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From: "652023330028 at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112746] New: Missed optimization for
 redundancy computation elimination (fre1(tree) for global variable)
Date: Tue, 28 Nov 2023 16:10:09 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112746

            Bug ID: 112746
           Summary: Missed optimization for redundancy computation
                    elimination (fre1(tree) for global variable)
           Product: gcc
           Version: 14.0
            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 maybe there is a missed optimization for redundancy
computation elimination.

When b is a local variable, the computation of m optimizes to 3 as expected.
When b appears as a global variable, it misses this optimization.

https://godbolt.org/z/j3cKsrKb6

int n,m;
int b;
void test(int b){
    b=3Dn+n;
    m=3D(n+b)/(n);
}
void test2(){
    b=3Dn+n;
    m=3D(n+b)/(n);
}

But GCC -O3 -fwrapv or GCC -O3:
test(int):
        mov     DWORD PTR m[rip], 3
        ret
test2():
        mov     ecx, DWORD PTR n[rip]
        lea     eax, [rcx+rcx]
        mov     DWORD PTR b[rip], eax
        add     eax, ecx
        cdq
        idiv    ecx
        mov     DWORD PTR m[rip], eax
        ret

Expected code (Clang):
test2():                              # @test2()
        mov     eax, dword ptr [rip + n]
        add     eax, eax
        mov     dword ptr [rip + b], eax
        mov     dword ptr [rip + m], 3
        ret

We see a difference in fre1(tree) for these two functions, which might help
with this issue:
void test (int b)
{
  int n.0_1;
  int _3;
  int _5;

  <bb 2> :
  # DEBUG BEGIN_STMT
  n.0_1 =3D n;
  b_7 =3D n.0_1 * 2;
  # DEBUG b =3D> b_7
  # DEBUG BEGIN_STMT
  _3 =3D n.0_1 * 3;
  _5 =3D 3;
  m =3D 3;
  return;

}

void test2 ()
{
  int n.3_1;
  int _2;
  int _5;
  int _7;

  <bb 2> :
  # DEBUG BEGIN_STMT
  n.3_1 =3D n;
  _2 =3D n.3_1 * 2;
  b =3D _2;
  # DEBUG BEGIN_STMT
  _5 =3D n.3_1 + _2;
  _7 =3D _5 / n.3_1;
  m =3D _7;
  return;

}


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