On 06/03/2012 11:35 PM, Mike Duigou wrote:
[I trimmed the distribution list]
On Jun 3 2012, at 13:44 , Peter Levart wrote:
On Thursday, May 31, 2012 03:22:35 AM mike.dui...@oracle.com wrote:
Changeset: 2c773daa825d
Author: mduigou
Date: 2012-05-17 10:06 -0700
URL: http://hg.openjdk.java.net/jdk8/tl/jdk/rev/2c773daa825d
6924259: Remove offset and count fields from java.lang.String
Summary: Removes the use of shared character array buffers by String along
with the two fields needed to support the use of shared buffers.
Wow, that's quite a change.
Indeed. It was a long time in development. It is a change which is expected to
be overall beneficial though and in the general case a positive win.
So .substring() is not O(1) any more?
No. Though with object allocation it probably was only ever roughly O(1) anyway.
Allocation fast path just bump a pointer, so it's O(1).
There are two advantages of the new code.
The String object and the array of chars are now co-located in memory
(at least for small/medium strings) so cpu caches are happy.
This fix a longstanding memory leak issue
see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4513622
that why some parsers already doesn't use the substring() trick.
BTW Mike, you can now close this bug.
Doesn't this have impact on the performance of parsers and such that rely on
the performance caracteristics of the .substring() ?
It does have an impact. We've seen as much as a couple of percent on some
benchmarks. Parsers which use substring for extraction are definitely impacted
by this change.
Have you considered then implementing .subSequence() not in terms of just
delegating to .substring() but returning a special CharSequence view over the
chars of the sub-sequence?
It does look that String.subSequence() returning a special view rather than a
substring would be a good optimization and probably a very good compromise for
parser developers. Please create an issue and if you have the time and
expertise a patch would speed things along (though unfortunately almost
certainly too late for inclusion in 7u6).
Given that Integer.parseInt() or Double.parseDouble() takes a String and
not a CharSequence, yes you can create a CharSequence view but the only
way to use it
is to call toString() on it.
Mike
cheers,
Rémi
