1 //===-- llvm/MC/MCSchedule.h - Scheduling -----------------------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the classes used to describe a subtarget's machine model
11 // for scheduling and other instruction cost heuristics.
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_MC_MCSCHEDULE_H
16 #define LLVM_MC_MCSCHEDULE_H
18 #include "llvm/ADT/Optional.h"
19 #include "llvm/Config/llvm-config.h"
20 #include "llvm/Support/DataTypes.h"
25 struct InstrItinerary;
26 class MCSubtargetInfo;
29 class InstrItineraryData;
31 /// Define a kind of processor resource that will be modeled by the scheduler.
32 struct MCProcResourceDesc {
34 unsigned NumUnits; // Number of resource of this kind
35 unsigned SuperIdx; // Index of the resources kind that contains this kind.
37 // Number of resources that may be buffered.
39 // Buffered resources (BufferSize != 0) may be consumed at some indeterminate
40 // cycle after dispatch. This should be used for out-of-order cpus when
41 // instructions that use this resource can be buffered in a reservaton
44 // Unbuffered resources (BufferSize == 0) always consume their resource some
45 // fixed number of cycles after dispatch. If a resource is unbuffered, then
46 // the scheduler will avoid scheduling instructions with conflicting resources
47 // in the same cycle. This is for in-order cpus, or the in-order portion of
48 // an out-of-order cpus.
51 // If the resource has sub-units, a pointer to the first element of an array
52 // of `NumUnits` elements containing the ProcResourceIdx of the sub units.
53 // nullptr if the resource does not have sub-units.
54 const unsigned *SubUnitsIdxBegin;
56 bool operator==(const MCProcResourceDesc &Other) const {
57 return NumUnits == Other.NumUnits && SuperIdx == Other.SuperIdx
58 && BufferSize == Other.BufferSize;
62 /// Identify one of the processor resource kinds consumed by a particular
63 /// scheduling class for the specified number of cycles.
64 struct MCWriteProcResEntry {
65 uint16_t ProcResourceIdx;
68 bool operator==(const MCWriteProcResEntry &Other) const {
69 return ProcResourceIdx == Other.ProcResourceIdx && Cycles == Other.Cycles;
73 /// Specify the latency in cpu cycles for a particular scheduling class and def
74 /// index. -1 indicates an invalid latency. Heuristics would typically consider
75 /// an instruction with invalid latency to have infinite latency. Also identify
76 /// the WriteResources of this def. When the operand expands to a sequence of
77 /// writes, this ID is the last write in the sequence.
78 struct MCWriteLatencyEntry {
80 uint16_t WriteResourceID;
82 bool operator==(const MCWriteLatencyEntry &Other) const {
83 return Cycles == Other.Cycles && WriteResourceID == Other.WriteResourceID;
87 /// Specify the number of cycles allowed after instruction issue before a
88 /// particular use operand reads its registers. This effectively reduces the
89 /// write's latency. Here we allow negative cycles for corner cases where
90 /// latency increases. This rule only applies when the entry's WriteResource
91 /// matches the write's WriteResource.
93 /// MCReadAdvanceEntries are sorted first by operand index (UseIdx), then by
95 struct MCReadAdvanceEntry {
97 unsigned WriteResourceID;
100 bool operator==(const MCReadAdvanceEntry &Other) const {
101 return UseIdx == Other.UseIdx && WriteResourceID == Other.WriteResourceID
102 && Cycles == Other.Cycles;
106 /// Summarize the scheduling resources required for an instruction of a
107 /// particular scheduling class.
109 /// Defined as an aggregate struct for creating tables with initializer lists.
110 struct MCSchedClassDesc {
111 static const unsigned short InvalidNumMicroOps = (1U << 14) - 1;
112 static const unsigned short VariantNumMicroOps = InvalidNumMicroOps - 1;
114 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
117 uint16_t NumMicroOps : 14;
120 uint16_t WriteProcResIdx; // First index into WriteProcResTable.
121 uint16_t NumWriteProcResEntries;
122 uint16_t WriteLatencyIdx; // First index into WriteLatencyTable.
123 uint16_t NumWriteLatencyEntries;
124 uint16_t ReadAdvanceIdx; // First index into ReadAdvanceTable.
125 uint16_t NumReadAdvanceEntries;
127 bool isValid() const {
128 return NumMicroOps != InvalidNumMicroOps;
130 bool isVariant() const {
131 return NumMicroOps == VariantNumMicroOps;
135 /// Specify the cost of a register definition in terms of number of physical
136 /// register allocated at register renaming stage. For example, AMD Jaguar.
137 /// natively supports 128-bit data types, and operations on 256-bit registers
138 /// (i.e. YMM registers) are internally split into two COPs (complex operations)
139 /// and each COP updates a physical register. Basically, on Jaguar, a YMM
140 /// register write effectively consumes two physical registers. That means,
141 /// the cost of a YMM write in the BtVer2 model is 2.
142 struct MCRegisterCostEntry {
143 unsigned RegisterClassID;
147 /// A register file descriptor.
