RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v46]

[Volodymyr Paprotski]

On Mon, 21 Oct 2024 20:31:28 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>>> @rkennke Could you post the full command you used please? And perhaps also the seed that gets printed.. having trouble getting it to fail..
>>> 
>>> So far I added a few options and perrmitations of: `./build/linux-x86_64-server-fastdebug/images/jdk/bin/java -Xcomp -XX:-TieredCompilation -XX:+UnlockDiagnosticVMOptions -XX:+EnableX86ECoreOpts -XX:+UseSerialGC -XX:+UnlockExperimentalVMOptions -XX:+UseCompactObjectHeaders test/jdk/java/lang/StringBuffer/ECoreIndexOf.java` and lo luck.. IndexOf.java test checks "all interesting" lengths of haystack and needle and can't get it to fail either.
>> 
>> I could reproduce on 3rd try with a fastdebug build with:
>> 
>> make test TEST=java/lang/StringBuffer/ECoreIndexOf.java TEST_VM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseCompactObjectHeaders -XX:+UseSerialGC"
>> 
>> 
>> It prints:
>> 
>> Seed set to 636754923980405411
>> 
>> 
>> It probably depends on GC operation: It would only fail when the array happens to be the very first object in the heap. The relevant GC/heap configs would be:
>> 
>> InitialHeapSize                          = 805306368
>> MaxHeapSize                              = 805306368
>> MaxNewSize                               = 268435456
>> 
>> 
>> So you should probably also add `-Xmx805306368 -Xms805306368 -Xmn268435456`
>
>> > Thanks @rkennke able to reproduce now.. Sandhya will have a patch soon and I will re-verify
>> 
>> @rkennke @vpaprotsk Please find attached the patch which should fix the problem.
>> 
>> [smallneedlefix.patch](https://github.com/user-attachments/files/17466073/smallneedlefix.patch)
> 
> Testing now. Runs the reproducer in a loop since half an hour without crashing. I'll let it run overnight, and if @vpaprotsk approves the changes, then I'll intergrate them tomorrow morning.

@rkennke I've been running the patch too, for about 2.5 hours now, looks good to me.

Also looked things over again, looks good. Just to explain/document what I reviewed.. 

- Looked at other uses of the needle (that code didn't change, so not exhaustive claim). Typically size of the needle being less then 16 'doesnt matter'.. i.e. broadcast first char, last char, if first/last character mask matches, switch-table for comparing middle 
     - i.e. no reading headers needed
- The case Sandhya fixes, handles UL special case (i.e. haystack unicode, needle regular). For cases of needle less then 32 bytes, copy the needle to the stack, and expand 8->16 so regular UU code can be used. Previous code looped 256bit loads at a time, now we loop 128 instead.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2427718674