llvm/MC/MCSchedule.h

//===-- llvm/MC/MCSchedule.h - Scheduling -----------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the classes used to describe a subtarget's machine model
// for scheduling and other instruction cost heuristics.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_MC_MCSCHEDMODEL_H
#define LLVM_MC_MCSCHEDMODEL_H

#include "llvm/Support/DataTypes.h"
#include <cassert>

namespace llvm {

struct InstrItinerary;

/// Define a kind of processor resource that will be modeled by the scheduler.
struct MCProcResourceDesc {
#ifndef NDEBUG
  const char *Name;
#endif
  unsigned NumUnits; // Number of resource of this kind
  unsigned SuperIdx; // Index of the resources kind that contains this kind.

  // Buffered resources may be consumed at some indeterminate cycle after
  // dispatch (e.g. for instructions that may issue out-of-order). Unbuffered
  // resources always consume their resource some fixed number of cycles after
  // dispatch (e.g. for instruction interlocking that may stall the pipeline).
  bool IsBuffered;

  bool operator==(const MCProcResourceDesc &Other) const {
    return NumUnits == Other.NumUnits && SuperIdx == Other.SuperIdx
      && IsBuffered == Other.IsBuffered;
  }
};

/// Identify one of the processor resource kinds consumed by a particular
/// scheduling class for the specified number of cycles.
struct MCWriteProcResEntry {
  unsigned ProcResourceIdx;
  unsigned Cycles;

  bool operator==(const MCWriteProcResEntry &Other) const {
    return ProcResourceIdx == Other.ProcResourceIdx && Cycles == Other.Cycles;
  }
};

/// Specify the latency in cpu cycles for a particular scheduling class and def
/// index. Also identify the WriteResources of this def. When the operand
/// expands to a sequence of writes, this ID is the last write in the sequence.
struct MCWriteLatencyEntry {
  unsigned Cycles;
  unsigned WriteResourceID;

  bool operator==(const MCWriteLatencyEntry &Other) const {
    return Cycles == Other.Cycles && WriteResourceID == Other.WriteResourceID;
  }
};

/// Specify the number of cycles allowed after instruction issue before a
/// particular use operand reads its registers. This effectively reduces the
/// write's latency. Here we allow negative cycles for corner cases where
/// latency increases. This rule only applies when the entry's WriteResource
/// matches the write's WriteResource.
///
/// MCReadAdvanceEntries are sorted first by operand index (UseIdx), then by
/// WriteResourceIdx.
struct MCReadAdvanceEntry {
  unsigned UseIdx;
  unsigned WriteResourceID;
  int Cycles;

  bool operator==(const MCReadAdvanceEntry &Other) const {
    return UseIdx == Other.UseIdx && WriteResourceID == Other.WriteResourceID
      && Cycles == Other.Cycles;
  }
};

/// Summarize the scheduling resources required for an instruction of a
/// particular scheduling class.
///
/// Defined as an aggregate struct for creating tables with initializer lists.
struct MCSchedClassDesc {
  static const unsigned short InvalidNumMicroOps = UINT16_MAX;
  static const unsigned short VariantNumMicroOps = UINT16_MAX - 1;

#ifndef NDEBUG
  const char* Name;
#endif
  unsigned short NumMicroOps;
  bool     BeginGroup;
  bool     EndGroup;
  unsigned WriteProcResIdx; // First index into WriteProcResTable.
  unsigned NumWriteProcResEntries;
  unsigned WriteLatencyIdx; // First index into WriteLatencyTable.
  unsigned NumWriteLatencyEntries;
  unsigned ReadAdvanceIdx; // First index into ReadAdvanceTable.
  unsigned NumReadAdvanceEntries;

  bool isValid() const {
    return NumMicroOps != InvalidNumMicroOps;
  }
  bool isVariant() const {
    return NumMicroOps == VariantNumMicroOps;
  }
};

/// Machine model for scheduling, bundling, and heuristics.
///
/// The machine model directly provides basic information about the
/// microarchitecture to the scheduler in the form of properties. It also
/// optionally refers to scheduler resource tables and itinerary
/// tables. Scheduler resource tables model the latency and cost for each
/// instruction type. Itinerary tables are an independant mechanism that
/// provides a detailed reservation table describing each cycle of instruction
/// execution. Subtargets may define any or all of the above categories of data
/// depending on the type of CPU and selected scheduler.
class MCSchedModel {
public:
  static MCSchedModel DefaultSchedModel; // For unknown processors.

