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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102982] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Thu, 28 Oct 2021 09:47:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102982

            Bug ID: 102982
           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

void foo(void);

static short a, b, d;
static int c[2][2][2] =3D {0};

int main() {
  c[0][0][0] =3D 0;
  if (c[1][1][1]) {
    d =3D a % b >=3D 1 ^ 2;
    foo();
  }
}


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

gcc-11 test.c -O3 -S -o /dev/stdout
...
main:
.LFB0:
        .cfi_startproc
        movl    $0, c(%rip)
        movl    c+28(%rip), %eax
        testl   %eax, %eax
        je      .L2
        ud2
        .p2align 4,,10
        .p2align 3
.L2:
        xorl    %eax, %eax
        ret
        .cfi_endproc



gcc-trunk test.c -O3 -S -o /dev/stdout
...
main:
.LFB0:
        .cfi_startproc
        movl    $0, c(%rip)
        movl    c+28(%rip), %ecx
        testl   %ecx, %ecx
        je      .L4
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
.L4:
        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 20211028 (experimental) (GCC)

Started with
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3Db58dc0b803057c0e6032e0d=
9bd92cd834f72c75c=