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//===--------------------- BottleneckAnalysis.h -----------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// This file implements the bottleneck analysis view.
/// This view internally observes backend pressure increase events in order to
/// identify problematic data dependencies and processor resource interferences.
/// Example of bottleneck analysis report for a dot-product on X86 btver2:
/// Cycles with backend pressure increase [ 40.76% ]
/// Throughput Bottlenecks:
/// Resource Pressure [ 39.34% ]
/// - JFPA [ 39.34% ]
/// - JFPU0 [ 39.34% ]
/// Data Dependencies: [ 1.42% ]
/// - Register Dependencies [ 1.42% ]
/// - Memory Dependencies [ 0.00% ]
/// According to the example, backend pressure increased during the 40.76% of
/// the simulated cycles. In particular, the major cause of backend pressure
/// increases was the contention on floating point adder JFPA accessible from
/// pipeline resource JFPU0.
/// At the end of each cycle, if pressure on the simulated out-of-order buffers
/// has increased, a backend pressure event is reported.
/// In particular, this occurs when there is a delta between the number of uOps
/// dispatched and the number of uOps issued to the underlying pipelines.
/// The bottleneck analysis view is also responsible for identifying and
/// printing the most "critical" sequence of dependent instructions according to
/// the simulated run.
/// Below is the critical sequence computed for the dot-product example on
/// btver2:
/// Instruction Dependency Information
/// +----< 2. vhaddps %xmm3, %xmm3, %xmm4
/// |
/// | < loop carried >
/// |
/// | 0. vmulps %xmm0, %xmm0, %xmm2
/// +----> 1. vhaddps %xmm2, %xmm2, %xmm3 ## RESOURCE interference: JFPA [ probability: 73% ]
/// +----> 2. vhaddps %xmm3, %xmm3, %xmm4 ## REGISTER dependency: %xmm3
/// |
/// | < loop carried >
/// |
/// +----> 1. vhaddps %xmm2, %xmm2, %xmm3 ## RESOURCE interference: JFPA [ probability: 73% ]
/// The algorithm that computes the critical sequence is very similar to a
/// critical path analysis.
/// A dependency graph is used internally to track dependencies between nodes.
/// Nodes of the graph represent instructions from the input assembly sequence,
/// and edges of the graph represent data dependencies or processor resource
/// interferences.
/// Edges are dynamically 'discovered' by observing instruction state
/// transitions and backend pressure increase events. Edges are internally
/// ranked based on their "criticality". A dependency is considered to be
/// critical if it takes a long time to execute, and if it contributes to
/// backend pressure increases. Criticality is internally measured in terms of
/// cycles; it is computed for every edge in the graph as a function of the edge
/// latency and the number of backend pressure increase cycles contributed by
/// that edge.
/// At the end of simulation, costs are propagated to nodes through the edges of
/// the graph, and the most expensive path connecting the root-set (a
/// set of nodes with no predecessors) to a leaf node is reported as critical
/// sequence.
#include "Views/InstructionView.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/MC/MCInstPrinter.h"
#include "llvm/MC/MCSchedule.h"
#include "llvm/MC/MCSubtargetInfo.h"
#include "llvm/Support/FormattedStream.h"
#include "llvm/Support/raw_ostream.h"
namespace llvm {
namespace mca {
class PressureTracker {
const MCSchedModel &SM;
// Resource pressure distribution. There is an element for every processor
// resource declared by the scheduling model. Quantities are number of cycles.
SmallVector<unsigned, 4> ResourcePressureDistribution;
// Each processor resource is associated with a so-called processor resource
// mask. This vector allows to correlate processor resource IDs with processor
// resource masks. There is exactly one element per each processor resource
// declared by the scheduling model.
SmallVector<uint64_t, 4> ProcResID2Mask;
// Maps processor resource state indices (returned by calls to
// `getResourceStateIndex(Mask)` to processor resource identifiers.
SmallVector<unsigned, 4> ResIdx2ProcResID;
// Maps Processor Resource identifiers to ResourceUsers indices.
SmallVector<unsigned, 4> ProcResID2ResourceUsersIndex;
// Identifies the last user of a processor resource unit.
