RFR: 8329570: G1: Excessive is_obj_dead_cond calls in verification

[Stefan Karlsson]

On Wed, 3 Apr 2024 10:42:27 GMT, Guoxiong Li <gli at openjdk.org> wrote:

>> While investigating [JDK-8329314](https://bugs.openjdk.org/browse/JDK-8329314) it was found that G1 was slower than Parallel when allocating objects because of the extra verification when running with fastdebug builds.
>> 
>> This tiny tests takes 30s with G1:
>> 
>> public class Test {
>>     static int n = 847734685;
>> 
>>     public static void main(String[] args) {
>>          String[] strs = new String[n + 1];
>>     }
>> }
>> 
>> 
>> A large portion of that time is spent inside this assert:
>> 
>>     assert(!G1CollectedHeap::heap()->is_obj_dead_cond(_containing_obj, _vo), "Precondition");
>> 
>> which is called for every single oop in the _containing_obj. If I move this assert to where _containing_obj is initialized, the test now completes in 4-5 seconds. I propose that we make this tiny change to improve the speed of the debug builds.
>
> src/hotspot/share/gc/g1/g1HeapRegion.cpp line 627:
> 
>> 625:   void set_containing_obj(oop const obj) {
>> 626:     assert(!G1CollectedHeap::heap()->is_obj_dead_cond(obj, _vo), "Precondition");
>> 627:     _containing_obj = obj;
> 
> Should the method `set_containing_obj` judge whether the `obj` is `null`? Just like the `do_oop_work`.

Sounds like a reasonable thing to do. I'll move that assert as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18595#discussion_r1549505155