Offload API definitions

Note: This is a work-in-progress. It is loosely based on equivalent tooling in Unified Runtime.

The Tablegen files in this directory are used to define the Offload API. They are used with the offload-tblgen tool to generate API headers, print headers, and other implementation details.

The root file is OffloadAPI.td - additional .td files can be included in this file to add them to the API.

API Objects

The API consists of a number of objects, which always have a name field and description field, and are one of the following types:

Function

Represents an API entry point function. Has a list of returns and parameters. Also has fields for details (representing a bullet-point list of information about the function that would otherwise be too detailed for the description), and analogues (equivalent functions in other APIs).

Parameter

Represents a parameter to a function, has type, name, and desc fields. Also has a flags field containing flags representing whether the parameter is in, out, or optional.

The type field is used to infer if the parameter is a pointer or handle type. A handle type is a pointer to an opaque struct, used to abstract over plugin-specific implementation details.

There are two special variants of a parameter:

• RangedParameter - Represents a parameter that has a range described by other parameters. Generally these are pointers to an arbitrary number of objects. The range is used for generating validation and printing code. E.g, a range might be between (0, NumDevices)
• TypeTaggedParameter - Represents a parameter (usually of void* type) that has the type and size of its pointee data described by other function parameters. The type is usually described by a type-tagged enum. This allows functions (e.g. olGetDeviceInfo) to return data of an arbitrary type.

Return

A return represents a possible return code from the function, and optionally a list of conditions in which this value may be returned. The conditions list is not expected to be exhaustive. A condition is considered free-form text, but if it is wrapped in `backticks` then it is treated as literal code representing an error condition (e.g. someParam < 1). These conditions are used to automatically create validation checks by the offload-tblgen validation generator.

Returns are automatically generated for functions with pointer or handle parameters, so API authors do not need to exhaustively add null checks for these types of parameters. All functions also get a number of default return values automatically.

Struct

Represents a struct. Contains a list of members, which each have a type, name, and desc.

Also optionally takes a base_class field. If this is either of the special offload_base_properties_t or offload_base_desc_t structs, then the struct will inherit members from those structs. The generated struct does not use actual C++ inheritance, but instead explicitly has those members copied in, which preserves ABI compatibility with C.

Enum

Represents a C-style enum. Contains a list of etor values, which have a name and description.

A TaggedEtor record type also exists which addtionally takes a type. This type is used when the enum is used as a parameter to a function with a type-tagged function parameter (e.g. olGetDeviceInfo).

All enums automatically get a <enum_name>_FORCE_UINT32 = 0x7fffffff value, which forces the underlying type to be uint32.

Handle

Represents a pointer to an opaque struct, as described in the Parameter section. It does not take any extra fields.

Typedef

Represents a typedef, contains only a value field.

Macro

Represents a C preprocessor #define. Contains a value field. Optionally takes a condition field, which allows the macro to be conditionally defined, and an alt_value field, which represents the value if the condition is false.

Macro arguments are presented in the name field (e.g. name = mymacro(arg)).

While there may seem little point generating a macro from tablegen, doing this allows the entire source of the header file to be generated from the tablegen files, rather than requiring a mix of C source and tablegen.

Generation

API header

./offload-tblgen -I <path-to-llvm>/offload/API  <path-to-llvm>/offload/API/OffloadAPI.td --gen-api

The comments in the generated header are in Doxygen format, although generating documentation from them hasn't been implemented yet.

The entirety of this header is generated by Tablegen, rather than having a predefined header file that includes one or more .inc files. This is because this header is expected to be part of the installation and distributed to end-users, so should be self-contained.

Entry Points

./offload-tblgen -I <path-to-llvm>/offload/API  <path-to-llvm>/offload/API/OffloadAPI.td --gen-entry-points

These functions form the actual Offload interface, and are wrappers over the functions that contain the actual implementation (see 'Adding a new entry point').

They implement automatically generated validation checks, and tracing of function calls with arguments and results. The tracing can be enabled with the OFFLOAD_TRACE environment variable.

Implementation function declarations

./offload-tblgen -I <path-to-llvm>/offload/API  <path-to-llvm>/offload/API/OffloadAPI.td --gen-impl-func-decls

Generates declarations of the implementation of functions of every entry point in the API, e.g. offloadDeviceFoo_impl for offloadDeviceFoo.

Print header

./offload-tblgen -I <path-to-llvm>/offload/API  <path-to-llvm>/offload/API/OffloadAPI.td --gen-print-header

This header contains std::ostream &operator<<(std::ostream&) definitions for various API objects, including function parameters.

As with the API header, it is expected that this header is part of the installed package, so it is entirely generated by Tablegen.

For ease of implementation, and since it is not strictly part of the API, this is a C++ header file. If a C version is desirable it could be added.

Future Tablegen backends

RecordTypes.hpp contains wrappers for all of the API object types, which will allow more backends to be easily added in future.

Adding to the API

A new object can be added to the API by adding to one of the existing .td files. It is also possible to add a new tablegen file to the API by adding it to the includes in OffloadAPI.td. When the offload target is rebuilt, the new definition will be included in the generated files.

Adding a new entry point

When a new entry point is added (e.g. offloadDeviceFoo), the actual entry point is automatically generated, which contains validation and tracing code. It expects an implementation function (offloadDeviceFoo_impl) to be defined, which it will call into. The definition of this implementation function should be added to src/offload_impl.cpp