[OMPD] The LLVM OMPD ompd_get_task_function() entry_point address return value is 0 for the outermost implicit task of a parallel region

[jdelsign]

Using the attach test program compiled with CLANG/LLVM trunk 9 as of a week or two ago, and using an OMP and OMPD built from the "ompd-devices" branch, I ran the following test under TotalView (which has support for LLVM's OMPD library):

• Set environment variable OMP_OMPD=on
• Set environment variable OMP_NUM_THREADS=1
• Set a breakpoint inside the parallel region in "case 0" in function "f" at line 109. (But note that that line actually falls inside the outlined function named "omp_outlined.debug_.10"
• Run to the breakpoint.

Here is the (unfiltered) stack trace for the one and only thread in the program at this point:

d1.<> w
>  0 omp_outlined._debug__.10 PC=0x004015b8, FP=0x7fffffffd540 [/home/jdelsign/ms2-demo/tx_omp_parallel_nested.c#109]
   1 omp_outlined..12 PC=0x00401628, FP=0x7fffffffd570 [/home/jdelsign/ms2-demo/tx_omp_parallel_nested.c#108]
   2 __kmp_invoke_microtask PC=0x7ffff7b9a1e1, FP=0x7fffffffd610 [/.../libomp.so]
   3 __kmp_fork_call  PC=0x7ffff7b01b84, FP=0x7fffffffdac0 [/.../kmp_runtime.cpp#1909]
   4 __kmpc_fork_call PC=0x7ffff7aec09a, FP=0x7fffffffdcb0 [/.../kmp_csupport.cpp#299]
   5 f                PC=0x004012f1, FP=0x7fffffffdd20 [/home/jdelsign/ms2-demo/tx_omp_parallel_nested.c#107]
   6 main             PC=0x00401c35, FP=0x7fffffffdd90 [/home/jdelsign/ms2-demo/tx_omp_parallel_nested.c#224]
   7 __libc_start_main PC=0x7ffff72bf443, FP=0x7fffffffde50 [/lib64/libc.so.6]
   8 _start           PC=0x00400ac4, FP=0x7fffffffde58 [/amd/home/jdelsign/ms2-demo/tx_omp_parallel_nested]
d1.<>

If I use TotalView to call into the LLVM OMPD library to unwind the parallel regions, and get the task handles and task functions for each one, I see the following TotalView debug output for the OMPD DLL calls it makes.

• Get the current parallel handle (0x3b225d0) for the thread (0x3c6a7e0):

OMPD DLL: ompd_get_curr_parallel_handle(thread_handle=0x3c6a7e0)->rc_ok: parallel_handle=0x3b225d0

• Get the task handle (0x3d641a0) for the implicit task associated with the parallel region (0x3b225d0):

OMPD DLL: ompd_get_task_in_parallel(parallel_handle=0x3b225d0, thread_num=0)->rc_ok: task_handle=0x3d641a0

• Get the task function for the task handle (0x3d641a0):

OMPD DLL: ompd_get_task_function(task_handle=0x3d641a0)->rc_ok: entry_point={segment=0,address=0x0}

NOTICE: The entry_point address returned is 0! Why? I would have expected that the address of "omp_outlined..12", which is 0x00401600. In fact, the compiler passes the "microtask" argument as "omp_outlined..12" into "__kmpc_fork_call" which in turn passes it into "__kmp_fork_call".

• Release the task handle (0x3d641a0):

OMPD DLL: ompd_rel_task_handle(task_handle=0x3d641a0)->rc_ok

• Get the enclosing parallel handle (0x3d641a0) and get its task handle and function:

OMPD DLL: ompd_get_enclosing_parallel_handle(parallel_handle=0x3b225d0)->rc_ok: enclosing_parallel_handle=0x3d641a0
OMPD DLL: ompd_get_task_in_parallel(parallel_handle=0x3d641a0, thread_num=0)->rc_ok: task_handle=0x3d960f0
OMPD DLL: ompd_get_task_function(task_handle=0x3d960f0)->rc_ok: entry_point={segment=0,address=0x0}
OMPD DLL: ompd_rel_task_handle(task_handle=0x3d960f0)->rc_ok

NOTICE: Again, 0 is returned. This one seems valid because I suspect that it's related to the "initial parallel region", and that does not correspond to any user code.

• Get the next enclosing handle, which I assume fails because we are at the root (though the OMP 5.0 spec doesn't say what's supposed to happen in that case).

OMPD DLL: ompd_get_enclosing_parallel_handle(parallel_handle=0x3d641a0)->rc_unsupported
OMPD DLL: ompd_rel_parallel_handle(parallel_handle=0x3d641a0)->rc_ok
OMPD DLL: ompd_rel_parallel_handle(parallel_handle=0x3b225d0)->rc_ok

If I allow the program to run into more deeply nested parallel regions, I do see valid results returned from ompd_get_task_function() for the innermost parallel regions, but DLL continues to return 0 for the outermost parallel region inside of "f".

tx_omp_parallel_nested.c.txt