  // IssueWidth is the maximum number of instructions that may be scheduled in
  // the same per-cycle group.
  unsigned IssueWidth;
  static const unsigned DefaultIssueWidth = 1;

  // MinLatency is the minimum latency between a register write
  // followed by a data dependent read. This determines which
  // instructions may be scheduled in the same per-cycle group. This
  // is distinct from *expected* latency, which determines the likely
  // critical path but does not guarantee a pipeline
  // hazard. MinLatency can always be overridden by the number of
  // InstrStage cycles.
  //
  // (-1) Standard in-order processor.
  //      Use InstrItinerary OperandCycles as MinLatency.
  //      If no OperandCycles exist, then use the cycle of the last InstrStage.
  //
  //  (0) Out-of-order processor, or in-order with bundled dependencies.
  //      RAW dependencies may be dispatched in the same cycle.
  //      Optional InstrItinerary OperandCycles provides expected latency.
  //
  // (>0) In-order processor with variable latencies.
  //      Use the greater of this value or the cycle of the last InstrStage.
  //      Optional InstrItinerary OperandCycles provides expected latency.
  //      TODO: can't yet specify both min and expected latency per operand.
  int MinLatency;
  static const unsigned DefaultMinLatency = -1;

  // LoadLatency is the expected latency of load instructions.
  //
  // If MinLatency >= 0, this may be overriden for individual load opcodes by
  // InstrItinerary OperandCycles.
  unsigned LoadLatency;
  static const unsigned DefaultLoadLatency = 4;

  // HighLatency is the expected latency of "very high latency" operations.
  // See TargetInstrInfo::isHighLatencyDef().
  // By default, this is set to an arbitrarily high number of cycles
  // likely to have some impact on scheduling heuristics.
  // If MinLatency >= 0, this may be overriden by InstrItinData OperandCycles.
  unsigned HighLatency;
  static const unsigned DefaultHighLatency = 10;

  // MispredictPenalty is the typical number of extra cycles the processor
  // takes to recover from a branch misprediction.
  unsigned MispredictPenalty;
  static const unsigned DefaultMispredictPenalty = 10;

private:
  unsigned ProcID;
  const MCProcResourceDesc *ProcResourceTable;
  const MCSchedClassDesc *SchedClassTable;
  unsigned NumProcResourceKinds;
  unsigned NumSchedClasses;
  // Instruction itinerary tables used by InstrItineraryData.
  friend class InstrItineraryData;
  const InstrItinerary *InstrItineraries;

public:
  // Default's must be specified as static const literals so that tablegenerated
  // target code can use it in static initializers. The defaults need to be
  // initialized in this default ctor because some clients directly instantiate
  // MCSchedModel instead of using a generated itinerary.
  MCSchedModel(): IssueWidth(DefaultIssueWidth),
                  MinLatency(DefaultMinLatency),
                  LoadLatency(DefaultLoadLatency),
                  HighLatency(DefaultHighLatency),
                  MispredictPenalty(DefaultMispredictPenalty),
                  ProcID(0), ProcResourceTable(0), SchedClassTable(0),
                  NumProcResourceKinds(0), NumSchedClasses(0),
                  InstrItineraries(0) {
    (void)NumProcResourceKinds;
    (void)NumSchedClasses;
  }

  // Table-gen driven ctor.
  MCSchedModel(unsigned iw, int ml, unsigned ll, unsigned hl, unsigned mp,
               unsigned pi, const MCProcResourceDesc *pr,
               const MCSchedClassDesc *sc, unsigned npr, unsigned nsc,
               const InstrItinerary *ii):
    IssueWidth(iw), MinLatency(ml), LoadLatency(ll), HighLatency(hl),
    MispredictPenalty(mp), ProcID(pi), ProcResourceTable(pr),
    SchedClassTable(sc), NumProcResourceKinds(npr), NumSchedClasses(nsc),
    InstrItineraries(ii) {}

  unsigned getProcessorID() const { return ProcID; }

  /// Does this machine model include instruction-level scheduling.
  bool hasInstrSchedModel() const { return SchedClassTable; }

  const MCProcResourceDesc *getProcResource(unsigned ProcResourceIdx) const {
    assert(hasInstrSchedModel() && "No scheduling machine model");

    assert(ProcResourceIdx < NumProcResourceKinds && "bad proc resource idx");
    return &ProcResourceTable[ProcResourceIdx];
  }

  const MCSchedClassDesc *getSchedClassDesc(unsigned SchedClassIdx) const {
    assert(hasInstrSchedModel() && "No scheduling machine model");

    assert(SchedClassIdx < NumSchedClasses && "bad scheduling class idx");
    return &SchedClassTable[SchedClassIdx];
  }
};

} // End llvm namespace

#endif