Added key_single to coding rules

latex/global/coding_rules.tex

@@ -612,6 +612,46 @@ definitions" to obtain the required numerical precision and range as well as the
 REAL(wp), DIMENSION(jpi,jpj,jpk) ::  zpc      ! power consumption
 \end{forlines}
 
+\subsubsection{The \key{single}}
+In the \path{./src/OCE/par_kind.F90} module the working precision
+for \forcode{REAL} variables is declared as follows:
+
+\begin{forlines}
+# if defined key_single
+INTEGER, PUBLIC, PARAMETER ::   wp = sp                              !: working precision
+# else
+INTEGER, PUBLIC, PARAMETER ::   wp = dp                              !: working precision
+# endif
+\end{forlines}
+The default is \forcode{dp}, so that all the real variables declared \forcode{wp} are assigned to be double precision.
+The \key{single} is available to switch the precision of the \forcode{wp} parameter
+to single precision. This does not mean that the code will be fully single precision,
+which can lead to instabilities in the model. In facts, the declaration of a set of variables
+has been hardcoded to \forcode{dp} and the key \key{single} enables to run in mixed-precision.
+Variables that need to stay double precision, no matter the value of the working precision are declared as:
+
+\begin{forlines}
+	REAL(dp), DIMENSION(jpi,jpj,jpk) ::  arr_name
+\end{forlines}
+This set of variables has been highlighted using the methodology described in Tinto et al. 2019. and this mixed-precision
+version of \NEMO is tested for the official configuration \forcode{ORCA2} compiled without the \forcode{ICE} module.
+Apart from changing the value of the \forcode{wp} parameter another macro will be activated by the usage of the \key{single}. Inside the
+\path{./src/OCE/single_precision_substitute.h90} two macros are defined:
+
+\begin{forlines}
+#if defined key_single
+#   define CASTSP(x) REAL(x,sp)
+#   define CASTDP(x) REAL(x,dp)
+#else
+#   define CASTSP(x) x
+#   define CASTDP(x) x
+#endif
+\end{forlines}
+When the \key{single} is used at compilation time all the variable
+to which this macro is applied will be put inside a cast, to double or single precision depending on the macro used.
+This is done to ensure coherence at compilation time between variables and dummy arguments in some function and subroutine calls.
+When the \key{single} is omitted the cast is no longer needed and the macro evaluates to the variable itself.
+
 \subsection{Structures}
 
 The \forcode{TYPE} structure allowing to declare some variables is more often used in \NEMO,