[OpenCL] Add clang extension for bitfields

[Anastasia]

Summary

Bitfields result in struct/union layout that is implementation dependent and therefore it was excluded from OpenCL originally for portability reasons. In kernel prameters use of bitfields can result in functionality issues.

However, use of bitfields has been desirable feature for some applications, there was a discussion about allowing this in the next standards (https://github.com/KhronosGroup/OpenCL-Docs/issues/301) but regardless to that clang can expose this functionality as a compiler extension for the application that can take advanatge of it at the price of reduced portability.

This addresses PR45339!

[svenvh]

clang/docs/LanguageExtensions.rst

1748

clang/lib/Sema/SemaDecl.cpp

16817–16818

clang/test/SemaOpenCL/unsupported.cl

11

The extension has "bitfields" in the name but most diagnostics (including this one) spell it as "bit-fields". I wonder what the least surprising name would be? It seems Clang tends to use the hyphenated form in diagnostics and the non-hyphenated form in e.g. option names (e.g. fsigned-bitfields), so using the non-hyphenated form in the extension name is probably fine.

[Anastasia]

clang/test/SemaOpenCL/unsupported.cl

11

Hyphenated words are often split so we could also go for __cl_clang_bit_fields? But I think this might be more confusing...

[Anastasia]

Added suggestions from Sven.

[Anastasia]

clang/lib/Sema/SemaDecl.cpp

16817–16818

Slightly modified to avoid repeating the extension name that can be read from the source code.

[svenvh]

LGTM!

clang/docs/LanguageExtensions.rst

 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_bitfields``
+--------------------------------
+
+With this extension it is possible to enable bitfields in structs
+or unions using the 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>`_.
+
+Use of bitfields in OpenCL kernels can result in reduced portability as struct
+layout is not guaranteed to be consistent when compiled by different compilers.
+If structs with bitfields are used as kernel function parameters, it can result
+in incorrect functionality when the layout is different between the host and
+device code.
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  #pragma OPENCL EXTENSION __cl_clang_bitfields : enable
+  struct with_bitfield {
+    unsigned int i : 5; // compiled - no diagnostic generated
+  };
+
+  #pragma OPENCL EXTENSION __cl_clang_bitfields : disable
+  struct without_bitfield {
+    unsigned int i : 5; // error - bitfields are not supported
+  };
+
 ``__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>`_.

clang/include/clang/Basic/OpenCLExtensions.def

 OPENCL_EXTENSION(cl_khr_srgb_image_writes, true, 200)
 OPENCL_EXTENSION(cl_khr_subgroups, true, 200)
 
 // Clang Extensions.
 OPENCL_EXTENSION(cl_clang_storage_class_specifiers, true, 100)
 OPENCL_EXTENSION(__cl_clang_function_pointers, true, 100)
 OPENCL_EXTENSION(__cl_clang_variadic_functions, true, 100)
 OPENCL_EXTENSION(__cl_clang_non_portable_kernel_param_types, true, 100)
+OPENCL_EXTENSION(__cl_clang_bitfields, true, 100)
 
 // AMD OpenCL extensions
 OPENCL_EXTENSION(cl_amd_media_ops, true, 100)
 OPENCL_EXTENSION(cl_amd_media_ops2, true, 100)
 
 // ARM OpenCL extensions
 OPENCL_EXTENSION(cl_arm_integer_dot_product_int8, true, 120)
 OPENCL_EXTENSION(cl_arm_integer_dot_product_accumulate_int8, true, 120)

clang/lib/Basic/Targets/AMDGPU.h

public:
   }
 
   void setSupportedOpenCLOpts() override {
     auto &Opts = getSupportedOpenCLOpts();
     Opts["cl_clang_storage_class_specifiers"] = true;
     Opts["__cl_clang_variadic_functions"] = true;
     Opts["__cl_clang_function_pointers"] = true;
     Opts["__cl_clang_non_portable_kernel_param_types"] = true;
+    Opts["__cl_clang_bitfields"] = true;
 
     bool IsAMDGCN = isAMDGCN(getTriple());
 
     Opts["cl_khr_fp64"] = hasFP64();
     Opts["__opencl_c_fp64"] = hasFP64();
 
     if (IsAMDGCN || GPUKind >= llvm::AMDGPU::GK_CEDAR) {
       Opts["cl_khr_byte_addressable_store"] = true;

clang/lib/Basic/Targets/NVPTX.h

public:
   }
 
   void setSupportedOpenCLOpts() override {
     auto &Opts = getSupportedOpenCLOpts();
     Opts["cl_clang_storage_class_specifiers"] = true;
     Opts["__cl_clang_function_pointers"] = true;
     Opts["__cl_clang_variadic_functions"] = true;
     Opts["__cl_clang_non_portable_kernel_param_types"] = true;
+    Opts["__cl_clang_bitfields"] = true;
 
