On Thu, 15 Apr 2021 08:33:47 GMT, gregcawthorne <github.com+73799211+gregcawthorne@openjdk.org> wrote:
Glibc 2.29 onwards provides optimised versions of log,log10,exp. These functions have an accuracy of 0.9ulp or better in glibc 2.29.
Therefore this patch adds code to parse, store and check the runtime glibcs version in os_linux.cpp/hpp. This is then used to select the glibcs implementation of log, log10, exp at runtime for c1 and c2, iff we have glibc 2.29 or greater.
This will ensure OpenJDK can benefit from future improvements to glibc.
Glibc adheres to the ieee754 standard, unless stated otherwise in its spec.
As there are no stated exceptions in the current glibc spec for dlog, dlog10 and dexp, we can assume they currently follow ieee754 (which testing confirms). As such, future version of glibc are unlikely to lose this compliance with ieee754 in future.
W.r.t performance this patch sees ~15-30% performance improvements for log and log10, with ~50-80% performance improvements for exp for the common input ranged (which output real numbers). However for the NaN and inf output ranges we see a slow down of up to a factor of 2 for some functions and architectures.
Due to this being the uncommon case we assert that this is a worthwhile tradeoff.
Where does the requirement for monotonicity come from? ------------- PR: https://git.openjdk.java.net/jdk/pull/3510