// This vector is updated on every instruction issued event.
// There is one entry for every processor resource unit declared by the
// processor model. An all_ones value is treated like an invalid instruction
// identifier.
using User = std::pair<unsigned, unsigned>;
SmallVector<User, 4> ResourceUsers;
struct InstructionPressureInfo {
unsigned RegisterPressureCycles;
unsigned MemoryPressureCycles;
unsigned ResourcePressureCycles;
DenseMap<unsigned, InstructionPressureInfo> IPI;
void updateResourcePressureDistribution(uint64_t CumulativeMask);
User getResourceUser(unsigned ProcResID, unsigned UnitID) const {
unsigned Index = ProcResID2ResourceUsersIndex[ProcResID];
return ResourceUsers[Index + UnitID];
PressureTracker(const MCSchedModel &Model);
ArrayRef<unsigned> getResourcePressureDistribution() const {
return ResourcePressureDistribution;
void getResourceUsers(uint64_t ResourceMask,
SmallVectorImpl<User> &Users) const;
unsigned getRegisterPressureCycles(unsigned IID) const {
assert(IPI.find(IID) != IPI.end() && "Instruction is not tracked!");
const InstructionPressureInfo &Info = IPI.find(IID)->second;
return Info.RegisterPressureCycles;
unsigned getMemoryPressureCycles(unsigned IID) const {
assert(IPI.find(IID) != IPI.end() && "Instruction is not tracked!");
const InstructionPressureInfo &Info = IPI.find(IID)->second;
return Info.MemoryPressureCycles;
unsigned getResourcePressureCycles(unsigned IID) const {
assert(IPI.find(IID) != IPI.end() && "Instruction is not tracked!");
const InstructionPressureInfo &Info = IPI.find(IID)->second;
return Info.ResourcePressureCycles;
const char *resolveResourceName(uint64_t ResourceMask) const {
unsigned Index = getResourceStateIndex(ResourceMask);
unsigned ProcResID = ResIdx2ProcResID[Index];
const MCProcResourceDesc &PRDesc = *SM.getProcResource(ProcResID);
return PRDesc.Name;
void onInstructionDispatched(unsigned IID);
void onInstructionExecuted(unsigned IID);
void handlePressureEvent(const HWPressureEvent &Event);
void handleInstructionIssuedEvent(const HWInstructionIssuedEvent &Event);
// A dependency edge.
struct DependencyEdge {
// Dependency edge descriptor.
// It specifies the dependency type, as well as the edge cost in cycles.
struct Dependency {
DependencyType Type;
uint64_t ResourceOrRegID;
uint64_t Cost;
Dependency Dep;
unsigned FromIID;
unsigned ToIID;
// Used by the bottleneck analysis to compute the interference
// probability for processor resources.
unsigned Frequency;
// A dependency graph used by the bottleneck analysis to describe data
// dependencies and processor resource interferences between instructions.
// There is a node (an instance of struct DGNode) for every instruction in the
// input assembly sequence. Edges of the graph represent dependencies between
// instructions.
// Each edge of the graph is associated with a cost value which is used
// internally to rank dependency based on their impact on the runtime
// performance (see field DependencyEdge::Dependency::Cost). In general, the
// higher the cost of an edge, the higher the impact on performance.
// The cost of a dependency is a function of both the latency and the number of
// cycles where the dependency has been seen as critical (i.e. contributing to
// back-pressure increases).
// Loop carried dependencies are carefully expanded by the bottleneck analysis
// to guarantee that the graph stays acyclic. To this end, extra nodes are
// pre-allocated at construction time to describe instructions from "past and
// future" iterations. The graph is kept acyclic mainly because it simplifies
// the complexity of the algorithm that computes the critical sequence.
