OpenJDK / jdk / jdk
changeset 54378:7c576e4d0afa
8221723: Avoid storing zero to String.hash
Reviewed-by: shade, prappo, jiangli
| author | redestad |
|---|---|
| date | Tue, 02 Apr 2019 11:24:40 +0200 |
| parents | 35794e8db61b |
| children | 40a7e2fc9beb |
| files | src/java.base/share/classes/java/lang/String.java test/micro/org/openjdk/bench/java/lang/StringHashCode.java |
| diffstat | 2 files changed, 20 insertions(+), 3 deletions(-) [+] |
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--- a/src/java.base/share/classes/java/lang/String.java Tue Apr 02 10:04:35 2019 +0200 +++ b/src/java.base/share/classes/java/lang/String.java Tue Apr 02 11:24:40 2019 +0200 @@ -1510,8 +1510,14 @@ public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { - hash = h = isLatin1() ? StringLatin1.hashCode(value) - : StringUTF16.hashCode(value); + h = isLatin1() ? StringLatin1.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; }
--- a/test/micro/org/openjdk/bench/java/lang/StringHashCode.java Tue Apr 02 10:04:35 2019 +0200 +++ b/test/micro/org/openjdk/bench/java/lang/StringHashCode.java Tue Apr 02 11:24:40 2019 +0200 @@ -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. * * This code is free software; you can redistribute it and/or modify it @@ -42,11 +42,13 @@ private String hashcode; private String hashcode0; + private String empty; @Setup public void setup() { hashcode = "abcdefghijkl"; 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 int notCached() { 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(); + } }