libgomp.texi

理解和实践操作系统的一本好书

@node GOMP_STACKSIZE
@section @env{GOMP_STACKSIZE} -- Set default thread stack size
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{Description}:
Set the default thread stack size in kilobytes. This is in opposition 
to @code{pthread_attr_setstacksize} which gets the number of bytes as an 
argument. If the stacksize can not be set due to system constraints, an 
error is reported and the initial stacksize is left unchanged. If undefined,
the stack size is system dependent.

@item @emph{Reference}: 
@uref{http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00493.html, 
GCC Patches Mailinglist}, 
@uref{http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00496.html,
GCC Patches Mailinglist}
@end table



@c ---------------------------------------------------------------------
@c The libgomp ABI
@c ---------------------------------------------------------------------

@node The libgomp ABI
@chapter The libgomp ABI

The following sections present notes on the external ABI as 
presented by libgomp. Only maintainers should need them.

@menu
* Implementing MASTER construct::
* Implementing CRITICAL construct::
* Implementing ATOMIC construct::
* Implementing FLUSH construct::
* Implementing BARRIER construct::
* Implementing THREADPRIVATE construct::
* Implementing PRIVATE clause::
* Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses::
* Implementing REDUCTION clause::
* Implementing PARALLEL construct::
* Implementing FOR construct::
* Implementing ORDERED construct::
* Implementing SECTIONS construct::
* Implementing SINGLE construct::
@end menu


@node Implementing MASTER construct
@section Implementing MASTER construct

@smallexample
if (omp_get_thread_num () == 0)
  block
@end smallexample

Alternately, we generate two copies of the parallel subfunction
and only include this in the version run by the master thread.
Surely that's not worthwhile though...



@node Implementing CRITICAL construct
@section Implementing CRITICAL construct

Without a specified name,

@smallexample
  void GOMP_critical_start (void);
  void GOMP_critical_end (void);
@end smallexample

so that we don't get COPY relocations from libgomp to the main
application.

With a specified name, use omp_set_lock and omp_unset_lock with
name being transformed into a variable declared like

@smallexample
  omp_lock_t gomp_critical_user_<name> __attribute__((common))
@end smallexample

Ideally the ABI would specify that all zero is a valid unlocked
state, and so we wouldn't actually need to initialize this at
startup.



@node Implementing ATOMIC construct
@section Implementing ATOMIC construct

The target should implement the @code{__sync} builtins.

Failing that we could add

@smallexample
  void GOMP_atomic_enter (void)
  void GOMP_atomic_exit (void)
@end smallexample

which reuses the regular lock code, but with yet another lock
object private to the library.



@node Implementing FLUSH construct
@section Implementing FLUSH construct

Expands to the @code{__sync_synchronize} builtin.



@node Implementing BARRIER construct
@section Implementing BARRIER construct

@smallexample
  void GOMP_barrier (void)
@end smallexample


@node Implementing THREADPRIVATE construct
@section Implementing THREADPRIVATE construct

In _most_ cases we can map this directly to @code{__thread}.  Except
that OMP allows constructors for C++ objects.  We can either
refuse to support this (how often is it used?) or we can 
implement something akin to .ctors.

Even more ideally, this ctor feature is handled by extensions
to the main pthreads library.  Failing that, we can have a set
of entry points to register ctor functions to be called.



@node Implementing PRIVATE clause
@section Implementing PRIVATE clause

In association with a PARALLEL, or within the lexical extent
of a PARALLEL block, the variable becomes a local variable in
the parallel subfunction.

In association with FOR or SECTIONS blocks, create a new
automatic variable within the current function.  This preserves
the semantic of new variable creation.



@node Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses
@section Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses

Seems simple enough for PARALLEL blocks.  Create a private 
struct for communicating between parent and subfunction.
In the parent, copy in values for scalar and "small" structs;
copy in addresses for others TREE_ADDRESSABLE types.  In the 
subfunction, copy the value into the local variable.

Not clear at all what to do with bare FOR or SECTION blocks.
The only thing I can figure is that we do something like

@smallexample
#pragma omp for firstprivate(x) lastprivate(y)
for (int i = 0; i < n; ++i)
  body;
@end smallexample

which becomes

@smallexample
@{
  int x = x, y;

  // for stuff

  if (i == n)
    y = y;
@}
@end smallexample

where the "x=x" and "y=y" assignments actually have different
uids for the two variables, i.e. not something you could write
directly in C.  Presumably this only makes sense if the "outer"
x and y are global variables.

COPYPRIVATE would work the same way, except the structure 
broadcast would have to happen via SINGLE machinery instead.



@node Implementing REDUCTION clause
@section Implementing REDUCTION clause

The private struct mentioned in the previous section should have 
a pointer to an array of the type of the variable, indexed by the 
thread's @var{team_id}.  The thread stores its final value into the 
array, and after the barrier the master thread iterates over the
array to collect the values.


