RFR: 8302976: C2 intrinsification of Float.floatToFloat16 and Float.float16ToFloat yields different result than the interpreter

[Joe Darcy]

On Wed, 22 Feb 2023 04:03:02 GMT, David Holmes <dholmes at openjdk.org> wrote:

> I'm also a bit concerned that we are rushing in to "fix" this. IIUC we have three mechanisms for implementing this functionality:
> 
>     1. The interpreted Java code
> 
>     2. The compiled non-intrinisc sharedRuntime code
> 
>     3. The compiler intrinsic that uses a hardware instruction.
> 
> 
> Unless the hardware instructions for all relevant CPUs behave exactly the same, then I don't see how we can have parity of behaviour across these three mechanisms.
> 
> The observed behaviour may be surprising but it seems not to be a bug. And is this even a real concern - would real programs actually need to peek at the raw bits and so see the difference, or does it suffice to handle Nan's opaquely?

>From the spec (https://download.java.net/java/early_access/jdk20/docs/api/java.base/java/lang/Float.html#float16ToFloat(short))

"Returns the float value closest to the numerical value of the argument, a floating-point binary16 value encoded in a short. The conversion is exact; all binary16 values can be exactly represented in float. Special cases:

    If the argument is zero, the result is a zero with the same sign as the argument.
    If the argument is infinite, the result is an infinity with the same sign as the argument.
    If the argument is a NaN, the result is a NaN. "

If the float argument is a NaN, you are supposed to get a float16 NaN as a result -- that is all the specification requires. However, the implementation makes stronger guarantees to try to preserve some non-zero NaN significand bits if they are set.

"NaN boxing" is a technique used to put extra information into the significand bits a NaN and pass the around. It is consistent with the intended use of the feature by IEEE 754 and used in various language runtimes: e.g.,

https://piotrduperas.com/posts/nan-boxing
https://leonardschuetz.ch/blog/nan-boxing/ 
https://anniecherkaev.com/the-secret-life-of-nan

The Java specs are careful to avoid mentioning quiet vs signaling NaNs in general discussion.

That said, I think it is reasonable on a given JVM invocation if Float.floatToFloat16(f) gave the same result for input f regardless of in what context it was called.

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PR: https://git.openjdk.org/jdk/pull/12704