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\chapter{Code Generation}

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\section{Introduction}

\PLASMA uses code generation to streamline the writing of similar code for multiple data
types. This has been done in the past: NAG Fortran tools were used for LAPACK development
and Clint Whaley's Extract for ATLAS and BLACS. Other solutions include use of C
preprocessor in Goto BLAS and m4 macros in the p4 messaging system that eventually
became the basis of the MPICH 1.

After looking at these tools, the \PLASMA team decided to use a simpler solution:
a custom Python script that resides in \texttt{tools/codegen.py}

\section{Basic Usage}
The usual workflow for \PLASMA team when developing a new computational routine is
as follows:
\begin{enumerate}
\item Write and debug the routine using double precision real data type using arbitrary
tools (editors, compilers, etc.) without worrying about \PLASMA's development tools.
\item Convert the double precision real routine so it works with double precision complex
data type.
\item Use the \PLASMA's code generation script to generate single and double precision
real versions and single precision complex version.
\item Compare the result of conversion with the initial version done in step 1.
\end{enumerate}

Of course, this is a typical workflow so there may be others that are equally good.
However, as a rule \PLASMA team only maintains double precision complex version
of all the computational routines: the remaining three are automatically generated
with the code generation script.

The code generation step is done through the various Makefiles using
the \texttt{generate} rule and a simple line stating what conversions 
are to be done in the file comments.  The file's header comments should include a
directive~\ref{codegen:header} like this:
\begin{verbatim}
@precisions normal z -> c d s
\end{verbatim}
A typical invocation of the script is:
\begin{verbatim}
./codegen.py -f core_zblas.c
\end{verbatim}
As a result of the above command three files will be generated:
\begin{verbatim}
core\_sblas.c
core\_dblas.c
core\_cblas.c
\end{verbatim}

\section{Advanced Usage}
By taking advantage of precision generation in your functions, many intelligent compilation
features are uncovered.  This section addresses several tricks that can be applied to your code
to reduce the amount of code written and thusly, debugged.

The header directive~\ref{codegen:header} from above can even be adapted to allow conversions of any kind.  For example,
mixed precision files take advantage of this by using a different directive:
\begin{verbatim}
@precisions mixed zc -> ds
\end{verbatim}

\subsection{Complex Value Passing with \texttt{CBLAS\_SADDR}}
The \texttt{CBLAS\_SADDR()} macro helps in dealing with old C code such as CBLAS that
passes real scalars by value and complex scalars by address. Consider the matrix-matrix
multiply routines:
\begin{verbatim}
double real_alpha = 1.0;
double _Complex complex_alpha = 1.0:

/* pass by value */
cblas_dgemm(col_major, trans, trans, M, N, K, real_alpha, ... );

/* pass by address */
cblas_zgemm(col_major, trans, trans, M, N, K, &complex_alpha, ... );
\end{verbatim}

The double precision complex code in \PLASMA looks like this:
\begin{verbatim}
PLASMA_Complex64_t alpha = 1.0:

cblas_zgemm( ..., CBLAS_SADDR(alpha), ... );
\end{verbatim}
The code generation script will:
\begin{enumerate}
\item change \texttt{PLASMA\_Complex64\_t} to \texttt{PLASMA\_Complex32\_t}
for single precision complex version of the code.
\item change \texttt{PLASMA\_Complex64\_t} to \texttt{double}
for double precision real version of the code and will remove \texttt{CBLAS\_SADDR}.
\item change \texttt{PLASMA\_Complex64\_t} to \texttt{float}
for single precision real version of the code and will remove \texttt{CBLAS\_SADDR}.
\end{enumerate}

\texttt{CBLAS\_SADDR} is defined as a single argument macro that returns an address
of its argument so it will do the right thing for both complex versions of the code.

\subsection{Conditional Code Generation}
It is possible to generate code conditionally. For example Hermitian routines
only make sense for complex data type:
\begin{verbatim}
#ifdef COMPLEX
void CORE_zherk(int uplo, int trans,
                int N, int K,
                double alpha, PLASMA_Complex64_t *A, int LDA,
                double beta, PLASMA_Complex64_t *C, int LDC)
{
/* ... */
}
#endif
\end{verbatim}

On the other hand, routines specific to floating-point arithmetic make
sense only for real data types. When code generation occurs, {\tt \#ifdef COMPLEX}
becomes {\tt \#ifdef REAL}.  More importantly, this feature requires the following
lines in the original double precision complex version:
\begin{verbatim}
#undef REAL
#define COMPLEX
\end{verbatim}
After generation, these two lines become:
\begin{verbatim}
#undef COMPLEX
#define REAL
\end{verbatim}
So that this code works to conditionally inserting logic for real data types:
\begin{verbatim}
#ifdef REAL
double
BLAS_dfpinfo(enum blas_cmach_type cmach)
{
/* ... */
}
#endif
\end{verbatim}

