CalcSpillWeights.cpp revision a77da0579bc141eba62760e21a216e5d3eafd792 
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/CodeGen/CalcSpillWeights.h"
13#include "llvm/CodeGen/LiveIntervalAnalysis.h"
14#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
15#include "llvm/CodeGen/MachineFunction.h"
16#include "llvm/CodeGen/MachineLoopInfo.h"
17#include "llvm/CodeGen/MachineRegisterInfo.h"
18#include "llvm/Support/Debug.h"
19#include "llvm/Support/raw_ostream.h"
20#include "llvm/Target/TargetInstrInfo.h"
21#include "llvm/Target/TargetMachine.h"
22#include "llvm/Target/TargetRegisterInfo.h"
23using namespace llvm;
24
25void llvm::calculateSpillWeights(LiveIntervals &LIS,
26                           MachineFunction &MF,
27                           const MachineLoopInfo &MLI,
28                           const MachineBlockFrequencyInfo &MBFI) {
29  DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
30               << "********** Function: " << MF.getName() << '\n');
31
32  MachineRegisterInfo &MRI = MF.getRegInfo();
33  VirtRegAuxInfo VRAI(MF, LIS, MLI, MBFI);
34  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
35    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
36    if (MRI.reg_nodbg_empty(Reg))
37      continue;
38    VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
39  }
40}
41
42// Return the preferred allocation register for reg, given a COPY instruction.
43static unsigned copyHint(const MachineInstr *mi, unsigned reg,
44                         const TargetRegisterInfo &tri,
45                         const MachineRegisterInfo &mri) {
46  unsigned sub, hreg, hsub;
47  if (mi->getOperand(0).getReg() == reg) {
48    sub = mi->getOperand(0).getSubReg();
49    hreg = mi->getOperand(1).getReg();
50    hsub = mi->getOperand(1).getSubReg();
51  } else {
52    sub = mi->getOperand(1).getSubReg();
53    hreg = mi->getOperand(0).getReg();
54    hsub = mi->getOperand(0).getSubReg();
55  }
56
57  if (!hreg)
58    return 0;
59
60  if (TargetRegisterInfo::isVirtualRegister(hreg))
61    return sub == hsub ? hreg : 0;
62
63  const TargetRegisterClass *rc = mri.getRegClass(reg);
64
65  // Only allow physreg hints in rc.
66  if (sub == 0)
67    return rc->contains(hreg) ? hreg : 0;
68
69  // reg:sub should match the physreg hreg.
70  return tri.getMatchingSuperReg(hreg, sub, rc);
71}
72
73// Check if all values in LI are rematerializable
74static bool isRematerializable(const LiveInterval &LI,
75                               const LiveIntervals &LIS,
76                               const TargetInstrInfo &TII) {
77  for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
78       I != E; ++I) {
79    const VNInfo *VNI = *I;
80    if (VNI->isUnused())
81      continue;
82    if (VNI->isPHIDef())
83      return false;
84
85    MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
86    assert(MI && "Dead valno in interval");
87
88    if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
89      return false;
90  }
91  return true;
92}
93
94void
95VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
96  MachineRegisterInfo &mri = MF.getRegInfo();
97  const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
98  MachineBasicBlock *mbb = 0;
99  MachineLoop *loop = 0;
100  bool isExiting = false;
101  float totalWeight = 0;
102  SmallPtrSet<MachineInstr*, 8> visited;
103
104  // Find the best physreg hint and the best virtreg hint.
105  float bestPhys = 0, bestVirt = 0;
106  unsigned hintPhys = 0, hintVirt = 0;
107
108  // Don't recompute a target specific hint.
109  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
110
111  // Don't recompute spill weight for an unspillable register.
112  bool Spillable = li.isSpillable();
113
114  for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
115       MachineInstr *mi = I.skipInstruction();) {
116    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
117      continue;
118    if (!visited.insert(mi))
119      continue;
120
121    float weight = 1.0f;
122    if (Spillable) {
123      // Get loop info for mi.
124      if (mi->getParent() != mbb) {
125        mbb = mi->getParent();
126        loop = Loops.getLoopFor(mbb);
127        isExiting = loop ? loop->isLoopExiting(mbb) : false;
128      }
129
130      // Calculate instr weight.
131      bool reads, writes;
132      tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
133      weight = LiveIntervals::getSpillWeight(
134          writes, reads, MBFI.getBlockFreq(mi->getParent()));
135
136      // Give extra weight to what looks like a loop induction variable update.
137      if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
138        weight *= 3;
139
140      totalWeight += weight;
141    }
142
143    // Get allocation hints from copies.
144    if (noHint || !mi->isCopy())
145      continue;
146    unsigned hint = copyHint(mi, li.reg, tri, mri);
147    if (!hint)
148      continue;
149    float hweight = Hint[hint] += weight;
150    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
151      if (hweight > bestPhys && mri.isAllocatable(hint))
152        bestPhys = hweight, hintPhys = hint;
153    } else {
154      if (hweight > bestVirt)
155        bestVirt = hweight, hintVirt = hint;
156    }
157  }
158
159  Hint.clear();
160
161  // Always prefer the physreg hint.
162  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
163    mri.setRegAllocationHint(li.reg, 0, hint);
164    // Weakly boost the spill weight of hinted registers.
165    totalWeight *= 1.01F;
166  }
167
168  // If the live interval was already unspillable, leave it that way.
169  if (!Spillable)
170    return;
171
172  // Mark li as unspillable if all live ranges are tiny.
173  if (li.isZeroLength(LIS.getSlotIndexes())) {
174    li.markNotSpillable();
175    return;
176  }
177
178  // If all of the definitions of the interval are re-materializable,
179  // it is a preferred candidate for spilling.
180  // FIXME: this gets much more complicated once we support non-trivial
181  // re-materialization.
182  if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
183    totalWeight *= 0.5F;
184
185  li.weight = normalizeSpillWeight(totalWeight, li.getSize());
186}
187