[OpenMP] Add variant extension that applies to declarations

[jhuber6]

Summary

This patch adds a new extension to the omp begin / end declare variant
support that causes it to apply to function declarations as well. This
is explicitly not done in the standard, but can be useful in some
situations so we should provide it as an extension. This will allow us
to uniquely bind and overload existing definitions with a simple
declaration using variants.

[jdoerfert]

LG. one nit.

clang/include/clang/Basic/AttrDocs.td

4349–4351

clang/include/clang/Basic/AttrDocs.td

``implementation={extension(EXTENSION)}``:
 
   .. code-block:: none
 
     match_all
     match_any
     match_none
     disable_implicit_base
     allow_templates
+    bind_to_declaration
 
 The match extensions change when the *entire* context selector is considered a
 match for an OpenMP context. The default is ``all``, with ``none`` no trait in the
 selector is allowed to be in the OpenMP context, with ``any`` a single trait in
 both the selector and OpenMP context is sufficient. Only a single match
 extension trait is allowed per context selector.
 The disable extensions remove default effects of the ``begin declare variant``
 applied to a definition. If ``disable_implicit_base`` is given, we will not
 introduce an implicit base function for a variant if no base function was
 found. The variant is still generated but will never be called, due to the
 absence of a base function and consequently calls to a base function.
 The allow extensions change when the ``begin declare variant`` effect is
 applied to a definition. If ``allow_templates`` is given, template function
 definitions are considered as specializations of existing or assumed template
 declarations with the same name. The template parameters for the base functions
-are used to instantiate the specialization.
-
+are used to instantiate the specialization. If ``bind_to_declartion`` is given,
+apply the same variant rules to function declarations. This allows the user to
+override declarations with only a function declaration.
   }];
 }
 
 def LeafDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
 
 The ``leaf`` attribute is used as a compiler hint to improve dataflow analysis

clang/lib/Parse/ParseOpenMP.cpp

static bool checkExtensionProperty(Parser &P, SourceLocation Loc,
   if (TIProperty.Kind ==
       TraitProperty::implementation_extension_disable_implicit_base)
     return true;
 
   if (TIProperty.Kind ==
       TraitProperty::implementation_extension_allow_templates)
     return true;
 
+  if (TIProperty.Kind ==
+      TraitProperty::implementation_extension_bind_to_declaration)
+    return true;
+
   auto IsMatchExtension = [](OMPTraitProperty &TP) {
     return (TP.Kind ==
                 llvm::omp::TraitProperty::implementation_extension_match_all ||
             TP.Kind ==
                 llvm::omp::TraitProperty::implementation_extension_match_any ||
             TP.Kind ==
                 llvm::omp::TraitProperty::implementation_extension_match_none);
   };

clang/lib/Sema/SemaDecl.cpp

if (Status != ReservedIdentifierStatus::NotReserved &&
       !isFromSystemHeader(Context.getSourceManager(), D)) {
     Diag(D->getLocation(), diag::warn_reserved_extern_symbol)
         << D << static_cast<int>(Status);
   }
 }
 
 Decl *Sema::ActOnDeclarator(Scope *S, Declarator &D) {
   D.setFunctionDefinitionKind(FunctionDefinitionKind::Declaration);
+
+  // Check if we are in an `omp begin/end declare variant` scope. Handle this
+  // declaration only if the `bind_to_declaration` extension is set.
+  SmallVector<FunctionDecl *, 4> Bases;
+  if (LangOpts.OpenMP && isInOpenMPDeclareVariantScope())
+    if (getOMPTraitInfoForSurroundingScope()->isExtensionActive(llvm::omp::TraitProperty::
+              implementation_extension_bind_to_declaration))
+    ActOnStartOfFunctionDefinitionInOpenMPDeclareVariantScope(
+        S, D, MultiTemplateParamsArg(), Bases);
+
   Decl *Dcl = HandleDeclarator(S, D, MultiTemplateParamsArg());
 
   if (OriginalLexicalContext && OriginalLexicalContext->isObjCContainer() &&
       Dcl && Dcl->getDeclContext()->isFileContext())
     Dcl->setTopLevelDeclInObjCContainer();
 
+  if (!Bases.empty())
+    ActOnFinishedFunctionDefinitionInOpenMPDeclareVariantScope(Dcl, Bases);
+
   return Dcl;
 }
 
 /// DiagnoseClassNameShadow - Implement C++ [class.mem]p13:
 ///   If T is the name of a class, then each of the following shall have a
 ///   name different from T:
 ///     - every static data member of class T;
 ///     - every member function of class T

clang/test/OpenMP/declare_variant_bind_to_decl.cpp

+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --function-signature --include-generated-funcs
+// RUN: %clang_cc1 -verify -fopenmp -x c++ -triple powerpc64le-unknown-unknown -emit-llvm %s -o - | FileCheck %s
+// expected-no-diagnostics
+
+#ifndef HEADER
+#define HEADER
+
+void foo() { }
+
+#pragma omp begin declare variant match( \
+    device = {arch(ppc64le, ppc64)},     \
+    implementation = {extension(match_any, bind_to_declaration)})
+
+void foo();
+
+#pragma omp end declare variant
+
+int main() {
+  foo();
+}
+
+#endif
+// CHECK-LABEL: define {{[^@]+}}@_Z3foov
+// CHECK-SAME: () #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    ret void
+//
+//
+// CHECK-LABEL: define {{[^@]+}}@main
+// CHECK-SAME: () #[[ATTR1:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    call void @"_Z74foo$ompvariant$S2$s7$Pppc64le$Pppc64$S3$s9$Pmatch_any$Pbind_to_declarationv"()
+// CHECK-NEXT:    ret i32 0
+//

llvm/include/llvm/Frontend/OpenMP/OMPKinds.def

 __OMP_TRAIT_PROPERTY(implementation, vendor, unknown)
 
 __OMP_TRAIT_SELECTOR(implementation, extension, true)
 __OMP_TRAIT_PROPERTY(implementation, extension, match_all)
 __OMP_TRAIT_PROPERTY(implementation, extension, match_any)
 __OMP_TRAIT_PROPERTY(implementation, extension, match_none)
 __OMP_TRAIT_PROPERTY(implementation, extension, disable_implicit_base)
 __OMP_TRAIT_PROPERTY(implementation, extension, allow_templates)
+__OMP_TRAIT_PROPERTY(implementation, extension, bind_to_declaration)
 
 __OMP_TRAIT_SET(user)
 
 __OMP_TRAIT_SELECTOR(user, condition, true)
 
 __OMP_TRAIT_PROPERTY(user, condition, true)
 __OMP_TRAIT_PROPERTY(user, condition, false)
 __OMP_TRAIT_PROPERTY(user, condition, unknown)