group.hpp

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//==-------------- group.hpp --- SYCL work group ---------------------------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
 
#pragma once
 
#include <CL/__spirv/spirv_ops.hpp>            // for __spirv_MemoryBarrier
#include <CL/__spirv/spirv_types.hpp>          // for Scope, __ocl_event_t
#include <sycl/access/access.hpp>              // for decorated, mode, addr...
#include <sycl/detail/common.hpp>              // for NDLoop, __SYCL_ASSERT
#include <sycl/detail/defines.hpp>             // for __SYCL_TYPE
#include <sycl/detail/defines_elementary.hpp>  // for __SYCL2020_DEPRECATED
#include <sycl/detail/generic_type_traits.hpp> // for convertToOpenCLType
#include <sycl/detail/helpers.hpp>             // for Builder, getSPIRVMemo...
#include <sycl/detail/item_base.hpp>           // for id, range
#include <sycl/detail/type_traits.hpp>         // for is_bool, change_base_...
#include <sycl/device_event.hpp>               // for device_event
#include <sycl/exception.hpp>                  // for make_error_code, errc
#include <sycl/h_item.hpp>                     // for h_item
#include <sycl/id.hpp>                         // for id
#include <sycl/item.hpp>                       // for item
#include <sycl/memory_enums.hpp>               // for memory_scope
#include <sycl/multi_ptr.hpp>                  // for multi_ptr, address_sp...
#include <sycl/pointers.hpp>                   // for decorated_global_ptr
#include <sycl/range.hpp>                      // for range
 
#include <memory>      // for unique_ptr
#include <stddef.h>    // for size_t
#include <stdint.h>    // for uint8_t, uint32_t
#include <type_traits> // for enable_if_t, remove_c...
 
namespace sycl {
inline namespace _V1 {
namespace detail {
class Builder;
 
// Implements a barrier accross work items within a work group.
inline void workGroupBarrier() {
#ifdef __SYCL_DEVICE_ONLY__
  constexpr uint32_t flags =
      static_cast<uint32_t>(
          __spv::MemorySemanticsMask::SequentiallyConsistent) |
      static_cast<uint32_t>(__spv::MemorySemanticsMask::WorkgroupMemory);
  __spirv_ControlBarrier(__spv::Scope::Workgroup, __spv::Scope::Workgroup,
                         flags);
#endif // __SYCL_DEVICE_ONLY__
}
 
} // namespace detail
 
// SYCL 1.2.1rev5, section "4.8.5.3 Parallel For hierarchical invoke":
// Quote:
//   ... To guarantee use of private per-work-item memory, the private_memory
//   class can be used to wrap the data. This class very simply constructs
//   private data for a given group across the entire group.The id of the
//   current work-item is passed to any access to grab the correct data.
template <typename T, int Dimensions = 1>
class __SYCL_TYPE(private_memory) private_memory {
public:
  // Construct based directly off the number of work-items
  private_memory(const group<Dimensions> &G) {
#ifndef __SYCL_DEVICE_ONLY__
    // serial host => one instance per work-group - allocate space for each WI
    // in the group:
    Val.reset(new T[G.get_local_range().size()]);
#endif // __SYCL_DEVICE_ONLY__
    (void)G;
  }
 
  // Access the instance for the current work-item
  T &operator()(const h_item<Dimensions> &Id) {
#ifndef __SYCL_DEVICE_ONLY__
    // Calculate the linear index of current WI and return reference to the
    // corresponding spot in the value array:
    size_t Ind = Id.get_physical_local().get_linear_id();
    return Val.get()[Ind];
#else
    (void)Id;
    return Val;
#endif // __SYCL_DEVICE_ONLY__
  }
 
private:
#ifdef __SYCL_DEVICE_ONLY__
  // On SYCL device private_memory<T> instance is created per physical WI, so
  // there is 1:1 correspondence betwen this class instances and per-WI memory.
  T Val;
#else
  // On serial host there is one private_memory<T> instance per work group, so
  // it must have space to hold separate value per WI in the group.
  std::unique_ptr<T[]> Val;
#endif // #ifdef __SYCL_DEVICE_ONLY__
};
 
template <int Dimensions = 1> class __SYCL_TYPE(group) group {
public:
#ifndef __DISABLE_SYCL_INTEL_GROUP_ALGORITHMS__
  using id_type = id<Dimensions>;
  using range_type = range<Dimensions>;
  using linear_id_type = size_t;
  static constexpr int dimensions = Dimensions;
#endif // __DISABLE_SYCL_INTEL_GROUP_ALGORITHMS__
 
  static constexpr sycl::memory_scope fence_scope =
      sycl::memory_scope::work_group;
 
  group() = delete;
 
