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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/102650] New: Dead Code Elimination Regression
 at -O3 (trunk vs 11.2.0)
Date: Fri, 08 Oct 2021 13:47:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102650

            Bug ID: 102650
           Summary: 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=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
      bisections
static int a =3D 2, b, c, d;
void foo(void);
int main() {
    short e;
    int f =3D -1;
    if (b)
        c =3D 0;
    c || (f =3D 2);
    for (; d < 1; d++)
        e =3D f + a;
    if (!e)
        foo();
    return 0;
}


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

gcc-11 -v
Target: x86_64-pc-linux-gnu
Configured with: ../configure --disable-multilib --disable-bootstrap
--enable-languages=3Dc,c++
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)

gcc-11 -O3 case.c -S -o /dev/stdout
...
main:
.LFB0:
        .cfi_startproc
        movl    d(%rip), %eax
        testl   %eax, %eax
        jg      .L2
        movl    $1, d(%rip)
.L2:
        xorl    %eax, %eax
        ret

gcc-trunk -v
Target: x86_64-pc-linux-gnu
Configured with: ../configure --disable-multilib --disable-bootstrap
--enable-languages=3Dc,c++=20
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211008 (experimental) (GCC)

gcc-trunk -O3 case.c -S -o /dev/stdout
...
main:
.LFB0:
        .cfi_startproc
        movl    d(%rip), %ecx
        testl   %ecx, %ecx
        jg      .L3
        movl    $1, d(%rip)
        xorl    %eax, %eax
        ret
.L3:
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret


18b88412069f51433e1b4f440d3c035bfc7b5cca
(https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3D18b88412069f51433e1b4f=
440d3c035bfc7b5cca)
introduced this regression=