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