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From: "carnet at student dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113422] New: Missed optimizations in the
 presence of pointer chains
Date: Tue, 16 Jan 2024 13:21:51 +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=3D113422

            Bug ID: 113422
           Summary: Missed optimizations in the presence of pointer chains
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carnet at student dot ethz.ch
  Target Milestone: ---

https://godbolt.org/z/hdWKn4bjc

All three functions write to the same variable (b). Clang is able to optimi=
ze
this. GCC -O3 cannot always do this.
Assembly for foo is writing to **d instead to directly write 1 to b. There =
is
similar behavior for bar and baz. Clang optimizes the code to directly writ=
e to
b.
Baz is fully optimized only if the foo and bar are removed.

int b =3D 0;
static int *c =3D &b;
static int **d =3D &c;
static int ***e =3D &d;

void foo() {***e =3D 1;}
void bar() {**d =3D 1;}
void baz() {*c =3D 1;}

Assembly code:
foo:
        movq    d(%rip), %rax
        movq    (%rax), %rax
        movl    $1, (%rax)
        ret=