Re: asm as "memory barrier"

[Paul Koning <pkoning at equallogic dot com>]

> From: Richard Henderson <rth at redhat dot com>
>>On Wed, Apr 23, 2003 at 03:55:57PM -0400, Paul Koning wrote:
>> But it also (as documented) causes the compiler to assume that memory
>> has changed since then, i.e., anything it loaded before is no longer
>> valid.  That's NOT what I want.
> 
> There doesn't appear to be any way to do this.  I'd have expected
> 
>   int x;
>   int foo()
>   {
>     int r = x;
>     {
>       char *p;
>       asm ("" : "=X"(p));
>       asm volatile ("" : : "X"(*p));
>     }
>     return x + r;
>   }
> 
> to work (as an observed read from a pointer with unknown value),
> but it would seem that CSE doesn't want to combine the two loads
> across the volatile asm.

Perhaps because of the aliasing rules?

I observed that simply asm volatile (""), i.e., "old style asm" *does*
do what I want.  My concern wasn't that -- it was the fact that there
isn't any documented property of old style asm.  Or at least, only
very little, and part of that has traditionally been wrong.

     paul