docs/HLSLSupport.rst - llvm-project/clang - Git at Google

 ============
 HLSL Support
 ============

 .. contents::
    :local:

 Introduction
 ============

 HLSL Support is under active development in the Clang codebase. This document
 describes the high level goals of the project, the guiding principles, as well
 as some idiosyncrasies of the HLSL language and how we intend to support them in
 Clang.

 Project Goals
 =============

 The long term goal of this project is to enable Clang to function as a
 replacement for the `DirectXShaderCompiler (DXC)
 <https://github.com/microsoft/DirectXShaderCompiler/>`_ in all its supported
 use cases. Accomplishing that goal will require Clang to be able to process most
 existing HLSL programs with a high degree of source compatibility.

 Non-Goals
 ---------

 HLSL ASTs do not need to be compatible between DXC and Clang. We do not expect
 identical code generation or that features will resemble DXC's implementation or
 architecture. In fact, we explicitly expect to deviate from DXC's implementation
 in key ways.

 Guiding Principles
 ==================

 This document lacks details for architectural decisions that are not yet
 finalized. Our top priorities are quality, maintainability, and flexibility. In
 accordance with community standards we are expecting a high level of test
 coverage, and we will engineer our solutions with long term maintenance in mind.
 We are also working to limit modifications to the Clang C++ code paths and
 share as much functionality as possible.

 Architectural Direction
 =======================

 HLSL support in Clang is expressed as C++ minus unsupported C and C++ features.
 This is different from how other Clang languages are implemented. Most languages
 in Clang are additive on top of C.

 HLSL is not a formally or fully specified language, and while our goals require
 a high level of source compatibility, implementations can vary and we have some
 flexibility to be more or less permissive in some cases. For modern HLSL DXC is
 the reference implementation.

 The HLSL effort prioritizes following similar patterns for other languages,
 drivers, runtimes and targets. Specifically, We will maintain separation between
 HSLS-specific code and the rest of Clang as much as possible following patterns
 in use in Clang code today (i.e. ParseHLSL.cpp, SemaHLSL.cpp, CGHLSL*.cpp...).
 We will use inline checks on language options where the code is simple and
 isolated, and prefer HLSL-specific implementation files for any code of
 reasonable complexity.

 In places where the HLSL language is in conflict with C and C++, we will seek to
 make minimally invasive changes guarded under the HLSL language options. We will
 seek to make HLSL language support as minimal a maintenance burden as possible.

 DXC Driver
 ----------

 A DXC driver mode will provide command-line compatibility with DXC, supporting
 DXC's options and flags. The DXC driver is HLSL-specific and will create an
 HLSLToolchain which will provide the basis to support targeting both DirectX and
 Vulkan.

 Parser
 ------

 Following the examples of other parser extensions HLSL will add a ParseHLSL.cpp
 file to contain the implementations of HLSL-specific extensions to the Clang
 parser. The HLSL grammar shares most of its structure with C and C++, so we will
 use the existing C/C++ parsing code paths.

 Sema
 ----

 HLSL's Sema implementation will also provide an ``ExternalSemaSource``. In DXC,
 an ``ExternalSemaSource`` is used to provide definitions for HLSL built-in data
 types and built-in templates. Clang is already designed to allow an attached
 ``ExternalSemaSource`` to lazily complete data types, which is a **huge**
 performance win for HLSL.

 CodeGen
 -------

 Like OpenCL, HLSL relies on capturing a lot of information into IR metadata.
 *hand wave* *hand wave* *hand wave* As a design principle here we want our IR to
 be idiomatic Clang IR as much as possible. We will use IR attributes wherever we
 can, and use metadata as sparingly as possible. One example of a difference from
 DXC already implemented in Clang is the use of target triples to communicate
 shader model versions and shader stages.

 Our HLSL CodeGen implementation should also have an eye toward generating IR
 that will map directly to targets other than DXIL. While IR itself is generally
 not re-targetable, we want to share the Clang CodeGen implementation for HLSL
 with other GPU graphics targets like SPIR-V and possibly other GPU and even CPU
 targets.

 HLSL Language
 =============

 The HLSL language is insufficiently documented, and not formally specified.
 Documentation is available on `Microsoft's website
 <https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl>`_.
 The language syntax is similar enough to C and C++ that carefully written C and
 C++ code is valid HLSL. HLSL has some key differences from C & C++ which we will
 need to handle in Clang.

 HLSL is not a conforming or valid extension or superset of C or C++. The
 language has key incompatibilities with C and C++, both syntactically and
 semantically.

