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diff --git a/manual/CHAPTER_Optimize.tex b/manual/CHAPTER_Optimize.tex index d09b3c47..eee92ef5 100644 --- a/manual/CHAPTER_Optimize.tex +++ b/manual/CHAPTER_Optimize.tex @@ -15,23 +15,23 @@ passes that each perform a simple optimization: \begin{itemize} \item Once at the beginning of {\tt opt}: \begin{itemize} -\item {\tt opt\_const} -\item {\tt opt\_share -nomux} +\item {\tt opt\_expr} +\item {\tt opt\_merge -nomux} \end{itemize} \item Repeat until result is stable: \begin{itemize} \item {\tt opt\_muxtree} \item {\tt opt\_reduce} -\item {\tt opt\_share} +\item {\tt opt\_merge} \item {\tt opt\_rmdff} \item {\tt opt\_clean} -\item {\tt opt\_const} +\item {\tt opt\_expr} \end{itemize} \end{itemize} The following section describes each of the {\tt opt\_*} passes. -\subsection{The opt\_const pass} +\subsection{The opt\_expr pass} This pass performs const folding on the internal combinational cell types described in Chap.~\ref{chapter:celllib}. This means a cell with all constant @@ -57,11 +57,11 @@ this pass can also optimize cells with some constant inputs. $a$ & 1 & $a$ \\ 1 & $b$ & $b$ \\ \end{tabular} - \caption{Const folding rules for {\tt\$\_AND\_} cells as used in {\tt opt\_const}.} - \label{tab:opt_const_and} + \caption{Const folding rules for {\tt\$\_AND\_} cells as used in {\tt opt\_expr}.} + \label{tab:opt_expr_and} \end{table} -Table~\ref{tab:opt_const_and} shows the replacement rules used for optimizing +Table~\ref{tab:opt_expr_and} shows the replacement rules used for optimizing an {\tt\$\_AND\_} gate. The first three rules implement the obvious const folding rules. Note that `any' might include dynamic values calculated by other parts of the circuit. The following three lines propagate undef (X) states. @@ -76,10 +76,10 @@ an undef value or a 1 and therefore the output can be set to undef. The last two lines simply replace an {\tt\$\_AND\_} gate with one constant-1 input with a buffer. -Besides this basic const folding the {\tt opt\_const} pass can replace 1-bit wide +Besides this basic const folding the {\tt opt\_expr} pass can replace 1-bit wide {\tt \$eq} and {\tt \$ne} cells with buffers or not-gates if one input is constant. -The {\tt opt\_const} pass is very conservative regarding optimizing {\tt \$mux} cells, +The {\tt opt\_expr} pass is very conservative regarding optimizing {\tt \$mux} cells, as these cells are often used to model decision-trees and breaking these trees can interfere with other optimizations. @@ -130,7 +130,7 @@ This pass identifies unused signals and cells and removes them from the design. creates an \B{unused\_bits} attribute on wires with unused bits. This attribute can be used for debugging or by other optimization passes. -\subsection{The opt\_share pass} +\subsection{The opt\_merge pass} This pass performs trivial resource sharing. This means that this pass identifies cells with identical inputs and replaces them with a single instance of the cell. @@ -222,6 +222,10 @@ This heuristic has proven to work very well. It is possible to overwrite it by s and setting \B{fsm\_encoding}{\tt = "none"} on registers that match the above criteria but should not be considered FSM state registers. +Note however that marking state registers with \B{fsm\_encoding} that are not +suitable for FSM recoding can cause synthesis to fail or produce invalid +results. + \subsection{FSM Extraction} The {\tt fsm\_extract} pass operates on all state signals marked with the |