> 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. ------------- 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
