RFR: 8354362: Use automatic indentation in CollectedHeap printing

[Joel Sikström]

On Wed, 16 Apr 2025 05:21:41 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Hello,
>> 
>>> This PR only focuses on fixing indentation and re-arranging some callsites. It does *not* change the contents of any output, apart from some (IMO relevant) indentation/whitespace additions.
>> 
>> Currently, the CollectedHeap printing code (print_on and print_on_error, with calls "below") prepends spaces in messages in a way that only makes sense if you write the code and then check the output to see if you've done everything correctly. To make writing and maintaining printing code easy, I propose we move to a system where each printing method, starting at callers of print_on and print_on_error, uses the indentation API in outputStream and does not rely on prepending spaces like is done right now.
>> 
>> What I propose is that any (GC) printing method should not make any assumptions of the indentation level of its caller(s). This means that each function shall:
>> 1. Not prepend any spaces to its printing, and instead expect that the caller(s) should handle any indentation before calling this function.
>> 2. Enforce its own indentation, by enabling auto indentation in its own context and for its "lower level" calls (which is often the desired outcome).
>> 
>> Combining these two rules means that *any* (GC) printing method can be called from anywhere and give sensible output, without (seemingly random) indentation of expectations elsewhere.
>> 
>> I have aggregated calls that print on the same indentation level to the same callsite. This makes it clear where to look in the code and also makes it easier to add/enforce indendation. To this end, I have re-arranged print_on_error so that it never includes print_on. The new system I propose is that print_on and print_on_error can be called separately for different information, which aligns well with having the same callsite for the same indentation. See changes in vmError.cpp for how this is implemented.
>> 
>> Instead of prepending spaces, I use StreamAutoIndentor, defined in ostream.hpp. To make using automatic indentation easier, I've made some changes to StreamAutoIndentor so that it inherits from streamIndentor and also add an *optional* argument to StreamAutoIndentor to apply an indentation. My reasoning for this is that most places that use streamIndentor also want to use StreamAutoIndentor (either immediately or some time before) so that it is automatically applied. A downside of this change is that any previous uses of StreamAutoIndentor now also needs to store an extra int worth of memory. To me, th...
>
> src/hotspot/share/gc/shared/collectedHeap.cpp line 119:
> 
>> 117:     heap->print_on(&st);
>> 118:     MetaspaceUtils::print_on(&st);
>> 119:   }
> 
> Pre-existing, the other cases of printing in this file have a preceding ResourceMark. It is either needed here or not needed there.

The ResourceMarks that are used in other places in this file are not needed anymore. The reason they are placed where they are is because previously (a long time ago, since before [this](https://github.com/openjdk/jdk/commit/d12604111ccd6a5da38602077f4574adc850d9b8#diff-f9496186f2b54da5514e073a08b00afe2e2f8fbae899b13c182c8fbccc7aa7a6) commit), they were next to creating a debug stream. When the debug stream was replaced with a LogStream, the ResourceMark should have followed the LogStream, but it didn't in the changes for print_heap_{before,after}_gc(), see universe.cpp in [this](https://github.com/openjdk/jdk/commit/d12604111ccd6a5da38602077f4574adc850d9b8#diff-f9496186f2b54da5514e073a08b00afe2e2f8fbae899b13c182c8fbccc7aa7a6) commit, where the printing methods were before being moved to collectedHeap.cpp. 

The ResourceMarks should be removed, like Casper has done in [JDK-8294954](https://github.com/openjdk/jdk/pull/24162). I talked with Casper about the ResourceMarks, as he have looked over why the ResourceMarks are there in his patch and he agrees that they should be removed from print_heap_{before,after}_gc(), as they are likely there only for the LogStream.

To summarise, no, ResourceMarks are not needed here, and they should be removed in the other places in this file.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24593#discussion_r2046931370