Add linkage definitions

specs/language/declarations.tex

@@ -1,8 +1,45 @@
 \Ch{Declarations}{Decl}
 \Sec{Function Definitions}{Decl.Function}
-\Sub{Visibility and Linkage}{Decl.Function.Visibility}
-
-\p Functions have \textit{internal linkage} by default.
-
 \Sec{Attributes}{Decl.Attr}
 \Sub{Entry Attributes}{Decl.Attr.Entry}
+\Sec{Linkage}{Decl.Linkage}
+
+\p An entity that denotes an object, reference, function, type, template, namespace, or value, may have a \textit{linkage}. If a name has \textit{linkage}, it refers to the same entity as the same name introduced by a declaration in another scope. If a variable, function, or another entity with the same name is declared in several scopes, but does not have sufficient \textit{linkage}, then several instances of the entity are generated.
+
+\p There are three linkages recognized: \textit{external linkage}, \textit{internal linkage} and \textit{no linkage}.
+
+\Sub{External Linkage}{Decl.Linkage.External}
+
+\p Entities with \textit{external linkage} can be referred to from the scopes in the other translation units and enable linking between them.
+
+\p The following entities in HLSL have \textit{external linkage}:
+\begin{itemize}
+    \item entry point functions
+    \item functions marked with \texttt{export} keyword
+    \item global variables that are not marked \texttt{static} or \texttt{groupshared} \footnote{These are not really linked with other translation units but rather their values are loaded indirectly based on cbuffer mapping.}
+    \item enumerations
+    \item classes, their member functions, static data members, nested classes and enumerations
+    \item template classes not declared \texttt{static}
+\end{itemize}
+
+\p Linkage of functions (including template functions) that are not entry points or marked with \texttt{export} keyword is implementation dependent. \footnote{In DXC today functions that are not entry points or exported have \textit{internal linkage} by default. This can be overriden by \texttt{-default-linkage} compiler option.}
+
+\Sub{Internal Linkage}{Decl.Linkage.Internal}
+
+\p Entities with \textit{internal linkage} can be referred to from all scopes in the current translation unit.
+
+\p The following entities in HLSL have \textit{internal linkage}:
+\begin{itemize}
+    \item global variables marked as \texttt{static} or \texttt{groupshared}
+\end{itemize}
+
+\Sub{No Linkage}{Decl.Linkage.NoLinkage}
+
+\p An entity with \textit{no linkage} can be referred to only from the scope it is in.
+
+\p Any of the following entites declared at block scope have no linkage:
+\begin{itemize}
+  \item local variables
+  \item local classes and their member functions
+  \item other names declared at block scope such as typedefs, enumerations, and enumerators.
+\end{itemize}