RFR: 8265768 [aarch64] Use glibc libm impl for dlog,dlog10,dexp iff 2.29 or greater on AArch64.

[Tobias Hartmann]

On Tue, 25 May 2021 15:32:40 GMT, gregcawthorne <duke at openjdk.java.net> 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.
>
> greg.cawthorne at arm.com
> 
> Should work

@gregcawthorne any plans to re-open and fix this?

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PR: https://git.openjdk.java.net/jdk/pull/3510