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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 #include "llvm/CodeGen/VirtRegMap.h"
     11 #include "llvm/CodeGen/CalcSpillWeights.h"
     12 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
     13 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
     14 #include "llvm/CodeGen/MachineFunction.h"
     15 #include "llvm/CodeGen/MachineLoopInfo.h"
     16 #include "llvm/CodeGen/MachineRegisterInfo.h"
     17 #include "llvm/Support/Debug.h"
     18 #include "llvm/Support/raw_ostream.h"
     19 #include "llvm/Target/TargetInstrInfo.h"
     20 #include "llvm/Target/TargetRegisterInfo.h"
     21 #include "llvm/Target/TargetSubtargetInfo.h"
     22 using namespace llvm;
     23 
     24 #define DEBUG_TYPE "calcspillweights"
     25 
     26 void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
     27                            MachineFunction &MF,
     28                            VirtRegMap *VRM,
     29                            const MachineLoopInfo &MLI,
     30                            const MachineBlockFrequencyInfo &MBFI,
     31                            VirtRegAuxInfo::NormalizingFn norm) {
     32   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
     33                << "********** Function: " << MF.getName() << '\n');
     34 
     35   MachineRegisterInfo &MRI = MF.getRegInfo();
     36   VirtRegAuxInfo VRAI(MF, LIS, VRM, MLI, MBFI, norm);
     37   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
     38     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
     39     if (MRI.reg_nodbg_empty(Reg))
     40       continue;
     41     VRAI.calculateSpillWeightAndHint(LIS.getInterval(Reg));
     42   }
     43 }
     44 
     45 // Return the preferred allocation register for reg, given a COPY instruction.
     46 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
     47                          const TargetRegisterInfo &tri,
     48                          const MachineRegisterInfo &mri) {
     49   unsigned sub, hreg, hsub;
     50   if (mi->getOperand(0).getReg() == reg) {
     51     sub = mi->getOperand(0).getSubReg();
     52     hreg = mi->getOperand(1).getReg();
     53     hsub = mi->getOperand(1).getSubReg();
     54   } else {
     55     sub = mi->getOperand(1).getSubReg();
     56     hreg = mi->getOperand(0).getReg();
     57     hsub = mi->getOperand(0).getSubReg();
     58   }
     59 
     60   if (!hreg)
     61     return 0;
     62 
     63   if (TargetRegisterInfo::isVirtualRegister(hreg))
     64     return sub == hsub ? hreg : 0;
     65 
     66   const TargetRegisterClass *rc = mri.getRegClass(reg);
     67 
     68   // Only allow physreg hints in rc.
     69   if (sub == 0)
     70     return rc->contains(hreg) ? hreg : 0;
     71 
     72   // reg:sub should match the physreg hreg.
     73   return tri.getMatchingSuperReg(hreg, sub, rc);
     74 }
     75 
     76 // Check if all values in LI are rematerializable
     77 static bool isRematerializable(const LiveInterval &LI,
     78                                const LiveIntervals &LIS,
     79                                VirtRegMap *VRM,
     80                                const TargetInstrInfo &TII) {
     81   unsigned Reg = LI.reg;
     82   unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
     83   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
     84        I != E; ++I) {
     85     const VNInfo *VNI = *I;
     86     if (VNI->isUnused())
     87       continue;
     88     if (VNI->isPHIDef())
     89       return false;
     90 
     91     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
     92     assert(MI && "Dead valno in interval");
     93 
     94     // Trace copies introduced by live range splitting.  The inline
     95     // spiller can rematerialize through these copies, so the spill
     96     // weight must reflect this.
     97     if (VRM) {
     98       while (MI->isFullCopy()) {
     99         // The copy destination must match the interval register.
    100         if (MI->getOperand(0).getReg() != Reg)
    101           return false;
    102 
    103         // Get the source register.
    104         Reg = MI->getOperand(1).getReg();
    105 
    106         // If the original (pre-splitting) registers match this
    107         // copy came from a split.
    108         if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
    109             VRM->getOriginal(Reg) != Original)
    110           return false;
    111 
    112         // Follow the copy live-in value.
