[Bug tree-optimization/114972] New: [11/12/13/14/15 Regression] Missed optimization for Dead Code Elimination

[Bug tree-optimization/114972] New: [11/12/13/14/15 Regression] Missed optimization for Dead Code Elimination

[652023330028 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114972

            Bug ID: 114972
           Summary: [11/12/13/14/15 Regression] Missed optimization for
                    Dead Code Elimination
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 652023330028 at smail dot nju.edu.cn
  Target Milestone: ---

Hello, we noticed that the code below can be optimized by Dead Code
Elimination, but gcc -O3 missed it.

This is a regression from GCC-6.1.

https://godbolt.org/z/fGzMeba66

int a, b, m;
void func() {
    m = a + b;
    a++;
    b--;
    if(a+b-m != 0) m=100;  //dead code
}

GCC -O3:
func():
        mov     eax, DWORD PTR a[rip]
        mov     edx, DWORD PTR b[rip]
        lea     ecx, [rax+rdx]
        add     eax, 1
        sub     edx, 1
        mov     DWORD PTR a[rip], eax
        add     eax, edx
        cmp     ecx, eax
        mov     eax, 100
        mov     DWORD PTR b[rip], edx
        cmovne  ecx, eax
        mov     DWORD PTR m[rip], ecx
        ret

Expected code (GCC-5.5)
func():
        mov     edx, DWORD PTR a[rip]
        mov     eax, DWORD PTR b[rip]
        lea     ecx, [rdx+rax]
        add     edx, 1
        sub     eax, 1
        mov     DWORD PTR a[rip], edx
        mov     DWORD PTR b[rip], eax
        mov     DWORD PTR m[rip], ecx
        ret

Thank you very much for your time and effort! We look forward to hearing from
you.

[Bug tree-optimization/114972] [11/12/13/14/15 Regression] Missed optimization for Dead Code Elimination

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114972

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.5
           Keywords|                            |missed-optimization

[Bug tree-optimization/114972] [11/12/13/14/15 Regression] Missed optimization for Dead Code Elimination

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114972

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-05-07

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

FRE was able to do:
RHS a.0_13 + b.1_14 simplified to a.0_3 + b.1_4 has constants 0

But then in GCC 6 it can't see that it is that.

Maybe when something moved over to match and simplify it is missed ...