[SYCL] Basic diagnostics for the sycl_kernel_entry_point attribute.

The `sycl_kernel_entry_point` attribute is used to declare a function that
defines a pattern for an offload kernel entry point. The attribute requires
a single type argument that specifies a class type that meets the requirements
for a SYCL kernel name as described in section 5.2, "Naming of kernels", of
the SYCL 2020 specification. A unique kernel name type is required for each
function declared with the attribute. The attribute may not first appear on a
declaration that follows a definition of the function. The function is
required to have a `void` return type. The function must not be a non-static
member function, be deleted or defaulted, be declared with the `constexpr` or
`consteval` specifiers, be declared with the `[[noreturn]]` attribute, be a
coroutine, or accept variadic arguments.

Diagnostics are not yet provided for the following:
- Use of a type as a kernel name that does not satisfy the forward
  declarability requirements specified in section 5.2, "Naming of kernels",
  of the SYCL 2020 specification.
- Use of a type as a parameter of the attributed function that does not
  satisfy the kernel parameter requirements specified in section 4.12.4,
  "Rules for parameter passing to kernels", of the SYCL 2020 specification
  (each such function parameter constitutes a kernel parameter).
- Use of language features that are not permitted in device functions as
  specified in section 5.4, "Language restrictions for device functions",
  of the SYCL 2020 specification.

There are several issues noted by various FIXME comments.
- The diagnostic generated for kernel name conflicts needs additional work
  to better detail the relevant source locations; such as the location of
  each declaration as well as the original source of each kernel name.
- A number of the tests illustrate spurious errors being produced due to
  attributes that appertain to function templates being instantiated too
  early (during overload resolution as opposed to after an overload is
  selected).

Author: tahonermann

clang/include/clang/AST/ASTContext.h

@@ -3360,6 +3360,16 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// this function.
   void registerSYCLEntryPointFunction(FunctionDecl *FD);
 
+  /// Given a type used as a SYCL kernel name, returns a reference to the
+  /// metadata generated from the corresponding SYCL kernel entry point.
+  /// Aborts if the provided type is not a registered SYCL kernel name.
+  const SYCLKernelInfo &getSYCLKernelInfo(QualType T) const;
+
+  /// Returns a pointer to the metadata generated from the corresponding
+  /// SYCLkernel entry point if the provided type corresponds to a registered
+  /// SYCL kernel name. Returns a null pointer otherwise.
+  const SYCLKernelInfo *findSYCLKernelInfo(QualType T) const;
+
   //===--------------------------------------------------------------------===//
   //                    Statistics
   //===--------------------------------------------------------------------===//

clang/include/clang/Basic/DiagnosticGroups.td

@@ -648,6 +648,7 @@ def PoundPragmaMessage : DiagGroup<"#pragma-messages">,
 def : DiagGroup<"redundant-decls">;
 def RedeclaredClassMember : DiagGroup<"redeclared-class-member">;
 def GNURedeclaredEnum : DiagGroup<"gnu-redeclared-enum">;
+def RedundantAttribute : DiagGroup<"redundant-attribute">;
 def RedundantMove : DiagGroup<"redundant-move">;
 def Register : DiagGroup<"register", [DeprecatedRegister]>;
 def ReturnTypeCLinkage : DiagGroup<"return-type-c-linkage">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12408,6 +12408,29 @@ def err_sycl_special_type_num_init_method : Error<
   "types with 'sycl_special_class' attribute must have one and only one '__init' "
   "method defined">;
 
