On Tue, 23 Feb 2021 07:00:07 GMT, Joe Darcy <da...@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. ------------- Marked as reviewed by bpb (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/2684
