[clang] [HLSL][Docs] Add documentation for HLSL functions (PR #75397)

[Tex Riddell via cfe-commits]

================
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+===================
+HLSL Function Calls
+===================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document describes the design and implementation of HLSL's function call
+semantics in Clang. This includes details related to argument conversion and
+parameter lifetimes.
+
+This document does not seek to serve as official documentation for HLSL's
+call semantics, but does provide an overview to assist a reader. The
+authoritative documentation for HLSL's language semantics is the `draft language
+specification <https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf>`_.
+
+Argument Semantics
+==================
+
+In HLSL, all function arguments are passed by value in and out of functions.
+HLSL has 3 keywords which denote the parameter semantics (``in``, ``out`` and
+``inout``). In a function declaration a parameter may be annotated any of the
+following ways:
+
+#. <no parameter annotation> - denotes input
+#. ``in`` - denotes input
+#. ``out`` - denotes output
+#. ``in out`` - denotes input and output
+#. ``out in`` - denotes input and output
+#. ``inout`` - denotes input and output
+
+Parameters that are exclusively input behave like C/C++ parameters that are
+passed by value.
+
+For parameters that are output (or input and output), a temporary value is
+created in the caller. The temporary value is then passed by-address. For
+output-only parameters, the temporary is uninitialized when passed (it is
+undefined behavior to not explicitly initialize an ``out`` parameter inside a
+function). For input and output parameters, the temporary is initialized from
----------------
tex3d wrote:

> (it is undefined behavior to not explicitly initialize an ``out`` parameter inside a function)

Perhaps it's better to say that an out parameter has an undefined value if not initialized inside the function, rather than this being considered undefined **_behavior_**?

https://github.com/llvm/llvm-project/pull/75397