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@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.ht...
[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