Intel OpenMP Runtime & LLVM [kmp_tasking.c]: Problem to access SHARED VARIABLE DATA LOCATION

Runtimes OpenMP
1

Dear concern,

Please consider the source code(openmp c code) snippet as below:
— code snippet —
int a1, b1, c1, N;

Initially address of a1:= 0x7fff9bd58304
omp_set_num_threads(2);
#pragma omp parallel
{
#pragma omp single
{
// task one: a1=b1+c1;
#pragma omp task shared(a1) firstprivate(b1,c1)
{
a1 = a1 + b1 + c1;
}
}
}

In the above openMP code task construct contains shared variable (a1), I AM INTERESTED TO FIND THE SHARED VARIABLE OF TASK DATA IN RUNTIME TASK ALLOCATION.

I am looking into the source code of Intel OpenMP Runtime, my openmp code is compiled with LLVM. In the LLVM IR, @__kmpc_omp_task_alloc( …) calls runtime function kmp_task_alloc(), source code is here: under the directory, Kmp_tasking.c

I am trying to access the “shared variable data location for a task” which is as below:

kmp_task_t *
__kmp_task_alloc(…){

kmp_task_t *task;
kmp_taskdata_t *taskdata;

// Calculate shared structure offset including padding after kmp_task_t struct
// to align pointers in shared struct
shareds_offset = sizeof( kmp_taskdata_t ) + sizeof_kmp_task_t;
shareds_offset = __kmp_round_up_to_val( shareds_offset, sizeof( void * ));

// Avoid double allocation here by combining shareds with taskdata
#if USE_FAST_MEMORY
taskdata = (kmp_taskdata_t ) __kmp_fast_allocate( thread, shareds_offset + sizeof_shareds );
#else / ! USE_FAST_MEMORY */
taskdata = (kmp_taskdata_t ) __kmp_thread_malloc( thread, shareds_offset + sizeof_shareds );
#endif / USE_FAST_MEMORY */

task = KMP_TASKDATA_TO_TASK(taskdata);

if (sizeof_shareds > 0) {
// Avoid double allocation here by combining shareds with taskdata
task->shareds = & ((char *) taskdata)[ shareds_offset ];
printf(“task->shareds address original:= %p address of pointer it points to := %p\n”, & ((char *) taskdata)[ shareds_offset ], *task->shareds);
}

}

here,
typedef struct kmp_task { /* GEH: Shouldn’t this be aligned somehow? */
void * shareds; /**< pointer to block of pointers to shared vars */
}

But, task->shareds = & ((char *) taskdata)[ shareds_offset ]; which is a pointer to block of pointers to shared variables, if I print the shared variable pointing address(which should be the address of a1:= 0x7fff9bd58304)
IT PRINTS DIFFERENT ADDRESS: <0x1fe0ef0>

NEED TO GET THE ADDRESS OF TASK’S SHARED VARIABLE ADDRESS FROM RUNTIME.

appreciate your comment and help. Thanking you.

2

Hello,

If you print information from __kmp_task_alloc() function, you cannot find your data in the structures.

The reason is that this routine only allocates space requested by the compiler and initializes its own runtime-related info, and if you examine the space allocated for shareds you can only find uninitialized memory there.

The addresses of shared variables are put into allocated memory by the compiler later, AFTER the __kmp_task_alloc() call. E.g. the array of shareds should already be initialized in the __kmp_omp_task() call. But I doubt you can rely on particular location of any shared variable in the array of shareds, it is compiler’s decision where to place your particular variable’s address, and it may or may not be the first in the array of shareds. E.g. compiler can insert its own auxiliary data before user’s data.

Regards,

Andrey

3

Hi Andrey,

Thank you very much for your time and nice explanation. You are right I couldn’t access the data location.
I am thinking to add a runtime call with shared data reference, let us see what I get. And please share better idea if you have any.

Best Regards,

  • Raju