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

[Jonathan Gibbons]

Yes, but is this really a big enough performance and footprint pain 
point to be worth addressing in javac itself?

We're now talking about some specific code construction like
     new StringBuilder().append(a + b + c)

Any other case where the string builder can be observed externally 
cannot be subject to the proposed optimization. The use case is now 
really getting pretty specific.

-- Jon

On 09/08/2014 10:41 AM, Guy Steele wrote:
> Good point, but counterpoint: it might be acceptable to have modified the string buffer in situations where throwing an exception would always cause the string buffer to become inaccessible.
>
> —Guy
>
> On Sep 8, 2014, at 1:30 PM, Jonathan Gibbons <jonathan.gibbons at oracle.com> wrote:
>
>> 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.