Re: [4/8] Add bit_field_mode_iterator

[Eric Botcazou <ebotcazou@adacore.com>]

> get_best_mode has various checks to decide what counts as an acceptable
> bitfield mode.  It actually has two copies of them, with slightly different
> alignment checks:
> 
>   MIN (unit, BIGGEST_ALIGNMENT) > align
> 
> vs.
> 
>   unit <= MIN (align, BIGGEST_ALIGNMENT)
> 
> The second looks more correct, since we can't necessarily guarantee
> larger alignments than BIGGEST_ALIGNMENT in all cases.

Under the assumption that integer modes really require maximal alignment, i.e. 
whatever BIGGEST_ALIGNMENT is, I agree.

> This patch adds a new iterator class that can be used to walk through
> the modes, and rewrites get_best_mode to use it.  I kept the existing
> checks with two changes:
> 
> - bitregion_start is now tested independently of bitregion_end

The comments needs to be updated then.

> - MAX_FIXED_MODE_SIZE is used as a limit even if a bitregion is defined

This makes sense I think.

> It shouldn't make any difference in practice, but both changes felt
> more in keeping with the documentation of bitregion_start and
> MAX_FIXED_MODE_SIZE, and the next patch wants the bitregion_end
> test to be separate from bitregion_start.
> 
> The behaviour of the Sequent i386 compiler probably isn't the
> issue it once was, but that's also dealt with in the next patch.
> 
> Tested as described in the covering note.  OK to install?
> 
> Richard
> 
> 
> gcc/
> 	* machmode.h (bit_field_mode_iterator): New class.
> 	(get_best_mode): Change final parameter to bool.
> 	* stor-layout.c (bit_field_mode_iterator::bit_field_mode_iterator)
> 	(bit_field_mode_iterator::next_mode): New functions, split out from...
> 	(get_best_mode): ...here.  Change final parameter to bool.
> 	Use bit_field_mode_iterator.

This looks good to me, modulo:

> +  volatilep_ (volatilep), count_ (0)
> +{
> +  if (bitregion_end_)
> +    bitregion_end_ += 1;
> +}

IMO this is confusing.  I think bitregion_end/bitregion_end_ should have a 
consistent meaning.


> +/* Calls to this function return successively larger modes that can be used
> +   to represent the bitfield.  Return true if another bitfield mode is +  
> available, storing it in *OUT_MODE if so.  */
> +
> +bool bit_field_mode_iterator::next_mode (enum machine_mode *out_mode)

'bool' on its own line I think.


I find the interface a bit awkward though.  Can't we model it on the existing 
iterators in basic-block.h or tree-flow.h?  get_best_mode would be written:

  FOR_EACH_BITFIELD_MODE (mode, iter, bitsize, bitpos,
			  bitregion_start, bitregion_end,
			  align, volatilep)
    {
	if (largest_mode != VOIDmode
	    && GET_MODE_SIZE (mode) > GET_MODE_SIZE (largest_mode)
	  break;

	if (iter.prefer_smaller_modes ())
	  return mode;

	widest_mode = mode; 
    }

  return widest_mode;

and the implementation entirely hidden.

-- 
Eric Botcazou