source: CIVL/examples/cuda/exitBarrier.cvl@ fbc5eb8

// Tyler Sorensen
//
// An attempt to create an "exit barrier"
// as seen in NVIDIA PTX instructions
// See "exit" and "bar.sync" instructions in PTX ISA
//
// The algorithm implemented uses the general CIVL barrier 
// module found in examples/concurrency/barrier2.cvl
//
// The exit semantics are a combination of barrier
// resignation as seen in KRoC release of occam and
// documented in the paper:
// Higher Level of Process Synchronization by:
// Peter H. Welch and David C. Wood.
// and a return statement (to exit the thread)
//
// The resignation algorithm is given briefly in 
// the paper cited above which I attempted to
// implement. However, this implementation assumes
// that a resigned thread does not access the barrier 
// again! It is implemented with the exit barrier
// semantics in mind, i.e. the thread will cease
// execution right after resignation. No attempts
// are made to guard against behavior deviating
// from this. 
// Commandline execution:
//              civl verify exitBarrier.cvl

// TODO: Right now, the thread termination is simply
// a "return" statement. Anyway for that to actually
// kill the thread?

#include <civlc.h>

struct _CIVL_Barrier {
  int numProcs;   //how many are enrolled to begin with, cannot change
  int numWaiting; //New variable for exit barriers, how many you need to wait for, can change
  int *in_barrier;
  int num_in_barrier;
  int lock;
};

typedef struct _CIVL_Barrier CIVL_Barrier;

void CIVL_Barrier_init(int numProcs, CIVL_Barrier *barrier, int *array) {  
  barrier->numProcs = numProcs;
  barrier->numWaiting = numProcs;
  barrier->num_in_barrier = 0;
  barrier->lock = 0;
  barrier->in_barrier = array;
  for (int i=0; i<numProcs; i++)
    barrier->in_barrier[i] = 0;
}

void CIVL_barrier(CIVL_Barrier *barrier, int tid) {
  $when (barrier->lock==0) barrier->lock = 1;
  barrier->in_barrier[tid] = 1;
  barrier->num_in_barrier++;

  //simply wait for how many need to be waited for, but
  //free them all. No exceptions.
  if (barrier->num_in_barrier == barrier->numWaiting) {
    for (int i=0; i<barrier->numProcs; i++)
      barrier->in_barrier[i] = 0;
    barrier->num_in_barrier = 0;
  }
  barrier->lock = 0;
  $when (barrier->in_barrier[tid] == 0);
}

//The resign function: Basically the same, but
//decrement the number waiting for. If 
//A barrier is waiting on the resigning thread, then free
//the barrier before leaving. No need to wait also.
void CIVL_barrier_resign(CIVL_Barrier *barrier, int tid) {
  $when (barrier->lock==0) barrier->lock = 1;
  barrier->in_barrier[tid] = 1;
  barrier->numWaiting--;
  if (barrier->num_in_barrier == barrier->numWaiting) {
    for (int i=0; i<barrier->numProcs; i++)
      barrier->in_barrier[i] = 0;
    barrier->num_in_barrier = 0;
  }
  barrier->lock = 0; 
}

#define CIVL_barrier_exit(barrier, tid) CIVL_barrier_resign(barrier, tid); return

void main() {
  int N=4;
  $proc threads[N];
  int counter = 0;
  CIVL_Barrier b;
  int barrier_array[N];
  
  void run_proc0(int tid) {  
    $assert(counter == 0);
    CIVL_barrier_exit(&b, tid);
  }

  void run_proc1(int tid) {
    $assert(counter == 0);
    CIVL_barrier(&b, tid);
    counter++;
    CIVL_barrier_exit(&b, tid);
  }

  void run_proc2(int tid) {  
    $assert(counter == 0);
    CIVL_barrier(&b, tid);
    counter++;
    CIVL_barrier(&b, tid);
    $assert(counter == 3);
    CIVL_barrier_exit(&b, tid);   
  }

  void run_proc3(int tid) {
    $assert(counter == 0);
    CIVL_barrier(&b, tid);
    counter++;
    CIVL_barrier(&b, tid);
    $assert(counter == 3);
    CIVL_barrier(&b, tid);
    counter--;
    CIVL_barrier_exit(&b, tid);
  }

  CIVL_Barrier_init(N, &b, barrier_array);

  //Spawn threads
  $proc proc0 = $spawn run_proc0(0);
  $proc proc1 = $spawn run_proc1(1);
  $proc proc2 = $spawn run_proc2(2);
  $proc proc3 = $spawn run_proc3(3);

  //wait for threads
  $wait(proc0);
  $wait(proc1);
  $wait(proc2);
  $wait(proc3);

  $assert(counter == 2);
}