Hi Fabian; This has been an excellent discussion and thank you for starting it! Even if there is disagreement on various aspects I don't believe that anyone is upset or angry.
Mike On Aug 30 2014, at 10:04 , Fabian Lange <lange.fab...@gmail.com> wrote: > Hello dear list, > > it was not my intention to steal precious developer time by annoying > you guys with my finding. > I really wanted to understand why there is a source difference. So I > went ahead and looked at it from the only aspect I could estimate > contributing to it. > I am well aware of the fact that JIT does an amazing job optimizing > code, so it surprises me how in some replies there is an negative > undertone and randomness attributed to JIT. > Right now I assume that my question upset some people, because it is > of course not nice that people question code which was written over a > decade ago. > If not discussing the question on a technical level, I do not know why > the argument of time wasting on micro level is made in multiple page > long e-mails, which for sure also took precious time to write. Having the right perspective is important to know how to handle specific situations. I believe the longer emails are mostly discussion of the general case and considerations involved in deciding what to do in a situation such as this. If they provide future guidance then they have done their job. > > So thanks to everybody who taught me a bit about inner workings of JIT, > and special thanks to Martin who did have the courage to submit a bug > and webrev! Indeed! He even found a few more cases to adjust. Mike > > Fabian: > > PS: Out of curiosity I looked at the contains implementation here: > https://android.googlesource.com/platform/libcore/+/master/luni/src/main/java/java/util/LinkedList.java > it is manually inlined to avoid counting the position at all. > I wonder if JIT would do that at any point as well. > > On Wed, Aug 27, 2014 at 4:02 PM, Fabian Lange <lange.fab...@gmail.com> wrote: >> Hi all, >> I have been involved recently in some theoretical or nonsensical >> discussions about microbenchmarking, jit compiling assemblies and so >> fort. >> One example was LinkedList vs ArrayList. >> >> What I noticed is that those two have a different implementation for >> contains(): >> >> ArrayList: >> >> public boolean contains(Object o) { >> return indexOf(o) >= 0; >> } >> >> LinkedList: >> >> public boolean contains(Object o) { >> return indexOf(o) != -1; >> } >> >> Logically this is of course identical due to the contract of contains >> which returns either -1 or the >=0 index of the element. >> >> This code has been like this almost forever, and I was wondering if >> this actually makes a difference in CPU cycles. >> >> And in fact this code compiles into different assembler instructions. >> The array list does a test against 0 and conditional move, while the >> linked list does a jump equals on -1. >> >> Again that is not surprising, because the actual java source is >> different. But I wonder if both options are equally good in cold >> performance and when jitted based on parameter values. >> >> Wouldn't one implementation be better than the other? And why is not >> the "better" implementation taken in both classes (and maybe other >> Collections which use indexOf) ? >> >> Is the answer that this has always been like this and the benefit is >> not worth the risk of touching ancient code? >> >> And if not for performance, would code clarify and similarity be an argument? >> >> (this message was posted to jdk8-dev initially, thanks to Dalibor >> Topic for the pointer to this list) >> >> Fabian
