RFR: 8353175: Eliminate double iteration of stream in FieldDescriptor reinitialization [v6]

[Aleksey Shipilev]

On Thu, 10 Apr 2025 08:16:25 GMT, Radim Vansa <rvansa at openjdk.org> wrote:

>> On the reproducer https://bugs.openjdk.org/secure/attachment/113985/CCC.java my local testing shows these numbers:
>> 
>> ### JDK-17
>> 
>> $ hyperfine -w 5 -r 10 '/path/to/jdk-17/bin/java -cp /tmp/ CCC'
>> Benchmark 1: /path/to/jdk-17/bin/java -cp /tmp/ CCC
>>   Time (mean ± σ):      32.5 ms ±   0.9 ms    [User: 27.5 ms, System: 10.6 ms]
>>   Range (min … max):    31.1 ms …  33.7 ms    10 runs
>> 
>> ### JDK-25 before the change applied
>> 
>> $ hyperfine -w 5 -r 10 '/path/to/jdk-25/build/linux-x86_64-server-release/images/jdk/bin/java -cp /tmp/ CCC'
>> Benchmark 1: /path/to/jdk-25/build/linux-x86_64-server-release/images/jdk/bin/java -cp /tmp/ CCC
>>   Time (mean ± σ):     101.6 ms ±   1.5 ms    [User: 96.8 ms, System: 14.6 ms]
>>   Range (min … max):    99.0 ms … 104.5 ms    10 runs
>> 
>> ### JDK-25 with this patch
>> 
>> $ hyperfine -w 5 -r 10 '/path/to/jdk-25/build/linux-x86_64-server-release/images/jdk/bin/java -cp /tmp/ CCC'
>> Benchmark 1: /path/to/jdk-25/build/linux-x86_64-server-release/images/jdk/bin/java -cp /tmp/ CCC
>>   Time (mean ± σ):      75.8 ms ±   1.2 ms    [User: 69.8 ms, System: 16.0 ms]
>>   Range (min … max):    73.8 ms …  78.2 ms    10 runs
>
> Radim Vansa has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove outdated comment

src/hotspot/share/oops/instanceKlass.cpp line 1945:

> 1943:     fields_sorted.sort(compare_fields_by_offset);
> 1944:     fieldDescriptor fd;
> 1945:     for (auto it = fields_sorted.begin(); it != fields_sorted.end(); ++it) {

Ah, that's not what I meant :) There is no need to use iterators here, just pull `length` out of `fields_stored.length()`, and use the same old indexed loop:


 int length = fields_stored.length();
 if (length > 0) {
    fields_sorted.sort(compare_fields_by_offset);
    for (int i = 0; i < length; i++) {
      ...

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/24290#discussion_r2036826130