RFR: 8295810: cleanup debug agent removeThread() api

[Serguei Spitsyn]

On Mon, 24 Oct 2022 20:43:48 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> The implementation of removeThread() is currently:
>> 
>> 
>> static void
>> removeThread(JNIEnv *env, ThreadList *list, jthread thread)
>> {
>>     ThreadNode *node;
>> 
>>     node = findThread(list, thread);
>>     if (node != NULL) {
>>         removeNode(list, node);
>>         clearThread(env, node);
>>     }
>> }
>> 
>> However, currently all callers already have the ThreadNode*, so they end up calling like the following:
>> 
>> `            removeThread(env, list, node->thread);`
>> 
>> So we go from a ThreadNode* to a jthread, only to do a findThread() to get the ThreadNode* again. Also, the list is stored in the ThreadNode. removeThread() can instead be implemented as:
>> 
>> 
>> static void
>> removeThread(JNIEnv *env, ThreadNode *node)
>> {
>>   JDI_ASSERT(node != NULL);
>>   removeNode(node->list, node);
>>   clearThread(env, node);
>> }
>> 
>> 
>> This is faster, although not by as much as you might think. TLS is used to map a jthread to a ThreadNode*, so the findThread() call is actually already pretty fast. The exception is when dealing with the otherThreads list. These threads have not yet been started, so TLS cannot be used, but it is rare for threads to appear on this list. Still, this is a good cleanup to do, and should be a bit faster.
>> 
>> This cleanup was initially implemented as part of the work for [JDK-8295376](https://bugs.openjdk.org/browse/JDK-8295376), but is being split out.
>
> src/jdk.jdwp.agent/share/native/libjdwp/threadControl.c line 322:
> 
>> 320:     ThreadNode *next;
>> 321: 
>> 322:     JDI_ASSERT(list == node->list);
> 
> "list" argument is redundant
> I suggest to drop it and replace this assert with
> `ThreadList *list == node->list;`

I was almost to suggest the same.

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PR: https://git.openjdk.org/jdk/pull/10828