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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102950] New: [11 Regression] Dead Code
 Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)
Date: Tue, 26 Oct 2021 15:21:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102950

            Bug ID: 102950
           Summary: [11 Regression] Dead Code Elimination Regression at
                    -O3 (trunk&11.2.0 vs 10.3.0)
           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 char a;
static short d(unsigned e) {
  char b;
  short c;
  a =3D b =3D e;
  if (b)
    return 0;
  if (1 >=3D e) {
    c =3D e =3D=3D 0;
    if (c)
      foo();
  }
  return 0;
}
int main() { d(a ^ 233); }

10.3.0 at -O3 can eliminate the call to foo but neither trunk nor 11.2.0 at=
 -O3
can:

gcc-10 -O3 -S test.c -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        xorb    $-23, a(%rip)
        xorl    %eax, %eax
        ret
        .cfi_endproc

gcc-11 -O3 -S test.c -o /dev/stdout
...
main:
.LFB1:
        .cfi_startproc
        movsbl  a(%rip), %eax
        xorb    $-23, %al
        movb    %al, a(%rip)
        cmpl    $1, %eax
        ja      .L10
        testb   %al, %al
        je      .L14
.L10:
        xorl    %eax, %eax
        ret
.L14:
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc


gcc-trunk -O3 -S test.c -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        movsbl  a(%rip), %eax
        xorb    $-23, %al
        movb    %al, a(%rip)
        cmpl    $1, %eax
        ja      .L10
        testb   %al, %al
        je      .L14
.L10:
        xorl    %eax, %eax
        ret
.L14:
        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
Using built-in specs.
Target: x86_64-pc-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211022 (experimental) (GCC)

Introduced with
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3Dfcae5121154d1c3382b056b=
cc2c563cedac28e74=