RFR: 8370044: TraceBytecodes shouldn't break up lines [v2]

[Chen Liang]

On Fri, 20 Feb 2026 13:32:59 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 one additional commit since the last revision:
> 
>   Johan's feedback.

test/hotspot/jtreg/runtime/interpreter/CoherentBytecodeTraceTest.java line 160:

> 158:     }
> 159: 
> 160:     public static interface Strategy {

static is trivial on interfaces, and static final are trivial on records.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29842#discussion_r2834136300