Re: [split] Add CFI ops, fix call/return prediction, other changes

[Paolo Bonzini <bonzini at gnu dot org>]

On 09/30/2009 05:15 PM, Ian Lance Taylor wrote:
> Paolo Bonzini<bonzini@gnu.org>  writes:
>
> > > movl 4(%ebp), %eax # Increment return address
> > > inc %eax
> > > movl %eax, -8(%ebp) # Store it in an unused slot
> > >
> > > movl -4(%ebp), %eax # Restore 3rd register
> > >
> > > call *-8(%ebp) # Call our caller via slot
> >
> > Also, would it make sense to reserve 3 bytes for the ret, so that
> > stdcall functions could have split stack?  Alternatively, you need to
> > disable split stack (with a sorry) for stdcall functions.
>
> stdcall functions don't work yet, but this isn't why.  The ret we're
> skipping here is just magic inserted to make the call/return predictor
> line up.  It's really the split_stack_return insn, not a regular
> return.  The actual stack popping ret is at the end of the function as
> usual.

Yes, understood.  But for stdcall functions to work you need a 
stack-popping ret here too (you are returning to the caller and need to 
clean up your arguments).  This would consume more than one byte.

Paolo