RFR: 8354362: Use automatic indentation in CollectedHeap printing

[Joel Sikström]

On Wed, 16 Apr 2025 08:28:22 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...
>
> Notes:
> 
> - We may want to simplify at some point and merge streamIndentor and streamAutoIndentor. That includes checking which existing call sites use streamIndentor *without* wanting auto indentation. Not sure but I guess there are none.
> I think the existing cases fall into two categories: where streamIndentor was used on a stream that had already autoindent enabled, and where the code uses "cr_indent()" or "indent" to manually indent.
> 
> - It would be nice to have a short comment in collectedHeap.hpp about when print_on resp print_on_error is used. From your explanation, I expect print_on_error to be used for information that should only be printed in case of a fatal error, right?
> 
> - To simplify and prevent mistakes, we should consider making set_autoindent in outputStream private and make the indentor RAII classes friends of outputStream.

Thank you for looking at this @tstuefe! 

I've addressed some of your comments with new commits. I agree that we likely want to merge streamIndentor and StreamAutoIndentor in a follow up RFE, where it also would be good to look at making set_autoindent() private. 

I haven't looked into it, but it feels weird to have an indentation level on an outputStream and use it only explicitly via indent() and not via a StreamAutoIndentor. I think a good solution would be to only allow indentation via the StreamAutoIndentor API like you're proposing, and look into whether there should be some API for temporarily disabling indentation with a RAII object (or just some parameters to StreamAutoIndentor) if there are cases that require it.

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PR Comment: https://git.openjdk.org/jdk/pull/24593#issuecomment-2809764389