linux-kernel - Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

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Message-Id: <200905251803.51478.lkml@morethan.org>
Date:	Mon, 25 May 2009 18:03:48 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Harald Welte <HaraldWelte@...tech.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

On Mon May 25 2009, H. Peter Anvin wrote:
> Michael S. Zick wrote:
> > On Mon May 25 2009, Michael S. Zick wrote:
> > 
> > In actual application, this *should not* make a difference.
> > 
> 
> No kidding.  This is a valid transformation for integers, since it is
> all done with 2's-complement arithmetic.
> 

Load Effective Address does two's complement arithmetic?
I'll take your word for it.

For example:

#include <stdio.h>

extern int diff_umask(int mask, int *cnt1, int *cnt2)
{ return (((mask - *cnt1) + *cnt2) & mask); }

int main() {
 int msk  = 0x7fffffff; /* max positive */
 int idx1 = 0x7ffffffd; /* max positive - 2 */
 int idx2 = 0x7fffffff; /* max positive */

 int rst;

 rst = diff_umask(msk, &idx1, &idx2);
 printf("\n\t%d\n", rst);  /* " 1 " - correct */
}

But that is because when it is compiled as a
single source file, gcc is hardcoding the lea
adjustment when it is not an external file:
(compare to the above listings)
Like I wrote - I don't use 31-bit ring buffers, so I don't care.

objdump -d testdiff:
- - - snip - - -
080483b0 <diff_umask>:
 80483b0:       8b 44 24 0c             mov    0xc(%esp),%eax
 80483b4:       8b 4c 24 04             mov    0x4(%esp),%ecx
 80483b8:       8b 10                   mov    (%eax),%edx
 80483ba:       8d 04 11                lea    (%ecx,%edx,1),%eax
 80483bd:       8b 54 24 08             mov    0x8(%esp),%edx
 80483c1:       2b 02                   sub    (%edx),%eax
 80483c3:       21 c8                   and    %ecx,%eax
 80483c5:       c3                      ret
- - - snip - - -

Mike

> Floating-point numbers is a whole other game.
> 
> 	-hpa
> 


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