This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

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

On Wed, Apr 24, 2013 at 4:29 PM, Richard Sandiford
<> wrote:
> Richard Biener <> 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:
> 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

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]