StringBuilding optimization bug since Java 7 update 4

[Damjan Jovanovic]

In JDK 1.7.0_04 amd64 and 1.7.0_05 x86, both on Ubuntu 12.04 amd64:

-XX:+OptimizeStringConcat reproduces the bug in the Javac example from
http://riven8192.blogspot.com/2012/06/hotspot-bug-in-java-7u4-7u5-7u6rc-with.html
-XX:-OptimizeStringConcat works correctly (it should infinite loop)

Regards

On Fri, Jun 22, 2012 at 3:47 PM, Charlie Hunt <chunt at salesforce.com> wrote:
> Could you try your experiment with -XX:-OptimizeStringConcat to see if the
> unexpected behavior goes away?
>
> IIRC, -XX:+OptimizeStringConcat was made the default in 7u4.
>
> charlie ...
>
> On Jun 21, 2012, at 4:39 PM, Skip Balk wrote:
>
> Dear HotSpot developers,
>
>
> This is the first time I post anything on any mailinglist, so please forgive
> me if my message is not quite up to the standards you are used to.
>
> Last week, I encountered a bug in simple String concatenation, like:
>
> String s = "test";
>     s = s+s;
>     s = s+s;
>     s = s+s;
>
> yielding the string: "nullnulltesttest" instead of
> "testtesttesttesttesttesttesttest". The first Java version that seems to
> suffer from this bug is Java 7u4, and is confirmed to occur in 7u5, 7u6(rc).
> It has been further reproduced on 32bit, 64bit, clientvm and servervm.
>
> After a few thousand (interpreted) runs of this code, it starts to give
> these incorrect results, leading me to assume that a HotSpot optimisation is
> the root cause of this problem, which is backed by the fact that when
> running the java process with the -Xint argument, the bug does not occur.
>
> Unfortunately, today I discovered that with my trivial sourcecode, the issue
> only occured with the Eclipse compiler. The Javac output seemed to be
> 'friendly' to HotSpot.
>
> Upon further investigation, it turned out that "s=s+s" was compiled to
> different bytecode by Javac and Eclipse:
>     Eclipse: s = new StringBuilder(String.valueOf(s)).append(s).toString();
>     Javac:   s = new StringBuilder().append(s).append(s).toString();
> When writing the version Eclipse produces in Java sourcecode, the javac
> compiler also produced the bytecode that made HotSpot trip.
>
> Without further ado: here are the full code-dumps (both for eclipse and
> javac)
>     http://riven8192.blogspot.com/2012/06/hotspot-bug-in-java-7u4-7u5-7u6rc-with.html
>
> "javap -c" output, with sourcecode containing "s=s+s"
>     Javac:   http://pastebin.com/raw.php?i=pC3kRC6c
>     Eclipse: http://pastebin.com/raw.php?i=Pbj0fyZ8
>
> Console output:
>     Java Version: 23.0-b21
>     Failed at iteration: 11983
>     Length mismatch: 16 <> 32
>     Expected: "testtesttesttesttesttesttesttest"
>     Actual:   "nullnulltesttest"
>
> Last but not least, this bug seems to be triggered by the empty-for-loop,
> which leads me to believe this is a case of too aggresive dead code removal.
> I hope you can
>
>
> With kind regards,
>     Riven (http://www.java-gaming.org/ administrator)
>
>