From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-help-return-25639-listarch-gcc-help=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 30963 invoked by alias); 20 Nov 2006 15:31:48 -0000
Received: (qmail 30954 invoked by uid 22791); 20 Nov 2006 15:31:47 -0000
X-Spam-Check-By: sourceware.org
Received: from mx2.redhat.com (HELO mx2.redhat.com) (66.187.237.31)     by sourceware.org (qpsmtpd/0.31) with ESMTP; Mon, 20 Nov 2006 15:31:39 +0000
Received: from int-mx2.corp.redhat.com (int-mx2.corp.redhat.com [172.16.27.26]) 	by mx2.redhat.com (8.12.11.20060308/8.12.11) with ESMTP id kAKFVbCQ023221 	for <gcc-help@gcc.gnu.org>; Mon, 20 Nov 2006 10:31:37 -0500
Received: from zebedee.littlepinkcloud.COM (vpn-14-158.rdu.redhat.com [10.11.14.158]) 	by int-mx2.corp.redhat.com (8.13.1/8.13.1) with ESMTP id kAKFVWUi009528; 	Mon, 20 Nov 2006 10:31:33 -0500
Received: from littlepinkcloud.COM (localhost.localdomain [127.0.0.1]) 	by zebedee.littlepinkcloud.COM (8.13.6/8.13.5) with ESMTP id kAKFVUtW010821; 	Mon, 20 Nov 2006 15:31:31 GMT
Received: (from aph@localhost) 	by littlepinkcloud.COM (8.13.6/8.13.5/Submit) id kAKFVU9t010816; 	Mon, 20 Nov 2006 15:31:30 GMT
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <17761.51794.41304.831893@zebedee.pink>
Date: Mon, 20 Nov 2006 15:31:00 -0000
From: Andrew Haley <aph@redhat.com>
To: "tom@womack.net" <tom@womack.net>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Register constraint 'not RAX or RDX'
In-Reply-To: <12693659.1164035234728.JavaMail.?@fh1035.dia.cp.net>
References: <12693659.1164035234728.JavaMail.?@fh1035.dia.cp.net>
X-Mailer: VM 7.19 under Emacs 21.4.1
X-IsSubscribed: yes
Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-help@gcc.gnu.org>
Sender: gcc-help-owner@gcc.gnu.org
X-SW-Source: 2006-11/txt/msg00249.txt.bz2

tom@womack.net writes:
 > What's the right way of writing the 'compute a*b%c' intrinsic?
 > 
 > I can do
 > 
 > #define mulmod(a,b,c,d,e) { asm ("mul %%rbx; div %%rsi;" : "=d" (d), 
 > "=a" (e) : "a" (a), "b" (b), "S" (c));}
 > 
 > but that causes quite a lot of ugly register-shuffling
 > 
 > If I do
 > 
 > #define mulmod(a,b,c,d,e) { asm ("mul %0; div %1;" : "=d" (d), "=a" 
 > (e) : "a" (a), "r" (b), "r" (c));}
 > 
 > then the program doesn't work; disassembling
 > 
 > u64 zul(u64 a, u64 b, u64 p)
 > {
 >   u64 x,y;
 >   mulmod(a,b,p,x,y);
 >   return x;
 > }
 > 
 > gives
 > 
 > 00000000004005b0 <_Z3zulyyy>:
 >   4005b0:       48 89 f8                mov    %rdi,%rax
 >   4005b3:       48 f7 e2                mul    %rdx
 >   4005b6:       48 f7 f0                div    %rax
 >   4005b9:       48 89 d0                mov    %rdx,%rax
 >   4005bc:       c3                      retq
 > 
 > which is clearly crazy since %rax from 4005b0 will have been 
 > overwritten by the mul command before it's used.

#define mulmod(a,b,c,d,e)			\
{						\
  asm ("mul %3\n\tdiv %4\n"			\
       : "=&d" (d), "=a" (e)			\
       : "1" (a), "g" (b), "g" (c));		\
}

Andrew.