CalcSpillWeights.cpp revision 1646071b23513b28e27f5029ccedd2fb1797a5e1 
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/Function.h"
13#include "llvm/ADT/SmallSet.h"
14#include "llvm/CodeGen/CalcSpillWeights.h"
15#include "llvm/CodeGen/LiveIntervalAnalysis.h"
16#include "llvm/CodeGen/MachineFunction.h"
17#include "llvm/CodeGen/MachineLoopInfo.h"
18#include "llvm/CodeGen/MachineRegisterInfo.h"
19#include "llvm/CodeGen/SlotIndexes.h"
20#include "llvm/Support/Debug.h"
21#include "llvm/Support/raw_ostream.h"
22#include "llvm/Target/TargetInstrInfo.h"
23#include "llvm/Target/TargetMachine.h"
24#include "llvm/Target/TargetRegisterInfo.h"
25using namespace llvm;
26
27char CalculateSpillWeights::ID = 0;
28INITIALIZE_PASS(CalculateSpillWeights, "calcspillweights",
29                "Calculate spill weights", false, false);
30
31void CalculateSpillWeights::getAnalysisUsage(AnalysisUsage &au) const {
32  au.addRequired<LiveIntervals>();
33  au.addRequired<MachineLoopInfo>();
34  au.setPreservesAll();
35  MachineFunctionPass::getAnalysisUsage(au);
36}
37
38bool CalculateSpillWeights::runOnMachineFunction(MachineFunction &fn) {
39
40  DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
41               << "********** Function: "
42               << fn.getFunction()->getName() << '\n');
43
44  LiveIntervals &lis = getAnalysis<LiveIntervals>();
45  VirtRegAuxInfo vrai(fn, lis, getAnalysis<MachineLoopInfo>());
46  for (LiveIntervals::iterator I = lis.begin(), E = lis.end(); I != E; ++I) {
47    LiveInterval &li = *I->second;
48    if (TargetRegisterInfo::isVirtualRegister(li.reg))
49      vrai.CalculateWeightAndHint(li);
50  }
51  return false;
52}
53
54// Return the preferred allocation register for reg, given a COPY instruction.
55static unsigned copyHint(const MachineInstr *mi, unsigned reg,
56                         const TargetRegisterInfo &tri,
57                         const MachineRegisterInfo &mri) {
58  unsigned sub, hreg, hsub;
59  if (mi->getOperand(0).getReg() == reg) {
60    sub = mi->getOperand(0).getSubReg();
61    hreg = mi->getOperand(1).getReg();
62    hsub = mi->getOperand(1).getSubReg();
63  } else {
64    sub = mi->getOperand(1).getSubReg();
65    hreg = mi->getOperand(0).getReg();
66    hsub = mi->getOperand(0).getSubReg();
67  }
68
69  if (!hreg)
70    return 0;
71
72  if (TargetRegisterInfo::isVirtualRegister(hreg))
73    return sub == hsub ? hreg : 0;
74
75  const TargetRegisterClass *rc = mri.getRegClass(reg);
76
77  // Only allow physreg hints in rc.
78  if (sub == 0)
79    return rc->contains(hreg) ? hreg : 0;
80
81  // reg:sub should match the physreg hreg.
82  return tri.getMatchingSuperReg(hreg, sub, rc);
83}
84
85void VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
86  MachineRegisterInfo &mri = mf_.getRegInfo();
87  const TargetRegisterInfo &tri = *mf_.getTarget().getRegisterInfo();
88  MachineBasicBlock *mbb = 0;
89  MachineLoop *loop = 0;
90  unsigned loopDepth = 0;
91  bool isExiting = false;
92  float totalWeight = 0;
93  SmallPtrSet<MachineInstr*, 8> visited;
94
95  // Find the best physreg hist and the best virtreg hint.
96  float bestPhys = 0, bestVirt = 0;
97  unsigned hintPhys = 0, hintVirt = 0;
98
99  // Don't recompute a target specific hint.
100  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
101
102  for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
103       MachineInstr *mi = I.skipInstruction();) {
104    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
105      continue;
106    if (!visited.insert(mi))
107      continue;
108
109    // Get loop info for mi.
110    if (mi->getParent() != mbb) {
111      mbb = mi->getParent();
112      loop = loops_.getLoopFor(mbb);
113      loopDepth = loop ? loop->getLoopDepth() : 0;
114      isExiting = loop ? loop->isLoopExiting(mbb) : false;
115    }
116
117    // Calculate instr weight.
118    bool reads, writes;
119    tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
120    float weight = LiveIntervals::getSpillWeight(writes, reads, loopDepth);
121
122    // Give extra weight to what looks like a loop induction variable update.
123    if (writes && isExiting && lis_.isLiveOutOfMBB(li, mbb))
124      weight *= 3;
125
126    totalWeight += weight;
127
128    // Get allocation hints from copies.
129    if (noHint || !mi->isCopy())
130      continue;
131    unsigned hint = copyHint(mi, li.reg, tri, mri);
132    if (!hint)
133      continue;
134    float hweight = hint_[hint] += weight;
135    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
136      if (hweight > bestPhys && lis_.isAllocatable(hint))
137        bestPhys = hweight, hintPhys = hint;
138    } else {
139      if (hweight > bestVirt)
140        bestVirt = hweight, hintVirt = hint;
141    }
142  }
143
144  hint_.clear();
145
146  // Always prefer the physreg hint.
147  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
148    mri.setRegAllocationHint(li.reg, 0, hint);
149    // Weakly boost the spill weifght of hinted registers.
150    totalWeight *= 1.01F;
151  }
152
153  // Mark li as unspillable if all live ranges are tiny.
154  if (li.isZeroLength()) {
155    li.markNotSpillable();
156    return;
157  }
158
159  // If all of the definitions of the interval are re-materializable,
160  // it is a preferred candidate for spilling. If none of the defs are
161  // loads, then it's potentially very cheap to re-materialize.
162  // FIXME: this gets much more complicated once we support non-trivial
163  // re-materialization.
164  bool isLoad = false;
165  SmallVector<LiveInterval*, 4> spillIs;
166  if (lis_.isReMaterializable(li, spillIs, isLoad)) {
167    if (isLoad)
168      totalWeight *= 0.9F;
169    else
170      totalWeight *= 0.5F;
171  }
172
173  li.weight = totalWeight;
174  lis_.normalizeSpillWeight(li);
175}
176
177