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

[Christian Hagedorn]

On Fri, 9 May 2025 14:57:54 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`.

Looks reasonable to me.

src/hotspot/share/runtime/globals.hpp line 655:

> 653:   product(bool, DeoptimizeOnAllocationException, false, DIAGNOSTIC,          \
> 654:           "Deoptimize on exception during allocation instead of using the"  \
> 655:           " compiled exception handlers")                                   \

For consistency with other flag definitions:
Suggestion:

  product(bool, DeoptimizeOnAllocationException, false, DIAGNOSTIC,         \
          "Deoptimize on exception during allocation instead of using the " \
          "compiled exception handlers")                                    \

-------------

Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25149#pullrequestreview-2843683815
PR Review Comment: https://git.openjdk.org/jdk/pull/25149#discussion_r2091166029