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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/24021] VRP does not work with floating points
Date: Mon, 01 Aug 2022 13:27:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D24021

--- Comment #25 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
Adding some notes here as I work through this PR...

Even with floating aware VRP, we won't be able to do much because SCEV (whi=
ch
ranger and VRP use) does not work with non-integers.

At EVRP time we see:

double BG_SplineLength ()
{
  double i;
  double lastPoint;

  <bb 2> :
  goto <bb 6>; [INV]

  <bb 3> :
  if (i_3 =3D=3D 0.0)
    goto <bb 5>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 4> :

  <bb 5> :
  # lastPoint_1 =3D PHI <i_3(3), 2.0e+0(4)>
  i_10 =3D i_3 + 1.00000001490116119384765625e-1;

  <bb 6> :
  # lastPoint_2 =3D PHI <lastPoint_5(D)(2), lastPoint_1(5)>
  # i_3 =3D PHI
<1.00000000000000002081668171172168513294309377670288085938e-2(2), i_10(5)>
  if (i_3 <=3D 1.0e+0)
    goto <bb 3>; [INV]
  else
    goto <bb 7>; [INV]

  <bb 7> :
  return lastPoint_2;

}

On the 6->3 edge we know that i_3 is [-INF, 1.0], so even adding a range-op
entry for the PLUS_EXPR, we'd know that i_3 is [-INF, 1.1], which is not en=
ough
to fold the i_3 =3D=3D 0.0 conditional.  If you convert this testcase to in=
teger
and turn off SCEV (and unrolling and FRE, etc), you'll notice that VRP does=
n't
do much here (in either legacy nor ranger mode).

We would need SCEV to give us [1.0, 1.1] in order to fold the i_3 =3D=3D 0.0
conditional.  For that matter, if I hack SCEV to give us the expected end
points, I can get evrp to fold the conditional.  I think once VRP is FP awa=
re,
we can close this PR because this particular testcase is SCEV specific.  So=
 for
this particular testcase, perhaps we could open a SCEV+FP PR?

As an aside, the second conditional will not be folded by VRP (legacy or
ranger), even with SCEV, even if you convert the above testcase to
integer...it's either gotten later by unrolling+FRE for FP, or cddce for
integers.

What I'm trying to say is that even with FP VRP, which we'll have shortly, =
the
first conditional won't be folded because SCEV doesn't do floats, and the
second one won't be, because it's not VRP's job.=