Don't cares in synthesis: theoretical pitfalls and practical solutions

The effective use of don't cares requires solving several theoretical and practical problems. The theoretical problems are caused by a need to have all tools in a methodology use a consistent semantics of don't cares, so as to guarantee correctness of the final implementation. Several common meanings of "don't care" will be considered, and their respective conditions for design correctness will be derived. The main theoretical result shows that in existing design languages, the following three desirable properties are mutually inconsistent: unrestricted use of non-Boolean values (e.g., X), implementing a large design one partition at a time, and assurance of correctness of the final implementation. A practical solution to this problem involves several issues: specifying don't cares in a language description, deriving them during high-level synthesis, and optimizing logic in their presence. Experimental results showing the impact of don't cares on logic quality are presented.