> This patch suggests a workaround to an issue with huge SCF MH expression > trees taking excessive JIT compilation resources by reviving (part of) the > simple bytecode-generating strategy that was originally available as an > all-or-nothing strategy choice. > > Instead of reintroducing a binary strategy choice I propose a threshold > parameter, controlled by > `-Djava.lang.invoke.StringConcat.highArityThreshold=<val>`: For expressions > below or at this threshold there's no change, for expressions with an arity > above it we use the `StringBuilder`-chain bytecode generator. > > There are a few trade-offs at play here which influence the choice of > threshold. The simple high arity strategy will for example not see any reuse > of LambdaForms but strictly always generate a class per indy callsite, which > means we might end up with a higher total number of classes generated and > loaded in applications if we set this value too low. It may also produce > worse performance on average. On the other hand there is the observed > increase in C2 resource usage as expressions grow unwieldy. On the other > other hand high arity expressions are likely rare to begin with, with less > opportunities for sharing than the more common low-arity expressions. > > I turned the submitted test case into a few JMH benchmarks and did some > experiments with `-XX:CompileCommand=MemStat,StringConcat::concat,print`: > > Baseline strategy: > 13 args: 6.3M > 23 args: 18M > 123 args: 868M > > `-Djava.lang.invoke.StringConcat.highArityThreshold=0`: > 13 args: 2.11M > 23 args: 3.67M > 123 args: 4.75M > > For 123 args the memory overhead of the baseline strategy is 180x, but for 23 > args we're down to a 5x memory overhead, and down to a 3x overhead for 13 > args. Since the absolute overhead for 23 is borderline acceptable (+15Mb) > I've conservatively chosen a threshold at arity 20. This keeps C2 resource > pressure at a reasonable level (< 18M) while avoiding perturbing performance > at the vast majority of call sites. > > I was asked to use the new class file API for mainline. There's a version of > this patch implemented using ASM in 7c52a9f which might be a reasonable basis > for a backport.
Claes Redestad has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits: - Merge master, resolve conflicts due pr/18688 - Dump using the hidden class name - Pre-size stringbuilders based on constant size and a small argument factor - @liach feedback - Bump threshold after experiments - Port ASM to CFA - Lower compilation overhead for large concat expressions, initial ASM-based version based on pre-existing implementation ------------- Changes: https://git.openjdk.org/jdk/pull/18690/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18690&range=04 Stats: 237 lines in 2 files changed: 229 ins; 2 del; 6 mod Patch: https://git.openjdk.org/jdk/pull/18690.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/18690/head:pull/18690 PR: https://git.openjdk.org/jdk/pull/18690
