1# MLIR C API
2
3**Current status: Under development, API unstable, built by default.**
4
5## Design
6
7Many languages can interoperate with C but have a harder time with C++ due to
8name mangling and memory model differences. Although the C API for MLIR can be
9used directly from C, it is primarily intended to be wrapped in higher-level
10language- or library-specific constructs. Therefore the API tends towards
11simplicity and feature minimalism.
12
13**Note:** while the C API is expected to be more stable than C++ API, it
14currently offers no stability guarantees.
15
16### Scope
17
18The API is provided for core IR components (attributes, blocks, operations,
19regions, types, values), Passes and some fundamental type and attribute kinds.
20The core IR API is intentionally low-level, e.g. exposes a plain list of
21operation's operands and attributes without attempting to assign "semantic"
22names to them. Users of specific dialects are expected to wrap the core API in a
23dialect-specific way, for example, by implementing an ODS backend.
24
25### Object Model
26
27Core IR components are exposed as opaque _handles_ to an IR object existing in
28C++. They are not intended to be inspected by the API users (and, in many cases,
29cannot be meaningfully inspected). Instead the users are expected to pass
30handles to the appropriate manipulation functions.
31
32The handle _may or may not_ own the underlying object.
33
34### Naming Convention and Ownership Model
35
36All objects are prefixed with `Mlir`. They are typedefs and should be used
37without `struct`.
38
39All functions are prefixed with `mlir`.
40
41Functions primarily operating on an instance of `MlirX` are prefixed with
42`mlirX`. They take the instance being acted upon as their first argument (except
43for creation functions). For example, `mlirOperationGetNumOperands` inspects an
44`MlirOperation`, which it takes as its first operand.
45
46The *ownership* model is encoded in the naming convention as follows.
47
48-   By default, the ownership is not transferred.
49-   Functions that transfer the ownership of the result to the caller can be in
50    one of two forms:
51    *   functions that create a new object have the name `mlirXCreate<...>`, for
52        example, `mlirOperationCreate`;
53    *   functions that detach an object from a parent object have the name
54        `mlirYTake<...>`, for example `mlirOperationStateTakeRegion`.
55-   Functions that take ownership of some of their arguments have the form
56    `mlirY<...>OwnedX<...>` where `X` can refer to the type or any other
57    sufficiently unique description of the argument, the ownership of which will
58    be taken by the callee, for example `mlirRegionAppendOwnedBlock`.
59-   Functions that create an object by default do not transfer its ownership to
60    the caller, i.e. one of other objects passed in as an argument retains the
61    ownership, they have the form `mlirX<...>Get`. For example,
62    `mlirTypeParseGet`.
63-   Functions that destroy an object owned by the caller are of the form
64    `mlirXDestroy`.
65
66If the code owns an object, it is responsible for destroying the object when it
67is no longer necessary. If an object that owns other objects is destroyed, any
68handles to those objects become invalid. Note that types and attributes are
69owned by the `MlirContext` in which they were created.
70
71### Nullity
72
73A handle may refer to a _null_ object. It is the responsibility of the caller to
74check if an object is null by using `mlirXIsNull(MlirX)`. API functions do _not_
75expect null objects as arguments unless explicitly stated otherwise. API
76functions _may_ return null objects.
77
78### Type Hierarchies
79
80MLIR objects can form type hierarchies in C++. For example, all IR classes
81representing types are derived from `mlir::Type`, some of them may also be also
82derived from common base classes such as `mlir::ShapedType` or dialect-specific
83base classes. Type hierarchies are exposed to C API through naming conventions
84as follows.
85
86-   Only the top-level class of each hierarchy is exposed, e.g. `MlirType` is
87    defined as a type but `MlirShapedType` is not. This avoids the need for
88    explicit upcasting when passing an object of a derived type to a function
89    that expects a base type (this happens more often in core/standard APIs,
90    while downcasting usually involves further checks anyway).
91-   A type `Y` that derives from `X` provides a function `int mlirXIsAY(MlirX)`
92    that returns a non-zero value if the given dynamic instance of `X` is also
93    an instance of `Y`. For example, `int MlirTypeIsAInteger(MlirType)`.
94-   A function that expects a derived type as its first argument takes the base
95    type instead and documents the expectation by using `Y` in its name
96    `MlirY<...>(MlirX, ...)`. This function asserts that the dynamic instance of
97    its first argument is `Y`, and it is the responsibility of the caller to
98    ensure it is indeed the case.
