null_if_double_check.eq(((check_from & (check_from - 1)) - 1) >> 63),

Interconnect      = 200
(check_from - 1)  = 1039 + 62*23 + 355 = 2820
Interconnect      = 200
(check_from & ^^) = 300
Interconnect      = 200
(^^ - 1)          = 1039 + 62*23 + 355 = 2820
Interconnect      = 200
                  = 6540ps

            null_if_double_check.eq(Repl((check_from & (check_from - 1)).bool(), 64)),

Interconnect      = 200
(check_from - 1)  = 1039 + 62*23 + 355 = 2820
Interconnect      = 200
(check_from & ^^) = 300
Interconnect      = 200
LUT6 (64 -> 11)   = 605
Interconnect      = 200
LUT6 (11 -> 2)    = 605
Interconnect      = 200
LUT2 (2 -> 1)     = 300
Interconnect      = 200
                  = 5840ps

	    tnt's idea

Interconnect      = 200
LUT6 (64 -> 22)   = 605
Interconnect      = 200
LUT6 (22 -> 4)    = 605
Interconnect      = 200
LUT4 (4 -> 1)     = 462
Interconnect      = 200
                  = 2272ps