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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99918] [9/10/11 Regression] suboptimal code
 for bool bitfield tests
Date: Tue, 06 Apr 2021 08:39:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99918

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-04-06
             Blocks|                            |85316
     Ever confirmed|0                           |1
   Target Milestone|---                         |9.4

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
The main issue is optimize_bit_field_compare in fold-const.c which produces
during GENERIC folding in .005t.original:

  if ((BIT_FIELD_REF <b, 8, 0> & 1) !=3D 0)
    {
      b.j =3D 0;
    }
  else
    {
      b.j =3D b.i;
    }
  return b.j;

that's premature in this place.  For f() it also takes until DOM3 to do
the folding unless you disable SRA which then makes EVRP recognize the
second store as a.j =3D 0.  With SRA we fail to derive ranges for a_10 in

  a_10 =3D MEM <unsigned char> [(struct A *)&a];
  a$1_11 =3D MEM <unsigned char> [(struct A *)&a + 1B];
  _1 =3D VIEW_CONVERT_EXPR<_Bool>(a_10);
  if (_1 !=3D 0)
    goto <bb 4>; [INV]
  else
    goto <bb 3>; [INV]

  <bb 3> :

  <bb 4> :
  # a$1_9 =3D PHI <0(2), a_10(3)>
  _7 =3D VIEW_CONVERT_EXPR<_Bool>(a$1_9);

thus we're missing looking through VIEW_CONVERT_EXPR in register_assert_for.
Amending that would eventually also allow optimizing the prematurely folded
vairant.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85316
[Bug 85316] [meta-bug] VRP range propagation missed cases=