RFR: 8224816: Provide os::processor_id() implementation for Mac OS

[Per Liden]

Hi Erik,

On 5/27/19 12:05 PM, Erik Österlund wrote:
> Hi,
> 
> In order to port ZGC to Mac OS, we need to implement os::processor_id(), 
> due to the use of CPU locals in ZGC.
> I built an implementation that uses CPUID to get the APIC id, and from 
> that determine a unique CPU id.
> Note though that the APIC ids are split over a potentially larger number 
> space than the number of processors.
> In order to reliably translate APIC ids to CPU ids, I first use CPUID to 
> query how many bits are being used
> for APIC ids, and then create an array of integers covering that space. 
> The array maps possible APIC id positions
> to a unique processor id. The array is lazily populated with APIC id -> 
> processor id mappings.
> The specific CPUID leaf used to query this topology information is 
> intel-specific, but that should be okay as all macs are Intel based.
> I have tried this out on a number of macs with a mac port of ZGC (to be 
> upstreamed soon), and it worked well.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8224816
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8224816/webrev.00/

Just a few comments:

* Maybe replace "cpu" with "processor" in variable names, since the 
function is named processor_id().

* Could we split out the setup of the array into a separate function 
helper, like:

   volatile int* mapping = get_apic_to_processor_mapping();

* Move the declaration of "apic_to_cpu_mapping" and "next_cpu_id" out of 
the function, to avoid the pthread_once initialization checks every call?

cheers,
/Per

> 
> Thanks,
> /Erik