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MLIR: Incremental Application to Graph Algorithms in ML Frameworks

The existing documentation about MLIR focuses on long term vision, how its pieces fit together, and the benefits of modular and composable infrastructure in the vast and distant future. While this viewpoint appeals to some, it causes concern for others who are more concerned about the "here and now" - why does it make sense to make a "revolutionary" change when any individual problem can be fixed in place?

This document explains that adoption of MLIR to solve graph based problems isn't a revolutionary change: it is an incremental series of steps which build on each other, each of which delivers local value. This document also addresses some points of confusion that keep coming up.

One note: even though a major advantage of MLIR is that it can span the full spectrum from graph algorithms down to low-level code generation, this document focuses on the use of MLIR for graph-level algorithms. MLIR will also unlock exciting code generation opportunities (particularly given its novel approach to integrating state of the art polyhedral techniques), but issues that touch on MLIR's relationship to XLA, Eigen, etc, are out of scope for this particular doc.

This document uses TensorFlow as the example given that it is the focus of our immediate work, but we believe that the same viewpoint could be useful for people working in the context of other ML frameworks that may consider adopting MLIR in the future.

How is MLIR relevant?

MLIR is an overloaded acronym which unpacks as "Multi-Level Intermediate Representation". Its high-level purpose is to provide mechanics for describing and transforming programs and computations in a flexible way. It provides common compiler infrastructure for things like constant folding, dead code elimination, graph rewriting, and others - which are independent of the representational choices picked by a given dialect (e.g. its concurrency semantics). It was built with a specific focus on compile time and memory efficiency, accurate propagation of source location information (important for reporting high quality errors and warnings) and is designed for testability.

TensorFlow has numerous subsystems (some of which are proprietary, e.g. Tensor-RT, nGraph, CoreML, etc) as well as translation layers between these different subsystems, and these translation layers face similar challenges. ((As an aside, the internals of each of these subsystems could often benefit from MLIR infrastructure, but that isn't a focus of this doc.))

A key observation that MLIR makes is that these subsystems often have two things going on: they are both particular data structures and encodings (e.g. HLO graphs, TF-Lite's flat buffer format, TensorFlow's Graph format, the ONNX abstraction, etc) as well as an abstraction of computation (a specific way of modeling a convolution, a set of supported operations etc).

MLIR uses a standard IR (i.e., a set of data structures) for representing these computations - this allows a huge amount of shared infrastructure across these problem domains. MLIR then allows the definition of domain-specific "dialects" that describe the set of operations that are legal and supported for a given application. This means that the actual translations between data structures are kept as simple as possible - and are thus relatively easy to make "correct". This allows the common compiler infrastructure to handle the mapping problems and the other issues within the domain.

MLIR's design is directly informed by the experience of building (and then living with) intermediate representations like the LLVM IR, LLVM SelectionDAG, the LLVM machine instruction representation, Swift SIL IR, and learns new lessons from TensorFlow and XLA HLO, as well as learning from building countless research and production systems on top of them. Our goal is to drag the state of the art in compilers forward, not to merely apply a few well-known techniques to the machine learning domain.

What does adoption mean?

The point of this document is not to advocate for rewriting any particular subsystem in TensorFlow - indeed, the burden required to justify a rewrite is high, and often very specific to that subsystem. That said, there are several subsystems that are about to get rewritten or substantially revised anyway, so we use those as examples to concretely describe the benefits that MLIR provides in these cases and what it will take. The subsystems discussed are:

the TF Lite TOCO translator, which we need to improve error reporting/reliability issues and generalize it to support more ops, and
the TF/XLA bridge which needs to improve usability by merging some of its usage models, support dynamic shapes and generalize guest subsystem support to Tensor-RT and nGraph.
Grappler is another subsystem that is likely to get substantial revisions in the future, and would definitely benefit from the MLIR framework, but there are no known plans to do that work at this point, so we don't discuss it further.

Adopting MLIR for these works the same way - and, in fact, the work to support TF Lite is mostly a subset of the larger work to support the functionality of the TF/XLA bridge. TF Lite and the TF/XLA bridge include several compiler passes (things like encapsulate, functionalize control flow, lowering of ops, fusion, constant folding, shape inference, etc).

MLIR supports converting from TensorFlow Graphs to MLIR and back, which means that we can start by putting in a no-op translation to MLIR and back into the pipeline, and verify that nothing breaks. Then we can work on replacing the compiler transformations one by one by reimplementing them (with the improved algorithms that we're planning).