+// SYCL kernel entry point diagnostics
+def err_sycl_entry_point_invalid : Error<
+  "'sycl_kernel_entry_point' attribute cannot be applied to a"
+  " %select{non-static member|variadic|deleted|defaulted|constexpr|consteval|"
+           "noreturn|coroutine}0 function">;
+def err_sycl_entry_point_invalid_redeclaration : Error<
+  "'sycl_kernel_entry_point' kernel name argument does not match prior"
+  " declaration%diff{: $ vs $|}0,1">;
+def err_sycl_kernel_name_conflict : Error<
+  "'sycl_kernel_entry_point' kernel name argument conflicts with a previous"
+  " declaration">;
+def warn_sycl_kernel_name_not_a_class_type : Warning<
+  "%0 is not a valid SYCL kernel name type; a class type is required">,
+  InGroup<DiagGroup<"nonportable-sycl">>, DefaultError;
+def warn_sycl_entry_point_redundant_declaration : Warning<
+  "redundant 'sycl_kernel_entry_point' attribute">, InGroup<RedundantAttribute>;
+def err_sycl_entry_point_after_definition : Error<
+  "'sycl_kernel_entry_point' attribute cannot be added to a function after the"
+  " function is defined">;
+def err_sycl_entry_point_return_type : Error<
+  "'sycl_kernel_entry_point' attribute only applies to functions with a"
+  " 'void' return type">;
+
 def warn_cuda_maxclusterrank_sm_90 : Warning<
   "maxclusterrank requires sm_90 or higher, CUDA arch provided: %0, ignoring "
   "%1 attribute">, InGroup<IgnoredAttributes>;

clang/include/clang/Sema/SemaSYCL.h

@@ -63,6 +63,8 @@ class SemaSYCL : public SemaBase {
 
   void handleKernelAttr(Decl *D, const ParsedAttr &AL);
   void handleKernelEntryPointAttr(Decl *D, const ParsedAttr &AL);
+
+  void CheckSYCLEntryPointFunctionDecl(FunctionDecl *FD);
 };
 
 } // namespace clang

clang/lib/AST/ASTContext.cpp

@@ -14463,6 +14463,19 @@ void ASTContext::registerSYCLEntryPointFunction(FunctionDecl *FD) {
       std::make_pair(KernelNameType, BuildSYCLKernelInfo(KernelNameType, FD)));
 }
 
+const SYCLKernelInfo &ASTContext::getSYCLKernelInfo(QualType T) const {
+  CanQualType KernelNameType = getCanonicalType(T);
+  return SYCLKernels.at(KernelNameType);
+}
+
+const SYCLKernelInfo *ASTContext::findSYCLKernelInfo(QualType T) const {
+  CanQualType KernelNameType = getCanonicalType(T);
+  auto IT = SYCLKernels.find(KernelNameType);
+  if (IT != SYCLKernels.end())
+    return &IT->second;
+  return nullptr;
+}
+
 OMPTraitInfo &ASTContext::getNewOMPTraitInfo() {
   OMPTraitInfoVector.emplace_back(new OMPTraitInfo());
   return *OMPTraitInfoVector.back();

clang/lib/Sema/SemaDecl.cpp

@@ -53,6 +53,7 @@
 #include "clang/Sema/SemaPPC.h"
 #include "clang/Sema/SemaRISCV.h"
 #include "clang/Sema/SemaSwift.h"
+#include "clang/Sema/SemaSYCL.h"
 #include "clang/Sema/SemaWasm.h"
 #include "clang/Sema/Template.h"
 #include "llvm/ADT/STLForwardCompat.h"
@@ -3018,6 +3019,15 @@ static void checkNewAttributesAfterDef(Sema &S, Decl *New, const Decl *Old) {
       // declarations after definitions.
       ++I;
       continue;
+    } else if (isa<SYCLKernelEntryPointAttr>(NewAttribute)) {
+      // Elevate latent uses of the sycl_kernel_entry_point attribute to an
+      // error since the definition will have already been created without
+      // the semantic effects of the attribute having been applied.
+      S.Diag(NewAttribute->getLocation(),
+             diag::err_sycl_entry_point_after_definition);
+      S.Diag(Def->getLocation(), diag::note_previous_definition);
+      ++I;
+      continue;
     }
 
     S.Diag(NewAttribute->getLocation(),
@@ -12146,7 +12156,7 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     OpenMP().ActOnFinishedFunctionDefinitionInOpenMPAssumeScope(NewFD);
 
   if (LangOpts.isSYCL() && NewFD->hasAttr<SYCLKernelEntryPointAttr>())
-    getASTContext().registerSYCLEntryPointFunction(NewFD);
+    SYCL().CheckSYCLEntryPointFunctionDecl(NewFD);
 
   // Semantic checking for this function declaration (in isolation).
 
