Re: x86: -Os -msse2 needs -maccumulate-outgoing-args

[Stuart Hastings <stuart at apple dot com>]

On Jan 13, 2004, at 12:51 PM, Jan Hubicka wrote:

> > This testcase
>
> Hi,
> this is not -maccumulate-outgoing-args that breaks, it is
> -mpreffered-stack-boundary=2 implied by -Os.  We still don't have the
> dynamic stack alignment code merged in, unfortunately.
> It would be probably sane to give some warning or hard error when SSE
> register is put onto stack, but I don't see easy way to do this, as we
> should not output such a warning for long doubles that have same
> alignment properties.
> Ideas?

Well, the hardware insists upon alignment for vectors, and -Os is only 
advisory ("optimize for space, please"), so I would expect either

1) -mmmx/-msse/-msse2 to override the -mpreferred-stack-boundary effect 
of -Os
2) dynamic stack alignment (but this is contrary to the spirit of -Os)
3) warning or hard error

Trying to think like a "typical GCC user," it's not clear to me why -Os 
could cause warnings or hard errors.  I'd always assumed that 
optimizations were optional; if they couldn't be done correctly, they 
won't be done (unless there's a bug).

I'll agree that "-mpreferred-stack-boundary=2 -msse2" is a recipe for 
bad code if obeyed literally, and grounds for a warning or error.  But 
only if they appear on the same commandline.

IMHO, #3 isn't a good solution.  Since we don't have 2), that leaves 
1).  Is there a 4) I didn't think of ?

stuart hastings
Apple Computer