Re: [dataflow] partial register handling

[Roman Zippel <zippel at linux-m68k dot org>]

Hi,

On Fri, 11 May 2007, Paolo Bonzini wrote:

> First of all, scrap my other message...
> 
> > There was a debate several months ago on this issue: how much should the
> > df scanner be a theorem prover with respect to how many bits survive an
> > operation.
> > For instance, I could easily add to your list, anding and oring
> > operations which also preserve bits. 
> 
> The rules should be simple.  Bits of the dest reg survive only if one of the
> following is true.
> 
> - there is a STRICT_LOW_PART (of a SUBREG)
> 
> - there is a ZERO_EXTRACT (not necessarily of a SUBREG!)
> 
> - the subreg is part of a multiword subreg
> 
> The last point is decided in other parts of the code than the one Roman is
> touching.  So, Roman's change to df_read_modify_subreg_p is wrong; other
> subregs, in particular (subreg:HI (reg:SI 123) 2), should not be affected.  At
> most, we might want there the more pedantic
> 
>   return (isize > osize
>           && isize > REGMODE_NATURAL_SIZE (GET_MODE (isize));
> 
> 
> It seems to me that all is missing is setting DF_REF_PARTIAL for ZERO_EXTRACT.
> That is
> 
> -      if (GET_CODE (dst) == STRICT_LOW_PART)
> -        dst_in_strict_lowpart = true;
> +      if (GET_CODE (dst) == STRICT_LOW_PART
> +          || GET_CODE (dst) == ZERO_EXTRACT)
> +        flags |= DF_REF_PARTIAL;

That clears up about df_read_modify_subreg_p, thanks.
But I don't think that's enough, with the current loop it would strip the 
subreg of a multiword subreg and there is some logic in df_ref_record, 
which wouldn't see it. An alternative might be:

  while (GET_CODE (dst) == STRICT_LOW_PART
	 || GET_CODE (dst) == ZERO_EXTRACT)
    {
      flags |= DF_REF_READ_WRITE | DF_REF_PARTIAL;
      loc = &XEXP (dst, 0);
      dst = *loc;
    }

  if (df_read_modify_subreg_p (dst))
    flags |= DF_REF_READ_WRITE | DF_REF_PARTIAL;

That would only leave singleword subreg and paradoxical subreg, if they 
should require anything.
One could also restrict STRICT_LOW_PART to subregs, as after reload the 
subreg part is usually gone.

bye, Roman