@@ -15978,6 +15988,24 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
       CheckCoroutineWrapper(FD);
   }
 
+  // Diagnose invalid SYCL kernel entry point function declarations.
+  if (FD && !FD->isInvalidDecl() && !FD->isTemplated() &&
+      FD->hasAttr<SYCLKernelEntryPointAttr>()) {
+    if (FD->isDeleted()) {
+      Diag(FD->getAttr<SYCLKernelEntryPointAttr>()->getLocation(),
+           diag::err_sycl_entry_point_invalid)
+          << /*deleted function*/2;
+    } else if (FD->isDefaulted()) {
+      Diag(FD->getAttr<SYCLKernelEntryPointAttr>()->getLocation(),
+           diag::err_sycl_entry_point_invalid)
+          << /*defaulted function*/3;
+    } else if (FSI->isCoroutine()) {
+      Diag(FD->getAttr<SYCLKernelEntryPointAttr>()->getLocation(),
+           diag::err_sycl_entry_point_invalid)
+          << /*coroutine*/7;
+    }
+  }
+
   {
     // Do not call PopExpressionEvaluationContext() if it is a lambda because
     // one is already popped when finishing the lambda in BuildLambdaExpr().

clang/lib/Sema/SemaSYCL.cpp

@@ -10,7 +10,9 @@
 
 #include "clang/Sema/SemaSYCL.h"
 #include "clang/AST/Mangle.h"
+#include "clang/AST/SYCLKernelInfo.h"
 #include "clang/AST/TypeOrdering.h"
+#include "clang/Basic/Diagnostic.h"
 #include "clang/Sema/Attr.h"
 #include "clang/Sema/ParsedAttr.h"
 #include "clang/Sema/Sema.h"
@@ -206,3 +208,127 @@ void SemaSYCL::handleKernelEntryPointAttr(Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (SemaRef.Context)
                  SYCLKernelEntryPointAttr(SemaRef.Context, AL, TSI));
 }
+
+static SourceLocation SourceLocationForType(QualType QT) {
+  SourceLocation Loc;
+  const Type *T = QT->getUnqualifiedDesugaredType();
+  if (const TagType *TT = dyn_cast<TagType>(T))
+    Loc = TT->getDecl()->getLocation();
+  else if (const ObjCInterfaceType *ObjCIT = dyn_cast<ObjCInterfaceType>(T))
+    Loc = ObjCIT->getDecl()->getLocation();
+  return Loc;
+}
+
+static bool CheckSYCLKernelName(Sema &S, SourceLocation Loc,
+                                QualType KernelName) {
+  assert(!KernelName->isDependentType());
+
+  if (!KernelName->isStructureOrClassType()) {
+    // SYCL 2020 section 5.2, "Naming of kernels", only requires that the
+    // kernel name be a C++ typename. However, the definition of "kernel name"
+    // in the glossary states that a kernel name is a class type. Neither
+    // section explicitly states whether the kernel name type can be
+    // cv-qualified. For now, kernel name types are required to be class types
+    // and that they may be cv-qualified. The following issue requests
+    // clarification from the SYCL WG.
+    //   https://github.com/KhronosGroup/SYCL-Docs/issues/568
+    S.Diag(Loc, diag::warn_sycl_kernel_name_not_a_class_type) << KernelName;
+    SourceLocation DeclTypeLoc = SourceLocationForType(KernelName);
+    if (DeclTypeLoc.isValid())
+      S.Diag(DeclTypeLoc, diag::note_entity_declared_at)
+          << KernelName;
+    return true;
+  }
+
+  return false;
+}
+
+void SemaSYCL::CheckSYCLEntryPointFunctionDecl(FunctionDecl *FD) {
+  // Ensure that all attributes present on the declaration are consistent
+  // and warn about any redundant ones.
+  const SYCLKernelEntryPointAttr *SKEPAttr = nullptr;
+  for (auto SAI = FD->specific_attr_begin<SYCLKernelEntryPointAttr>();
+       SAI != FD->specific_attr_end<SYCLKernelEntryPointAttr>();
