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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103281] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Tue, 16 Nov 2021 18:31:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103281

            Bug ID: 103281
           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 foo(void);

static unsigned b;

int main() {
  for (; b < 3; b++) {
    char c =3D b;
    char a =3D c ? c : c << 1;
    if (!(a < 1 ^ b))
      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
        movl    b(%rip), %eax
        cmpl    $2, %eax
        ja      .L6
        testl   %eax, %eax
        movl    $1, %edx
        cmove   %edx, %eax
.L8:
        addl    $1, %eax
        movl    %eax, b(%rip)
        cmpl    $2, %eax
        je      .L8
.L6:
        xorl    %eax, %eax
        ret
        .cfi_endproc


gcc-trunk -O3 -S -o /dev/stdout case.c
main:
.LFB0:
        .cfi_startproc
        movl    b(%rip), %eax
        cmpl    $2, %eax
        ja      .L13
        .p2align 4,,10
        .p2align 3
.L12:
        xorl    %edx, %edx
        testb   %al, %al
        setle   %dl
        cmpl    %eax, %edx
        je      .L16
        addl    $1, %eax
        movl    %eax, b(%rip)
        cmpl    $3, %eax
        jne     .L12
.L13:
        xorl    %eax, %eax
        ret
        .p2align 4,,10
        .p2align 3
.L16:
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
.L14:
        call    foo
        movl    b(%rip), %eax
        addl    $1, %eax
        movl    %eax, b(%rip)
        cmpl    $2, %eax
        ja      .L7
.L2:
        xorl    %edx, %edx
        testb   %al, %al
        setle   %dl
        cmpl    %eax, %edx
        je      .L14
        addl    $1, %eax
        movl    %eax, b(%rip)
        cmpl    $3, %eax
        jne     .L2
.L7:
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc


gcc-trunk -v
Using built-in specs.
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=3Dgcc.git;a=3Dcommit;h=3D7d6979197274a662da7bdc5=
64314afe8415865c1=