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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102705] New: [12 Regression] Dead Code
 Elimination Regression at -O3 (trunk vs 11.2.0)
Date: Tue, 12 Oct 2021 13:41:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102705

            Bug ID: 102705
           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 int b;
static char c;
static short a(short d, short e) { return e =3D=3D 0 || d && e =3D=3D 1 ? 0=
 : d % e; }
int main() {
  b =3D c =3D b >=3D 2 ? 0 : 1 >> b;
  short f =3D a(0 >=3D 0 ^ c, 5);
  if (f =3D=3D c)
    foo();
  a(0, 9);
}

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

gcc-11.2.0 test.c -S -O3 -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        movl    b(%rip), %ecx
        movl    $1, %eax
        sarl    %cl, %eax
        movl    %eax, %edx
        xorl    %eax, %eax
        cmpl    $2, %ecx
        cmovl   %edx, %eax
        movsbl  %al, %eax
        movl    %eax, b(%rip)
        xorl    %eax, %eax
        ret
        .cfi_endproc


gcc-trunk test.c -S -O3 -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        movl    b(%rip), %ecx
        cmpl    $1, %ecx
        jle     .L2
        xorl    %ecx, %ecx
        movl    %ecx, b(%rip)
.L5:
        xorl    %eax, %eax
        ret
.L2:
        movl    $1, %eax
        sarl    %cl, %eax
        movl    %eax, %edx
        movl    %eax, b(%rip)
        xorl    $1, %edx
        movsbw  %dl, %dx
        cmpw    %ax, %dx
        jne     .L5
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc

gcc-trunk -v=20=20=20
Using built-in specs.
Target: x86_64-pc-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211012 (experimental) (GCC)

It started with
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3D145bc41dae7c7bfa093d61e=
77346f98e6a595a0e=