 An Aside on GPU Languages
 -------------------------

 Due to HLSL being a GPU targeted language HLSL is a Single Program Multiple Data
 (SPMD) language relying on the implicit parallelism provided by GPU hardware.
 Some language features in HLSL enable programmers to take advantage of the
 parallel nature of GPUs in a hardware abstracted language.

 HLSL also prohibits some features of C and C++ which can have catastrophic
 performance or are not widely supportable on GPU hardware or drivers. As an
 example, register spilling is often excessively expensive on GPUs, so HLSL
 requires all functions to be inlined during code generation, and does not
 support a runtime calling convention.

 Pointers & References
 ---------------------

 HLSL does not support referring to values by address. Semantically all variables
 are value-types and behave as such. HLSL disallows the pointer dereference
 operators (unary ``*``, and ``->``), as well as the address of operator (unary
 &). While HLSL disallows pointers and references in the syntax, HLSL does use
 reference types in the AST, and we intend to use pointer decay in the AST in
 the Clang implementation.

 HLSL ``this`` Keyword
 ---------------------

 HLSL does support member functions, and (in HLSL 2021) limited operator
 overloading. With member function support, HLSL also has a ``this`` keyword. The
 ``this`` keyword is an example of one of the places where HLSL relies on
 references in the AST, because ``this`` is a reference.

 Bitshifts
 ---------

 In deviation from C, HLSL bitshifts are defined to mask the shift count by the
 size of the type. In DXC, the semantics of LLVM IR were altered to accommodate
 this, in Clang we intend to generate the mask explicitly in the IR. In cases
 where the shift value is constant, this will be constant folded appropriately,
 in other cases we can clean it up in the DXIL target.

 Non-short Circuiting Logical Operators
 --------------------------------------

 In HLSL 2018 and earlier, HLSL supported logical operators (and the ternary
 operator) on vector types. This behavior required that operators not short
 circuit. The non-short circuiting behavior applies to all data types until HLSL
 2021. In HLSL 2021, logical and ternary operators do not support vector types
 instead builtin functions ``and``, ``or`` and ``select`` are available, and
 operators short circuit matching C behavior.

 Precise Qualifier
 -----------------

 HLSL has a ``precise`` qualifier that behaves unlike anything else in the C
 language. The support for this qualifier in DXC is buggy, so our bar for
 compatibility is low.

 The ``precise`` qualifier applies in the inverse direction from normal
 qualifiers. Rather than signifying that the declaration containing ``precise``
 qualifier be precise, it signifies that the operations contributing to the
 declaration's value be ``precise``. Additionally, ``precise`` is a misnomer:
 values attributed as ``precise`` comply with IEEE-754 floating point semantics,
 and are prevented from optimizations which could decrease *or increase*
 precision.

 Differences in Templates
 ------------------------

 HLSL uses templates to define builtin types and methods, but disallowed
 user-defined templates until HLSL 2021. HLSL also allows omitting empty template
 parameter lists when all template parameters are defaulted. This is an ambiguous
 syntax in C++, but Clang detects the case and issues a diagnostic. This makes
 supporting the case in Clang minimally invasive.

 Vector Extensions
 -----------------

 HLSL uses the OpenCL vector extensions, and also provides C++-style constructors
 for vectors that are not supported by Clang.

 Standard Library
 ----------------

 HLSL does not support the C or C++ standard libraries. Like OpenCL, HLSL
 describes its own library of built in types, complex data types, and functions.

 Unsupported C & C++ Features
 ----------------------------

 HLSL does not support all features of C and C++. In implementing HLSL in Clang
 use of some C and C++ features will produce diagnostics under HLSL, and others
 will be supported as language extensions. In general, any C or C++ feature that
 can be supported by the DXIL and SPIR-V code generation targets could be treated
 as a clang HLSL extension. Features that cannot be lowered to DXIL or SPIR-V,
 must be diagnosed as errors.

 HLSL does not support the following C features:

 * Pointers
 * References
 * ``goto`` or labels
 * Variable Length Arrays
 * ``_Complex`` and ``_Imaginary``
 * C Threads or Atomics (or Obj-C blocks)
 * ``union`` types `(in progress for HLSL 202x) <https://github.com/microsoft/DirectXShaderCompiler/pull/4132>`_
 * Most features C11 and later

 HLSL does not support the following C++ features:

 * RTTI
 * Exceptions
 * Multiple inheritance
 * Access specifiers
 * Anonymous or inline namespaces
 * ``new`` & ``delete`` operators in all of their forms (array, placement, etc)
 * Constructors and destructors
 * Any use of the ``virtual`` keyword
 * Most features C++11 and later
	============
	HLSL Support
	============

	.. contents::
	:local:

	Introduction
	============

	HLSL Support is under active development in the Clang codebase. This document
	describes the high level goals of the project, the guiding principles, as well
	as some idiosyncrasies of the HLSL language and how we intend to support them in
	Clang.

