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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104526] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs. 11.2.0)
Date: Mon, 14 Feb 2022 10:07:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104526

            Bug ID: 104526
           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 #138750
void foo(void);

static int a, b =3D 1, *c =3D &b;
int main() {
  for (; a; a--) {
    int d =3D 2 >> (1 / *c);
    if (!d)
      foo();
  }
}

gcc-58aeb75d4097010ad9bb72b964265b18ab284f93 (trunk) -O3 can not eliminate =
foo
but gcc-11.2.0 -O3 can.

gcc-58aeb75d4097010ad9bb72b964265b18ab284f93 (trunk) -O3 -S -o /dev/stdout
case.c
--------- OUTPUT ---------
main:
.LFB0:
        .cfi_startproc
        movl    a(%rip), %eax
        testl   %eax, %eax
        je      .L12
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        xorl    %ebx, %ebx
.L2:
        movl    b(%rip), %ecx
        leal    1(%rcx), %eax
        cmpl    $2, %eax
        movl    $2, %eax
        cmova   %ebx, %ecx
        sarl    %cl, %eax
        testl   %eax, %eax
        je      .L3
        movl    $0, a(%rip)
.L10:
        xorl    %eax, %eax
        popq    %rbx
        .cfi_remember_state
        .cfi_def_cfa_offset 8
        ret
        .p2align 4,,10
        .p2align 3
.L3:
        .cfi_restore_state
        call    foo
        subl    $1, a(%rip)
        jne     .L2
        jmp     .L10
.L12:
        .cfi_def_cfa_offset 8
        .cfi_restore 3
        xorl    %eax, %eax
        ret
---------- END OUTPUT ---------


gcc-11.2.0 -O3 -S -o /dev/stdout case.c
--------- OUTPUT ---------
main:
.LFB0:
        .cfi_startproc
        movl    a(%rip), %eax
        testl   %eax, %eax
        je      .L2
        movl    $0, a(%rip)
.L2:
        xorl    %eax, %eax
        ret
---------- END OUTPUT ---------


Bisects to:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3Dc2b610e7c6c89fd422c5c31=
f01023bcddf3cf4a5

----- Build information -----
----- 58aeb75d4097010ad9bb72b964265b18ab284f93 (trunk)
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 12.0.1 20220213 (experimental) (GCC)

----- releases/gcc-11.2.0
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 11.2.0 (GCC)=