opt_expr - perform const folding and simple expression rewriting

yosys> help opt_expr

opt_expr [options] [selection]

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

-mux_undef

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

-mux_bool

replace $mux cells with inverters or buffers when possible

-undriven

replace undriven nets with undef (x) constants

-noclkinv

do not optimize clock inverters by changing FF types

-fine

perform fine-grain optimizations

-full

alias for -mux_undef -mux_bool -undriven -fine

-keepdc

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.