Re: i386.h fix for 4.0 branch (pr16888 fallout)

[Michael Matz <matz at suse dot de>]

Hi,

On Mon, 9 May 2005, Richard Henderson wrote:

> On Mon, May 09, 2005 at 11:09:26PM +0200, Michael Matz wrote:
> > 2005-04-28  James E Wilson  <wilson@specifixinc.com>
> > 
> > 	* config/i386/i386.h (HI_REGISTER_NAMES): Fix typos in comment.
> > 	(ADDITIONAL_REGISTER_NAMES): Delete obsolete mmx register entries.
> 
> Ok to backport anywhere you like.

In 4.0 now.  In retrospect I actually think that Alexandres change might 
not have been such a good idea after all.  The problem is, that now you 
can't compile anymore programs which do the checking for MMX capability at 
runtime, i.e. something like:
   if (cpu_supports_mmx()) {
     asm do_stuff_with_mmx;
   } else {
     do_stuff_in_plain_c();
   }

like is done in some media code.  With Alexandres changes you have to 
specify -mmmx (or another option of that effect) now to even compile the 
above.  The problem being that this activates TARGET_MMX (or _SSE{,2} for 
that matter), and hence activates that instruction set also generally, 
i.e. potentially on code paths not guarded by the above if written by the 
programmer.  Hence you are now unable to compile such code which still 
runs (slower) on non-mmx/sse capable processors.


Ciao,
Michael.