RFR: JDK-8224963: Char-Byte Performance Enhancement

[Andrew Dinn]

Hi Adam,

On 11/06/2019 10:04, Adam Farley8 wrote:
> Merely patching the class library code will not effect any performance
> enhancement.
> 
> Quite the opposite. :)
> 
> The changes in the class library were designed to work in tandem with
> some changes to the OpenJ9 JIT compiler, which is what accelerates
> performance.
> 
> Naturally, investigating and sharing OpenJ9 logic is a bad idea due to
> contamination, so we're stuck with investigating the CL changes to
> identify any potential value to OpenJDK with Hotspot.

This seems a slightly odd way to go about arriving at an improvement in
the generated code.

What you probably ought to do in order to make progress here -- and
/without/ exposing any secrets about the J9 implementation -- is to take
an assembly dump of the x86 code that is currently being generated and
then identify some transformation to that x86 code which 1) brings it
nearer to what J9 produces and/or 2) clearly improves performance (you
appear to be assuming 1 and 2 go together here but that is not actually
required). n.b. you could identify such a generated code transformation
starting either from the existing Java source or from your modified Java
source.

If you /can/ specify what the improved machine code needs to look like
relative to the current code then it would be possible and, perhaps,
relatively easy for someone who does understand the C2 compiler to
determine whether there is a way to adopt this alternative
code-generation strategy simply by tweaking the C2 compiler.

Of course, the alternative might be that effecting such a change in
generated code would require major changes to C2 -- it might even be (as
the Arkansas farmer says) that you can't get there from here. Whatever,
proceeding as suggested above won't expose what J9 is doing and won't
require you to know what C2 is doing.

regards,


Andrew Dinn
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