RegisterClassInfo.cpp

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//===- RegisterClassInfo.cpp - Dynamic Register Class Info ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the RegisterClassInfo class which provides dynamic
// information about target register classes. Callee-saved vs. caller-saved and
// reserved registers depend on calling conventions and other dynamic
// information, so some things cannot be determined statically.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
 
using namespace llvm;
 
#define DEBUG_TYPE "regalloc"
 
static cl::opt<unsigned>
StressRA("stress-regalloc", cl::Hidden, cl::init(0), cl::value_desc("N"),
         cl::desc("Limit all regclasses to N registers"));
 
RegisterClassInfo::RegisterClassInfo() = default;
 
void RegisterClassInfo::runOnMachineFunction(const MachineFunction &mf,
                                             bool Rev) {
  bool Update = false;
  MF = &mf;
 
  auto &STI = MF->getSubtarget();
 
  // Allocate new array the first time we see a new target.
  if (STI.getRegisterInfo() != TRI || Reverse != Rev) {
    Reverse = Rev;
    TRI = STI.getRegisterInfo();
    RegClass.reset(new RCInfo[TRI->getNumRegClasses()]);
    Update = true;
  }
 
  // Test if CSRs have changed from the previous function.
  const MachineRegisterInfo &MRI = MF->getRegInfo();
  const MCPhysReg *CSR = MRI.getCalleeSavedRegs();
  bool CSRChanged = true;
  if (!Update) {
    CSRChanged = false;
    size_t LastSize = LastCalleeSavedRegs.size();
    for (unsigned I = 0;; ++I) {
      if (CSR[I] == 0) {
        CSRChanged = I != LastSize;
        break;
      }
      if (I >= LastSize) {
        CSRChanged = true;
        break;
      }
      if (CSR[I] != LastCalleeSavedRegs[I]) {
        CSRChanged = true;
        break;
      }
    }
  }
 
  // Get the callee saved registers.
  if (CSRChanged) {
    LastCalleeSavedRegs.clear();
    // Build a CSRAlias map. Every CSR alias saves the last
    // overlapping CSR.
    CalleeSavedAliases.assign(TRI->getNumRegUnits(), 0);
    for (const MCPhysReg *I = CSR; *I; ++I) {
      for (MCRegUnit U : TRI->regunits(*I))
        CalleeSavedAliases[static_cast<unsigned>(U)] = *I;
      LastCalleeSavedRegs.push_back(*I);
    }
 
    Update = true;
  }
 
  // Even if CSR list is same, we could have had a different allocation order
  // if the target's CSR allocation-order mask changes.
  BitVector CSRHintsForAllocOrder;
  STI.getCSRAllocationOrderMask(mf, CSRHintsForAllocOrder);
  if (IgnoreCSRForAllocOrder != CSRHintsForAllocOrder) {
    Update = true;
    IgnoreCSRForAllocOrder = std::move(CSRHintsForAllocOrder);
  }
 
  RegCosts = TRI->getRegisterCosts(*MF);
 
  // Different reserved registers?
  const BitVector &RR = MF->getRegInfo().getReservedRegs();
  if (RR != Reserved) {
    Update = true;
    Reserved = RR;
  }
 
  // Invalidate cached information from previous function.
  if (Update) {
    unsigned NumPSets = TRI->getNumRegPressureSets();
    PSetLimits.reset(new unsigned[NumPSets]);
    std::fill(&PSetLimits[0], &PSetLimits[NumPSets], 0);
    ++Tag;
  }
}
 
bool RegisterClassInfo::hasCurrentReservedRegs() const {
  return Reserved == MF->getRegInfo().getReservedRegs();
}
 
void RegisterClassInfo::updateReservedRegs(const BitVector &ReservedInput) {
  assert(MF && TRI && RegClass &&
         "RegisterClassInfo must be initialized before updating reserved regs");
  assert(ReservedInput.size() == Reserved.size() &&
         "Reserved register bit vectors must have the same size");
  if (ReservedInput == Reserved)
    return;
 
