CalcSpillWeights.cpp revision d94998f52574eacef148bd856de701af2c594b03 
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/TargetRegisterInfo.h"
24
25using namespace llvm;
26
27char CalculateSpillWeights::ID = 0;
28static RegisterPass<CalculateSpillWeights> X("calcspillweights",
29                                             "Calculate spill weights");
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  MachineLoopInfo *loopInfo = &getAnalysis<MachineLoopInfo>();
46  const TargetInstrInfo *tii = fn.getTarget().getInstrInfo();
47  MachineRegisterInfo *mri = &fn.getRegInfo();
48
49  SmallSet<unsigned, 4> processed;
50  for (MachineFunction::iterator mbbi = fn.begin(), mbbe = fn.end();
51       mbbi != mbbe; ++mbbi) {
52    MachineBasicBlock* mbb = mbbi;
53    SlotIndex mbbEnd = lis->getMBBEndIdx(mbb);
54    MachineLoop* loop = loopInfo->getLoopFor(mbb);
55    unsigned loopDepth = loop ? loop->getLoopDepth() : 0;
56    bool isExiting = loop ? loop->isLoopExiting(mbb) : false;
57
58    for (MachineBasicBlock::const_iterator mii = mbb->begin(), mie = mbb->end();
59         mii != mie; ++mii) {
60      const MachineInstr *mi = mii;
61      if (tii->isIdentityCopy(*mi))
62        continue;
63
64      if (mi->getOpcode() == TargetInstrInfo::IMPLICIT_DEF)
65        continue;
66
67      if (mi->getOpcode() == TargetInstrInfo::DEBUG_VALUE)
68        continue;
69
70      for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
71        const MachineOperand &mopi = mi->getOperand(i);
72        if (!mopi.isReg() || mopi.getReg() == 0)
73          continue;
74        unsigned reg = mopi.getReg();
75        if (!TargetRegisterInfo::isVirtualRegister(mopi.getReg()))
76          continue;
77        // Multiple uses of reg by the same instruction. It should not
78        // contribute to spill weight again.
79        if (!processed.insert(reg))
80          continue;
81
82        bool hasDef = mopi.isDef();
83        bool hasUse = !hasDef;
84        for (unsigned j = i+1; j != e; ++j) {
85          const MachineOperand &mopj = mi->getOperand(j);
86          if (!mopj.isReg() || mopj.getReg() != reg)
87            continue;
88          hasDef |= mopj.isDef();
89          hasUse |= mopj.isUse();
90          if (hasDef && hasUse)
91            break;
92        }
93
94        LiveInterval &regInt = lis->getInterval(reg);
95        float weight = lis->getSpillWeight(hasDef, hasUse, loopDepth);
96        if (hasDef && isExiting) {
97          // Looks like this is a loop count variable update.
98          SlotIndex defIdx = lis->getInstructionIndex(mi).getDefIndex();
99          const LiveRange *dlr =
100            lis->getInterval(reg).getLiveRangeContaining(defIdx);
101          if (dlr->end >= mbbEnd)
102            weight *= 3.0F;
103        }
104        regInt.weight += weight;
105      }
106      processed.clear();
107    }
108  }
109
110  for (LiveIntervals::iterator I = lis->begin(), E = lis->end(); I != E; ++I) {
111    LiveInterval &li = *I->second;
112    if (TargetRegisterInfo::isVirtualRegister(li.reg)) {
113      // If the live interval length is essentially zero, i.e. in every live
114      // range the use follows def immediately, it doesn't make sense to spill
115      // it and hope it will be easier to allocate for this li.
116      if (isZeroLengthInterval(&li)) {
117        li.weight = HUGE_VALF;
118        continue;
119      }
120
121      bool isLoad = false;
122      SmallVector<LiveInterval*, 4> spillIs;
123      if (lis->isReMaterializable(li, spillIs, isLoad)) {
124        // If all of the definitions of the interval are re-materializable,
125        // it is a preferred candidate for spilling. If non of the defs are
126        // loads, then it's potentially very cheap to re-materialize.
127        // FIXME: this gets much more complicated once we support non-trivial
128        // re-materialization.
129        if (isLoad)
130          li.weight *= 0.9F;
131        else
132          li.weight *= 0.5F;
133      }
134
135      // Slightly prefer live interval that has been assigned a preferred reg.
136      std::pair<unsigned, unsigned> Hint = mri->getRegAllocationHint(li.reg);
137      if (Hint.first || Hint.second)
138        li.weight *= 1.01F;
139
140      // Divide the weight of the interval by its size.  This encourages
141      // spilling of intervals that are large and have few uses, and
142      // discourages spilling of small intervals with many uses.
143      li.weight /= lis->getApproximateInstructionCount(li) * SlotIndex::NUM;
144    }
145  }
146
147  return false;
148}
149
150/// Returns true if the given live interval is zero length.
151bool CalculateSpillWeights::isZeroLengthInterval(LiveInterval *li) const {
152  for (LiveInterval::Ranges::const_iterator
153       i = li->ranges.begin(), e = li->ranges.end(); i != e; ++i)
154    if (i->end.getPrevIndex() > i->start)
155      return false;
156  return true;
157}
158