RFR: 8215017: Improve String::equals warmup characteristics

Thu Apr 11 16:06:12 UTC 2019

I'm confused.
This change seems to remove the one and only call to StringUTF16.equals,
making that dead code?
Looking more closely at StringUTF16.equals and StringLatin1.equals, both
seem to boil down to equivalent """do the byte arrays contain the same
bytes?"""?
Which suggests we can coalesce these two methods and have String.equals
simply check that the two coders and bytes are the same?
Arrays.equals(byte[],byte[]) is also an intrinsic but has more checks.
Hmmmm ... there's been performance work on ArraysSupport.mismatch.  Maybe
we're not using it for Strings because we expect Strings to be short?
I don't know if hotspot these days needs to keep intrinsics around for
library code that has been deleted.

On Thu, Apr 11, 2019 at 7:09 AM Claes Redestad <claes.redestad at oracle.com>
wrote:

> Hi,
>
> please review this improved version, which drops the new sameCoder()
> method and insteads inlines that test in the one place where
> its use is performance sensitive.
>
> Webrev: http://cr.openjdk.java.net/~redestad/8215017/jdk.01/
>
> Microbenchmark show perfectly matching results, except an additional
> 8-20% improvement in interpreter results (which means a more
> noticeable improvement in startup/warmup tests):
>
> Benchmark                      Mode  Cnt    Score    Error  Units
> StringEquals.almostEqual       avgt    5  740.164 ±  2.586  ns/op
> StringEquals.almostEqualUTF16  avgt    5  707.969 ±  2.796  ns/op
> StringEquals.different         avgt    5  357.230 ±  9.751  ns/op
> StringEquals.differentCoders   avgt    5  287.026 ±  2.112  ns/op
> StringEquals.equal             avgt    5  762.012 ±  5.500  ns/op
> StringEquals.equalsUTF16       avgt    5  767.058 ±  4.619  ns/op
>
> Thanks!
>
> /Claes
>
> On 2018-12-08 01:11, Claes Redestad 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
>