  __SYCL2020_DEPRECATED("use sycl::group::get_group_id() instead")
  id<Dimensions> get_id() const { return index; }
 
  __SYCL2020_DEPRECATED("use sycl::group::get_group_id() instead")
  size_t get_id(int dimension) const { return index[dimension]; }
 
  id<Dimensions> get_group_id() const { return index; }
 
  size_t get_group_id(int dimension) const { return index[dimension]; }
 
  __SYCL2020_DEPRECATED("calculate sycl::group::get_group_range() * "
                        "sycl::group::get_max_local_range() instead")
  range<Dimensions> get_global_range() const { return globalRange; }
 
  size_t get_global_range(int dimension) const {
    return globalRange[dimension];
  }
 
  id<Dimensions> get_local_id() const {
#ifdef __SYCL_DEVICE_ONLY__
    return __spirv::initLocalInvocationId<Dimensions, id<Dimensions>>();
#else
    throw sycl::exception(make_error_code(errc::feature_not_supported),
                          "get_local_id() is not implemented on host");
#endif
  }
 
  size_t get_local_id(int dimention) const { return get_local_id()[dimention]; }
 
  size_t get_local_linear_id() const {
    return get_local_linear_id_impl<Dimensions>();
  }
 
  range<Dimensions> get_local_range() const { return localRange; }
 
  size_t get_local_range(int dimension) const { return localRange[dimension]; }
 
  size_t get_local_linear_range() const {
    return get_local_linear_range_impl();
  }
 
  range<Dimensions> get_group_range() const { return groupRange; }
 
  size_t get_group_range(int dimension) const {
    return get_group_range()[dimension];
  }
 
  size_t get_group_linear_range() const {
    return get_group_linear_range_impl();
  }
 
  range<Dimensions> get_max_local_range() const { return get_local_range(); }
 
  size_t operator[](int dimension) const { return index[dimension]; }
 
  __SYCL2020_DEPRECATED("use sycl::group::get_group_linear_id() instead")
  size_t get_linear_id() const { return get_group_linear_id(); }
 
  size_t get_group_linear_id() const { return get_group_linear_id_impl(); }
 
  bool leader() const { return (get_local_linear_id() == 0); }
 
  template <typename WorkItemFunctionT>
  void parallel_for_work_item(WorkItemFunctionT Func) const {
    // need barriers to enforce SYCL semantics for the work item loop -
    // compilers are expected to optimize when possible
    detail::workGroupBarrier();
#ifdef __SYCL_DEVICE_ONLY__
    range<Dimensions> GlobalSize{
        __spirv::initGlobalSize<Dimensions, range<Dimensions>>()};
    range<Dimensions> LocalSize{
        __spirv::initWorkgroupSize<Dimensions, range<Dimensions>>()};
    id<Dimensions> GlobalId{
        __spirv::initGlobalInvocationId<Dimensions, id<Dimensions>>()};
    id<Dimensions> LocalId{
        __spirv::initLocalInvocationId<Dimensions, id<Dimensions>>()};
 
    // no 'iterate' in the device code variant, because
    // (1) this code is already invoked by each work item as a part of the
    //     enclosing parallel_for_work_group kernel
    // (2) the range this pfwi iterates over matches work group size exactly
    item<Dimensions, false> GlobalItem =
        detail::Builder::createItem<Dimensions, false>(GlobalSize, GlobalId);
    item<Dimensions, false> LocalItem =
        detail::Builder::createItem<Dimensions, false>(LocalSize, LocalId);
    h_item<Dimensions> HItem =
        detail::Builder::createHItem<Dimensions>(GlobalItem, LocalItem);
 
    Func(HItem);
#else
    id<Dimensions> GroupStartID = index * id<Dimensions>{localRange};
 
