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 #include "llvm/CodeGen/CalcSpillWeights.h"
  11 #include "llvm/ADT/SmallPtrSet.h"
  12 #include "llvm/CodeGen/LiveInterval.h"
  13 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
  14 #include "llvm/CodeGen/MachineFunction.h"
  15 #include "llvm/CodeGen/MachineInstr.h"
  16 #include "llvm/CodeGen/MachineLoopInfo.h"
  17 #include "llvm/CodeGen/MachineOperand.h"
  18 #include "llvm/CodeGen/MachineRegisterInfo.h"
  19 #include "llvm/CodeGen/VirtRegMap.h"
  20 #include "llvm/Support/Debug.h"
  21 #include "llvm/Support/raw_ostream.h"
  22 #include "llvm/Target/TargetInstrInfo.h"
  23 #include "llvm/Target/TargetRegisterInfo.h"
  24 #include "llvm/Target/TargetSubtargetInfo.h"
  25 #include <cassert>
  26 #include <tuple>
  27
  28 using namespace llvm;
  29
  30 #define DEBUG_TYPE "calcspillweights"
  31
  32 void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
  33                            MachineFunction &MF,
  34                            VirtRegMap *VRM,
  35                            const MachineLoopInfo &MLI,
  36                            const MachineBlockFrequencyInfo &MBFI,
  37                            VirtRegAuxInfo::NormalizingFn norm) {
  38   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
  39                << "********** Function: " << MF.getName() << '\n');
  40
  41   MachineRegisterInfo &MRI = MF.getRegInfo();
  42   VirtRegAuxInfo VRAI(MF, LIS, VRM, MLI, MBFI, norm);
  43   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
  44     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
  45     if (MRI.reg_nodbg_empty(Reg))
  46       continue;
  47     VRAI.calculateSpillWeightAndHint(LIS.getInterval(Reg));
  48   }
  49 }
  50
  51 // Return the preferred allocation register for reg, given a COPY instruction.
  52 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
  53                          const TargetRegisterInfo &tri,
  54                          const MachineRegisterInfo &mri) {
  55   unsigned sub, hreg, hsub;
  56   if (mi->getOperand(0).getReg() == reg) {
  57     sub = mi->getOperand(0).getSubReg();
  58     hreg = mi->getOperand(1).getReg();
  59     hsub = mi->getOperand(1).getSubReg();
  60   } else {
  61     sub = mi->getOperand(1).getSubReg();
  62     hreg = mi->getOperand(0).getReg();
  63     hsub = mi->getOperand(0).getSubReg();
  64   }
  65
  66   if (!hreg)
  67     return 0;
  68
  69   if (TargetRegisterInfo::isVirtualRegister(hreg))
  70     return sub == hsub ? hreg : 0;
  71
  72   const TargetRegisterClass *rc = mri.getRegClass(reg);
  73
  74   // Only allow physreg hints in rc.
  75   if (sub == 0)
  76     return rc->contains(hreg) ? hreg : 0;
  77
  78   // reg:sub should match the physreg hreg.
  79   return tri.getMatchingSuperReg(hreg, sub, rc);
  80 }
  81
  82 // Check if all values in LI are rematerializable
  83 static bool isRematerializable(const LiveInterval &LI,
  84                                const LiveIntervals &LIS,
  85                                VirtRegMap *VRM,
  86                                const TargetInstrInfo &TII) {
  87   unsigned Reg = LI.reg;
  88   unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
  89   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
  90        I != E; ++I) {
  91     const VNInfo *VNI = *I;
  92     if (VNI->isUnused())
  93       continue;
  94     if (VNI->isPHIDef())
  95       return false;
  96
  97     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
  98     assert(MI && "Dead valno in interval");
  99
 100     // Trace copies introduced by live range splitting.  The inline
 101     // spiller can rematerialize through these copies, so the spill
 102     // weight must reflect this.
 103     if (VRM) {
 104       while (MI->isFullCopy()) {
 105         // The copy destination must match the interval register.
 106         if (MI->getOperand(0).getReg() != Reg)
 107           return false;
 108
 109         // Get the source register.
 110         Reg = MI->getOperand(1).getReg();
 111
 112         // If the original (pre-splitting) registers match this
 113         // copy came from a split.
 114         if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
 115             VRM->getOriginal(Reg) != Original)
 116           return false;
 117
 118         // Follow the copy live-in value.