99
100### Auxiliary Types
101
102#### `StringRef`
103
104Numerous MLIR functions return instances of `StringRef` to refer to a non-owning
105segment of a string. This segment may or may not be null-terminated. In C API,
106these are represented as instances of `MlirStringRef` structure that contains a
107pointer to the first character of the string fragment (`str`) and the fragment
108length (`length`). Note that the fragment is _not necessarily_ null-terminated,
109the `length` field must be used to identify the last character. `MlirStringRef`
110is a non-owning pointer, the caller is in charge of performing the copy or
111ensuring that the pointee outlives all uses of `MlirStringRef`.
112
113### Printing
114
115IR objects can be printed using `mlirXPrint(MlirX, MlirStringCallback, void *)`
116functions. These functions accept take arguments a callback with signature `void
117(*)(const char *, intptr_t, void *)` and a pointer to user-defined data. They
118call the callback and supply it with chunks of the string representation,
119provided as a pointer to the first character and a length, and forward the
120user-defined data unmodified. It is up to the caller to allocate memory if the
121string representation must be stored and perform the copy. There is no guarantee
122that the pointer supplied to the callback points to a null-terminated string,
123the size argument should be used to find the end of the string. The callback may
124be called multiple times with consecutive chunks of the string representation
125(the printing itself is buffered).
126
127*Rationale*: this approach allows the caller to have full control of the
128allocation and avoid unnecessary allocation and copying inside the printer.
129
130For convenience, `mlirXDump(MlirX)` functions are provided to print the given
131object to the standard error stream.
132
133## Common Patterns
134
135The API adopts the following patterns for recurrent functionality in MLIR.
136
137### Indexed Components
138
139An object has an _indexed component_ if it has fields accessible using a
140zero-based contiguous integer index, typically arrays. For example, an
141`MlirBlock` has its arguments as an indexed component. An object may have
142several such components. For example, an `MlirOperation` has attributes,
143operands, regions, results and successors.
144
145For indexed components, the following pair of functions is provided.
146
147-   `intptr_t mlirXGetNum<Y>s(MlirX)` returns the upper bound on the index.
148-   `MlirY mlirXGet<Y>(MlirX, intptr_t pos)` returns 'pos'-th subobject.
149
150The sizes are accepted and returned as signed pointer-sized integers, i.e.
151`intptr_t`. This typedef is available in C99.
152
153Note that the name of subobject in the function does not necessarily match the
154type of the subobject. For example, `mlirOperationGetOperand` returns an
155`MlirValue`.
156
157### Iterable Components
158
159An object has an _iterable component_ if it has iterators accessing its fields
160in some order other than integer indexing, typically linked lists. For example,
161an `MlirBlock` has an iterable list of operations it contains. An object may
162have several iterable components.
163
164For iterable components, the following triple of functions is provided.
165
166-   `MlirY mlirXGetFirst<Y>(MlirX)` returns the first subobject in the list.
167-   `MlirY mlirYGetNextIn<X>(MlirY)` returns the next subobject in the list that
168    contains the given object, or a null object if the given object is the last
169    in this list.
170-   `int mlirYIsNull(MlirY)` returns 1 if the given object is null.
171
172Note that the name of subobject in the function may or may not match its type.
173
174This approach enables one to iterate as follows.
175
176```c++
177MlirY iter;
178for (iter = mlirXGetFirst<Y>(x); !mlirYIsNull(iter);
179     iter = mlirYGetNextIn<X>(iter)) {
180  /* User 'iter'. */
181}
182```
183
184## Extending the API
185
186### Extensions for Dialect Attributes and Types
187
188Dialect attributes and types can follow the example of builtin attributes and
189types, provided that implementations live in separate directories, i.e.
190`include/mlir-c/<...>Dialect/` and `lib/CAPI/<...>Dialect/`. The core APIs
191provide implementation-private headers in `include/mlir/CAPI/IR` that allow one
192to convert between opaque C structures for core IR components and their C++
193counterparts. `wrap` converts a C++ class into a C structure and `unwrap` does
194the inverse conversion. Once the C++ object is available, the API implementation
195should rely on `isa` to implement `mlirXIsAY` and is expected to use `cast`
196inside other API calls.
197