
210  Q.collectInterferingVRegs(1);
211  assert(Q.interferingVRegs().size() == 1 &&
212         "expected at least one interference");
213
214  // Do not attempt reassignment unless we find only a single interference.
215  if (!Q.seenAllInterferences())
216    return false;
217
218  // Don't allow any interferences on aliases.
219  for (const unsigned *AliasI = TRI->getAliasSet(PhysReg); *AliasI; ++AliasI) {
220    if (query(VirtReg, *AliasI).checkInterference())
221      return false;
222  }
223
224  return reassignVReg(*Q.interferingVRegs()[0], PhysReg);
225}
226
227unsigned RAGreedy::selectOrSplit(LiveInterval &VirtReg,
228                                SmallVectorImpl<LiveInterval*> &SplitVRegs) {
229  // Populate a list of physical register spill candidates.
230  SmallVector<unsigned, 8> PhysRegSpillCands, ReassignCands;
231
232  // Check for an available register in this class.
233  const TargetRegisterClass *TRC = MRI->getRegClass(VirtReg.reg);
234  DEBUG(dbgs() << "RegClass: " << TRC->getName() << ' ');
235
236  AllocationOrder Order(VirtReg.reg, *VRM, ReservedRegs);
237  while (unsigned PhysReg = Order.next()) {
238    // Check interference and as a side effect, intialize queries for this
239    // VirtReg and its aliases.
240    unsigned InterfReg = checkPhysRegInterference(VirtReg, PhysReg);
241    if (InterfReg == 0) {
242      // Found an available register.
243      return PhysReg;
244    }
245    assert(!VirtReg.empty() && "Empty VirtReg has interference");
246    LiveInterval *InterferingVirtReg =
247      Queries[InterfReg].firstInterference().liveUnionPos().value();
248
249    // The current VirtReg must either be spillable, or one of its interferences
250    // must have less spill weight.
251    if (InterferingVirtReg->weight < VirtReg.weight ) {
252      // For simplicity, only consider reassigning registers in the same class.
253      if (InterfReg == PhysReg)
254        ReassignCands.push_back(PhysReg);
255      else
256        PhysRegSpillCands.push_back(PhysReg);
257    }
258  }
259
260  // Try to reassign interfering physical register. Priority among
261  // PhysRegSpillCands does not matter yet, because the reassigned virtual
262  // registers will still be assigned to physical registers.
263  for (SmallVectorImpl<unsigned>::iterator PhysRegI = ReassignCands.begin(),
264         PhysRegE = ReassignCands.end(); PhysRegI != PhysRegE; ++PhysRegI) {
265    if (reassignInterferences(VirtReg, *PhysRegI))
266      // Reassignment successfull. The caller may allocate now to this PhysReg.
267      return *PhysRegI;
268  }
269
270  PhysRegSpillCands.insert(PhysRegSpillCands.end(), ReassignCands.begin(),
271                           ReassignCands.end());
272
273  // Try to spill another interfering reg with less spill weight.
274  //
275  // FIXME: do this in two steps: (1) check for unspillable interferences while
276  // accumulating spill weight; (2) spill the interferences with lowest
277  // aggregate spill weight.
278  for (SmallVectorImpl<unsigned>::iterator PhysRegI = PhysRegSpillCands.begin(),
279         PhysRegE = PhysRegSpillCands.end(); PhysRegI != PhysRegE; ++PhysRegI) {
280
281    if (!spillInterferences(VirtReg, *PhysRegI, SplitVRegs)) continue;
282
283    assert(checkPhysRegInterference(VirtReg, *PhysRegI) == 0 &&
284           "Interference after spill.");
285    // Tell the caller to allocate to this newly freed physical register.
286    return *PhysRegI;
287  }
288
289  // No other spill candidates were found, so spill the current VirtReg.
290  DEBUG(dbgs() << "spilling: " << VirtReg << '\n');
291  SmallVector<LiveInterval*, 1> pendingSpills;
292
293  spiller().spill(&VirtReg, SplitVRegs, pendingSpills);
294
295  // The live virtual register requesting allocation was spilled, so tell
296  // the caller not to allocate anything during this round.
297  return 0;
298}
299
300bool RAGreedy::runOnMachineFunction(MachineFunction &mf) {
301  DEBUG(dbgs() << "********** GREEDY REGISTER ALLOCATION **********\n"
302               << "********** Function: "
303               << ((Value*)mf.getFunction())->getName() << '\n');
304
305  MF = &mf;
306  TM = &mf.getTarget();
307  MRI = &mf.getRegInfo();
308
309  const TargetRegisterInfo *TRI = TM->getRegisterInfo();
310  RegAllocBase::init(*TRI, getAnalysis<VirtRegMap>(),
311                     getAnalysis<LiveIntervals>());
312
313  ReservedRegs = TRI->getReservedRegs(*MF);
314  SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM));
315  allocatePhysRegs();
316  addMBBLiveIns(MF);
317
318  // Run rewriter
319  std::auto_ptr<VirtRegRewriter> rewriter(createVirtRegRewriter());
320  rewriter->runOnMachineFunction(*MF, *VRM, LIS);
321
322  // The pass output is in VirtRegMap. Release all the transient data.
323  releaseMemory();
324
325  return true;
326}
327
328