RFR: 8362394: C2: Repeated stacked string concatenation fails with "Hit MemLimit" and other resourcing errors

[Roberto Castañeda Lozano]

On Mon, 11 Aug 2025 08:05:58 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> This PR addresses a bug in the stringopts phase. During string concatenation, repeated stacking of concatenations can lead to excessive compilation resource use and generation of questionable code as the merging of two StringBuilder-append-toString links sc1 and sc2 can result in a new StringBuilder with the size sc1->num_arguments() * sc2->num_arguments().
>> 
>> In the attached test, the size of the successively merged StringBuilder doubles on each merge -- there's 24 of them -- as the toString result of the first component is used twice in the second component [1], etc. Not only does the compiler hang on this test case, but the string concat optimization seems to give an arbitrary amount of back-to-back stores in the generated code depending on the number of stacked concatenations.
>> 
>> The proposed solution is to put an upper bound on the size of a merged concatenation, which guards against this case of repeated concatenations on the same string variable, and potentially other edge cases. 100 seems like a generous limit, and higher limits could be insufficient as each argument corresponds to about 20 new nodes later in replace_string_concat [2].
>> 
>> [1] https://github.com/openjdk/jdk/blob/0ceb366dc26e2e4f6252da9dd8930b016a5d46ba/src/hotspot/share/opto/stringopts.cpp#L303
>> 
>> [2] https://github.com/openjdk/jdk/blob/0ceb366dc26e2e4f6252da9dd8930b016a5d46ba/src/hotspot/share/opto/stringopts.cpp#L1806
>> 
>> Testing: T1-4.
>> 
>> Extra testing: verified that no method in T1-4 is being compiled with a merged concat candidate exceeding the suggested limit of 100 aguments, regardless of whether or not the later checks verify_control_flow() and verify_mem_flow pass.
>
> test/hotspot/jtreg/compiler/stringopts/TestStackedConcatsMany.java line 42:
> 
>> 40:         for (int i = 0; i < 10; i++) {
>> 41:             String s = f(" ");
>> 42:         }
> 
> This could be simplified into:
> Suggestion:
> 
>         new StringBuilder(); // Trigger loading of the StringBuilder class.
>         String s = f(" ");
> 
> Then you can narrow down the CompileOnly command to `-XX:CompileOnly=compiler.stringopts.TestStackedConcatsMany::f`.

It would be good to assert that `f(" ")` returns the expected result.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26685#discussion_r2265974097