Clarify backend and introduce "backend::none"

adoc/chapters/architecture.adoc

@@ -3,36 +3,25 @@
 [[architecture]]
 = SYCL architecture
 
-This chapter describes the structure of a SYCL application, and how the SYCL
-generic programming model lays out on top of a number of <<backend>>s.
+This chapter describes the structure of a SYCL application and its programming
+model.
 
 
 == Overview
 
 SYCL is an open industry standard for programming a heterogeneous system.
 The design of SYCL allows standard {cpp} source code to be written such that it
-can run on either an heterogeneous device or on the <<host>>.
-
-The terminology used for SYCL inherits historically from OpenCL with some
-SYCL-specific additions.
-However SYCL is a generic {cpp} programming model that can be laid out on top of
-other heterogeneous APIs apart from OpenCL.
-SYCL implementations can provide <<backend>>s for various heterogeneous APIs,
-implementing the SYCL general specification on top of them.
-We refer to this heterogeneous API as the <<backend-api>>.
-The SYCL general specification defines the behavior that all SYCL
-implementations must expose to SYCL users for a SYCL application to behave as
-expected.
-
-A function object that can execute on a <<device>> exposed by a <<backend-api>>
-is called a <<sycl-kernel-function>>.
-
-To ensure maximum interoperability with different <<backend-api>>s, software
-developers can access the <<backend-api>> alongside the SYCL general API
-whenever they include the <<backend>> interoperability headers.
-However, interoperability is a <<backend>>-specific feature.
-An application that uses interoperability does not conform to the SYCL general
-application model, since it is not portable across backends.
+can run on either an heterogeneous device such as a GPU or on a general purpose
+CPU.
+
+Some terminology from SYCL derives from OpenCL.
+However, OpenCL is not a prerequisite for implementing SYCL.
+A SYCL implementation may layer on top of a heterogeneous programming API such
+as OpenCL, but this is not necessary.
+It is also possible to implement SYCL directly on hardware.
+
+A function object that can execute on a <<device>> is called a
+<<sycl-kernel-function>>.
 
 // Note below I leave the reference to OpenCL intentionally
 
@@ -194,8 +183,8 @@ An implementation may also expose empty <<platform, platforms>> that do not
 contain any <<device,devices>>.
 
 A SYCL <<context>> is constructed, either directly by the user or implicitly
-when creating a <<queue>>, to hold all the runtime information required by the
-SYCL runtime and the <<backend>> to operate on a device, or group of devices.
+when creating a <<queue>>, to hold all the runtime information required to
+operate on a device, or group of devices.
 When a group of devices can be grouped together on the same context, they have
 some visibility of each other's memory objects.
 The SYCL runtime can assume that memory is visible across all devices in the
@@ -204,26 +193,23 @@ Not all devices exposed from the same <<platform>> can be grouped together in
 the same <<context>>.
 
 A SYCL application executes on the host as a standard {cpp} program.
-<<device,Devices>> are exposed through different <<backend, SYCL backends>> to
-the SYCL application.
+The <<sycl-runtime>> exposes <<device,devices>> to the application.
 The SYCL application submits <<command-group-function-object,command group
-function objects>> to <<queue,queues>>.
-Each <<queue>> enables execution on a given device.
+function objects>> to <<queue,queues>>, and each <<queue>> enables execution on
+a given device.
 
 The <<sycl-runtime>> then extracts operations from the
 <<command-group-function-object>>, e.g. an explicit copy operation or a
 <<sycl-kernel-function>>.
-When the operation is a <<sycl-kernel-function>>, the <<sycl-runtime>> uses a
-<<backend>>-specific mechanism to extract the device binary from the SYCL
+When the operation is a <<sycl-kernel-function>>, the <<sycl-runtime>> uses an
+implementation-specific mechanism to extract the device binary from the SYCL
 application and pass it to the heterogeneous API for execution on the
 <<device>>.
 
 A SYCL <<device>> is divided into one or more compute units (CUs) which are each
 divided into one or more processing elements (PEs).
 Computations on a device occur within the processing elements.
-How computation is mapped to PEs is <<backend>> and <<device>> specific.
-Two devices exposed via two different backends can map computations differently
-to the same device.
+How computation is mapped to PEs is specific to the implementation.
 
