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performancecompiler-constructioncompiler-optimization

Compile time optimization of Math.pow

I've read that it's possible to optimize multiplication by a known constant at compile time by generating code which makes clever use of bit shifting and compiler-generated magic constants.

I'm interested in possibilities for optimizing exponentiation in a similar hacky manner. I know about exponentiation by squaring, so I guess you could aggressively optimize 

pow(CONSTANT, n)

by embedding precomputed successive squares of CONSTANT into the executable. I'm not sure whether this is actually a good idea.

But when it comes to

pow(n, CONSTANT)

I can't think of anything. Is there a known way to do this efficiently? Do the minds of StackOverflow have ideas, on either problem?

Solution

  • Assuming pow(a,b) is implemented as exp(b * log(a)) (which it probably is), if a is a constant then you can precompute its log. If b is a constant, it only helps if it is also an integer.