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-
-\chapter{The Verilog and AST Frontends}
-\label{chapter:verilog}
-
-This chapter provides an overview of the implementation of the Yosys Verilog
-and AST frontends. The Verilog frontend reads Verilog-2005 code and creates
-an abstract syntax tree (AST) representation of the input. This AST representation
-is then passed to the AST frontend that converts it to RTLIL data, as illustrated
-in Fig.~\ref{fig:Verilog_flow}.
-
-\begin{figure}[b!]
-	\hfil
-	\begin{tikzpicture}
-		\tikzstyle{process} = [draw, fill=green!10, rectangle, minimum height=3em, minimum width=10em, node distance=5em, font={\ttfamily}]
-		\tikzstyle{data} = [draw, fill=blue!10, ellipse, minimum height=3em, minimum width=7em, node distance=5em, font={\ttfamily}]
-
-		\node[data]    (n1)               {Verilog Source};
-		\node[process] (n2) [below of=n1] {Verilog Frontend};
-		\node[data]    (n3) [below of=n2] {AST};
-		\node[process] (n4) [below of=n3] {AST Frontend};
-		\node[data]    (n5) [below of=n4] {RTLIL};
-
-		\draw[-latex] (n1) -- (n2);
-		\draw[-latex] (n2) -- (n3);
-		\draw[-latex] (n3) -- (n4);
-		\draw[-latex] (n4) -- (n5);
-
-		\tikzstyle{details} = [draw, fill=yellow!5, rectangle, node distance=6cm, font={\ttfamily}]
-
-		\node[details] (d1) [right of=n2] {\begin{minipage}{5cm}
-			\hfil
-			\begin{tikzpicture}
-				\tikzstyle{subproc} = [draw, fill=green!10, rectangle, minimum height=2em, minimum width=10em, node distance=3em, font={\ttfamily}]
-				\node (s0) {};
-				\node[subproc] (s1) [below of=s0] {Preprocessor};
-				\node[subproc] (s2) [below of=s1] {Lexer};
-				\node[subproc] (s3) [below of=s2] {Parser};
-				\node[node distance=3em] (s4) [below of=s3] {};
-				\draw[-latex] (s0) -- (s1);
-				\draw[-latex] (s1) -- (s2);
-				\draw[-latex] (s2) -- (s3);
-				\draw[-latex] (s3) -- (s4);
-			\end{tikzpicture}
-		\end{minipage}};
-
-		\draw[dashed] (n2.north east) -- (d1.north west);
-		\draw[dashed] (n2.south east) -- (d1.south west);
-
-		\node[details] (d2) [right of=n4] {\begin{minipage}{5cm}
-			\hfil
-			\begin{tikzpicture}
-				\tikzstyle{subproc} = [draw, fill=green!10, rectangle, minimum height=2em, minimum width=10em, node distance=3em, font={\ttfamily}]
-				\node (s0) {};
-				\node[subproc] (s1) [below of=s0] {Simplifier};
-				\node[subproc] (s2) [below of=s1] {RTLIL Generator};
-				\node[node distance=3em] (s3) [below of=s2] {};
-				\draw[-latex] (s0) -- (s1);
-				\draw[-latex] (s1) -- (s2);
-				\draw[-latex] (s2) -- (s3);
-			\end{tikzpicture}
-		\end{minipage}};
-
-		\draw[dashed] (n4.north east) -- (d2.north west);
-		\draw[dashed] (n4.south east) -- (d2.south west);
-
-	\end{tikzpicture}
-	\caption{Simplified Verilog to RTLIL data flow}
-	\label{fig:Verilog_flow}
-\end{figure}
-
-
-\section{Transforming Verilog to AST}
-
-The {\it Verilog frontend} converts the Verilog sources to an internal AST representation that closely resembles
-the structure of the original Verilog code. The Verilog frontend consists of three components, the
-{\it Preprocessor}, the {\it Lexer} and the {\it Parser}.
-
-The source code to the Verilog frontend can be found in {\tt frontends/verilog/} in the Yosys source tree.
-
-\subsection{The Verilog Preprocessor}
-
-The Verilog preprocessor scans over the Verilog source code and interprets some of the Verilog compiler
-directives such as \lstinline[language=Verilog]{`include}, \lstinline[language=Verilog]{`define} and
-\lstinline[language=Verilog]{`ifdef}.
-
-It is implemented as a C++ function that is passed a file descriptor as input and returns the
-pre-processed Verilog code as a \lstinline[language=C++]{std::string}.
-
-The source code to the Verilog Preprocessor can be found in {\tt
-frontends/verilog/preproc.cc} in the Yosys source tree.
-
-\subsection{The Verilog Lexer}
-
-\begin{sloppypar}
-The Verilog Lexer is written using the lexer generator {\it flex} \citeweblink{flex}. Its source code
-can be found in {\tt frontends/verilog/verilog\_lexer.l} in the Yosys source tree.
-The lexer does little more than identifying all keywords and literals
-recognised by the Yosys Verilog frontend.
-\end{sloppypar}
-
-The lexer keeps track of the current location in the Verilog source code using
-some global variables. These variables are used by the constructor of AST nodes
-to annotate each node with the source code location it originated from.
