llvm-project/llvm/lib/MCA/Context.cpp

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//===---------------------------- Context.cpp -------------------*- 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
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This file defines a class for holding ownership of various simulated
/// hardware units. A Context also provides a utility routine for constructing
/// a default out-of-order pipeline with fetch, dispatch, execute, and retire
/// stages.
///
//===----------------------------------------------------------------------===//
#include "llvm/MCA/Context.h"
#include "llvm/MCA/HardwareUnits/RegisterFile.h"
#include "llvm/MCA/HardwareUnits/RetireControlUnit.h"
#include "llvm/MCA/HardwareUnits/Scheduler.h"
#include "llvm/MCA/Stages/DispatchStage.h"
#include "llvm/MCA/Stages/EntryStage.h"
#include "llvm/MCA/Stages/ExecuteStage.h"
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#include "llvm/MCA/Stages/MicroOpQueueStage.h"
#include "llvm/MCA/Stages/RetireStage.h"
namespace llvm {
namespace mca {
std::unique_ptr<Pipeline>
Context::createDefaultPipeline(const PipelineOptions &Opts, SourceMgr &SrcMgr) {
const MCSchedModel &SM = STI.getSchedModel();
// Create the hardware units defining the backend.
auto RCU = std::make_unique<RetireControlUnit>(SM);
auto PRF = std::make_unique<RegisterFile>(SM, MRI, Opts.RegisterFileSize);
auto LSU = std::make_unique<LSUnit>(SM, Opts.LoadQueueSize,
[llvm-mca][MC] Add the ability to declare which processor resources model load/store queues (PR36666). This patch adds the ability to specify via tablegen which processor resources are load/store queue resources. A new tablegen class named MemoryQueue can be optionally used to mark resources that model load/store queues. Information about the load/store queue is collected at 'CodeGenSchedule' stage, and analyzed by the 'SubtargetEmitter' to initialize two new fields in struct MCExtraProcessorInfo named `LoadQueueID` and `StoreQueueID`. Those two fields are identifiers for buffered resources used to describe the load queue and the store queue. Field `BufferSize` is interpreted as the number of entries in the queue, while the number of units is a throughput indicator (i.e. number of available pickers for loads/stores). At construction time, LSUnit in llvm-mca checks for the presence of extra processor information (i.e. MCExtraProcessorInfo) in the scheduling model. If that information is available, and fields LoadQueueID and StoreQueueID are set to a value different than zero (i.e. the invalid processor resource index), then LSUnit initializes its LoadQueue/StoreQueue based on the BufferSize value declared by the two processor resources. With this patch, we more accurately track dynamic dispatch stalls caused by the lack of LS tokens (i.e. load/store queue full). This is also shown by the differences in two BdVer2 tests. Stalls that were previously classified as generic SCHEDULER FULL stalls, are not correctly classified either as "load queue full" or "store queue full". About the differences in the -scheduler-stats view: those differences are expected, because entries in the load/store queue are not released at instruction issue stage. Instead, those are released at instruction executed stage. This is the main reason why for the modified tests, the load/store queues gets full before PdEx is full. Differential Revision: https://reviews.llvm.org/D54957 llvm-svn: 347857
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Opts.StoreQueueSize, Opts.AssumeNoAlias);
auto HWS = std::make_unique<Scheduler>(SM, *LSU);
// Create the pipeline stages.
