[PATCH] D63543: [MCA][Bottleneck Analysis] Teach how to compute a critical sequence of instructions based on the simulation.

Wed Jun 19 05:00:36 PDT 2019

andreadb created this revision.
andreadb added reviewers: RKSimon, courbet, lebedev.ri, mattd.
Herald added a subscriber: gbedwell.

This patch teaches the bottleneck analysis how to identify and print the most
expensive sequence of instructions according to the simulation. Fixes PR37494.

The goal is to help users identify the sequence of instruction which is most
critical for performance.

A dependency graph is internally used by the bottleneck analysis to describe data
dependencies and processor resource interferences between instructions.

There is one node in the graph for every instruction in the input assembly
sequence. The number of nodes in the graph is independent from the number of
iterations simulated by the tool. It means that a single node of the graph
represents all the possible instances of a same instruction contributed by
the simulated iterations.

Edges are dynamically "discovered" by the bottleneck analysis by observing
instruction state transitions  and "backend pressure increase" events generated
by the Execute stage. Information from the events is used to identify critical
dependencies, and materialize edges in the graph. A dependency edge is uniquely
identified by a pair of node identifiers plus an instance of struct
DependencyEdge::Dependency (which provides more details about the
actual dependency kind).

The bottleneck analysis internally ranks dependency edges based on their impact
on the runtime (see field DependencyEdge::Dependency::Cost). To this end, each
edge of the graph has an associated cost. By default, the cost of an edge is a
function of its latency (in cycles). In practice, the cost of an edge is also a
function of the number of cycles where the dependency has been seen as
'contributing to backend pressure increases'. The idea is that the higher the
cost of an edge, the higher is the impact of the dependency on performance. To put
it in another way, the cost of an edge is a measure of criticality for performance.

Note how a same edge may be found in multiple iteration of the
simulated loop. The logic that adds new edges to the graph checks if an equivalent
dependency already exists (duplicate edges are not allowed). If an equivalent
dependency edge is found, field DependencyEdge::Frequency of that edge is
incremented by one, and the new cost is cumulatively added to the existing edge cost.

At the end of simulation, costs are propagated to nodes through the edges of the
graph. The goal is to identify a critical sequence from a node of the root-set
(composed by node of the graph with no predecessors) to a 'sink node' with no
successors.
Note that the graph is intentionally kept acyclic to minimize the complexity of
the critical sequence computation algorithm (complexity is currently linear in the
number of the graph).

The critical path is finally computed as a sequence of dependency edges. For edges
describing processor resource interferences, the view also prints a
so-called "interference probability" value (by dividing field DependencyEdge::Frequency
by the total number of iterations).

Examples of critical sequence computations can be found in tests added/modified
by this patch.

On output streams that support colored output, instructions from the critical
sequence are rendered with a different color.

Strictly speaking the analysis conducted by the bottleneck analysis view is not
a critical path analysis. The cost of an edge doesn't only depend on the
dependency latency. More importantly, the cost of a same edge may be computed
differently by different iterations.

The number of dependencies is discovered dynamically based on the events
generated by the simulator. However, their number is not fixed. This is
especially true for edges that model processor resource interferences; an
interference may not occur in every iteration. For that reason, it makes
sense to also print out a "probability of interference".

By construction, the accuracy of this analysis (as always) is strongly dependent
on the simulation (and therefore the quality of the information available in the
scheduling model).

That being said, the critical sequence effectively identifies a performance
criticality. Instructions from that sequence are expected to have a very big
impact on performance. So, users can take advantage of this information to focus
their attention on specific interactions between instructions.
In my experience, it works quite well in practice, and produces useful
output (in a reasonable amount time). So, I'd like to know your opinion on this.

-Andrea

https://reviews.llvm.org/D63543

Files:
  test/tools/llvm-mca/X86/BtVer2/bottleneck-hints-1.s
  test/tools/llvm-mca/X86/BtVer2/bottleneck-hints-2.s
  test/tools/llvm-mca/X86/BtVer2/bottleneck-hints-3.s
  test/tools/llvm-mca/X86/BtVer2/bottleneck-hints-4.s
  test/tools/llvm-mca/X86/SkylakeClient/bottleneck-analysis.s
  test/tools/llvm-mca/X86/option-all-views-1.s
  test/tools/llvm-mca/X86/option-all-views-2.s
  test/tools/llvm-mca/X86/option-no-stats-1.s
  tools/llvm-mca/Views/BottleneckAnalysis.cpp
  tools/llvm-mca/Views/BottleneckAnalysis.h

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