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.zi...@cosoco.de
<mailto:ulf.zi...@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;
}
