CalcSpillWeights.cpp revision 986d76d7b3844b9a2f3d01a48975952749267a93 
1//===------------------------ CalcSpillWeights.cpp ------------------------===//
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#define DEBUG_TYPE "calcspillweights"
11
12#include "llvm/ADT/SmallSet.h"
13#include "llvm/CodeGen/CalcSpillWeights.h"
14#include "llvm/CodeGen/LiveIntervalAnalysis.h"
15#include "llvm/CodeGen/MachineFunction.h"
16#include "llvm/CodeGen/MachineLoopInfo.h"
17#include "llvm/CodeGen/MachineRegisterInfo.h"
18#include "llvm/CodeGen/SlotIndexes.h"
19#include "llvm/Support/Debug.h"
20#include "llvm/Support/raw_ostream.h"
21#include "llvm/Target/TargetInstrInfo.h"
22#include "llvm/Target/TargetMachine.h"
23#include "llvm/Target/TargetRegisterInfo.h"
24using namespace llvm;
25
26char CalculateSpillWeights::ID = 0;
27INITIALIZE_PASS_BEGIN(CalculateSpillWeights, "calcspillweights",
28                "Calculate spill weights", false, false)
29INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
30INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
31INITIALIZE_PASS_END(CalculateSpillWeights, "calcspillweights",
32                "Calculate spill weights", false, false)
33
34void CalculateSpillWeights::getAnalysisUsage(AnalysisUsage &au) const {
35  au.addRequired<LiveIntervals>();
36  au.addRequired<MachineLoopInfo>();
37  au.setPreservesAll();
38  MachineFunctionPass::getAnalysisUsage(au);
39}
40
41bool CalculateSpillWeights::runOnMachineFunction(MachineFunction &MF) {
42
43  DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
44               << "********** Function: " << MF.getName() << '\n');
45
46  LiveIntervals &LIS = getAnalysis<LiveIntervals>();
47  MachineRegisterInfo &MRI = MF.getRegInfo();
48  VirtRegAuxInfo VRAI(MF, LIS, getAnalysis<MachineLoopInfo>());
49  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
50    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
51    if (MRI.reg_nodbg_empty(Reg))
52      continue;
53    VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
54  }
55  return false;
56}
57
58// Return the preferred allocation register for reg, given a COPY instruction.
59static unsigned copyHint(const MachineInstr *mi, unsigned reg,
60                         const TargetRegisterInfo &tri,
61                         const MachineRegisterInfo &mri) {
62  unsigned sub, hreg, hsub;
63  if (mi->getOperand(0).getReg() == reg) {
64    sub = mi->getOperand(0).getSubReg();
65    hreg = mi->getOperand(1).getReg();
66    hsub = mi->getOperand(1).getSubReg();
67  } else {
68    sub = mi->getOperand(1).getSubReg();
69    hreg = mi->getOperand(0).getReg();
70    hsub = mi->getOperand(0).getSubReg();
71  }
72
73  if (!hreg)
74    return 0;
75
76  if (TargetRegisterInfo::isVirtualRegister(hreg))
77    return sub == hsub ? hreg : 0;
78
79  const TargetRegisterClass *rc = mri.getRegClass(reg);
80
81  // Only allow physreg hints in rc.
82  if (sub == 0)
83    return rc->contains(hreg) ? hreg : 0;
84
85  // reg:sub should match the physreg hreg.
86  return tri.getMatchingSuperReg(hreg, sub, rc);
87}
88
89// Check if all values in LI are rematerializable
90static bool isRematerializable(const LiveInterval &LI,
91                               const LiveIntervals &LIS,
92                               const TargetInstrInfo &TII) {
93  for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
94       I != E; ++I) {
95    const VNInfo *VNI = *I;
96    if (VNI->isUnused())
97      continue;
98    if (VNI->isPHIDef())
99      return false;
100
101    MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
102    assert(MI && "Dead valno in interval");
103
104    if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
105      return false;
106  }
107  return true;
108}
109
110void VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
111  MachineRegisterInfo &mri = MF.getRegInfo();
112  const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
113  MachineBasicBlock *mbb = 0;
114  MachineLoop *loop = 0;
115  unsigned loopDepth = 0;
116  bool isExiting = false;
117  float totalWeight = 0;
118  SmallPtrSet<MachineInstr*, 8> visited;
119
120  // Find the best physreg hist and the best virtreg hint.
121  float bestPhys = 0, bestVirt = 0;
122  unsigned hintPhys = 0, hintVirt = 0;
123
124  // Don't recompute a target specific hint.
125  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
126
127  // Don't recompute spill weight for an unspillable register.
128  bool Spillable = li.isSpillable();
129
130  for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
131       MachineInstr *mi = I.skipInstruction();) {
132    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
133      continue;
134    if (!visited.insert(mi))
135      continue;
136
137    float weight = 1.0f;
138    if (Spillable) {
139      // Get loop info for mi.
140      if (mi->getParent() != mbb) {
141        mbb = mi->getParent();
142        loop = Loops.getLoopFor(mbb);
143        loopDepth = loop ? loop->getLoopDepth() : 0;
144        isExiting = loop ? loop->isLoopExiting(mbb) : false;
145      }
146
147      // Calculate instr weight.
148      bool reads, writes;
149      tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
150      weight = LiveIntervals::getSpillWeight(writes, reads, loopDepth);
151
152      // Give extra weight to what looks like a loop induction variable update.
153      if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
154        weight *= 3;
155
156      totalWeight += weight;
157    }
158
159    // Get allocation hints from copies.
160    if (noHint || !mi->isCopy())
161      continue;
162    unsigned hint = copyHint(mi, li.reg, tri, mri);
163    if (!hint)
164      continue;
165    float hweight = Hint[hint] += weight;
166    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
167      if (hweight > bestPhys && LIS.isAllocatable(hint))
168        bestPhys = hweight, hintPhys = hint;
169    } else {
170      if (hweight > bestVirt)
171        bestVirt = hweight, hintVirt = hint;
172    }
173  }
174
175  Hint.clear();
176
177  // Always prefer the physreg hint.
178  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
179    mri.setRegAllocationHint(li.reg, 0, hint);
180    // Weakly boost the spill weight of hinted registers.
181    totalWeight *= 1.01F;
182  }
183
184  // If the live interval was already unspillable, leave it that way.
185  if (!Spillable)
186    return;
187
188  // Mark li as unspillable if all live ranges are tiny.
189  if (li.isZeroLength(LIS.getSlotIndexes())) {
190    li.markNotSpillable();
191    return;
192  }
193
194  // If all of the definitions of the interval are re-materializable,
195  // it is a preferred candidate for spilling.
196  // FIXME: this gets much more complicated once we support non-trivial
197  // re-materialization.
198  if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
199    totalWeight *= 0.5F;
200
201  li.weight = normalizeSpillWeight(totalWeight, li.getSize());
202}
203