	Project Goals
	=============

	The long term goal of this project is to enable Clang to function as a
	replacement for the `DirectXShaderCompiler (DXC)
	<https://github.com/microsoft/DirectXShaderCompiler/>`_ in all its supported
	use cases. Accomplishing that goal will require Clang to be able to process most
	existing HLSL programs with a high degree of source compatibility.

	Non-Goals
	---------

	HLSL ASTs do not need to be compatible between DXC and Clang. We do not expect
	identical code generation or that features will resemble DXC's implementation or
	architecture. In fact, we explicitly expect to deviate from DXC's implementation
	in key ways.

	Guiding Principles
	==================

	This document lacks details for architectural decisions that are not yet
	finalized. Our top priorities are quality, maintainability, and flexibility. In
	accordance with community standards we are expecting a high level of test
	coverage, and we will engineer our solutions with long term maintenance in mind.
	We are also working to limit modifications to the Clang C++ code paths and
	share as much functionality as possible.

	Architectural Direction
	=======================

	HLSL support in Clang is expressed as C++ minus unsupported C and C++ features.
	This is different from how other Clang languages are implemented. Most languages
	in Clang are additive on top of C.

	HLSL is not a formally or fully specified language, and while our goals require
	a high level of source compatibility, implementations can vary and we have some
	flexibility to be more or less permissive in some cases. For modern HLSL DXC is
	the reference implementation.

	The HLSL effort prioritizes following similar patterns for other languages,
	drivers, runtimes and targets. Specifically, We will maintain separation between
	HSLS-specific code and the rest of Clang as much as possible following patterns
	in use in Clang code today (i.e. ParseHLSL.cpp, SemaHLSL.cpp, CGHLSL*.cpp...).
	We will use inline checks on language options where the code is simple and
	isolated, and prefer HLSL-specific implementation files for any code of
	reasonable complexity.

	In places where the HLSL language is in conflict with C and C++, we will seek to
	make minimally invasive changes guarded under the HLSL language options. We will
	seek to make HLSL language support as minimal a maintenance burden as possible.

	DXC Driver
	----------

	A DXC driver mode will provide command-line compatibility with DXC, supporting
	DXC's options and flags. The DXC driver is HLSL-specific and will create an
	HLSLToolchain which will provide the basis to support targeting both DirectX and
	Vulkan.

	Parser
	------

	Following the examples of other parser extensions HLSL will add a ParseHLSL.cpp
	file to contain the implementations of HLSL-specific extensions to the Clang
	parser. The HLSL grammar shares most of its structure with C and C++, so we will
	use the existing C/C++ parsing code paths.

	Sema
	----

	HLSL's Sema implementation will also provide an ``ExternalSemaSource``. In DXC,
	an ``ExternalSemaSource`` is used to provide definitions for HLSL built-in data
	types and built-in templates. Clang is already designed to allow an attached
	``ExternalSemaSource`` to lazily complete data types, which is a huge
	performance win for HLSL.

	CodeGen
	-------

	Like OpenCL, HLSL relies on capturing a lot of information into IR metadata.
	hand wave hand wave hand wave As a design principle here we want our IR to
	be idiomatic Clang IR as much as possible. We will use IR attributes wherever we
	can, and use metadata as sparingly as possible. One example of a difference from
	DXC already implemented in Clang is the use of target triples to communicate
	shader model versions and shader stages.

	Our HLSL CodeGen implementation should also have an eye toward generating IR
	that will map directly to targets other than DXIL. While IR itself is generally
	not re-targetable, we want to share the Clang CodeGen implementation for HLSL
	with other GPU graphics targets like SPIR-V and possibly other GPU and even CPU
	targets.

	HLSL Language
	=============

	The HLSL language is insufficiently documented, and not formally specified.
	Documentation is available on `Microsoft's website
	<https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl>`_.
	The language syntax is similar enough to C and C++ that carefully written C and
	C++ code is valid HLSL. HLSL has some key differences from C & C++ which we will
	need to handle in Clang.

	HLSL is not a conforming or valid extension or superset of C or C++. The
	language has key incompatibilities with C and C++, both syntactically and
	semantically.

