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$m is assigned the most significant set bit of 64-bit integer $z, for 
+the cost of 6 ANDs and 6 truth tests.

...
                  4:                     2
    :             2                      ;

my $z=17;
my $m;
...
                        0xFFFFFFFFFFFFFFFC&$z?  61:     62
                    :   0xFFFFFFFFFFFFFFFF&$z?  63:     64;
                    }

my $m=0xFFFF8000&$z?
        0xFF800000&$z?
...
            :   0xFFFFFFFE&$z?
                    0xFFFFFFFC&$z?  29: 30
                :   0xFFFFFFFF&$z?  31: 32;