     Opts["cl_khr_fp64"] = true;
     Opts["__opencl_c_fp64"] = true;
     Opts["cl_khr_byte_addressable_store"] = true;
     Opts["cl_khr_global_int32_base_atomics"] = true;
     Opts["cl_khr_global_int32_extended_atomics"] = true;
     Opts["cl_khr_local_int32_base_atomics"] = true;
     Opts["cl_khr_local_int32_extended_atomics"] = true;

clang/lib/Sema/SemaDecl.cpp

FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType T,
   if (LangOpts.OpenCL) {
     // OpenCL v1.2 s6.9b,r & OpenCL v2.0 s6.12.5 - The following types cannot be
     // used as structure or union field: image, sampler, event or block types.
     if (T->isEventT() || T->isImageType() || T->isSamplerT() ||
         T->isBlockPointerType()) {
       Diag(Loc, diag::err_opencl_type_struct_or_union_field) << T;
       Record->setInvalidDecl();
       InvalidDecl = true;
     }
-    // OpenCL v1.2 s6.9.c: bitfields are not supported.
-    if (BitWidth) {
+    // OpenCL v1.2 s6.9.c: bitfields are not supported, unless Clang extension
+    // is enabled.
+    if (BitWidth && !getOpenCLOptions().isAvailableOption(
+                        "__cl_clang_bitfields", LangOpts)) {
       Diag(Loc, diag::err_opencl_bitfields);
       InvalidDecl = true;
     }
   }
 
   // Anonymous bit-fields cannot be cv-qualified (CWG 2229).
   if (!InvalidDecl && getLangOpts().CPlusPlus && !II && BitWidth &&
       T.hasQualifiers()) {

clang/test/Misc/amdgcn.languageOptsOpenCL.cl

 #endif
 #pragma OPENCL EXTENSION __cl_clang_variadic_functions : enable
 
 #ifndef __cl_clang_non_portable_kernel_param_types
 #error "Missing __cl_clang_non_portable_kernel_param_types define"
 #endif
 #pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
 
+#ifndef __cl_clang_bitfields
+#error "Missing __cl_clang_bitfields define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_bitfields : enable
+
 #ifndef cl_khr_fp16
 #error "Missing cl_khr_fp16 define"
 #endif
 #pragma OPENCL EXTENSION cl_khr_fp16: enable
 
 #ifndef cl_khr_int64_base_atomics
 #error "Missing cl_khr_int64_base_atomics define"
 #endif

clang/test/Misc/nvptx.languageOptsOpenCL.cl

 #endif
 #pragma OPENCL EXTENSION __cl_clang_variadic_functions : enable
 
 #ifndef __cl_clang_non_portable_kernel_param_types
 #error "Missing __cl_clang_non_portable_kernel_param_types define"
 #endif
 #pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
 
+#ifndef __cl_clang_bitfields
+#error "Missing __cl_clang_bitfields define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_bitfields : enable
+
 #ifdef cl_khr_fp16
 #error "Incorrect cl_khr_fp16 define"
 #endif
 #pragma OPENCL EXTENSION cl_khr_fp16: enable
 // expected-warning@-1{{unsupported OpenCL extension 'cl_khr_fp16' - ignoring}}
 
 #ifdef cl_khr_int64_base_atomics
 #error "Incorrect cl_khr_int64_base_atomics define"

clang/test/Misc/r600.languageOptsOpenCL.cl

 #endif
 #pragma OPENCL EXTENSION __cl_clang_variadic_functions : enable
 
 #ifndef __cl_clang_non_portable_kernel_param_types
 #error "Missing __cl_clang_non_portable_kernel_param_types define"
 #endif
 #pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
 
+#ifndef __cl_clang_bitfields
+#error "Missing __cl_clang_bitfields define"
+#endif
+#pragma OPENCL EXTENSION __cl_clang_bitfields : enable
+
 #ifdef cl_khr_fp16
 #error "Incorrect cl_khr_fp16 define"
 #endif
 #pragma OPENCL EXTENSION cl_khr_fp16: enable
 // expected-warning@-1{{unsupported OpenCL extension 'cl_khr_fp16' - ignoring}}
 
 #ifdef cl_khr_int64_base_atomics
 #error "Incorrect cl_khr_int64_base_atomics define"

clang/test/SemaOpenCL/unsupported.cl

 // RUN: %clang_cc1 -verify %s
+// RUN: %clang_cc1 -verify %s -DBITFIELDS_EXT
 
-struct {
-  int a : 1; // expected-error {{bit-fields are not supported in OpenCL}}
+#ifdef BITFIELDS_EXT
+#pragma OPENCL EXTENSION __cl_clang_bitfields : enable
+#endif
+
+struct test {
+  int a : 1;
+#ifndef BITFIELDS_EXT
+// expected-error@-2 {{bit-fields are not supported in OpenCL}}
+#endif
 };
 
 void no_vla(int n) {
   int a[n]; // expected-error {{variable length arrays are not supported in OpenCL}}
 }
 
 void no_logxor(int n) {
   int logxor = n ^^ n; // expected-error {{^^ is a reserved operator in OpenCL}}
 }