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      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"
     25 using namespace llvm;
     26 
     27 char CalculateSpillWeights::ID = 0;
     28 INITIALIZE_PASS_BEGIN(CalculateSpillWeights, "calcspillweights",
     29                 "Calculate spill weights", false, false)
     30 INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
     31 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
     32 INITIALIZE_PASS_END(CalculateSpillWeights, "calcspillweights",
     33                 "Calculate spill weights", false, false)
     34 
     35 void 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 
     43 bool 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.
     62 static 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
     93 static 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 
    113 void
    114 VirtRegAuxInfo::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