RFR: JDK-8197594: String#repeat

[Jim Laskey]

Peter,

Very reasonable and worth considering. The main reason the power of 2 copy works well is that the source is (almost) always cached.

I thought about this a bit at the beginning and wondered about introducing Arrays.fill(T[] dst, T[] src) where dst is filled repeatedly from src. This can then become an intrinsic and “do the right thing” based on hardware. String::repeat can be adapted later. I didn’t pursue because I lacked other use cases.

File a performance against the implementation (once checked in) and we can see what the performance team thinks.

Cheers,

— Jim


> On Mar 1, 2018, at 7:50 AM, Peter Levart <peter.levart at gmail.com> wrote:
> 
> Hi,
> 
> On 03/01/2018 03:13 AM, Paul Sandoz wrote:
>> Hi Jim,
>> 
>> Looks good. I like the power of 2 copying.
> 
> Is this really the fastest way? Say you are doing this:
> 
> String s = ... 64 bytes ...
> 
> s.repeat(16384);
> 
> ...the last arraycopy will be copying 512 KiB from one part of memory to the other part. It means that the source 512 KiB range will not fit into L1 cache. Neither fully into L2 cache.
> 
> The fastest way might be to employ power-of-two copying until the range reaches L1 caches size / 2 for example (16 K ?) and then use the same source range repeatedly as a "stamp" until the rounded end.
> 
> What do you think?
> 
> Regards, Peter
>