 When a SYCL application contains <<sycl-kernel-function>> objects, the SYCL
 implementation must provide an offline compilation mechanism that enables the
@@ -233,29 +219,36 @@ as SPIR-V, that will be finalized during execution or a final device ISA.
 
 A device may expose special purpose functionality as a _built-in_ function.
 The SYCL API exposes functions to query and dispatch said _built-in_ functions.
-Some <<backend, SYCL backends>> and <<device,devices>> may not support
-programmable kernels, and only support _built-in_ functions.
+Some <<device,devices>> may not support programmable kernels, and only support
+_built-in_ functions.
 // TODO: Conformance of these custom-devices?
 
 
 == The SYCL backend model
 
-SYCL is a generic programming model for the {cpp} language that can target
-multiple heterogeneous APIs, such as OpenCL.
-
-SYCL implementations enable these target APIs by implementing <<backend, SYCL
-backends>>.
-For a SYCL implementation to be conformant on said <<backend>>, it must execute
-the SYCL generic programming model on the backend.
-All SYCL implementations must provide at least one backend.
-
-The present document covers the SYCL generic interface available to all
-<<backend, SYCL backends>>.
-How the SYCL generic interface maps to a particular <<backend>> is defined
-either by a separate <<backend>> specification document, provided by the Khronos
-SYCL group, or by the SYCL implementation documentation.
-Whenever there is a <<backend>> specification document, this takes precedence
-over SYCL implementation documentation.
+A SYCL implementation may expose devices by layering on top of a heterogeneous
+API such as OpenCL or it may expose devices in some other way.
+An implementation may even combine these methods, exposing devices through
+several different heterogeneous APIs or through other mechanisms.
+Regardless of how the devices are exposed, they must implement the semantics of
+the SYCL APIs defined in this document.
+
+When a SYCL implementation layers on top of a heterogeneous API such as OpenCL,
+we refer to this as a <<backend>>.
+In this case, the implementation may choose to document how the backend APIs
+correlate to SYCL and provide interoperation APIs that allow a SYCL application
+to make calls to both the SYCL APIs and the backend APIs.
+However, applications that make use of this ability are no longer conformant to
+the <<core-spec>> and may not be portable to other implementations of SYCL.
+
+In order to aid with this sort of backend interoperation, SYCL provides the
+[code]#backend# enumeration, and most SYCL objects provide a member function
+[code]#get_backend#.
+If a SYCL implementation wants to expose backend interoperation, it must define
+one or more enumerators in the [code]#backend# enumeration, and each of those
+enumerators must have a corresponding backend interoperation specification.
+These enumerators and specifications may be defined either by the Khronos SYCL
+group or by the vendor of the SYCL implementation.
 
 When a SYCL user builds their SYCL application, she decides which of the
 <<backend, SYCL backends>> will be used to build the SYCL application.
@@ -306,22 +299,21 @@ across <<backend, SYCL backends>>.
 
 USM allocations are subject to the limitations described in <<sec:usm>>.
 
-When a SYCL application runs on any number of <<backend, SYCL backends>> without
-relying on any <<backend>>-specific behavior or interoperability, it is said to
-be a SYCL general application, and it is expected to run in any SYCL-conformant
+When a SYCL application runs without relying on any <<backend>>-specific
+behavior or interoperability, it is expected to run on any SYCL-conformant
 implementation that supports the required features for the application.
 
 
 === Platform mixed version support
 
-The SYCL generic programming model exposes a number of <<platform,platforms>>,
-each of them either empty or exposing a number of <<device,devices>>.
-Each <<platform>> is bound to a certain <<backend>>.
-SYCL <<device,devices>> associated with said <<platform>> are associated with
-that <<backend>>.
+The SYCL programming model exposes a number of <<platform,platforms>>, each of
+them either empty or exposing a number of <<device,devices>>.
+Each platform is bound to no more than one <<backend>>.
+If the platform is bound to a backend, the devices associated with that platform
+are associated with that backend.
 
