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there are many adders that might be useful to you... try reading them from text books... any good digital design book will have these stuff... then you can model these adders easily yourself... and your question is out of scope of reply in this short space....:|:!:
First make out an n bit adder, once you decide how will the circuit look like, then only proceed with the entity and architecture of the code. You can use structural code for simplicity in the beginning and then try to agressively write in other forms.
You cannot do anything in vhdl to do a high speed adder. Instead, You will have to think an architecture, which is fast, or get it from somewhere, and then use vhdl to code it.