On Tue, 2 Jul 2024, Alex Coplan wrote:
> On 02/07/2024 10:46, Alex Coplan wrote:
> > On 02/07/2024 10:01, Richard Biener wrote:
> > > On Mon, 1 Jul 2024, Tamar Christina wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Tamar Christina <tamar.christ...@arm.com>
> > > > > Sent: Monday, July 1, 2024 9:14 PM
> > > > > To: gcc-patches@gcc.gnu.org
> > > > > Cc: nd <n...@arm.com>; rguent...@suse.de; j...@ventanamicro.com
> > > > > Subject: [PATCH 1/2]middle-end: fix wide_int_constant_multiple_p when
> > > > > VAL and
> > > > > DIV are 0. [PR114932]
> > > > >
> > > > > Hi All,
> > > > >
> > > > > wide_int_constant_multiple_p tries to check if for two tree
> > > > > expressions a and b
> > > > > that there is a multiplier which makes a == b * c.
> > > > >
> > > > > This code however seems to think that there's no c where a=0 and b=0
> > > > > are equal
> > > > > which is of course wrong.
> > > > >
> > > > > This fixes it and also fixes the comment.
> > > > >
> > > > > Bootstrapped Regtested on aarch64-none-linux-gnu,
> > > > > x86_64-pc-linux-gnu -m32, -m64 and no issues.
> > > > >
> > > > > Ok for master?
> > > > >
> > > > > Thanks,
> > > > > Tamar
> > > > >
> > > > > gcc/ChangeLog:
> > > > >
> > > > > PR tree-optimization/114932
> > > > > * tree-affine.cc (wide_int_constant_multiple_p): Support 0 and
> > > > > 0 being
> > > > > multiples.
> > > > >
> > > > > ---
> > > > > diff --git a/gcc/tree-affine.cc b/gcc/tree-affine.cc
> > > > > index
> > > > > d6309c4390362b680f0aa97a41fac3281ade66fd..bfea0fe826a6affa0ace154e3ca
> > > > > 38c9ef632fcba 100644
> > > > > --- a/gcc/tree-affine.cc
> > > > > +++ b/gcc/tree-affine.cc
> > > > > @@ -880,11 +880,10 @@ free_affine_expand_cache (hash_map<tree,
> > > > > name_expansion *> **cache)
> > > > > *cache = NULL;
> > > > > }
> > > > >
> > > > > -/* If VAL != CST * DIV for any constant CST, returns false.
> > > > > - Otherwise, if *MULT_SET is true, additionally compares CST and
> > > > > MULT,
> > > > > - and if they are different, returns false. Finally, if neither of
> > > > > these
> > > > > - two cases occur, true is returned, and CST is stored to MULT and
> > > > > MULT_SET
> > > > > - is set to true. */
> > > > > +/* If VAL == CST * DIV for any constant CST, returns true.
> > > > > + and if *MULT_SET is true, additionally compares CST and MULT
> > > > > + and if they are different, returns false. If true is returned,
> > > > > CST is
> > > > > + stored to MULT and MULT_SET is set to true. */
> > > > >
> > > > > static bool
> > > > > wide_int_constant_multiple_p (const poly_widest_int &val,
> > > > > @@ -895,6 +894,12 @@ wide_int_constant_multiple_p (const
> > > > > poly_widest_int
> > > > > &val,
> > > > >
> > > > > if (known_eq (val, 0))
> > > > > {
> > > > > + if (maybe_eq (div, 0))
> > > > > + {
> > > > > + *mult = 1;
> > > > > + return true;
> > > > > + }
> > > > > +
> > > >
> > > > Note, I also tested known_eq here, and also no regression on what I can
> > > > test.
> > > > I picked maybe_eq since that's what the lines after this one tests.
> > >
> > > I think the maybe_eq (div, 0) is because otherwise multiple_p might
> > > crash? I'm not sure if there's a difference between
> > > maybe_eq (x, 0) and known_eq (x, 0) though - how does a maybe_eq
> > > POLY_INT look like that's not known_eq?
> >
> > Take:
> >
> > A = POLY_INT_CST [16,0]
> > B = POLY_INT_CST [8,8]
> >
> > then these represent polynomials:
> >
> > A = 16
> > B = 8 + 8x
> >
> > where x is only known at runtime. We have maybe_eq (A,B) since there is
> > a value of x (= 1) which makes these equal at runtime, but clearly
> > !known_eq (A,B) (take x = 0, for example).
>
> So specifically in the case of:
>
> maybe_eq (x, 0) vs known_eq (x, 0)
>
> I suppose x = POLY_INT_CST [-4,4] would satisfy the first (again with x
> = 1) but not the second.
Ah yeah - I wasn't aware that a negative offset is a thing. I think
that at least we know x > 0, right, so [0, 4] is never zero, likewise
[4, 4] never is?
Richard.
> Thanks,
> Alex
>
> >
> > That is my understanding at least, hopefully that makes sense.
> >
> > Thanks,
> > Alex
> >
> > >
> > > > I'm not sure I fully understand why one tests known and the other
> > > > maybe. It seems to me
> > > > that both should test known. But I tested both so which ever one is
> > > > felt to be more correct
> > > > I can commit If ok.
> > > >
> > > > Thanks,
> > > > Tamar
> > > >
> > > > > if (*mult_set && maybe_ne (*mult, 0))
> > > > > return false;
> > > > > *mult_set = true;
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > --
> > > >
> > >
> > > --
> > > Richard Biener <rguent...@suse.de>
> > > SUSE Software Solutions Germany GmbH,
> > > Frankenstrasse 146, 90461 Nuernberg, Germany;
> > > GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