-Although the APIs in the SYCL generic programming model are defined according to
-this specification and their version is indicated by the macro
+Although most APIs in the SYCL programming model are defined according to this
+specification and their version is indicated by the macro
 [code]#SYCL_LANGUAGE_VERSION#, this does not apply to APIs exposed by the
 <<backend, SYCL backends>>.
 Each <<backend>> provides its own document that defines its APIs, and that
@@ -1608,22 +1600,22 @@ queue constructor.
 Device topology may be cached by the <<sycl-runtime>>, but this is not required.
 
 Device discovery will return all <<device,devices>> from all
-<<platform,platforms>> exposed by all the supported <<backend, SYCL backends>>.
+<<platform,platforms>>.
 
 === Interfacing with the SYCL backend API
 
 There are two styles of developing a SYCL application:
 
-. writing a pure SYCL generic application;
+. writing a SYCL application that works with any <<backend>>;
 . writing a SYCL application that relies on some <<backend>> specific behavior.
 
 When users follow 1., there is no assumption about what <<backend>> will be used
 during compilation or execution of the SYCL application.
 Therefore, the <<backend-api>> is not assumed to be available to the developer.
 Only standard {cpp} types and interfaces are assumed to be available, as
 described in <<sec:progmodel.cpp>>.
-Users only need to include the [code]#<sycl/sycl.hpp># header to write a SYCL
-generic application.
+Users only need to include the [code]#<sycl/sycl.hpp># header to write such an
+application.
 
 On the other hand, when users follow 2., they must know what <<backend-api>>s
 they are using.

adoc/chapters/device_compiler.adoc

@@ -676,13 +676,13 @@ associated with an aspect that is not listed in the attribute.
 == Address-space deduction
 
 {cpp} has no type-level support to represent address spaces.
-As a consequence, the SYCL generic programming model does not directly affect
-the {cpp} type of unannotated pointers and references.
+As a consequence, the SYCL programming model does not directly affect the {cpp}
+type of unannotated pointers and references.
 
-Source level guarantees about address spaces in the SYCL generic programming
-model can only be achieved using pointer classes (instances of
-[code]#multi_ptr#), which are regular classes that represent pointers to data
-stored in the corresponding address spaces.
+Source level guarantees about address spaces in the SYCL programming model can
+only be achieved using pointer classes (instances of [code]#multi_ptr#), which
+are regular classes that represent pointers to data stored in the corresponding
+address spaces.
 
 In SYCL, the address space of pointer and references are derived from:
 

adoc/chapters/host_backend.adoc

@@ -49,7 +49,7 @@ kernel execution model.
 Kernel math library functions on the host must conform to OpenCL math precision
 requirements.
 The SYCL host device needs to be queried for the capabilities it provides.
-This ensures consistency when executing any SYCL general application.
+This ensures consistency when executing any SYCL application.
 
 The <<host-device>> must report as supporting images and therefore support the
 minimum image formats.

adoc/chapters/opencl_backend.adoc

@@ -4,10 +4,10 @@
 [[chapter:opencl-backend]]
 = OpenCL backend specification
 
-This chapter describes how the SYCL general programming model is mapped on top
-of OpenCL, and how the SYCL generic interoperability interface must be
-implemented by vendors providing SYCL for OpenCL implementations to ensure SYCL
-applications written for the OpenCL backend are interoperable.
+This chapter describes how the SYCL programming model is mapped on top of
+OpenCL, and how the SYCL interoperability interface must be implemented by
+vendors providing SYCL for OpenCL implementations to ensure SYCL applications
+written for the OpenCL backend are interoperable.
 
 
 [[sec:opencl:native-interop-application]]
@@ -669,8 +669,8 @@ bool has_extension(const sycl::device& syclDevice, const std::string& extension)
 === Reference counting
 
 Most OpenCL objects are reference counted.
-The SYCL general programming model doesn't require that native objects are
-reference counted.
+The SYCL programming model doesn't require that native objects are reference
+counted.
 However, for convenience, the following function is provided in the
 [code]#sycl::opencl# namespace.