  // Cached orders cannot regain unreserved registers; recompute them lazily.
  bool OnlyNewReservations = Reserved.subsetOf(ReservedInput);
 
  Reserved = ReservedInput;
 
  // Pressure limits depend on the number of allocatable registers.
  std::fill_n(PSetLimits.get(), TRI->getNumRegPressureSets(), 0);
 
  // NumRegs may hide entries beyond the stress limit, so those orders cannot
  // safely be compacted using only their visible prefix.
  if (!OnlyNewReservations || StressRA) {
    ++Tag;
    return;
  }
 
  for (const TargetRegisterClass &RC : TRI->regclasses()) {
    RCInfo &Info = RegClass[RC.getID()];
 
    // Skip stale class information.
    if (Info.Tag != Tag)
      continue;
 
    unsigned NewNumRegs = 0;
    uint8_t MinCost = uint8_t(~0u);
    uint8_t LastCost = uint8_t(~0u);
    unsigned LastCostChange = 0;
 
    for (unsigned I = 0; I != Info.NumRegs; ++I) {
      MCPhysReg PhysReg = Info.Order[I];
      if (Reserved.test(PhysReg))
        continue;
 
      uint8_t Cost = RegCosts[PhysReg];
      MinCost = std::min(MinCost, Cost);
      if (Cost != LastCost)
        LastCostChange = NewNumRegs;
 
      Info.Order[NewNumRegs++] = PhysReg;
      LastCost = Cost;
    }
 
    Info.NumRegs = NewNumRegs;
    Info.MinCost = MinCost;
    Info.LastCostChange = LastCostChange;
 
    Info.ProperSubClass = false;
    if (const TargetRegisterClass *Super =
            TRI->getLargestLegalSuperClass(&RC, *MF))
      if (Super != &RC && getNumAllocatableRegs(Super) > Info.NumRegs)
        Info.ProperSubClass = true;
  }
}
 
/// compute - Compute the preferred allocation order for RC with reserved
/// registers filtered out. Volatile registers come first followed by CSR
/// aliases ordered according to the CSR order specified by the target.
void RegisterClassInfo::compute(const TargetRegisterClass *RC) const {
  assert(RC && "no register class given");
  RCInfo &RCI = RegClass[RC->getID()];
 
  // Raw register count, including all reserved regs.
  unsigned NumRegs = RC->getNumRegs();
 
  if (!RCI.Order)
    RCI.Order.reset(new MCPhysReg[NumRegs]);
 
  unsigned N = 0;
  SmallVector<MCPhysReg, 16> CSRAlias;
  uint8_t MinCost = uint8_t(~0u);
  uint8_t LastCost = uint8_t(~0u);
  unsigned LastCostChange = 0;
 
  // FIXME: Once targets reserve registers instead of removing them from the
  // allocation order, we can simply use begin/end here.
  ArrayRef<MCPhysReg> RawOrder = TRI->getRawAllocationOrder(*RC, *MF, Reverse);
  for (unsigned PhysReg : reverse_conditionally(RawOrder, Reverse)) {
    // Remove reserved registers from the allocation order.
    if (Reserved.test(PhysReg))
      continue;
    uint8_t Cost = RegCosts[PhysReg];
    MinCost = std::min(MinCost, Cost);
 
    if (getLastCalleeSavedAlias(PhysReg) &&
        (IgnoreCSRForAllocOrder.empty() ||
         !IgnoreCSRForAllocOrder.test(PhysReg)))
      // PhysReg aliases a CSR, save it for later.
      CSRAlias.push_back(PhysReg);
    else {
      if (Cost != LastCost)
        LastCostChange = N;
      RCI.Order[N++] = PhysReg;
      LastCost = Cost;
    }
  }
  RCI.NumRegs = N + CSRAlias.size();
  assert(RCI.NumRegs <= NumRegs && "Allocation order larger than regclass");
 
