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

[Joe Darcy]

> 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.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2684/files
  - new: https://git.openjdk.java.net/jdk/pull/2684/files/9d26b312..ee2ea23a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2684&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2684&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2684.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2684/head:pull/2684

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