Missing trivial optimization in String equals (original) (raw)

Claes Redestad claes.redestad at oracle.com
Tue Jan 23 00:13:49 UTC 2018

Hi,

do you have an example application that is spending significant time comparing the contents of such identical arrays and would benefit? I would guess such comparisons are about twice as fast as typical ones due to having to load half as much from memory. ;-)

If there's a valid optimization case, though, then it would only be realizable when UseStringDeduplication is enabled. Thus it's crucial not to regress all the other cases that won't benefit (Parallel, Serial, ...), and as String::equals is often a very hot method then even a few added instructions that only compare data we've already loaded into registers might be too much. Or maybe not.

As String::equals is routinely replaced by a C2 intrinsic[1] that's where we should look to optimize (not so much in the JDK library code). I think it'd be straightforward to train that intrinsic to only emit the test when the related feature is enabled. Maybe the kind folks over at hotspot-compiler-dev at openjdk.java.net might be able to give some pointers. If so I'd invite you to set up a set of experiments and see if this can have a benefit.

Regards

/Claes

[1] http://hg.openjdk.java.net/jdk/jdk/file/e1876e6b57b6/src/hotspot/share/opto/library_call.cpp#l1116

On 2018-01-23 00:14, Martin Grajcar wrote:

It looks like when testing equality of Strings, there's no test if the two arrays are actually the same object. This was impossible before Java 8u20 (as no two strings used to share arrays), but it's well possible with -XX:+UseStringDeduplication.

Maybe someone here cares... Regards, Martin. http://hg.openjdk.java.net/jdk10/jdk10/jdk/file/777356696811/src/java.base/ share/classes/java/lang/String.java#l1014 http://hg.openjdk.java.net/jdk10/jdk10/jdk/file/777356696811/src/java.base/ share/classes/java/lang/StringLatin1.java#l89 http://hg.openjdk.java.net/jdk10/jdk10/jdk/file/777356696811/src/java.base/ share/classes/java/lang/StringUTF16.java#l258

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