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

[Daniel Skantz]

On Fri, 8 Aug 2025 06:10:56 GMT, Daniel Skantz <dskantz 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.

src/hotspot/share/opto/stringopts.cpp line 688:

> 686:             // which is a problem in the case of repeated stacked concats.
> 687:             // Put a limit at 100 arguments to guard against excessive resource use.
> 688:             bool n_args_is_bounded = merged->num_arguments() < 100;

This check can be done in the `merge` method instead. It's also possible to increase the bound but it will need a live node check later in `replace_string_concat`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26685#discussion_r2265763910