linux-kernel - Re: [PATCH 1/2] bitreversal program

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Message-Id: <1211229736.5915.86.camel@brick>
Date:	Mon, 19 May 2008 13:42:15 -0700
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Soumyadip Das Mahapatra <kernelhacker@...ualserver.org>
Cc:	linux-kernel@...r.kernel.org, akinobu.mita@...il.com
Subject: Re: [PATCH 1/2] bitreversal program

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

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