strictfp in Java

Question

I'm implementing some neural network library in java , and there are intensive double (not Double) matrix operations, Matrices are large and performance is required of course.

So I came to read about strictfp keyword I honestly didn't understand what it does exactly and I was looking for simple explanation about If i should be using it or not and why

Answer 1

strictfp indicates that floating point calculations should use the exact IEEE754 standard. Without strictfp, the VM is free to use other (but platform dependent) representations of intermediate float and double values, in order to increase precision.

Use strictfp if you need the exact same results on multiple platforms. Avoid it if you want the best precision your current platform can give you.

E.g. in the following simple addition:

  2.0 + 1.1 + 3.0

Do you want the intermediate results (e.g. 2.0 + 1.1) to be represented as an IEEE754 standard double, or with the best possible precision your platform allows. strictfp ensures the first, not using strictfp allows the VM to use the second alternative.

Not using strictfp will not hurt performance, and may on platforms where the native float types don't map to IEEE754 increase performance, since the VM isn't required to convert back and forth in between native and IEEE754 formats. The answer is platform dependent, you'll need to measure.