Re: [PATCH] Rename nonzero_bits to known_zero_bits.

[Aldy Hernandez <aldyh@redhat.com>]

Folks.  I have decided to put this aside until the next release.  I
originally wanted a simple rename, and reimplementing things to align
with rtl, etc, is beyond what I want to tackle on this late.

I'll archive this away, and revisit it when we implement the
irange::known_ones mask.

Thanks for your input.
Aldy

On Fri, Oct 21, 2022 at 8:01 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Fri, Oct 21, 2022 at 06:54:32PM +0200, Jakub Jelinek wrote:
> > On Fri, Oct 21, 2022 at 06:51:19PM +0200, Jakub Jelinek wrote:
> > > Agreed.
> > >
> > > I think maybe_nonzero_bits would be fine.
> >
> > Or yet another option is to change what we track and instead of
> > having just one bitmask have 2 as tree-ssa-ccp.cc does,
> > one bitmask says which bits are known to be always the same
> > and the other which specifies the values of those bits.
> > "For X with a CONSTANT lattice value X & ~mask == value & ~mask.  The
> > zero bits in the mask cover constant values.  The ones mean no
> > information."
>
> I am still working on making the RTL nonzero_bits use DF (and indeed I
> do a known_zero instead :-) ).  This makes the special version in
> combine unnecessary: instead of working better than the generic version
> it is strictly weaker then.  This change then makes it possible to use
> nonzero_bits in instruction conditions (without causing ICEs as now --
> passes after combine return a subset of the nonzero_bits the version in
> combine does, which can make insns no longer match in later passes).
>
> My fear is tracking twice as many bits might become expensive.  OTOH
> ideally we can get rid of combine's reg_stat completely at some point
> in the future (which has all the same problems as combine's version of
> nonzero_bits: the values it returns depend on the order combine tried
> possible combinations).
>
> Storage requirements are the same for known_zero_bits and known_one_bits
> vs. known_bits and known_bit_values, but the latter is a bit more
> costly to compute, but more importantly it is usually a lot less
> convenient in use.  (A third option is known_bits and known_zero_bits?)
>
>
> Segher
>