This is a development plan, we wouldn't actually ship a TensorFlow that just uses MLIR for a single pass. In practice, we'll have the MLIR flag gated under an option, build out a replacement for an entire subsystem (e.g. the TOCO translator) and when the time is right, we'll do A/B comparisons and eventually make a switch and phase out the old code over time.

What benefit does MLIR provide?

The adoption plan above might sound like it only makes things worse in the immediate term - we have two implementations of the same functionality, we are dividing our efforts, etc. In order for this to be worth it, we should have a good sense that we are building towards an improved future that will make customers and TensorFlow engineers happier when it lands. Here we describe a few of the benefits that MLIR provides, in no particular order:

A Lossless Human Editable Textual Representation

The MLIR in-memory data structure has a human readable and writable format, as well as a specification for that format - built just like any other programming language. Important properties of this format are that it is compact, easy to read, and lossless. You can dump an MLIR program out to disk and munge around with it, then send it through a few more passes.

If you haven't worked with a system that works this way, it is hard to overstate how big of a deal this in practice: it means that you can call foo->dump() on an IR object to see its full contents, it means you can diff the IR before and after a change, delta reduce IR files, and many other things.

A Graph Verification Pass

Like many other popular compiler infrastructures, MLIR provides infrastructure and implementation for a "verifier" which checks that the IR is well formed. The MLIR verifier is a simple framework that makes it easy to provide a single source of truth for those correctness properties and is general across all Dialects (e.g. TF Graph, TF Lite flat buffer, XLA HLO, etc).

A verifier pass is sort of like a 'super assertion' that catches mistakes in program transformations early, making you as an engineer more productive, making the product more reliable, and making it easier to track down bugs when they appear - because the verifier can be run at any time, either as a compiler pass or with a single function call.

While MLIR provides a well-considered infrastructure for IR verification, and has simple checks for existing TensorFlow operations, there is a lot that should be added here and lots of opportunity to get involved!

Designed for Testability

There are many aspects of this in MLIR, but we'll focus on compiler transformations since they are the easiest to understand. Compiler transformations are modeled as subclasses of the Pass C++ class, which are driven by an mlir-opt tool. When combined with a lossless textual representation, it becomes really easy to write unit tests for compiler transformations, for example, this is a simple test that shows "x-x" is being turned into zero:

  // RUN: mlir-opt %s -canonicalize | FileCheck %s
  func @test_subi_zero_cfg(%arg0: i32) -> i32 {
    %y = subi %arg0, %arg0 : i32
    return %y: i32
  }
  // CHECK-LABEL: func @test_subi_zero_cfg(%arg0: i32)
  // CHECK-NEXT: %c0_i32 = constant 0 : i32
  // CHECK-NEXT: return %c0

The "CHECK" comments are interpreted by the LLVM FileCheck tool, which is sort of like a really advanced grep. This test is fully self-contained: it feeds the input into the canonicalize pass, and checks that the output matches the CHECK lines. See the test/Transforms directory for more examples. In contrast, standard unit testing exposes the API of the underlying framework to lots and lots of tests (making it harder to refactor and move the API), typically requires a lot more code, and exacerbates issues with link time. For examples, see the TEST_F functions in TensorFlow's testsuite.

MLIR has been pervasively designed with this sort of design by testability, allowing us to put in place a culture that expects every behavior changing commit to include a test case, and for these test cases to be stable and reliable over time, since they are testing exactly what they are supposed to. End to end integration tests are still super useful for some things of course!

Infrastructure for Warnings and Error Diagnostics and Location Tracking

MLIR benefits from the lessons learned from building other compilers - including Clang which [set the standard](http://blog.llvm.org/2010/04/amazing-feats-of-clang-error-recovery.html) for quality of implementation in C/C++ compiler diagnostics. Drawing from this experience (and fixing mistakes in LLVM), MLIR requires that operations and functions carry abstract location information, that transformations propagate this information, and provides standardized mechanisms to emit errors and warnings, as well as for clients to hook into them to capture and report them in custom ways.

Why is this important? In practice, many graph-to-graph translators can fail (e.g. TF Lite when an unsupported op is used) and it is important to be able to report the error up through to the user in the most precise way possible, in order for it to be actionable. This includes tracking rewrites through fusions and fissions of ops, mapping back into language / API specific domains, etc.

