Comparator , |A-B|

Which is a better implementation of
1. A comparator
a) Subtractor along with sign examination logic.
b) Comparator from its truth table.
How does XST and DC implement a description of a comparator in Verilog.

2. mod (A-B)
do we just describe this as
[ comparator -> select line of mux
b-a and a-b as two inputs of a mux ]

if(a <= b)
out <= b-a;
else
out <= a-b;


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