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Re: [JGIT PATCH v2] FindBugs: don't use new String(String) in RefDatabase

From
Shawn O. Pearce <spearce@spearce.org>
Date
Jul 21, 2009, 15:03 UTC
Message-ID
<20090721150337.GP11191@spearce.org>
In-Reply-To
<200907211650.36831.robin.rosenberg.lists@dewire.com>
Robin Rosenberg <robin.rosenberg.lists@dewire.com> wrote:
> Shawn, any references for the ability of JIT's abilities to stack allocate in this context? For
> me learning, will commit anyway.

See [1] for example. I read a presentation from a HotSpot engineer at Sun a year or two ago that talked about it as a feature in the Sun Java 6 runtime, but I can't track that down now.

Its a pretty simple concept. Folks realized that some types, e.g. StringBuilder, are often used only within a single stack frame, and that escape analysis can be used to prove that the StringBuilder instance is only visible within that stack frame. Doing a stack allocation instead of a heap allocation would allow the JVM to avoid creating unnecessary garbage.

Ah, according to [2] the feature is only in 6u14 and later, and is an option you still need to enable on the command line. But its the direction the Sun JVM is going. I imagine it would be on by default in the future, various performance tests seem to indicate its a worthwhile optimization.

[1] http://www.ibm.com/developerworks/java/library/j-jtp09275.html?ca=dgr-lnxw01JavaUrbanLegends [2] http://java.sun.com/javase/6/webnotes/6u14.html

-- 
Shawn.
Previous: Robin RosenbergNext: Robin Rosenberg
Message 9 of 10 in “Method invokes inefficient new String(String) constructor”
  1. Method invokes inefficient new String(String) constructorYann Simon, Mar 19, 2009
  2. Shawn O. PearceMar 19, 2009
  3. Yann SimonMar 19, 2009
  4. Yann SimonJul 9, 2009
  5. FindBugs: don't use new String(String) in RefDatabaseShawn O. Pearce, Jul 10, 2009
  6. Yann SimonJul 13, 2009
  7. FindBugs: don't use new String(String) in RefDatabaseShawn O. Pearce, Jul 13, 2009
  8. Robin RosenbergJul 21, 2009
  9. Shawn O. PearceJul 21, 2009
  10. Robin RosenbergJul 21, 2009

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