tools/llvm-mca/Dispatch.h

//===----------------------- Dispatch.h -------------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This file implements classes that are used to model register files,
/// reorder buffers and the hardware dispatch logic.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_TOOLS_LLVM_MCA_DISPATCH_H
#define LLVM_TOOLS_LLVM_MCA_DISPATCH_H

#include "Instruction.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/MC/MCSubtargetInfo.h"
#include <map>

namespace mca {

class WriteState;
class DispatchUnit;
class Scheduler;
class Backend;

/// \brief Manages hardware register files, and tracks data dependencies
/// between registers.
class RegisterFile {
  const llvm::MCRegisterInfo &MRI;

  // Each register file is described by an instance of RegisterMappingTracker.
  // RegisterMappingTracker tracks the number of register mappings dynamically
  // allocated during the execution.
  struct RegisterMappingTracker {
    // Total number of register mappings that are available for register
    // renaming. A value of zero for this field means: this register file has
    // an unbounded number of registers.
    const unsigned TotalMappings;
    // Number of mappings that are currently in use.
    unsigned NumUsedMappings;

    RegisterMappingTracker(unsigned NumMappings)
        : TotalMappings(NumMappings), NumUsedMappings(0) {}
  };

  // This is where information related to the various register files is kept.
  // This set always contains at least one register file at index #0. That
  // register file "sees" all the physical registers declared by the target, and
  // (by default) it allows an unbound number of mappings.
  // Users can limit the number of mappings that can be created by register file
  // #0 through the command line flag `-register-file-size`.
  llvm::SmallVector<RegisterMappingTracker, 4> RegisterFiles;

  // RegisterMapping objects are mainly used to track physical register
  // definitions. A WriteState object describes a register definition, and it is
  // used to track RAW dependencies (see Instruction.h).  A RegisterMapping
  // object also specifies the set of register files.  The mapping between
  // physreg and register files is done using a "register file mask".
  //
  // A register file mask identifies a set of register files. Each bit of the
  // mask representation references a specific register file.
  // For example:
  //    0b0001     -->  Register file #0
  //    0b0010     -->  Register file #1
  //    0b0100     -->  Register file #2
  //
  // Note that this implementation allows register files to overlap.
  // The maximum number of register files allowed by this implementation is 32.
  using RegisterMapping = std::pair<WriteState *, unsigned>;

  // This map contains one entry for each physical register defined by the
  // processor scheduling model.
  std::vector<RegisterMapping> RegisterMappings;

  // This method creates a new RegisterMappingTracker for a register file that
  // contains all the physical registers specified by the register classes in
  // the 'RegisterClasses' set.
  //
  // The long term goal is to let scheduling models optionally describe register
  // files via tablegen definitions. This is still a work in progress.
  // For example, here is how a tablegen definition for a x86 FP register file
  // that features AVX might look like:
  //
  //    def FPRegisterFile : RegisterFile<[VR128RegClass, VR256RegClass], 60>
  //
  // Here FPRegisterFile contains all the registers defined by register class
  // VR128RegClass and VR256RegClass. FPRegisterFile implements 60
  // registers which can be used for register renaming purpose.
  //
  // The list of register classes is then converted by the tablegen backend into
  // a list of register class indices. That list, along with the number of
  // available mappings, is then used to create a new RegisterMappingTracker.
  void addRegisterFile(llvm::ArrayRef<unsigned> RegisterClasses,
                       unsigned NumTemps);

  // Allocates a new register mapping in every register file specified by the
  // register file mask. This method is called from addRegisterMapping.
  void createNewMappings(unsigned RegisterFileMask,
                         llvm::MutableArrayRef<unsigned> UsedPhysRegs);

  // Removes a previously allocated mapping from each register file in the
  // RegisterFileMask set. This method is called from invalidateRegisterMapping.
  void removeMappings(unsigned RegisterFileMask,
                      llvm::MutableArrayRef<unsigned> FreedPhysRegs);

public:
  RegisterFile(const llvm::MCRegisterInfo &mri, unsigned TempRegs = 0)
      : MRI(mri), RegisterMappings(MRI.getNumRegs(), {nullptr, 0U}) {
    addRegisterFile({}, TempRegs);
    // TODO: teach the scheduling models how to specify multiple register files.
  }