auto Fetch = std::make_unique<EntryStage>(SrcMgr);
auto Dispatch = std::make_unique<DispatchStage>(STI, MRI, Opts.DispatchWidth,
*RCU, *PRF);
[MCA] Highlight kernel bottlenecks in the summary view. This patch adds a new flag named -bottleneck-analysis to print out information about throughput bottlenecks. MCA knows how to identify and classify dynamic dispatch stalls. However, it doesn't know how to analyze and highlight kernel bottlenecks. The goal of this patch is to teach MCA how to correlate increases in backend pressure to backend stalls (and therefore, the loss of throughput). From a Scheduler point of view, backend pressure is a function of the scheduler buffer usage (i.e. how the number of uOps in the scheduler buffers changes over time). Backend pressure increases (or decreases) when there is a mismatch between the number of opcodes dispatched, and the number of opcodes issued in the same cycle. Since buffer resources are limited, continuous increases in backend pressure would eventually leads to dispatch stalls. So, there is a strong correlation between dispatch stalls, and how backpressure changed over time. This patch teaches how to identify situations where backend pressure increases due to: - unavailable pipeline resources. - data dependencies. Data dependencies may delay execution of instructions and therefore increase the time that uOps have to spend in the scheduler buffers. That often translates to an increase in backend pressure which may eventually lead to a bottleneck. Contention on pipeline resources may also delay execution of instructions, and lead to a temporary increase in backend pressure. Internally, the Scheduler classifies instructions based on whether register / memory operands are available or not. An instruction is marked as "ready to execute" only if data dependencies are fully resolved. Every cycle, the Scheduler attempts to execute all instructions that are ready to execute. If an instruction cannot execute because of unavailable pipeline resources, then the Scheduler internally updates a BusyResourceUnits mask with the ID of each unavailable resource. ExecuteStage is responsible for tracking changes in backend pressure. If backend pressure increases during a cycle because of contention on pipeline resources, then ExecuteStage sends a "backend pressure" event to the listeners. That event would contain information about instructions delayed by resource pressure, as well as the BusyResourceUnits mask. Note that ExecuteStage also knows how to identify situations where backpressure increased because of delays introduced by data dependencies. The SummaryView observes "backend pressure" events and prints out a "bottleneck report". Example of bottleneck report: ``` Cycles with backend pressure increase [ 99.89% ] Throughput Bottlenecks: Resource Pressure [ 0.00% ] Data Dependencies: [ 99.89% ] - Register Dependencies [ 0.00% ] - Memory Dependencies [ 99.89% ] ``` A bottleneck report is printed out only if increases in backend pressure eventually caused backend stalls. About the time complexity: Time complexity is linear in the number of instructions in the Scheduler::PendingSet. The average slowdown tends to be in the range of ~5-6%. For memory intensive kernels, the slowdown can be significant if flag -noalias=false is specified. In the worst case scenario I have observed a slowdown of ~30% when flag -noalias=false was specified. We can definitely recover part of that slowdown if we optimize class LSUnit (by doing extra bookkeeping to speedup queries). For now, this new analysis is disabled by default, and it can be enabled via flag -bottleneck-analysis. Users of MCA as a library can enable the generation of pressure events through the constructor of ExecuteStage. This patch partially addresses https://bugs.llvm.org/show_bug.cgi?id=37494 Differential Revision: https://reviews.llvm.org/D58728 llvm-svn: 355308
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auto Execute =
std::make_unique<ExecuteStage>(*HWS, Opts.EnableBottleneckAnalysis);
auto Retire = std::make_unique<RetireStage>(*RCU, *PRF);
// Pass the ownership of all the hardware units to this Context.
addHardwareUnit(std::move(RCU));
addHardwareUnit(std::move(PRF));
addHardwareUnit(std::move(LSU));
addHardwareUnit(std::move(HWS));
// Build the pipeline.
