CalcSpillWeights.cpp revision 96601ca332ab388754ca4673be8973396fea2ddd 
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: "
45               << MF.getName() << '\n');
46
47  LiveIntervals &LIS = getAnalysis<LiveIntervals>();
48  MachineRegisterInfo &MRI = MF.getRegInfo();
49  VirtRegAuxInfo VRAI(MF, LIS, getAnalysis<MachineLoopInfo>());
50  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
51    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
52    if (MRI.reg_nodbg_empty(Reg))
53      continue;
54    VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
55  }
56  return false;
57}
58
59// Return the preferred allocation register for reg, given a COPY instruction.
60static unsigned copyHint(const MachineInstr *mi, unsigned reg,
61                         const TargetRegisterInfo &tri,
62                         const MachineRegisterInfo &mri) {
63  unsigned sub, hreg, hsub;
64  if (mi->getOperand(0).getReg() == reg) {
65    sub = mi->getOperand(0).getSubReg();
66    hreg = mi->getOperand(1).getReg();
67    hsub = mi->getOperand(1).getSubReg();
68  } else {
69    sub = mi->getOperand(1).getSubReg();
70    hreg = mi->getOperand(0).getReg();
71    hsub = mi->getOperand(0).getSubReg();
72  }
73
74  if (!hreg)
75    return 0;
76
77  if (TargetRegisterInfo::isVirtualRegister(hreg))
78    return sub == hsub ? hreg : 0;
79
80  const TargetRegisterClass *rc = mri.getRegClass(reg);
81
82  // Only allow physreg hints in rc.
83  if (sub == 0)
84    return rc->contains(hreg) ? hreg : 0;
85
86  // reg:sub should match the physreg hreg.
87  return tri.getMatchingSuperReg(hreg, sub, rc);
88}
89
90// Check if all values in LI are rematerializable
91static bool isRematerializable(const LiveInterval &LI,
92                               const LiveIntervals &LIS,
93                               const TargetInstrInfo &TII) {
94  for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
95       I != E; ++I) {
96    const VNInfo *VNI = *I;
97    if (VNI->isUnused())
98      continue;
99    if (VNI->isPHIDef())
100      return false;
101
102    MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
103    assert(MI && "Dead valno in interval");
104
105    if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
106      return false;
107  }
108  return true;
109}
110
111void VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
112  MachineRegisterInfo &mri = MF.getRegInfo();
113  const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
114  MachineBasicBlock *mbb = 0;
115  MachineLoop *loop = 0;
116  unsigned loopDepth = 0;
117  bool isExiting = false;
118  float totalWeight = 0;
119  SmallPtrSet<MachineInstr*, 8> visited;
120
121  // Find the best physreg hist and the best virtreg hint.
122  float bestPhys = 0, bestVirt = 0;
123  unsigned hintPhys = 0, hintVirt = 0;
124
125  // Don't recompute a target specific hint.
126  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
127
128  // Don't recompute spill weight for an unspillable register.
129  bool Spillable = li.isSpillable();
130
131  for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
132       MachineInstr *mi = I.skipInstruction();) {
133    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
134      continue;
135    if (!visited.insert(mi))
136      continue;
137
138    float weight = 1.0f;
139    if (Spillable) {
140      // Get loop info for mi.
141      if (mi->getParent() != mbb) {
142        mbb = mi->getParent();
143        loop = Loops.getLoopFor(mbb);
144        loopDepth = loop ? loop->getLoopDepth() : 0;
145        isExiting = loop ? loop->isLoopExiting(mbb) : false;
146      }
147
148      // Calculate instr weight.
149      bool reads, writes;
150      tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
151      weight = LiveIntervals::getSpillWeight(writes, reads, loopDepth);
152
153      // Give extra weight to what looks like a loop induction variable update.
154      if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
155        weight *= 3;
156
157      totalWeight += weight;
158    }
159
160    // Get allocation hints from copies.
161    if (noHint || !mi->isCopy())
162      continue;
163    unsigned hint = copyHint(mi, li.reg, tri, mri);
164    if (!hint)
165      continue;
166    float hweight = Hint[hint] += weight;
167    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
168      if (hweight > bestPhys && LIS.isAllocatable(hint))
169        bestPhys = hweight, hintPhys = hint;
170    } else {
171      if (hweight > bestVirt)
172        bestVirt = hweight, hintVirt = hint;
173    }
174  }
175
176  Hint.clear();
177
178  // Always prefer the physreg hint.
179  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
180    mri.setRegAllocationHint(li.reg, 0, hint);
181    // Weakly boost the spill weight of hinted registers.
182    totalWeight *= 1.01F;
183  }
184
185  // If the live interval was already unspillable, leave it that way.
186  if (!Spillable)
187    return;
188
189  // Mark li as unspillable if all live ranges are tiny.
190  if (li.isZeroLength(LIS.getSlotIndexes())) {
191    li.markNotSpillable();
192    return;
193  }
194
195  // If all of the definitions of the interval are re-materializable,
196  // it is a preferred candidate for spilling.
197  // FIXME: this gets much more complicated once we support non-trivial
198  // re-materialization.
199  if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
200    totalWeight *= 0.5F;
201
202  li.weight = normalizeSpillWeight(totalWeight, li.getSize());
203}
204