-
-\begin{sloppypar}
-Finally the lexer identifies and handles special comments such as
-``\lstinline[language=Verilog]{// synopsys translate_off}'' and
-``\lstinline[language=Verilog]{// synopsys full_case}''. (It is recommended to
-use \lstinline[language=Verilog]{`ifdef} constructs instead of the Synsopsys
-translate\_on/off comments and attributes such as
-\lstinline[language=Verilog]{(* full_case *)} over ``\lstinline[language=Verilog]{// synopsys full_case}''
-whenever possible.)
-\end{sloppypar}
-
-\subsection{The Verilog Parser}
-
-The Verilog Parser is written using the parser generator {\it bison} \citeweblink{bison}. Its source code
-can be found in {\tt frontends/verilog/verilog\_parser.y} in the Yosys source tree.
-
-It generates an AST using the \lstinline[language=C++]{AST::AstNode} data structure
-defined in {\tt frontends/ast/ast.h}. An \lstinline[language=C++]{AST::AstNode} object has
-the following properties:
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-
-\begin{table}[b!]
-\hfil
-\begin{tabular}{>{\raggedright\arraybackslash}p{7cm}>{\raggedright\arraybackslash}p{8cm}}
-AST Node Type & Corresponding Verilog Construct \\
-\hline
-\hline
-\arrayrulecolor{gray}
-{\tt AST\_NONE} & This Node type should never be used. \\
-\hline
-%
-{\tt AST\_DESIGN} & This node type is used for the top node of the AST tree. It
-has no corresponding Verilog construct. \\
-\hline
-%
-{\tt AST\_MODULE},
-{\tt AST\_TASK},
-{\tt AST\_FUNCTION} &
-\lstinline[language=Verilog];module;,
-\lstinline[language=Verilog];task; and
-\lstinline[language=Verilog];function; \\
-\hline
-%
-{\tt AST\_WIRE} &
-\lstinline[language=Verilog];input;,
-\lstinline[language=Verilog];output;,
-\lstinline[language=Verilog];wire;,
-\lstinline[language=Verilog];reg; and
-\lstinline[language=Verilog];integer; \\
-\hline
-%
-{\tt AST\_MEMORY}  &
-Verilog Arrays \\
-\hline
-%
-{\tt AST\_AUTOWIRE} &
-Created by the simplifier when an undeclared signal name is used. \\
-\hline
-%
-{\tt AST\_PARAMETER},
-{\tt AST\_LOCALPARAM} &
-\lstinline[language=Verilog];parameter; and
-\lstinline[language=Verilog];localparam; \\
-\hline
-%
-{\tt AST\_PARASET} &
-Parameter set in cell instantiation \\
-\hline
-%
-{\tt AST\_ARGUMENT} &
-Port connection in cell instantiation \\
-\hline
-%
-{\tt AST\_RANGE} &
-Bit-Index in a signal or element index in array \\
-\hline
-%
-{\tt AST\_CONSTANT} &
-A literal value \\
-\hline
-%
-{\tt AST\_CELLTYPE} &
-The type of cell in cell instantiation \\
-\hline
-%
-{\tt AST\_IDENTIFIER} &
-An Identifier (signal name in expression or cell/task/etc. name in other contexts) \\
-\hline
-%
-{\tt AST\_PREFIX} &
-Construct an identifier in the form {\tt <prefix>[<index>].<suffix>} (used only in
-advanced generate constructs) \\
-\hline
-%
-{\tt AST\_FCALL},
-{\tt AST\_TCALL} &
-Call to function or task \\
-\hline
-%
-{\tt AST\_TO\_SIGNED},
-{\tt AST\_TO\_UNSIGNED} &
-The \lstinline[language=Verilog];$signed(); and
-\lstinline[language=Verilog];$unsigned(); functions \\
-\hline
-\end{tabular}
-\caption{AST node types with their corresponding Verilog constructs. \\ (continued on next page)}
-\label{tab:Verilog_AstNodeType}
-\end{table}
-
-\begin{table}[t!]
