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

Vladimir Kozlov vladimir.kozlov at oracle.com
Mon Jun 25 09:16:51 PDT 2012

I have a fix which I tested over weekend, I will send review request today.

Vladimir

On 6/25/12 5:33 AM, Krystal Mok wrote:

Hi all,

Is anybody fixing this bug already? I looked at this issue earlier today. It looks like the problem had something to do with coalescing stacked string concats. When I comment out the "try to coalesce separate concats" block from PhaseStringOpts's constructor, the bug doesn't reproduce any more. There was a lot of noise in the original test case, that it was hard to analyze what went wrong on the IR level. Here's a smaller repro case derived from Riven's: https://gist.github.com/2987732 It still reproduces on JDK7u4/HS23 and JDK8b43/HS24, both with OSR and standard compilation. Strangely, it doesn't reproduce when OSR is disabled (with either -UseOnStackReplacement or -CICompileOSR): $ java -XX:-UseOnStackReplacement Main CompilerOracle: exclude Main.logError Java Version: 24.0-b14-internal-jvmg try OSR compilation 1071 1 b java.lang.String::length (6 bytes) 1135 2 n java.lang.System::arraycopy (0 bytes) (static) 1148 3 b java.lang.Object:: (1 bytes) 1178 4 b java.lang.AbstractStringBuilder::ensureCapacityInternal (16 bytes) 1229 5 b java.lang.String::getChars (62 bytes) 1397 6 b java.lang.AbstractStringBuilder::append (48 bytes) 2013 7 b java.lang.StringBuilder::append (8 bytes) 2682 8 b java.lang.Math::min (11 bytes) 2702 9 b java.lang.String:: (67 bytes) 3140 10 b java.util.Arrays::copyOfRange (63 bytes) 3331 11 b java.lang.AbstractStringBuilder:: (12 bytes) 3390 12 b java.lang.StringBuilder::toString (17 bytes) 3993 13 b java.lang.String::valueOf (14 bytes) 4028 14 b java.lang.StringBuilder:: (18 bytes) try standard compilation 5563 15 b Main::foo (98 bytes) $ ... Regards, Kris On Sat, Jun 23, 2012 at 1:06 AM, Vladimir Kozlov <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote: Thank you for finding this issue, I filed bug report: 7179138: Incorrect result with String concatenation optimization Few questions: Can we add your tests to Hotspot VM regression tests? http://hg.openjdk.java.net/_hsx/hotspot-main/hotspot/file/_1c280e5b8d31/test/compiler/ <http://hg.openjdk.java.net/hsx/hotspot-main/hotspot/file/1c280e5b8d31/test/compiler/> What should we put into the test's Copyright line? And what reference you want in test's @author line? Thanks, Vladimir

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 "testtesttesttesttesttesttestte_st". 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 <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 <http://pastebin.com/raw.php?i=pC3kRC6c> Eclipse: http://pastebin.com/raw.php?i=_Pbj0fyZ8 <http://pastebin.com/raw.php?i=Pbj0fyZ8> Console output: Java Version: 23.0-b21 Failed at iteration: 11983 Length mismatch: 16 <> 32 _Expected: "testtesttesttesttesttesttestte_st" 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)

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