Hi Christoph,
Can you take another look at the code just ahead of those instances.
Suppose when limit == 0, the empty elements were removed from the tail
of list.
It would seem then that the matchList would always be exactly the size.
Can you compare the performance?
Adding a check for an optimization usually is a bit slower in every
non-optimized case and that's worth watching out for.
Given the overhead of proposing and reviewing a change, will this be
worth it?
$.02, Roger
On 10/20/2018 01:43 PM, Christoph Dreis wrote:
Hi,
while looking through the code of String.split() and Pattern.split(), I
wondered if we could get rid of the calls to list.subList(0, resultSize) if
resultSize matches the size of the list. If that's the case, subList() seems
redundant.
JMH Benchmarks in single-shot mode show the following results with a new
revised version (attached below):
Benchmark Mode
Cnt Score Error Units
MyBenchmark.testNew ss 10 12957,900
± 7800,863 ns/op
MyBenchmark.testNew:·gc.alloc.rate ss 10 6,248 ±
3,231 MB/sec
MyBenchmark.testNew:·gc.alloc.rate.norm ss 10 1384,000 ± 0,001
B/op
MyBenchmark.testNew:·gc.count ss 10 ? 0
counts
MyBenchmark.testOld ss 10
25017,800 ± 19178,411 ns/op
MyBenchmark.testOld:·gc.alloc.rate ss 10 4,887 ±
3,764 MB/sec
MyBenchmark.testOld:·gc.alloc.rate.norm ss 10 1464,000 ± 0,001
B/op
MyBenchmark.testOld:·gc.count ss 10 ? 0
counts
Other benchmark modes show inconclusive results from the throughput
perspective, yet it's clearly visible that less objects are allocated.
Could anybody verify if that's a worthwhile improvement? And in case it is,
potentially sponsor it?
Thanks in advance.
Cheers,
Christoph
===== PATCH =====
diff -r 5e894b0f5e63 src/java.base/share/classes/java/lang/String.java
--- a/src/java.base/share/classes/java/lang/String.java Fri Oct 19 16:29:45
2018 -0700
+++ b/src/java.base/share/classes/java/lang/String.java Sat Oct 20 18:49:02
2018 +0200
@@ -2315,6 +2315,9 @@
}
}
String[] result = new String[resultSize];
+ if (list.size() == resultSize) {
+ return list.toArray(result);
+ }
return list.subList(0, resultSize).toArray(result);
}
return Pattern.compile(regex).split(this, limit);
diff -r 5e894b0f5e63
src/java.base/share/classes/java/util/regex/Pattern.java
--- a/src/java.base/share/classes/java/util/regex/Pattern.java Fri Oct 19
16:29:45 2018 -0700
+++ b/src/java.base/share/classes/java/util/regex/Pattern.java Sat Oct 20
18:49:02 2018 +0200
@@ -1293,6 +1293,9 @@
while (resultSize > 0 &&
matchList.get(resultSize-1).equals(""))
resultSize--;
String[] result = new String[resultSize];
+ if (matchList.size() == resultSize) {
+ return matchList.toArray(result);
+ }
return matchList.subList(0, resultSize).toArray(result);
}
