[OpenCL] Add clang extension for function pointers.

The new clang internal extension '__cl_clang_function_pointers'
allows use of function pointers and other features that have
the same functionality:
- Use of member function pointers;
- Unrestricted use of references to functions;
- Virtual member functions.

This not a vendor extension and therefore it doesn't require any
special target support. Exposing this functionality fully
will require vendor or Khronos extension.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D94021

Author: Anastasia Stulova

clang/docs/LanguageExtensions.rst

@@ -1722,6 +1722,58 @@ syntax to be used with ``std::complex`` with the same meaning.)
 For GCC compatibility, ``__builtin_complex(re, im)`` can also be used to
 construct a complex number from the given real and imaginary components.
 
+OpenCL Features
+===============
+
+Clang supports internal OpenCL extensions documented below.
+
+``__cl_clang_function_pointers``
+--------------------------------
+
+With this extension it is possible to enable various language features that
+are relying on function pointers using regular OpenCL extension pragma
+mechanism detailed in `the OpenCL Extension Specification,
+section 1.2
+<https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview>`_.
+
+In C++ for OpenCL this also enables:
+
+- Use of member function pointers;
+
+- Unrestricted use of references to functions;
+
+- Virtual member functions.
+
+Such functionality is not conformant and does not guarantee to compile
+correctly in any circumstances. It can be used if:
+
+- the kernel source does not contain call expressions to (member-) function
+  pointers, or virtual functions. For example this extension can be used in
+  metaprogramming algorithms to be able to specify/detect types generically.
+
+- the generated kernel binary does not contain indirect calls because they
+  are eliminated using compiler optimizations e.g. devirtualization. 
+
+- the selected target supports the function pointer like functionality e.g.
+  most CPU targets.
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  #pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
+  void foo()
+  {
+    void (*fp)(); // compiled - no diagnostic generated
+  }
+
+  #pragma OPENCL EXTENSION __cl_clang_function_pointers : disable
+  void bar()
+  {
+    void (*fp)(); // error - pointers to function are not allowed
+  }
+
+
 Builtin Functions
 =================
 

clang/include/clang/Basic/OpenCLExtensions.def

@@ -69,6 +69,7 @@ OPENCLEXT_INTERNAL(cl_khr_subgroups, 200, ~0U)
 
 // Clang Extensions.
 OPENCLEXT_INTERNAL(cl_clang_storage_class_specifiers, 100, ~0U)
+OPENCLEXT_INTERNAL(__cl_clang_function_pointers, 100, ~0U)
 
 // AMD OpenCL extensions
 OPENCLEXT_INTERNAL(cl_amd_media_ops, 100, ~0U)

clang/lib/Basic/Targets/AMDGPU.h

@@ -285,6 +285,7 @@ class LLVM_LIBRARY_VISIBILITY AMDGPUTargetInfo final : public TargetInfo {
   void setSupportedOpenCLOpts() override {
     auto &Opts = getSupportedOpenCLOpts();
     Opts.support("cl_clang_storage_class_specifiers");
+    Opts.support("__cl_clang_function_pointers");
 
     bool IsAMDGCN = isAMDGCN(getTriple());
 

clang/lib/Basic/Targets/NVPTX.h

@@ -128,6 +128,7 @@ class LLVM_LIBRARY_VISIBILITY NVPTXTargetInfo : public TargetInfo {
   void setSupportedOpenCLOpts() override {
     auto &Opts = getSupportedOpenCLOpts();
     Opts.support("cl_clang_storage_class_specifiers");
+    Opts.support("__cl_clang_function_pointers");
 
