llvm-project/mlir/docs/CAPI.md

MLIR C API

Current status: Under development, API unstable, built by default.

Design

Many languages can interoperate with C but have a harder time with C++ due to name mangling and memory model differences. Although the C API for MLIR can be used directly from C, it is primarily intended to be wrapped in higher-level language- or library-specific constructs. Therefore the API tends towards simplicity and feature minimalism.

Note: while the C API is expected to be more stable than C++ API, it currently offers no stability guarantees.

Scope

The API is provided for core IR components (attributes, blocks, operations, regions, types, values), Passes and some fundamental type and attribute kinds. The core IR API is intentionally low-level, e.g. exposes a plain list of operation's operands and attributes without attempting to assign "semantic" names to them. Users of specific dialects are expected to wrap the core API in a dialect-specific way, for example, by implementing an ODS backend.

Object Model

Core IR components are exposed as opaque handles to an IR object existing in C++. They are not intended to be inspected by the API users (and, in many cases, cannot be meaningfully inspected). Instead the users are expected to pass handles to the appropriate manipulation functions.

The handle may or may not own the underlying object.

Naming Convention and Ownership Model

All objects are prefixed with Mlir. They are typedefs and should be used without struct.

All functions are prefixed with mlir.

Functions primarily operating on an instance of MlirX are prefixed with mlirX. They take the instance being acted upon as their first argument (except for creation functions). For example, mlirOperationGetNumOperands inspects an MlirOperation, which it takes as its first operand.

The ownership model is encoded in the naming convention as follows.

• By default, the ownership is not transerred.
• Functions that tranfer the ownership of the result to the caller can be in one of two forms:
  • functions that create a new object have the name mlirXCreate<...>, for example, mlirOperationCreate;
  • functions that detach an object from a parent object have the name mlirYTake<...>, for example mlirOperationStateTakeRegion.
• Functions that take ownership of some of their arguments have the form mlirY<...>OwnedX<...> where X can refer to the type or any other sufficiently unique description of the argument, the ownership of which will be taken by the callee, for example mlirRegionAppendOwnedBlock.
• Functions that create an object by default do not transfer its ownership to the caller, i.e. one of other objects passed in as an argument retains the ownership, they have the form mlirX<...>Get. For example, mlirTypeParseGet.
• Functions that destroy an object owned by the caller are of the form mlirXDestroy.

If the code owns an object, it is responsible for destroying the object when it is no longer necessary. If an object that owns other objects is destroyed, any handles to those objects become invalid. Note that types and attributes are owned by the MlirContext in which they were created.

Nullity

A handle may refer to a null object. It is the responsibility of the caller to check if an object is null by using MlirXIsNull(MlirX). API functions do not expect null objects as arguments unless explicitly stated otherwise. API functions may return null objects.

Common Patterns

The API adopts the following patterns for recurrent functionality in MLIR.

Indexed Components

An object has an indexed component if it has fields accessible using a zero-based contiguous integer index, typically arrays. For example, an MlirBlock has its arguments as a indexed component. An object may have several such components. For example, an MlirOperation has attributes, operands, regions, results and successors.

For indexed components, the following pair of functions is provided.

• unsigned mlirXGetNum<Y>s(MlirX) returns the upper bound on the index.
• MlirY mlirXGet<Y>(MlirX, unsigned pos) returns 'pos'-th subobject.

Note that the name of subobject in the function does not necessarily match the type of the subobject. For example, mlirOperationGetOperand returns a MlirValue.

Iterable Components

An object has an iterable component if it has iterators accessing its fields in some order other than integer indexing, typically linked lists. For example, an MlirBlock has an iterable list of operations it contains. An object may have several iterable components.

For iterable components, the following triple of functions is provided.

• MlirY mlirXGetFirst<Y>(MlirX) returns the first subobject in the list.
• MlirY mlirYGetNextIn<X>(MlirY) returns the next subobject in the list that contains the given object, or a null object if the given object is the last in this list.
• int mlirYIsNull(MlirY) returns 1 if the given object is null.

Note that the name of subobject in the function may or may not match its type.

This approach enables one to iterate as follows.

MlirY iter;
for (iter = mlirXGetFirst<Y>(x); !mlirYIsNull(iter);
     iter = mlirYGetNextIn<X>(iter)) {
  /* User 'iter'. */
}