StringBuilding optimization bug since Java 7 update 4 (original) (raw)

Charlie Hunt chunt at salesforce.com
Fri Jun 22 07:30:51 PDT 2012

Thanks for confirming. Fwiw, I added a "work around" comment to the blog entry.

I'm sure one of the HotSpot compiler engineers will file a bug for it.

On Jun 22, 2012, at 9:12 AM, Damjan Jovanovic wrote:

In JDK 1.7.004 amd64 and 1.7.005 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)

More information about the hotspot-compiler-dev mailing list