RFR: 8303078: Reduce allocations when pretty printing JCTree during compilation

[Christoph Dreis]

On Wed, 22 Feb 2023 14:52:32 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

>> Hi,
>> 
>> I've been recently optimizing a few project pipelines and always noticed that their compilation step is somewhat "expensive".  Zooming into the issue always revealed Lombok to be a larger contributor. Unfortunately, I can't get rid of Lombok in this customer project (before you ask).
>> 
>> Apparently, Lombok is printing the respective `JCTree` here and there to check for type matches. I can't imagine this to be super efficient, but that's how it is at the moment.
>> <img width="948" alt="image" src="https://user-images.githubusercontent.com/6304496/220134737-bd9b508c-a908-448b-b0d1-e960db28b24a.png">
>> 
>> Anyhow, regardless of the Lombok inefficiencies I think there are some optimization opportunities in the JDK itself. 
>> 
>> 1. Overall, `Pretty.visitSelect` accounts for 8-10% of the total allocations in this project. And among those there are StringBuilder allocations coming from the following:
>> 
>> 
>>     public void visitSelect(JCFieldAccess tree) {
>>         try {
>>             printExpr(tree.selected, TreeInfo.postfixPrec);
>>             // StringBuilder allocations hiding in here.
>>             print("." + tree.name);
>>         } catch (IOException e) {
>>             throw new UncheckedIOException(e);
>>         }
>>     }
>> 
>> 
>> This PR splits the `print` calls into two separate ones to avoid this String concatenation.
>> 
>> ...
>>             printExpr(tree.selected, TreeInfo.postfixPrec);
>>             print('.');
>>             print(tree.name);
>> ...
>> 
>> 
>> Secondly, the `print` method takes an `Object` which seems like a good fit for another (private?) variant of it that only takes a `char`. By this we would probably avoid any eventual boxing and avoid any conversion with `Convert.escapeUnicode(s.toString())` that seems superfluous for chars like `.`, ` `, or any braces like `(`, `{` etc. 
>> 
>> This is currently a draft PR as long as the scope is not clarified. It currently only includes the necessary changes that would optimize the particular use-case. But there are more cases where e.g. the new `char` variant could be used and/or any String concatenation could be split into separate `print` calls.
>> 
>> Let me know what you think and if I should include the other cases as well. If you think this is worthwhile, I'd appreciate if this is is sponsored. (Including creating an issue as I can't do this myself apparently. I will of course squash everything together with the proper issue ID once available.)
>> 
>> I've contributed before, so the CLA should be signed.
>> 
>> Cheers,
>> Christoph
>
> If we proceed with this, I would recommend looking for other possible uses of the new method.
> 
> A simple `grep` shows 129 single character strings, of which 97 are arguments to `print`.
> 
> Separate but related, I note that a similar class, `DocPretty.java`, has 75 single character strings, of which  68 are arguments to print.
> 
> Overall changing the coding style to use `out.print(char)` seems worthwhile.

@jonathan-gibbons Thanks for your review.
> If we proceed with this, I would recommend looking for other possible uses of the new method.

That's the idea - there are multiple cases where this could be used. I'll push an updated version that already changes things to make reviewing a bit easier.

EDIT: Done.

> There are 19 instances of string concatenation in `print` arguments, for your consideration as well.

@jonathan-gibbons Made the adjustments to `Pretty`.

> src/jdk.compiler/share/classes/com/sun/tools/javac/tree/Pretty.java line 139:
> 
>> 137:     private void print(char c) throws IOException {
>> 138:         out.write(c);
>> 139:     }
> 
> This bypasses the `Convert.escapeUnicode` code on line 132.   That's not bad, but should be clearly documented ... in that this is a special method for known ASCII characters, and not a general purpose method that could be used on the characters of a string

Adjusted the comment in that regards.

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PR: https://git.openjdk.org/jdk/pull/12667