Constructing the Control Dependence Graph form Dominance Frontier in SSA ?

[QiangHuang <qianghuang87 at gmail dot com>]

    In the process of translating the GIMPLE into SSA, gcc first
computed the dominance frontier of the CFG. I want to compute the
control dependence graph from the dominance frontiers computed just
now.

   The computed dominance frontier for all basic blocks is stored in
the array of bitmap structure -"dfs", which means dfs[X] is the
dominance frontier of basic block X. The process  in the function
"rewite_into_ssa()".

    However I don't understand clearly about what a certain bit in
dfs[x] stands for.Without the clear notation of dfs, I can't change it
to the CDG.

   Would someone tell me how the "dfs" represents dominance frontier ,
or another way to compute control dependence graph ?

   Thanks in advance~^_^