llvm/lib/CodeGen/RegisterClassInfo.cpp

   1 //===- RegisterClassInfo.cpp - Dynamic Register Class Info ----------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the RegisterClassInfo class which provides dynamic
  10 // information about target register classes. Callee-saved vs. caller-saved and
  11 // reserved registers depend on calling conventions and other dynamic
  12 // information, so some things cannot be determined statically.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/CodeGen/RegisterClassInfo.h"
  17 #include "llvm/ADT/ArrayRef.h"
  18 #include "llvm/ADT/BitVector.h"
  19 #include "llvm/ADT/SmallVector.h"
  20 #include "llvm/CodeGen/MachineFunction.h"
  21 #include "llvm/CodeGen/MachineRegisterInfo.h"
  22 #include "llvm/CodeGen/TargetFrameLowering.h"
  23 #include "llvm/CodeGen/TargetRegisterInfo.h"
  24 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  25 #include "llvm/MC/MCRegisterInfo.h"
  26 #include "llvm/Support/CommandLine.h"
  27 #include "llvm/Support/Debug.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include <algorithm>
  30 #include <cassert>
  31 #include <cstdint>
  32
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "regalloc"
  36
  37 static cl::opt<unsigned>
  38 StressRA("stress-regalloc", cl::Hidden, cl::init(0), cl::value_desc("N"),
  39          cl::desc("Limit all regclasses to N registers"));
  40
  41 RegisterClassInfo::RegisterClassInfo() = default;
  42
  43 void RegisterClassInfo::runOnMachineFunction(const MachineFunction &mf) {
  44   bool Update = false;
  45   MF = &mf;
  46
  47   // Allocate new array the first time we see a new target.
  48   if (MF->getSubtarget().getRegisterInfo() != TRI) {
  49     TRI = MF->getSubtarget().getRegisterInfo();
  50     RegClass.reset(new RCInfo[TRI->getNumRegClasses()]);
  51     Update = true;
  52   }
  53
  54   // Does this MF have different CSRs?
  55   assert(TRI && "no register info set");
  56
  57   // Get the callee saved registers.
  58   const MCPhysReg *CSR = MF->getRegInfo().getCalleeSavedRegs();
  59   if (Update || CSR != CalleeSavedRegs) {
  60     // Build a CSRAlias map. Every CSR alias saves the last
  61     // overlapping CSR.
  62     CalleeSavedAliases.assign(TRI->getNumRegs(), 0);
  63     for (const MCPhysReg *I = CSR; *I; ++I)
  64       for (MCRegAliasIterator AI(*I, TRI, true); AI.isValid(); ++AI)
  65         CalleeSavedAliases[*AI] = *I;
  66
  67     Update = true;
  68   }
  69   CalleeSavedRegs = CSR;
  70
  71   RegCosts = TRI->getRegisterCosts(*MF);
  72
  73   // Different reserved registers?
  74   const BitVector &RR = MF->getRegInfo().getReservedRegs();
  75   if (Reserved.size() != RR.size() || RR != Reserved) {
  76     Update = true;
  77     Reserved = RR;
  78   }
  79
  80   // Invalidate cached information from previous function.
  81   if (Update) {
  82     unsigned NumPSets = TRI->getNumRegPressureSets();
  83     PSetLimits.reset(new unsigned[NumPSets]);
  84     std::fill(&PSetLimits[0], &PSetLimits[NumPSets], 0);
  85     ++Tag;
  86   }
  87 }
  88
  89 /// compute - Compute the preferred allocation order for RC with reserved
  90 /// registers filtered out. Volatile registers come first followed by CSR
  91 /// aliases ordered according to the CSR order specified by the target.
  92 void RegisterClassInfo::compute(const TargetRegisterClass *RC) const {
  93   assert(RC && "no register class given");
  94   RCInfo &RCI = RegClass[RC->getID()];
  95   auto &STI = MF->getSubtarget();
  96
  97   // Raw register count, including all reserved regs.
  98   unsigned NumRegs = RC->getNumRegs();
  99
 100   if (!RCI.Order)
 101     RCI.Order.reset(new MCPhysReg[NumRegs]);
 102
 103   unsigned N = 0;
 104   SmallVector<MCPhysReg, 16> CSRAlias;
 105   uint8_t MinCost = uint8_t(~0u);
 106   uint8_t LastCost = uint8_t(~0u);
 107   unsigned LastCostChange = 0;
 108
 109   // FIXME: Once targets reserve registers instead of removing them from the
 110   // allocation order, we can simply use begin/end here.
