On Tue, 6 Jun 2023 18:48:35 GMT, John R Rose <jr...@openjdk.org> wrote:
>> This reasoning seems invalid. There are method calls in there, and you rely >> on inlining heuristics for this not to break. Please use reachabilityFence >> instead. > > It appears you are assuming that some combination of bytecodes constitutes a > critical section that excludes the GC. But the JVMS makes no guarantees > about GC exclusion across bytecodes. > > A thread can be pre-empted at any bytecode (even in the middle of a > bytecode). Another thread can trigger a GC. Maybe the first thread will be > rolled forward to a place convenient to the JVM, but you cannot predict what > that will be, because the JVMS does not give you any contract about that > matter. > > For example, some JITs deoptimize (branch to the interpreter) at > unpredictable points for reasons no Java programmer should ever think about > (because it’s not in the JVMS contract). Deoptimizing can allocate storage > (for example, to materialize objects whose allocation was deferred by escape > analysis). Thus, it is not safe to assume that any particular bytecode is > immune from GC. > > Also, some JITs (like C2) inject synthetic safepoints injected as part of > arbitrarily complex loop transformations. A GC at such a safepoint might > possibly appear to be tied to a bytecode which is simply a fall-through from > a previous bytecode. This can happen if the loop is rotated, and a > fallthrough point begins to function as a back-branch in the IR. > > The rule of thumb for non-JIT engineers is, if you find yourself trying to > predict what how “the JIT must work”, stop. > > The net of this is that, if you need to preserve an object across a critical > section, don’t try to read the JIT’s mind or expect it to read yours. Put in > a reachability fence that spans that critical section. I don't know what the right fix for this is. If it involves reachabilityFence, then a try-finally statement should be used, with the "critical section" being within the try-clause and the reachabilityFence within the finally-clause. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/14211#discussion_r1220682487
