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/MachineBlockFrequencyInfo.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_BEGIN(CalculateSpillWeights, "calcspillweights",
29                "Calculate spill weights", false, false)
30INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
31INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
32INITIALIZE_PASS_END(CalculateSpillWeights, "calcspillweights",
33                "Calculate spill weights", false, false)
34
35void CalculateSpillWeights::getAnalysisUsage(AnalysisUsage &au) const {
36  au.addRequired<LiveIntervals>();
37  au.addRequired<MachineBlockFrequencyInfo>();
38  au.addRequired<MachineLoopInfo>();
39  au.setPreservesAll();
40  MachineFunctionPass::getAnalysisUsage(au);
41}
42
43bool CalculateSpillWeights::runOnMachineFunction(MachineFunction &MF) {
44
45  DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
46               << "********** Function: " << MF.getName() << '\n');
47
48  LiveIntervals &LIS = getAnalysis<LiveIntervals>();
49  MachineRegisterInfo &MRI = MF.getRegInfo();
50  VirtRegAuxInfo VRAI(MF, LIS, getAnalysis<MachineLoopInfo>(),
51                      getAnalysis<MachineBlockFrequencyInfo>());
52  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
53    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
54    if (MRI.reg_nodbg_empty(Reg))
55      continue;
56    VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
57  }
58  return false;
59}
60
61// Return the preferred allocation register for reg, given a COPY instruction.
62static unsigned copyHint(const MachineInstr *mi, unsigned reg,
63                         const TargetRegisterInfo &tri,
64                         const MachineRegisterInfo &mri) {
65  unsigned sub, hreg, hsub;
66  if (mi->getOperand(0).getReg() == reg) {
67    sub = mi->getOperand(0).getSubReg();
68    hreg = mi->getOperand(1).getReg();
69    hsub = mi->getOperand(1).getSubReg();
70  } else {
71    sub = mi->getOperand(1).getSubReg();
72    hreg = mi->getOperand(0).getReg();
73    hsub = mi->getOperand(0).getSubReg();
74  }
75
76  if (!hreg)
77    return 0;
78
79  if (TargetRegisterInfo::isVirtualRegister(hreg))
80    return sub == hsub ? hreg : 0;
81
82  const TargetRegisterClass *rc = mri.getRegClass(reg);
83
84  // Only allow physreg hints in rc.
85  if (sub == 0)
86    return rc->contains(hreg) ? hreg : 0;
87
88  // reg:sub should match the physreg hreg.
89  return tri.getMatchingSuperReg(hreg, sub, rc);
90}
91
92// Check if all values in LI are rematerializable
93static bool isRematerializable(const LiveInterval &LI,
94                               const LiveIntervals &LIS,
95                               const TargetInstrInfo &TII) {
96  for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
97       I != E; ++I) {
98    const VNInfo *VNI = *I;
99    if (VNI->isUnused())
100      continue;
101    if (VNI->isPHIDef())
102      return false;
103
104    MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
105    assert(MI && "Dead valno in interval");
106
107    if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
108      return false;
109  }
110  return true;
111}
112
113void
114VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
115  MachineRegisterInfo &mri = MF.getRegInfo();
116  const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
117  MachineBasicBlock *mbb = 0;
118  MachineLoop *loop = 0;
119  bool isExiting = false;
120  float totalWeight = 0;
121  SmallPtrSet<MachineInstr*, 8> visited;
122
123  // Find the best physreg hint and the best virtreg hint.
124  float bestPhys = 0, bestVirt = 0;
125  unsigned hintPhys = 0, hintVirt = 0;
126
127  // Don't recompute a target specific hint.
128  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
129
130  // Don't recompute spill weight for an unspillable register.
131  bool Spillable = li.isSpillable();
132
133  for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
134       MachineInstr *mi = I.skipInstruction();) {
135    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
136      continue;
137    if (!visited.insert(mi))
138      continue;
139
140    float weight = 1.0f;
141    if (Spillable) {
142      // Get loop info for mi.
143      if (mi->getParent() != mbb) {
144        mbb = mi->getParent();
145        loop = Loops.getLoopFor(mbb);
146        isExiting = loop ? loop->isLoopExiting(mbb) : false;
147      }
148
149      // Calculate instr weight.
150      bool reads, writes;
151      tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
152      weight = LiveIntervals::getSpillWeight(
153          writes, reads, MBFI.getBlockFreq(mi->getParent()));
154
155      // Give extra weight to what looks like a loop induction variable update.
156      if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
157        weight *= 3;
158
159      totalWeight += weight;
160    }
161
162    // Get allocation hints from copies.
163    if (noHint || !mi->isCopy())
164      continue;
165    unsigned hint = copyHint(mi, li.reg, tri, mri);
166    if (!hint)
167      continue;
168    float hweight = Hint[hint] += weight;
169    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
170      if (hweight > bestPhys && mri.isAllocatable(hint))
171        bestPhys = hweight, hintPhys = hint;
172    } else {
173      if (hweight > bestVirt)
174        bestVirt = hweight, hintVirt = hint;
175    }
176  }
177
178  Hint.clear();
179
180  // Always prefer the physreg hint.
181  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
182    mri.setRegAllocationHint(li.reg, 0, hint);
183    // Weakly boost the spill weight of hinted registers.
184    totalWeight *= 1.01F;
185  }
186
187  // If the live interval was already unspillable, leave it that way.
188  if (!Spillable)
189    return;
190
191  // Mark li as unspillable if all live ranges are tiny.
192  if (li.isZeroLength(LIS.getSlotIndexes())) {
193    li.markNotSpillable();
194    return;
195  }
196
197  // If all of the definitions of the interval are re-materializable,
198  // it is a preferred candidate for spilling.
199  // FIXME: this gets much more complicated once we support non-trivial
200  // re-materialization.
201  if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
202    totalWeight *= 0.5F;
203
204  li.weight = normalizeSpillWeight(totalWeight, li.getSize());
205}
206