Hello!
It's quite possible that when we concatenate two strings, one of them
appears to be empty. We cannot simply return another string in this
case, as JLS 15.18.1 explicitly says (for unknown to me reason) about
the result of the string concatenation expression that 'The String
object is newly created'. However, it's still possible to reuse the
internal array of another string to reduce allocations:
--- src/java.base/share/classes/java/lang/StringConcatHelper.java
(revision 58998:04e3d254c76be87788a40cbd66d013140ea951d8)
+++ src/java.base/share/classes/java/lang/StringConcatHelper.java
(revision 58998+:04e3d254c76b+)
@@ -420,6 +420,12 @@
static String simpleConcat(Object first, Object second) {
String s1 = stringOf(first);
String s2 = stringOf(second);
+ if (s1.isEmpty()) {
+ return new String(s2.value(), s2.coder());
+ }
+ if (s2.isEmpty()) {
+ return new String(s1.value(), s1.coder());
+ }
// start "mixing" in length and coder or arguments, order is not
// important
long indexCoder = mix(initialCoder(), s2);
Very simple benchmark like this validates that the concatenation
became faster if one of the strings is empty:
@Param({"", "longlonglongline"})
String data;
@Param({"", "longlonglongline"})
String data2;
@Benchmark
public String plus() {
return data + data2;
}
Without patch I observe on VM 15-ea+20-899:
Benchmark (data) (data2) Mode Cnt Score Error Units
plus avgt 30 15,335 ± 0,186 ns/op
plus longlonglongline avgt 30 19,867 ± 0,109 ns/op
plus longlonglongline avgt 30 20,283 ± 0,230 ns/op
plus longlonglongline longlonglongline avgt 30 26,047 ± 0,230 ns/op
With patch:
Benchmark (data) (data2) Mode Cnt Score Error Units
plus avgt 30 6,668 ± 0,055 ns/op
plus longlonglongline avgt 30 6,708 ± 0,114 ns/op
plus longlonglongline avgt 30 7,003 ± 0,064 ns/op
plus longlonglongline longlonglongline avgt 30 25,126 ± 0,392 ns/op
There could be an added cost of up to two branches for the normal case
(I believe, if one of the strings is constant, then decent JIT can
eliminate one of the branches). However, I believe, the benefit could
outweigh it, as empty strings are not that uncommon and in this case,
we reduce O(N) time and memory consumption to O(1).
What do you think? Is this a reasonable thing to do? I can file an
issue and submit a proper webrev if it looks like a useful patch.
With best regards,
Tagir Valeev.
