RFR: 8264123: add ThreadsList.is_valid() support

[Daniel D.Daugherty]

On Thu, 1 Apr 2021 17:34:24 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi Dan,
>> 
>> I have seen some other verification code which also might be performance impacting, can we got over and measure what the cost is for different verification when you gotten your bake time here? So we can take an informed decision based on what the cost vs debuggability in product is.
>> 
>> Looks good, thanks.
>
>> @robehn - Thanks for the review!
>> 
>> Are you saying that I should measure the cost of these `guarantee()` calls before
>> I switch them to `assert()` calls in the future? If that's what you mean, I'll include
>> a note for that when I file the new bug. If that's not what you mean, please let me
>> know what you do mean.
> 
> Yes

I've filed the following new bug:

    JDK-8264624 change the guarantee() calls added by JDK-8264123 to assert() calls

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PR: https://git.openjdk.java.net/jdk/pull/3255