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From: "yann at ywg dot ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108358] New: Dead Code Elimination Regression
 at -Os (trunk vs. 12.2.0)
Date: Tue, 10 Jan 2023 13:03:03 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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X-Bugzilla-Who: yann at ywg dot ch
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108358

            Bug ID: 108358
           Summary: Dead Code Elimination Regression at -Os (trunk vs.
                    12.2.0)
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yann at ywg dot ch
  Target Milestone: ---

Created attachment 54233
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D54233&action=3Dedit
presented code as file

cat case.c #25
struct a {
  int b;
  int c;
  short d;
  int e;
  int f;
};
struct g {
  struct a f;
  struct a h;
};
int i;
void foo();
void bar31_(void);
int main() {
  struct g j, l =3D {2, 1, 6, 1, 1, 7, 5, 1, 0, 1};
  for (; i; ++i)
    bar31_();
  j =3D l;
  struct g m =3D j;
  struct g k =3D m;
  if (k.h.b)
    ;
  else
    foo();
}

`gcc-cb93c5f8008b95743b741d6f1842f9be50c6985c (trunk) -Os` can not eliminate
`foo` but `gcc-releases/gcc-12.2.0 -Os` can.

`gcc-cb93c5f8008b95743b741d6f1842f9be50c6985c (trunk) -Os -S -o /dev/stdout
case.c`
--------- OUTPUT ---------
main:
.LFB0:
        .cfi_startproc
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $.LC0, %esi
        movl    $10, %ecx
        subq    $80, %rsp
        .cfi_def_cfa_offset 96
        leaq    40(%rsp), %rdi
        leaq    40(%rsp), %rbx
        rep movsl
.L2:
        cmpl    $0, i(%rip)
        je      .L7
        call    bar31_
        incl    i(%rip)
        jmp     .L2
.L7:
        movq    %rsp, %rdi
        movl    $10, %ecx
        movq    %rbx, %rsi
        rep movsl
        cmpl    $0, 20(%rsp)
        jne     .L4
        xorl    %eax, %eax
        call    foo
.L4:
        addq    $80, %rsp
        .cfi_def_cfa_offset 16
        xorl    %eax, %eax
        popq    %rbx
        .cfi_def_cfa_offset 8
        ret
---------- END OUTPUT ---------


`gcc-releases/gcc-12.2.0 -Os -S -o /dev/stdout case.c`
--------- OUTPUT ---------
main:
.LFB0:
        .cfi_startproc
        cmpl    $0, i(%rip)
        jne     .L7
        xorl    %eax, %eax
        ret
.L7:
        pushq   %rax
        .cfi_def_cfa_offset 16
.L3:
        call    bar31_
        incl    i(%rip)
        cmpl    $0, i(%rip)
        jne     .L3
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
---------- END OUTPUT ---------


Bisects to:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3Df6c168f8c06047bfaa3005e=
570126831b8855dcc=