    // ... host variant needs explicit 'iterate' because it is serial
    detail::NDLoop<Dimensions>::iterate(
        localRange, [&](const id<Dimensions> &LocalID) {
          item<Dimensions, false> GlobalItem =
              detail::Builder::createItem<Dimensions, false>(
                  globalRange, GroupStartID + LocalID);
          item<Dimensions, false> LocalItem =
              detail::Builder::createItem<Dimensions, false>(localRange,
                                                             LocalID);
          h_item<Dimensions> HItem =
              detail::Builder::createHItem<Dimensions>(GlobalItem, LocalItem);
          Func(HItem);
        });
#endif // __SYCL_DEVICE_ONLY__
    // Need both barriers here - before and after the parallel_for_work_item
    // (PFWI). There can be work group scope code after the PFWI which reads
    // work group local data written within this PFWI. Back Ends are expected to
    // optimize away unneeded barriers (e.g. two barriers in a row).
    detail::workGroupBarrier();
  }
 
  template <typename WorkItemFunctionT>
  void parallel_for_work_item(range<Dimensions> flexibleRange,
                              WorkItemFunctionT Func) const {
    detail::workGroupBarrier();
#ifdef __SYCL_DEVICE_ONLY__
    range<Dimensions> GlobalSize{
        __spirv::initGlobalSize<Dimensions, range<Dimensions>>()};
    range<Dimensions> LocalSize{
        __spirv::initWorkgroupSize<Dimensions, range<Dimensions>>()};
    id<Dimensions> GlobalId{
        __spirv::initGlobalInvocationId<Dimensions, id<Dimensions>>()};
    id<Dimensions> LocalId{
        __spirv::initLocalInvocationId<Dimensions, id<Dimensions>>()};
 
    item<Dimensions, false> GlobalItem =
        detail::Builder::createItem<Dimensions, false>(GlobalSize, GlobalId);
    item<Dimensions, false> LocalItem =
        detail::Builder::createItem<Dimensions, false>(LocalSize, LocalId);
    h_item<Dimensions> HItem = detail::Builder::createHItem<Dimensions>(
        GlobalItem, LocalItem, flexibleRange);
 
    // iterate over flexible range with work group size stride; each item
    // performs flexibleRange/LocalSize iterations (if the former is divisible
    // by the latter)
    detail::NDLoop<Dimensions>::iterate(
        LocalId, LocalSize, flexibleRange,
        [&](const id<Dimensions> &LogicalLocalID) {
          HItem.setLogicalLocalID(LogicalLocalID);
          Func(HItem);
        });
#else
    id<Dimensions> GroupStartID = index * localRange;
 
    detail::NDLoop<Dimensions>::iterate(
        localRange, [&](const id<Dimensions> &LocalID) {
          item<Dimensions, false> GlobalItem =
              detail::Builder::createItem<Dimensions, false>(
                  globalRange, GroupStartID + LocalID);
          item<Dimensions, false> LocalItem =
              detail::Builder::createItem<Dimensions, false>(localRange,
                                                             LocalID);
          h_item<Dimensions> HItem = detail::Builder::createHItem<Dimensions>(
              GlobalItem, LocalItem, flexibleRange);
 
          detail::NDLoop<Dimensions>::iterate(
              LocalID, localRange, flexibleRange,
              [&](const id<Dimensions> &LogicalLocalID) {
                HItem.setLogicalLocalID(LogicalLocalID);
                Func(HItem);
              });
        });
#endif // __SYCL_DEVICE_ONLY__
    detail::workGroupBarrier();
  }
 
  template <access::mode accessMode = access::mode::read_write>
  void mem_fence(
      typename std::enable_if_t<accessMode == access::mode::read ||
                                    accessMode == access::mode::write ||
                                    accessMode == access::mode::read_write,
                                access::fence_space>
          accessSpace = access::fence_space::global_and_local) const {
    uint32_t flags = detail::getSPIRVMemorySemanticsMask(accessSpace);
    // TODO: currently, there is no good way in SPIR-V to set the memory
    // barrier only for load operations or only for store operations.
    // The full read-and-write barrier is used and the template parameter
    // 'accessMode' is ignored for now. Either SPIR-V or SYCL spec may be
    // changed to address this discrepancy between SPIR-V and SYCL,
    // or if we decide that 'accessMode' is the important feature then
    // we can fix this later, for example, by using OpenCL 1.2 functions
    // read_mem_fence() and write_mem_fence().
    __spirv_MemoryBarrier(__spv::Scope::Workgroup, flags);
  }
 