More selfishly for infrastructure hackers, this is a huge boon because it means that it is easy to write good tests for this: the testing tools for MLIR capture the diagnostics produced by passes (using the standard diagnostic hooks) and check that they match the expected diagnostics in the testcase. For example, to test the dependence analysis infra in the code generator, Andy Davis wrote a simple pass that checks dependencies and emits them as "notes", allowing him to write tests like this:

  // RUN: mlir-opt %s -memref-dependence-check -verify-diagnostics
  func @different_memrefs() {
    %m.a = alloc() : memref<100xf32>
    %m.b = alloc() : memref<100xf32>
    %c0 = constant 0 : index
    %c1 = constant 1.0 : f32
    store %c1, %m.a[%c0] : memref<100xf32>
    // expected-note@-1 {{dependence from memref access 0 to access 1 = false}}
    %v0 = load %m.b[%c0] : memref<100xf32>
    return
  }

Note that a major limitation of this is that MLIR suffers from a problem of "garbage in, garbage out": if the input locations to MLIR are imprecise, then there is nothing that it can do to recover them. There is work underway in TensorFlow/Python to improve the situation, and Swift for TensorFlow already has perfect location tracking due to its design.

Shape Information Captured in the IR

In TensorFlow Graphs, each op takes and returns values using a very simple type system (TF_DataType) in which each value is a tensor of unknown rank and dimensions. At the same time, many graphs have static shapes easily knowable for wide swaths of the computation, and even dynamically shaped operations often have statically knowable dimensions. Many analyses and transformations benefit and use this information when available, but because TensorFlow graphs don't capture this (e.g. serialize it to proto), passes have to recompute it on demand with ShapeRefiner.

The MLIR Tensor Type directly captures shape information, so you can have things like:

  %x = tf.Add %x, %y : tensor<128 x 8 x ? x f32>

Capturing this in the IR is expected to speed up transformations (avoiding recomputing the same info over and over again) which therefore makes it practical to apply stronger shape analysis algorithms. It also makes it easier to work with the IR, because on-the-side representations can get out of date, and the API is easier to work with from an ergonomics perspective.

Unified Graph Rewriting Infrastructure

This is still a work in progress, but we have sightlines towards a general rewriting infrastructure for transforming DAG tiles into other DAG tiles, using a declarative pattern format. DAG to DAG rewriting is a generalized solution for many common compiler optimizations, lowerings, and other rewrites and having an IR enables us to invest in building a single high-quality implementation.

Declarative pattern rules are preferable to imperative C++ code for a number of reasons: they are more compact, easier to reason about, can have checkers written against them, and new tools can be built that inspect and manipulate the declarative patterns in interesting ways - e.g. applying theorem provers to them. It will be exciting to see this ecosystem develop as the infrastructure matures.

Clarified Semantics for TensorFlow Operations

One of the challenging things about working with TensorFlow is that there are many invariants and behaviors that need to be preserved and known about when working with Graphs, and these can be difficult to reason about and lead to bugs. Things like 'dead values', Switch and Merge nodes, concurrency semantics, nodes that execute even when passed a dead value, multiple device program representation - etc... all add complexities that can make it challenging to reason about whether a transformation or analysis is correct in general. Even something as simple as constant folding or transforming integer x-x into 0 is non-trivial because you need to consider control dependence edges.

One of our major goals for the TensorFlow dialect of MLIR is to sort out these situations and upgrade existing TensorFlow graphs to semantics that are easier to reason about. The solutions to these problems are all still being debated, but those discussions have already yielded a lot of potential answers: introducing a tf_dead_or<x> types for switch/merge, modeling of TF operations using futures/async semantics etc. None of these particular battles are critical or important for MLIR to succeed (because of its "meta" nature, the abstraction decisions of any given dialect are up for it to decide), but each one that works out will make it easier to work with and transform TensorFlow operations. We expect these issues to get nailed down in the next couple of months when MLIR effort moves beyond TF Lite / TOCO support. The discussions that are happening now are super valuable and making progress.

Ergonomics

A minor-in-theory, but important-in-practice point is that MLIR is designed to make it easy, memory efficient, and less error prone to transform code than other systems. TensorFlow::Graph has implementation issues where the same information is stored redundantly in different places (which must be manually kept up to date), has somewhat unusual representation of certain constructs (e.g. the function library, which makes it very difficult to add or remove functions, e.g. during interprocedural transformations), and stores information in the graph that is used by the executor, but isn't necessary for program transformation.

