src/llvm/lib/CodeGen/CalcSpillWeights.cpp

   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 }