  template <typename dataT>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  std::enable_if_t<!detail::is_bool<dataT>::value,
                   device_event> async_work_group_copy(local_ptr<dataT> dest,
                                                       global_ptr<dataT> src,
                                                       size_t numElements,
                                                       size_t srcStride) const {
    __ocl_event_t E = __SYCL_OpGroupAsyncCopyGlobalToLocal(
        __spv::Scope::Workgroup, detail::convertToOpenCLType(dest),
        detail::convertToOpenCLType(src), numElements, srcStride, 0);
    return device_event(E);
  }
 
  template <typename dataT>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  std::enable_if_t<!detail::is_bool<dataT>::value,
                   device_event> async_work_group_copy(global_ptr<dataT> dest,
                                                       local_ptr<dataT> src,
                                                       size_t numElements,
                                                       size_t destStride)
      const {
    __ocl_event_t E = __SYCL_OpGroupAsyncCopyLocalToGlobal(
        __spv::Scope::Workgroup, detail::convertToOpenCLType(dest),
        detail::convertToOpenCLType(src), numElements, destStride, 0);
    return device_event(E);
  }
 
  template <typename DestDataT, typename SrcDataT>
  std::enable_if_t<!detail::is_bool<DestDataT>::value &&
                       std::is_same_v<std::remove_const_t<SrcDataT>, DestDataT>,
                   device_event>
  async_work_group_copy(decorated_local_ptr<DestDataT> dest,
                        decorated_global_ptr<SrcDataT> src, size_t numElements,
                        size_t srcStride) const {
    __ocl_event_t E = __SYCL_OpGroupAsyncCopyGlobalToLocal(
        __spv::Scope::Workgroup, detail::convertToOpenCLType(dest),
        detail::convertToOpenCLType(src), numElements, srcStride, 0);
    return device_event(E);
  }
 
  template <typename DestDataT, typename SrcDataT>
  std::enable_if_t<!detail::is_bool<DestDataT>::value &&
                       std::is_same_v<std::remove_const_t<SrcDataT>, DestDataT>,
                   device_event>
  async_work_group_copy(decorated_global_ptr<DestDataT> dest,
                        decorated_local_ptr<SrcDataT> src, size_t numElements,
                        size_t destStride) const {
    __ocl_event_t E = __SYCL_OpGroupAsyncCopyLocalToGlobal(
        __spv::Scope::Workgroup, detail::convertToOpenCLType(dest),
        detail::convertToOpenCLType(src), numElements, destStride, 0);
    return device_event(E);
  }
 
  template <typename T, access::address_space DestS, access::address_space SrcS>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  std::enable_if_t<
      detail::is_scalar_bool<T>::value,
      device_event> async_work_group_copy(multi_ptr<T, DestS,
                                                    access::decorated::legacy>
                                              Dest,
                                          multi_ptr<T, SrcS,
                                                    access::decorated::legacy>
                                              Src,
                                          size_t NumElements,
                                          size_t Stride) const {
    static_assert(sizeof(bool) == sizeof(uint8_t),
                  "Async copy to/from bool memory is not supported.");
    auto DestP = multi_ptr<uint8_t, DestS, access::decorated::legacy>(
        reinterpret_cast<uint8_t *>(Dest.get()));
    auto SrcP = multi_ptr<uint8_t, SrcS, access::decorated::legacy>(
        reinterpret_cast<uint8_t *>(Src.get()));
    return async_work_group_copy(DestP, SrcP, NumElements, Stride);
  }
 
