Coercing floating-point to be deterministic in .NET?

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Just what is this “hint” to the runtime?

As you conjecture, the compiler tracks whether a conversion to double or float was actually present in the source code, and if it was, it always inserts the appropriate conv opcode.

Does the C# spec stipulate that an explicit cast to float causes the insertion of a conv.r4 in the IL?

No, but I assure you that there are unit tests in the compiler test cases that ensure that it does. Though the specification does not demand it, you can rely on this behaviour.

The specification’s only comment is that any floating point operation may be done in a higher precision than required at the whim of the runtime, and that this can make your results unexpectedly more accurate. See section 4.1.6.

Does the CLR spec stipulate that a conv.r4 instruction causes a value to be narrowed down to its native size?

Yes, in Partition I, section 12.1.3, which I note you could have looked up yourself rather than asking the internet to do it for you. These specifications are free on the web.

A question you didn’t ask but probably should have:

Is there any operation other than casting that truncates floats out of high precision mode?

Yes. Assigning to a static field, instance field or element of a double[] or float[] array truncates.

Is consistent truncation enough to guarantee reproducibility across machines?

No. I encourage you to read section 12.1.3, which has much interesting to say on the subject of denormals and NaNs.

And finally, another question you did not ask but probably should have:

How can I guarantee reproducible arithmetic?

Use integers.

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