starpu-max/doc/c-extensions.texi

@c This is part of the StarPU Handbook.
@c Copyright (C) 2011 Institut National de Recherche en Informatique et Automatique

@node C Extensions
@chapter C Extensions

@cindex C extensions
@cindex GCC plug-in

When configured with @code{--enable-gcc-extensions}, StarPU builds a
plug-in for the GNU Compiler Collection (GCC), which defines extensions
to the C language that make it easier to write StarPU code@footnote{This
feature is only available for GCC 4.5 and later.}.  Those extensions
include syntactic sugar for defining tasks and their implementations,
invoking a task, and manipulating data buffers.

This section does not require detailed knowledge of the StarPU library.

Note: as of StarPU @value{VERSION}, this is still an area under
development and subject to change.

@menu
* Defining Tasks::              Defining StarPU tasks
* Registered Data Buffers::     Manipulating data buffers
@end menu

@node Defining Tasks
@section Defining Tasks

@cindex task
@cindex task implementation

The StarPU GCC plug-in views @dfn{tasks} as ``extended'' C functions:

@enumerate
@item
tasks may have several implementations---e.g., one for CPUs, one written
in OpenCL, one written in CUDA;
@item
when a task is invoked, it may run in parallel, and StarPU is free to
choose any of its implementations.
@end enumerate

Tasks and their implementations must be @emph{declared}.  These
declarations are annotated with @dfn{attributes} (@pxref{Attribute
Syntax, attributes in GNU C,, gcc, Using the GNU Compiler Collection
(GCC)}): the declaration of a task is a regular C function declaration
with an additional @code{task} attribute, and task implementations are
declared with a @code{task_implementation} attribute.

The following function attributes are provided:

@table @code

@item task
@cindex @code{task} attribute
Declare the given function as a StarPU task.  Its return type must be
@code{void}, and it must not be defined---instead, a definition will
automatically be provided by the compiler.

Under the hood, declaring a task leads to the declaration of the
corresponding @code{codelet} (@pxref{Codelet and Tasks}).  If one or
more task implementations are declared in the same compilation unit,
then the codelet and the function itself are also defined; they inherit
the scope of the task.

Scalar arguments to the task are passed by value and copied to the
target device if need be---technically, they are passed as the
@code{cl_arg} buffer (@pxref{Codelets and Tasks, @code{cl_arg}}).

Pointer arguments are assumed to be registered data buffers---the
@code{buffers} argument of a task (@pxref{Codelets and Tasks,
@code{buffers}}); @code{const}-qualified pointer arguments are viewed as
read-only buffers (@code{STARPU_R}), and non-@code{const}-qualified
buffers are assumed to be used read-write (@code{STARPU_RW}).

@item task_implementation (@var{target}, @var{task})
@cindex @code{task_implementation} attribute
Declare the given function as an implementation of @var{task} to run on
@var{target}.  @var{target} must be a string, currently one of
@code{"cpu"} or @code{"cuda"}.
@c FIXME: Update when OpenCL support is ready.

@end table

Here is an example:

@example
static void matmul (const float *A, const float *B, float *C,
		    size_t nx, size_t ny, size_t nz)
  __attribute__ ((task));

static void matmul_cpu (const float *A, const float *B, float *C,
			size_t nx, size_t ny, size_t nz)
  __attribute__ ((task_implementation ("cpu", matmul)));


static void
matmul_cpu (const float *A, const float *B, float *C,
	    size_t nx, size_t ny, size_t nz)
@{
  size_t i, j, k;

  for (j = 0; j < ny; j++)
    for (i = 0; i < nx; i++)
      @{
	for (k = 0; k < nz; k++)
	  C[j * nx + i] += A[j * nz + k] * B[k * nx + i];
      @}
@}
@end example

@noindent
A @code{matmult} task is defined; it has only one implementation,
@code{matmult_cpu}, which runs on the CPU.  Variables @var{A} and
@var{B} are input buffers, whereas @var{C} is considered an input/output
buffer.  The task can be invoked like a regular C function:

@example
matmul (&A[i * zdim * bydim + k * bzdim * bydim],
        &B[k * xdim * bzdim + j * bxdim * bzdim],
        &C[i * xdim * bydim + j * bxdim * bydim],
        bxdim, bydim, bzdim);
@end example

@noindent
This leads to an @dfn{asynchronous invocation}, whereby @code{matmult}'s
implementation may run in parallel with the continuation of the caller.

The next section describes how memory buffers must be handled in
StarPU-GCC code.


@node Registered Data Buffers
@section Registered Data Buffers

Data buffers such as matrices and vectors that are to be passed to tasks
must be @dfn{registered}.  Registration allows StarPU to handle data
transfers among devices---e.g., transferring an input buffer from the
CPU's main memory to a task scheduled to run a GPU (@pxref{StarPU Data
Management Library}).

The following pragmas are provided:

@table @code

@item #pragma starpu register @var{ptr} [@var{size}]
Register @var{ptr} as a @var{size}-element buffer.

@item #pragma starpu unregister @var{ptr}
@item #pragma starpu acquire @var{ptr}

@end table

FIXME: finish

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