A DESCRIPTION OF THE PROBLEM :
A pattern like:
var low = a *b;
var high = Math.multiplyHigh(a, b);

generates two x86_64 multiplications even the hardware can do it in one instruction.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
java -XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,MultiplyTest::doMath MultiplyTest.java

---------- BEGIN SOURCE ----------
public class MultiplyTest {
  public static void main(String[] args) {
    long b = 987654321L;
    var acc = 0L;
    for (int i = 0; i < 200_000; i++) {
      acc += doMath(i, b);
    }
    System.exit((int) acc);
  }

  private static long doMath(long a, long b) {
    long low = a * b;
    long high = Math.multiplyHigh(a, b);
    return low + high;
  }
}
---------- END SOURCE ----------

FREQUENCY :
ALWAYS