Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!hpda!hpcuhb!hpcilzb!hpcea!hpausla!jcl
From: jcl@hpausla.HP.COM (Jeff Laing)
Newsgroups: comp.lang.forth
Subject: Re: Re: Re: Re: Assembly or .... (two-result functions)
Message-ID: <2560002@hpausla.HP.COM>
Date: 11 Dec 88 22:06:30 GMT
References: <3459@geaclib.UUCP>
Organization: HP Australian Software Operation
Lines: 20

>	      applications. One example is an operation you need for a
>	      fft program. There you have to reverse the bitorder of a word.
>	      In assembly language there's a very elegant and fast solution :
>             ... code deleted ...
>	      If you want to write the same program in a high-level language
>	      you have to work with bit-masks and test operations, which
>	      is (compared to the program above) very slow and complicated.

Of course, if you want it to run FAST, you drop back to a nice high level
language and do something like

static int rev15[16] = { 0, 8, 2, 12, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15}

flipped = ( ( rev15[(orig    ) & 0x000F] << 24 ) |
            ( rev15[(orig>>8 ) & 0x000F] << 16 ) |
            ( rev15[(orig>>16) & 0x000F] << 8  ) |
            ( rev15[(orig>>24) & 0x000F]       )  )

Not a loop in sight, and your compiler should be able to make all those ANDs,
ORs and SHIFTS real quick as well ...