From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-154620-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 5597 invoked by alias); 12 Jun 2009 16:32:48 -0000
Received: (qmail 5587 invoked by uid 22791); 12 Jun 2009 16:32:47 -0000
X-SWARE-Spam-Status: No, hits=-2.6 required=5.0 	tests=AWL,BAYES_00,SPF_PASS
X-Spam-Check-By: sourceware.org
Received: from mail-fx0-f222.google.com (HELO mail-fx0-f222.google.com) (209.85.220.222)     by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 12 Jun 2009 16:32:33 +0000
Received: by fxm22 with SMTP id 22so2669865fxm.8         for <gcc@gcc.gnu.org>; Fri, 12 Jun 2009 09:32:31 -0700 (PDT)
Received: by 10.86.31.19 with SMTP id e19mr3991714fge.24.1244824350728;         Fri, 12 Jun 2009 09:32:30 -0700 (PDT)
Received: from yakj.usersys.redhat.com (nat-pool-brq.redhat.com [62.40.79.66])         by mx.google.com with ESMTPS id 4sm3004592fge.18.2009.06.12.09.32.29         (version=TLSv1/SSLv3 cipher=RC4-MD5);         Fri, 12 Jun 2009 09:32:30 -0700 (PDT)
Message-ID: <4A328311.2020803@gmail.com>
Date: Fri, 12 Jun 2009 16:32:00 -0000
From: Paolo Bonzini <paolo.bonzini@gmail.com>
User-Agent: Thunderbird 2.0.0.17 (X11/20081009)
MIME-Version: 1.0
Newsgroups: gmane.comp.gcc.devel
To: Zachary Turner <divisortheory@gmail.com>
CC: gcc@gcc.gnu.org
Subject: Re: naked functions on x86 architecture
References: <478231340906120920m74bd9c0i88861d5e759a8fb1@mail.gmail.com>
In-Reply-To: <478231340906120920m74bd9c0i88861d5e759a8fb1@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
X-SW-Source: 2009-06/txt/msg00274.txt.bz2

> This is one example, but it illustrates a general concept that I think
> is really useful and I personally have used numerous times for lots of
> other instructions than SCAS.  If there is a way to achieve this
> without using a naked function then please advise.

Keeping the __asm syntax, I'd be surprised if this did not work:

template<typename T>
int find_first_nonzero_scas(T* x, int cnt)
{
     int result = 0;
     __asm {
         xor eax, eax
         mov edi, x
         mov ecx, cnt
     }
     if (sizeof (T) == 1)
       __asm { rep scasb; mov result, edi }
     if (sizeof (T) == 2)
       __asm { rep scasw; mov result, edi }
     if (sizeof (T) == 4)
       __asm { rep scasl; mov result, edi }
     result -= reinterpret_cast<int>(x);
     result /= sizeof(T);
     return --result;
}

Paolo