On Wed, 22 Feb 2023 04:03:02 GMT, David Holmes <dhol...@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. ------------- PR: https://git.openjdk.org/jdk/pull/12704
