RFR(S): 8187964: [s390][ppc]: Intrinsify Math.multiplyHigh(long, long)

[Schmidt, Lutz]

Dear all,

I would like to request reviews for this s390-only enhancement (ppc support was already implemented for other purpose):

Bug:    https://bugs.openjdk.java.net/browse/JDK-8187964
Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8187964.00/index.html

This change provides platform-specific implementations for the Math.multiplyHigh method, exploiting 64bit x 64bit => 128bit multiply instructions available on these platforms. Microbenchmark performance shows improvement of 4x to 5x for [s390] and 10x to 15x for [ppc].

Thank you!
Lutz

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