  // CSR aliases go after the volatile registers, preserve the target's order.
  for (unsigned PhysReg : CSRAlias) {
    uint8_t Cost = RegCosts[PhysReg];
    if (Cost != LastCost)
      LastCostChange = N;
    RCI.Order[N++] = PhysReg;
    LastCost = Cost;
  }
 
  // Register allocator stress test.  Clip register class to N registers.
  if (StressRA && RCI.NumRegs > StressRA)
    RCI.NumRegs = StressRA;
 
  // Check if RC is a proper sub-class.
  if (const TargetRegisterClass *Super =
          TRI->getLargestLegalSuperClass(RC, *MF))
    if (Super != RC && getNumAllocatableRegs(Super) > RCI.NumRegs)
      RCI.ProperSubClass = true;
 
  RCI.MinCost = MinCost;
  RCI.LastCostChange = LastCostChange;
 
  LLVM_DEBUG({
    dbgs() << "AllocationOrder(" << TRI->getRegClassName(RC) << ") = [";
    for (unsigned I = 0; I != RCI.NumRegs; ++I)
      dbgs() << ' ' << printReg(RCI.Order[I], TRI);
    dbgs() << (RCI.ProperSubClass ? " ] (sub-class)\n" : " ]\n");
  });
 
  // RCI is now up-to-date.
  RCI.Tag = Tag;
}
 
/// This is not accurate because two overlapping register sets may have some
/// nonoverlapping reserved registers. However, computing the allocation order
/// for all register classes would be too expensive.
unsigned RegisterClassInfo::computePSetLimit(unsigned Idx) const {
  const TargetRegisterClass *RC = TRI->getLargestRegClassForRegPressureSet(Idx);
  assert(RC && "Failed to find register class");
  unsigned NAllocatableRegs = getNumAllocatableRegs(RC);
  unsigned RegPressureSetLimit = TRI->getRegPressureSetLimit(*MF, Idx);
  // If all the regs are reserved, return raw RegPressureSetLimit.
  // One example is VRSAVERC in PowerPC.
  // Avoid returning zero, getRegPressureSetLimit(Idx) assumes computePSetLimit
  // return non-zero value.
  if (NAllocatableRegs == 0)
    return RegPressureSetLimit;
  unsigned NReserved = RC->getNumRegs() - NAllocatableRegs;
  unsigned ReservedRegWeight = TRI->getRegClassWeight(RC).RegWeight * NReserved;
  // A target-provided limit may already account for restricted register
  // availability, such as an AMDGPU occupancy requirement. If the additional
  // reserved-register adjustment would not leave a positive limit, preserve the
  // target's nonzero limit; zero is the PSetLimits cache sentinel.
  if (ReservedRegWeight >= RegPressureSetLimit)
    return RegPressureSetLimit;
  return RegPressureSetLimit - ReservedRegWeight;
}
 
INITIALIZE_PASS(MachineRegisterClassInfoWrapperPass,
                "machine-register-class-info",
                "Machine Register Class Info Analysis", true, true)
 
MachineRegisterClassAnalysis::Result
MachineRegisterClassAnalysis::run(MachineFunction &MF,
                                  MachineFunctionAnalysisManager &) {
  RegisterClassInfo RCI;
  RCI.runOnMachineFunction(MF);
  return RCI;
}
 
char MachineRegisterClassInfoWrapperPass::ID = 0;
 
MachineRegisterClassInfoWrapperPass::MachineRegisterClassInfoWrapperPass()
    : MachineFunctionPass(ID), RCI() {
  PassRegistry &Registry = *PassRegistry::getPassRegistry();
  initializeMachineRegisterClassInfoWrapperPassPass(Registry);
}
 
bool MachineRegisterClassInfoWrapperPass::runOnMachineFunction(
    MachineFunction &MF) {
  RCI.runOnMachineFunction(MF);
  return false;
}
 
void MachineRegisterClassInfoWrapperPass::anchor() {}
 
AnalysisKey MachineRegisterClassAnalysis::Key;