If there's a clean improvement in the java code, it's worth putting in. You can try benchmarking with -Xint. Are we talking about this method? static int indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex) { It does look like we can tighten the code up a little... On Thu, Jan 8, 2015 at 3:05 PM, Zoltan Sziladi <kissziszi@gmail.com> wrote:
Thanks for the info. So that basically means we have 2 implementations of indexOf currently, one is in HotSpot, the other is in the JDK itself, which rarely gets executed. Aside from this later fact, isn't it still worth improving the JDK implementation if it is possible? I know that the intrinsified method gets executed most of the time, but still, if we can improve the JDK implementation also, why not? I don't know much about other JVMs but maybe a few don't intrinsify it?
Is there any existing test suite which is considered conclusive enough that if an implementation beats the naive algorithm in those testcases then it could be considered as a replacement in the JDK?
Regards, Zoltan
On Thu, Jan 8, 2015 at 12:42 PM, Vitaly Davidovich <vitalyd@gmail.com> wrote:
The java impl you saw would be called by (a) interpreter, (b) if you explicitly disable intrinsification of this function, or (c) some other JVM that doesn't intrinsify this method (or any method).
People don't usually disable intrinsics; if they do, it's because they hit some JIT bug and may disable it.
On Thu, Jan 8, 2015 at 3:34 PM, Zoltan Sziladi <kissziszi@gmail.com> wrote:
Hi,
Thanks everyone for all the info. So, just to go step by step in understanding this. Andrew said HotSpot would ignore my implementation. So why is there an implementation of indexOf at all in the JDK, if that's not the code that's executed? Is it just a default fallback? When is the indexOf function not intrinsified? When do people usually disable intrinsification? Sorry if these are newbie questions, I'm new to this part of Java.
Regards, Zoltan
On Tue, Jan 6, 2015 at 1:28 AM, Andrew Haley <aph@redhat.com> wrote:
Hi,
On 05/01/15 18:59, Zoltan Sziladi wrote:
This discussion was a long time ago, I was just reading through it to check again what was the last state of the discussion about the String.indexOf. There is one part which I still do not understand, hopefully someone could shed some light on it. A few emails ago Martin mentioned
"Hotspot seems to have some intrinsification of String.indexOf, which confuses me. Hotspot seems the right place to provide more optimizations for this, since there has been a fair amount of work creating high-performance low-level implementations of this idea in C."
Then Ivan asked what that actually meant, whether hotspot actually replaced the jdk implementation with a low level optimized C implementation, but I never saw an answer to that.
You can have a look at an implementation of MacroAssembler::string_indexof in
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/file/b6b89b8f8531/src/cpu/x86/v...
Can someone please explain this? If we somehow found an algorithm
that
beat
the naive implementation in the average case, would it be possible to just implement it in the JDK?
No, because HotSpot would ignore it. Any speed improvements have to be done in the architecture-dependent files.
Andrew.