Constraint functions for JVM flags G1SATBBufferSize and G1UpdateBufferSize are not enforced if UseG1GC is not explicitly enabled before them in the command line arguments. This is due to the constraint functions relying on UseG1GC being set, but during early argument parsing (AtParse phase), the order of flag evaluation is not guaranteed. As a result, invalid values for G1-specific flags can pass unchecked and lead to JVM crashes.

java -XX:G1UpdateBufferSize=0 -version
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# A fatal error has been detected by the Java Runtime Environment:
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# SIGSEGV (0xb) at pc=0x00007fdfa35c7ba9, pid=3095399, tid=3095400
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# JRE version: (26.0) (build )
# Java VM: Java HotSpot(TM) 64-Bit Server VM (26-internal-2025-06-16-1150094.opc..., mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xea4ba9] PlaceholderTable::get_entry(Symbol*, ClassLoaderData*)+0x89
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