Alex Coplan <alex.cop...@arm.com> writes:
> On 02/07/2024 13:41, Richard Biener wrote:
>> 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.
FWIW, the reason for maybe_eq here:
if (maybe_eq (div, 0))
return false;
if (!multiple_p (val, div, &cst))
return false;
is that the division is undefined when div *might* be zero.
>> > > >
>> > > > 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?
>
> I don't think so, I think the only guarantee is that the
> x >= 0. From doc/poly-int.texi:
>
> @code{poly_int} makes the simplifying requirement that each indeterminate
> must be a nonnegative integer.
>
> For SVE the unknown x is the number of 128-bit blocks beyond the minimum
> of 128, so in particular the indeterminate x = 0 for 128-bit SVE, and we
> would have [0,4] = 0 and [4,4] = 4 at runtime in that case.
Yeah, just wanted to +1 everything Alex said above :)
Thanks,
Richard
