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8221836: Avoid recalculating String.hash when zero

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Co-authored-by: Peter Levart <peter.levart@gmail.com>
Reviewed-by: jrose, adinn
This commit is contained in:
Claes Redestad 2019-04-10 12:05:50 +02:00
parent d40aa622cf
commit 89a267ca46
6 changed files with 66 additions and 40 deletions
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23
src/java.base/share/classes/java/lang/String.java
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@ -164,6 +164,12 @@ public final class String
/** Cache the hash code for the string */
private int hash; // Default to 0
/**
* Cache if the hash has been calculated as actually being zero, enabling
* us to avoid recalculating this.
*/
private boolean hashIsZero; // Default to false;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
@ -1508,14 +1514,21 @@ public final class String
* @return a hash code value for this object.
*/
public int hashCode() {
// The hash or hashIsZero fields are subject to a benign data race,
// making it crucial to ensure that any observable result of the
// calculation in this method stays correct under any possible read of
// these fields. Necessary restrictions to allow this to be correct
// without explicit memory fences or similar concurrency primitives is
// that we can ever only write to one of these two fields for a given
// String instance, and that the computation is idempotent and derived
// from immutable state
int h = hash;
if (h == 0 && value.length > 0) {
if (h == 0 && !hashIsZero) {
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) {
if (h == 0) {
hashIsZero = true;
} else {
hash = h;
}
}