//===-- llvm/MC/MCSchedule.h - Scheduling -----------------------*- C++ -*-===//
//
// 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 defines the classes used to describe a subtarget's machine model
// for scheduling and other instruction cost heuristics.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_MC_MCSCHEDULE_H
#define LLVM_MC_MCSCHEDULE_H
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/DataTypes.h"
#include <cassert>
namespace llvm {
template <typename T> class ArrayRef;
struct InstrItinerary;
class MCSubtargetInfo;
class MCInstrInfo;
class MCInst;
class InstrItineraryData;
/// Define a kind of processor resource that will be modeled by the scheduler.
struct MCProcResourceDesc {
const char *Name;
unsigned NumUnits; // Number of resource of this kind
unsigned SuperIdx; // Index of the resources kind that contains this kind.
// Number of resources that may be buffered.
//
// Buffered resources (BufferSize != 0) may be consumed at some indeterminate
// cycle after dispatch. This should be used for out-of-order cpus when
// instructions that use this resource can be buffered in a reservaton
// station.
//
// Unbuffered resources (BufferSize == 0) always consume their resource some
// fixed number of cycles after dispatch. If a resource is unbuffered, then
// the scheduler will avoid scheduling instructions with conflicting resources
// in the same cycle. This is for in-order cpus, or the in-order portion of
// an out-of-order cpus.
int BufferSize;
// If the resource has sub-units, a pointer to the first element of an array
// of `NumUnits` elements containing the ProcResourceIdx of the sub units.
// nullptr if the resource does not have sub-units.
const unsigned *SubUnitsIdxBegin;
bool operator==(const MCProcResourceDesc &Other) const {
return NumUnits == Other.NumUnits && SuperIdx == Other.SuperIdx
&& BufferSize == Other.BufferSize;
}
};
/// Identify one of the processor resource kinds consumed by a particular
/// scheduling class for the specified number of cycles.
struct MCWriteProcResEntry {
uint16_t ProcResourceIdx;
uint16_t 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. -1 indicates an invalid latency. Heuristics would typically consider
/// an instruction with invalid latency to have infinite latency. 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 {
int16_t Cycles;
uint16_t 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 = (1U << 13) - 1;
static const unsigned short VariantNumMicroOps = InvalidNumMicroOps - 1;
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
const char* Name;
#endif
uint16_t NumMicroOps : 13;
uint16_t BeginGroup : 1;
uint16_t EndGroup : 1;
uint16_t RetireOOO : 1;
uint16_t WriteProcResIdx; // First index into WriteProcResTable.
uint16_t NumWriteProcResEntries;
uint16_t WriteLatencyIdx; // First index into WriteLatencyTable.
uint16_t NumWriteLatencyEntries;
uint16_t ReadAdvanceIdx; // First index into ReadAdvanceTable.
uint16_t NumReadAdvanceEntries;
bool isValid() const {
return NumMicroOps != InvalidNumMicroOps;
}
bool isVariant() const {
return NumMicroOps == VariantNumMicroOps;
}
};
/// Specify the cost of a register definition in terms of number of physical
/// register allocated at register renaming stage. For example, AMD Jaguar.
/// natively supports 128-bit data types, and operations on 256-bit registers
/// (i.e. YMM registers) are internally split into two COPs (complex operations)
/// and each COP updates a physical register. Basically, on Jaguar, a YMM
/// register write effectively consumes two physical registers. That means,
/// the cost of a YMM write in the BtVer2 model is 2.
struct MCRegisterCostEntry {
unsigned RegisterClassID;
unsigned Cost;
bool AllowMoveElimination;
};
/// A register file descriptor.
///
/// This struct allows to describe processor register files. In particular, it
/// helps describing the size of the register file, as well as the cost of
/// allocating a register file at register renaming stage.