+       ++SAI) {
+    if (!SKEPAttr) {
+      SKEPAttr = *SAI;
+      continue;
+    }
+    if (!getASTContext().hasSameType(SAI->getKernelName(),
+                                     SKEPAttr->getKernelName())) {
+      Diag(SAI->getLocation(), diag::err_sycl_entry_point_invalid_redeclaration)
+          << SAI->getKernelName() << SKEPAttr->getKernelName();
+      Diag(SKEPAttr->getLocation(), diag::note_previous_attribute);
+    } else {
+      Diag(SAI->getLocation(),
+           diag::warn_sycl_entry_point_redundant_declaration);
+      Diag(SKEPAttr->getLocation(), diag::note_previous_attribute);
+    }
+  }
+  assert(SKEPAttr && "Missing sycl_kernel_entry_point attribute");
+
+  // Ensure the kernel name type is valid.
+  if (!SKEPAttr->getKernelName()->isDependentType()) {
+    CheckSYCLKernelName(SemaRef, SKEPAttr->getLocation(),
+                        SKEPAttr->getKernelName());
+  }
+
+  // Ensure that an attribute present on the previous declaration
+  // matches the one on this declaration.
+  FunctionDecl *PrevFD = FD->getPreviousDecl();
+  if (PrevFD && !PrevFD->isInvalidDecl()) {
+    const auto *PrevSKEPAttr = PrevFD->getAttr<SYCLKernelEntryPointAttr>();
+    if (PrevSKEPAttr) {
+      if (!getASTContext().hasSameType(SKEPAttr->getKernelName(),
+                                       PrevSKEPAttr->getKernelName())) {
+        Diag(SKEPAttr->getLocation(),
+             diag::err_sycl_entry_point_invalid_redeclaration)
+            << SKEPAttr->getKernelName() << PrevSKEPAttr->getKernelName();
+        Diag(PrevSKEPAttr->getLocation(), diag::note_previous_decl)
+            << PrevFD;;
+      }
+    }
+  }
+
+  if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
+    if (!MD->isStatic()) {
+      Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_invalid)
+          << /*non-static member function*/0;
+    }
+  }
+  if (FD->isVariadic()) {
+    Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_invalid)
+        << /*variadic function*/1;
+  }
+  if (FD->isConsteval()) {
+    Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_invalid)
+        << /*consteval function*/5;
+  } else if (FD->isConstexpr()) {
+    Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_invalid)
+        << /*constexpr function*/4;
+  }
+  if (FD->isNoReturn()) {
+    Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_invalid)
+        << /*noreturn function*/6;
+  }
+
+  if (!FD->getReturnType()->isVoidType()) {
+    Diag(SKEPAttr->getLocation(), diag::err_sycl_entry_point_return_type);
+  }
+
+  if (!FD->isInvalidDecl() && !FD->isTemplated()) {
+    const SYCLKernelInfo *SKI =
+        getASTContext().findSYCLKernelInfo(SKEPAttr->getKernelName());
+    if (SKI) {
+      if (!declaresSameEntity(FD, SKI->getKernelEntryPointDecl())) {
+        // FIXME: This diagnostic should include the origin of the kernel
+        // FIXME: names; not just the locations of the conflicting declarations.
+        Diag(FD->getLocation(), diag::err_sycl_kernel_name_conflict);
+        Diag(SKI->getKernelEntryPointDecl()->getLocation(),
+             diag::note_previous_declaration);
+      }
+    } else {
+      getASTContext().registerSYCLEntryPointFunction(FD);
+    }
+  }
+}

clang/lib/Sema/SemaTemplate.cpp

@@ -1550,6 +1550,8 @@ NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
     IdResolver.AddDecl(Param);
   }
 