class DependencyGraph {
struct DGNode {
unsigned NumPredecessors;
unsigned NumVisitedPredecessors;
uint64_t Cost;
unsigned Depth;
DependencyEdge CriticalPredecessor;
SmallVector<DependencyEdge, 8> OutgoingEdges;
SmallVector<DGNode, 16> Nodes;
DependencyGraph(const DependencyGraph &) = delete;
DependencyGraph &operator=(const DependencyGraph &) = delete;
void addDependency(unsigned From, unsigned To,
DependencyEdge::Dependency &&DE);
void pruneEdges(unsigned Iterations);
void initializeRootSet(SmallVectorImpl<unsigned> &RootSet) const;
void propagateThroughEdges(SmallVectorImpl<unsigned> &RootSet,
unsigned Iterations);
#ifndef NDEBUG
void dumpDependencyEdge(raw_ostream &OS, const DependencyEdge &DE,
MCInstPrinter &MCIP) const;
DependencyGraph(unsigned Size) : Nodes(Size) {}
void addRegisterDep(unsigned From, unsigned To, unsigned RegID,
unsigned Cost) {
addDependency(From, To, {DependencyEdge::DT_REGISTER, RegID, Cost});
void addMemoryDep(unsigned From, unsigned To, unsigned Cost) {
addDependency(From, To, {DependencyEdge::DT_MEMORY, /* unused */ 0, Cost});
void addResourceDep(unsigned From, unsigned To, uint64_t Mask,
unsigned Cost) {
addDependency(From, To, {DependencyEdge::DT_RESOURCE, Mask, Cost});
// Called by the bottleneck analysis at the end of simulation to propagate
// costs through the edges of the graph, and compute a critical path.
void finalizeGraph(unsigned Iterations) {
SmallVector<unsigned, 16> RootSet;
propagateThroughEdges(RootSet, Iterations);
// Returns a sequence of edges representing the critical sequence based on the
// simulated run. It assumes that the graph has already been finalized (i.e.
// method `finalizeGraph()` has already been called on this graph).
void getCriticalSequence(SmallVectorImpl<const DependencyEdge *> &Seq) const;
#ifndef NDEBUG
void dump(raw_ostream &OS, MCInstPrinter &MCIP) const;
/// A view that collects and prints a few performance numbers.
class BottleneckAnalysis : public InstructionView {
PressureTracker Tracker;
DependencyGraph DG;
unsigned Iterations;
unsigned TotalCycles;
bool PressureIncreasedBecauseOfResources;
bool PressureIncreasedBecauseOfRegisterDependencies;
bool PressureIncreasedBecauseOfMemoryDependencies;
// True if throughput was affected by dispatch stalls.
bool SeenStallCycles;
struct BackPressureInfo {
// Cycles where backpressure increased.
unsigned PressureIncreaseCycles;
// Cycles where backpressure increased because of pipeline pressure.
unsigned ResourcePressureCycles;
// Cycles where backpressure increased because of data dependencies.
unsigned DataDependencyCycles;
// Cycles where backpressure increased because of register dependencies.
unsigned RegisterDependencyCycles;
// Cycles where backpressure increased because of memory dependencies.
unsigned MemoryDependencyCycles;
BackPressureInfo BPI;
// Used to populate the dependency graph DG.
void addRegisterDep(unsigned From, unsigned To, unsigned RegID, unsigned Cy);
void addMemoryDep(unsigned From, unsigned To, unsigned Cy);
void addResourceDep(unsigned From, unsigned To, uint64_t Mask, unsigned Cy);
void printInstruction(formatted_raw_ostream &FOS, const MCInst &MCI,
bool UseDifferentColor = false) const;
// Prints a bottleneck message to OS.
void printBottleneckHints(raw_ostream &OS) const;
void printCriticalSequence(raw_ostream &OS) const;
BottleneckAnalysis(const MCSubtargetInfo &STI, MCInstPrinter &MCIP,
ArrayRef<MCInst> Sequence, unsigned Iterations);
void onCycleEnd() override;
void onEvent(const HWStallEvent &Event) override { SeenStallCycles = true; }
void onEvent(const HWPressureEvent &Event) override;
void onEvent(const HWInstructionEvent &Event) override;
void printView(raw_ostream &OS) const override;
StringRef getNameAsString() const override { return "BottleneckAnalysis"; }
bool isSerializable() const override { return false; }
#ifndef NDEBUG
void dump(raw_ostream &OS, MCInstPrinter &MCIP) const { DG.dump(OS, MCIP); }
} // namespace mca
} // namespace llvm