  template <typename T, access::address_space DestS, access::address_space SrcS>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  std::enable_if_t<
      detail::is_vector_bool<T>::value,
      device_event> async_work_group_copy(multi_ptr<T, DestS,
                                                    access::decorated::legacy>
                                              Dest,
                                          multi_ptr<T, SrcS,
                                                    access::decorated::legacy>
                                              Src,
                                          size_t NumElements,
                                          size_t Stride) const {
    static_assert(sizeof(bool) == sizeof(uint8_t),
                  "Async copy to/from bool memory is not supported.");
    using VecT = detail::change_base_type_t<T, uint8_t>;
    auto DestP = address_space_cast<DestS, access::decorated::legacy>(
        reinterpret_cast<VecT *>(Dest.get()));
    auto SrcP = address_space_cast<SrcS, access::decorated::legacy>(
        reinterpret_cast<VecT *>(Src.get()));
    return async_work_group_copy(DestP, SrcP, NumElements, Stride);
  }
 
  template <typename DestT, access::address_space DestS, typename SrcT,
            access::address_space SrcS>
  std::enable_if_t<detail::is_scalar_bool<DestT>::value &&
                       std::is_same_v<std::remove_const_t<SrcT>, DestT>,
                   device_event>
  async_work_group_copy(multi_ptr<DestT, DestS, access::decorated::yes> Dest,
                        multi_ptr<SrcT, SrcS, access::decorated::yes> Src,
                        size_t NumElements, size_t Stride) const {
    static_assert(sizeof(bool) == sizeof(uint8_t),
                  "Async copy to/from bool memory is not supported.");
    using QualSrcT =
        std::conditional_t<std::is_const_v<SrcT>, const uint8_t, uint8_t>;
    auto DestP = multi_ptr<uint8_t, DestS, access::decorated::yes>(
        detail::cast_AS<typename multi_ptr<uint8_t, DestS,
                                           access::decorated::yes>::pointer>(
            Dest.get_decorated()));
    auto SrcP = multi_ptr<QualSrcT, SrcS, access::decorated::yes>(
        detail::cast_AS<typename multi_ptr<QualSrcT, SrcS,
                                           access::decorated::yes>::pointer>(
            Src.get_decorated()));
    return async_work_group_copy(DestP, SrcP, NumElements, Stride);
  }
 
  template <typename DestT, access::address_space DestS, typename SrcT,
            access::address_space SrcS>
  std::enable_if_t<detail::is_vector_bool<DestT>::value &&
                       std::is_same_v<std::remove_const_t<SrcT>, DestT>,
                   device_event>
  async_work_group_copy(multi_ptr<DestT, DestS, access::decorated::yes> Dest,
                        multi_ptr<SrcT, SrcS, access::decorated::yes> Src,
                        size_t NumElements, size_t Stride) const {
    static_assert(sizeof(bool) == sizeof(uint8_t),
                  "Async copy to/from bool memory is not supported.");
    using VecT = detail::change_base_type_t<DestT, uint8_t>;
    using QualSrcVecT =
        std::conditional_t<std::is_const_v<SrcT>, std::add_const_t<VecT>, VecT>;
    auto DestP = multi_ptr<VecT, DestS, access::decorated::yes>(
        detail::cast_AS<
            typename multi_ptr<VecT, DestS, access::decorated::yes>::pointer>(
            Dest.get_decorated()));
    auto SrcP = multi_ptr<QualSrcVecT, SrcS, access::decorated::yes>(
        detail::cast_AS<typename multi_ptr<QualSrcVecT, SrcS,
                                           access::decorated::yes>::pointer>(
            Src.get_decorated()));
    return async_work_group_copy(DestP, SrcP, NumElements, Stride);
  }
 
  template <typename dataT>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  device_event
      async_work_group_copy(local_ptr<dataT> dest, global_ptr<dataT> src,
                            size_t numElements) const {
    return async_work_group_copy(dest, src, numElements, 1);
  }
 
  template <typename dataT>
  __SYCL2020_DEPRECATED("Use decorated multi_ptr arguments instead")
  device_event
      async_work_group_copy(global_ptr<dataT> dest, local_ptr<dataT> src,
                            size_t numElements) const {
    return async_work_group_copy(dest, src, numElements, 1);
  }
 
  template <typename DestDataT, typename SrcDataT>
  typename std::enable_if_t<
      std::is_same_v<DestDataT, std::remove_const_t<SrcDataT>>, device_event>
  async_work_group_copy(decorated_local_ptr<DestDataT> dest,
                        decorated_global_ptr<SrcDataT> src,
                        size_t numElements) const {
    return async_work_group_copy(dest, src, numElements, 1);
  }
 
