Mathematica 9 is now available
Technology Guide--The Technology Inside Matters
Common Subexpression Detection and Collection

Before evaluating a large expression, Mathematica often identifies common subexpressions, which it collects and caches in linear time to avoid repeated work and to improve accuracy. This technique can significantly cut both the computation time and the memory footprint for large, complex computations. Note that numerical-precision control is implemented to work with common subexpression collection and caching.

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