Re: [PATCH 1/2]middle-end: fix wide_int_constant_multiple_p when VAL and DIV are 0. [PR114932]

[Richard Sandiford <richard.sandiford@arm.com>]

Alex Coplan <alex.coplan@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.christina@arm.com>
>> > > > > > Sent: Monday, July 1, 2024 9:14 PM
>> > > > > > To: gcc-patches@gcc.gnu.org
>> > > > > > Cc: nd <nd@arm.com>; rguenther@suse.de; jlaw@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