Messages in this thread | | | Date | Tue, 20 May 2008 13:01:27 +0200 (CEST) | From | Soumyadip Das Mahapatra <> | Subject | Re: [PATCH 1/2] bitreversal program |
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On Mon, 19 May 2008, Harvey Harrison wrote:
> On Mon, 2008-05-19 at 19:04 +0200, Soumyadip Das Mahapatra wrote: >> --- a/include/linux/bitrev.h 2008-04-17 08:19:44.000000000 +0530 >> +++ b/include/linux/bitrev.h 2008-05-19 21:49:46.000000000 +0530 >> @@ -3,11 +3,32 @@ >> >> #include <linux/types.h> >> >> -extern u8 const byte_rev_table[256]; >> +/** >> + * Here is a generalised bit reversal program >> + * @x: word to get bits reversed >> + * @k: key, explained below >> + * for k = 31, it reverses the bits of word(32 bit) >> + * for k = 24, it reverses the bytes in word >> + * for k = 7, it reverses the bits in every byte without >> + * changing the positions of bytes in a word >> + * and for k = 16 it swaps the left and right halves of a >> + * word >> + */ >> >> -static inline u8 bitrev8(u8 byte) > > What about anybody who currently uses bitrev8? > >> +static inline u32 gen_bit_rev(u32 x, u32 k) >> { >> - return byte_rev_table[byte]; >> + if(k & 1) >> + x = (x & 0x55555555) << 1 | (x & 0xaaaaaaaa) >> 1; >> + if(k & 2) >> + x = (x & 0x33333333) << 2 | (x & 0xcccccccc) >> 2; >> + if(k & 4) >> + x = (x & 0x0f0f0f0f) << 4 | (x & 0xf0f0f0f0) >> 4; >> + if(k & 8) >> + x = (x & 0x00ff00ff) << 8 | (x & 0xff00ff00) >> 8; >> + if(k & 16) >> + x = (x & 0x0000ffff) << 16 | (x & 0xffff0000) >> 16; >> + >> + return x; >> } > > Why is this better than a single 256 byte table? > > Harvey > > > -- > This message has been scanned for viruses and > dangerous content by MailScanner, and is > believed to be clean. > >
Thanks for reviewing Harvey :-) please look at the line below >> -static inline u8 bitrev8(u8 byte) It is a static function, so you cant use it from outside of this file. So there should not be anyone using this function.
>Why is this better than a single 256 byte table?
Why store those things if stuffs can be done in smoother and cleaner (using less memeory ofcourse) way!
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