Are doubles faster than floats in C#?

Question

I\'m writing an application which reads large arrays of floats and performs some simple operations with them. I\'m using floats, because I thought it\'d be faster than doubl

Answer 1

The short answer is, "use whichever precision is required for acceptable results."

Your one guarantee is that operations performed on floating point data are done in at least the highest precision member of the expression. So multiplying two float's is done with at least the precision of float, and multiplying a float and a double would be done with at least double precision. The standard states that "[floating-point] operations may be performed with higher precision than the result type of the operation."

Given that the JIT for .NET attempts to leave your floating point operations in the precision requested, we can take a look at documentation from Intel for speeding up our operations. On the Intel platform your floating point operations may be done in an intermediate precision of 80 bits, and converted down to the precision requested.

From Intel's guide to C++ Floating-point Operations¹ (sorry only have dead tree), they mention:

• Use a single precision type (for example, float) unless the extra precision obtained through double or long double is required. Greater precision types increase memory size and bandwidth requirements. ...
• Avoid mixed data type arithmetic expressions

That last point is important as you can slow yourself down with unnecessary casts to/from float and double, which result in JIT'd code which requests the x87 to cast away from its 80-bit intermediate format in between operations!

1. Yes, it says C++, but the C# standard plus knowledge of the CLR lets us know the information for C++ should be applicable in this instance.

Answer 2

Floats should be faster on a 32-bit system, but profile the code to make sure you're optimizing the right thing.

Answer 3

With 387 FPU arithmetic, float is only faster than double for certain long iterative operations like pow, log, etc (and only if the compiler sets the FPU control word appropriately).

With packed SSE arithmetic, it makes a big difference though.

Answer 4

I'm writing a ray tracer, and replacing the floats with doubles for my Color class gives me a 5% speedup. Replacing the Vectors floats with doubles is another 5% faster! Pretty cool :)

That's with a Core i7 920