+  ProcessDeclAttributes(S, Param, D);
+
   // C++0x [temp.param]p9:
   //   A default template-argument may be specified for any kind of
   //   template-parameter that is not a template parameter pack.

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -1134,8 +1134,19 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   // the presence of a sycl_kernel_entry_point attribute, register it so that
   // associated metadata is recreated.
   if (FD->hasAttr<SYCLKernelEntryPointAttr>()) {
+    const auto *SKEPAttr = FD->getAttr<SYCLKernelEntryPointAttr>();
     ASTContext &C = Reader.getContext();
-    C.registerSYCLEntryPointFunction(FD);
+    const SYCLKernelInfo *SKI =
+        C.findSYCLKernelInfo(SKEPAttr->getKernelName());
+    if (SKI) {
+      if (!declaresSameEntity(FD, SKI->getKernelEntryPointDecl())) {
+        Reader.Diag(FD->getLocation(), diag::err_sycl_kernel_name_conflict);
+        Reader.Diag(SKI->getKernelEntryPointDecl()->getLocation(),
+             diag::note_previous_declaration);
+      }
+    } else {
+      C.registerSYCLEntryPointFunction(FD);
+    }
   }
 }
 

clang/test/ASTSYCL/ast-dump-sycl-kernel-entry-point.cpp

@@ -107,14 +107,29 @@ void skep5<KN<5,2>>(long) {
 // CHECK:      | |-TemplateArgument type 'long'
 // CHECK:      | `-SYCLKernelEntryPointAttr {{.*}} KN<5, 2>
 
+// FIXME: C++23 [temp.expl.spec]p12 states:
+// FIXME:   ... Similarly, attributes appearing in the declaration of a template
+// FIXME:   have no effect on an explicit specialization of that template.
+// FIXME: Clang currently instantiates a function template specialization from
+// FIXME: the function template declaration and links it as a previous
+// FIXME: declaration of an explicit specialization. The instantiated
+// FIXME: declaration includes attributes instantiated from the function
+// FIXME: template declaration. When the instantiated declaration and the
+// FIXME: explicit specialization both specify a sycl_kernel_entry_point
+// FIXME: attribute with different kernel name types, a spurious diagnostic
+// FIXME: is issued. The following test case is incorrectly diagnosed as
+// FIXME: having conflicting kernel name types (KN<5,3> vs the incorrectly
+// FIXME: inherited KN<5,-1>).
+#if 0
 template<>
 [[clang::sycl_kernel_entry_point(KN<5,3>)]]
 void skep5<KN<5,-1>>(long long) {
 }
-// CHECK:      |-FunctionDecl {{.*}} prev {{.*}} skep5 'void (long long)' explicit_specialization
-// CHECK-NEXT: | |-TemplateArgument type 'KN<5, -1>'
-// CHECK:      | |-TemplateArgument type 'long long'
-// CHECK:      | `-SYCLKernelEntryPointAttr {{.*}} KN<5, 3>
+// FIXME-CHECK:      |-FunctionDecl {{.*}} prev {{.*}} skep5 'void (long long)' explicit_specialization
+// FIXME-CHECK-NEXT: | |-TemplateArgument type 'KN<5, -1>'
+// FIXME-CHECK:      | |-TemplateArgument type 'long long'
+// FIXME-CHECK:      | `-SYCLKernelEntryPointAttr {{.*}} KN<5, 3>
+#endif
 
 template void skep5<KN<5,4>>(int);
 // Checks are located with the primary template declaration above.