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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	[thread overview]
Message-ID: <bug-99987-4-3S3uBCGgDt@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99987-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99987

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 = 123+5;
  for(unsigned g = 123; g != 0; g--)
  {
    h--;
    if(h == 0)
       foo();
  }
}
```
The relationship between h and g is `h = g + 5` so you can never get `h == 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.

  parent reply	other threads:[~2023-08-18  6:36 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-09  9:54 [Bug ipa/99987] New: " zhendong.su at inf dot ethz.ch
2021-04-09 10:52 ` [Bug tree-optimization/99987] " rguenth at gcc dot gnu.org
2023-05-05  7:50 ` [Bug tree-optimization/99987] [12/13/14 Regression] " pinskia at gcc dot gnu.org
2023-05-08 12:21 ` rguenth at gcc dot gnu.org
2023-05-09 11:39 ` marxin at gcc dot gnu.org
2023-08-18  6:13 ` pinskia at gcc dot gnu.org
2023-08-18  6:36 ` pinskia at gcc dot gnu.org [this message]
2024-03-10  2:45 ` law at gcc dot gnu.org

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