RFR(s): 8231630: Optimize boot loader with no bootclasspath append entry

[Ioi Lam]

Hi Jiangli,

The changes look good. Thanks for fixing this.

- Ioi

On 10/5/19 1:28 PM, Jiangli Zhou wrote:
> Please review the following small optimization for boot loader.
>
> webrev: http://cr.openjdk.java.net/~jiangli/8231630/webrev.00/
> RFE: https://bugs.openjdk.java.net/browse/JDK-8231630
>
> During classloading, the boot loader in the delegation path calls
> SystemDictionary::load_instance_class to load a requested class. After
> the module system is initialized, the VM checks if the requested
> class' package is in a module defined to the boot loader. If the class
> is either in the unnamed package or unnamed module, or in a module not
> defined to the boot loader, the VM only searches the bootloader's
> append entry (or entries). As a simple optimization, if there is no
> boot append entry, we can immediately return NULL in that case without
> any additional work. This particular call path is heavily exercised in
> some microbenchmarks, such as Dacapo 'pmd'. The improvement is mostly
> within noise level and may be un-noticeable for real world
> applications, but does show a ~3% difference with 'pmd' in
> Before/After comparison. As 'pmd' results can be noisy, the comparison
> is done with a large number of benchmark runs.
>
> Testing with submit-repo tests.
>
> Best regards,
> Jiangli