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Re: patch to fix constant math -5th patch, rtl


On 04/24/2013 10:42 AM, Richard Biener wrote:
On Wed, Apr 24, 2013 at 4:29 PM, Richard Sandiford
<rdsandiford@googlemail.com> wrote:
Richard Biener <richard.guenther@gmail.com> writes:
I suppose the above should use immed_double_int_const (v, mode), too,
In practice it doesn't matter, because...

which oddly only ever truncates to mode for modes <= HOST_BITS_PER_WIDE_INT
via gen_int_mode.
...right.  That's because there's not really any proper support for
non-power-of-2 modes.  Partial modes like PDI are specified by things like:

PARTIAL_INT_MODE (DI);

which is glaringly absent of any bit width.  So if the constant is big
enough to need 2 HWIs, it in practice must be exactly 2 HWIs wide.
Ah, of course.

One of the advantages of wide_int is that it allows us to relax this
restriction: we can have both (a) mode widths greater than
HOST_BITS_PER_WIDE_INT*2 and (b) mode widths that are greater than
HOST_BITS_PER_WIDE_INT while not being a multiple of it.

In other words, one of the reasons wide_int can't be exactly 1:1
in practice is because it is clearing out these mistakes (GEN_INT
rather than gen_int_mode) and missing features (non-power-of-2 widths).
Note that the argument should be about CONST_WIDE_INT here,
not wide-int.  Indeed CONST_WIDE_INT has the desired feature
and can be properly truncated/extended according to mode at the time we build it
via immed_wide_int_cst (w, mode).  I don't see the requirement that
wide-int itself is automagically providing that truncation/extension
(though it is a possibility, one that does not match existing behavior of
HWI for CONST_INT or double-int for CONST_DOUBLE).

Richard.

yes but you still have the problem with partial ints having no length. Our plan was to be very careful and make sure that at no point were we doing anything that makes it harder to put modes in const_ints, but that is different from going thru everything and doing it.

Partially because of this discussion and some issues that you brought up with patch 4, i am removing the trick of being able to say 'wi + rtl' because there is no mode for the rtl. i am leaving the 'wi + tree' because there is enough to info in the treecst to make this work.
but you are going to have to say something wi::add (mode, rtl)

see, i am willing to do things that work better in the tree world than in the rtl world.

kenny

Richard


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