-\ContinuedFloat
-\hfil
-\begin{tabular}{>{\raggedright\arraybackslash}p{7cm}>{\raggedright\arraybackslash}p{8cm}}
-AST Node Type & Corresponding Verilog Construct \\
-\hline
-\hline
-\arrayrulecolor{gray}
-{\tt AST\_CONCAT}
-{\tt AST\_REPLICATE} &
-The \lstinline[language=Verilog];{...}; and
-\lstinline[language=Verilog];{...{...}}; operators \\
-\hline
-%
-{\tt AST\_BIT\_NOT},
-{\tt AST\_BIT\_AND},
-{\tt AST\_BIT\_OR},
-{\tt AST\_BIT\_XOR},
-{\tt AST\_BIT\_XNOR} &
-The bitwise operators \break
-\lstinline[language=Verilog];~;,
-\lstinline[language=Verilog];&;,
-\lstinline[language=Verilog];|;,
-\lstinline[language=Verilog];^; and
-\lstinline[language=Verilog];~^; \\
-\hline
-%
-{\tt AST\_REDUCE\_AND},
-{\tt AST\_REDUCE\_OR},
-{\tt AST\_REDUCE\_XOR},
-{\tt AST\_REDUCE\_XNOR} &
-The unary reduction operators \break
-\lstinline[language=Verilog];~;,
-\lstinline[language=Verilog];&;,
-\lstinline[language=Verilog];|;,
-\lstinline[language=Verilog];^; and
-\lstinline[language=Verilog];~^; \\
-\hline
-%
-{\tt AST\_REDUCE\_BOOL} &
-Conversion from multi-bit value to boolean value
-(equivalent to {\tt AST\_REDUCE\_OR}) \\
-\hline
-%
-{\tt AST\_SHIFT\_LEFT},
-{\tt AST\_SHIFT\_RIGHT},
-{\tt AST\_SHIFT\_SLEFT},
-{\tt AST\_SHIFT\_SRIGHT} &
-The shift operators \break
-\lstinline[language=Verilog];<<;,
-\lstinline[language=Verilog];>>;,
-\lstinline[language=Verilog];<<<; and
-\lstinline[language=Verilog];>>>; \\
-\hline
-%
-{\tt AST\_LT},
-{\tt AST\_LE},
-{\tt AST\_EQ},
-{\tt AST\_NE},
-{\tt AST\_GE},
-{\tt AST\_GT} &
-The relational operators \break
-\lstinline[language=Verilog];<;,
-\lstinline[language=Verilog];<=;,
-\lstinline[language=Verilog];==;,
-\lstinline[language=Verilog];!=;,
-\lstinline[language=Verilog];>=; and
-\lstinline[language=Verilog];>; \\
-\hline
-%
-{\tt AST\_ADD},
-{\tt AST\_SUB},
-{\tt AST\_MUL},
-{\tt AST\_DIV},
-{\tt AST\_MOD},
-{\tt AST\_POW} &
-The binary operators \break
-\lstinline[language=Verilog];+;,
-\lstinline[language=Verilog];-;,
-\lstinline[language=Verilog];*;,
-\lstinline[language=Verilog];/;,
-\lstinline[language=Verilog];%; and
-\lstinline[language=Verilog];**; \\
-\hline
-%
-{\tt AST\_POS},
-{\tt AST\_NEG} &
-The prefix operators
-\lstinline[language=Verilog];+; and
-\lstinline[language=Verilog];-; \\
-\hline
-%
-{\tt AST\_LOGIC\_AND},
-{\tt AST\_LOGIC\_OR},
-{\tt AST\_LOGIC\_NOT} &
-The logic operators
-\lstinline[language=Verilog];&&;,
-\lstinline[language=Verilog];||; and
-\lstinline[language=Verilog];!; \\
-\hline
-%
-{\tt AST\_TERNARY} &
-The ternary \lstinline[language=Verilog];?:;-operator \\
-\hline
-%
-{\tt AST\_MEMRD}
-{\tt AST\_MEMWR} &
-Read and write memories. These nodes are generated by
-the AST simplifier for writes/reads to/from Verilog arrays. \\
-\hline
-%
-{\tt AST\_ASSIGN} &
-An \lstinline[language=Verilog];assign; statement \\
-\hline
-%
-{\tt AST\_CELL} &
-A cell instantiation \\
-\hline
-%
-{\tt AST\_PRIMITIVE} &
-A primitive cell (\lstinline[language=Verilog];and;,
-\lstinline[language=Verilog];nand;,
-\lstinline[language=Verilog];or;, etc.) \\
-\hline
-%
-{\tt AST\_ALWAYS},
-{\tt AST\_INITIAL} &
-Verilog \lstinline[language=Verilog];always;- and \lstinline[language=Verilog];initial;-blocks \\
-\hline
-%
-{\tt AST\_BLOCK} &
-A \lstinline[language=Verilog];begin;-\lstinline[language=Verilog];end;-block \\
-\hline
-%
-{\tt AST\_ASSIGN\_EQ}.
-{\tt AST\_ASSIGN\_LE} &
-Blocking (\lstinline[language=Verilog];=;) and nonblocking (\lstinline[language=Verilog];<=;)
-assignments within an \lstinline[language=Verilog];always;- or \lstinline[language=Verilog];initial;-block \\
-\hline
-%
-{\tt AST\_CASE}.
-{\tt AST\_COND},
-{\tt AST\_DEFAULT} &
-The \lstinline[language=Verilog];case; (\lstinline[language=Verilog];if;) statements, conditions within a case
-and the default case respectively \\
-\hline
-%
-{\tt AST\_FOR} &
-A \lstinline[language=Verilog];for;-loop with an
-\lstinline[language=Verilog];always;- or
-\lstinline[language=Verilog];initial;-block \\
-\hline
-%
-{\tt AST\_GENVAR},
-{\tt AST\_GENBLOCK},
-{\tt AST\_GENFOR},
-{\tt AST\_GENIF} &
-The \lstinline[language=Verilog];genvar; and
-\lstinline[language=Verilog];generate; keywords and
-\lstinline[language=Verilog];for; and \lstinline[language=Verilog];if; within a
-generate block. \\
-\hline
-%
-{\tt AST\_POSEDGE},
-{\tt AST\_NEGEDGE},
-{\tt AST\_EDGE} &
-Event conditions for \lstinline[language=Verilog];always; blocks. \\
-\hline
-\end{tabular}
-\caption{AST node types with their corresponding Verilog constructs. \\ (continuation from previous page)}
-\label{tab:Verilog_AstNodeTypeCont}
-\end{table}
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-
-\begin{itemize}
-\item {\bf The node type} \\
-This enum (\lstinline[language=C++]{AST::AstNodeType}) specifies the role of the node.
