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From: Kemal <kemalinanc8@gmail.com>
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X-Bullshit: scale-out dependency replication descriptor database 
Subject: Re: [9front] [PATCH] branchless assembly abs() and labs() for amd64
Reply-To: 9front@9front.org
Precedence: bulk

hello again,

on maintainability issue, we can do it on c with bit shifts like this:

#include <u.h>
#include <libc.h>

int
abs(int a)
{
 return (a ^ (a >> 8*sizeof(int)-1)) - (a >> 8*sizeof(int)-1));
}

so that we don't have to maintain more platform specific code.

on size, the assembly one seems to be the clear winner. the original
abs's object file (unlinked) is around 400~ bytes, assembly one is
120-140~, the branchless c one is 300~.

on performance, well i couldn't do much tests on this and i think my
test program doesn't compare these 3 implementations properly. i might
try performance benchmarks if i can setup a better suite.

on if abs is commonly used, well i couldn't find a program that uses
it heavily so having branchless abs is not important.