     Opts.support("cl_khr_fp64");
     Opts.support("cl_khr_byte_addressable_store");

clang/lib/Parse/ParseDecl.cpp

@@ -3630,12 +3630,13 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
     case tok::kw_virtual:
       // C++ for OpenCL does not allow virtual function qualifier, to avoid
       // function pointers restricted in OpenCL v2.0 s6.9.a.
-      if (getLangOpts().OpenCLCPlusPlus) {
+      if (getLangOpts().OpenCLCPlusPlus &&
+          !getActions().getOpenCLOptions().isEnabled(
+              "__cl_clang_function_pointers")) {
         DiagID = diag::err_openclcxx_virtual_function;
         PrevSpec = Tok.getIdentifierInfo()->getNameStart();
         isInvalid = true;
-      }
-      else {
+      } else {
         isInvalid = DS.setFunctionSpecVirtual(Loc, PrevSpec, DiagID);
       }
       break;

clang/lib/Sema/SemaDecl.cpp

@@ -6748,14 +6748,16 @@ static bool diagnoseOpenCLTypes(Scope *S, Sema &Se, Declarator &D,
   }
 
   // OpenCL v1.0 s6.8.a.3: Pointers to functions are not allowed.
-  QualType NR = R;
-  while (NR->isPointerType() || NR->isMemberFunctionPointerType()) {
-    if (NR->isFunctionPointerType() || NR->isMemberFunctionPointerType()) {
-      Se.Diag(D.getIdentifierLoc(), diag::err_opencl_function_pointer);
-      D.setInvalidType();
-      return false;
+  if (!Se.getOpenCLOptions().isEnabled("__cl_clang_function_pointers")) {
+    QualType NR = R;
+    while (NR->isPointerType() || NR->isMemberFunctionPointerType()) {
+      if (NR->isFunctionPointerType() || NR->isMemberFunctionPointerType()) {
+        Se.Diag(D.getIdentifierLoc(), diag::err_opencl_function_pointer);
+        D.setInvalidType();
+        return false;
+      }
+      NR = NR->getPointeeType();
     }
-    NR = NR->getPointeeType();
   }
 
   if (!Se.getOpenCLOptions().isEnabled("cl_khr_fp16")) {

clang/lib/Sema/SemaType.cpp

@@ -2089,7 +2089,8 @@ QualType Sema::BuildPointerType(QualType T,
     return QualType();
   }
 
-  if (T->isFunctionType() && getLangOpts().OpenCL) {
+  if (T->isFunctionType() && getLangOpts().OpenCL &&
+      !getOpenCLOptions().isEnabled("__cl_clang_function_pointers")) {
     Diag(Loc, diag::err_opencl_function_pointer);
     return QualType();
   }

clang/test/Misc/amdgcn.languageOptsOpenCL.cl

@@ -12,7 +12,12 @@
 #ifndef cl_clang_storage_class_specifiers
 #error "Missing cl_clang_storage_class_specifiers define"
 #endif
-#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers: enable
+#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers : enable
+
+#ifndef __cl_clang_function_pointers
+#error "Missing __cl_clang_function_pointers define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
 
 #ifndef cl_khr_fp16
 #error "Missing cl_khr_fp16 define"

clang/test/Misc/nvptx.languageOptsOpenCL.cl

@@ -20,7 +20,12 @@
 #ifndef cl_clang_storage_class_specifiers
 #error "Missing cl_clang_storage_class_specifiers define"
 #endif
-#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers: enable
+#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers : enable
+
+#ifndef __cl_clang_function_pointers
+#error "Missing __cl_clang_function_pointers define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
 
 #ifdef cl_khr_fp16
 #error "Incorrect cl_khr_fp16 define"

clang/test/Misc/r600.languageOptsOpenCL.cl

@@ -28,7 +28,12 @@
 #ifndef cl_clang_storage_class_specifiers
 #error "Missing cl_clang_storage_class_specifiers define"
 #endif
-#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers: enable
+#pragma OPENCL EXTENSION cl_clang_storage_class_specifiers : enable
+
+#ifndef __cl_clang_function_pointers
+#error "Missing __cl_clang_function_pointers define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
 
 #ifdef cl_khr_fp16
 #error "Incorrect cl_khr_fp16 define"