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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101179] y % (x ? 16 : 4) and y % (4 << (2 *
 (bool)x)) produce different code
Date: Thu, 24 Jun 2021 06:40:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101179

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |85316

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> On IRC Richi said: "VRP has code to do that but maybe for some reason shi=
fts
> are not handled"

Specifically simplify_using_ranges::simplify has

      /* Convert:
         LHS =3D CST BINOP VAR
         Where VAR is two-valued and LHS is used in GIMPLE_COND only
         To:
         LHS =3D VAR =3D=3D VAL1 ? (CST BINOP VAL1) : (CST BINOP VAL2)

         Also handles:
         LHS =3D VAR BINOP CST
         Where VAR is two-valued and LHS is used in GIMPLE_COND only
         To:
         LHS =3D VAR =3D=3D VAL1 ? (VAL1 BINOP CST) : (VAL2 BINOP CST) */

restrictions that stand in the way:

      if (TREE_CODE_CLASS (rhs_code) =3D=3D tcc_binary
...
          && single_imm_use (lhs, &use_p, &use_stmt)
          && gimple_code (use_stmt) =3D=3D GIMPLE_COND)

VRP1 sees

  <bb 2> [local count: 1073741824]:
  _1 =3D y_7(D) % 100;
  x_8 =3D _1 =3D=3D 0;
  _2 =3D (int) x_8;
  _3 =3D _2 * 2;
  _4 =3D 4 << _3;
  _5 =3D y_7(D) % _4;
  _6 =3D _5 =3D=3D 0;=20
  _9 =3D (int) _6;
  return _9;=20

so it would consider _2 * 2 but its single use is in a shift, the
condition is after another op, the modulo, and there the condition
is in an assignment, not in a GIMPLE_COND.


Referenced Bugs:

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