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#include "llvm/CodeGen/VirtRegMap.h"
11#include "llvm/CodeGen/CalcSpillWeights.h"
12#include "llvm/CodeGen/LiveIntervalAnalysis.h"
13#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
14#include "llvm/CodeGen/MachineFunction.h"
15#include "llvm/CodeGen/MachineLoopInfo.h"
16#include "llvm/CodeGen/MachineRegisterInfo.h"
17#include "llvm/Support/Debug.h"
18#include "llvm/Support/raw_ostream.h"
19#include "llvm/Target/TargetInstrInfo.h"
20#include "llvm/Target/TargetRegisterInfo.h"
21#include "llvm/Target/TargetSubtargetInfo.h"
22using namespace llvm;
23
24#define DEBUG_TYPE "calcspillweights"
25
26void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
27                           MachineFunction &MF,
28                           VirtRegMap *VRM,
29                           const MachineLoopInfo &MLI,
30                           const MachineBlockFrequencyInfo &MBFI,
31                           VirtRegAuxInfo::NormalizingFn norm) {
32  DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
33               << "********** Function: " << MF.getName() << '\n');
34
35  MachineRegisterInfo &MRI = MF.getRegInfo();
36  VirtRegAuxInfo VRAI(MF, LIS, VRM, MLI, MBFI, norm);
37  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
38    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
39    if (MRI.reg_nodbg_empty(Reg))
40      continue;
41    VRAI.calculateSpillWeightAndHint(LIS.getInterval(Reg));
42  }
43}
44
45// Return the preferred allocation register for reg, given a COPY instruction.
46static unsigned copyHint(const MachineInstr *mi, unsigned reg,
47                         const TargetRegisterInfo &tri,
48                         const MachineRegisterInfo &mri) {
49  unsigned sub, hreg, hsub;
50  if (mi->getOperand(0).getReg() == reg) {
51    sub = mi->getOperand(0).getSubReg();
52    hreg = mi->getOperand(1).getReg();
53    hsub = mi->getOperand(1).getSubReg();
54  } else {
55    sub = mi->getOperand(1).getSubReg();
56    hreg = mi->getOperand(0).getReg();
57    hsub = mi->getOperand(0).getSubReg();
58  }
59
60  if (!hreg)
61    return 0;
62
63  if (TargetRegisterInfo::isVirtualRegister(hreg))
64    return sub == hsub ? hreg : 0;
65
66  const TargetRegisterClass *rc = mri.getRegClass(reg);
67
68  // Only allow physreg hints in rc.
69  if (sub == 0)
70    return rc->contains(hreg) ? hreg : 0;
71
72  // reg:sub should match the physreg hreg.
73  return tri.getMatchingSuperReg(hreg, sub, rc);
74}
75
76// Check if all values in LI are rematerializable
77static bool isRematerializable(const LiveInterval &LI,
78                               const LiveIntervals &LIS,
79                               VirtRegMap *VRM,
80                               const TargetInstrInfo &TII) {
81  unsigned Reg = LI.reg;
82  unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
83  for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
84       I != E; ++I) {
85    const VNInfo *VNI = *I;
86    if (VNI->isUnused())
87      continue;
88    if (VNI->isPHIDef())
89      return false;
90
91    MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
92    assert(MI && "Dead valno in interval");
93
94    // Trace copies introduced by live range splitting.  The inline
95    // spiller can rematerialize through these copies, so the spill
96    // weight must reflect this.
97    if (VRM) {
98      while (MI->isFullCopy()) {
99        // The copy destination must match the interval register.
100        if (MI->getOperand(0).getReg() != Reg)
101          return false;
102
103        // Get the source register.
104        Reg = MI->getOperand(1).getReg();
105
106        // If the original (pre-splitting) registers match this
107        // copy came from a split.
108        if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
109            VRM->getOriginal(Reg) != Original)
110          return false;
111
112        // Follow the copy live-in value.
113        const LiveInterval &SrcLI = LIS.getInterval(Reg);
114        LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
115        VNI = SrcQ.valueIn();
116        assert(VNI && "Copy from non-existing value");
117        if (VNI->isPHIDef())
118          return false;
119        MI = LIS.getInstructionFromIndex(VNI->def);
120        assert(MI && "Dead valno in interval");
121      }
122    }
123
124    if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
125      return false;
126  }
127  return true;
128}
129
130void
131VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
132  MachineRegisterInfo &mri = MF.getRegInfo();
133  const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
134  MachineBasicBlock *mbb = nullptr;
135  MachineLoop *loop = nullptr;
136  bool isExiting = false;
137  float totalWeight = 0;
138  unsigned numInstr = 0; // Number of instructions using li
139  SmallPtrSet<MachineInstr*, 8> visited;
140
141  // Find the best physreg hint and the best virtreg hint.
142  float bestPhys = 0, bestVirt = 0;
143  unsigned hintPhys = 0, hintVirt = 0;
144
145  // Don't recompute a target specific hint.
146  bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
147
148  // Don't recompute spill weight for an unspillable register.
149  bool Spillable = li.isSpillable();
150
151  for (MachineRegisterInfo::reg_instr_iterator
152       I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
153       I != E; ) {
154    MachineInstr *mi = &*(I++);
155    numInstr++;
156    if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
157      continue;
158    if (!visited.insert(mi).second)
159      continue;
160
161    float weight = 1.0f;
162    if (Spillable) {
163      // Get loop info for mi.
164      if (mi->getParent() != mbb) {
165        mbb = mi->getParent();
166        loop = Loops.getLoopFor(mbb);
167        isExiting = loop ? loop->isLoopExiting(mbb) : false;
168      }
169
170      // Calculate instr weight.
171      bool reads, writes;
172      std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
173      weight = LiveIntervals::getSpillWeight(
174        writes, reads, &MBFI, mi);
175
176      // Give extra weight to what looks like a loop induction variable update.
177      if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
178        weight *= 3;
179
180      totalWeight += weight;
181    }
182
183    // Get allocation hints from copies.
184    if (noHint || !mi->isCopy())
185      continue;
186    unsigned hint = copyHint(mi, li.reg, tri, mri);
187    if (!hint)
188      continue;
189    // Force hweight onto the stack so that x86 doesn't add hidden precision,
190    // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
191    //
192    // FIXME: we probably shouldn't use floats at all.
193    volatile float hweight = Hint[hint] += weight;
194    if (TargetRegisterInfo::isPhysicalRegister(hint)) {
195      if (hweight > bestPhys && mri.isAllocatable(hint))
196        bestPhys = hweight, hintPhys = hint;
197    } else {
198      if (hweight > bestVirt)
199        bestVirt = hweight, hintVirt = hint;
200    }
201  }
202
203  Hint.clear();
204
205  // Always prefer the physreg hint.
206  if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
207    mri.setRegAllocationHint(li.reg, 0, hint);
208    // Weakly boost the spill weight of hinted registers.
209    totalWeight *= 1.01F;
210  }
211
212  // If the live interval was already unspillable, leave it that way.
213  if (!Spillable)
214    return;
215
216  // Mark li as unspillable if all live ranges are tiny.
217  if (li.isZeroLength(LIS.getSlotIndexes())) {
218    li.markNotSpillable();
219    return;
220  }
221
222  // If all of the definitions of the interval are re-materializable,
223  // it is a preferred candidate for spilling.
224  // FIXME: this gets much more complicated once we support non-trivial
225  // re-materialization.
226  if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
227    totalWeight *= 0.5F;
228
229  li.weight = normalize(totalWeight, li.getSize(), numInstr);
230}
231