RFR: 8224986: (str) optimize StringBuilder.append(CharSequence, int, int) for String arguments

[Ivan Gerasimov]

Hi Claes!

It looks like for the cases when this.isLatin1() != s.isLatin1() the 
code is essentially the same as in appendChars(CharSequence, int, int).
So, to save some byte codes (which can actually help with inlining), it 
might have been written as:

private final void appendChars(String s, int off, int end) {
     if (isLatin1() ^ s.isLatin1()) {
         appendChars((CharSequence)s, off, end);
     } else {
         System.arraycopy(s.value(), off << coder, this.value, 
this.count << coder, (end - off) << coder);
         count += end - off;
     }
}

With kind regards,
Ivan

On 5/29/19 8:36 AM, Claes Redestad wrote:
> Hi,
>
> it's been pointed out[1] that append(string.substring(start, end)) can
> outperform append(string, start, end) in some circumstances. In part due
> the former being intrinsified, but also I think due it tickling some
> string concat optimizations that allow for eliding the StringBuilder
> itself completely when there's only one non-constant argument.
>
> While not ruling out intrinsification of append(String, int, int), I was
> able to achieve a reasonable speed-up by peeling off String arguments
> to append(CharSequence, int, int) and dropping into a routine that uses
> System.arraycopy when appropriate.
>
> This seems like a reasonable improvement in the interim, and an
> opportunity to incorporate the relevant microbenchmarks.
>
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8224986
> Webrev: http://cr.openjdk.java.net/~redestad/8224986/open.00/
>
> I extended and incorporated the microbenchmark provided in [1] to also 
> include variants with UTF16 Strings and mixes of arguments, ensuring
> speedups on all appendBounds* variants[2].
>
> Thanks!
>
> /Claes
>
> [1] 
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2019-January/057834.html
> [2]
> Baseline:
>
> Benchmark            (length)  Mode  Cnt     Score     Error Units
> appendBounds             1000  avgt   10   576.939 ±  21.125 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  2144.000 ±   0.001 B/op
> appendBoundsMix          1000  avgt   10   855.308 ±  27.071 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  3160.000 ±   0.001 B/op
> appendBoundsUtf16        1000  avgt   10  1424.518 ±  33.319 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  5192.000 ±   0.001 B/op
>
> Patch:
>
> Benchmark            (length)  Mode  Cnt     Score     Error Units
> appendBounds             1000  avgt   10   466.640 ±  15.069 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  2120.000 ±   0.001 B/op
> appendBoundsMix          1000  avgt   10   758.886 ±  22.983 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  3160.000 ±   0.001 B/op
> appendBoundsUtf16        1000  avgt   10  1320.360 ±  49.097 ns/op
>  ·gc.alloc.rate.norm     1000  avgt   10  5192.000 ±   0.001 B/op
>

-- 
With kind regards,
Ivan Gerasimov