RFR: 8353638: C2: deoptimization and re-execution cycle with StringBuilder [v2]

[Tobias Hartmann]

On Fri, 16 May 2025 08:00:37 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> Unlike what was assumed at first, it is quite different from [JDK-8346989](https://bugs.openjdk.org/browse/JDK-8346989). The problem is actually unrelated to `StringBuilder`, but has to do with the underlying array allocation. 
>> 
>> Here, the problem is that the array allocation function, that is throwing when given a negative length, causes a deopt rather than using the compiled exception handlers. This is an old workaround, and the flag `StressCompiledExceptionHandlers` to rather use compiled handlers instead of deopting was added in [JDK-8004741](https://bugs.openjdk.org/browse/JDK-8004741) in 2012. This flag is used in testing since october 2022.
>> 
>> So maybe it's time to use the compiled exception handlers! I propose to turn them on by default, and instead, add a diagnostic flag to deopt instead, in case something goes wrong. Doing so improve the performance to match the ones of C1 (both for direct array allocation, and `StringBuilder` construction). For instance, with the case given in the JBS issue:
>> 
>> Stop at level 0
>> CompileCommand: compileonly C.test* bool compileonly = true
>> 
>> real	0m4,277s
>> user	0m4,214s
>> sys	0m0,117s
>> 
>> Stop at level 1
>> CompileCommand: compileonly C.test* bool compileonly = true
>> 
>> real	0m4,104s
>> user	0m4,079s
>> sys	0m0,106s
>> 
>> Stop at level 2
>> CompileCommand: compileonly C.test* bool compileonly = true
>> 
>> real	0m4,308s
>> user	0m4,239s
>> sys	0m0,145s
>> 
>> Stop at level 3
>> CompileCommand: compileonly C.test* bool compileonly = true
>> 
>> real	0m4,304s
>> user	0m4,247s
>> sys	0m0,132s
>> 
>> Default (Stop at level 4)
>> CompileCommand: compileonly C.test* bool compileonly = true
>> 
>> real	0m4,086s
>> user	0m4,059s
>> sys	0m0,122s
>> 
>> 
>> 
>> I've run some tests (up to tier10), it seems all fine, ignoring the usual noise. I've checked with @dougxc, it shouldn't impact Graal as it doesn't use `OptoRuntime`.
>
> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix spaces

Marked as reviewed by thartmann (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/25149#pullrequestreview-2846276527