Re: inline asm and status flags on x86

[Richard Earnshaw <rearnsha at arm dot com>]

> On Thu, May 04, 2000 at 01:42:31PM -0400, Geoff Berry wrote:
> > How's this?
> > 
> > 2000-05-04  Geoff Berry  <geoffb@bops.com>
> > 
> > 	* extend.texi (Extended Asm): Document inability to give asm
> > 	statements access to condition codes.
> > 
> > Index: extend.texi
> > ===================================================================
> > RCS file: /cvs/gcc/egcs/gcc/extend.texi,v
> > retrieving revision 1.44
> > diff -c -3 -p -r1.44 extend.texi
> > *** extend.texi	2000/03/14 08:24:21	1.44
> > --- extend.texi	2000/05/04 17:39:52
> > *************** instructions would alter the condition c
> > *** 2662,2667 ****
> > --- 2662,2671 ----
> >   test it.  This problem doesn't arise for ordinary ``test'' and
> >   ``compare'' instructions because they don't have any output operands.
> >   
> > + For reasons similar to those described above, it is not possible to give
> > + an assembler instruction access to the condition code left by previous
> > + instructions.
> > + 
> >   If you are writing a header file that should be includable in ANSI C
> >   programs, write @code{__asm__} instead of @code{asm}.  @xref{Alternate
> >   Keywords}.
> 
> We probably should at least think about redoing the wording of previous
> paragraph for non-CC0 machines.
> 

I don't see how you would do it even for non-CC0 machines.  The inputs to 
an ASM are 'C' (or other language) variables, or expressions.  There are 
no expressions that directly represent the state of the condition-codes at 
this level, so there is no way to specify to the ASM what was intended.

R.