149 /// This struct allows to describe processor register files. In particular, it
150 /// helps describing the size of the register file, as well as the cost of
151 /// allocating a register file at register renaming stage.
152 /// FIXME: this struct can be extended to provide information about the number
153 /// of read/write ports to the register file. A value of zero for field
154 /// 'NumPhysRegs' means: this register file has an unbounded number of physical
156 struct MCRegisterFileDesc {
158 uint16_t NumPhysRegs;
159 uint16_t NumRegisterCostEntries;
160 // Index of the first cost entry in MCExtraProcessorInfo::RegisterCostTable.
161 uint16_t RegisterCostEntryIdx;
164 /// Provide extra details about the machine processor.
166 /// This is a collection of "optional" processor information that is not
167 /// normally used by the LLVM machine schedulers, but that can be consumed by
168 /// external tools like llvm-mca to improve the quality of the peformance
170 struct MCExtraProcessorInfo {
171 // Actual size of the reorder buffer in hardware.
172 unsigned ReorderBufferSize;
173 // Number of instructions retired per cycle.
174 unsigned MaxRetirePerCycle;
175 const MCRegisterFileDesc *RegisterFiles;
176 unsigned NumRegisterFiles;
177 const MCRegisterCostEntry *RegisterCostTable;
178 unsigned NumRegisterCostEntries;
180 struct PfmCountersInfo {
181 // An optional name of a performance counter that can be used to measure
183 const char *CycleCounter;
185 // For each MCProcResourceDesc defined by the processor, an optional list of
186 // names of performance counters that can be used to measure the resource
188 const char **IssueCounters;
190 PfmCountersInfo PfmCounters;
193 /// Machine model for scheduling, bundling, and heuristics.
195 /// The machine model directly provides basic information about the
196 /// microarchitecture to the scheduler in the form of properties. It also
197 /// optionally refers to scheduler resource tables and itinerary
198 /// tables. Scheduler resource tables model the latency and cost for each
199 /// instruction type. Itinerary tables are an independent mechanism that
200 /// provides a detailed reservation table describing each cycle of instruction
201 /// execution. Subtargets may define any or all of the above categories of data
202 /// depending on the type of CPU and selected scheduler.
204 /// The machine independent properties defined here are used by the scheduler as
205 /// an abstract machine model. A real micro-architecture has a number of
206 /// buffers, queues, and stages. Declaring that a given machine-independent
207 /// abstract property corresponds to a specific physical property across all
208 /// subtargets can't be done. Nonetheless, the abstract model is
209 /// useful. Futhermore, subtargets typically extend this model with processor
210 /// specific resources to model any hardware features that can be exploited by
211 /// sceduling heuristics and aren't sufficiently represented in the abstract.
213 /// The abstract pipeline is built around the notion of an "issue point". This
214 /// is merely a reference point for counting machine cycles. The physical
215 /// machine will have pipeline stages that delay execution. The scheduler does
216 /// not model those delays because they are irrelevant as long as they are
217 /// consistent. Inaccuracies arise when instructions have different execution
218 /// delays relative to each other, in addition to their intrinsic latency. Those
219 /// special cases can be handled by TableGen constructs such as, ReadAdvance,
220 /// which reduces latency when reading data, and ResourceCycles, which consumes
221 /// a processor resource when writing data for a number of abstract
224 /// TODO: One tool currently missing is the ability to add a delay to
225 /// ResourceCycles. That would be easy to add and would likely cover all cases
226 /// currently handled by the legacy itinerary tables.
228 /// A note on out-of-order execution and, more generally, instruction
229 /// buffers. Part of the CPU pipeline is always in-order. The issue point, which
230 /// is the point of reference for counting cycles, only makes sense as an
231 /// in-order part of the pipeline. Other parts of the pipeline are sometimes
232 /// falling behind and sometimes catching up. It's only interesting to model
233 /// those other, decoupled parts of the pipeline if they may be predictably
234 /// resource constrained in a way that the scheduler can exploit.
236 /// The LLVM machine model distinguishes between in-order constraints and
237 /// out-of-order constraints so that the target's scheduling strategy can apply
238 /// appropriate heuristics. For a well-balanced CPU pipeline, out-of-order
239 /// resources would not typically be treated as a hard scheduling
240 /// constraint. For example, in the GenericScheduler, a delay caused by limited
241 /// out-of-order resources is not directly reflected in the number of cycles
242 /// that the scheduler sees between issuing an instruction and its dependent
243 /// instructions. In other words, out-of-order resources don't directly increase
244 /// the latency between pairs of instructions. However, they can still be used
245 /// to detect potential bottlenecks across a sequence of instructions and bias
246 /// the scheduling heuristics appropriately.
247 struct MCSchedModel {
248 // IssueWidth is the maximum number of instructions that may be scheduled in
249 // the same per-cycle group. This is meant to be a hard in-order constraint
250 // (a.k.a. "hazard"). In the GenericScheduler strategy, no more than
251 // IssueWidth micro-ops can ever be scheduled in a particular cycle.
