RFR: 8370044: TraceBytecodes shouldn't break up lines [v3]

[duke]

On Mon, 23 Feb 2026 09:13:45 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> Hi all, 
>> 
>> This patch addresses bytecodes being torn/mangled when printing concurrently (i.e., tracing the bytecodes of concurrently executing threads). Taking `invokeinterface` on `foo` as an example, the output could look something like the following two, ranging from mangled but legible to completely illegible:
>> 
>> jint[27651] [27395]   696080    64  invokeinterface,  116 <CoherentBytecodeTraceTest$Strategy.foo(I)V> 
>> 
>> 
>> [43267]   696692    64  invokeinterface
>> )
>> [33283] [27395] [27907]   696694    29  fast_agetfield 116 <CoherentBytecodeTraceTest$Strategy.foo(I)V> 
>> 
>> 
>> The solution is to buffer each bytecode print into a `stringStream` before giving it to the tty. This avoids having to take a tty lock. With this solution, the print looks as follows (note: different occurrence so surrounding bytecodes differ):
>> 
>> [25347]   685730    61  iconst_1
>> [25091]   685717    64  invokeinterface 190 <CoherentBytecodeTraceTest$Strategy.foo(I)V> 
>> [24579]   685732    20  invokestatic 125 <java/util/concurrent/ConcurrentHashMap.spread(I)I> 
>> [24579]   685734    23  istore #4
>> 
>> 
>> I introduce a test case that finds `<CoherentBytecodeTraceTest$Strategy.foo(I)V>` and ensures the bytecode preceding it is correct. This reliably failed every time previously, and does not fail after my change.
>> 
>> Parsing the code, I noticed that bytecode printing takes a parameter `bool buffer = true` which internally buffer to a `stringStream` before flushing to the provided `outputStream`. There were numerous places where the calling code was performing buffering itself without passing `buffer = false`, effectively double buffering. This has been addressed.
>> 
>> Testing: tiers 1-4. on Linux (x64, AArch64), macOS (x64, AArch64), Windows (x64).
>
> Paul Hübner has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - David review comments.
>  - Johan and Chen review comments.

@Arraying 
Your change (at version a7f2eca4dbe75d1f6ebabb15edd850a71911db30) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/29842#issuecomment-3949969045