From: Nicolas Pitre <nico@cam.org>
Date: Tue, 18 Aug 2009 00:39:30 GMT
Subject: Re: [PATCH] block-sha1: make the size member first in the context struct
Message-ID: <alpine.LFD.2.00.0908172035320.6044@xanadu.home>
In-Reply-To: <alpine.DEB.1.00.0908180223280.8306@pacific.mpi-cbg.de>

On Tue, 18 Aug 2009, Johannes Schindelin wrote:

> Hi,
> 
> On Mon, 17 Aug 2009, Nicolas Pitre wrote:
> 
> > This is a 64-bit value, hence having it first provides a better 
> > alignment.
> > 
> > Signed-off-by: Nicolas Pitre <nico@cam.org>
> > 
> > diff --git a/block-sha1/sha1.h b/block-sha1/sha1.h
> > index 6ff59b2..b864df6 100644
> > --- a/block-sha1/sha1.h
> > +++ b/block-sha1/sha1.h
> > @@ -7,9 +7,9 @@
> >   */
> >  
> >  typedef struct {
> > +	unsigned long long size;
> >  	unsigned int H[5];
> >  	unsigned int W[16];
> > -	unsigned long long size;
> >  } blk_SHA_CTX;
> 
> By the reasoning suggested in the commit message, should H[5] not go to 
> the end?

Both arrays are based of unsigned ints which have the same alignment 
rules.  Furthermore the size and H members are much more used than the W 
member, and keeping them close should help with CPU cache locality.


Nicolas