Re: [patch] arca-vm-2.2.5

Arvind Sankar
1999-04-07T21:08:46Z

On Wed, Apr 07, 1999 at 05:59:04PM +0200, Gabriel Paubert wrote:
>
>
> On Tue, 6 Apr 1999, David Miller wrote:
>
> > Date: Wed, 7 Apr 1999 00:49:18 +0200 (CEST)
> > From: Ingo Molnar <mingo@chiara.csoma.elte.hu>
> >
> > It should be 'inode >> 8' (which is done by the log2
> > solution). Unless i'm misunderstanding something.
> >
> > Consider that:
> >
> > (((unsigned long) inode) >> (sizeof(struct inode) & ~ (sizeof(struct inode) - 1)))
> >
> > sort of approximates this and avoids the funny looking log2 macro. :-)
>
> May I disagree ? Compute this expression in the case sizeof(struct inode)
> is a large power of 2. Say 0x100, the shift count becomes (0x100 & ~0xff),
> or 0x100. Shifts by amounts larger than or equal to the word size are
> undefined in C AFAIR (and in practice on most architectures which take
> the shift count modulo some power of 2).
>

typo there, I guess. the >> should be an integer division. Since the divisor is
a constant power of 2, the compiler will optimize it into a shift.

-- arvind