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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/106217] New: [11/12/13 Regression] sinking of
 loads prevents vectorization
Date: Wed, 06 Jul 2022 18:19:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106217

            Bug ID: 106217
           Summary: [11/12/13 Regression] sinking of loads prevents
                    vectorization
           Product: gcc
           Version: 11.3.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---

The following example

void f(int n, float * __restrict x, float *mass, float max, float &ax)
{
    float lax =3D 0.0f;

    for (int i =3D 0; i < n; ++i) {
        float f =3D x[i] * mass[i];
        lax +=3D x[i] >=3D max ? 0 : f * x[i];
    }

  ax +=3D lax;
  return;
}

vectorizes with GCC 10 with -Ofast but fails starting with GCC 11.
The difference is that in the sink1 pass we now sink the unconditional load=
 of
mass[i] into the conditional.

Sinking # VUSE <.MEM_14(D)>
_7 =3D *_6;
 from bb 3 to bb 4
Sinking _6 =3D mass_18(D) + _2;
 from bb 3 to bb 4

consequently this causes if-convert to no longer be able to if-convert the =
loop
because the load is now conditional, and we only accept unconditional loads=
 in
ifcvt_memrefs_wont_trap.

-------------------------
_6 =3D mass_18(D) + _2;
-------------------------
_7 =3D *_6;
tree could trap...

perhaps during the sinking we should mark the load as non-trapping? as it w=
as
originally unconditional anyway.=