[Bug tree-optimization/110502] New: [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[Bug tree-optimization/110502] New: [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

            Bug ID: 110502
           Summary: [14 Regression] Dead Code Elimination Regression at
                    -Os since r14-1656-g55fcaa9a8bd
           Product: gcc
           Version: 14.0
            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: ---

https://godbolt.org/z/T9rv1nsGr

Given the following code:

void foo(void);
static char e = -3L, k;
static int f;
static int *g;
static signed char *h = &e;
static signed char **i = &h;
static unsigned j;
static char(a)(char b, char c) { return b + c; }
static int(d)(int b, int c) { return b && b < -c ? b : b + c; }
static char l(char *n, unsigned short o) {
    char p;
    int q, s = 4104765887;
    int *r = &f;
    if (!(((o) >= 65533) && ((o) <= 65533))) {
        __builtin_unreachable();
    }
    for (; s <= 4; s = a(s, 5)) {
        int **t = &g;
        *t = &s;
        j++;
        p = *n;
        q = d(*g, p);
        *r = q;
    }
    if (g || f)
        ;
    else
        foo();
    return 0;
}
int main() {
    l(&e, e);
    char m = &k == (*i = &m);
}

gcc-trunk -Os does not eliminate the call to foo:

main:
        pushq   %rbx
        movl    $-190201409, %edx
        xorl    %esi, %esi
        subq    $16, %rsp
        movl    j(%rip), %ecx
        movl    $-190201409, 12(%rsp)
        leal    15(%rcx), %r8d
.L2:
        leal    1(%rcx), %edi
        cmpl    %r8d, %edi
        je      .L8
        testl   %edx, %edx
        movb    $1, %sil
        sete    %al
        cmpl    $2, %edx
        setg    %cl
        orl     %ecx, %eax
        movl    %edi, %ecx
        movzbl  %al, %eax
        negl    %eax
        andl    $-3, %eax
        addl    %edx, %eax
        addl    $5, %edx
        movsbl  %dl, %edx
        jmp     .L2
.L8:
        testb   %sil, %sil
        leaq    12(%rsp), %rbx
        je      .L4
        movq    %rbx, g(%rip)
        movl    %ecx, j(%rip)
        movl    %eax, f(%rip)
        jmp     .L5
.L4:
        cmpq    $0, g(%rip)
        jne     .L5
        cmpl    $0, f(%rip)
        jne     .L5
        call    foo
.L5:
        movq    %rbx, h(%rip)
        addq    $16, %rsp
        xorl    %eax, %eax
        popq    %rbx
        ret

gcc-13.1.0 -Os eliminates the call to foo:

main:
        movl    j(%rip), %edx
        movl    $-190201409, %eax
        leal    14(%rdx), %esi
.L4:
        testl   %eax, %eax
        je      .L6
        movl    %eax, %ecx
        cmpl    $2, %eax
        jle     .L2
.L6:
        leal    -3(%rax), %ecx
.L2:
        addl    $5, %eax
        incl    %edx
        movsbl  %al, %eax
        cmpl    %esi, %edx
        jne     .L4
        leaq    -4(%rsp), %rax
        movl    %ecx, f(%rip)
        movq    %rax, g(%rip)
        movq    %rax, h(%rip)
        xorl    %eax, %eax
        movl    %edx, j(%rip)
        ret

Bisects to r14-1656-g55fcaa9a8bd

[Bug tree-optimization/110502] [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
   Target Milestone|---                         |14.0
             Target|                            |x86_64-linux-gnu

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note on aarch64, it is still optimized (not you need -fsigned-char there or
change all char to be `signed char`).

Note also with -fno-ivopts, even GCC 13 is not optimizing out the call to foo.

[Bug tree-optimization/110502] [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-06-30
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/110502] [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note LIM2 has way different IR selection which is kinda interesting since there
are no stores that happened in the bbs that were removed:
  if (_2 != 0)
    goto <bb 4>; [75.00%]
  else
    goto <bb 5>; [25.00%]

  <bb 4> [local count: 6515660663]:
  iftmp.8_23 = s.5_14 + -3;

  <bb 5> [local count: 8687547565]:
  # iftmp.8_24 = PHI <s.5_14(3), iftmp.8_23(4)>

[Bug tree-optimization/110502] [14 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org
           Priority|P3                          |P2

[Bug tree-optimization/110502] [14/15 Regression] Dead Code Elimination Regression at -Os since r14-1656-g55fcaa9a8bd

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|14.0                        |14.2

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 14.1 is being released, retargeting bugs to GCC 14.2.