auto StagePipeline = std::make_unique<Pipeline>();
[llvm-mca] Refactor how execution is orchestrated by the Pipeline. This patch changes how instruction execution is orchestrated by the Pipeline. In particular, this patch makes it more explicit how instructions transition through the various pipeline stages during execution. The main goal is to simplify both the stage API and the Pipeline execution. At the same time, this patch fixes some design issues which are currently latent, but that are likely to cause problems in future if people start defining custom pipelines. The new design assumes that each pipeline stage knows the "next-in-sequence". The Stage API has gained three new methods: - isAvailable(IR) - checkNextStage(IR) - moveToTheNextStage(IR). An instruction IR can be executed by a Stage if method `Stage::isAvailable(IR)` returns true. Instructions can move to next stages using method moveToTheNextStage(IR). An instruction cannot be moved to the next stage if method checkNextStage(IR) (called on the current stage) returns false. Stages are now responsible for moving instructions to the next stage in sequence if necessary. Instructions are allowed to transition through multiple stages during a single cycle (as long as stages are available, and as long as all the calls to `checkNextStage(IR)` returns true). Methods `Stage::preExecute()` and `Stage::postExecute()` have now become redundant, and those are removed by this patch. Method Pipeline::runCycle() is now simpler, and it correctly visits stages on every begin/end of cycle. Other changes: - DispatchStage no longer requires a reference to the Scheduler. - ExecuteStage no longer needs to directly interact with the RetireControlUnit. Instead, executed instructions are now directly moved to the next stage (i.e. the retire stage). - RetireStage gained an execute method. This allowed us to remove the dependency with the RCU in ExecuteStage. - FecthStage now updates the "program counter" during cycleBegin() (i.e. before we start executing new instructions). - We no longer need Stage::Status to be returned by method execute(). It has been dropped in favor of a more lightweight llvm::Error. Overally, I measured a ~11% performance gain w.r.t. the previous design. I also think that the Stage interface is probably easier to read now. That being said, code comments have to be improved, and I plan to do it in a follow-up patch. Differential revision: https://reviews.llvm.org/D50849 llvm-svn: 339923
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StagePipeline->appendStage(std::move(Fetch));
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if (Opts.MicroOpQueueSize)
StagePipeline->appendStage(std::make_unique<MicroOpQueueStage>(
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Opts.MicroOpQueueSize, Opts.DecodersThroughput));
[llvm-mca] Refactor how execution is orchestrated by the Pipeline. This patch changes how instruction execution is orchestrated by the Pipeline. In particular, this patch makes it more explicit how instructions transition through the various pipeline stages during execution. The main goal is to simplify both the stage API and the Pipeline execution. At the same time, this patch fixes some design issues which are currently latent, but that are likely to cause problems in future if people start defining custom pipelines. The new design assumes that each pipeline stage knows the "next-in-sequence". The Stage API has gained three new methods: - isAvailable(IR) - checkNextStage(IR) - moveToTheNextStage(IR). An instruction IR can be executed by a Stage if method `Stage::isAvailable(IR)` returns true. Instructions can move to next stages using method moveToTheNextStage(IR). An instruction cannot be moved to the next stage if method checkNextStage(IR) (called on the current stage) returns false. Stages are now responsible for moving instructions to the next stage in sequence if necessary. Instructions are allowed to transition through multiple stages during a single cycle (as long as stages are available, and as long as all the calls to `checkNextStage(IR)` returns true). Methods `Stage::preExecute()` and `Stage::postExecute()` have now become redundant, and those are removed by this patch. Method Pipeline::runCycle() is now simpler, and it correctly visits stages on every begin/end of cycle. Other changes: - DispatchStage no longer requires a reference to the Scheduler. - ExecuteStage no longer needs to directly interact with the RetireControlUnit. Instead, executed instructions are now directly moved to the next stage (i.e. the retire stage). - RetireStage gained an execute method. This allowed us to remove the dependency with the RCU in ExecuteStage. - FecthStage now updates the "program counter" during cycleBegin() (i.e. before we start executing new instructions). - We no longer need Stage::Status to be returned by method execute(). It has been dropped in favor of a more lightweight llvm::Error. Overally, I measured a ~11% performance gain w.r.t. the previous design. I also think that the Stage interface is probably easier to read now. That being said, code comments have to be improved, and I plan to do it in a follow-up patch. Differential revision: https://reviews.llvm.org/D50849 llvm-svn: 339923
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StagePipeline->appendStage(std::move(Dispatch));
StagePipeline->appendStage(std::move(Execute));
StagePipeline->appendStage(std::move(Retire));
return StagePipeline;
}
} // namespace mca
} // namespace llvm