[OpenMP] Silence warning about fallthrough

[protze.joachim]

Summary

The three switch fallthrough generate a warning with -Wimplicit-fallthrough.
Two are documented as fallthrough, one is not, but I think the intention is to also fallthrough in kmp_tasking.cpp.

Not sure whether kmp.h is the best place to define the macro.

[jlpeyton]

runtime/src/kmp.h

3973–3984

Move this to kmp_os.h around the noreturn attribute definition which looks very similar to this one, and rename it KMP_FALLTHROUGH.

[protze.joachim]

Updated as requested

[jlpeyton]

LGTM

[david2050]

This change breaks builds fort compilers because the "&&" in the preprocessor does not protect the case that __has_cpp_attribute is not defined. Better something like

#if __cplusplus > 201402L  
#if __has_cpp_attribute(fallthrough)
#  define KMP_FALLTHROUGH() [[fallthrough]]
#elif __has_cpp_attribute(clang::fallthrough)  
#  define KMP_FALLTHROUGH() [[clang::fallthrough]]
#elif __has_attribute(fallthough) || _GNUC_VER >= 700
#  define KMP_FALLTHROUGH() __attribute__((__fallthrough__))
#endif
#endif
#if !defined(KMP_FALLTHROUGH)
#  define KMP_FALLTHROUGH() ((void)0)
#endif

[david2050]

This comment was removed by david2050.

runtime/src/kmp.h

   tgt_mandatory = 2
 };
 typedef enum kmp_target_offload_kind kmp_target_offload_kind_t;
 // Set via OMP_TARGET_OFFLOAD if specified, defaults to tgt_default otherwise
 extern kmp_target_offload_kind_t __kmp_target_offload;
 extern int __kmpc_get_target_offload();
 #endif
 
+// from libcxx/include/__config
+// Use a function like macro to imply that it must be followed by a semicolon
+#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#  define _LIBOMP_FALLTHROUGH() [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#  define _LIBOMP_FALLTHROUGH() [[clang::fallthrough]]
+#elif __has_attribute(fallthough) || _GNUC_VER >= 700
+#  define _LIBOMP_FALLTHROUGH() __attribute__((__fallthrough__))
+#else
+#  define _LIBOMP_FALLTHROUGH() ((void)0)
+#endif
+
 #ifdef __cplusplus
 }
 #endif
 
 #endif /* KMP_H */

runtime/src/kmp_dispatch.cpp

case kmp_sch_static_steal: {
     } else {
       KD_TRACE(100, ("__kmp_dispatch_init_algorithm: T#%d falling-through to "
                      "kmp_sch_static_balanced\n",
                      gtid));
       schedule = kmp_sch_static_balanced;
       /* too few iterations: fall-through to kmp_sch_static_balanced */
     } // if
     /* FALL-THROUGH to static balanced */
+    _LIBOMP_FALLTHROUGH();
   } // case
 #endif
   case kmp_sch_static_balanced: {
     T init, limit;
 
     KD_TRACE(
         100,
         ("__kmp_dispatch_init_algorithm: T#%d kmp_sch_static_balanced case\n",

runtime/src/kmp_runtime.cpp

         }
       }
       switch (neighbor) {
       case 0: // Cannot parse environment variable -- neighbor status unknown.
         // Assume it is the incompatible format of future version of the
         // library. Assume the other library is alive.
         // WARN( ... ); // TODO: Issue a warning.
         file_name = "unknown library";
+        _LIBOMP_FALLTHROUGH();
       // Attention! Falling to the next case. That's intentional.
       case 1: { // Neighbor is alive.
         // Check it is allowed.
         char *duplicate_ok = __kmp_env_get("KMP_DUPLICATE_LIB_OK");
         if (!__kmp_str_match_true(duplicate_ok)) {
           // That's not allowed. Issue fatal error.
           __kmp_fatal(KMP_MSG(DuplicateLibrary, KMP_LIBRARY_FILE, file_name),
                       KMP_HNT(DuplicateLibrary), __kmp_msg_null);

runtime/src/kmp_tasking.cpp

     num_tasks_min =
         KMP_MIN(thread->th.th_team_nproc * 10, INITIAL_TASK_DEQUE_SIZE);
 
   // compute num_tasks/grainsize based on the input provided
   switch (sched) {
   case 0: // no schedule clause specified, we can choose the default
     // let's try to schedule (team_size*10) tasks
     grainsize = thread->th.th_team_nproc * 10;
+    _LIBOMP_FALLTHROUGH();
   case 2: // num_tasks provided
     if (grainsize > tc) {
       num_tasks = tc; // too big num_tasks requested, adjust values
       grainsize = 1;
       extras = 0;
     } else {
       num_tasks = grainsize;
       grainsize = tc / num_tasks;