[clang] HLSL availability diagnostics design doc (PR #92207)

[Xiang Li via cfe-commits]

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+=============================
+HLSL Availability Diagnostics
+=============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+HLSL availability diagnostics emits errors or warning when unavailable shader APIs are used. Unavailable shader APIs are APIs that are exposed in HLSL code but are not available in the target shader stage or shader model version.
+
+There are three modes of HLSL availability diagnostic:
+
+#. **Default mode** - compiler emits an error when an unavailable shader API is found in a code that is reachable from the shader entry point function or from an exported library function (when compiling a shader library)
+
+#. **Relaxed mode** - same as default mode except the compiler emits a warning. This mode is enabled by ``-Wno-error=hlsl-availability``.
+
+#. **Strict mode** - compiler emits an error when when an unavailable API is found in parsed code regardless of whether it can be reached from the shader entry point or exported functions, or not. This mode is enabled by ``-fhlsl-strict-diagnostics``.
+
+Implementation Details
+======================
+
+Environment Parameter
+---------------------
+
+In order to encode API availability based on the shader model version and shader model stage a new ``environment`` parameter was added to the existing Clang ``availability`` attribute. 
+
+The values allowed for this parameter are a subset of values allowed as the ``llvm::Triple`` environment component. If the environment parameters is present, the declared availability attribute applies only to targets with the same platform and environment.
+
+Default and Relaxed Diagnostic Modes
+------------------------------------
+
+This mode is implemeted in ``DiagnoseHLSLAvailability`` class in ``SemaHLSL.cpp`` and it is invoked after the whole translation unit is parsed (from ``Sema::ActOnEndOfTranslationUnit``). The implementation iterates over all shader entry points and exported library functions in the translation unit and performs an AST traversal of each function body.
+
+When a reference to another function is found and it has a body, the AST of the referenced function is also scanned. This chain of AST traversals will reach all of the code that is reachable from the initial shader entry point or exported library function.
+
+All shader APIs have an availability attribute that specifies the shader model version (and environment, if applicable) when this API was first introduced.When a reference to a function without a definition is found and it has an availability attribute, the version of the attribute is checked against the target shader model version and shader stage (if shader stage context is known), and an appropriate diagnostic is generated as needed.
+
+All shader entry functions have ``HLSLShaderAttr`` attribute that specifies what type of shader this function represents. However, for exported library functions the target shader stage is unknown, so in this case the HLSL API availability will be only checked against the shader model version.
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python3kgae wrote:

For an exported function used pixel shader only intrinsic, then linked with a pixel shader entry to create a pixel shader, will linker report same error?

https://github.com/llvm/llvm-project/pull/92207