RFR: 8215017: Improve String::equals warmup characteristics

[Martin Buchholz]

Claes, make sure you've mind-melded with Aleksey on this, e.g. via
https://www.youtube.com/watch?v=wIyeOaitmWM

On Fri, Dec 7, 2018 at 4:07 PM Claes Redestad <claes.redestad at oracle.com>
wrote:

> Hi,
>
> following up from discussions during review of JDK-8214971[1], I
> examined the startup and peak performance of a few different variant of
> writing String::equals.
>
> Webrev: http://cr.openjdk.java.net/~redestad/8215017/jdk.00/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8215017
>
> - folding coder() == aString.coder() into sameCoder(aString) helps
> interpreter without adversely affecting higher optimization levels
>
> - Jim's proposal to use Arrays.equals is _interesting_: it improves
> peak performance on some inputs but regress it on others. I'll defer
> that to a future RFE as it needs a more thorough examination.
>
> - what we can do is simplify to only use StringLatin1.equals. If I'm not
> completely mistaken these are all semantically neutral (and
> StringUTF16.equals effectively redundant). If I'm completely wrong here
> I'll welcome the learning opportunity. :-)
>
> This removes a branch and two method calls, and for UTF16 Strings we'll
> use a simpler algorithm early, which turns out to be beneficial during
> interpreter and C1 level.
>
> I added a simple microbenchmark to explore this, results show 1.2-2.5x
> improvements in interpreter performance, while remaining perfectly
> neutral results for optimized code on this simple micro[2].
>
> This could be extended to clean up and move StringLatin1.equals back
> into String and remove StringUTF16, but we'd also need to rearrange the
> intrinsics on the VM side. Let me know what you think.
>
> Thanks!
>
> /Claes
>
> [1]
>
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-December/057162.html
>
> [2]
> ========== Baseline =================
>
> -Xint
> Benchmark                            Mode  Cnt     Score    Error  Units
> StringEquals.equalsAlmostEqual       avgt    4   968.640 ±  1.337  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  2082.007 ±  5.303  ns/op
> StringEquals.equalsDifferent         avgt    4   583.166 ± 29.461  ns/op
> StringEquals.equalsDifferentCoders   avgt    4   422.993 ±  1.291  ns/op
> StringEquals.equalsEqual             avgt    4   988.671 ±  1.492  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  2103.060 ±  5.705  ns/op
>
> -XX:+CompactStrings
> Benchmark                            Mode  Cnt   Score   Error  Units
> StringEquals.equalsAlmostEqual       avgt    4  23.896 ± 0.089  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  23.935 ± 0.562  ns/op
> StringEquals.equalsDifferent         avgt    4  15.086 ± 0.044  ns/op
> StringEquals.equalsDifferentCoders   avgt    4  12.572 ± 0.008  ns/op
> StringEquals.equalsEqual             avgt    4  25.143 ± 0.025  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  25.148 ± 0.021  ns/op
>
> -XX:-CompactStrings
> Benchmark                            Mode  Cnt   Score   Error  Units
> StringEquals.equalsAlmostEqual       avgt    4  24.539 ± 0.127  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  22.638 ± 0.047  ns/op
> StringEquals.equalsDifferent         avgt    4  13.930 ± 0.835  ns/op
> StringEquals.equalsDifferentCoders   avgt    4  13.836 ± 0.025  ns/op
> StringEquals.equalsEqual             avgt    4  26.420 ± 0.020  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  23.889 ± 0.037  ns/op
>
> ========== Fix ======================
>
> -Xint
> Benchmark                            Mode  Cnt    Score     Error  Units
> StringEquals.equalsAlmostEqual       avgt    4  811.859 ±   8.663  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  802.784 ± 352.884  ns/op
> StringEquals.equalsDifferent         avgt    4  431.837 ±   1.884  ns/op
> StringEquals.equalsDifferentCoders   avgt    4  358.244 ±   1.208  ns/op
> StringEquals.equalsEqual             avgt    4  832.056 ±   3.541  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  832.434 ±   3.516  ns/op
>
> -XX:+CompactStrings
> Benchmark                            Mode  Cnt   Score   Error  Units
> StringEquals.equalsAlmostEqual       avgt    4  23.906 ± 0.151  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  23.905 ± 0.123  ns/op
> StringEquals.equalsDifferent         avgt    4  15.088 ± 0.023  ns/op
> StringEquals.equalsDifferentCoders   avgt    4  12.575 ± 0.030  ns/op
> StringEquals.equalsEqual             avgt    4  25.149 ± 0.059  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  25.149 ± 0.033  ns/op
>
> -XX:-CompactStrings
> Benchmark                            Mode  Cnt   Score   Error  Units
> StringEquals.equalsAlmostEqual       avgt    4  24.521 ± 0.050  ns/op
> StringEquals.equalsAlmostEqualUTF16  avgt    4  22.639 ± 0.035  ns/op
> StringEquals.equalsDifferent         avgt    4  13.831 ± 0.020  ns/op
> StringEquals.equalsDifferentCoders   avgt    4  13.884 ± 0.345  ns/op
> StringEquals.equalsEqual             avgt    4  26.395 ± 0.066  ns/op
> StringEquals.equalsEqualsUTF16       avgt    4  23.904 ± 0.112  ns/op
>