add a spec for loop unroll (#263)

* add a spec for loop unroll

specs/language/statements.tex

@@ -1,2 +1,33 @@
 \Ch{Statements}{Stmt}
+\begin{grammar}
+    \define{statement}\br
+    labeled-statement\br
+    \opt{attribute-specifier-sequence} expression-statement\br
+    \opt{attribute-specifier-sequence} compound-statement\br
+    \opt{attribute-specifier-sequence} iteration-statement\br
+    \opt{attribute-specifier-sequence} selection-statement\br
+    declaration-statement
+\end{grammar}
 \Sec{Label Statements}{Stmt.Label}
+\p The optional \textit{attribute-specifier-sequence} applies to the statement 
+that immediately follows it.
+\Sec{Attributes}{Stmt.Attr}
+\Sub{Unroll Attribute}{Stmt.Attr.Unroll}
+\p The \textit{[unroll]}  attribute is only valid when applied to 
+\textit{iteration-statements}. It is used to indicate that 
+\textit{iteration-statements} like \texttt{for}, \texttt{while} and 
+\texttt{do while} can be unrolled. This attribute qualifier can be used to 
+specify full unrolling or partial unrolling by a specified amount. This is a 
+compiler hint  and the compiler may ignore this directive.
+
+\p The unroll attribute may optionally have an unroll factor represented as a 
+single argument \texttt{n}  that is an integer constant expression value 
+greater than zero. If n is not  specified, the compiler determines the 
+unrolling factor for the loop. The \textit{[unroll]}  attribute can not be 
+applied to the same \textit{iteration-statement} as the \texttt{[loop]} 
+attribute.
+
+\Sub{Loop Attribute}{Stmt.Attr.Loop}
+\p The Attribute \texttt{[loop]} tells the compiler to execute each iteration 
+of the loop. In other words, its a hint to indicate a loop should not be 
+unrolled. Therefore it is not compatible with the \texttt{[unroll]} attribute.