[SYCL][Doc] Add draft group collectives extension (#838)

Exposes work-group functions from OpenCL 2.0:
- any
- all
- broadcast
- reduce
- scan

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/doc/extensions/GroupCollectives/GroupCollectives.md

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+# SYCL(TM) Proposals: Group Collectives for NDRange Parallelism
+
+**IMPORTANT**: This specification is a draft.
+
+**NOTE**: Khronos(R) is a registered trademark and SYCL(TM) is a trademark of the Khronos Group, Inc. OpenCL(TM) is a trademark of Apple Inc. used by permission by Khronos.
+
+This proposal exposes the work-group functions from OpenCL 2.0 (any, all, broadcast, reductions and scans) to the NDRange variant of `parallel_for`, and does not address hierarchical parallelism.
+
+The new functions are added to the `cl::sycl::group` class, and guarded by the `__SYCL_INTEL_GROUP_COLLECTIVES__` macro.
+
+## Alignment with OpenCL vs C++
+
+Where a feature is common to both OpenCL and C++, this proposal opts for C++-like naming:
+- Collective operators are named as in `<functional>` (e.g. `plus` instead of `sum`) and to avoid clashes with names in `<algorithm>` (e.g. `minimum` instead of `min`).
+- Scan operations are named as in `<algorithm>` (e.g. `inclusive_scan` instead of `scan_inclusive`).
+
+## Data Types
+
+All functions are supported for the fundamental scalar types supported by SYCL and instances of the SYCL `vec` class.  The fundamental scalar types (as defined in Section 6.5 of the SYCL 1.2.1 specification) are: `bool`, `char`, `signed char`, `unsigned char`, `short int`, `unsigned short int`, `int`, `unsigned int`, `long int`, `unsigned long int`, `long long int`, `unsigned long long int`, `size_t`, `float`, `double`, `half`.
+
+Functions with arguments of type `vec<T,N>` are applied component-wise: they are semantically equivalent to `N` calls to a scalar function of type `T`.
+
+## Function Objects
+
+A number of function objects are provided in the `cl::sycl::intel` namespace that are equivalent to those found in the `<functional>` header from the C++ standard library.  These function objects are used for all interfaces requiring an operator to be specified.
+
+The parameter types and return type for all function objects will be deduced if `T` is not specified.
+
+|Function object|Description|
+|----------------|-----------|
+|`template <typename T=void> struct plus;`|`T operator(const T&, const T&) const` calls `operator+` on its arguments.|
+|`template <typename T=void> struct minimum;`|`T operator(const T&, const T&) const` applies `std::less` to its arguments, in the same order, then returns the lesser argument unchanged.|
+|`template <typename T=void> struct maximum;`|`T operator(const T&, const T&) const` applies `std::greater` to its arguments, in the same order, then returns the greater argument unchanged.|
+
+# Functions
+
+The member functions of the `group` class described in this section act as a work-group barrier, and it is undefined behavior for these functions to be invoked within a `parallel_for_work_group` or `parallel_for_work_item` context.
+
+## Vote / Ballot
+
+|Member functions|Description|
+|----------------|-----------|
+| `bool any(bool predicate) const` | Return `true` if `predicate` evaluates to `true` for any work-item in the work-group.|
+| `bool all(bool predicate) const` | Return `true` if `predicate` evaluates to `true` for all work-items in the work-group.|
+
+## Collectives
+
+|Member functions|Description|
+|----------------|-----------|
+|`template <typename T>T broadcast(T x, id<1> local_id) const` | Broadcast the value of `x` from the work-item with the specified id to all work-items within the work-group. The value of `local_id` must be the same for all work-items in the work-group.|
+|`template <typename T, class BinaryOp> T reduce(T x, BinaryOp binary_op) const;`|Combine the values of `x` from all work-items in the work-group using the specified operator, which must be one of `plus`, `minimum` or `maximum`.|
+|`template <typename T, class BinaryOp> T reduce(T x, T init, BinaryOp binary_op) const;`|Combine the values of `x` from all work-items in the work-group using an initial value of `init` and the specified operator, which must be one of `plus`, `minimum` or `maximum`.|
+|`template <typename T, class BinaryOp> T exclusive_scan(T x, BinaryOp binary_op) const;`|Perform an exclusive scan over the values of `x` from all work-items in the work-group using the specified operator, which must be one of: `plus`, `minimum` or `maximum`.  The value returned on work-item `i` is the exclusive scan of the first `i` work items in the work-group and the `init` value. For multi-dimensional work-groups, the order of work-items in the group is determined by their linear id. The initial value is the identity value of the operator.|
+|`template <typename T, class BinaryOp> T exclusive_scan(T x, T init, BinaryOp binary_op) const;`|Perform an exclusive scan over the values of `x` from all work-items in the work-group using the specified operator, which must be one of: `plus`, `minimum` or `maximum`.  The value returned on work-item `i` is the exclusive scan of the first `i` work items in the work-group and the `init` value. For multi-dimensional work-groups, the order of work-items in the group is determined by their linear id. The initial value is specified by `init`.|
+|`template <typename T, class BinaryOp> T inclusive_scan(T x, BinaryOp binary_op) const;`|Perform an inclusive scan over the values of `x` from all work-items in the work-group using the specified operator, which must be one of: `plus`, `minimum` or `maximum`.  The value returned on work-item `i` is the inclusive scan of the first `i` work items in the work-group. For multi-dimensional work-groups, the order of work-items in the group is determined by their linear id.|
+|`template <typename T, class BinaryOp> T inclusive_scan(T x, BinaryOp binary_op, T init) const;`|Perform an inclusive scan over the values of `x` from all work-items in the work-group using the specified operator, which must be one of: `plus`, `minimum` or `maximum`.  The value returned on work-item `i` is the inclusive scan of the first `i` work items in the work-group and the `init` value. For multi-dimensional work-groups, the order of work-items in the group is determined by their linear id.|