Optimization 2.0 for composing strings - Was: Replace concat String to append in StringBuilder parameters

Jonathan Gibbons jonathan.gibbons at oracle.com
Mon Sep 8 17:30:31 UTC 2014

It would be inappropriate/incorrect to apply the optimization as described.

The JLS requires that the argument to a method call should be computed 
before invoking the method.

Consider the case when one of the expressions in the series of string 
concatenations throws an exception. It would be incorrect to have 
already partially modified the string buffer.

-- Jon

On 08/29/2014 01:53 PM, Ulf Zibis wrote:
> Hi compiler people,
> is there some chance that javac could be enhanced to optimize better 
> as discussed in this thread? Than refactoring of up to now better 
> readable code to ugly StringBuilder at append() code would be superfluous.
> I really like the String concatenation syntax, but unfortunately it 
> often causes slower code and bigger footprint.
> Optimally javac would optimize mixed use of StringBuilder, 
> StringJoiner, concatenation, toString(), append(String), 
> append(Collection) etc. to a single StringBuilder instance, so that 
> the resulting code, JITed or not, will have better performance.
> Additionally javac could guess a reasonable initial capacity from the 
> given source code.
> Am 29.08.2014 um 10:01 schrieb Wang Weijun:
>> So it's not that the optimization fails but there is no optimization 
>> on them yet.
>> I do see the .append("x") case will be easy to deal with, but it 
>> looks like historically javac has not been a place to do many 
>> optimizations. It mostly converts the java source to byte codes in a 
>> 1-to-1 mapping and let VM do whatever it wants (to optimize). When 
>> you talked about compiling multiple concatenation into using a single 
>> StringBuilder, it's more like choosing the correct implementation 
>> rather than an optimization.
>> I don't expect to see big change on this in the near future, so shall 
>> we go on with the current enhancement?
>> Thanks
>> Max
>> On Aug 29, 2014, at 2:17, Ulf Zibis <Ulf.Zibis at CoSoCo.de> wrote:
>>> I mean:
>>> It does not output byte code that only uses a single char array to 
>>> compose the entire String in question.
>>> With "optimization fails", I also mean, there is used an additional 
>>> "StringComposer" e.g. another StringBuilder or a StringJoiner in 
>>> addition to the 1st StringBuilder.
>>> -Ulf
>>> Am 27.08.2014 um 14:02 schrieb Pavel Rappo:
>>>> Could you please explain what you mean by "javac optimization 
>>>> fails" here?
>>>> -Pavel
>>>> On 27 Aug 2014, at 10:41, Ulf Zibis <Ulf.Zibis at CoSoCo.de> wrote:
>>>>> 4.) Now we see, that javac optimization fails again if 
>>>>> StringBuilder, concatenation, toString(), append(String), 
>>>>> append(Collection) etc. and StringJoiner use is mixed.

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