Want Knuth style line numbering in algorithm2e algorithms

If your captions are below the algorithm,

\documentclass{article}
\usepackage[linesnumbered]{algorithm2e}

\makeatletter
\renewcommand{\theAlgoLine}{%
  \@arabic{\numexpr\value{algocf}+1\relax}.\arabic{AlgoLine}}
\makeatother

\begin{document}

\begin{algorithm}
\SetKwData{Left}{left}\SetKwData{This}{this}\SetKwData{Up}{up}
\SetKwFunction{Union}{Union}\SetKwFunction{FindCompress}{FindCompress}
\SetKwInOut{Input}{input}\SetKwInOut{Output}{output}
\Input{A bitmap $Im$ of size $w\times l$}
\Output{A partition of the bitmap}
\BlankLine
\emph{special treatment of the first line}\;
\For{$i\leftarrow 2$ \KwTo $l$}{
  \emph{special treatment of the first element of line $i$}\;
  \For{$j\leftarrow 2$ \KwTo $w$}{\label{forins}
    \Left$\leftarrow$ \FindCompress{$Im[i,j-1]$}\;
    \Up$\leftarrow$ \FindCompress{$Im[i-1,]$}\;
    \This$\leftarrow$ \FindCompress{$Im[i,j]$}\;
    \If(\tcp*[h]{O(\Left,\This)==1}){\Left compatible with \This}{\label{lt}
      \lIf{\Left $<$ \This}{\Union{\Left,\This}}
      \lElse{\Union{\This,\Left}}
    }
    \If(\tcp*[f]{O(\Up,\This)==1}){\Up compatible with \This}{\label{ut}
      \lIf{\Up $<$ \This}{\Union{\Up,\This}}
      \tcp{\This is put under \Up to keep tree as flat as possible}\label{cmt}
      \lElse{\Union{\This,\Up}}\tcp*[h]{\This linked to \Up}\label{lelse}
    }
  }
  \lForEach{element $e$ of the line $i$}{\FindCompress{p}}
}
\caption{disjoint decomposition}\label{algo_disjdecomp}
\end{algorithm}

\end{document}

If you place captions above, then \thealgocf should be used instead of \@arabic{\numexpr\value{algocf}+1\relax}.