/// FIXME: this struct can be extended to provide information about the number
/// of read/write ports to the register file. A value of zero for field
/// 'NumPhysRegs' means: this register file has an unbounded number of physical
/// registers.
struct MCRegisterFileDesc {
const char *Name;
uint16_t NumPhysRegs;
uint16_t NumRegisterCostEntries;
// Index of the first cost entry in MCExtraProcessorInfo::RegisterCostTable.
uint16_t RegisterCostEntryIdx;
// A value of zero means: there is no limit in the number of moves that can be
// eliminated every cycle.
uint16_t MaxMovesEliminatedPerCycle;
// Ture if this register file only knows how to optimize register moves from
// known zero registers.
bool AllowZeroMoveEliminationOnly;
};
/// Provide extra details about the machine processor.
///
/// This is a collection of "optional" processor information that is not
/// normally used by the LLVM machine schedulers, but that can be consumed by
/// external tools like llvm-mca to improve the quality of the peformance
/// analysis.
struct MCExtraProcessorInfo {
// Actual size of the reorder buffer in hardware.
unsigned ReorderBufferSize;
// Number of instructions retired per cycle.
unsigned MaxRetirePerCycle;
const MCRegisterFileDesc *RegisterFiles;
unsigned NumRegisterFiles;
const MCRegisterCostEntry *RegisterCostTable;
unsigned NumRegisterCostEntries;
unsigned LoadQueueID;
unsigned StoreQueueID;
};
/// 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 independent 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.
///
/// The machine independent properties defined here are used by the scheduler as
/// an abstract machine model. A real micro-architecture has a number of
/// buffers, queues, and stages. Declaring that a given machine-independent
/// abstract property corresponds to a specific physical property across all
/// subtargets can't be done. Nonetheless, the abstract model is
/// useful. Futhermore, subtargets typically extend this model with processor
/// specific resources to model any hardware features that can be exploited by
/// scheduling heuristics and aren't sufficiently represented in the abstract.
///
/// The abstract pipeline is built around the notion of an "issue point". This
/// is merely a reference point for counting machine cycles. The physical
/// machine will have pipeline stages that delay execution. The scheduler does
/// not model those delays because they are irrelevant as long as they are
/// consistent. Inaccuracies arise when instructions have different execution
/// delays relative to each other, in addition to their intrinsic latency. Those
/// special cases can be handled by TableGen constructs such as, ReadAdvance,
/// which reduces latency when reading data, and ResourceCycles, which consumes
/// a processor resource when writing data for a number of abstract
/// cycles.
///
/// TODO: One tool currently missing is the ability to add a delay to
/// ResourceCycles. That would be easy to add and would likely cover all cases
/// currently handled by the legacy itinerary tables.
///
/// A note on out-of-order execution and, more generally, instruction
/// buffers. Part of the CPU pipeline is always in-order. The issue point, which
/// is the point of reference for counting cycles, only makes sense as an
/// in-order part of the pipeline. Other parts of the pipeline are sometimes
/// falling behind and sometimes catching up. It's only interesting to model
/// those other, decoupled parts of the pipeline if they may be predictably
/// resource constrained in a way that the scheduler can exploit.
///
/// The LLVM machine model distinguishes between in-order constraints and
/// out-of-order constraints so that the target's scheduling strategy can apply
/// appropriate heuristics. For a well-balanced CPU pipeline, out-of-order
/// resources would not typically be treated as a hard scheduling
/// constraint. For example, in the GenericScheduler, a delay caused by limited
/// out-of-order resources is not directly reflected in the number of cycles
/// that the scheduler sees between issuing an instruction and its dependent
/// instructions. In other words, out-of-order resources don't directly increase
/// the latency between pairs of instructions. However, they can still be used
/// to detect potential bottlenecks across a sequence of instructions and bias
/// the scheduling heuristics appropriately.
struct MCSchedModel {
// IssueWidth is the maximum number of instructions that may be scheduled in
// the same per-cycle group. This is meant to be a hard in-order constraint
// (a.k.a. "hazard"). In the GenericScheduler strategy, no more than
// IssueWidth micro-ops can ever be scheduled in a particular cycle.