  // Creates a new register mapping for RegID.
  // This reserves a microarchitectural register in every register file that
  // contains RegID.
  void addRegisterMapping(WriteState &WS,
                          llvm::MutableArrayRef<unsigned> UsedPhysRegs);

  // Invalidates register mappings associated to the input WriteState object.
  // This releases previously allocated mappings for the physical register
  // associated to the WriteState.
  void invalidateRegisterMapping(const WriteState &WS,
                                 llvm::MutableArrayRef<unsigned> FreedPhysRegs);

  // Checks if there are enough microarchitectural registers in the register
  // files.  Returns a "response mask" where each bit is the response from a
  // RegisterMappingTracker.
  // For example: if all register files are available, then the response mask
  // is a bitmask of all zeroes. If Instead register file #1 is not available,
  // then the response mask is 0b10.
  unsigned isAvailable(llvm::ArrayRef<unsigned> Regs) const;
  void collectWrites(llvm::SmallVectorImpl<WriteState *> &Writes,
                     unsigned RegID) const;
  void updateOnRead(ReadState &RS, unsigned RegID);

  unsigned getNumRegisterFiles() const { return RegisterFiles.size(); }

#ifndef NDEBUG
  void dump() const;
#endif
};

/// \brief tracks which instructions are in-flight (i.e. dispatched but not
/// retired) in the OoO backend.
///
/// This class checks on every cycle if/which instructions can be retired.
/// Instructions are retired in program order.
/// In the event of instruction retired, the DispatchUnit object that owns
/// this RetireControlUnit gets notified.
/// On instruction retired, register updates are all architecturally
/// committed, and any temporary registers originally allocated for the
/// retired instruction are freed.
struct RetireControlUnit {
  // A "token" (object of class RUToken) is created by the retire unit for every
  // instruction dispatched to the schedulers.  Flag 'Executed' is used to
  // quickly check if an instruction has reached the write-back stage.  A token
  // also carries information related to the number of entries consumed by the
  // instruction in the reorder buffer. The idea is that those entries will
  // become available again once the instruction is retired.  On every cycle,
  // the RCU (Retire Control Unit) scans every token starting to search for
  // instructions that are ready to retire.  retired. Instructions are retired
  // in program order. Only 'Executed' instructions are eligible for retire.
  // Note that the size of the reorder buffer is defined by the scheduling model
  // via field 'NumMicroOpBufferSize'.
  struct RUToken {
    unsigned Index;    // Instruction index.
    unsigned NumSlots; // Slots reserved to this instruction.
    bool Executed;     // True if the instruction is past the WB stage.
  };

private:
  unsigned NextAvailableSlotIdx;
  unsigned CurrentInstructionSlotIdx;
  unsigned AvailableSlots;
  unsigned MaxRetirePerCycle; // 0 means no limit.
  std::vector<RUToken> Queue;
  DispatchUnit *Owner;

public:
  RetireControlUnit(unsigned NumSlots, unsigned RPC, DispatchUnit *DU)
      : NextAvailableSlotIdx(0), CurrentInstructionSlotIdx(0),
        AvailableSlots(NumSlots), MaxRetirePerCycle(RPC), Owner(DU) {
    assert(NumSlots && "Expected at least one slot!");
    Queue.resize(NumSlots);
  }

  bool isFull() const { return !AvailableSlots; }
  bool isEmpty() const { return AvailableSlots == Queue.size(); }
  bool isAvailable(unsigned Quantity = 1) const {
    // Some instructions may declare a number of uOps which exceedes the size
    // of the reorder buffer. To avoid problems, cap the amount of slots to
    // the size of the reorder buffer.
    Quantity = std::min(Quantity, static_cast<unsigned>(Queue.size()));
    return AvailableSlots >= Quantity;
  }

  // Reserves a number of slots, and returns a new token.
  unsigned reserveSlot(unsigned Index, unsigned NumMicroOps);

  /// Retires instructions in program order.
  void cycleEvent();

  void onInstructionExecuted(unsigned TokenID);