-Table~\ref{tab:Verilog_AstNodeType} contains a list of all node types.
-\item {\bf The child nodes} \\
-This is a list of pointers to all children in the abstract syntax tree.
-\item {\bf Attributes} \\
-As almost every AST node might have Verilog attributes assigned to it, the
-\lstinline[language=C++]{AST::AstNode} has direct support for attributes. Note that the
-attribute values are again AST nodes.
-\item {\bf Node content} \\
-Each node might have additional content data. A series of member variables exist to hold such data.
-For example the member \lstinline[language=C++]{std::string str} can hold a string value and is
-used e.g.~in the {\tt AST\_IDENTIFIER} node type to store the identifier name.
-\item {\bf Source code location} \\
-Each \lstinline[language=C++]{AST::AstNode} is automatically annotated with the current
-source code location by the \lstinline[language=C++]{AST::AstNode} constructor. It is
-stored in the \lstinline[language=C++]{std::string filename} and \lstinline[language=C++]{int linenum}
-member variables.
-\end{itemize}
-
-The \lstinline[language=C++]{AST::AstNode} constructor can be called with up to
-two child nodes that are automatically added to the list of child nodes for the new object.
-This simplifies the creation of AST nodes for simple expressions a bit. For example the bison
-code for parsing multiplications:
-
-\begin{lstlisting}[numbers=left,frame=single]
-        basic_expr '*' attr basic_expr {
-                $$ = new AstNode(AST_MUL, $1, $4);
-                append_attr($$, $3);
-        } |
-\end{lstlisting}
-
-The generated AST data structure is then passed directly to the AST frontend
-that performs the actual conversion to RTLIL.
-
-Note that the Yosys command {\tt read\_verilog} provides the options {\tt -yydebug}
-and {\tt -dump\_ast} that can be used to print the parse tree or abstract syntax tree
-respectively.
-
-\section{Transforming AST to RTLIL}
-
-The {\it AST Frontend} converts a set of modules in AST representation to
-modules in RTLIL representation and adds them to the current design. This is done
-in two steps: {\it simplification} and {\it RTLIL generation}.
-
-The source code to the AST frontend can be found in {\tt frontends/ast/} in the Yosys source tree.
-
-\subsection{AST Simplification}
-
-A full-featured AST is too complex to be transformed into RTLIL directly. Therefore it must
-first be brought into a simpler form. This is done by calling the \lstinline[language=C++]{AST::AstNode::simplify()}
-method of all {\tt AST\_MODULE} nodes in the AST. This initiates a recursive process that performs the following transformations
-on the AST data structure:
-
-\begin{itemize}
-\item Inline all task and function calls.
-\item Evaluate all \lstinline[language=Verilog]{generate}-statements and unroll all \lstinline[language=Verilog]{for}-loops.
-\item Perform const folding where it is necessary (e.g.~in the value part of {\tt AST\_PARAMETER}, {\tt AST\_LOCALPARAM},
-{\tt AST\_PARASET} and {\tt AST\_RANGE} nodes).
-\item Replace {\tt AST\_PRIMITIVE} nodes with appropriate {\tt AST\_ASSIGN} nodes.
-\item Replace dynamic bit ranges in the left-hand-side of assignments with {\tt AST\_CASE} nodes with {\tt AST\_COND} children
-for each possible case.
-\item Detect array access patterns that are too complicated for the {\tt RTLIL::Memory} abstraction and replace them
-with a set of signals and cases for all reads and/or writes.
-\item Otherwise replace array accesses with {\tt AST\_MEMRD} and {\tt AST\_MEMWR} nodes.
-\end{itemize}
-
-In addition to these transformations, the simplifier also annotates the AST with additional information that is needed
-for the RTLIL generator, namely:
-
-\begin{itemize}
-\item All ranges (width of signals and bit selections) are not only const folded but (when a constant value
-is found) are also written to member variables in the {\tt AST\_RANGE} node.
-\item All identifiers are resolved and all {\tt AST\_IDENTIFIER} nodes are annotated with a pointer to the AST node
-that contains the declaration of the identifier. If no declaration has been found, an {\tt AST\_AUTOWIRE} node
-is created and used for the annotation.
-\end{itemize}
-
-This produces an AST that is fairly easy to convert to the RTLIL format.
-
-\subsection{Generating RTLIL}
-
-After AST simplification, the \lstinline[language=C++]{AST::AstNode::genRTLIL()} method of each {\tt AST\_MODULE} node
-in the AST is called. This initiates a recursive process that generates equivalent RTLIL data for the AST data.