253 // In practice, IssueWidth is useful to model any bottleneck between the
254 // decoder (after micro-op expansion) and the out-of-order reservation
255 // stations or the decoder bandwidth itself. If the total number of
256 // reservation stations is also a bottleneck, or if any other pipeline stage
257 // has a bandwidth limitation, then that can be naturally modeled by adding an
258 // out-of-order processor resource.
260 static const unsigned DefaultIssueWidth = 1;
262 // MicroOpBufferSize is the number of micro-ops that the processor may buffer
263 // for out-of-order execution.
265 // "0" means operations that are not ready in this cycle are not considered
266 // for scheduling (they go in the pending queue). Latency is paramount. This
267 // may be more efficient if many instructions are pending in a schedule.
269 // "1" means all instructions are considered for scheduling regardless of
270 // whether they are ready in this cycle. Latency still causes issue stalls,
271 // but we balance those stalls against other heuristics.
273 // "> 1" means the processor is out-of-order. This is a machine independent
274 // estimate of highly machine specific characteristics such as the register
275 // renaming pool and reorder buffer.
276 unsigned MicroOpBufferSize;
277 static const unsigned DefaultMicroOpBufferSize = 0;
279 // LoopMicroOpBufferSize is the number of micro-ops that the processor may
280 // buffer for optimized loop execution. More generally, this represents the
281 // optimal number of micro-ops in a loop body. A loop may be partially
282 // unrolled to bring the count of micro-ops in the loop body closer to this
284 unsigned LoopMicroOpBufferSize;
285 static const unsigned DefaultLoopMicroOpBufferSize = 0;
287 // LoadLatency is the expected latency of load instructions.
288 unsigned LoadLatency;
289 static const unsigned DefaultLoadLatency = 4;
291 // HighLatency is the expected latency of "very high latency" operations.
292 // See TargetInstrInfo::isHighLatencyDef().
293 // By default, this is set to an arbitrarily high number of cycles
294 // likely to have some impact on scheduling heuristics.
295 unsigned HighLatency;
296 static const unsigned DefaultHighLatency = 10;
298 // MispredictPenalty is the typical number of extra cycles the processor
299 // takes to recover from a branch misprediction.
300 unsigned MispredictPenalty;
301 static const unsigned DefaultMispredictPenalty = 10;
303 bool PostRAScheduler; // default value is false
308 const MCProcResourceDesc *ProcResourceTable;
309 const MCSchedClassDesc *SchedClassTable;
310 unsigned NumProcResourceKinds;
311 unsigned NumSchedClasses;
312 // Instruction itinerary tables used by InstrItineraryData.
313 friend class InstrItineraryData;
314 const InstrItinerary *InstrItineraries;
316 const MCExtraProcessorInfo *ExtraProcessorInfo;
318 bool hasExtraProcessorInfo() const { return ExtraProcessorInfo; }
320 unsigned getProcessorID() const { return ProcID; }
322 /// Does this machine model include instruction-level scheduling.
323 bool hasInstrSchedModel() const { return SchedClassTable; }
325 const MCExtraProcessorInfo &getExtraProcessorInfo() const {
326 assert(hasExtraProcessorInfo() &&
327 "No extra information available for this model");
328 return *ExtraProcessorInfo;
331 /// Return true if this machine model data for all instructions with a
332 /// scheduling class (itinerary class or SchedRW list).
333 bool isComplete() const { return CompleteModel; }
335 /// Return true if machine supports out of order execution.
336 bool isOutOfOrder() const { return MicroOpBufferSize > 1; }
338 unsigned getNumProcResourceKinds() const {
339 return NumProcResourceKinds;
342 const MCProcResourceDesc *getProcResource(unsigned ProcResourceIdx) const {
343 assert(hasInstrSchedModel() && "No scheduling machine model");
345 assert(ProcResourceIdx < NumProcResourceKinds && "bad proc resource idx");
346 return &ProcResourceTable[ProcResourceIdx];
349 const MCSchedClassDesc *getSchedClassDesc(unsigned SchedClassIdx) const {
350 assert(hasInstrSchedModel() && "No scheduling machine model");
352 assert(SchedClassIdx < NumSchedClasses && "bad scheduling class idx");
353 return &SchedClassTable[SchedClassIdx];
356 /// Returns the latency value for the scheduling class.
357 static int computeInstrLatency(const MCSubtargetInfo &STI,
358 const MCSchedClassDesc &SCDesc);
360 int computeInstrLatency(const MCSubtargetInfo &STI, unsigned SClass) const;
361 int computeInstrLatency(const MCSubtargetInfo &STI, const MCInstrInfo &MCII,
362 const MCInst &Inst) const;
364 // Returns the reciprocal throughput information from a MCSchedClassDesc.
366 getReciprocalThroughput(const MCSubtargetInfo &STI,
367 const MCSchedClassDesc &SCDesc);
370 getReciprocalThroughput(unsigned SchedClass, const InstrItineraryData &IID);
373 getReciprocalThroughput(const MCSubtargetInfo &STI, const MCInstrInfo &MCII,
374 const MCInst &Inst) const;
376 /// Returns the default initialized model.
377 static const MCSchedModel &GetDefaultSchedModel() { return Default; }
378 static const MCSchedModel Default;