  template <typename DestDataT, typename SrcDataT>
  typename std::enable_if_t<
      std::is_same_v<DestDataT, std::remove_const_t<SrcDataT>>, device_event>
  async_work_group_copy(decorated_global_ptr<DestDataT> dest,
                        decorated_local_ptr<SrcDataT> src,
                        size_t numElements) const {
    return async_work_group_copy(dest, src, numElements, 1);
  }
 
  template <typename... eventTN> void wait_for(eventTN... Events) const {
    waitForHelper(Events...);
  }
 
  bool operator==(const group<Dimensions> &rhs) const {
    bool Result = (rhs.globalRange == globalRange) &&
                  (rhs.localRange == localRange) && (rhs.index == index);
    __SYCL_ASSERT(rhs.groupRange == groupRange &&
                  "inconsistent group class fields");
    return Result;
  }
 
  bool operator!=(const group<Dimensions> &rhs) const {
    return !((*this) == rhs);
  }
 
private:
  range<Dimensions> globalRange;
  range<Dimensions> localRange;
  range<Dimensions> groupRange;
  id<Dimensions> index;
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 1), size_t>
  get_local_linear_id_impl() const {
    id<Dimensions> localId = get_local_id();
    return localId[0];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 2), size_t>
  get_local_linear_id_impl() const {
    id<Dimensions> localId = get_local_id();
    return localId[0] * localRange[1] + localId[1];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 3), size_t>
  get_local_linear_id_impl() const {
    id<Dimensions> localId = get_local_id();
    return (localId[0] * localRange[1] * localRange[2]) +
           (localId[1] * localRange[2]) + localId[2];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 1), size_t>
  get_local_linear_range_impl() const {
    auto localRange = get_local_range();
    return localRange[0];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 2), size_t>
  get_local_linear_range_impl() const {
    auto localRange = get_local_range();
    return localRange[0] * localRange[1];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 3), size_t>
  get_local_linear_range_impl() const {
    auto localRange = get_local_range();
    return localRange[0] * localRange[1] * localRange[2];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 1), size_t>
  get_group_linear_range_impl() const {
    auto groupRange = get_group_range();
    return groupRange[0];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 2), size_t>
  get_group_linear_range_impl() const {
    auto groupRange = get_group_range();
    return groupRange[0] * groupRange[1];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 3), size_t>
  get_group_linear_range_impl() const {
    auto groupRange = get_group_range();
    return groupRange[0] * groupRange[1] * groupRange[2];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 1), size_t>
  get_group_linear_id_impl() const {
    return index[0];
  }
 
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 2), size_t>
  get_group_linear_id_impl() const {
    return index[0] * groupRange[1] + index[1];
  }
 
  // SYCL specification 1.2.1rev5, section 4.7.6.5 "Buffer accessor":
  //    Whenever a multi-dimensional index is passed to a SYCL accessor the
  //    linear index is calculated based on the index {id1, id2, id3} provided
  //    and the range of the SYCL accessor {r1, r2, r3} according to row-major
  //    ordering as follows:
  //      id3 + (id2 · r3) + (id1 · r3 · r2)            (4.3)
  // section 4.8.1.8 "group class":
  //    size_t get_linear_id()const
  //    Get a linearized version of the work-group id. Calculating a linear
  //    work-group id from a multi-dimensional index follows the equation 4.3.
  template <int dims = Dimensions>
  typename std::enable_if_t<(dims == 3), size_t>
  get_group_linear_id_impl() const {
    return (index[0] * groupRange[1] * groupRange[2]) +
           (index[1] * groupRange[2]) + index[2];
  }
 
  void waitForHelper() const {}
 
  void waitForHelper(device_event Event) const { Event.wait(); }
 
  template <typename T, typename... Ts>
  void waitForHelper(T E, Ts... Es) const {
    waitForHelper(E);
    waitForHelper(Es...);
  }
 
protected:
  friend class detail::Builder;
  group(const range<Dimensions> &G, const range<Dimensions> &L,
        const range<Dimensions> GroupRange, const id<Dimensions> &I)
      : globalRange(G), localRange(L), groupRange(GroupRange), index(I) {}
};
} // namespace _V1
} // namespace sycl