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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/100112] missed optimization for dead code
 elimination at -O3, -Os (vs. -O1, -O2)
Date: Sat, 25 Sep 2021 07:51:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100112

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias
          Component|ipa                         |tree-optimization

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm, the difference is in fre, for some reason -fstrict-aliasing makes a
difference.

ealias at -O1:
  c.1_4 =3D c;
  c =3D c.1_4;
  c.2_5 =3D c;
  _6 =3D c.2_5 !=3D 0B;

ealias at -O3:
  *a.0_1 =3D _3;
  c.1_4 =3D c;
  c =3D c.1_4;
  c.2_5 =3D c;
  _6 =3D c.2_5 !=3D 0B;

fre1 at -O1:
  *a.0_1 =3D _3;
  c.1_4 =3D c;
  _6 =3D c.1_4 !=3D 0B;

fre1 at -O3:
  *a.0_1 =3D _3;
  c.1_4 =3D c;
  c =3D c.1_4;
  _6 =3D c.1_4 !=3D 0B;


fre1 at -O1 dump:
Value numbering stmt =3D c =3D c.1_4;
Store matched earlier value, value numbering store vdefs to matching vuses.

fre1 at -O3 dump:
Value numbering stmt =3D c.1_4 =3D c;
Setting value number of c.1_4 to c.1_4 (changed)
Making available beyond BB2 c.1_4 for value c.1_4
Value numbering stmt =3D c =3D c.1_4;
No store match
Value numbering store c to c.1_4
Setting value number of .MEM_11 to .MEM_11 (changed)

If I use -O3 -fno-strict-aliasing, I get the same result as -O1.=