Re: (subreg:SF (reg:SC)) question - 20020227-1.c failure

[Jan Hubicka <jh at suse dot cz>]

> Hi Ulrich,
> 
> Thanks for sending the patch.
> 
> > In addition to this patch, I needed to add a movstrictsf insn to
> > the s390 backend (kind of obvious ...), and also tweak it to
> > force complex values to always use at least two registers, 
> > even when they would theoretically fit into a single one 
> > (allowing that breaks horribly in reload for some reason I don't 
> > quite understand -- something to do with subreg_offset_representable_p 
> > failing ...).
> 
> I've been looking at subreg_offset_representable_p.  I still
> don't understand what "representable" means completely but I think
Representable means that the register is either low part, or a portion
of multi-reg value accessible via reading (possibly low part of) of one
of the registers.
> the routine is broken for big endian targets.  In particular,
> this adjustment seems very strange:
> 
>   /* The XMODE value can be seen as an vector of NREGS_XMODE
>      values.  The subreg must represent an lowpart of given field.
>      Compute what field it is.  */
>   offset -= subreg_lowpart_offset (ymode, 
> 				   mode_for_size (GET_MODE_BITSIZE (xmode)
> 						  / nregs_xmode,
> 					 	  MODE_INT, 0));
> 
> When the routine is passed an offset of 0 on a big endian target, this
> results in y_offset being -1 (xmode=SCmode, ymode=SFmode).  As a result,
> the routine returns false on a 64-bit target with mode_multiple = 2
> and nregs_multiple = 1.

Sorry for jumping in late, but because I wrote that function, I probably
should fix any bugs regarding that.
So you have (subreg:SF reg:SC 0) and nregs_multiple = 1, so you have
single 64bit register holding the whole SC value.  Why do you think you
can access directly the upper part of it?

Honza
> 
> If I change the routine so that it returns true for an offset 0
> (the high part of the SCmode value) and provide the patterns in the
> backend to handle the strict_low_part subreg insns, then things seem
> to work with a single register.
> 
> Dave
> -- 
> J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
> National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)