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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/99987] [12/13/14 Regression] missed
 optimization for dead code elimination at -O3 (vs. -O2)
Date: Fri, 18 Aug 2023 06:36:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99987

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |7.5.0

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So this is bascially the missed optimization:
```
void foo(void);
int main()
{
  unsigned h =3D 123+5;
  for(unsigned g =3D 123; g !=3D 0; g--)
  {
    h--;
    if(h =3D=3D 0)
       foo();
  }
}
```
The relationship between h and g is `h =3D g + 5` so you can never get `h =
=3D=3D 0`
in the loop as the range of g is [123,0].

Note In the original testcase, only GCC 8-11 dom3 (on x86_64; but not on
aarch64 due to IVOPTS differences) could optimize away the call to foo.

Also note LLVM can optimize it away both the above testcase and the original
one.=