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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/MachineFunction.h"
     16 #include "llvm/CodeGen/MachineLoopInfo.h"
     17 #include "llvm/CodeGen/MachineRegisterInfo.h"
     18 #include "llvm/CodeGen/SlotIndexes.h"
     19 #include "llvm/Support/Debug.h"
     20 #include "llvm/Support/raw_ostream.h"
     21 #include "llvm/Target/TargetInstrInfo.h"
     22 #include "llvm/Target/TargetMachine.h"
     23 #include "llvm/Target/TargetRegisterInfo.h"
     24 using namespace llvm;
     25 
     26 char CalculateSpillWeights::ID = 0;
     27 INITIALIZE_PASS_BEGIN(CalculateSpillWeights, "calcspillweights",
     28                 "Calculate spill weights", false, false)
     29 INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
     30 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
     31 INITIALIZE_PASS_END(CalculateSpillWeights, "calcspillweights",
     32                 "Calculate spill weights", false, false)
     33 
     34 void CalculateSpillWeights::getAnalysisUsage(AnalysisUsage &au) const {
     35   au.addRequired<LiveIntervals>();
     36   au.addRequired<MachineLoopInfo>();
     37   au.setPreservesAll();
     38   MachineFunctionPass::getAnalysisUsage(au);
     39 }
     40 
     41 bool CalculateSpillWeights::runOnMachineFunction(MachineFunction &MF) {
     42 
     43   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
     44                << "********** Function: " << MF.getName() << '\n');
     45 
     46   LiveIntervals &LIS = getAnalysis<LiveIntervals>();
     47   MachineRegisterInfo &MRI = MF.getRegInfo();
     48   VirtRegAuxInfo VRAI(MF, LIS, getAnalysis<MachineLoopInfo>());
     49   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
     50     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
     51     if (MRI.reg_nodbg_empty(Reg))
     52       continue;
     53     VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
     54   }
     55   return false;
     56 }
     57 
     58 // Return the preferred allocation register for reg, given a COPY instruction.
     59 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
     60                          const TargetRegisterInfo &tri,
     61                          const MachineRegisterInfo &mri) {
     62   unsigned sub, hreg, hsub;
     63   if (mi->getOperand(0).getReg() == reg) {
     64     sub = mi->getOperand(0).getSubReg();
     65     hreg = mi->getOperand(1).getReg();
     66     hsub = mi->getOperand(1).getSubReg();
     67   } else {
     68     sub = mi->getOperand(1).getSubReg();
     69     hreg = mi->getOperand(0).getReg();
     70     hsub = mi->getOperand(0).getSubReg();
     71   }
     72 
     73   if (!hreg)
     74     return 0;
     75 
     76   if (TargetRegisterInfo::isVirtualRegister(hreg))
     77     return sub == hsub ? hreg : 0;
     78 
     79   const TargetRegisterClass *rc = mri.getRegClass(reg);
     80 
     81   // Only allow physreg hints in rc.
     82   if (sub == 0)
     83     return rc->contains(hreg) ? hreg : 0;
     84 
     85   // reg:sub should match the physreg hreg.
     86   return tri.getMatchingSuperReg(hreg, sub, rc);
     87 }
     88 
     89 // Check if all values in LI are rematerializable
     90 static bool isRematerializable(const LiveInterval &LI,
     91                                const LiveIntervals &LIS,
     92                                const TargetInstrInfo &TII) {
     93   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
     94        I != E; ++I) {
     95     const VNInfo *VNI = *I;
     96     if (VNI->isUnused())
     97       continue;
     98     if (VNI->isPHIDef())
     99       return false;
    100 
    101     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
    102     assert(MI && "Dead valno in interval");
    103 
    104     if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
    105       return false;
    106   }
    107   return true;
    108 }
    109 
    110 void VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
    111   MachineRegisterInfo &mri = MF.getRegInfo();
    112   const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
    113   MachineBasicBlock *mbb = 0;
    114   MachineLoop *loop = 0;
    115   unsigned loopDepth = 0;
    116   bool isExiting = false;
    117   float totalWeight = 0;
    118   SmallPtrSet<MachineInstr*, 8> visited;
    119 
    120   // Find the best physreg hist and the best virtreg hint.
    121   float bestPhys = 0, bestVirt = 0;
    122   unsigned hintPhys = 0, hintVirt = 0;
    123 
    124   // Don't recompute a target specific hint.
    125   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
    126 
    127   // Don't recompute spill weight for an unspillable register.
    128   bool Spillable = li.isSpillable();
    129 
    130   for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
    131        MachineInstr *mi = I.skipInstruction();) {
    132     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
    133       continue;
    134     if (!visited.insert(mi))
    135       continue;
    136 
    137     float weight = 1.0f;
    138     if (Spillable) {
    139       // Get loop info for mi.
    140       if (mi->getParent() != mbb) {
    141         mbb = mi->getParent();
    142         loop = Loops.getLoopFor(mbb);
    143         loopDepth = loop ? loop->getLoopDepth() : 0;
    144         isExiting = loop ? loop->isLoopExiting(mbb) : false;
    145       }
    146 
    147       // Calculate instr weight.
    148       bool reads, writes;
    149       tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
    150       weight = LiveIntervals::getSpillWeight(writes, reads, loopDepth);
    151 
    152       // Give extra weight to what looks like a loop induction variable update.
    153       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
    154         weight *= 3;
    155 
    156       totalWeight += weight;
    157     }
    158 
    159     // Get allocation hints from copies.
    160     if (noHint || !mi->isCopy())
    161       continue;
    162     unsigned hint = copyHint(mi, li.reg, tri, mri);
    163     if (!hint)
    164       continue;
    165     float hweight = Hint[hint] += weight;
    166     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
    167       if (hweight > bestPhys && mri.isAllocatable(hint))
    168         bestPhys = hweight, hintPhys = hint;
    169     } else {
    170       if (hweight > bestVirt)
    171         bestVirt = hweight, hintVirt = hint;
    172     }
    173   }
    174 
    175   Hint.clear();
    176 
    177   // Always prefer the physreg hint.
    178   if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
    179     mri.setRegAllocationHint(li.reg, 0, hint);
    180     // Weakly boost the spill weight of hinted registers.
    181     totalWeight *= 1.01F;
    182   }
    183 
    184   // If the live interval was already unspillable, leave it that way.
    185   if (!Spillable)
    186     return;
    187 
    188   // Mark li as unspillable if all live ranges are tiny.
    189   if (li.isZeroLength(LIS.getSlotIndexes())) {
    190     li.markNotSpillable();
    191     return;
    192   }
    193 
    194   // If all of the definitions of the interval are re-materializable,
    195   // it is a preferred candidate for spilling.
    196   // FIXME: this gets much more complicated once we support non-trivial
    197   // re-materialization.
    198   if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
    199     totalWeight *= 0.5F;
    200 
    201   li.weight = normalizeSpillWeight(totalWeight, li.getSize());
    202 }
    203