String.indexOf optimization

[Andrew Haley]

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/vm/macroAssembler_x86.cpp

> 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.