-
-The \lstinline[language=C++]{AST::AstNode::genRTLIL()} method returns an \lstinline[language=C++]{RTLIL::SigSpec} structure.
-For nodes that represent expressions (operators, constants, signals, etc.), the cells needed to implement the calculation
-described by the expression are created and the resulting signal is returned. That way it is easy to generate the circuits
-for large expressions using depth-first recursion. For nodes that do not represent an expression (such as {\tt
-AST\_CELL}), the corresponding circuit is generated and an empty \lstinline[language=C++]{RTLIL::SigSpec} is returned.
-
-\section{Synthesizing Verilog always Blocks}
-
-For behavioural Verilog code (code utilizing \lstinline[language=Verilog]{always}- and
-\lstinline[language=Verilog]{initial}-blocks) it is necessary to also generate \lstinline[language=C++]{RTLIL::Process}
-objects. This is done in the following way:
-
-\begin{itemize}
-\item Whenever \lstinline[language=C++]{AST::AstNode::genRTLIL()} encounters an \lstinline[language=Verilog]{always}-
-or \lstinline[language=Verilog]{initial}-block, it creates an instance of
-\lstinline[language=Verilog]{AST_INTERNAL::ProcessGenerator}. This object then generates the
-\lstinline[language=C++]{RTLIL::Process} object for the block. It also calls \lstinline[language=C++]{AST::AstNode::genRTLIL()}
-for all right-hand-side expressions contained within the block.
-%
-\begin{sloppypar}
-\item First the  \lstinline[language=Verilog]{AST_INTERNAL::ProcessGenerator} creates a list of all signals assigned
-within the block. It then creates a set of temporary signals using the naming scheme {\tt \$\it<number>\tt
-\textbackslash\it <original\_name>} for each of the assigned signals.
-\end{sloppypar}
-%
-\item Then an \lstinline[language=C++]{RTLIL::Process} is created that assigns all intermediate values for each left-hand-side
-signal to the temporary signal in its \lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule} tree.
-%
-\item Finally a \lstinline[language=C++]{RTLIL::SyncRule} is created for the \lstinline[language=C++]{RTLIL::Process} that
-assigns the temporary signals for the final values to the actual signals.
-%
-\item A process may also contain memory writes. A \lstinline[language=C++]{RTLIL::MemWriteAction} is created for each of them.
-%
-\item Calls to \lstinline[language=C++]{AST::AstNode::genRTLIL()} are generated for right hand sides as needed. When blocking
-assignments are used, \lstinline[language=C++]{AST::AstNode::genRTLIL()} is configured using global variables to use
-the temporary signals that hold the correct intermediate values whenever one of the previously assigned signals is used
-in an expression.
-\end{itemize}
-
-Unfortunately the generation of a correct \lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule}
-tree for behavioural code is a non-trivial task. The AST frontend solves the problem using the approach described on the following
-pages. The following example illustrates what the algorithm is supposed to do. Consider the following Verilog code:
-
-\begin{lstlisting}[numbers=left,frame=single,language=Verilog]
-always @(posedge clock) begin
-	out1 = in1;
-	if (in2)
-		out1 = !out1;
-	out2 <= out1;
-	if (in3)
-		out2 <= out2;
-	if (in4)
-		if (in5)
-			out3 <= in6;
-		else
-			out3 <= in7;
-	out1 = out1 ^ out2;
-end
-\end{lstlisting}
-
-This is translated by the Verilog and AST frontends into the following RTLIL code (attributes, cell parameters
-and wire declarations not included):
-
-\begin{lstlisting}[numbers=left,frame=single,language=rtlil]
-cell $logic_not $logic_not$<input>:4$2
-  connect \A \in1
-  connect \Y $logic_not$<input>:4$2_Y
-end
-cell $xor $xor$<input>:13$3
-  connect \A $1\out1[0:0]
-  connect \B \out2
-  connect \Y $xor$<input>:13$3_Y
-end
-process $proc$<input>:1$1
-  assign $0\out3[0:0] \out3
-  assign $0\out2[0:0] $1\out1[0:0]
-  assign $0\out1[0:0] $xor$<input>:13$3_Y
-  switch \in2
-    case 1'1
-      assign $1\out1[0:0] $logic_not$<input>:4$2_Y
-    case
-      assign $1\out1[0:0] \in1
-  end
-  switch \in3
-    case 1'1
-      assign $0\out2[0:0] \out2
-    case
-  end
-  switch \in4
-    case 1'1
-      switch \in5
-        case 1'1
-          assign $0\out3[0:0] \in6
-        case
-          assign $0\out3[0:0] \in7
-      end
-    case
-  end
-  sync posedge \clock
-    update \out1 $0\out1[0:0]
-    update \out2 $0\out2[0:0]
-    update \out3 $0\out3[0:0]
-end
-\end{lstlisting}
-
-Note that the two operators are translated into separate cells outside the generated process. The signal
-\lstinline[language=Verilog]{out1} is assigned using blocking assignments and therefore \lstinline[language=Verilog]{out1}
-has been replaced with a different signal in all expressions after the initial assignment. The signal
-\lstinline[language=Verilog]{out2} is assigned using nonblocking assignments and therefore is not substituted
-on the right-hand-side expressions.
