[OpenCL] Add clang extension for bit-fields.

Allow use of bit-fields as a clang extension
in OpenCL. The extension can be enabled using
pragma directives.

This fixes PR45339!

Differential Revision: https://reviews.llvm.org/D101843

clang/docs/LanguageExtensions.rst

@@ -1741,6 +1741,34 @@ 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``
 --------------------------------
 

clang/include/clang/Basic/OpenCLExtensions.def

@@ -88,6 +88,7 @@ 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)

clang/lib/Basic/Targets/AMDGPU.h

@@ -288,6 +288,7 @@ public:
     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());
 

clang/lib/Basic/Targets/NVPTX.h

@@ -136,6 +136,7 @@ public:
     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;

clang/lib/Sema/SemaDecl.cpp

@@ -16815,8 +16815,10 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType 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;
     }

clang/test/Misc/amdgcn.languageOptsOpenCL.cl

@@ -29,6 +29,11 @@
 #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

clang/test/Misc/nvptx.languageOptsOpenCL.cl

@@ -33,6 +33,11 @@
 #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

clang/test/Misc/r600.languageOptsOpenCL.cl

@@ -39,6 +39,11 @@
 #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

clang/test/SemaOpenCL/unsupported.cl

@@ -1,7 +1,15 @@
 // 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) {