[Bug tree-optimization/103280] New: [12 Regression] Dead Code Elimination Regression at -O3 (trunk vs 11.2.0)

[Bug tree-optimization/103280] New: [12 Regression] Dead Code Elimination Regression at -O3 (trunk vs 11.2.0)

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103280

            Bug ID: 103280
           Summary: [12 Regression] Dead Code Elimination Regression at
                    -O3 (trunk vs 11.2.0)
           Product: gcc
           Version: 12.0
            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 case.c
void bar(void);
void foo(void);

static char a;
int main() {
  short b = -25536;
  for (; a; a = 1) {
    bar();
    if (b)
      b = 1;
    else
      foo();
  }
}

trunk cannot eliminate the call to foo but 11.2.0 can:

gcc-11.2.0 -O3 -S -o /dev/stdout case.c
main:
.LFB0:
        .cfi_startproc
        cmpb    $0, a(%rip)
        je      .L2
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    bar
        .p2align 4,,10
        .p2align 3
.L3:
        movb    $1, a(%rip)
        call    bar
        movb    $1, a(%rip)
        call    bar
        jmp     .L3
        .p2align 4,,10
        .p2align 3
.L2:
        .cfi_def_cfa_offset 8
        xorl    %eax, %eax
        ret
        .cfi_endproc

gcc-trunk -O3 -S -o /dev/stdout case.c
main:
.LFB0:
        .cfi_startproc
        cmpb    $0, a(%rip)
        je      .L2
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $-25536, %ebx
        jmp     .L4
        .p2align 4,,10
        .p2align 3
.L5:
        movl    $1, %ebx
.L3:
        movb    $1, a(%rip)
.L4:
        call    bar
        testw   %bx, %bx
        jne     .L5
        call    foo
        jmp     .L3
        .p2align 4,,10
        .p2align 3
.L2:
        .cfi_def_cfa_offset 8
        .cfi_restore 3
        xorl    %eax, %eax
        ret
        .cfi_endproc

gcc-trunk -v
Using built-in specs.
Target: x86_64-pc-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211116 (experimental) (GCC)

Started with
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=e53fbb17839723ea1697fcbaf76b1c092675eaaf

[Bug tree-optimization/103280] [12 Regression] Dead Code Elimination Regression at -O3 (trunk vs 11.2.0)

[aldyh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103280

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
Duplicate.

There's a threading candidate in 2->6->3->5 that would elide the call to foo(),
but doing so would peel off an iteration.  This is another case of the first
iteration of a loop having dead code.

*** This bug has been marked as a duplicate of bug 102981 ***