-
-The \lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule}
-tree must be interpreted the following way:
-
-\begin{itemize}
-\item On each case level (the body of the process is the {\it root case}), first the actions on this level are
-evaluated and then the switches within the case are evaluated. (Note that the last assignment on line 13 of the
-Verilog code has been moved to the beginning of the RTLIL process to line 13 of the RTLIL listing.)
-
-I.e.~the special cases deeper in the switch hierarchy override the defaults on the upper levels. The assignments
-in lines 12 and 22 of the RTLIL code serve as an example for this.
-
-Note that in contrast to this, the order within the \lstinline[language=C++]{RTLIL::SwitchRule} objects
-within a \lstinline[language=C++]{RTLIL::CaseRule} is preserved with respect to the original AST and
-Verilog code.
-%
-\item \begin{sloppypar}
-The whole \lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule} tree
-describes an asynchronous circuit. I.e.~the decision tree formed by the switches can be seen independently for
-each assigned signal. Whenever one assigned signal changes, all signals that depend on the changed signals
-are to be updated. For example the assignments in lines 16 and 18 in the RTLIL code in fact influence the assignment
-in line 12, even though they are in the ``wrong order''.
-\end{sloppypar}
-\end{itemize}
-
-The only synchronous part of the process is in the \lstinline[language=C++]{RTLIL::SyncRule} object generated at line
-35 in the RTLIL code. The sync rule is the only part of the process where the original signals are assigned. The
-synchronization event from the original Verilog code has been translated into the synchronization type ({\tt posedge})
-and signal ({\tt \textbackslash clock}) for the \lstinline[language=C++]{RTLIL::SyncRule} object. In the case of
-this simple example the \lstinline[language=C++]{RTLIL::SyncRule} object is later simply transformed into a set of
-d-type flip-flops and the \lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule} tree
-to a decision tree using multiplexers.
-
-\begin{sloppypar}
-In more complex examples (e.g.~asynchronous resets) the part of the
-\lstinline[language=C++]{RTLIL::CaseRule}/\lstinline[language=C++]{RTLIL::SwitchRule}
-tree that describes the asynchronous reset must first be transformed to the
-correct \lstinline[language=C++]{RTLIL::SyncRule} objects. This is done by the {\tt proc\_adff} pass.
-\end{sloppypar}
-
-\subsection{The ProcessGenerator Algorithm}
-
-The \lstinline[language=C++]{AST_INTERNAL::ProcessGenerator} uses the following internal state variables:
-
-\begin{itemize}
-\item \begin{sloppypar}
-\lstinline[language=C++]{subst_rvalue_from} and \lstinline[language=C++]{subst_rvalue_to} \\
-These two variables hold the replacement pattern that should be used by \lstinline[language=C++]{AST::AstNode::genRTLIL()}
-for signals with blocking assignments. After initialization of \lstinline[language=C++]{AST_INTERNAL::ProcessGenerator}
-these two variables are empty.
-\end{sloppypar}
-%
-\item \lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to} \\
-These two variables contain the mapping from left-hand-side signals ({\tt \textbackslash \it <name>}) to the current
-temporary signal for the same thing (initially {\tt \$0\textbackslash \it <name>}).
-%
-\item \lstinline[language=C++]{current_case} \\
-A pointer to a \lstinline[language=C++]{RTLIL::CaseRule} object. Initially this is the root case of the
-generated \lstinline[language=C++]{RTLIL::Process}.
-\end{itemize}
-
-As the algorithm runs these variables are continuously modified as well as pushed
-to the stack and later restored to their earlier values by popping from the stack.
-
-On startup the ProcessGenerator generates a new
-\lstinline[language=C++]{RTLIL::Process} object with an empty root case and
-initializes its state variables as described above. Then the \lstinline[language=C++]{RTLIL::SyncRule} objects
-are created using the synchronization events from the {\tt AST\_ALWAYS} node and the initial values of
-\lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to}. Then the
-AST for this process is evaluated recursively.
-
-During this recursive evaluation, three different relevant types of AST nodes can be discovered:
-{\tt AST\_ASSIGN\_LE} (nonblocking assignments), {\tt AST\_ASSIGN\_EQ} (blocking assignments) and
-{\tt AST\_CASE} (\lstinline[language=Verilog]{if} or \lstinline[language=Verilog]{case} statement).
-
-\subsubsection{Handling of Nonblocking Assignments}
-
-When an {\tt AST\_ASSIGN\_LE} node is discovered, the following actions are performed by the
-ProcessGenerator:
-
-\begin{itemize}
-\item The left-hand-side is evaluated using \lstinline[language=C++]{AST::AstNode::genRTLIL()} and mapped to
-a temporary signal name using \lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to}.
-%
-\item The right-hand-side is evaluated using \lstinline[language=C++]{AST::AstNode::genRTLIL()}. For this call,
-the values of \lstinline[language=C++]{subst_rvalue_from} and \lstinline[language=C++]{subst_rvalue_to} are used to
-map blocking-assigned signals correctly.
-%
-\item Remove all assignments to the same left-hand-side as this assignment from the \lstinline[language=C++]{current_case}
-and all cases within it.
