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Date: Wed, 16 Feb 2011 10:33:00 -0000
From: Andrew Haley <aph@redhat.com>
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To: gcc-help@gcc.gnu.org
Subject: Re: need volatile for asm?
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On 02/15/2011 11:58 PM, Andi Hellmund wrote:
> On 02/16/2011 12:11 AM, kevin diggs wrote:
>> Hi,
>>
>> Does the asm in:
>>
>> #include<stdio.h>
>>
>> int main(int argc, char *argv[])
>> {
>> unsigned int pc;
>>
>> asm("\n\t"
>> "call 1f\n\t"
>> "1: pop %0\n"
>> :"=g"(pc)
>> );
>>
>> printf(__FILE__"`%s()-%d: %%pc is %p\n",__func__,__LINE__,pc);
>>
>> return 0;
>> }
>>
>> need volatile?

>
> no, you don't need the volatile keyword since the output operand
> (pc) is live after the asm statement which puts a side-effect on the
> asm statement.

I'm not sure about that.  Because there is no input operand, gcc is
free to move the asm.  Volatile will prevent that from happening.

I'd do this:

#include <stdio.h>

int main(int argc, char *argv[])
{
   unsigned int pc;

   pc = (unsigned int)&&label;

  label:
   printf(__FILE__"`%s()-%d: %%pc is %p\n",__func__,__LINE__,pc);

   return 0;
}

Andrew.