	An Aside on GPU Languages
	-------------------------

	Due to HLSL being a GPU targeted language HLSL is a Single Program Multiple Data
	(SPMD) language relying on the implicit parallelism provided by GPU hardware.
	Some language features in HLSL enable programmers to take advantage of the
	parallel nature of GPUs in a hardware abstracted language.

	HLSL also prohibits some features of C and C++ which can have catastrophic
	performance or are not widely supportable on GPU hardware or drivers. As an
	example, register spilling is often excessively expensive on GPUs, so HLSL
	requires all functions to be inlined during code generation, and does not
	support a runtime calling convention.

	Pointers & References
	---------------------

	HLSL does not support referring to values by address. Semantically all variables
	are value-types and behave as such. HLSL disallows the pointer dereference
	operators (unary ``*``, and ``->``), as well as the address of operator (unary
	&). While HLSL disallows pointers and references in the syntax, HLSL does use
	reference types in the AST, and we intend to use pointer decay in the AST in
	the Clang implementation.

	HLSL ``this`` Keyword
	---------------------

	HLSL does support member functions, and (in HLSL 2021) limited operator
	overloading. With member function support, HLSL also has a ``this`` keyword. The
	``this`` keyword is an example of one of the places where HLSL relies on
	references in the AST, because ``this`` is a reference.

	Bitshifts
	---------

	In deviation from C, HLSL bitshifts are defined to mask the shift count by the
	size of the type. In DXC, the semantics of LLVM IR were altered to accommodate
	this, in Clang we intend to generate the mask explicitly in the IR. In cases
	where the shift value is constant, this will be constant folded appropriately,
	in other cases we can clean it up in the DXIL target.

	Non-short Circuiting Logical Operators
	--------------------------------------

	In HLSL 2018 and earlier, HLSL supported logical operators (and the ternary
	operator) on vector types. This behavior required that operators not short
	circuit. The non-short circuiting behavior applies to all data types until HLSL
	2021. In HLSL 2021, logical and ternary operators do not support vector types
	instead builtin functions ``and``, ``or`` and ``select`` are available, and
	operators short circuit matching C behavior.

	Precise Qualifier
	-----------------

	HLSL has a ``precise`` qualifier that behaves unlike anything else in the C
	language. The support for this qualifier in DXC is buggy, so our bar for
	compatibility is low.

	The ``precise`` qualifier applies in the inverse direction from normal
	qualifiers. Rather than signifying that the declaration containing ``precise``
	qualifier be precise, it signifies that the operations contributing to the
	declaration's value be ``precise``. Additionally, ``precise`` is a misnomer:
	values attributed as ``precise`` comply with IEEE-754 floating point semantics,
	and are prevented from optimizations which could decrease or increase
	precision.

	Differences in Templates
	------------------------

	HLSL uses templates to define builtin types and methods, but disallowed
	user-defined templates until HLSL 2021. HLSL also allows omitting empty template
	parameter lists when all template parameters are defaulted. This is an ambiguous
	syntax in C++, but Clang detects the case and issues a diagnostic. This makes
	supporting the case in Clang minimally invasive.

	Vector Extensions
	-----------------

	HLSL uses the OpenCL vector extensions, and also provides C++-style constructors
	for vectors that are not supported by Clang.

	Standard Library
	----------------

	HLSL does not support the C or C++ standard libraries. Like OpenCL, HLSL
	describes its own library of built in types, complex data types, and functions.

	Unsupported C & C++ Features
	----------------------------

	HLSL does not support all features of C and C++. In implementing HLSL in Clang
	use of some C and C++ features will produce diagnostics under HLSL, and others
	will be supported as language extensions. In general, any C or C++ feature that
	can be supported by the DXIL and SPIR-V code generation targets could be treated
	as a clang HLSL extension. Features that cannot be lowered to DXIL or SPIR-V,
	must be diagnosed as errors.

	HLSL does not support the following C features:

	* Pointers
	* References
	* ``goto`` or labels
	* Variable Length Arrays
	* ``_Complex`` and ``_Imaginary``
	* C Threads or Atomics (or Obj-C blocks)
	* ``union`` types `(in progress for HLSL 202x) <https://github.com/microsoft/DirectXShaderCompiler/pull/4132>`_
	* Most features C11 and later

	HLSL does not support the following C++ features:

	* RTTI
	* Exceptions
	* Multiple inheritance
	* Access specifiers
	* Anonymous or inline namespaces
	* ``new`` & ``delete`` operators in all of their forms (array, placement, etc)
	* Constructors and destructors
	* Any use of the ``virtual`` keyword
	* Most features C++11 and later