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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109546] New: [13 Regression]  Missed Dead
 Code Elimination when using __builtin_unreachable
Date: Tue, 18 Apr 2023 15:07:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109546

            Bug ID: 109546
           Summary: [13 Regression]  Missed Dead Code Elimination when
                    using __builtin_unreachable
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: theodort at inf dot ethz.ch
  Target Milestone: ---

cat input.c=20

void foo(void);
static int a, c;
static int *b =3D &a;
static int **d =3D &b;
void __assert_fail() __attribute__((__noreturn__));
int main() {
  int *e =3D *d;
  if (e =3D=3D &a || e =3D=3D &c);
  else {
  __assert_fail();
  }
  if (e =3D=3D &a || e =3D=3D &c);
  else
    foo();
}

both gcc 12 and trunk at -O3 generate the following code:

main:
        movq    b(%rip), %rax
        cmpq    $c, %rax
        je      .L2
        cmpq    $a, %rax
        jne     .L7
.L2:
        xorl    %eax, %eax
        ret
.L7:
        pushq   %rax
        xorl    %eax, %eax
        call    __assert_fail
b:
        .quad   a

the call to foo is eliminated but the call to __assert_fail is missed, even
though=20
both if statement conditions are identical.=20

If add a __builtin_unreachable before the call to __assert_fail the opposite
happens:

void foo(void);
static int a, c;
static int *b =3D &a;
static int **d =3D &b;
void assert_fail() __attribute__((__noreturn__));
int main() {
  int *e =3D *d;
  if (e =3D=3D &a || e =3D=3D &c);
  else {
    __builtin_unreachable();=20
  assert_fail();
  }
  if (e =3D=3D &a || e =3D=3D &c);
  else
    foo();
}

gcc-trunk -O3 generates:

main:
        movq    b(%rip), %rax
        cmpq    $c, %rax
        je      .L4
        cmpq    $a, %rax
        je      .L4
        pushq   %rax
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        ret
.L4:
        xorl    %eax, %eax
        ret
b:
        .quad   a

the call to __assert_fail is now properly eliminated but now the call to fo=
o is
missed.=20

This is a regression as gcc-12 generates:=20

main:
        xorl    %eax, %eax
        ret=