TensorFlow has made a lot of progress in this area over the years, and there are lots of ideas about further improvements in the future, we are happy that MLIR addresses these needs (making it much easier to implement correct program transformations) today, and are committed to pushing hard to make it better.

Compile Time Performance and Memory Use

MLIR has been designed to be memory and compile-time efficient in its algorithms and data structures, using immutable and uniqued structures, low level bit-packing, and other well-known techniques to avoid unnecessary heap allocations, and allow simple and safe multithreaded optimization of MLIR programs. There are other reasons to believe that the MLIR implementations of common transformations will be more efficient than the Python and C++ TensorFlow::Graph implementations of the same things, given the current implementation details of TensorFlow.

That said, this is very much a theory at this point. When the new implementation of various subsystems are available, we will see what happens in practice: there will be no reason to speculate - we can measure.

Common Questions and Concerns

Here we address some frequently asked questions and concerns.

Isn't MLIR a big dependency to take on?

We've heard that at least some people are concerned that MLIR is a "big" dependency to take on, and could result in large code size. Here are some key points MLIR:

The entire MLIR codebase is a pretty small C++ code base in absolute terms compared to what goes into a modern ML framework.
Like LLVM, MLIR is designed as a set of libraries that clients can link in or ignore as they wish. For example, the transformations in MLIR kept separate from the core IR abstractions, and dialect specific code (e.g. TensorFlow, TF-Lite, XLA, etc) is all independently selectable by the build system. Clients that don't care about XLA don't link in that code, whether they are a TF-Lite system or a client that is completely unrelated to TensorFlow.
MLIR's only third party dependency is on LLVM, but it doesn't depend on LLVM IR or any other heavy dependency - it just depends on LLVM's support library which provides efficient hash tables and other memory efficient data structures that the STL does not. There have been discussions about splitting this set of libraries out to its own subproject in LLVM that the LLVM IR project depends on. This would be great for MLIR as well as other LLVM subprojects.
TensorFlow and many other frameworks already use LLVM - if so, MLIR would not be pulling in an additional dependency at all.

How does MLIR represent {control flow, concurrency, …} semantics in TensorFlow?

MLIR provides a dialect that is an isomorphic 1-1 mapping between TensorFlow graphs and MLIR, as well as a pretty complete translator back and forth (the only known gap is that a few TF_DataType enums aren't handled yet). MLIR is a "Multi-Level IR", which allows it to represent code with different abstraction levels, so the ability to faithfully represent TensorFlow code in a completely backwards compatible way (even if there are some historical warts!) is critical.

In addition to the isomorphic mapping, we are actively working on efforts to raise the abstraction level for working with TensorFlow graphs in MLIR. Doing so would make it even easier to write TensorFlow transformations than it is today, and would provide a path to migrating TF 1.x graphs forward into the TF 2.x world. For example, because MLIR has an extensible type system, we can directly model whether it is impossible for a Tensor value to be a "dead" value - similar to the use of optional types in modern programming languages.

These discussions occasionally cause confusion because there are several issues being mixed up into one:

What are the current semantics of TensorFlow graphs, and what invariants can we rely on?
What should the semantics be in TensorFlow 2.0?
What do programs rely on in practice, and if it is unfriendly, can we migrate it?
Can we find a way to make it so transforms don't have to worry about the complexities of Switch/Merge, by using higher level control flow representations? (tentative answer: yes)
How should MLIR represent async vs sync operations, what invariants are provided, how does this dovetail with control flow?
When is it safe and beneficial to perform optimizations that might reduce parallelism?

All of these questions have a "conservative/safe fallback": we can continue providing exactly the same abstractions that TensorFlow always has. That said, we are trying hard to level-up the representation (taking advantage of the "Multi-Level" part of MLIR) because doing so will make it much much easier to write analyses and transformations than it currently is in TensorFlow.

Non Goals

It is important to point out things that MLIR does not aim to do. For example, there is no runtime component to MLIR: the TensorFlow executor, the TF Lite FlatBuffer interpreter, or other existing runtime should be used as-is.

Another non-goal is that MLIR currently doesn't support a stable binary encoding. We will certainly add this at some point, but existing formats should be used for serialization and distribution in the meantime.

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