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/LiveIntervals.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/TargetInstrInfo.h"
  20 #include "llvm/CodeGen/TargetRegisterInfo.h"
  21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  22 #include "llvm/CodeGen/VirtRegMap.h"
  23 #include "llvm/Support/Debug.h"
  24 #include "llvm/Support/raw_ostream.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   LLVM_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   if (!tri.enableMultipleCopyHints()) {
  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   unsigned CopiedPReg = (hsub ? tri.getSubReg(hreg, hsub) : hreg);
  83   if (rc->contains(CopiedPReg))
  84     return CopiedPReg;
  85
  86   // Check if reg:sub matches so that a super register could be hinted.
  87   if (sub)
  88     return tri.getMatchingSuperReg(CopiedPReg, sub, rc);
  89
  90   return 0;
  91 }
  92
  93 // Check if all values in LI are rematerializable
  94 static bool isRematerializable(const LiveInterval &LI,
  95                                const LiveIntervals &LIS,
  96                                VirtRegMap *VRM,
  97                                const TargetInstrInfo &TII) {
  98   unsigned Reg = LI.reg;
  99   unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
 100   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
 101        I != E; ++I) {
 102     const VNInfo *VNI = *I;
 103     if (VNI->isUnused())
 104       continue;
 105     if (VNI->isPHIDef())
 106       return false;
 107
 108     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
 109     assert(MI && "Dead valno in interval");
 110
 111     // Trace copies introduced by live range splitting.  The inline
 112     // spiller can rematerialize through these copies, so the spill
 113     // weight must reflect this.
 114     if (VRM) {
 115       while (MI->isFullCopy()) {
 116         // The copy destination must match the interval register.
 117         if (MI->getOperand(0).getReg() != Reg)
 118           return false;
 119
 120         // Get the source register.
 121         Reg = MI->getOperand(1).getReg();
 122
 123         // If the original (pre-splitting) registers match this
 124         // copy came from a split.
 125         if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
 126             VRM->getOriginal(Reg) != Original)
 127           return false;
 128
 129         // Follow the copy live-in value.
 130         const LiveInterval &SrcLI = LIS.getInterval(Reg);
 131         LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
 132         VNI = SrcQ.valueIn();
 133         assert(VNI && "Copy from non-existing value");
 134         if (VNI->isPHIDef())
 135           return false;
 136         MI = LIS.getInstructionFromIndex(VNI->def);
 137         assert(MI && "Dead valno in interval");
 138       }
 139     }
 140
 141     if (!TII.isTriviallyReMaterializable(*MI, LIS.getAliasAnalysis()))
 142       return false;
 143   }
 144   return true;
 145 }
 146
 147 void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
 148   float weight = weightCalcHelper(li);
 149   // Check if unspillable.
 150   if (weight < 0)
 151     return;
 152   li.weight = weight;
 153 }
 154
 155 float VirtRegAuxInfo::futureWeight(LiveInterval &li, SlotIndex start,
 156                                    SlotIndex end) {
 157   return weightCalcHelper(li, &start, &end);
 158 }
 159
 160 float VirtRegAuxInfo::weightCalcHelper(LiveInterval &li, SlotIndex *start,
 161                                        SlotIndex *end) {
 162   MachineRegisterInfo &mri = MF.getRegInfo();
 163   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
 164   MachineBasicBlock *mbb = nullptr;
 165   MachineLoop *loop = nullptr;
 166   bool isExiting = false;
 167   float totalWeight = 0;
 168   unsigned numInstr = 0; // Number of instructions using li
 169   SmallPtrSet<MachineInstr*, 8> visited;
 170
 171   std::pair<unsigned, unsigned> TargetHint = mri.getRegAllocationHint(li.reg);
 172
 173   // Don't recompute spill weight for an unspillable register.
 174   bool Spillable = li.isSpillable();
 175
 176   bool localSplitArtifact = start && end;
 177
 178   // Do not update future local split artifacts.
 179   bool updateLI = !localSplitArtifact;
 180
 181   if (localSplitArtifact) {
 182     MachineBasicBlock *localMBB = LIS.getMBBFromIndex(*end);
 183     assert(localMBB == LIS.getMBBFromIndex(*start) &&
 184            "start and end are expected to be in the same basic block");
 185
 186     // Local split artifact will have 2 additional copy instructions and they
 187     // will be in the same BB.
 188     // localLI = COPY other
 189     // ...
 190     // other   = COPY localLI
 191     totalWeight += LiveIntervals::getSpillWeight(true, false, &MBFI, localMBB);
 192     totalWeight += LiveIntervals::getSpillWeight(false, true, &MBFI, localMBB);
 193
 194     numInstr += 2;
 195   }
 196
 197   // CopyHint is a sortable hint derived from a COPY instruction.
