opt_expr - perform const folding and simple expression rewriting

    opt_expr [options] [selection]

This pass performs const folding on internal cell types with constant inputs.
It also performs some simple expression rewriting.

        remove 'undef' inputs from $mux, $pmux and $_MUX_ cells

        replace $mux cells with inverters or buffers when possible

        replace undriven nets with undef (x) constants

        do not optimize clock inverters by changing FF types

        perform fine-grain optimizations

        alias for -mux_undef -mux_bool -undriven -fine

        some optimizations change the behavior of the circuit with respect to
        don't-care bits. for example in 'a+0' a single x-bit in 'a' will cause
        all result bits to be set to x. this behavior changes when 'a+0' is
        replaced by 'a'. the -keepdc option disables all such optimizations.