9.0 KiB
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 transferred.
- Functions that transfer 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 examplemlirOperationStateTakeRegion
.
- functions that create a new object have the name
- Functions that take ownership of some of their arguments have the form
mlirY<...>OwnedX<...>
whereX
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 examplemlirRegionAppendOwnedBlock
. - 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.
Type Hierarchies
MLIR objects can form type hierarchies in C++. For example, all IR classes
representing types are derived from mlir::Type
, some of them may also be also
derived from common base classes such as mlir::ShapedType
or dialect-specific
base classes. Type hierarchies are exposed to C API through naming conventions
as follows.
- Only the top-level class of each hierarchy is exposed, e.g.
MlirType
is defined as a type butMlirShapedType
is not. This avoids the need for explicit upcasting when passing an object of a derived type to a function that expects a base type (this happens more often in core/standard APIs, while downcasting usually involves further checks anyway). - A type
Y
that derives fromX
provides a functionint mlirXIsAY(MlirX)
that returns a non-zero value if the given dynamic instance ofX
is also an instance ofY
. For example,int MlirTypeIsAInteger(MlirType)
. - A function that expects a derived type as its first argument takes the base
type instead and documents the expectation by using
Y
in its nameMlirY<...>(MlirX, ...)
. This function asserts that the dynamic instance of its first argument isY
, and it is the responsibility of the caller to ensure it is indeed the case.
Auxiliary Types
StringRef
Numerous MLIR functions return instances of StringRef
to refer to a non-owning
segment of a string. This segment may or may not be null-terminated. In C API,
these are represented as instances of MlirStringRef
structure that contains a
pointer to the first character of the string fragment (str
) and the fragment
length (length
). Note that the fragment is not necessarily null-terminated,
the length
field must be used to identify the last character. MlirStringRef
is a non-owning pointer, the caller is in charge of perfoming the copy or
ensuring that the pointee outlives all uses of MlirStringRef
.
Printing
IR objects can be printed using mlirXPrint(MlirX, MlirStringCallback, void *)
functions. These functions accept take arguments a callback with signature void (*)(const char *, intptr_t, void *)
and a pointer to user-defined data. They
call the callback and supply it with chunks of the string representation,
provided as a pointer to the first character and a length, and forward the
user-defined data unmodified. It is up to the caller to allocate memory if the
string representation must be stored and perform the copy. There is no guarantee
that the pointer supplied to the callback points to a null-terminated string,
the size argument should be used to find the end of the string. The callback may
be called multiple times with consecutive chunks of the string representation
(the printing itself is buffered).
Rationale: this approach allows the caller to have full control of the allocation and avoid unnecessary allocation and copying inside the printer.
For convenience, mlirXDump(MlirX)
functions are provided to print the given
object to the standard error stream.
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 an 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.
intptr_t mlirXGetNum<Y>s(MlirX)
returns the upper bound on the index.MlirY mlirXGet<Y>(MlirX, intptr_t pos)
returns 'pos'-th subobject.
The sizes are accepted and returned as signed pointer-sized integers, i.e.
intptr_t
. This typedef is available in C99.
Note that the name of subobject in the function does not necessarily match the
type of the subobject. For example, mlirOperationGetOperand
returns an
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'. */
}
Extending the API
Extensions for Dialect Attributes and Types
Dialect attributes and types can follow the example of standard attributes and
types, provided that implementations live in separate directories, i.e.
include/mlir-c/<...>Dialect/
and lib/CAPI/<...>Dialect/
. The core APIs
provide implementation-private headers in include/mlir/CAPI/IR
that allow one
to convert between opaque C structures for core IR components and their C++
counterparts. wrap
converts a C++ class into a C structure and unwrap
does
the inverse conversion. Once the C++ object is available, the API
implementation should rely on isa
to implement mlirXIsAY
and is expected to
use cast
inside other API calls.