\subsection{Code Dependent on Data Type}
Sometimes, the code for different data types needs to be different.
This can be coded in plain C without any preprocessor intervention.
Here is a sample:
\begin{verbatim}
if (sizeof(PLASMA_Complex64_t) == sizeof(double))
  tmult = 1; /* testing with real data types */
else
  tmult = 2; /* testing with complex data types */
\end{verbatim}
For complex versions the \texttt{else} branch of the \texttt{if}
statement is used but the compiler and \texttt{tmult} is set
to \texttt{2}. For single precision real version the code generating
script will produce:
\begin{verbatim}
if (sizeof(float) == sizeof(float))
  tmult = 1; /* testing with real data types */
else
  tmult = 2; /* testing with complex data types */
\end{verbatim}
and \texttt{tmult} will be set to \texttt{1}.
For double precision real version the code generating
script will produce:
\begin{verbatim}
if (sizeof(double) == sizeof(double))
  tmult = 1; /* testing with real data types */
else
  tmult = 2; /* testing with complex data types */
\end{verbatim}
and \texttt{tmult} will be set to \texttt{1} as well.

Introduction of \texttt{if} statements might have adverse effects
on performance. But modern compilers will likely remove the above
\texttt{if} statements because their conditional expression is known
compiled time.  If preferred, the same can be accomplished \textit{with} the
preprocessor using a technique similar to the previously mentioned method. 
In example:
\begin{verbatim}
#define DCOMPLEX
#ifdef DCOMPLEX
...
#endif
\end{verbatim}
or, similarly:
\begin{verbatim}
#define DCOMPLEX 1
if(DCOMPLEX){
	...
}
\end{verbatim}
Both of the above examples require only a single simple rule be added 
to the code generator substitution module~\ref{codegen:module}:
\begin{verbatim}
('SINGLE','DOUBLE','COMPLEX','DCOMPLEX')
\end{verbatim}

\section{Specifying Code Generation in Files}
\label{codegen:header}
A special keyword is used to enable code generation in your files.  A single line 
will indicate not only that generation is required, but what kind(s) of generation,
and which types should be done. 
\subsection{Forward Example}
The indicator line has a very specific structure
(explained in section \ref{codegen:headerdescription}).  The indicator line should be 
included in a front-closed comment line.
%% KEYWORD CONV_TYPE[,CONV\_TYPE]* ORIGIN\_TYPE -> OUTPUT\_TYPE[ OUTPUT\_TYPE]*
\begin{verbatim}
KEYWORD CONV_TYPE[,CONV_TYPE]* ORIGIN_TYPE -> OUTPUT_TYPE[ OUTPUT_TYPE]*
@precisions normal z -> c d s
@precisions normal,specialz z -> c
\end{verbatim}
The first line is the normal conversion line.  The second line uses a special conversion {\tt specialz}
and only goes from double complex to single complex.
\subsection{Header Description}
\label{codegen:headerdescription}
\begin{description}
\item[KEYWORD]
The keyword is {\tt @precisions} to work within Doxygen comments.
\item[CONV\_TYPE]
You can specify one or more conversion sets to be used.
\item[ORIGIN\_TYPE]
This is the origin type to use as the search needle for replacements. There can only be one.
\item[OUTPUT\_TYPE]
There can be one or more of these specified.  These are the precisions used as the output.
For each entry, either zero or one file will be generated. This is dependent on some replacement
causing a change in the original filename.
\end{description}

\section{Code Generator Substitution Module}
\label{codegen:module}
The substitution module specifies the rule types and substitutions for each
of the precision types during generation.  This module is called {\tt subs.py}
and is located in the {\tt tools} directory.  
\subsection{Forward Example}
Modules have a very specific structure
(explained in section \ref{codegen:moddescription}):
\begin{verbatim}
subs = {
  'all' : [ ## Special key
            ## Changes are applied to all applicable conversions automatically
    [None,None]
  ],
  'mixed' : [
    ['zc','ds'],
    ('PLASMA_Complex64_t','double'),
    ('PLASMA_Complex32_t','float'),
    ## This is a deletion on conversion from zc -> ds
    ('COMPLEXONLY',''), 
  ],
  'normal' : [
    ['s','d','c','z'],
    ('float','double','PLASMA_Complex32_t','PLASMA_Complex64_t'),
    ## There is no replacement here from z -> d
    ('NOTDOUBLE',None,'NOTDOUBLE','DOUBLE'), 
  ],
}
\end{verbatim}


\subsection{Description of Members}
\label{codegen:moddescription}
\begin{description}
\item[subs]
A dictionary listing all of the replacement types.
\item[subs['all']]
This is a special set of replacements executed on {\em all} files matching types in {\tt subs['all'][0]}.
\item[subs[x] or CONVERSION\_TYPE]
These are special sets of replacemnts for designation in the file generation header.
\item[subs[x][0]]
This is a special list specifying the conversion types.  These types are those used in the
header specification. For example this may be {\tt ['z','c','d','s']}.
\item[subs[x][1:n]]
These are tuples that are replacements made during generation.  They can be {\tt ''}, {\tt None}, or any
regular expression string.  These replacements are done using Python's regular expression engine.
If the replacement value is {\tt ''}, then the search needle is deleted from the haystack.
If the replacement value is {\tt None}, then no replacement is made.

\end{description}

	
