RFR: 8354362: Use automatic indentation in CollectedHeap printing

[Thomas Stuefe]

On Fri, 11 Apr 2025 11:28:12 GMT, Joel Sikström <jsikstro 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, this is a trade-off worth makin...

Hi @jsikstro, good cleanup, some small nits remain.

Cheers, Thomas

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.

src/hotspot/share/memory/metaspace.cpp line 221:

> 219:   MetaspaceCombinedStats stats = get_combined_statistics();
> 220:   out->print("Metaspace");
> 221:   out->fill_to(17);

We rely on absolute position here? Will not work well with different indentation levels.

src/hotspot/share/utilities/vmError.cpp line 1399:

> 1397:       st->cr();
> 1398:     }
> 1399:     Universe::heap()->print_on_error(st);

Why is print_on_error called outside the indentation scope?

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PR Review: https://git.openjdk.org/jdk/pull/24593#pullrequestreview-2770781675
PR Review Comment: https://git.openjdk.org/jdk/pull/24593#discussion_r2046093409
PR Review Comment: https://git.openjdk.org/jdk/pull/24593#discussion_r2046096635
PR Review Comment: https://git.openjdk.org/jdk/pull/24593#discussion_r2046084544