On Fri, Aug 27, 2021 at 6:50 AM Hongtao Liu <crazy...@gmail.com> wrote:
>
> On Thu, Aug 26, 2021 at 7:09 PM Richard Biener via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > On Thu, Aug 26, 2021 at 12:50 PM Richard Sandiford
> > <richard.sandif...@arm.com> wrote:
> > >
> > > Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > > > On Thu, Aug 26, 2021 at 11:06 AM Richard Sandiford
> > > > <richard.sandif...@arm.com> wrote:
> > > >>
> > > >> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > > >> > One thought I had is whether we can "fix" validate_subreg to have
> > > >> > less
> > > >> > "weird" allowed float-int
> > > >> > special cases. As said upthread I think that we either should allow
> > > >> > all of those, implying that
> > > >> > subregs work semantically as if there's subregs to same-sized integer
> > > >> > modes inbetween or
> > > >> > disallow them all and make sure we're actually doing that
> > > >> > explicitely.
> > > >> >
> > > >> > For example
> > > >> >
> > > >> > /* ??? Similarly, e.g. with (subreg:DF (reg:TI)). Though
> > > >> > store_bit_field
> > > >> > is the culprit here, and not the backends. */
> > > >> > else if (known_ge (osize, regsize) && known_ge (isize, osize))
> > > >> > ;
> > > >> >
> > > >> > I can't decipther rtl.text as to what the semantics of such a subreg
> > > >> > is
> > > >> > given the docs hand-wave about WORDS_BIG_ENDIAN vs.
> > > >> > FLOAT_WORDS_BIG_ENDIAN but don't actually say what happens
> > > >> > when you mix those in a subreg. So maybe the above should
> > > >> > have explicitely have WORDS_BIG_ENDIAN == FLOAT_WORDS_BIG_ENDIAN.
> > > >> >
> > > >> > But then the world would be much simpler if subregs of non-same size
> > > >> > modes have explicit documentation for the mode kinds we have.
> > > >>
> > > >> Yeah. Although validate_subreg was a good idea, some of the mode
> > > >> checks
> > > >> are IMO a failed experiment. The hope was that eventually we'd remove
> > > >> all those special exceptions once the culprit has been fixed. However,
> > > >> the code is over 16 years old at this point and those changes never
> > > >> happened.
> > > >>
> > > >> Nested subregs aren't a thing (thankfully) and one of the big
> > > >> disadvantages
> > > >> of the current validate_subreg mode-changing rules is that they aren't
> > > >> transitive. This can artificially require temporary pseudos for things
> > > >> that could be expressed directly as a single subreg.
> > > >
> > > > And that's what the proposed patch does (add same-mode size integer mode
> > > > punning intermediate subregs).
> > > >
> > > > So if that's not supposed to be necessary then why restrict subregs at
> > > > all?
> > >
> > > I was trying to say: I'm not sure we should.
> > >
> > > > I mean you seem to imply that the semantics would be clear and
> > > > well-defined
> > > > (to you - not to me). The only thing is that of course not all subregs
> > > > are
> > > > "implemented" by a target (or can be, w/o spilling).
> > >
> > > Yeah. That's for TARGET_CAN_CHANGE_MODE_CLASS to decide.
> > > But it only comes in to play during RA or when trying to take
> > > the subreg of a particular hard register. Transitivity doesn't
> > > matter so much for the hard register case since the result of
> > > simplify_gen_subreg should then be another hard register.
> > >
> > > > Which means - we should adjust validate_subreg with another special-case
> > > > or rather generalize the existing ones to an overall set that makes more
> > > > sense?
> > >
> > > Maybe it's too radical, but I would whether we should just get rid of:
> > >
> > > /* ??? This should not be here. Temporarily continue to allow word_mode
> > > subregs of anything. The most common offender is (subreg:SI
> > > (reg:DF)).
> > > Generally, backends are doing something sketchy but it'll take time
> > > to
> > > fix them all. */
> > > if (omode == word_mode)
> > > ;
> > > /* ??? Similarly, e.g. with (subreg:DF (reg:TI)). Though
> > > store_bit_field
> > > is the culprit here, and not the backends. */
> > > else if (known_ge (osize, regsize) && known_ge (isize, osize))
> > > ;
> > > /* Allow component subregs of complex and vector. Though given the
> > > below
> > > extraction rules, it's not always clear what that means. */
> > > else if ((COMPLEX_MODE_P (imode) || VECTOR_MODE_P (imode))
> > > && GET_MODE_INNER (imode) == omode)
> > > ;
> > > /* ??? x86 sse code makes heavy use of *paradoxical* vector subregs,
> > > i.e. (subreg:V4SF (reg:SF) 0) or (subreg:V4SF (reg:V2SF) 0). This
> > > surely isn't the cleanest way to represent this. It's questionable
> > > if this ought to be represented at all -- why can't this all be
> > > hidden
> > > in post-reload splitters that make arbitrarily mode changes to the
> > > registers themselves. */
> > > else if (VECTOR_MODE_P (omode)
> > > && GET_MODE_INNER (omode) == GET_MODE_INNER (imode))
> > > ;
> > > /* Subregs involving floating point modes are not allowed to
> > > change size. Therefore (subreg:DI (reg:DF) 0) is fine, but
> > > (subreg:SI (reg:DF) 0) isn't. */
> > > else if (FLOAT_MODE_P (imode) || FLOAT_MODE_P (omode))
> > > {
> > > if (! (known_eq (isize, osize)
> > > /* LRA can use subreg to store a floating point value in
> > > an integer mode. Although the floating point and the
> > > integer modes need the same number of hard registers,
> > > the size of floating point mode can be less than the
> > > integer mode. LRA also uses subregs for a register
> > > should be used in different mode in on insn. */
> > > || lra_in_progress))
> > > return false;
> > > }
> > >
> > > altogether.
> >
> > Yeah, I would fully support this. Maybe replace it with a comment
> > but I don't know what it should say.
> >
> > Richard.
> >
> > >
> > > Thanks,
> > > Richard
>
> I'm going to upstream the patch.
Hmm, so looks like you pushed the variant massaging extract_bit_field. Above
we supported to instead "fix" validate_subreg to allow the HFmode subreg.
So maybe we should revert and try that?
Richard.
> --
> BR,
> Hongtao
