[OpenMP] Fix performance regression reported in bug llvm#51235

shiltian · shiltian · commit 9f5d6ea52eb1 · 2021-08-04T12:34:49.000-04:00
This patch fixes the "performance regression" reported in https://bugs.llvm.org/show_bug.cgi?id=51235. In fact it has nothing to do with performance. The root cause is, the stolen task is not allowed to execute by another thread because by default it is tied task. Since hidden helper task will always be executed by hidden helper threads, it should be untied. Reviewed By: protze.joachim Differential Revision: https://reviews.llvm.org/D107121
diff --git a/openmp/runtime/src/kmp_tasking.cpp b/openmp/runtime/src/kmp_tasking.cpp
@@ -1444,6 +1444,7 @@ kmp_task_t *__kmpc_omp_target_task_alloc(ident_t *loc_ref, kmp_int32 gtid,
   if (__kmp_enable_hidden_helper) {
     auto &input_flags = reinterpret_cast<kmp_tasking_flags_t &>(flags);
     input_flags.hidden_helper = TRUE;
+    input_flags.tiedness = TASK_UNTIED;
   }
 
   return __kmpc_omp_task_alloc(loc_ref, gtid, flags, sizeof_kmp_task_t,

Original file line number	Diff line number	Diff line change
`@@ -1444,6 +1444,7 @@ kmp_task_t __kmpc_omp_target_task_alloc(ident_t loc_ref, kmp_int32 gtid,`
`1444`	`1444`	`if (__kmp_enable_hidden_helper) {`
`1445`	`1445`	`auto &input_flags = reinterpret_cast<kmp_tasking_flags_t &>(flags);`
`1446`	`1446`	`input_flags.hidden_helper = TRUE;`
	`1447`	`+ input_flags.tiedness = TASK_UNTIED;`
`1447`	`1448`	`}`
`1448`	`1449`
`1449`	`1450`	`return __kmpc_omp_task_alloc(loc_ref, gtid, flags, sizeof_kmp_task_t,`