//
// In practice, IssueWidth is useful to model any bottleneck between the
// decoder (after micro-op expansion) and the out-of-order reservation
// stations or the decoder bandwidth itself. If the total number of
// reservation stations is also a bottleneck, or if any other pipeline stage
// has a bandwidth limitation, then that can be naturally modeled by adding an
// out-of-order processor resource.
unsigned IssueWidth;
static const unsigned DefaultIssueWidth = 1;
// MicroOpBufferSize is the number of micro-ops that the processor may buffer
// for out-of-order execution.
//
// "0" means operations that are not ready in this cycle are not considered
// for scheduling (they go in the pending queue). Latency is paramount. This
// may be more efficient if many instructions are pending in a schedule.
//
// "1" means all instructions are considered for scheduling regardless of
// whether they are ready in this cycle. Latency still causes issue stalls,
// but we balance those stalls against other heuristics.
//
// "> 1" means the processor is out-of-order. This is a machine independent
// estimate of highly machine specific characteristics such as the register
// renaming pool and reorder buffer.
unsigned MicroOpBufferSize;
static const unsigned DefaultMicroOpBufferSize = 0;
// LoopMicroOpBufferSize is the number of micro-ops that the processor may
// buffer for optimized loop execution. More generally, this represents the
// optimal number of micro-ops in a loop body. A loop may be partially
// unrolled to bring the count of micro-ops in the loop body closer to this
// number.
unsigned LoopMicroOpBufferSize;
static const unsigned DefaultLoopMicroOpBufferSize = 0;
// LoadLatency is the expected latency of load instructions.
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.
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;
bool PostRAScheduler; // default value is false
bool CompleteModel;
unsigned ProcID;
const MCProcResourceDesc *ProcResourceTable;
const MCSchedClassDesc *SchedClassTable;
unsigned NumProcResourceKinds;
unsigned NumSchedClasses;
// Instruction itinerary tables used by InstrItineraryData.
friend class InstrItineraryData;
const InstrItinerary *InstrItineraries;
const MCExtraProcessorInfo *ExtraProcessorInfo;
bool hasExtraProcessorInfo() const { return ExtraProcessorInfo; }
unsigned getProcessorID() const { return ProcID; }
/// Does this machine model include instruction-level scheduling.
bool hasInstrSchedModel() const { return SchedClassTable; }
const MCExtraProcessorInfo &getExtraProcessorInfo() const {
assert(hasExtraProcessorInfo() &&
"No extra information available for this model");
return *ExtraProcessorInfo;
}
/// Return true if this machine model data for all instructions with a
/// scheduling class (itinerary class or SchedRW list).
bool isComplete() const { return CompleteModel; }
/// Return true if machine supports out of order execution.
bool isOutOfOrder() const { return MicroOpBufferSize > 1; }
unsigned getNumProcResourceKinds() const {
return NumProcResourceKinds;
}
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];
}
/// Returns the latency value for the scheduling class.
static int computeInstrLatency(const MCSubtargetInfo &STI,
const MCSchedClassDesc &SCDesc);
int computeInstrLatency(const MCSubtargetInfo &STI, unsigned SClass) const;
int computeInstrLatency(const MCSubtargetInfo &STI, const MCInstrInfo &MCII,
const MCInst &Inst) const;
// Returns the reciprocal throughput information from a MCSchedClassDesc.
static double
getReciprocalThroughput(const MCSubtargetInfo &STI,
const MCSchedClassDesc &SCDesc);
static double
getReciprocalThroughput(unsigned SchedClass, const InstrItineraryData &IID);
double
getReciprocalThroughput(const MCSubtargetInfo &STI, const MCInstrInfo &MCII,
const MCInst &Inst) const;
/// Returns the maximum forwarding delay for register reads dependent on
/// writes of scheduling class WriteResourceIdx.
static unsigned getForwardingDelayCycles(ArrayRef<MCReadAdvanceEntry> Entries,
unsigned WriteResourceIdx = 0);
/// Returns the default initialized model.
static const MCSchedModel &GetDefaultSchedModel() { return Default; }
static const MCSchedModel Default;
};
} // namespace llvm
#endif