[OpenMP] Issue warning if a simd construct nested inside another simd construct

[kkwli0]

Summary

Currently, the following nested scenario is not allowed in the OpenMP spec but clang allows it. Issue a warning.

#pragma omp simd
  for (i=0; i<10; i++)
#pragma omp simd  // warning
    for (j=0; j<10; j++)
      ;

The spec only allows the following case

#pragma omp simd
  for (i=0; i<10; i++)
#pragma omp ordered simd // OK
    for (j=0; j<10; j++)
      ;

[ABataev]

Kelvin, I allowed it intentionally. I don't see a reason why nested 'simd' directives are not allowed, they just allow to vectorize both outer and inner loops. I think it's nice to have such feature.

[kkwli0]

Thanks for explanation. As this is an extension, hould we issue a warning message? Code compiled successfully with clang will fail with other compilers.

[ABataev]

icc accepts such code. Not sure about gcc. But I agree, that it would be good to emit a warning here.

[kkwli0]

GCC60 issues an error message.

[kkwli0]

Update the patch to issue warning instead.

[ABataev]

lib/Sema/SemaOpenMP.cpp

2950–2959

Maybe this code can be changed a little bit:

SemaRef.Diag(StartLoc,
             (CurrentRegion != OMPD_simd) ?
               diag::err_omp_prohibited_region_simd : 
               diag::warn_omp_nesting_simd);
return CurrentRegion != OMPD_simd;

2952–2959

The code is not formatted properly

[kkwli0]

lib/Sema/SemaOpenMP.cpp

2952–2959

Will update it.

[kkwli0]

Update the patch as comment in the review. Thanks.

[ABataev]

LG

[kkwli0]

Committed revision 274352.

include/clang/Basic/DiagnosticSemaKinds.td

 def err_omp_param_or_this_in_clause : Error<
   "expected reference to one of the parameters of function %0%select{| or 'this'}1">;
 def err_omp_expected_uniform_param : Error<
   "expected a reference to a parameter specified in a 'uniform' clause">;
 def err_omp_expected_int_param : Error<
   "expected a reference to an integer-typed parameter">;
 def err_omp_at_least_one_motion_clause_required : Error<
   "expected at least one 'to' clause or 'from' clause specified to '#pragma omp target update'">;
+def warn_omp_nesting_simd : Warning<
+  "OpenMP only allows an ordered construct with the simd clause nested in a simd construct">,
+  InGroup<SourceUsesOpenMP>;
 } // end of OpenMP category
 
 let CategoryName = "Related Result Type Issue" in {
 // Objective-C related result type compatibility
 def warn_related_result_type_compatibility_class : Warning<
   "method is expected to return an instance of its class type "
   "%diff{$, but is declared to return $|"
   ", but is declared to return different type}0,1">;

lib/Sema/SemaOpenMP.cpp

if (Stack->getCurScope()) {
     bool CloseNesting = true;
     enum {
       NoRecommend,
       ShouldBeInParallelRegion,
       ShouldBeInOrderedRegion,
       ShouldBeInTargetRegion,
       ShouldBeInTeamsRegion
     } Recommend = NoRecommend;
-    if (isOpenMPSimdDirective(ParentRegion) && CurrentRegion != OMPD_ordered &&
-        CurrentRegion != OMPD_simd) {
+    if (isOpenMPSimdDirective(ParentRegion) && CurrentRegion != OMPD_ordered) {
+      if (CurrentRegion != OMPD_simd) {
         // OpenMP [2.16, Nesting of Regions]
         // OpenMP constructs may not be nested inside a simd region.
         // OpenMP [2.8.1,simd Construct, Restrictions]
-      // An ordered construct with the simd clause is the only OpenMP construct
-      // that can appear in the simd region.
+        // An ordered construct with the simd clause is the only OpenMP
+        // construct that can appear in the simd region.
         SemaRef.Diag(StartLoc, diag::err_omp_prohibited_region_simd);
         return true;
       }
+      else {
+        // Allowing a SIMD consruct nested in another SIMD construct is an
+        // extension. The OpenMP 4.5 spec does not allow it. Issue a
+        // warning message.
+        SemaRef.Diag(StartLoc, diag::warn_omp_nesting_simd);
+        return false;
+      }
+    }
     if (ParentRegion == OMPD_atomic) {
       // OpenMP [2.16, Nesting of Regions]
       // OpenMP constructs may not be nested inside an atomic region.
       SemaRef.Diag(StartLoc, diag::err_omp_prohibited_region_atomic);
       return true;
     }
     if (CurrentRegion == OMPD_section) {
       // OpenMP [2.7.2, sections Construct, Restrictions]

test/OpenMP/nesting_of_regions.cpp

 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
#pragma omp flush // expected-error {{OpenMP constructs may not be nested inside a simd region}}
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp ordered // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     bar();
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
+#pragma omp ordered simd // OK
+    bar();
+  }
+#pragma omp simd
+  for (int i = 0; i < 10; ++i) {
+#pragma omp ordered threads // expected-error {{OpenMP constructs may not be nested inside a simd region}}
+    bar();
+  }
+#pragma omp simd
+  for (int i = 0; i < 10; ++i) {
 #pragma omp atomic // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     ++a;
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp target // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     ++a;
   }

 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
#pragma omp flush // expected-error {{OpenMP constructs may not be nested inside a simd region}}
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp ordered // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     bar();
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
+#pragma omp ordered simd // OK
+    bar();
+  }
+#pragma omp for simd
+  for (int i = 0; i < 10; ++i) {
+#pragma omp ordered threads // expected-error {{OpenMP constructs may not be nested inside a simd region}}
+    bar();
+  }
+#pragma omp for simd
+  for (int i = 0; i < 10; ++i) {
 #pragma omp atomic // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     ++a;
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp target // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     ++a;
   }

 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;

 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;

 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;

 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
-#pragma omp simd
+#pragma omp simd // expected-warning {{OpenMP only allows an ordered construct with the simd clause nested in a simd construct}}
     for (int i = 0; i < 10; ++i)
       ;
   }
 #pragma omp parallel for simd
   for (int i = 0; i < 10; ++i) {
 #pragma omp for simd // expected-error {{OpenMP constructs may not be nested inside a simd region}}
     for (int i = 0; i < 10; ++i)
       ;