.net - Is Multiplying the Inverse Better or Worse?

Question

When dealing with double data types is multiplying by the inverse better or worse?

Which way is faster? Which way uses less memory? Which way is preferred?

How does MSIL handle this?

SquareInches = MMSquared / 645.16 
SquareInches = MMSquared * 0.0015500031000062000124000248000496

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Related Question:

Should I use multiplication or division?

Answer 1

Multiplying by the inverse is faster. Compilers don't optimize this automatically because it can result in a small loss of precision. (This actually came up on a D newsgroup Walter Bright frequents, and he made it clear that compilers do not do this automatically.) You should normally divide because it is more readable and accurate.

If you are executing a piece of floating point code a billion times in a loop and you don't care about a small loss of precision and you will be dividing by the same number several times, then multiplying by the inverse can be a good optimization. I have actually gotten significant real world speedup in a few cases like the ones described by multiplying by the inverse, but these are extreme edge cases in loops executed several billion times that pretty much do nothing but multiply floats.