#ifndef NDEBUG
  void dump() const;
#endif
};

// \brief Implements the hardware dispatch logic.
//
// This class is responsible for the dispatch stage, in which instructions are
// dispatched in groups to the Scheduler.  An instruction can be dispatched if
// functional units are available.
// To be more specific, an instruction can be dispatched to the Scheduler if:
//  1) There are enough entries in the reorder buffer (implemented by class
//     RetireControlUnit) to accomodate all opcodes.
//  2) There are enough temporaries to rename output register operands.
//  3) There are enough entries available in the used buffered resource(s).
//
// The number of micro opcodes that can be dispatched in one cycle is limited by
// the value of field 'DispatchWidth'. A "dynamic dispatch stall" occurs when
// processor resources are not available (i.e. at least one of the
// abovementioned checks fails). Dispatch stall events are counted during the
// entire execution of the code, and displayed by the performance report when
// flag '-verbose' is specified.
//
// If the number of micro opcodes of an instruction is bigger than
// DispatchWidth, then it can only be dispatched at the beginning of one cycle.
// The DispatchUnit will still have to wait for a number of cycles (depending on
// the DispatchWidth and the number of micro opcodes) before it can serve other
// instructions.
class DispatchUnit {
  unsigned DispatchWidth;
  unsigned AvailableEntries;
  unsigned CarryOver;
  Scheduler *SC;

  std::unique_ptr<RegisterFile> RAT;
  std::unique_ptr<RetireControlUnit> RCU;
  Backend *Owner;

  bool checkRAT(unsigned Index, const Instruction &Desc);
  bool checkRCU(unsigned Index, const InstrDesc &Desc);
  bool checkScheduler(unsigned Index, const InstrDesc &Desc);

  void updateRAWDependencies(ReadState &RS, const llvm::MCSubtargetInfo &STI);
  void notifyInstructionDispatched(unsigned IID,
                                   llvm::ArrayRef<unsigned> UsedPhysRegs);

public:
  DispatchUnit(Backend *B, const llvm::MCRegisterInfo &MRI,
               unsigned MicroOpBufferSize, unsigned RegisterFileSize,
               unsigned MaxRetirePerCycle, unsigned MaxDispatchWidth,
               Scheduler *Sched)
      : DispatchWidth(MaxDispatchWidth), AvailableEntries(MaxDispatchWidth),
        CarryOver(0U), SC(Sched),
        RAT(llvm::make_unique<RegisterFile>(MRI, RegisterFileSize)),
        RCU(llvm::make_unique<RetireControlUnit>(MicroOpBufferSize,
                                                 MaxRetirePerCycle, this)),
        Owner(B) {}

  unsigned getDispatchWidth() const { return DispatchWidth; }

  bool isAvailable(unsigned NumEntries) const {
    return NumEntries <= AvailableEntries || AvailableEntries == DispatchWidth;
  }

  bool isRCUEmpty() const { return RCU->isEmpty(); }

  bool canDispatch(unsigned Index, const Instruction &Inst) {
    const InstrDesc &Desc = Inst.getDesc();
    assert(isAvailable(Desc.NumMicroOps));
    return checkRCU(Index, Desc) && checkRAT(Index, Inst) &&
           checkScheduler(Index, Desc);
  }

  void dispatch(unsigned IID, Instruction *I, const llvm::MCSubtargetInfo &STI);

  void collectWrites(llvm::SmallVectorImpl<WriteState *> &Vec,
                     unsigned RegID) const {
    return RAT->collectWrites(Vec, RegID);
  }

  void cycleEvent(unsigned Cycle) {
    RCU->cycleEvent();
    AvailableEntries =
        CarryOver >= DispatchWidth ? 0 : DispatchWidth - CarryOver;
    CarryOver = CarryOver >= DispatchWidth ? CarryOver - DispatchWidth : 0U;
  }

  void notifyInstructionRetired(unsigned Index);

  void notifyDispatchStall(unsigned Index, unsigned EventType);

  void onInstructionExecuted(unsigned TokenID) {
    RCU->onInstructionExecuted(TokenID);
  }

#ifndef NDEBUG
  void dump() const;
#endif
};
} // namespace mca

#endif