[docs][mlgo] Document MLModelRunner (#139205)

Author: mtrofin

llvm/docs/MLGO.rst

@@ -1,28 +1,176 @@
-====
-MLGO
-====
+=============================================
+Machine Learning - Guided Optimization (MLGO)
+=============================================
 
 Introduction
 ============
 
-MLGO is a framework for integrating ML techniques systematically in LLVM. It is
-designed primarily to replace heuristics within LLVM with machine learned
-models. Currently there is upstream infrastructure for the following
-heuristics:
+MLGO refers to integrating ML techniques (primarily) to replace heuristics within
+LLVM with machine learned models.
+
+Currently the following heuristics feature such integration:
 
 * Inlining for size
 * Register allocation (LLVM greedy eviction heuristic) for performance
 
-This document is an outline of the tooling that composes MLGO.
+This document is an outline of the tooling and APIs facilitating MLGO.
+
+Note that tools for orchestrating ML training are not part of LLVM, as they are
+dependency-heavy - both on the ML infrastructure choice, as well as choices of
+distrubuted computing. For the training scenario, LLVM only contains facilities
+enabling it, such as corpus extraction, training data extraction, and evaluation
+of models during training.
+
+
+.. contents::
 
 Corpus Tooling
 ==============
 
 ..
     TODO(boomanaiden154): Write this section.
 
