RFR: 8253409: Double-rounding possibility in float fma [v2]

[Brian Burkhalter]

On Tue, 23 Feb 2021 07:00:07 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> In floating-point, usually doing an operation to double precision and then rounding to float gives the right result in float precision. One exception to this is fused multiply add (fma) where "a * b + c" is computed with a single rounding. This requires the equivalent of extra intermediate precision inside the operation. If a float fma is implemented using a double fma rounded to float, for some well-chosen arguments where the final result is near a half-way result in *float*, an incorrect answer will be computed due to double rounding. In more detail, the double result will round up and then the cast to float will round up again whereas a single rounding of the exact answer to float would only round-up once.
>> 
>> The new float fma implementation does the exact arithmetic using BigDecimal where possible, with guard to handle the non-finite and signed zero IEEE 754 details.
>
> Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add a jtreg run command to disable any fma instrinic so the Java code is tested.

Looks fine. Presumably the updated test fails without the source change.

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Marked as reviewed by bpb (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2684