Single Precision Math Operations in .NET?

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+3 Q:

Single Precision Math Operations in .NET?

The .NET framework's Math functions mostly operate on double precision floats, there are no single precision (float) overloads. When working with single precision data in a high performance scenario this results in unnecessary casting and also calculating functions with more precision than is required, thus performance is affected to some degree.

Is there any way of avoiding some of this additional CPU overhead? E.g. is there an open source math library with float overloads that calls underlying FPU instructions directly? (My understanding is that this would require support in the CLR). And actually I'm not sure if modern CPUs even have single precision instructions.

This question has been partly inspired by this question about optimizing a sigmoid function:

Math optimization in C#

+2 A:

To my knowledge, the .NET Framework does not include an API with direct access to math intrinsics. The Mono libraries do include working support for intrinsics, but I'm not sure the state of them.

[Edit: This paragraph is commentary on why you don't see overloads for float parameters.] One trouble is the CLI evaluation stack (per ECMA-335) does not distinguish between the float and double types. A valid implementation could treat everything as a double for math operations, but I imagine the CLR (Microsoft's implementation of the CLI) performs optimizations for single-precision arithmetic where it can.

I think it's somewhat unfortunate that the issue of intrinsics (in particular SIMD extensions) hasn't been addressed [in a released product] yet. My outsider's-guess is support for intrinsics would require significant alterations to the VM that pose unacceptable risks at this point in the .NET Framework release cycle. The garbage collector (and I think the exception handling mechanisms) is tightly coupled with the register allocator, and supporting intrinsics adds a radical new variable to that area.

280Z28 2009-08-18 09:58:25

When you refer to instrinsics is this the same as external functions? - which is how the Math functions are implemented, versus intrinsics defined in the language. AFAIK the CLR could easily be extended to implement e.g. public static extern double Sin(float a) in addition to the double precision version.

locster 2009-08-19 12:40:24

Are you talking about implementing it in native code? The P/Invoke overhead of doing so for a single floating point operation would completely negate trying to avoid the performance impact of working with `Sin(double)`. Intrinsics are declared `extern` but are not P/Invoke - the native code for them is internally handled by the JIT itself to make them as efficient as possible.

280Z28 2009-08-19 16:25:14

So perhaps some performance could be gained if your algorithm was such that you could make a single P/Invoke that does many float operations.

Fantius 2009-08-24 18:30:19

Yes, we build a high performance forward scaling math library that natively uses single precision floating point computations if that's all you need. And you are correct, if implemented right, they can be much faster than when using double precision.

Check out this math library from CenterSpace Software.

Paul

Paul 2009-11-06 06:06:04

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Single Precision Math Operations in .NET?

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