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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102892] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Fri, 22 Oct 2021 09:39:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102892

            Bug ID: 102892
           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 test.c
static long b[2][1] =3D {0};
void bar(void);
void foo(void);
int main() {
  long c =3D 0;
  for (long a; a < 1; ++a)
    for (; c <=3D 1; c++) {
      bar();
      if (1 =3D=3D b[c][0])
        foo();
    }
}

11.2.0 at -O3 can eliminate the call to foo but trunk at -O3 cannot:

gcc-11 -O3 -S test.c -o /dev/stdout
...
main:
.LFB0:
        .cfi_startproc
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
        call    bar
        call    bar
        xorl    %eax, %eax
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc

gcc-trunk-O3 -S test.c -o /dev/stdout
main:
.LFB0:
        .cfi_startproc
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        xorl    %ebx, %ebx
        call    bar
        cmpq    $1, b(,%rbx,8)
        je      .L11
        .p2align 4,,10
        .p2align 3
.L3:
        cmpq    $1, %rbx
        je      .L12
.L5:
        call    bar
        movl    $1, %ebx
        cmpq    $1, b(,%rbx,8)
        jne     .L3
.L11:
        call    foo
        cmpq    $1, %rbx
        jne     .L5
.L12:
        xorl    %eax, %eax
        popq    %rbx
        .cfi_def_cfa_offset 8
        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 20211022 (experimental) (GCC)

Introduced with
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3D01b5038718056b024b370b7=
4a874fbd92c5bbab3=