GOMP compatibility: add missing OMP4.0 taskdeps handling code

[pawosm01]

Summary

This patch adds OpenMP 4.0 task dependencies handling code that is
missing in LLVM libomp's GOMP compatibility layer. The absence of this
code is responsible for weird behaviour of the taskdep benchmarks from
Kastors suite whenever they are compiled using GCC and linked against LLVM libomp
instead of GOMP.

[AndreyChurbanov]

This change breaks codes compiled by gcc 4.8 or earlier, because it is unsafe to unconditionally read the new parameter that earlier compilers don't provide. Note that new libgomp library works fine with old compilers, so there should probably be some flag (bit in existing gomp_flags) responsible for providing newly added parameter.

[Hahnfeld]

runtime/src/kmp_gsupport.cpp

923 ↗

(On Diff #92124)

priority is probably an OpenMP 4.5 thing?

[pawosm01]

This comment was removed by pawosm01.

[pawosm01]

now using gomp_flags

[jcownie]

Is "dep_list[i].flags.in = 1;" really supposed to be set in both of the if cases (lines 980 and 983)?
If so, can't we lift that out of the if?

[pawosm01]

In D31071#708667, @jcownie wrote:

Is "dep_list[i].flags.in = 1;" really supposed to be set in both of the if cases (lines 980 and 983)?
If so, can't we lift that out of the if?

Yes and maybe. It must me set to 1 for both due to `KMP_DEBUG_ASSERT(dep->flags.in);` checked for each task dependency processed by __kmp_process_deps() in kmp_taskdeps.cpp. Setting all flags in both branches seemed to me as better expressing my intents, but I can change it if you insist.

[AndreyChurbanov]

LGTM

runtime/src/kmp_gsupport.cpp

923 ↗

(On Diff #92124)

Right, the priority is 4.5 addition, and we can add its implementation later if we want to.

[jcownie]

Setting all flags in both branches seemed to me as better expressing my intents, but I can change it if you insist.

I am not going to insist. I just find it odd to have a duplicate line of code, and the fact that it is the same in both if branches made me suspicious that there was bug lurking there. So, personally, I would find code like this clearer.

dep_list[i].len = 0U;
dep_list[i].flags.in = 1;
dep_list[i].flags.out = i<nout;

[pawosm01]

In D31071#708701, @jcownie wrote:

Setting all flags in both branches seemed to me as better expressing my intents, but I can change it if you insist.

I am not going to insist. I just find it odd to have a duplicate line of code, and the fact that it is the same in both if branches made me suspicious that there was bug lurking there. So, personally, I would find code like this clearer.

dep_list[i].len = 0U;
dep_list[i].flags.in = 1;
dep_list[i].flags.out = i<nout;

Yep, that looks better.

[jcownie]

Thanks, LGTM.

openmp/trunk/runtime/src/kmp_gsupport.cpp

 
 
 //
 // Tasking constructs
 //
 
 void
 xexpand(KMP_API_NAME_GOMP_TASK)(void (*func)(void *), void *data, void (*copy_func)(void *, void *),
-  long arg_size, long arg_align, bool if_cond, unsigned gomp_flags)
+  long arg_size, long arg_align, bool if_cond, unsigned gomp_flags
+#if OMP_40_ENABLED
+  , void **depend
+#endif
+)
 {
     MKLOC(loc, "GOMP_task");
     int gtid = __kmp_entry_gtid();
     kmp_int32 flags = 0;
     kmp_tasking_flags_t *input_flags = (kmp_tasking_flags_t *) & flags;
 
     KA_TRACE(20, ("GOMP_task: T#%d\n", gtid));
 
if (arg_size > 0) {
             (*copy_func)(task->shareds, data);
         }
         else {
             KMP_MEMCPY(task->shareds, data, arg_size);
         }
     }
 
     if (if_cond) {
+#if OMP_40_ENABLED
+        if (gomp_flags & 8) {
+            KMP_ASSERT(depend);
+            const size_t ndeps = (kmp_intptr_t)depend[0];
+            const size_t nout = (kmp_intptr_t)depend[1];
+            kmp_depend_info_t dep_list[ndeps];
+
+            for (size_t i = 0U; i < ndeps; i++) {
+                dep_list[i].base_addr = (kmp_intptr_t)depend[2U + i];
+                dep_list[i].len = 0U;
+                dep_list[i].flags.in = 1;
+                dep_list[i].flags.out = (i < nout);
+            }
+            __kmpc_omp_task_with_deps(&loc, gtid, task, ndeps, dep_list, 0, NULL);
+        }
+        else
+#endif
         __kmpc_omp_task(&loc, gtid, task);
     }
     else {
 #if OMPT_SUPPORT
         ompt_thread_info_t oldInfo;
         kmp_info_t *thread;
         kmp_taskdata_t *taskdata;
         if (ompt_enabled) {