Integrated: 8370044: TraceBytecodes shouldn't break up lines

[Paul Hübner]

On Fri, 20 Feb 2026 11:12:48 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).

This pull request has now been integrated.

Changeset: 32a61971
Author:    Paul Hübner <phubner at openjdk.org>
Committer: Johan Sjölen <jsjolen at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/32a619715e9d34e45b9bd6c954a03ead34be5a48
Stats:     173 lines in 8 files changed: 163 ins; 0 del; 10 mod

8370044: TraceBytecodes shouldn't break up lines

Reviewed-by: dholmes, coleenp, jsjolen

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PR: https://git.openjdk.org/jdk/pull/29842