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8221723: Avoid storing zero to String.hash

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Reviewed-by: shade, prappo, jiangli
This commit is contained in:
Claes Redestad 2019-04-02 11:24:40 +02:00
parent 6bce53795b
commit 1908897b93
2 changed files with 20 additions and 3 deletions
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10
src/java.base/share/classes/java/lang/String.java
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@ -1510,8 +1510,14 @@ public final class String
public int hashCode() { public int hashCode() {
int h = hash; int h = hash;
if (h == 0 && value.length > 0) { if (h == 0 && value.length > 0) {
hash = h = isLatin1() ? StringLatin1.hashCode(value) h = isLatin1() ? StringLatin1.hashCode(value)
: StringUTF16.hashCode(value); : StringUTF16.hashCode(value);
// Avoid issuing a store if the calculated value is also zero:
// in addition to a minor performance benefit, this allows storing
// Strings with zero hash code in read-only memory.
if (h != 0) {
hash = h;
}
} }
return h; return h;
} }

13
test/micro/org/openjdk/bench/java/lang/StringHashCode.java
View File

@ -1,5 +1,5 @@
/* /*
* Copyright (c) 2014 Oracle and/or its affiliates. All rights reserved. * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
* *
* This code is free software; you can redistribute it and/or modify it * This code is free software; you can redistribute it and/or modify it
@ -42,11 +42,13 @@ public class StringHashCode {
private String hashcode; private String hashcode;
private String hashcode0; private String hashcode0;
private String empty;
@Setup @Setup
public void setup() { public void setup() {
hashcode = "abcdefghijkl"; hashcode = "abcdefghijkl";
hashcode0 = new String(new char[]{72, 90, 100, 89, 105, 2, 72, 90, 100, 89, 105, 2}); hashcode0 = new String(new char[]{72, 90, 100, 89, 105, 2, 72, 90, 100, 89, 105, 2});
empty = new String();
} }
/** /**
@ -66,4 +68,13 @@ public class StringHashCode {
public int notCached() { public int notCached() {
return hashcode0.hashCode(); return hashcode0.hashCode();
} }
/**
* Benchmark testing String.hashCode() with the empty string. Since the
* empty String has hashCode = 0, this value is always recalculated.
*/
@Benchmark
public int empty() {
return empty.hashCode();
}
} }