Re: [0/10] Replace host_integerp and tree_low_cst

[Richard Sandiford <rdsandiford at googlemail dot com>]

Jeff Law <law@redhat.com> writes:
> On 11/16/13 05:53, Richard Sandiford wrote:
>> After the patch that went in yesterday, all calls to host_integerp and
>> tree_low_cst pass a constant "pos" argument.  This series replaces each
>> function with two separate ones:
> [ ... ]
> So I've almost entirely ignored the whole wide-int conversion discussion 
> and I suspect I'm not entirely alone.
>
> Can you briefly summarize what's y'all are trying to accomplish with the 
> wide-int changes?

At the moment, we can only handle tree and rtl integer constants that
fit in 2 HOST_WIDE_INTs.  The idea is to remove that limit.  E.g. things
like OImode (used in a few ports) will become a first-class citizen,
with all OImode values being representable.

Besides that headline reason, there are various side benefits.  E.g.:

- All INTEGER_CSTs can be viewed either in their TYPE_PRECISION or in
  "infinite" precision, which isn't possible for 128-bit constants today.
  (I.e. there's no way to distinguish signed and unsigned 128-bit constants
  in a double_int.)

- Wider-than-2-HWI intermediate results can be represented as a single
  integer.  I'm told this is useful for VRP.  (wide-int is mostly Kenny
  and Mike's work, I've just been butting in recently.)

- rtl-level constant folding can use the same code to handle all
  combinations of CONST_INT and CONST_DOUBLE (and CONST_WIDE_INT,
  on converted ports).  At the moment we handle CONST_INT cases
  specially, and don't try as hard with CONST_DOUBLEs.

Implementation-wise, it tries to make it so that the common single-HWI
cases are still fast.

Thanks,
Richard