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Diffstat (limited to 'manual/CHAPTER_Optimize.tex')
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1 files changed, 14 insertions, 14 deletions
diff --git a/manual/CHAPTER_Optimize.tex b/manual/CHAPTER_Optimize.tex index af8e22497..4b09c2231 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. @@ -69,17 +69,17 @@ These are the only three cases in which it is allowed to propagate an undef according to Sec.~5.1.10 of IEEE Std. 1364-2005 \cite{Verilog2005}. The next two lines assume the value 0 for undef states. These two rules are only -used if no other subsitutions are possible in the current module. If other substitutions +used if no other substitutions are possible in the current module. If other substitutions are possible they are performed first, in the hope that the `any' will change to 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. @@ -241,7 +241,7 @@ by identifying the driver for the state signal. From there the {\tt \$mux}-tree driving the state register inputs is recursively traversed. All select inputs are control signals and the leaves of the -{\tt \$mux}-tree are the states. The algorithm fails if a non-constant leaf +{\tt \$mux}-tree are the states. The algorithm fails if a non-constant leaf that is not the state signal itself is found. The list of control outputs is initialized with the bits from the state signal. @@ -296,7 +296,7 @@ table altered to give the same performance without the external feedback path. \item Entries in the transition table that yield the same output and only differ in the value of a single control input bit are merged and the different bit is removed from the sensitivity list (turned into a don't-care bit). -\item Constant inputs are removed and the transition table is alterered to give an unchanged behaviour. +\item Constant inputs are removed and the transition table is altered to give an unchanged behaviour. \item Unused inputs are removed. \end{itemize} |