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