Finding a JVM JIT Bug the Hard Way
· 13 min read
In early 2025, a user reported intermittent wrong results using the build-time compiler. Not a crash, just a quietly wrong answer. A year of debugging, dead ends, and creative workarounds later, we traced the problem to a bug in the JVM's C2 JIT compiler, and had to build an entire compiler from scratch in 3 days just to prove it.