 198   struct CopyHint {
 199     unsigned Reg;
 200     float Weight;
 201     bool IsPhys;
 202     unsigned HintOrder;
 203     CopyHint(unsigned R, float W, bool P, unsigned HR) :
 204       Reg(R), Weight(W), IsPhys(P), HintOrder(HR) {}
 205     bool operator<(const CopyHint &rhs) const {
 206       // Always prefer any physreg hint.
 207       if (IsPhys != rhs.IsPhys)
 208         return (IsPhys && !rhs.IsPhys);
 209       if (Weight != rhs.Weight)
 210         return (Weight > rhs.Weight);
 211
 212       // This is just a temporary way to achive NFC for targets that don't
 213       // enable multiple copy hints. HintOrder should be removed when all
 214       // targets return true in enableMultipleCopyHints().
 215       return (HintOrder < rhs.HintOrder);
 216
 217 #if 0 // Should replace the HintOrder check, see above.
 218       // (just for the purpose of maintaining the set)
 219       return Reg < rhs.Reg;
 220 #endif
 221     }
 222   };
 223   std::set<CopyHint> CopyHints;
 224
 225   // Temporary: see comment for HintOrder above.
 226   unsigned CopyHintOrder = 0;
 227   for (MachineRegisterInfo::reg_instr_iterator
 228        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
 229        I != E; ) {
 230     MachineInstr *mi = &*(I++);
 231
 232     // For local split artifacts, we are interested only in instructions between
 233     // the expected start and end of the range.
 234     SlotIndex si = LIS.getInstructionIndex(*mi);
 235     if (localSplitArtifact && ((si < *start) || (si > *end)))
 236       continue;
 237
 238     numInstr++;
 239     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugInstr())
 240       continue;
 241     if (!visited.insert(mi).second)
 242       continue;
 243
 244     float weight = 1.0f;
 245     if (Spillable) {
 246       // Get loop info for mi.
 247       if (mi->getParent() != mbb) {
 248         mbb = mi->getParent();
 249         loop = Loops.getLoopFor(mbb);
 250         isExiting = loop ? loop->isLoopExiting(mbb) : false;
 251       }
 252
 253       // Calculate instr weight.
 254       bool reads, writes;
 255       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
 256       weight = LiveIntervals::getSpillWeight(writes, reads, &MBFI, *mi);
 257
 258       // Give extra weight to what looks like a loop induction variable update.
 259       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
 260         weight *= 3;
 261
 262       totalWeight += weight;
 263     }
 264
 265     // Get allocation hints from copies.
 266     if (!mi->isCopy() ||
 267         (TargetHint.first != 0 && !tri.enableMultipleCopyHints()))
 268       continue;
 269     unsigned hint = copyHint(mi, li.reg, tri, mri);
 270     if (!hint)
 271       continue;
 272     // Force hweight onto the stack so that x86 doesn't add hidden precision,
 273     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
 274     //
 275     // FIXME: we probably shouldn't use floats at all.
 276     volatile float hweight = Hint[hint] += weight;
 277     if (TargetRegisterInfo::isVirtualRegister(hint) || mri.isAllocatable(hint))
 278       CopyHints.insert(CopyHint(hint, hweight, tri.isPhysicalRegister(hint),
 279                      (tri.enableMultipleCopyHints() ? hint : CopyHintOrder++)));
 280   }
 281
 282   Hint.clear();
 283
 284   // Pass all the sorted copy hints to mri.
 285   if (updateLI && CopyHints.size()) {
 286     // Remove a generic hint if previously added by target.
 287     if (TargetHint.first == 0 && TargetHint.second)
 288       mri.clearSimpleHint(li.reg);
 289
 290     for (auto &Hint : CopyHints) {
 291       if (TargetHint.first != 0 && Hint.Reg == TargetHint.second)
 292         // Don't add again the target-type hint.
 293         continue;
 294       mri.addRegAllocationHint(li.reg, Hint.Reg);
 295       if (!tri.enableMultipleCopyHints())
 296         break;
 297     }
 298
 299     // Weakly boost the spill weight of hinted registers.
 300     totalWeight *= 1.01F;
 301   }
 302
 303   // If the live interval was already unspillable, leave it that way.
 304   if (!Spillable)
 305     return -1.0;
 306
 307   // Mark li as unspillable if all live ranges are tiny and the interval
 308   // is not live at any reg mask.  If the interval is live at a reg mask
 309   // spilling may be required.
 310   if (updateLI && li.isZeroLength(LIS.getSlotIndexes()) &&
 311       !li.isLiveAtIndexes(LIS.getRegMaskSlots())) {
 312     li.markNotSpillable();
 313     return -1.0;
 314   }
 315
 316   // If all of the definitions of the interval are re-materializable,
 317   // it is a preferred candidate for spilling.
 318   // FIXME: this gets much more complicated once we support non-trivial
 319   // re-materialization.
 320   if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
 321     totalWeight *= 0.5F;
 322
 323   if (localSplitArtifact)
 324     return normalize(totalWeight, start->distance(*end), numInstr);
 325   return normalize(totalWeight, li.getSize(), numInstr);
 326 }