Re: Algoritmic Complexity Attacks and 2.4.20 the dcache code

[Alex Riesen]

David S. Miller, Fri, May 30, 2003 08:24:40 +0200:
>    From: Scott A Crosby <[e-mail]>
>    Date: 30 May 2003 00:04:24 -0500
>    
>    Have you seen the current dcache function?
>    
>    /* Linux dcache */
>    #define HASH_3(hi,ho,c)  ho=(hi + (c << 4) + (c >> 4)) * 11
>    
> Awesome, moving the Jenkins will actually save us some
> cycles :-)

    static
    int hash_3(int hi, int c)
    {
	return (hi + (c << 4) + (c >> 4)) * 11;
    }

gcc-3.2.1 -O2 -march=pentium

    hash_3:
	    pushl	%ebp
	    movl	%esp, %ebp
	    movl	12(%ebp), %eax
	    movl	8(%ebp), %ecx
	    movl	%eax, %edx
	    popl	%ebp
	    sall	$4, %edx
	    sarl	$4, %eax
	    addl	%ecx, %edx
	    addl	%eax, %edx
	    leal	(%edx,%edx,4), %eax
	    leal	(%edx,%eax,2), %eax
	    ret

It is not guaranteed to be this way on all architectures, of course.
But still - no multiplications.