Re: New back end ia16: 16-bit Intel x86

[Rask Ingemann Lambertsen <rask@sygehus.dk>]

On Sat, Aug 18, 2007 at 06:32:20PM -0700, H. Peter Anvin wrote:
> Rask Ingemann Lambertsen wrote:
> > 
> >    It is not different in your case! You're targetting the i386, so you can
> > use i386 addressing modes at just the cost of a 0x67 prefix:
> 
> Well, then you're effectively doing nothing other than the .code16gcc hack.

> >    Obviously you want to save the 0x67 prefix whenever possible; use
> > "20(%bp)" instead of "20(%ebp)", for example.

   I think that counts as "other than the .code16gcc hack". Using 16-bit
integers when possible does too. That's the opposite of what the i386 back
end does.

> >> It is also much harder to eliminate the frame pointer in
> >> 16-bit mode, since there is no %sp-based addressing at all.
> > 
> >    With disp(%esp) being valid on i386, you don't have that problem either.
> 
> If your environment isn't clean w.r.t. the top half of %esp, you can't
> *ever* use this mode.  The stack pointer especially...

   It doesn't seem to be a likely problem to me. Code operating on %sp can't
clobber the top half[1]. Code operating on %esp will need to restore it or run
into trouble. It would be very unusual to not restore the stack pointer. If
this was to be a problem, you wouldn't be able to just use ".code16gcc".

[1] Strictly speaking, only bits 16-19 are of any concern.

-- 
Rask Ingemann Lambertsen