@node Implementing PARALLEL construct
@section Implementing PARALLEL construct

@smallexample
  #pragma omp parallel
  @{
    body;
  @}
@end smallexample

becomes

@smallexample
  void subfunction (void *data)
  @{
    use data;
    body;
  @}

  setup data;
  GOMP_parallel_start (subfunction, &data, num_threads);
  subfunction (&data);
  GOMP_parallel_end ();
@end smallexample

@smallexample
  void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)
@end smallexample

The @var{FN} argument is the subfunction to be run in parallel.

The @var{DATA} argument is a pointer to a structure used to 
communicate data in and out of the subfunction, as discussed
above with respect to FIRSTPRIVATE et al.

The @var{NUM_THREADS} argument is 1 if an IF clause is present
and false, or the value of the NUM_THREADS clause, if
present, or 0.

The function needs to create the appropriate number of
threads and/or launch them from the dock.  It needs to
create the team structure and assign team ids.

@smallexample
  void GOMP_parallel_end (void)
@end smallexample

Tears down the team and returns us to the previous @code{omp_in_parallel()} state.



@node Implementing FOR construct
@section Implementing FOR construct

@smallexample
  #pragma omp parallel for
  for (i = lb; i <= ub; i++)
    body;
@end smallexample

becomes

@smallexample
  void subfunction (void *data)
  @{
    long _s0, _e0;
    while (GOMP_loop_static_next (&_s0, &_e0))
    @{
      long _e1 = _e0, i;
      for (i = _s0; i < _e1; i++)
        body;
    @}
    GOMP_loop_end_nowait ();
  @}

  GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);
  subfunction (NULL);
  GOMP_parallel_end ();
@end smallexample

@smallexample
  #pragma omp for schedule(runtime)
  for (i = 0; i < n; i++)
    body;
@end smallexample

becomes

@smallexample
  @{
    long i, _s0, _e0;
    if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))
      do @{
        long _e1 = _e0;
        for (i = _s0, i < _e0; i++)
          body;
      @} while (GOMP_loop_runtime_next (&_s0, _&e0));
    GOMP_loop_end ();
  @}
@end smallexample

Note that while it looks like there is trickyness to propagating
a non-constant STEP, there isn't really.  We're explicitly allowed
to evaluate it as many times as we want, and any variables involved
should automatically be handled as PRIVATE or SHARED like any other
variables.  So the expression should remain evaluable in the 
subfunction.  We can also pull it into a local variable if we like,
but since its supposed to remain unchanged, we can also not if we like.

If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be
able to get away with no work-sharing context at all, since we can
simply perform the arithmetic directly in each thread to divide up
the iterations.  Which would mean that we wouldn't need to call any
of these routines.

There are separate routines for handling loops with an ORDERED
clause.  Bookkeeping for that is non-trivial...



@node Implementing ORDERED construct
@section Implementing ORDERED construct

@smallexample
  void GOMP_ordered_start (void)
  void GOMP_ordered_end (void)
@end smallexample



@node Implementing SECTIONS construct
@section Implementing SECTIONS construct

A block as 

@smallexample
  #pragma omp sections
  @{
    #pragma omp section
    stmt1;
    #pragma omp section
    stmt2;
    #pragma omp section
    stmt3;
  @}
@end smallexample

becomes

@smallexample
  for (i = GOMP_sections_start (3); i != 0; i = GOMP_sections_next ())
    switch (i)
      @{
      case 1:
        stmt1;
        break;
      case 2:
        stmt2;
        break;
      case 3:
        stmt3;
        break;
      @}
  GOMP_barrier ();
@end smallexample


@node Implementing SINGLE construct
@section Implementing SINGLE construct

A block like 

@smallexample
  #pragma omp single
  @{
    body;
  @}
@end smallexample

becomes

@smallexample
  if (GOMP_single_start ())
    body;
  GOMP_barrier ();
@end smallexample

while 

@smallexample
  #pragma omp single copyprivate(x)
    body;
@end smallexample

becomes

@smallexample
  datap = GOMP_single_copy_start ();
  if (datap == NULL)
    @{
      body;
      data.x = x;
      GOMP_single_copy_end (&data);
    @}
  else
    x = datap->x;
  GOMP_barrier ();
@end smallexample



@c ---------------------------------------------------------------------
@c 
@c ---------------------------------------------------------------------

@node Reporting Bugs
@chapter Reporting Bugs

Bugs in the GNU OpenMP implementation should be reported via 
@uref{http://gcc.gnu.org/bugzilla/, bugzilla}. In all cases, please add 
"openmp" to the keywords field in the bug report.



@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------

@include gpl.texi



@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------

@include fdl.texi



@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------

@include funding.texi

@c ---------------------------------------------------------------------
@c Index
@c ---------------------------------------------------------------------

@node Index
@unnumbered Index

@printindex cp

@bye