Impact of code difference in Collection#contains() worth improving?

[Martin Buchholz]

Just think - one whole byte saved for each individual change!
I have a webrev!
http://cr.openjdk.java.net/~martin/webrevs/openjdk9/pico-optimize-contains/
https://bugs.openjdk.java.net/browse/JDK-8056951

Can haz review please?


On Fri, Aug 29, 2014 at 1:05 PM, Ulf Zibis <Ulf.Zibis at cosoco.de> wrote:

>
> Am 28.08.2014 um 19:46 schrieb Vitaly Davidovich:
>
>
>> There's no register pressure - the immediate (I.e. -1) is encoded
>> directly into the instruction, just like 0 would be.  The time when 0 is
>> particularly useful is when you test for it in the zero bit of the flags
>> register (e.g. dec followed by jz, such as when counting a loop down to
>> 0).  Otherwise, I don't see any advantage from machine code perspective.
>>
>>
> Thanks for explaining this, but a very little nit: the immediate (I.e. -1)
> uses additional 32/64 bits in code which must be loaded from memory and
> wastes space in CPU cache or am I wrong? This could be saved with >= 0.
>
> So if unifying the code I agree to Martin's opinion.
>
> -Ulf
>
>  The aforementioned cmov instruction is not without its own downsides, so
>> it's unclear which is better when branch probability isn't known a priori.
>>
>> The 1 byte code is unlikely to make any difference, unless jit is turned
>> off and you're running this through a tight loop in the interpreter (but if
>> one does that, perf conversation is moot :)).
>>
>> Sent from my phone
>>
>> On Aug 28, 2014 1:28 PM, "Ulf Zibis" <Ulf.Zibis at cosoco.de <mailto:
>> Ulf.Zibis at cosoco.de>> wrote:
>>
>>
>>     Am 27.08.2014 um 17:51 schrieb Martin Buchholz:
>>
>>         The ArrayList version saves one byte of bytecode, and is
>> therefore very
>>         slightly better.  We should bless that version and use it
>> consistently.
>>
>>
>>     +1
>>     Additional argument:
>>     The LinkedList code requires to load 32/64-Bit -1 into CPU. This may
>> take some time on some
>>     CPU and at least wastes memory footprint.
>>     Additionally register pressure increases.
>>     Vitaly, please correct me, if I'm wrong, just for learning more.
>>
>>     Another advantage is that there is no problem if some implementation
>> of indexOf() erroneously
>>     returns another negative value than -1. I remember some compare()
>> implementations, which
>>     sometimes return different values than only -1, 0, +1.
>>
>>     -Ulf
>>
>>             ArrayList:
>>
>>                  public boolean contains(Object o) {
>>                      return indexOf(o) >= 0;
>>                  }
>>
>>             LinkedList:
>>
>>                  public boolean contains(Object o) {
>>                      return indexOf(o) != -1;
>>                  }
>>
>>
>>
>