 111   ArrayRef<MCPhysReg> RawOrder = RC->getRawAllocationOrder(*MF);
 112   for (unsigned PhysReg : RawOrder) {
 113     // Remove reserved registers from the allocation order.
 114     if (Reserved.test(PhysReg))
 115       continue;
 116     uint8_t Cost = RegCosts[PhysReg];
 117     MinCost = std::min(MinCost, Cost);
 118
 119     if (CalleeSavedAliases[PhysReg] &&
 120         !STI.ignoreCSRForAllocationOrder(*MF, PhysReg))
 121       // PhysReg aliases a CSR, save it for later.
 122       CSRAlias.push_back(PhysReg);
 123     else {
 124       if (Cost != LastCost)
 125         LastCostChange = N;
 126       RCI.Order[N++] = PhysReg;
 127       LastCost = Cost;
 128     }
 129   }
 130   RCI.NumRegs = N + CSRAlias.size();
 131   assert(RCI.NumRegs <= NumRegs && "Allocation order larger than regclass");
 132
 133   // CSR aliases go after the volatile registers, preserve the target's order.
 134   for (unsigned i = 0, e = CSRAlias.size(); i != e; ++i) {
 135     unsigned PhysReg = CSRAlias[i];
 136     uint8_t Cost = RegCosts[PhysReg];
 137     if (Cost != LastCost)
 138       LastCostChange = N;
 139     RCI.Order[N++] = PhysReg;
 140     LastCost = Cost;
 141   }
 142
 143   // Register allocator stress test.  Clip register class to N registers.
 144   if (StressRA && RCI.NumRegs > StressRA)
 145     RCI.NumRegs = StressRA;
 146
 147   // Check if RC is a proper sub-class.
 148   if (const TargetRegisterClass *Super =
 149           TRI->getLargestLegalSuperClass(RC, *MF))
 150     if (Super != RC && getNumAllocatableRegs(Super) > RCI.NumRegs)
 151       RCI.ProperSubClass = true;
 152
 153   RCI.MinCost = MinCost;
 154   RCI.LastCostChange = LastCostChange;
 155
 156   LLVM_DEBUG({
 157     dbgs() << "AllocationOrder(" << TRI->getRegClassName(RC) << ") = [";
 158     for (unsigned I = 0; I != RCI.NumRegs; ++I)
 159       dbgs() << ' ' << printReg(RCI.Order[I], TRI);
 160     dbgs() << (RCI.ProperSubClass ? " ] (sub-class)\n" : " ]\n");
 161   });
 162
 163   // RCI is now up-to-date.
 164   RCI.Tag = Tag;
 165 }
 166
 167 /// This is not accurate because two overlapping register sets may have some
 168 /// nonoverlapping reserved registers. However, computing the allocation order
 169 /// for all register classes would be too expensive.
 170 unsigned RegisterClassInfo::computePSetLimit(unsigned Idx) const {
 171   const TargetRegisterClass *RC = nullptr;
 172   unsigned NumRCUnits = 0;
 173   for (const TargetRegisterClass *C : TRI->regclasses()) {
 174     const int *PSetID = TRI->getRegClassPressureSets(C);
 175     for (; *PSetID != -1; ++PSetID) {
 176       if ((unsigned)*PSetID == Idx)
 177         break;
 178     }
 179     if (*PSetID == -1)
 180       continue;
 181
 182     // Found a register class that counts against this pressure set.
 183     // For efficiency, only compute the set order for the largest set.
 184     unsigned NUnits = TRI->getRegClassWeight(C).WeightLimit;
 185     if (!RC || NUnits > NumRCUnits) {
 186       RC = C;
 187       NumRCUnits = NUnits;
 188     }
 189   }
 190   assert(RC && "Failed to find register class");
 191   compute(RC);
 192   unsigned NAllocatableRegs = getNumAllocatableRegs(RC);
 193   unsigned RegPressureSetLimit = TRI->getRegPressureSetLimit(*MF, Idx);
 194   // If all the regs are reserved, return raw RegPressureSetLimit.
 195   // One example is VRSAVERC in PowerPC.
 196   // Avoid returning zero, getRegPressureSetLimit(Idx) assumes computePSetLimit
 197   // return non-zero value.
 198   if (NAllocatableRegs == 0)
 199     return RegPressureSetLimit;
 200   unsigned NReserved = RC->getNumRegs() - NAllocatableRegs;
 201   return RegPressureSetLimit - TRI->getRegClassWeight(RC).RegWeight * NReserved;
 202 }