-Model Runner Interfaces
-=======================
+Interacting with ML models
+==========================
+
+We interact with ML models in 2 primary scenarios: one is to train such a model.
+The other, inference, is to use a model during compilation, to make optimization
+decisions.
+
+For a specific optimization problem - i.e. inlining, or regalloc eviction - we
+first separate correctness - preserving decisions from optimization decisions.
+For example, not inlining functions marked "no inline" is an example of the
+former. Same is not evicting an unevictable live range. An exmple of the latter
+is deciding to inline a function that will bloat the caller size, just because
+we have reason to believe that later, the effect will be some constant
+propagation that will actually reduce the size (or dynamic instruction count).
+
+ML models can be understood as functions. Their inputs are tensors - buffers of
+scalars. The output (in our case, singular) is a scalar. For example, for
+inlining, the inputs are properties of the caller, callee, and the callsite
+being analyzed for inlining. The output is a boolean.
+
+Inputs and outputs are named, have a scalar type (e.g. int32_t) and a shape
+(e.g. 3x4). These are the elements that we use to bind to a ML model.
+
+In both training and inference, we want to expose to ML (training algorithms or
+trained model, respectively) the features we want to make optimization
+decisions on. In that regard, the interface from the compiler side to the ML
+side is the same: pass features, and get a decision. It's essentially a function
+call, where the parameters and result are bound by name and are described by
+name, scalar type, and shape tuples.
+
+The main types in LLVM are:
+- ``MLModelRunner`` - an abstraction for the decision making mechanism
+- ``TensorSpec`` which describes a tensor.
+
+TensorSpec
+----------
+
+See ``llvm/Analysis/TensorSpec.h``. This is a simple data bag, identifying a
+tensor by name (a string), scalar type, and shape (a vector of ints). The scalar
+type can only be int (8, 16, 32, or 64), signed or unsigned; float; or double.
+
+MLModelRunner
+-------------
+
+See ``llvm/Analysis/MLModelRunner.h``. The abstraction has a pure virtual,
+``evaluateUntyped``, but the contract with implementers is a bit more involved:
+
+Implementers
+^^^^^^^^^^^^
+
+At construction, the implementer is expected to receive a list of ``TensorSpec``
+for input features and the ``TensorSpec`` of the output (e.g. 
+``std::vector<TensorSpec>``). The list type is not contractual, but it must be
+a 0-based indexing array-like container. Given a ``TensorSpec`` at index "I" in
+the input list, that has a name "N", shape "D1 x D2x ... Dn", and scalar type
+"T", the implementer must:
+
+- set up a contiguous buffer sized ``sizeof(T) * D1 * D2 * ... * Dn``. This
+  buffer's lifetime must be the same as the lifetime of the implementer object.
+- call ``MLModelRunner::setUpBufferForTensor`` passing I, the ``TensorSpec``,
+  and the buffer above.
+
+Internally, the expectation is that the implementer uses the name (and maybe
+shape) of a ``TensorSpec`` for binding (e.g. lookup in an underlying ML model).
+
+``MLModelRunner::setUpBufferForTensor`` stores each buffer at the corresponding
+index (i.e. its position in the list used at construction). The expectation is
+that the user will use that position when calling ``MLModelRunner::getTensor``
+to retrieve the underlying buffer (more on that in a bit).
+
+The implementation of ``evaluateUntyped`` is expected to use the value in the
+buffers described above, carry out whatever computation (e.g. evaluate a ML
+model) and then place the outcome in an output buffer which will be returned to
+the caller. Importantly, ``evaluateUntyped`` must not reset the input buffers.
+This is because during training we may want to log the features and decisions,
+and since the data is already buffered, there's no reason to force backing it
+up elsewhere.
+
+Users
+^^^^^
+
+The users must pass the input ``TensorSpec`` list at the construction of a
+specific ``MLModelRunner`` object. After that, users can be agnostic of the
+specific implementation, and would typically follow the following workflow:
+
+- call ``getTensor`` or ``getTensorUntyped``, for each input tensor, identified
+  by its index (i.e. the index of the corresponding ``TensorSpec`` in the list
+  used at construction).
+- populate the tensor buffer of each input tensor with values. Users can take
+  advantage of the stability of the tensor buffers like set only once those that
+  don't change, or cache the buffer address
+- call ``evaluate`` and use its result.
+
+Versioning
+^^^^^^^^^^
+
+We support a model "knowing" less inputs than the compiler. This is supported by
+``MLModelRunner::setUpBufferForTensor``. If a ``TensorSpec`` requested by the
+compiler is not supported by the underlying model, the ``MLModelRunner``
+implementer must still call ``setUpBufferForTensor`` with a ``nullptr`` value
+for the buffer. In turn, ``MLModelRunner`` will allocate an appropriately - sized
+buffer and track its lifetime. The user can safely populate that buffer. Since
+the rest of the inputs are still provided, this allows an evolution model where
+we first add features to the compiler and continue using older models without
+regressing. Then, the new compiler can be used to train new models. Deprecating
+features in the compiler involves, then, training first a model without those
+features.
+
+``MLModelRunner`` implementations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We currently feature 3 implementations:
+
+- ``ModelUnderTrainingRunner``. This requires the compiler be built with TFLite
+  support. It allows loading a TFLite model dynamically and is primarily
+  intended for training scenarios, but it can be used relatively easily in
+  production build environments, as it does not change how the compiler operates
+  (why this remark is necessary will become clear in a few paragraphs)
+
+- ``ReleaseModeModelRunner``. This is intended for inference scenarios. This
+  uses the rules defined in ``llvm/cmake/modules/TensorFlowCompile.cmake`` to
+  convert, at the time the compiler is built, TensorFlow Saved Models into a
+  header (.h) and native object (.o). The latter is a CPU-based implementation of
+  the neural network, together with its weights (essentially, loops performing
+  matrix multiplications)
+
+NOTE: we are actively working on replacing this with an EmitC implementation
+requiring no out of tree build-time dependencies.
+
+- ``InteractiveModelRunner``. This is intended for training scenarios where the
+  training algorithm drives compilation. This model runner has no special
+  dependencies, and relies on I/O pipes to communicate with a separate process
+- presumably a python training algorithm. We do not envision using this in a
+  production environment.
+
+Note that training leaves it to the training infrastructure to handle
+distributed computing. The assumed architecture has python processes
+communicating remotely between themselves, but managing local communication with
+clang.
 
 ..
-    TODO(mtrofin): Write this section.
+    TODO(mtrofin): 
+        - logging, and the use in interactive mode.
+        - discuss an example (like the inliner)