Patch: RFC: String sharing heuristic

[David Daney]

Bryce McKinlay wrote:
> I think this patch makes sense. In fact I suspect that its very common 
> for the original string to be thrown out when substring() is used, so 
> perhaps we could make the threshold even more conservative, eg (2 * 
> newCount) ?

How about a non-linear huristic?

Under 100 chars always share, More than 100 do something else.

David Daney


> 
> 
> Tom Tromey wrote:
> 
>> Please comment on this.
>>
>> Christopher Lansdown  showed me  an example program  that unexpectedly
>> used  a lot of  memory (as  compared to  running on  jamvm).  Analysis
>> showed that  he was  using substring(), which  was keeping  the entire
>> contents  of  his 300M  log  file live  --  whereas  the heuristic  in
>> Classpath's substring() avoided this.
>>
>> This is obviously an extreme example.  I'm inclined to put this patch
>> in, but I was hoping someone out there had a better argument for or
>> against than "this one program benefited".  Obviously any heuristic
>> here can't please everyone...
>>
>> Tom
>>
>> Index: ChangeLog
>> from  Tom Tromey  <tromey@redhat.com>
>>
>>     * java/lang/natString.cc (substring): Changed sharing heuristic.
>>
>> Index: java/lang/natString.cc
>> ===================================================================
>> RCS file: /cvs/gcc/gcc/libjava/java/lang/natString.cc,v
>> retrieving revision 1.37
>> diff -u -r1.37 natString.cc
>> --- java/lang/natString.cc 22 Apr 2005 19:02:41 -0000 1.37
>> +++ java/lang/natString.cc 11 May 2005 21:00:41 -0000
>> @@ -1,6 +1,6 @@
>> // natString.cc - Implementation of java.lang.String native methods.
>>
>> -/* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004  Free 
>> Software Foundation
>> +/* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005  Free 
>> Software Foundation
>>
>>    This file is part of libgcj.
>>
>> @@ -829,7 +829,7 @@
>>   if (beginIndex == 0 && endIndex == count)
>>     return this;
>>   jint newCount = endIndex - beginIndex;
>> -  if (newCount <= 8)  // Optimization, mainly for GC.
>> +  if (newCount <= 8  || (4 * newCount) < count)    // Optimization, 
>> mainly for GC.
>>     return JvNewString(JvGetStringChars(this) + beginIndex, newCount);
>>   jstring s = new String();
>>   s->data = data;
>>  
>>
>