netdev - Re: [PATCH] NET : change layout of ehash table

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <200702091036.58474.dada1@cosmosbay.com>
Date:	Fri, 9 Feb 2007 10:36:58 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
Cc:	ak@...e.de, linux@...izon.com, akepner@....com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] NET : change layout of ehash table

On Friday 09 February 2007 10:15, David Miller wrote:
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Fri, 9 Feb 2007 10:06:24 +0100
>
> > Yes, but a decent C compiler for such targets should not use a
> > multiply instruction to perform a (idx * 12) operation... :)
>
> Good point.
>
> Actually, I could never get GCC to avoid a divide on sparc64 for
> certain kinds of pointer arithmetic when the elements were not
> a power of two.  It probably has something to do with signedness.
>
> I think I narrowed is down to the fact that you can't legally replace
> a signed divide with shift/add/subtract.  But I could be remembering
> things wrong.

Thats strange, because pointer arithmetic is unsigned...
I dont know when gcc started to use reciprocal division, maybe your gcc was 
very old ?

$ cat div.c
struct s1 {        int pad[3];        };

unsigned long diffptr(struct s1 *a, struct s1 *b)
{
return a - b;
}

If compiled on i386 , gcc-4.1.1 :

$ gcc -O2 -fomit-frame-pointer -S div.c

diffptr:
        movl    4(%esp), %eax
        subl    8(%esp), %eax
        sarl    $2, %eax
        imull   $-1431655765, %eax, %eax
        ret

If compiled on x86_64 , gcc-4.1.1:

diffptr:
        subq    %rsi, %rdi
        movabsq $-6148914691236517205, %rax
        sarq    $2, %rdi
        imulq   %rax, %rdi
        movq    %rdi, %rax
        ret
-
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html