Re: A plan for eliminating cc0

[<tm_gccmail at kloo dot net>]

On 25 Mar 2005, Ian Lance Taylor wrote:

> Paul Schlie <schlie@comcast.net> writes:
> 
> > > 2) Reload requires the ability to insert arbitrary instructions
> > >    between any pair of instructions.  The instructions inserted by
> > >    reload will load registers from memory, store registers to memory,
> > >    and possibly, depending on the port, move values between different
> > >    classes of registers and add constants to base registers.  If
> > >    reload can't do that without affecting the dependencies between
> > >    instructions, then it will break.  And I don't think reload will be
> > >    able to do that between your two instructions above, on a typical
> > >    cc0 machine in which every move affects the condition codes.
> > 
> > - Understood, however along a similar line of though; it would seem "safe"
> >   to simply "save/restore" the global condition-state around all potentially
> >   destructive memory operations.
> 
> Safe but very costly.  It assumes that every processor has a cheap way
> to save and restore the condition codes in user mode, which is not
> necessarily the case.  And it assumes that the save and restore can be
> removed when not required, which is not obvious to me.

Not necessarily.

You need the ability to regenerate the condition code. There's at least
two ways of doing this:

1. Saving/restoring the condition code

2. Rematerializing the condition code. This would usually be a simple load
   which is faster than the save/load combo of #1.

Toshi