/* fibers.c -- extremely simple lightweight thread (fiber) implementation
   Copyright (C) 2016-2021 Free Software Foundation, Inc.
   Contributed by Pekka Jaaskelainen <pekka.jaaskelainen@parmance.com>
   for General Processor Tech.

   Copyright (C) 2015-2021 Free Software Foundation, Inc.
   Contributed by Pekka Jaaskelainen <pekka.jaaskelainen@parmance.com>
   for General Processor Tech.

   Permission is hereby granted, free of charge, to any person obtaining a
   copy of this software and associated documentation files
   (the "Software"), to deal in the Software without restriction, including
   without limitation the rights to use, copy, modify, merge, publish,
   distribute, sublicense, and/or sell copies of the Software, and to
   permit persons to whom the Software is furnished to do so, subject to
   the following conditions:

   The above copyright notice and this permission notice shall be included
   in all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
   IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
   DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
   OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
   USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>

#include "target-config.h"

#include "fibers.h"

void
phsa_fatal_error (int code);

ucontext_t main_context;

/* The last fiber in the linked list.  */
static fiber_t *tail_fiber = NULL;
/* The first fiber in the linked list.  */
static fiber_t *head_fiber = NULL;
/* The fiber currently being executed.  */
static fiber_t *current_fiber = NULL;

/* Makecontext accepts only integer arguments.  We need to split the
   pointer argument in case pointer does not fit into int.  This helper
   function can be used to restore the pointer from the arguments.  */

void *
fiber_int_args_to_ptr (int arg0, int arg1)
{
  void *ptr = NULL;
#if SIZEOF_VOIDP == 8 && SIZEOF_INT == 4
  ptr = (void*)(((uint64_t) arg0 & (uint64_t) 0xFFFFFFFF)
		| ((uint64_t) arg1 << 32));
#elif SIZEOF_VOIDP == 4 && SIZEOF_INT == 4
  ptr = (void*)arg0;
#else
# error Unsupported pointer/int size.
#endif
  return ptr;
}

void
fiber_init (fiber_t *fiber, fiber_function_t start_function, void *arg,
	    size_t stack_size, size_t stack_align)
{
  int arg0, arg1;
  if (getcontext (&fiber->context) != 0)
    phsa_fatal_error (3);
  if (posix_memalign (&fiber->context.uc_stack.ss_sp, stack_align, stack_size)
      != 0)
    phsa_fatal_error (4);
  fiber->context.uc_stack.ss_size = stack_size;
  fiber->context.uc_link = &main_context;

  /* makecontext () accepts only integer arguments.  Split the
     pointer argument to two args in the case pointer does not fit
     into one int.  */
#if SIZEOF_VOIDP == 8 && SIZEOF_INT == 4
  arg0 = (int32_t) 0xFFFFFFFF & (uint64_t)arg;
  arg1 = (int32_t) 0xFFFFFFFF & ((uint64_t)arg >> 32);
#elif SIZEOF_VOIDP == 4 && SIZEOF_INT == 4
  arg0 = (int)arg;
  arg1 = 0;
#else
# error Unsupported pointer/int size.
#endif

  makecontext (&fiber->context, (void*)start_function, 2, arg0, arg1);

  fiber->status = FIBER_STATUS_READY;
  fiber->next = NULL;
  fiber->prev = NULL;

  /* Create a linked list of the created fibers.  Append the new one at
     the end.  */
  if (tail_fiber == NULL)
    tail_fiber = fiber;
  else
    {
      tail_fiber->next = fiber;
      fiber->prev = tail_fiber;
      tail_fiber = fiber;
    }

  if (head_fiber == NULL)
    head_fiber = fiber;
}

void
fiber_exit ()
{
  fiber_status_t old_status = current_fiber->status;
  current_fiber->status = FIBER_STATUS_EXITED;
  if (old_status == FIBER_STATUS_JOINED)
    /* In case this thread has been joined, return back to the joiner.  */
    swapcontext (&current_fiber->context, &main_context);
  else
    /* In case the thread exited while being yielded from another thread,
       switch back to another fiber.  */
    fiber_yield ();
}

void
fiber_join (fiber_t *fiber)
{
  fiber_t *next_ready_fiber = NULL;
  current_fiber = fiber;
  if (fiber->status != FIBER_STATUS_EXITED)
    {
      fiber->status = FIBER_STATUS_JOINED;
      while (fiber->status != FIBER_STATUS_EXITED)
	swapcontext (&main_context, &fiber->context);
    }

  /* Remove the successfully joined fiber from the linked list so we won't
     access it later (the fiber itself might be freed after the join).  */
  if (fiber->prev != NULL)
    fiber->prev->next = fiber->next;

  if (fiber->next != NULL)
    fiber->next->prev = fiber->prev;

  if (head_fiber == fiber)
    head_fiber = fiber->next;

  if (tail_fiber == fiber)
    tail_fiber = fiber->prev;

  free (fiber->context.uc_stack.ss_sp);
}

void
fiber_yield ()
{
  fiber_t *next_ready_fiber = current_fiber;

  if (current_fiber == head_fiber
      && current_fiber == tail_fiber)
    {
      /* If the last fiber exits independently, there is no
	 fiber to switch to.  Switch to the main context in that
	 case.  */
      if (current_fiber->status == FIBER_STATUS_EXITED)
	swapcontext (&current_fiber->context, &main_context);
    }

  do {
    next_ready_fiber = next_ready_fiber->next != NULL
      ? next_ready_fiber->next : head_fiber;
  } while (next_ready_fiber != current_fiber
	   && next_ready_fiber->status == FIBER_STATUS_EXITED);

  fiber_t *old_current_fiber = current_fiber;
  current_fiber = next_ready_fiber;
  swapcontext (&old_current_fiber->context, &next_ready_fiber->context);
}

size_t
fiber_barrier_reach (fiber_barrier_t *barrier)
{
  /* Yield once to ensure that there are no fibers waiting for
     a previous triggering of the barrier in the waiting_count
     loop.  This should release them before we update the reached
     counter again.  */
  fiber_yield ();

  barrier->reached++;
  ++barrier->waiting_count;
  while (barrier->reached < barrier->threshold)
    fiber_yield ();
  --barrier->waiting_count;

  /* Wait until all the fibers have reached this point.  */
  while (barrier->waiting_count > 0)
    fiber_yield ();

  /* Now all fibers have been released from the barrier waiting
     loop.  We can now safely reset the reach count for new triggering.  */
  if (barrier->reached > 0)
    {
      barrier->reached = 0;
      return 0;
    }
  return 1;
}

void
fiber_barrier_init (fiber_barrier_t *barrier, size_t threshold)
{
  barrier->threshold = threshold;
  barrier->waiting_count = 0;
  barrier->reached = 0;
}
