Re: Branchless binary search in Java?

[Michael McCandless <lu...@mikemccandless.com>]

On Thu, Jul 27, 2023 at 8:44 AM Uwe Schindler <uw...@thetaphi.de> wrote:

> Hi Mike,
>
> actually Hotspot is using CMOV. Some nodes after bytecode analysis are
> converted to CMoveNode and it has implementations to generate machine code
> (as far as i see) for x86, s390 and arm.
>
> The generic code is here:
> https://github.com/openjdk/jdk/blob/486c7844f902728ce580c3994f58e3e497834952/src/hotspot/share/opto/movenode.cpp
>
Thanks Uwe, this is wild.  I read the comment at the top of that source
file and couldn't understand it!  And it made it sound like there was no
solution (all five bullets have problems).

> Actually it is used in some cases, but I did not find out when it uses it
> to generate instructions from bytecode. It also has some code to optimize
> the cmov away if the result is known before (or not, e.g. for floats it
> does not do this).
>
> I think best would be to ask on the hotspot compiler list on suggestions
> how to write Java code to trigger the JVM to insert the CMOV.
>
Thanks.  I wonder if there is any practical performance impact of targeting
CMOV from javaland...

Mike McCandless

http://blog.mikemccandless.com