-%
-\item Add the new assignment to the \lstinline[language=C++]{current_case}.
-\end{itemize}
-
-\subsubsection{Handling of Blocking Assignments}
-
-When an {\tt AST\_ASSIGN\_EQ} node is discovered, the following actions are performed by
-the ProcessGenerator:
-
-\begin{itemize}
-\item Perform all the steps that would be performed for a nonblocking assignment (see above).
-%
-\item Remove the found left-hand-side (before lvalue mapping) from
-\lstinline[language=C++]{subst_rvalue_from} and also remove the respective
-bits from \lstinline[language=C++]{subst_rvalue_to}.
-%
-\item Append the found left-hand-side (before lvalue mapping) to \lstinline[language=C++]{subst_rvalue_from}
-and append the found right-hand-side to \lstinline[language=C++]{subst_rvalue_to}.
-\end{itemize}
-
-\subsubsection{Handling of Cases and if-Statements}
-
-\begin{sloppypar}
-When an {\tt AST\_CASE} node is discovered, the following actions are performed by
-the ProcessGenerator:
-
-\begin{itemize}
-\item The values of \lstinline[language=C++]{subst_rvalue_from}, \lstinline[language=C++]{subst_rvalue_to},
-\lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to} are pushed to the stack.
-%
-\item A new \lstinline[language=C++]{RTLIL::SwitchRule} object is generated, the selection expression is evaluated using
-\lstinline[language=C++]{AST::AstNode::genRTLIL()} (with the use of \lstinline[language=C++]{subst_rvalue_from} and
-\lstinline[language=C++]{subst_rvalue_to}) and added to the \lstinline[language=C++]{RTLIL::SwitchRule} object and the
-object is added to the \lstinline[language=C++]{current_case}.
-%
-\item All lvalues assigned to within the {\tt AST\_CASE} node using blocking assignments are collected and
-saved in the local variable \lstinline[language=C++]{this_case_eq_lvalue}.
-%
-\item New temporary signals are generated for all signals in \lstinline[language=C++]{this_case_eq_lvalue} and stored
-in \lstinline[language=C++]{this_case_eq_ltemp}.
-%
-\item The signals in \lstinline[language=C++]{this_case_eq_lvalue} are mapped using \lstinline[language=C++]{subst_rvalue_from}
-and \lstinline[language=C++]{subst_rvalue_to} and the resulting set of signals is stored in
-\lstinline[language=C++]{this_case_eq_rvalue}.
-\end{itemize}
-
-Then the following steps are performed for each {\tt AST\_COND} node within the {\tt AST\_CASE} node:
-
-\begin{itemize}
-\item Set \lstinline[language=C++]{subst_rvalue_from}, \lstinline[language=C++]{subst_rvalue_to},
-\lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to} to the values
-that have been pushed to the stack.
-%
-\item Remove \lstinline[language=C++]{this_case_eq_lvalue} from
-\lstinline[language=C++]{subst_lvalue_from}/\lstinline[language=C++]{subst_lvalue_to}.
-%
-\item Append \lstinline[language=C++]{this_case_eq_lvalue} to \lstinline[language=C++]{subst_lvalue_from} and append
-\lstinline[language=C++]{this_case_eq_ltemp} to \lstinline[language=C++]{subst_lvalue_to}.
-%
-\item Push the value of \lstinline[language=C++]{current_case}.
-%
-\item Create a new \lstinline[language=C++]{RTLIL::CaseRule}. Set \lstinline[language=C++]{current_case} to the
-new object and add the new object to the \lstinline[language=C++]{RTLIL::SwitchRule} created above.
-%
-\item Add an assignment from \lstinline[language=C++]{this_case_eq_rvalue} to \lstinline[language=C++]{this_case_eq_ltemp}
-to the new \lstinline[language=C++]{current_case}.
-%
-\item Evaluate the compare value for this case using \lstinline[language=C++]{AST::AstNode::genRTLIL()} (with the use of
-\lstinline[language=C++]{subst_rvalue_from} and \lstinline[language=C++]{subst_rvalue_to}) modify the new
-\lstinline[language=C++]{current_case} accordingly.
-%
-\item Recursion into the children of the {\tt AST\_COND} node.
-%
-\item Restore \lstinline[language=C++]{current_case} by popping the old value from the stack.
-\end{itemize}
-
-Finally the following steps are performed:
-
-\begin{itemize}
-\item The values of \lstinline[language=C++]{subst_rvalue_from}, \lstinline[language=C++]{subst_rvalue_to},
-\lstinline[language=C++]{subst_lvalue_from} and \lstinline[language=C++]{subst_lvalue_to} are popped from the stack.
-%
-\item The signals from \lstinline[language=C++]{this_case_eq_lvalue} are removed from the
-\lstinline[language=C++]{subst_rvalue_from}/\lstinline[language=C++]{subst_rvalue_to}-pair.
-%
-\item The value of \lstinline[language=C++]{this_case_eq_lvalue} is appended to \lstinline[language=C++]{subst_rvalue_from}
-and the value of \lstinline[language=C++]{this_case_eq_ltemp} is appended to \lstinline[language=C++]{subst_rvalue_to}.
