CalcSpillWeights.cpp revision 6374c3d4d7b1fece8ed9acb590f809a0e6fb17ee 
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/TargetMachine.h"
24#include "llvm/Target/TargetRegisterInfo.h"
25using namespace llvm;
26
27char CalculateSpillWeights::ID = 0;
28INITIALIZE_PASS(CalculateSpillWeights, "calcspillweights",
29                "Calculate spill weights", false, false);
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  MachineRegisterInfo *mri = &fn.getRegInfo();
47
48  SmallSet<unsigned, 4> processed;
49  for (MachineFunction::iterator mbbi = fn.begin(), mbbe = fn.end();
50       mbbi != mbbe; ++mbbi) {
51    MachineBasicBlock* mbb = mbbi;
52    SlotIndex mbbEnd = lis->getMBBEndIdx(mbb);
53    MachineLoop* loop = loopInfo->getLoopFor(mbb);
54    unsigned loopDepth = loop ? loop->getLoopDepth() : 0;
55    bool isExiting = loop ? loop->isLoopExiting(mbb) : false;
56
57    for (MachineBasicBlock::const_iterator mii = mbb->begin(), mie = mbb->end();
58         mii != mie; ++mii) {
59      const MachineInstr *mi = mii;
60      if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
61        continue;
62
63      for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
64        const MachineOperand &mopi = mi->getOperand(i);
65        if (!mopi.isReg() || mopi.getReg() == 0)
66          continue;
67        unsigned reg = mopi.getReg();
68        if (!TargetRegisterInfo::isVirtualRegister(mopi.getReg()))
69          continue;
70        // Multiple uses of reg by the same instruction. It should not
71        // contribute to spill weight again.
72        if (!processed.insert(reg))
73          continue;
74
75        bool hasDef = mopi.isDef();
76        bool hasUse = !hasDef;
77        for (unsigned j = i+1; j != e; ++j) {
78          const MachineOperand &mopj = mi->getOperand(j);
79          if (!mopj.isReg() || mopj.getReg() != reg)
80            continue;
81          hasDef |= mopj.isDef();
82          hasUse |= mopj.isUse();
83          if (hasDef && hasUse)
84            break;
85        }
86
87        LiveInterval &regInt = lis->getInterval(reg);
88        float weight = lis->getSpillWeight(hasDef, hasUse, loopDepth);
89        if (hasDef && isExiting) {
90          // Looks like this is a loop count variable update.
91          SlotIndex defIdx = lis->getInstructionIndex(mi).getDefIndex();
92          const LiveRange *dlr =
93            lis->getInterval(reg).getLiveRangeContaining(defIdx);
94          if (dlr->end >= mbbEnd)
95            weight *= 3.0F;
96        }
97        regInt.weight += weight;
98      }
99      processed.clear();
100    }
101  }
102
103  for (LiveIntervals::iterator I = lis->begin(), E = lis->end(); I != E; ++I) {
104    LiveInterval &li = *I->second;
105    if (TargetRegisterInfo::isVirtualRegister(li.reg)) {
106      // If the live interval length is essentially zero, i.e. in every live
107      // range the use follows def immediately, it doesn't make sense to spill
108      // it and hope it will be easier to allocate for this li.
109      if (isZeroLengthInterval(&li)) {
110        li.weight = HUGE_VALF;
111        continue;
112      }
113
114      bool isLoad = false;
115      SmallVector<LiveInterval*, 4> spillIs;
116      if (lis->isReMaterializable(li, spillIs, isLoad)) {
117        // If all of the definitions of the interval are re-materializable,
118        // it is a preferred candidate for spilling. If none of the defs are
119        // loads, then it's potentially very cheap to re-materialize.
120        // FIXME: this gets much more complicated once we support non-trivial
121        // re-materialization.
122        if (isLoad)
123          li.weight *= 0.9F;
124        else
125          li.weight *= 0.5F;
126      }
127
128      // Slightly prefer live interval that has been assigned a preferred reg.
129      std::pair<unsigned, unsigned> Hint = mri->getRegAllocationHint(li.reg);
130      if (Hint.first || Hint.second)
131        li.weight *= 1.01F;
132
133      lis->normalizeSpillWeight(li);
134    }
135  }
136
137  return false;
138}
139
140/// Returns true if the given live interval is zero length.
141bool CalculateSpillWeights::isZeroLengthInterval(LiveInterval *li) const {
142  for (LiveInterval::Ranges::const_iterator
143       i = li->ranges.begin(), e = li->ranges.end(); i != e; ++i)
144    if (i->end.getPrevIndex() > i->start)
145      return false;
146  return true;
147}
148