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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/104530] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs. 11.2.0)
Date: Mon, 14 Feb 2022 16:40:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104530

            Bug ID: 104530
           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: ---

Dead Code Elimination Regression at -O3 (trunk vs.
a7fede6704dd207fb32b97bc30f945acc7b953c5) 138751
---------------
cat case.c #138751
void foo(void);

static int a, *b =3D &a, c, d =3D 1;

int main() {
    c =3D 0 =3D=3D b;
    a =3D *b;
    if (c % d)
        for (; d; --d)
            foo();
    b =3D 0;
}

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
        movq    b(%rip), %rax
        testq   %rax, %rax
        movl    (%rax), %edx
        sete    %al
        movzbl  %al, %eax
        movl    %edx, a(%rip)
        cltd
        idivl   d(%rip)
        testl   %edx, %edx
        je      .L9
        pushq   %rax
        .cfi_def_cfa_offset 16
        .p2align 4,,10
        .p2align 3
.L3:
        call    foo
        subl    $1, d(%rip)
        jne     .L3
        xorl    %ecx, %ecx
        xorl    %eax, %eax
        movq    %rcx, b(%rip)
        popq    %rsi
        .cfi_def_cfa_offset 8
        ret
.L9:
        xorl    %edx, %edx
        xorl    %eax, %eax
        movq    %rdx, b(%rip)
        ret
---------- END OUTPUT ---------


gcc-11.2.0 -O3 -S -o /dev/stdout case.c
--------- OUTPUT ---------

main:
.LFB0:
        .cfi_startproc
        movq    b(%rip), %rax
        movq    $0, b(%rip)
        movl    (%rax), %eax
        movl    %eax, a(%rip)
        xorl    %eax, %eax
        ret
---------- END OUTPUT ---------


Bisects to:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3Dc6bb1db76b3ac127aff7dac=
f391fc1798a94bb7d

----- Build information -----
----- 58aeb75d4097010ad9bb72b964265b18ab284f93 (trunk)
Target: x86_64-pc-linux-gnu
Configured with: ../configure --disable-multilib --disable-bootstrap
--enable-languages=3Dc,c++
--prefix=3D/zdata/compiler_cache/gcc-58aeb75d4097010ad9bb72b964265b18ab284f=
93
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++
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0   (GCC)=