-%
-\item Map the signals in \lstinline[language=C++]{this_case_eq_lvalue} using
-\lstinline[language=C++]{subst_lvalue_from}/\lstinline[language=C++]{subst_lvalue_to}.
-%
-\item Remove all assignments to signals in \lstinline[language=C++]{this_case_eq_lvalue} in \lstinline[language=C++]{current_case}
-and all cases within it.
-%
-\item Add an assignment from \lstinline[language=C++]{this_case_eq_ltemp} to \lstinline[language=C++]{this_case_eq_lvalue}
-to \lstinline[language=C++]{current_case}.
-\end{itemize}
-\end{sloppypar}
-
-\subsubsection{Further Analysis of the Algorithm for Cases and if-Statements}
-
-With respect to nonblocking assignments the algorithm is easy: later assignments invalidate earlier assignments.
-For each signal assigned using nonblocking assignments exactly one temporary variable is generated (with the
-{\tt \$0}-prefix) and this variable is used for all assignments of the variable.
-
-Note how all the \lstinline[language=C++]{_eq_}-variables become empty when no blocking assignments are used
-and many of the steps in the algorithm can then be ignored as a result of this.
-
-For a variable with blocking assignments the algorithm shows the following behaviour: First a new temporary variable
-is created. This new temporary variable is then registered as the assignment target for all assignments for this
-variable within the cases for this {\tt AST\_CASE} node. Then for each case the new temporary variable is first
-assigned the old temporary variable. This assignment is overwritten if the variable is actually assigned in this
-case and is kept as a default value otherwise.
-
-This yields an \lstinline[language=C++]{RTLIL::CaseRule} that assigns the new temporary variable in all branches.
-So when all cases have been processed a final assignment is added to the containing block that assigns the new
-temporary variable to the old one. Note how this step always overrides a previous assignment to the old temporary
-variable. Other than nonblocking assignments, the old assignment could still have an effect somewhere
-in the design, as there have been calls to \lstinline[language=C++]{AST::AstNode::genRTLIL()} with a
-\lstinline[language=C++]{subst_rvalue_from}/\lstinline[language=C++]{subst_rvalue_to}-tuple that contained
-the right-hand-side of the old assignment.
-
-\subsection{The proc pass}
-
-The ProcessGenerator converts a behavioural model in AST representation to a behavioural model in
-\lstinline[language=C++]{RTLIL::Process} representation. The actual conversion from a behavioural
-model to an RTL representation is performed by the {\tt proc} pass and the passes it launches:
-
-\begin{itemize}
-\item {\tt proc\_clean} and {\tt proc\_rmdead} \\
-These two passes just clean up the \lstinline[language=C++]{RTLIL::Process} structure. The {\tt proc\_clean}
-pass removes empty parts (eg. empty assignments) from the process and {\tt proc\_rmdead} detects and removes
-unreachable branches from the process's decision trees.
-%
-\item {\tt proc\_arst} \\
-This pass detects processes that describe d-type flip-flops with asynchronous
-resets and rewrites the process to better reflect what they are modelling:
-Before this pass, an asynchronous reset has two edge-sensitive sync rules and
-one top-level \C{RTLIL::SwitchRule} for the reset path. After this pass the
-sync rule for the reset is level-sensitive and the top-level
-\C{RTLIL::SwitchRule} has been removed.
-%
-\item {\tt proc\_mux} \\
-This pass converts the \C{RTLIL::CaseRule}/\C{RTLIL::SwitchRule}-tree to a tree
-of multiplexers per written signal. After this, the \C{RTLIL::Process} structure only contains
-the \C{RTLIL::SyncRule}s that describe the output registers.
-%
-\item {\tt proc\_dff} \\
-This pass replaces the \C{RTLIL::SyncRule}s to d-type flip-flops (with
-asynchronous resets if necessary).
-%
-\item {\tt proc\_dff} \\
-This pass replaces the \C{RTLIL::MemWriteActions}s with {\tt \$memwr} cells.
-%
-\item {\tt proc\_clean} \\
-A final call to {\tt proc\_clean} removes the now empty \C{RTLIL::Process} objects.
-\end{itemize}
-
-Performing these last processing steps in passes instead of in the Verilog frontend has two important benefits:
-
-First it improves the transparency of the process. Everything that happens in a separate pass is easier to debug,
-as the RTLIL data structures can be easily investigated before and after each of the steps.
-
-Second it improves flexibility. This scheme can easily be extended to support other types of storage-elements, such
-as sr-latches or d-latches, without having to extend the actual Verilog frontend.
-
-\section{Synthesizing Verilog Arrays}
-
-\begin{fixme}
-Add some information on the generation of {\tt \$memrd} and {\tt \$memwr} cells
-and how they are processed in the {\tt memory} pass.
-\end{fixme}
-
-\section{Synthesizing Parametric Designs}
-
-\begin{fixme}
-Add some information on the \lstinline[language=C++]{RTLIL::Module::derive()} method and how it
-is used to synthesize parametric modules via the {\tt hierarchy} pass.
-\end{fixme}
-