    113         const LiveInterval &SrcLI = LIS.getInterval(Reg);
    114         LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
    115         VNI = SrcQ.valueIn();
    116         assert(VNI && "Copy from non-existing value");
    117         if (VNI->isPHIDef())
    118           return false;
    119         MI = LIS.getInstructionFromIndex(VNI->def);
    120         assert(MI && "Dead valno in interval");
    121       }
    122     }
    123 
    124     if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
    125       return false;
    126   }
    127   return true;
    128 }
    129 
    130 void
    131 VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
    132   MachineRegisterInfo &mri = MF.getRegInfo();
    133   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
    134   MachineBasicBlock *mbb = nullptr;
    135   MachineLoop *loop = nullptr;
    136   bool isExiting = false;
    137   float totalWeight = 0;
    138   unsigned numInstr = 0; // Number of instructions using li
    139   SmallPtrSet<MachineInstr*, 8> visited;
    140 
    141   // Find the best physreg hint and the best virtreg hint.
    142   float bestPhys = 0, bestVirt = 0;
    143   unsigned hintPhys = 0, hintVirt = 0;
    144 
    145   // Don't recompute a target specific hint.
    146   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
    147 
    148   // Don't recompute spill weight for an unspillable register.
    149   bool Spillable = li.isSpillable();
    150 
    151   for (MachineRegisterInfo::reg_instr_iterator
    152        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
    153        I != E; ) {
    154     MachineInstr *mi = &*(I++);
    155     numInstr++;
    156     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
    157       continue;
    158     if (!visited.insert(mi).second)
    159       continue;
    160 
    161     float weight = 1.0f;
    162     if (Spillable) {
    163       // Get loop info for mi.
    164       if (mi->getParent() != mbb) {
    165         mbb = mi->getParent();
    166         loop = Loops.getLoopFor(mbb);
    167         isExiting = loop ? loop->isLoopExiting(mbb) : false;
    168       }
    169 
    170       // Calculate instr weight.
    171       bool reads, writes;
    172       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
    173       weight = LiveIntervals::getSpillWeight(
    174         writes, reads, &MBFI, mi);
    175 
    176       // Give extra weight to what looks like a loop induction variable update.
    177       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
    178         weight *= 3;
    179 
    180       totalWeight += weight;
    181     }
    182 
    183     // Get allocation hints from copies.
    184     if (noHint || !mi->isCopy())
    185       continue;
    186     unsigned hint = copyHint(mi, li.reg, tri, mri);
    187     if (!hint)
    188       continue;
    189     // Force hweight onto the stack so that x86 doesn't add hidden precision,
    190     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
    191     //
    192     // FIXME: we probably shouldn't use floats at all.
    193     volatile float hweight = Hint[hint] += weight;
    194     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
    195       if (hweight > bestPhys && mri.isAllocatable(hint))
    196         bestPhys = hweight, hintPhys = hint;
    197     } else {
    198       if (hweight > bestVirt)
    199         bestVirt = hweight, hintVirt = hint;
    200     }
    201   }
    202 
    203   Hint.clear();
    204 
    205   // Always prefer the physreg hint.
    206   if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
    207     mri.setRegAllocationHint(li.reg, 0, hint);
    208     // Weakly boost the spill weight of hinted registers.
    209     totalWeight *= 1.01F;
    210   }
    211 
    212   // If the live interval was already unspillable, leave it that way.
    213   if (!Spillable)
    214     return;
    215 
    216   // Mark li as unspillable if all live ranges are tiny.
    217   if (li.isZeroLength(LIS.getSlotIndexes())) {
    218     li.markNotSpillable();
    219     return;
    220   }
    221 
    222   // If all of the definitions of the interval are re-materializable,
    223   // it is a preferred candidate for spilling.
    224   // FIXME: this gets much more complicated once we support non-trivial
    225   // re-materialization.
    226   if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
    227     totalWeight *= 0.5F;
    228 
    229   li.weight = normalize(totalWeight, li.getSize(), numInstr);
    230 }
    231