Alex Herbert created RNG-191:
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Summary: Use java.lang.invoke to dynamically call Math multiply
high methods
Key: RNG-191
URL: https://issues.apache.org/jira/browse/RNG-191
Project: Commons RNG
Issue Type: Improvement
Components: core
Reporter: Alex Herbert
The following generators rely on a 128-bit multiplication result from two
64-bit longs:
* L128_X128_MIX
* L128_X256_MIX
* L128_X1024_MIX
* PHILOX_4X64
Java added support to java.lang.Math to compute the upper 64-bit result with
potential intrinsic calls to native functions:
||Method||JDK||Notes||
|[multiplyHigh (Javadoc JDK
21)|https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Math.html#multiplyHigh(long,long)]|9|Convert
to unsigned using:
Math.multiplyHigh(a, b) + ((a >> 63) & b) + ((b >> 63) & a)|
|[unsignedMultiplyHigh (Javadoc JDK
21)|https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Math.html#unsignedMultiplyHigh(long,long)]|18|
|
Since Commons RNG targets Java 8 these methods cannot be used. The current RNGs
use a software implementation to compose the upper bits of the 128-bit result.
However the methods can be used with a
[MethodHandle|https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/invoke/MethodHandle.html]
introduced in Java 7:
{code:java}
// find the method
MethodHandle h = MethodHandles.publicLookup()
.findStatic(Math.class,
"unsignedMultiplyHigh",
MethodType.methodType(long.class, long.class, long.class));
// invoke (snippet)
long a, b;
try {
long r = (long) h2.invokeExact(a, b);
} catch (Throwable e) {
throw new RuntimeException(e);
}
{code}
Investigate the use of MethodHandle within the named RNGs.
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