 119         const LiveInterval &SrcLI = LIS.getInterval(Reg);
 120         LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
 121         VNI = SrcQ.valueIn();
 122         assert(VNI && "Copy from non-existing value");
 123         if (VNI->isPHIDef())
 124           return false;
 125         MI = LIS.getInstructionFromIndex(VNI->def);
 126         assert(MI && "Dead valno in interval");
 127       }
 128     }
 129
 130     if (!TII.isTriviallyReMaterializable(*MI, LIS.getAliasAnalysis()))
 131       return false;
 132   }
 133   return true;
 134 }
 135
 136 void
 137 VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
 138   MachineRegisterInfo &mri = MF.getRegInfo();
 139   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
 140   MachineBasicBlock *mbb = nullptr;
 141   MachineLoop *loop = nullptr;
 142   bool isExiting = false;
 143   float totalWeight = 0;
 144   unsigned numInstr = 0; // Number of instructions using li
 145   SmallPtrSet<MachineInstr*, 8> visited;
 146
 147   // Find the best physreg hint and the best virtreg hint.
 148   float bestPhys = 0, bestVirt = 0;
 149   unsigned hintPhys = 0, hintVirt = 0;
 150
 151   // Don't recompute a target specific hint.
 152   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
 153
 154   // Don't recompute spill weight for an unspillable register.
 155   bool Spillable = li.isSpillable();
 156
 157   for (MachineRegisterInfo::reg_instr_iterator
 158        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
 159        I != E; ) {
 160     MachineInstr *mi = &*(I++);
 161     numInstr++;
 162     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
 163       continue;
 164     if (!visited.insert(mi).second)
 165       continue;
 166
 167     float weight = 1.0f;
 168     if (Spillable) {
 169       // Get loop info for mi.
 170       if (mi->getParent() != mbb) {
 171         mbb = mi->getParent();
 172         loop = Loops.getLoopFor(mbb);
 173         isExiting = loop ? loop->isLoopExiting(mbb) : false;
 174       }
 175
 176       // Calculate instr weight.
 177       bool reads, writes;
 178       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
 179       weight = LiveIntervals::getSpillWeight(writes, reads, &MBFI, *mi);
 180
 181       // Give extra weight to what looks like a loop induction variable update.
 182       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
 183         weight *= 3;
 184
 185       totalWeight += weight;
 186     }
 187
 188     // Get allocation hints from copies.
 189     if (noHint || !mi->isCopy())
 190       continue;
 191     unsigned hint = copyHint(mi, li.reg, tri, mri);
 192     if (!hint)
 193       continue;
 194     // Force hweight onto the stack so that x86 doesn't add hidden precision,
 195     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
 196     //
 197     // FIXME: we probably shouldn't use floats at all.
 198     volatile float hweight = Hint[hint] += weight;
 199     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
 200       if (hweight > bestPhys && mri.isAllocatable(hint)) {
 201         bestPhys = hweight;
 202         hintPhys = hint;
 203       }
 204     } else {
 205       if (hweight > bestVirt) {
 206         bestVirt = hweight;
 207         hintVirt = hint;
 208       }
 209     }
 210   }
 211
 212   Hint.clear();
 213
 214   // Always prefer the physreg hint.
 215   if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
 216     mri.setRegAllocationHint(li.reg, 0, hint);
 217     // Weakly boost the spill weight of hinted registers.
 218     totalWeight *= 1.01F;
 219   }
 220
 221   // If the live interval was already unspillable, leave it that way.
 222   if (!Spillable)
 223     return;
 224
 225   // Mark li as unspillable if all live ranges are tiny and the interval
 226   // is not live at any reg mask.  If the interval is live at a reg mask
 227   // spilling may be required.
 228   if (li.isZeroLength(LIS.getSlotIndexes()) &&
 229       !li.isLiveAtIndexes(LIS.getRegMaskSlots())) {
 230     li.markNotSpillable();
 231     return;
 232   }
 233
 234   // If all of the definitions of the interval are re-materializable,
 235   // it is a preferred candidate for spilling.
 236   // FIXME: this gets much more complicated once we support non-trivial
 237   // re-materialization.
 238   if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
 239     totalWeight *= 0.5F;
 240
 241   li.weight = normalize(totalWeight, li.getSize(), numInstr);
 242 }